Journal of Volcanology and Geothermal Research 207 (2011) 47–66

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Journal of Volcanology and Geothermal Research

j ourna l homepage: www.e lsev ie r.com/ locate / jvo lgeores

Evolution of shield-building and rejuvenescent volcanism of Mauritius

Jacob Moore a,1, William M. White a,⁎, Debajyoti Paul b,2, Robert A. Duncan c,Wafa Abouchami d, Stephen J.G. Galer d

a Department of Earth and Atmospheric Sciences, Cornell University, Snee Hall, Ithaca, NY 14583, USAb Department of Earth and Environmental Science, The University of Texas San Antonio, One UTSA Circle, San Antonio, TX 78249, USAc College of Oceanic and Atmospheric Sciences, Oregon State University, Ocean Administration Building 104, Corvallis, OR 97331, USAd Max-Planck-Institut für Chemie, Abteilung Biogeochemie, Postfach 3060, D-55020 Mainz, Germany

⁎ Corresponding author. Tel.: +1 607 255 7466; fax:E-mail address: wmw4@cornell.edu (W.M. White).

1 Now at Ellington & Associates, Inc., 1414 Lumpkin R2 Now at Department of Civil Engineering (Geos

Technology Kanpur, 208016 (UP), Kanpur, India.

0377-0273/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2011.07.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 November 2010Accepted 16 July 2011Available online 30 July 2011

We report chemical and isotopic analyses of 68 samples and 40Ar/39Ar ages of 47 samples from Mauritiusundertaken to understand the compositional evolution of the volcano and its causes through time. New40Ar/39Ar ages show that construction of the Mauritius shield was well underway by 8.9 Ma, 1.1 m.y. earlierthan previously thought and that the hiatus between the Intermediate and Younger Series was shorter thanpreviously thought, as eruption of the rejuvenescent Intermediate Series continued through at least 1.66 Maand Younger Series volcanism began by at least 1.0 Ma. Eruption frequency over the last 50 ka has been rathertypical of Younger Series volcanism over the last 400 ka and future eruptions are possible. Although outcropsof the Intermediate Series lavas are confined to the Southwest, Intermediate Series are present beneathYounger Series flows in drill cores throughout the island. We estimate the total volume of rejuvenescent lavasat ~35 km3 or about 0.05% of the volume of the volcano, similar to rejuvenescent volume fractions onHawaiian volcanoes. As earlier studies found, Older Series lavas, which on average are slightly normativelysilica-saturated, are somewhat more incompatible-element enriched than are the Intermediate and YoungerSeries, which are both slightly silica-undersaturated on average. Mean Sr and Nd isotope ratios of theIntermediate and Younger Series are nearly identical, but mean Pb isotope ratios, La/Sm, Nb/Y, and Nb/Zr ofthe Intermediate Series are higher than in the Younger Series. New high precision Pb isotope data, whichshows considerably less scatter than previously published data, plot between the Older Series and basalts ofthe Central Indian Ridge, allowing the possibility that the source of the rejuvenescent lavas is a mixture of thisplume and depleted mantle. We propose two possible explanations for the composition of rejuvenescentlavas. The first is that plume-derived melts reacted with deep lithosphere to form pyroxenite veins during theearly shield-building stage. Later, these veins melted as a consequence of conductive heating of thelithosphere by the plume to produce the rejuvenescent lavas. Alternatively, rejuvenescent lavas may bederived from a sheath of thermally entrained mantle that surrounds the plume and is a mixture of plumematerial and depleted upper mantle.

+1 607 254 4780.

oad, Houston, TX 77043, USA.ciences), Indian Institute of

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction and regional setting

“You gather the idea that Mauritius was made first, and then heaven;and that heaven was copied after Mauritius.”

–Mark Twain, “Following the Equator”, 1897

Oceanic island volcanoes are thought to be produced as convectivecolumns of hot, rising mantle approach the Earth's surface and melt(Morgan, 1971). As the lithospheric plate upon which it rides movessteadily away from the mantle plume, the volcano's connection to it isbrokenandthevolcanobecomesextinct as anewvolcanobegins to growjust upstream (Wilson, 1963). This progressive growth and death ofvolcanoes produces the volcanic chains that decorate the oceanfloor likestrings of pearls. Best known are the island chains of the Pacific such asHawaii and Samoa, but they occur in the other oceans as well. Mauritiusis the penultimate island of the Réunion hotspot track (e.g., Morgan,1981), inferred to have erupted from magmas of the deep-seatedRéunion mantle plume (Courtillot et al., 2003; Montelli et al., 2004) asthe Indian plate moved northeastward over the stable plume.

In all three of these examples – Hawaii, Samoa, and Mauritius – thissimplemantleplumemodel fails in an important respect: volcanic activity

48 J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

renews following hiatuses of a million years or more, long after newupstream volcanoes have emerged. In Hawaii, Stearns (1940) andMacdonald and Katsura (1964) referred to this phase of volcanism as“post-erosional”. Similar “post-erosional” or “rejuvenescent” volcanismhas subsequently been recognized on other oceanic island chains,including the Society Islands, Samoa, Madeira, the Canary Islands, andthe Austral Islands as well as seamount chains such as Louisville andMagellan. Particularly problematic is the observation that the chemicalcompositions of themain shield stage and the rejuvenescent volcanics areinvariably different, with the rejuvenescent phase being generally moresilica-undersaturated and incompatible element-enriched. This com-positional shift is accompanied by a shift in isotopic compositions,paradoxically, to more long-term incompatible element-depleted signa-tures (e.g., Chen and Frey, 1983; Roden et al., 1984; White and Duncan,1996).

Mauritius represents a particularly intriguing example of rejuve-nescent volcanism because there appears to be two separate epi-

M6

M17

M22M36

M39

B1B2

57°20’E 57°30’E

57°20’E 57°30’E20°00’S

20°10’S

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20°30’S

N

Tru

e N

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Older Series (8.4 — 5.5 Ma)

Intermediate Series (3.5 — 1.9 Ma

Younger Series (1.00 — 0.00 Ma)

Caldera Limit0 5 km

Drillcore sample

Surface sample

Crater

M1

M16

B2

B

M21C46

C2

C

Sediments

C

M37

M12M35

M15

C18M20

Fig. 1. Geologic map of Mauritius modified from Giorgi et al. (1999) and Baxter (1972) depic(orange). Locations of drill core (square) and surficial samples (circle) are shown. Inset sho

sodes of rejuvenescent volcanism (Simpson, 1950; McDougall andChamalaun, 1969; Baxter, 1972) and because the volumes of rejuve-nescent lavas appear to be particularly large, covering in excess of75% of the island's surface. Mauritius is located on the MascarenePlateau of the western Indian Ocean (20° 20′ S, 57° 35′ E; Fig. 1),approximately 1000 km east of Madagascar and 215 km northeastof Réunion Island. NUVEL 1A calculations of the modern Africanplate vector indicate Mauritius is drifting northeastward at 051° ata rate of approximately 24.3 mm/yr (DeMets et al., 1994), consistentwith the southwesterly younging ages of the plume-related volcanics(Duncan et al., 1989). Plate reconstructions of the Mascarene basinindicate Mauritius erupted through oceanic crust created at theCentral Indian Ridge (CIR) at about 60–65 Ma (Royer et al., 1992;Fretzdorff et al., 1998). The thickness of oceanic lithosphere of thisage is approximately 75 km. In addition to Mauritius, the Réunionhotspot is believed to be responsible for producing the Deccan Trapsof western India, the Chagos–Maldives–Laccadive Ridge system, the

M29

M33B6

B19

57°40’E20°00’S

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M32

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0

18 M30

62

C82

C76

?

?

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?

78B5

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RéunionMauritius

Rodriguez

Chagos-LaccadiveRidge

1000 m

3000 m

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Ridge

10°S

20°S

60°E 70°E

Mas

care

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au

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ting the Older Series (dark green), Intermediate Series (light blue), and Younger Seriesws regional geology of the western Indian Ocean.

49J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

Mascarene Plateau, and Réunion Island (Duncan et al., 1989; Duncanand Hargraves, 1990).

In the first systematic study of the island, Simpson (1950)recognized three episodes of eruptive activity interrupted by periodsof volcanic quiescence and erosion: the Older Series and the Early andLate lavas of the Younger Series. Baxter (1972) subsequently renamedthese the Older, Intermediate, and Younger Series. McDougalland Chamalaun (1969) found that the Older Series, which are theremnants of a massive shield volcano but are now restricted totopographic highs and the outer flanks of the island, were eruptedbetween 7.9 and 4.73 Ma. This period ended with the emplacement ofhighly differentiated trachytes. The Intermediate Series lavas eruptedfollowing a million year hiatus, with ages ranging from 3.5 to 1.9 Ma(McDougall and Chamalaun, 1969; Nohda et al., 2005). The YoungerSeries lavas erupted following a second hiatus, and have been dated as0.78 to 0.03 Ma (McDougall and Chamalaun, 1969; Nohda et al.,2005). These lavas emanate from a NE–SW aligned chain of ventsalong the spine of the island and form caldera- and valley-fillingflows (Fig. 1). Baxter (1972) found that the Older Series lavas weretransitional between tholeiites and alkali basalts (as are the basalts ofRéunion), while the two rejuvenescent lavas suites are somewhatmore alkaline than the Older Series lavas. Although the rejuvenescentseries now cover much of the island's surface, Baxter (1976) con-cluded that they are volumetrically minor in comparison to the OlderSeries lavas that make up the bulk of the shield.

Paul et al. (2007) divided the Older Series into an early primitiveseries and a later differentiated series. The former comprised samplesexclusively found in boreholes at depths of greater than 45 m.Identification of these samples as Older Series was based onconclusions drawn by hydrologists at the Mauritian Water ResourcesUnit that, “Below about 100 ft depth, the Older Series lavas are presentin all the boreholes” (Paul et al., 2007). However, no dating of theseborehole samples had been undertaken, and we were particularlyinterested in confirming that these borehole samples were indeedprimitive members of the Older Series. If so, it implies a volcanicevolution somewhat similar to Hawaii, where the extent of differen-tiation tends to increase in the post-shield stage (Frey et al., 1990). Asis the case in Hawaii and elsewhere, Mauritian rejuvenescent lavashave more depleted isotopic signatures than do the shield-buildinglavas of the Older Series (Sheth et al., 2003; Nohda et al., 2005; Paulet al., 2005).

The overall evolution of Mauritius shows strong similarities toHawaii but there are also some significant differences, most obviouslythat the rejuvenescent volcanism in Mauritius appears to haveproduced two distinct series separated by a long hiatus. Additionally,the shield lavas are tholeiitic in Hawaii but transitional in Mauritius.Furthermore, although the Younger and Intermediate Series arealkalic and strongly undersaturated lavas, such as nephelinites andfoidites that occur in Hawaiian rejuvenescent series, are absent.Finally, while the Hawaiian rejuvenescent lavas are much moreincompatible-element enriched than the shield lavas, this is not thecase in Mauritius; indeed the Younger and Intermediate Series lavasare on average less incompatible-element enriched than the OlderSeries (Sheth et al., 2003; Nohda et al., 2005; Paul et al., 2005).

Rejuvenescent volcanism on Hawaii, Mauritius, and other oceanicisland chains represents a significant challenge to the conventionalmantle plume model of oceanic island volcanism. What causesvolcanism to begin again after it has ceased? What accounts for thedifferences in isotopic composition between the shield stage and therejuvenescent stage? While a variety of answers to these broadquestions have been proposed (e.g., Roden et al., 1984; Chen and Frey,1985; Ribe and Christensen, 1999; Sheth et al., 2003; Bianco et al.,2005; Paul et al., 2005; Fekiacova et al., 2007) a consensus has yet toemerge. The present study was undertaken to address these and thefollowing questions: Is the hiatus between the Younger andIntermediate Series real or an artifact of sampling? Was there

chemical evolution through time in the Older Series from moreprimitive to more evolved, as Paul et al. (2007) suggested? Was therean associated isotopic evolution? Were there any systematic changesin composition with time in the Younger and Intermediate Series? Toanswer these questions, we undertook new sampling, new chemicaland isotopic analyses, and 40Ar–39Ar age determinations of lavasfrom all three series in Mauritius.

2. Analytical methodology

2.1. Sampling

Fifty-four of the samples reported on here were collected fromMauritius Island during 2007, including 37 cores samples fromboreholes previously drilled by the Mauritius Water Resources Unitand 17 surface outcrop samples. In addition, we also analyzed 10samples – 3 from outcrops and 7 from drill cores – collected by one ofus (DP) in 2003, and 7 outcrop samples of Younger Series lavasoriginally collected by A. N. Baxter (Baxter, 1972). The surficialsamples represent lavas from the eroded remnants of the shieldvolcano and the rejuvenescent cones and flows. Samples M35, M36,M37, and M39 are hyaloclasite breccias, i.e., the products of theexplosive interaction of magma and water. The drill core samplesconsist of interlayered subareal lava and agglomerate flow unitsrecovered from as deep as 220 m. Some of these samples are morethan 100 m below present sea level. The absence of pillow lavas andhyaloclastites among them provides direct evidence of the subsidenceof the island. Drill cores were selected in an effort to maximizepenetration into the Older Series at depth. In total, this study included13 samples from the Older Series, 20 from the Intermediate Series,and 38 from the Younger Series. Locations of the samples are listed inAppendix A and shown in Fig. 1. A KML file with sample locations isalso available on-line.

2.2. Analytical methods

Powders of the 2007 sample set were prepared for analysis atCornell University and the University of Texas at San Antonio byceramic disk mill. Major element concentrations were measured atthe GeoAnalytical Lab at Washington State University by XRF analysisfollowing the methods of Johnson et al. (1999). Geostandards BHVO-1, BCR-2, and BE-N were measured to monitor precision. Duplicateanalyses and analytical uncertainty of samples are given in AppendixB. Preparation for trace element analysis follows themethods outlinedin Cheatham et al. (1993). Samples were diluted to approximately0.025% total dissolved solids, with the addition of an internalcalibration standard after Gao et al. (2009). Thirty-seven traceelement concentrations were measured at the University of Houstonusing a Varian quadrupole-ion with a Varian SPS3 Auto-sampleroperating in normal resolution mode. Raw ICP-MS data were reducedby the methods of Gao et al. (2009), and includes an oxideinterference correction (K. Hollocher, 2011; http://www.union.edu/PUBLIC/GEODEPT/hollocher/icpms/ree_corrections .htm). StandardBCR-2 (n=28) was run to monitor precision and BIR-1, B-EN, andin-house standard PAL-889 (each n=4) were run as unknowns.Analytical uncertainties for samples and standards are shown inAppendix B along with recommended values of Govindaraju (1994).

Sample preparation and analytical methodology for Sr and Ndisotopic ratios was after White and Duncan (1996), though Nd elutionwas conducted following a modified version of Pin and Zalduegui(1997). Most Sr and Nd isotopic ratios were measured at theKeck Isotope Laboratory at Cornell University using a VG Sector 54thermal ionization mass spectrometer (TIMS) operating in dynamicmulti-collector mode; eight samples were analyzed at the Max-Planck-Institut für Chemie in Mainz. Isotopic ratios were normalizedto 88Sr/86Sr=0.11940 and 146Nd/144Nd=0.72190, respectively, using

Table 1Major element concentrations for Mauritius samples determined by XRF (oxides in wt.%).

B14-1 B14-2 B18-1 M-1 M6 M12 M30 M35 M36 M37 M39 MP-50* MP-24*

Older SeriesSiO2 48.18 45.70 44.07 46.38 46.00 45.34 47.11 46.74 45.92 45.34 46.54 47.01 46.19TiO2 2.70 2.78 2.96 3.47 3.23 2.78 2.86 3.12 2.87 3.08 2.93 3.14 2.84Al2O3 13.58 13.58 13.64 15.34 15.05 13.54 16.14 14.94 14.12 13.88 14.57 14.55 13.63FeOT* 12.17 10.53 11.14 11.32 11.80 11.35 10.45 9.87 11.23 11.27 11.88 11.40 12.02MnO 0.13 0.16 0.17 0.18 0.17 0.17 0.18 0.16 0.17 0.18 0.15 0.17 0.18MgO 5.04 6.64 6.57 5.35 5.39 7.33 5.53 3.53 6.69 7.11 5.09 6.83 9.15CaO 9.25 11.27 10.30 9.97 9.92 11.37 8.64 6.82 9.93 9.90 9.63 9.62 10.11Na2O 3.82 2.35 2.52 3.19 2.77 2.78 3.29 3.89 2.72 2.38 2.65 2.99 2.64K2O 0.97 1.05 1.05 1.41 1.32 0.93 1.52 1.60 1.08 1.21 1.06 1.45 0.98P2O5 0.26 0.39 0.40 0.58 0.47 0.35 0.47 0.59 0.38 0.43 0.37 0.52 0.37Sum 96.11 94.44 92.82 97.18 96.13 95.94 96.20 91.26 95.10 94.78 94.86 97.68 98.11LOI (%) 2.20 3.71 5.46 0.78 2.39 1.15 1.45 6.31 2.00 3.23 3.12 0.61 0.37

B1-1 B1-2 B1-3 B1-4 B1-5 B6-3 B6-4 B12-4 B14-3 B14-5 B14-6 B14-7 B18-8 B18-9 B19-4 B19-5 M15 M16 M17

Intermediate SeriesSiO2 45.04 40.28 43.32 45.85 44.19 44.91 43.15 43.45 44.08 45.72 41.37 44.29 45.77 43.13 42.82 43.84 46.00 44.88 46.86TiO2 1.77 1.99 1.85 1.52 1.64 1.82 1.83 2.06 1.67 1.81 2.53 2.07 1.37 2.26 1.52 2.17 1.90 1.7 1.99Al2O3 13.11 12.82 14.01 14.68 13.89 14.19 14.48 12.91 13.80 13.45 14.17 15.47 13.69 12.92 11.67 13.13 15.78 13.39 12.69FeOT 10.98 11.04 10.97 12.13 10.84 11.42 11.26 11.76 11.83 11.94 12.46 11.41 12.29 11.89 12.57 11.72 11.68 11.68 11.26MnO 0.15 0.21 0.22 0.18 0.17 0.17 0.19 0.18 0.18 0.18 0.17 0.18 0.19 0.20 0.19 0.18 0.18 0.18 0.16MgO 9.38 7.36 7.25 10.94 9.54 9.74 9.29 13.10 12.38 12.34 8.85 8.77 14.19 12.69 15.74 11.52 8.19 13.17 11.44CaO 8.22 10.41 9.78 10.22 9.85 8.94 9.65 10.12 9.00 10.17 8.55 10.87 9.11 10.28 8.83 10.03 11.02 9.89 9.92Na2O 2.16 2.05 1.94 2.50 2.07 2.27 2.00 2.15 1.84 2.27 1.34 2.06 2.37 1.85 1.83 1.89 2.69 2.55 2.72K2O 0.42 0.58 0.55 0.26 0.37 0.37 0.39 0.66 0.33 0.41 0.88 0.34 0.32 0.57 0.31 0.72 0.37 0.4 0.66P2O5 0.20 0.26 0.23 0.14 0.16 0.20 0.17 0.31 0.19 0.19 0.32 0.20 0.17 0.31 0.19 0.25 0.17 0.2 0.31Sum 91.44 87.01 90.12 98.42 92.72 94.02 92.40 96.70 95.31 98.48 90.62 95.65 99.47 96.08 95.67 95.45 97.98 98.04 98.01LOI 6.87 11.10 7.84 0.23 6.14 4.82 6.03 1.47 4.11 0.57 7.64 3.03 0.08 2.78 3.72 3.22 0.07 0.00 0.85

B1-1 B1-2 B1-3 B1-4 B1-5 B6-3 B6-4 B12-4 B14-3 B14-5 B14-6 B14-7 B18-8 B18-9 B19-4 B19-5 M15 M16 M17

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Table 1 (continued)

B1-1 B1-2 B1-3 B1-4 B1-5 B6-3 B6-4 B12-4 B14-3 B14-5 B14-6 B14-7 B18-8 B18-9 B19-4 B19-5 M15 M16 M17

Younger SeriesSiO2 46.79 46.95 45.23 44.71 44.78 46.37 45.82 44.59 42.57 45.85 45.69 45.71 45.75 43.05 45.72 46.05 44.97 43.801 43.98TiO2 1.88 1.72 2.03 1.73 2.02 1.63 1.59 2.09 2.17 1.91 1.75 1.74 2.16 2.02 1.82 1.69 1.86 1.98 2.01Al2O3 16.70 16.18 14.51 14.58 14.87 14.90 14.95 14.91 13.98 14.51 14.78 14.78 16.24 14.52 14.61 14.74 14.41 14.47 14.45FeOT 11.67 11.61 12.83 11.86 12.06 12.55 12.66 12.02 11.78 12.32 12.59 12.44 11.68 13.21 12.42 12.33 12.69 12.30 12.55MnO 0.18 0.18 0.19 0.18 0.18 0.19 0.19 0.18 0.17 0.18 0.19 0.19 0.18 0.20 0.19 0.19 0.19 0.19 0.19MgO 6.97 8.36 11.13 11.70 11.05 11.00 10.52 8.87 11.95 11.35 11.19 11.88 7.70 11.02 11.53 11.42 11.17 11.14 10.92CaO 10.96 10.69 9.87 10.17 10.54 10.03 9.84 10.96 9.80 10.23 9.82 10.15 11.12 8.16 9.92 9.94 9.90 9.69 9.96Na2O 2.81 2.88 3.05 2.06 2.77 2.85 2.54 2.34 1.55 2.78 3.08 2.69 3.00 1.69 2.70 2.72 2.78 2.56 2.29K2O 0.35 0.38 0.52 0.42 0.51 0.28 0.27 0.31 0.49 0.38 0.45 0.32 0.51 0.40 0.36 0.29 0.39 0.57 0.44P2O5 0.17 0.15 0.27 0.18 0.21 0.15 0.16 0.17 0.31 0.19 0.22 0.15 0.23 0.21 0.18 0.14 0.19 0.26 0.21Sum 98.47 99.08 99.62 97.58 98.99 99.95 98.54 96.43 94.79 99.71 99.76 100.05 98.56 94.48 99.45 99.50 98.54 96.96 97.01LOI 0.67 0.09 −0.55 1.35 −0.10 −0.65 0.30 1.85 3.79 −0.66 −0.76 −0.75 0.20 4.36 −0.52 −0.75 −0.06 1.08 1.09

M21 M22 M29 M32 M33 BH-2a BH-9a BH13b BH19bb BH25bb BH26bb C2c C18c C46c C62c C76c C78c C82c

SiO2 38.35 45.12 44.55 45.02 45.85 46.31 47.47 46.18 44.28 46.24 45.5 46.27 46.23 44.77 44.8 45.47 45.89 45.6TiO2 2.39 2.60 2.41 2.07 1.49 3.09 1.87 1.39 2.33 1.56 2.14 2.33 1.69 2.15 1.66 1.73 2 1.77Al2O3 17.94 15.50 14.05 15.92 15.14 14.27 16.34 14.81 13.29 14.6 14.03 15.99 13.76 15.05 15.25 15.24 16.01 14.2FeOT 13.60 11.92 13.64 11.97 11.90 11.682 11.916 12.285 12.825 12.24 12.85 12.63 11.32 11.50 11.92 11.56 11.75 12.39MnO 0.20 0.19 0.21 0.18 0.18 0.17 0.17 0.18 0.18 0.18 0.18 0.16 0.18 0.16 0.16 0.17 0.15 0.17MgO 7.22 7.53 10.00 7.74 10.59 5.83 7.66 11.23 12.32 11.16 10.62 6.31 11.34 9.15 8.92 9.72 7.23 11.17CaO 7.07 12.01 10.36 10.94 10.08 9.95 10.97 9.61 10.15 10.01 9.91 10.75 9.96 12.25 11.51 11.43 11.62 10.62Na2O 1.77 2.97 2.59 2.69 2.49 2.6 2.68 2.48 2.75 2.72 2.88 3.12 2.87 2.97 2.68 2.70 3.10 2.96K2O 0.34 0.65 0.35 0.38 0.27 1.17 0.36 0.21 0.72 0.29 0.47 0.46 0.35 0.59 0.35 0.34 0.44 0.44P2O5 0.22 0.29 0.21 0.20 0.13 0.4 0.17 0.11 0.28 0.12 0.22 0.21 0.21 0.30 0.17 0.22 0.25 0.30Sum 89.11 98.78 98.36 97.12 98.12 95.47 99.61 98.49 99.13 99.12 98.80 98.23 97.91 98.89 97.42 98.58 98.44 99.62LOI 8.90 0.01 0.08 0.58 0.17 4.00 3.41 0.01 −0.27 −0.9 −0.63 0.81 0.53 0.68 0.77 0.46 1.03 0.48

*FeOT is total Fe expressed as FeO.a Analyzed in MPI Mainz using method of Paul et al. (2007).b From Paul et al. (2007).c From Baxter (1972).

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an exponential correction. During the period of analysis the NIST-SRM-987 Sr standard yielded 87Sr/86Sr=0.71025±0.00001 (2σ)(n=47) and the Ames Nd standard yielded 143Nd/144Nd=0.512131±0.000006 (2σ) (n=13).

Analysis for Pb isotopic ratios utilized rock chips (~150 mg), with allsamples subjected to the cleaning and leachingprocedure ofAbouchamiet al. (1999). The triple spike technique of Galer and Abouchami (1998)and Galer (1999) was used for determination of Pb isotopic composi-tions and was done at the Max-Planck-Institut für Chemie in Mainzusing a Thermo Fisher Triton TIMS operating in static mode. The datareduction procedures of Galer (1997, 1999) were followed to yieldthe fractionation-corrected Pb isotopic ratios. Repeat analysis (n=25)of the NBS-981 Pb standard yielded 206Pb/204Pb=16.9436±0.0026(2σ) (152 ppm), 207Pb/204Pb=15.5016±0.0030 (2σ) (195 ppm), and208Pb/204Pb=36.7316±0.0085 (2σ) (231 ppm). This reproducibilityis better by a factor of nearly 6 than that in our previous study (Paulet al., 2005). Results from 206Pb/204Pb and 208Pb/204Pb are consistentwithin error with the ratios reported by Galer and Abouchami (1998),but 207Pb/204Pb slightly exceeds these values. Leadblanks (n=4)variedbetween 15 and 40 pg, and are negligible.

Crystallization ages for forty samples were measured by 40Ar–39Arincremental heating methods at Oregon State University, with theisotopic composition of Ar released at each heating step measuredon a MAP 215/50 mass spectrometer (Duncan and Keller, 2004).Samples were irradiated at the OSU TRIGA reactor, using FCT-3 biotite(28.03 Ma) as a neutron flux monitor used to determine J, the fluencefactor that relates 39Ar production to monitor age. Samples wereheated in 50–250 °C increments, from 400 °C to fusion in seven tofourteen steps. Isotopic data are reduced as age spectra (step agesplotted against % gas released) and isochrons (40Ar/36Ar vs 40Ar/39Ar).Plateau ages are the weighted mean of concordant, contiguous stepages (n=steps included in plateau/total steps). Isochron ages arecalculated from the slope of co-linear step compositions, whose

SiO2

Na 2

O +

K2O

551

2

3

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5

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7

42 47 52

8

10

12

14

16

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0 5 10 15 20

MgO

Al 2

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a b

c d

Fig. 2.Major element oxide plots for Mauritius lavas. Open symbols represent previously pub2005, 2007). Alkali-tholeiite divide in (a) after Macdonald and Katsura (1964), names are froIndian Ridge mid-ocean ridge basalts (CIR MORB) (data drawn from the Earthchem databa

40Ar/36Ar intercept gives the composition of initial Ar in the rock(atmospheric value=295.5). The Total Fusion Age is calculated bysumming all step compositions. In the absence of reliable plateauand isochron age, this is the best estimate of the sample age. Sevensamples originally collected by A. N. Baxter (Baxter, 1972) were datedearlier by conventional K–Ar methods at Oregon State University.

3. Analytical results

3.1. Major elements

Major elements are reported in Table 1; we include data from Paulet al. (2007) and Baxter (1972) for completeness. Most Mauritiuslavas fall on the alkali basalt side of theMacdonald and Katsura (1964)alkali basalt–tholeiite divide, although some do fall on the tholeiiticside. Their alkalic nature is less clear-cut when C.I.P.W. norms are usedto distinguish between alkali basalt and tholeiite; as Fig. 3 shows, all 3series include both hypersthene- and nepheline-normative samples.A compilation of our data and 46 previously published analysesof Older Series lavas has an average silica saturation (normativehypersthene less normative nepheline and leucite) of 0.02, i.e., theyare on average just silica saturated. In this respect, they are similarto lavas from Réunion, which were termed “transitional” basalts byUpton and Wadsworth (1966) because they straddle the tholeiite–alkali basalt divide. Compilations of Intermediate and Younger Seriesanalyses have nearly identical average silica saturation valuesof −1.45 and −1.43, respectively; i.e., they are on average alkalic,although they too are perhaps better considered “transitional”. Thedifference in silica saturation between Older Series lavas and therejuvenescent lavas is statistically significant at the 5% level (usingthe Student-T test for means assuming unequal variances). A fewpreviously published analyses of the Intermediate Series basalts,specifically Baxter's (1972) samples B-5, B-33, and B-34, fall in the

0

1

2

3

0 5 10 15 20

MgO

K2O

6

7

8

9

10

11

12

13

14

0 5 10 15 20

CaO

MgO

lished data for Mauritius (Baxter, 1972; Sheth et al., 2003; Nohda et al., 2005; Paul et al.,m Le Bas and Strekeisen (Le Bas and Streckeisen, 1991). Fields for Réunion and Centralses (http://earthchem.org) shown for comparison.

Fre

qu

ency

Normative Silica Saturation, %

-10 -5 0 5 10 15 20Older

Intermediate

Younger

0

10

20

30

40

50

60

70

Fig. 3. Histogram of CIPW normative silica saturation (percent normative hyperstheneminus percent normative nepheline; no Mauritius lavas have either quartz or leucite inthe norm) for Older, Intermediate, and Younger Series lavas of Mauritius. Based on datafrom Baxter (1972), Sheth et al. (2003), Nohda et al. (2005), Paul et al. (2007), and thiswork. Normative compositions were calculated assuming ferric iron is 10% of total iron.All three series have average silica saturation close to 0, but the Older series is skewedto silica saturation while the Intermediate and Younger Series are skewed to silicaundersaturation.

53J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

basanite field in Fig. 2a and contain more than 10% normativenepheline and would be classified as basanites on that basis. Incontrast, the rejuvenescent Honolulu Series of Oahu is dominated byhighly undersaturated nephelinites and melilitites that have anaverage silica saturation index of −18.6 (based on norms calculatedfrom 64 analyses of Honolulu Series lavas compiled from theEarthChem3 database). Rejuvenescent Koloa Series of Kaua'i are alsohighly undersaturated: 47 samples analyzed by Garcia et al. (2010)have an average silica saturation index of −13.4. Highly undersatu-rated rocks such as nephelinites have not been found among therejuvenescent lavas of Mauritius.

Fig. 2b, c, and d suggest compositional variation within all threeseries is controlled largely by fractional crystallization of olivineabove 12% MgO and olivine and pyroxene together below 12%MgO, consistent with the conclusions of Baxter (1972). Olivine andpyroxene are common phenocryst phases in these lavas andexperiments suggest they are important fractionating phases inlavas from Réunion (Fisk et al., 1988). The Older Series appear to bethe most fractionated, but there is considerable overlap in this respectwith the rejuvenescent series. Importantly, these figures suggestthat the parental compositions of the Older Series basalts on the onehand and the two rejuvenescent series basalts on the other aredistinct: the Older Series parent is poorer in Al2O3 and richer in K2O,TiO2, and P2O5 than the rejuvenescent series.

3.2. Trace elements

Trace element compositions are presented in Table 2 and showsimilar enrichment and depletion trends to those reported in theliterature (Fig. 4). The lavas of the Older Series have, on average, thehighest incompatible trace element concentrations of the three suitesas well as the highest ratios of more incompatible to less incompatibleelements (e.g., La/Sm, Nb/Y), although there is considerable overlap

3 The EarthChem database is available at http://earthchem.org.

with the rejuvenescent lavas. Chondrite-normalized rare earthelement (REE) patterns for the Older Series are nearly parallel withmoderate light-REE enrichment and chondrite-normalized La/Smratios ((La/Sm)N) of 2.2 to 2.7. Incompatible element and rare earthpatterns of Intermediate and Younger Series overlap considerably, butthe Intermediate Series ranges extend to somewhat more enrichedcompositions than the Younger Series. The rejuvenescent lavas showless, but more variable, light-REE enrichment than the Older Series,with (La/Sm)N ratios of 1.6–2.5 for Intermediate and 1.3–2.3 for theYounger Series. Mean Zr/Nb ratios decrease in the order: YoungerSeries (9.8), Intermediate Series (8.2), Older Series (8.1).

Incompatible element patterns show K-depletion in some cases,which is most likely a result of weathering. Some of the new anal-yses we report here show rather extreme Pb depletions andenrichments. This is found only in drill core samples of the Youngerand Intermediate Series, as noted by Paul et al. (2007). One possibleexplanation is that it reflects contamination by drilling fluid; however,this would not explain the depletions. In addition, samples that showeither Pb depletion or enrichment have Pb isotopic compositionscompletely typical of Mauritian rejuvenescent lavas. Pb mobility ingroundwater is well documented in the environmental literature anddepends on a number of factors including pH, speciation, and ligandavailability (e.g., Sadiq and Alam, 1997; Santos et al., 2002). We haveno information on groundwater chemistry, but strongly suspectthat the most likely explanation for these anomalies is mobility ofPb in groundwater: Pb has been leached from some rocks and hasprecipitated in others. We also observe some enrichment anddepletion in Sr content, which we suspect also results from mobilityin the presence of groundwater. The other alkalis and alkaline earths,Rb, Cs, and Ba, may also have been mobile.

3.3. Isotopic compositions

Fifty-four new Sr and Nd and 42 new Pb isotope ratios are presentedin Table 3. Overall, the results are consistent with published results(e.g., Sheth et al., 2003; Nohda et al., 2005; Paul et al., 2005), althoughthey extend the range of several fields. The Older Series lavas havethe most radiogenic Pb and Sr isotopic ratios and the lowest εNd values(Figs. 5, 6, and 7). These lavas span a relatively limited range of ratiosand are distinct from those of the Intermediate and Younger Series.

While our data are consistent with earlier results, the Pb isotopedata show much less scatter on the 207Pb/204Pb–206Pb/204Pb plot(Fig. 6). The new data define a tight – albeit not perfect – linearcorrelation. Quantitatively, the new 207Pb/204Pb–206Pb/204Pb data hasan r2 value (r is the correlation coefficient) of 0.94 (n=38), whereasthe previously published data has an r2 value of 0.55 (n=47). There isalso a remarkable reduction of scatter on 206Pb/204Pb–87Sr/86Sr and206Pb/204Pb-εNd plots (Fig. 7), which we also believe reflect the higherquality of the new data. These results demonstrate a clear benefit tousing techniques that reduce isotopic fractionation during Pb isotopicanalysis, such as the triple spike technique we used, as well as theimportance of strongly pre-leaching these sometimes altered rocksprior to analysis (e.g., Nobre Silva et al., 2009). The new data lead tointerpretations, discussed in Section 4.4 below, that differ significantlyfrom earlier conclusions of Paul et al. (2005) about the nature ofthe source of Mauritius volcanism.

On Figs. 5–7 samples dredged from the Gasitao Seamounts, locatednear the projected intersection of the Rodriques Ridge with the (CIR)(Nauret et al., 2006), are plotted as separate symbols. These lavas haveisotopic compositions quite similar to those of the Mauritiusrejuvenescent lavas. Nauret et al. (2006) found that these samplesplotted on an extension of the 208Pb/204Pb–207Pb/204Pb–206Pb/204PbRéunion array and concluded their source contained a Réunion plumecomponent. Samples dredged from the CIR ridge axis in this region,however, defined a different 208Pb/204Pb–207Pb/204Pb–206Pb/204Pb

Table 2Trace element concentrations for Mauritius samples (in ppm).

Older Series Intermediate Series

B18-1 M6 M12 M30 M35 M36 M37 M39 MP-50a

MP-24a

B2-1 B5-1 B6-1 B6-2 B12-1 B18-2 B18-3 B18-4 B18-5 B18-6 B18-7 B19-1 B19-2 B19-3 M20

Li 5.03 7.25 7.22 5.27 8.46 5.55 6.08 4.45 3.30 4.39 3.76 7.81 4.23 4.43 3.90 4.26 7.95 10.14 4.78 3.97 3.72 5.56 4.36Be 1.3 1.6 1.2 1.6 2.5 1.4 1.3 1.4 0.6 0.5 0.5 0.5 0.6 0.9 0.5 0.6 1.2 0.7 0.5 0.8 0.4 0.9 0.6B 0.6 0.3 0.7 0.8 0.3 1.3 1.0 0.3 0.2 0.2 0.5 63.0 0.5 0.4 0.5 0.6 0.9 68.9 0.9 0.6 0.4 0.5 0.5Sc 29.34 25.48 32.18 19.99 16.86 29.70 28.46 31.67 24.1 27.5 25.21 29.42 27.10 25.86 28.22 28.08 27.05 25.83 26.90 32.15 25.61 25.28 24.65 25.88 29.60Ti 16,506 19,698 16,305 18,904 21,826 15,361 17,374 19,539 8632 8530 10,794 10,950 9529 11,932 8559 9270 16,433 12,805 6677 12,149 8747 14,231 9161V 302.4 340.0 326.4 254.4 281.8 309.2 334.3 393.5 194.1 290.7 280.0 270.5 274.8 276.7 238.5 277.1 356.7 327.8 212.6 278.1 240.7 344.8 271.9Cr 203.4 79.0 394.1 92.7 13.2 143.5 155.4 98.5 280.8 404.0 443.8 382.0 263.2 552.1 364.2 225.7 396.6 225.3 531.5 496.9 532.6 488.4 249.4Co 48.62 45.49 55.73 38.53 31.21 49.99 48.90 41.48 42.2 49.4 57.86 73.85 61.81 61.48 56.77 68.51 64.26 67.55 67.45 57.31 79.83 70.26 85.99 66.49 58.69Ni 115.71 79.62 137.44 76.97 16.42 112.30 108.52 71.15 127 189 265.78 365.99 263.02 239.42 184.04 357.24 327.14 353.64 216.54 149.61 500.40 346.91 470.55 304.31 127.11Cu 110.66 78.75 67.33 40.60 33.10 61.13 59.63 69.84 69.6 62 86.53 100.05 79.42 71.38 77.90 86.63 67.73 88.65 86.04 84.91 82.77 81.83 67.96 87.32 77.28Zn 124.30 124.38 110.86 124.89 129.97 120.64 118.73 114.64 142 122 96.41 102.23 93.17 149.04 92.48 101.92 90.61 97.51 123.34 156.96 104.33 103.86 99.47 112.08 101.67Ga 24.96 25.93 22.53 26.91 25.93 25.26 23.97 24.46 18.94 19.99 18.79 19.43 20.51 19.44 17.43 19.02 23.15 23.55 18.51 20.20 14.02 21.05 21.28Rb 25.3 31.7 20.8 36.1 47.2 28.5 27.1 24.4 38.3 22.1 7.2 4.9 7.0 6.2 10.8 18.2 6.9 9.1 23.6 5.7 6.4 14.1 6.7 17.6 6.8Sr 459.8 522.6 437.8 543.7 831.4 434.4 410.7 404.3 528 418 202.5 240.2 255.8 246.6 672.4 388.8 350.1 340.3 331.6 345.2 262.3 1008.9 272.0 372.7 336.4Y 28.83 31.95 27.40 33.33 37.18 29.80 28.40 43.15 30.6 28.8 20.00 20.04 18.14 19.31 20.82 20.80 18.77 18.52 25.20 22.19 19.42 19.87 15.97 19.97 24.62Zr 236.2 282.3 230.8 302.6 459.1 219.8 234.8 241.0 284 219 89.0 96.6 104.0 114.8 90.1 146.6 89.3 104.9 190.2 116.8 91.4 142.5 89.0 160.3 88.2Nb 27.12 37.10 25.60 39.79 60.06 27.01 28.34 32.51 43.1 32.1 9.51 7.87 12.53 12.78 8.46 27.28 11.17 9.73 34.27 16.87 7.36 24.08 13.09 25.14 9.30Cs 0.092 0.206 0.061 0.295 0.665 0.161 0.090 0.089 0.43 0.13 0.053 0.053 0.087 0.080 0.058 0.256 0.094 0.062 0.311 0.081 0.072 0.143 0.063 0.135 0.043Ba 229.5 331.2 213.5 310.0 309.7 225.8 253.9 255.1 326 235 127.6 78.6 193.2 317.0 114.9 198.6 121.1 129.9 347.0 130.2 96.4 220.2 118.8 200.9 102.7La 27.14 32.90 25.55 33.10 42.19 26.72 29.44 42.17 35.4 29.5 10.69 7.79 9.41 9.99 10.80 19.50 9.31 11.22 22.21 12.32 9.60 19.04 11.20 16.39 10.78Ce 61.83 68.68 57.32 73.78 89.47 59.50 67.45 80.69 76.7 63.6 23.23 17.52 21.02 21.92 23.18 40.96 21.06 24.45 46.20 27.11 21.20 39.95 22.82 35.43 22.66Pr 7.99 9.10 7.30 9.53 11.11 7.82 8.62 11.64 9.58 8.08 3.09 2.45 2.80 2.93 3.11 5.22 2.81 3.24 6.03 3.62 2.83 5.09 2.91 4.56 3.18Nd 33.44 37.44 30.54 39.13 45.32 33.01 35.21 50.05 39.7 33.5 13.81 11.24 12.39 13.14 13.75 21.12 12.45 14.34 25.56 16.20 12.52 21.38 12.48 19.66 14.44Sm 7.70 8.28 7.12 8.80 9.83 7.46 7.85 11.50 8.72 7.48 3.77 3.15 3.29 3.47 3.62 4.84 3.28 3.65 5.93 4.14 3.18 4.87 3.10 4.63 3.80Eu 2.48 2.71 2.29 2.80 3.09 2.43 2.48 3.60 2.78 2.5 1.36 1.15 1.22 1.27 1.30 1.59 1.18 1.27 2.02 1.48 1.14 1.64 1.10 1.57 1.37Gd 7.44 7.94 7.06 8.34 9.15 7.39 7.35 11.67 7.94 7.13 4.32 3.71 3.68 3.96 4.13 4.82 3.66 4.00 5.95 4.39 3.66 4.85 3.40 4.58 4.41Tb 1.06 1.13 1.02 1.20 1.32 1.08 1.05 1.65 1.12 1.04 0.67 0.61 0.58 0.63 0.65 0.70 0.58 0.62 0.88 0.69 0.58 0.71 0.52 0.70 0.70Dy 5.55 5.95 5.46 6.10 6.40 5.97 5.42 6.81 6.32 5.93 4.33 3.94 3.63 3.70 4.20 4.22 3.83 4.03 5.05 4.35 4.09 3.91 3.34 3.58 4.12Ho 1.09 1.17 1.08 1.24 1.36 1.15 1.07 1.63 1.14 1.07 0.78 0.77 0.67 0.74 0.79 0.77 0.72 0.71 0.90 0.81 0.72 0.75 0.62 0.75 0.86Er 2.78 2.91 2.75 3.14 3.43 2.93 2.74 3.97 3.01 2.89 2.03 2.09 1.80 1.94 2.12 1.98 1.96 1.87 2.25 2.18 1.96 1.90 1.65 1.90 2.33Yb 2.12 2.19 2.17 2.40 2.57 2.35 2.07 2.69 2.44 2.4 1.68 1.81 1.49 1.60 1.83 1.66 1.67 1.59 1.75 1.81 1.76 1.49 1.40 1.41 1.84Lu 0.30 0.32 0.31 0.34 0.38 0.33 0.30 0.40 0.33 0.32 0.23 0.26 0.22 0.23 0.26 0.24 0.25 0.22 0.24 0.26 0.25 0.21 0.20 0.21 0.28Hf 5.34 6.15 5.37 6.72 9.05 4.83 5.39 5.60 6.90 5.49 2.09 2.21 2.43 2.59 2.11 3.20 2.02 2.44 4.06 2.56 2.02 3.11 2.06 3.45 2.05Ta 1.67 2.22 1.59 2.46 3.50 1.59 1.69 1.94 2.70 2.07 0.49 0.39 0.69 0.64 0.43 1.53 0.60 0.53 1.84 0.85 0.38 1.32 0.61 1.41 0.51Pb 2.68 4.70 4.61 4.89 4.72 3.18 3.25 2.82 4.71 2.64 20.74 1.92 7.22 16.64 0.98 1.95 0.80 1.28 2.20 2.16 1.13 1.88 1.66 1.79 3.35Th 3.29 3.92 3.31 4.15 5.62 3.08 3.66 2.81 4.19 3.09 1.21 0.84 1.09 1.17 1.19 2.54 1.00 1.30 2.67 1.67 0.93 2.26 1.28 1.96 1.04U 0.828 0.891 0.644 0.931 1.588 0.750 0.621 0.493 1.170 0.850 0.113 0.207 0.224 0.155 0.282 0.627 0.238 0.319 0.508 0.682 0.237 0.587 0.296 0.504 0.211

aFrom Paul et al. (2007).bAnalyzed in MPI Mainz by XRF.

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Table 2Trace element concentrations for Mauritius samples (in ppm).

IntermediateSeries

Younger Series

BH30ba MP-9a

B1-1 B1-2 B1-3 B1-4 B1-5 B6-3 B6-4 B18-8 B18-9 B19-4 B19-5 M15 M17 M22 M29 M33 BH-2b BH-9b BH13a BH19ba BH25ba BH26ba

Li 4.75 4.49 5.31 4.26 3.88 4.81 4.65 5.23 5.33 4.76 4.29 6.10 8.89 4.46 4.94 4.34Be 0.6 0.5 0.8 0.6 0.6 0.5 0.5 0.7 0.7 0.6 0.5 0.7 0.8 0.9 0.6 0.5B 0.4 0.4 0.9 0.5 0.7 1.1 0.6 0.6 0.5 0.3 0.5 0.9 0.9 0.6 11.0 0.6Sc 27.7 24.5 30.90 28.12 27.76 29.09 30.79 28.99 27.50 31.11 27.94 25.44 26.83 28.22 30.36 31.55 29.21 26.20 26 26 26.00 25.80 29.60 26.20Ti 11,166 9497 12,998 9868 13,986 8376 8632 15,199 9841 10,051 11,864 7590 8238 14,781 12,225 7111V 311.5 267.0 278.9 290.4 347.6 248.2 244.6 332.1 284.6 260.7 322.5 214.6 212.8 353.1 286.0 215.8 316 246Cr 233.3 217.1 377.3 455.4 212.7 347.2 370.4 127.8 394.5 262.7 369.9 387.3 381.0 152.5 332.9 337.7 70 254Co 65.2 54.7 53.49 59.61 69.61 73.63 68.28 71.34 70.59 54.28 73.65 66.76 69.31 73.21 79.92 55.01 63.28 69.20 49 48 64.90 64.00 60.80 59.00Ni 369 336 100.61 137.83 281.51 329.95 279.08 315.21 322.73 108.48 338.30 284.12 300.30 323.45 310.61 88.21 246.72 303.89 68 117 337.00 345.00 302.00 284.00Cu 69.1 73.4 76.06 75.36 89.37 91.97 82.95 98.32 90.93 87.32 85.48 76.24 80.91 87.07 89.29 95.55 102.02 87.97 90 87 86.30 76.30 69.10 81.10Zn 112 116 100.96 99.73 112.12 106.88 99.11 112.39 105.43 98.51 112.58 102.47 105.48 106.80 116.44 102.82 120.38 98.53 107 92 122.00 120.00 112.00 125.00Ga 22.92 21.74 21.26 20.90 19.05 21.04 20.81 22.26 20.70 19.31 19.32 20.86 21.26 22.45 20.45 19.69 24 21Rb 8.3 12.9 6.3 6.6 10.7 8.5 10.0 5.3 3.5 10.4 8.6 6.7 5.1 7.3 7.3 13.3 5.3 4.4 32 6 2.2 18.4 5.6 8.6Sr 300 390 343.2 297.7 370.1 400.1 354.3 245.7 274.0 360.0 291.9 298.0 247.8 320.4 358.1 454.0 272.7 249.0 378 278 205.0 382.0 235.0 328.0Y 19.5 22.6 22.60 21.02 21.83 20.65 21.10 20.70 21.02 22.31 21.84 19.10 19.06 21.75 22.25 21.73 30.62 18.57 30 26 20.30 20.30 18.50 21.70Zr 92.2 131 107.2 94.5 135.0 113.7 112.2 86.1 99.2 142.9 107.5 107.4 100.7 96.7 91.7 142.6 104.6 83.1 220 99 68.1 120.0 74.3 109.0Nb 15 29.9 10.99 9.47 18.62 11.93 18.28 5.80 8.43 22.98 9.66 11.00 11.50 7.50 10.62 22.36 14.33 5.78 32 14 7.30 26.40 8.30 14.50Cs 0.1 0.09 0.086 0.076 0.092 0.068 0.113 0.045 0.039 0.117 0.098 0.060 0.048 0.086 0.051 0.080 0.039 0.043 0.010 0.180 0.070 0.170Ba 121 255 109.2 490.7 140.4 133.0 161.8 68.3 92.0 145.5 145.5 87.4 71.2 123.1 135.7 201.6 129.9 71.7 175 63 53.0 203.0 66.0 110.0La 11.90 22.1 9.80 9.25 13.86 11.39 13.76 6.93 7.98 13.52 13.79 9.22 7.26 11.18 12.52 17.92 14.71 7.27 5.57 17.50 6.39 11.80Ce 25.7 44.3 21.50 20.38 30.99 25.81 28.71 17.18 18.57 29.61 28.78 21.52 17.15 25.31 28.25 38.75 28.03 16.96 13.30 36.50 15.20 27.40Pr 3.33 5.41 2.90 2.75 4.07 3.44 3.72 2.43 2.64 3.90 3.80 2.95 2.43 3.42 3.72 4.90 3.95 2.34 1.91 4.55 2.15 3.67Nd 14.6 22.5 13.25 12.43 17.55 14.74 15.69 11.40 12.05 17.20 16.42 13.21 11.31 15.30 16.46 20.64 17.04 10.88 8.85 19.40 10.20 16.50Sm 3.69 5.4 3.56 3.34 4.34 3.81 3.94 3.23 3.28 4.31 4.12 3.45 3.15 3.95 4.17 4.92 4.43 2.97 2.50 4.62 2.95 4.23Eu 1.29 1.85 1.32 1.27 1.48 1.35 1.38 1.18 1.19 1.52 1.46 1.24 1.16 1.40 1.44 1.69 1.54 1.11 0.98 1.54 1.10 1.50Gd 3.94 5.44 4.17 3.95 4.65 4.11 4.23 3.78 3.76 4.61 4.61 3.73 3.75 4.39 4.60 5.04 5.17 3.57 3.33 4.67 3.45 4.57Tb 0.62 0.8 0.68 0.64 0.71 0.65 0.66 0.62 0.61 0.72 0.71 0.60 0.60 0.68 0.71 0.76 0.82 0.58 0.56 0.70 0.56 0.71Dy 3.79 4.66 3.97 4.00 4.90 3.48 3.63 3.91 4.05 4.65 4.29 3.07 3.90 4.84 4.81 4.13 4.76 4.05 3.53 4.02 3.56 4.25Ho 0.73 0.85 0.85 0.81 0.83 0.78 0.79 0.77 0.75 0.84 0.85 0.71 0.76 0.82 0.85 0.83 1.04 0.72 0.71 0.74 0.70 0.82Er 2.06 2.27 2.30 2.21 2.20 2.09 2.16 2.11 2.06 2.23 2.26 1.91 2.07 2.18 2.28 2.17 2.76 1.95 2.00 2.00 2.01 2.28Yb 1.79 1.8 1.87 1.82 1.91 1.65 1.73 1.80 1.77 1.88 1.88 1.49 1.77 1.97 1.97 1.70 2.21 1.71 1.81 1.63 1.77 1.98Lu 0.25 0.25 0.28 0.27 0.27 0.26 0.27 0.27 0.26 0.27 0.27 0.23 0.26 0.28 0.28 0.25 0.34 0.24 0.26 0.22 0.25 0.27Hf 2.41 3.31 2.46 2.16 3.03 2.55 2.60 1.99 2.22 3.06 2.52 2.39 2.40 2.22 2.13 3.17 2.52 1.98 1.71 3.04 2.04 2.93Ta 0.88 1.61 0.54 0.48 1.04 0.56 0.77 0.30 0.47 1.11 0.51 0.55 0.57 0.45 0.58 1.21 0.81 0.31 0.41 1.56 0.52 0.98Pb 1.13 1.93 1.33 26.46 1.92 1.94 2.04 1.37 2.39 1.49 1.21 0.92 1.44 1.25 1.42 2.37 1.22 0.56 0.40 1.36 0.61 1.32Th 1.32 2.56 1.11 1.06 1.42 1.19 1.75 0.61 0.72 1.48 1.60 0.93 0.71 1.07 1.32 2.29 1.45 0.71 0.59 2.28 0.69 1.13U 0.350 0.650 0.216 0.255 0.370 0.255 0.401 0.167 0.176 0.352 0.373 0.229 0.188 0.266 0.295 0.492 0.308 0.167 0.100 0.550 0.190 0.320

Table 2 (continued)

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Cs Rb Ba Th U K Nb Ta La Ce Pr Pb Sr Nd Zr Hf Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu

10

100

1

Sam

ple/

Prim

itive

Man

tle

Older Series

Intermediate Series

Younger Series

200

b

5

10

100

200

Sam

ple/

Cho

ndrit

es

Older Series

Intermediate Series

Younger Series

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

a

Fig. 4. Normalized trace element data for representative samples from Mauritius. Fields show range of previously published data (Sheth et al., 2003; Nohda et al., 2005; Paul et al.,2005). (a) Primitive mantle normalized spider diagram. Large Pb anomalies occur only in drill core samples and likely result from Pb mobility in groundwater. (b) Chondrite mantlenormalized rare earth element data for representative samples of Mauritius lavas. Older Series patterns are parallel and relatively steep suggesting large degree melts. Bothrejuvenescent suites show crossing patterns suggestive of lower degree melts. Primitive mantle and chondrite values used for normalization are from McDonough and Sun (1995).

56 J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

array that did not pass through the Réunion array. They concludedfrom this that the Réunion plume did not influence the CIR in this area.

3.4. Radiometric dating

Forty new samples, including 26 drill core samples, were datedby 40Ar–39Ar methods and an additional 7 samples of A. N. Baxterwere dated by the conventional K–Ar method (38Ar spiked, totalfusion) (Table 4). Reliable ages (plateau and isochron) were obtainedfor 38 of the 40 samples from which 40Ar–39Ar data were collected,based on significant plateau ages and corroborating isochron ageswith 40Ar/36Ar intercepts consistent with equilibration with atmo-

Notes to Table:Strontium,Nd, and Pb isotope ratio errors are 2 standard errors of themean based on in-run statiσ is the standard deviation. Errors should be read as the last digit; for example, the Nd isotope raparts per 10,000, from thepresent day chondritic bulk Earth ratio, which is taken as 0.512638. Erand Abouchami (1998; G & A) are given for comparison.

a Sr and Nd isotope ratios determined at MPI Mainz.

spheric Ar at the time of crystallization (Table 4). MSWD is an F-statistic that compares analytical uncertainty between step ages tothat within step ages. Values over about 2.5 indicate that variationin step ages exceeds that expected for concordance (samples B14-1and B18-1 only). Several of our new ages for the Older Series exceedthe maximum age (7.9 Ma) for Mauritius found by McDougall andChamalaun (1969) and extend the minimum time of onset of volcanicactivity to 8.89 Ma. These old samples include breccia samples fromTrois Mammelle (M37) and Long Mountain (M39) at roughly 8 Maand samples B18-1 and B14-2, at nearly 8.41 and 8.89 Ma, respec-tively. Stratigraphically, sample B14-1 (isochron age of 7.48±0.21 Ma) underlies B14-2 (plateau age of 8.89±0.17 Ma) and would

stics. Standard error is given as (σ/√n)/mean,wheren is thenumber ofmeasurements andtio forM12 should be read as 0.512840±0.000013. εNd is a fractional deviation, in units ofrors of standards are given as 2 standard deviations of analytical runs. Pb results fromGaler

Table 3Isotope ratios in Mauritius Lavas.

87Sr/86Sr 143Nd/144Nd εNd 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

Older SeriesB14-1 0.70416 ±1 .5128578 ±15 4.29B14-2 0.70415 ±1 .5128606 ±9 4.35B18-1 0.70422 ±1 0.512848 ±8 4.09 19.0199 ±32 15.6152 ±39 39.1338 ±130M6 0.70420 ±1 – – 19.0532 ±34 15.6195 ±41 39.1730 ±137M12 0.70438 ±1 0.512840 ±13 3.94 19.0374 ±23 15.6192 ±28 39.1765 ±91M30 0.70415 ±1 0.512857 ±10 4.27 19.1205 ±31 15.6491 ±31 39.2682 ±93M35 0.70423 ±1 0.512870 ±13 4.53 19.0678 ±22 15.6218 ±27 39.1747 ±89M36 0.70429 ±1 – – 18.9896 ±34 15.6094 ±36 39.0961 ±110M36 leachate 0.70427 ±1M37 0.70428 ±1 – – 19.0163 ±25 15.6137 ±25 39.1518 ±74M37 leachate 0.70427 ±1M39 0.70419 ±1 0.512861 ±10 4.35 19.0494 ±48 15.6156 ±28 39.1495 ±93MP-50 0.70416 ±1 0.512874 ±10 4.61 19.0555 ±20 15.6165 ±23 39.1810 ±74MP-24 0.70424 ±1 0.512855 ±10 4.23 19.1997 ±25 15.6350 ±30 39.2697 ±99

Intermediate SeriesB2-1 0.70371 ±1 18.9374 ±39 15.5812 ±44 38.9479 ±144B5-1 0.70381 ±1 18.8474 ±27 15.5804 ±31 38.8772 ±99B5-2 0.70379 ±1B6-1 0.70385 ±1 0.512912 ±11 5.34B6-2 0.70387 ±1 0.512925 ±15 5.60 18.8273 ±33 15.5785 ±41 38.8581 ±135B12-1 0.70411 ±1B18-2 0.70357 ±2 0.512914 ±8 5.39 18.8683 ±42 15.5883 ±52 38.9323 ±173B18-3 0.70385 ±1 0.512900 ±9 5.11B18-4 0.70390 ±1 0.512888 ±16 4.87 18.8538 ±37 15.5810 ±38 38.9007 ±118B18-5 0.70374 ±1 0.512918 ±12 5.47 18.8034 ±25 15.5687 ±32 38.8556 ±104B18-6 0.70385 ±1 0.512889 ±12 4.90 18.8193 ±32 15.5731 ±39 38.8887 ±131B18-7 0.70369 ±1 0.512889 ±10 5.33B19-1 0.70378 ±1 0.512926 ±10 5.62 18.8367 ±39 15.5791 ±39 38.8744 ±125B19-2 0.70382 ±1 0.512899 ±12 5.09B19-3 0.70382 ±1 0.512914 ±9 5.39 18.8169 ±32 15.5734 ±40 38.8572 ±133M20 0.70377 ±1 0.512899 ±12 5.09 18.8486 ±26 15.5829 ±32 38.8954 ±104BH19ba 0.70375 ±1 0.512914 ±10 5.38 18.8603 ±25 15.5783 ±27 38.8932 ±84BH30ba 0.70376 ±1 0.512897 ±10 5.05 18.7641 ±17 15.5642 ±19 38.7880 ±63MP-9a 0.70376 ±1 18.8591 ±49 15.5778 ±54 38.9368 ±174

Younger SeriesB1-1 0.70376 ±1 0.512923 ±13 5.57B1-2 0.70382 ±1 0.512915 ±13 5.41 18.7548 ±25 15.5712 ±30 38.8031 ±99B1-3 0.70375 ±1 0.512904 ±13 5.18B1-4 0.70375 ±1 0.512901 ±8 5.12 18.7745 ±47 15.5723 ±48 38.7828 ±143B1-5 0.70390 ±1 0.512882 ±8 4.76 18.8144 ±20 15.5747 ±24 38.9015 ±80B6-3 0.70375 ±1 0.512899 ±12 5.09 18.7758 ±40 15.5703 ±41 38.7583 ±122B6-4 0.70378 ±1 0.512889 ±15 4.90 18.7381 ±24 15.5656 ±30 38.7367 ±98B18-7 0.70369 ±1 0.512911 ±10 5.33B18-8 0.70380 ±1 0.512884 ±9 4.80 18.8167 ±33 15.5772 ±41 38.8883 ±136B18-9 0.70369 ±1 0.512915 ±13 5.40 18.7438 ±28 15.5626 ±35 38.7726 ±116B19-4 0.70375 ±2 0.512896 ±11 5.04B19-5 0.70377 ±1 0.512906 ±9 5.23M15 0.70375 ±1 0.512898 ±9 5.07 18.7430 ±27 15.5727 ±33 38.7719 ±109M17 0.70374 ±1 0.512887 ±13 4.85 18.6559 ±38 15.5555 ±37 38.6736 ±112M22 0.70377 ±1 0.512910 ±12 5.31 18.7455 ±23 15.5675 ±25 38.8123 ±80M29 0.70375 ±1 0.512905 ±13 5.20 18.7726 ±23 15.5904 ±28 38.8720 ±94M33 0.70374 ±1 – –

C-2 0.70383 ±1 0.512892 ±11 4.96C-18 0.70392 ±1 – –

C-46 0.70386 ±1 0.512874 ±11 4.60C-62 0.70375 ±1 0.512890 ±11 4.91C-76 0.70388 ±1 0.512882 ±9 4.77C-78 0.70406 ±1C-82 0.70377 ±1 0.512896 ±11 5.04BH-9a 0.70371 ±1 0.512916 ±10 5.42 18.8221 ±26 15.5801 ±32 38.8563 ±105BH13a 0.70366 ±1 0.512928 ±10 5.65BH16aa 0.70366 ±1 0.512904 ±10 5.19 18.8063 ±40 15.5806 ±49 38.8139 ±160BH-2a 0.70371 ±1 18.7551 ±21 15.5719 ±26 38.7855 ±84BH25ba 0.70366 ±1 0.512934 ±10 5.77BH26ba 0.70374 ±1 0.512917 ±10 5.45 18.7750 ±19 15.5699 ±24 38.7988 ±77NIST 987 (n=47) 0.71025 ±1Ames Nd (n=13) 0.512131 ±6NIST 981 (n=25) 16.9436 ±26 15.5016 ±30 36.7316 ±85NIST 981 G & A 16.9405 ±15 15.4963 ±16 36.7219 ±44

57J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

CIR MORB

Piton des Neiges (Reunion)

Piton de la Fournaise(Reunion)

Piton des Neiges(Reunion)

Piton de la Fournaise(Reunion)

CIR MORB

Older Series

Intermediate Series

Younger Series

Older Series (others)

Intermediate Series (others)

Younger Series (others)

Gasitao Ridge

206Pb/204Pb

15.40

15.45

15.50

15.55

15.60

15.65

18.0 18.5 19.0 19.5

206Pb/204Pb18.0 18.5 19.0 19.5

37.5

38.0

38.5

39.0

39.5

208 P

b/20

4 Pb

207 P

b/20

4 Pb

Fig. 6. Pb isotope ratios in Mauritius lavas. Fields for CIR MORB and Réunion shown forcomparison. Data sources are the same as in Fig. 5. Solids and dashed lines are plume–depleted mantle mixing lines discussed in Section 4.4. The dot-dash line is a regressionline through the data reported here and has slope corresponding to an age of 2.5 Ga.High precision Pb isotope ratios obtained by the triple spike technique showconsiderably less scatter from linear arrays than do previously published data. Pbisotope ratios in Mauritius have decreased with time such that mean values decrease inthe order Older SeriesN Intermediate SeriesNYounger Series.

Reunion

CIR MORBCIR MORB

Older Series

Intermediate Series

Younger Series

Older (others)

Intermediate (others)

Younger (others)

Gasitao Ridge

εNd

2

3

4

5

6

7

8

9

10

0.7028 0.7030 0.7032 0.7034 0.7036 0.7038 0.7040 0.7042 0.704487Sr/86Sr

Fig. 5. Sr and Nd isotope ratios Mauritius lavas. Open symbols represent previouslypublished data for Mauritius (Sheth et al., 2003; Nohda et al., 2005; Paul et al., 2005).Fields for CIR MORB and Réunion shown for comparison are based on data in theEarthchem database (www.earthchem.org). Gasitao Ridge data is from Nauret et al.(2006). Lines are plume–depleted mantle mixing lines discussed in Section 4.4. TheOlder Series lavas have more enriched isotopic signatures than the rejuvenescent lavas.

58 J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

be expected to be older. However, B14-1 presents clear evidence ofexcess Ar in the age spectrum in the form of a “saddle-shaped” agespectrum and in a 40Ar/36Ar intercept of 312, significantly higher thanthe atmospheric value of 295. It is possible to calculate a plateau age of8.43±0.12 Ma (2σ) from 4 intermediate temperature steps, but thatage is still inconsistent with its stratigraphic position. Perhaps themost reasonable interpretation is that B14-1 is an intrusive dike or sill,an interpretation that would be consistent with its excess argon.

The Intermediate Series lavas dated here (n=11) consist entirely ofdrill core samples. In general, ages increase with depth as expected, butthe age of EauBleu core samples B19-1 throughB19-3 are not consistentwith their stratigraphic positions. The uncertainty on the plateau age ofB19-2 at 72 m depth, however, is particularly large (±0.5 Ma), and theuncertainty on the isochron age is even larger, allowing it to be youngerthan the sample below it at the limit of the 95% confidence interval.The age determined for sample B12-1 of 1.66 Ma is younger than anypreviously reported age for the Intermediate Series (3.5–1.9 Ma), thusextending the period for activity of series activity by 240 ka. The agedetermined for samples B18-8 and C76 of 0.795 Ma and 1.00 Ma,respectively, are older than any previously reported age for the YoungerSeries (0.78 Ma), extending the period of Younger Series activity back220,000 years. Sample C78 has an age of zero, however, the error on theage is quite large (± 240 ka).

4. Discussion

4.1. The geology and volcanic history of Mauritius revisited

New ages for samples M37, M39, B18-1 and B14-2 extend the age ofthe earliest known volcanism on Mauritius back in time from 7.9 Ma to8.9 Ma (Table 4). These samples are found at present elevations of 150–280 m about sea level. Given that thermal contraction and lithostaticrelaxation has undoubtedly occurred over the last 9 m.y., this suggeststhat a substantial volcano was present at that time and therefore thatinitiationof volcanismonMauritiuswasevenearlier.M36,M37, andM39samples are hyaloclastite breccias: lavas erupted in the presence ofwater. Perroud (1982) suggested that these hyaloclastites erupted in thesubmarinephaseof thevolcano. Twoobservations showthisnot tobe thecase, however. First, both B18-1 and B14-2 are older and are subareallavas, demonstrating that the island had emerged above sea level bythis time. Second, hyaloclastite breccias produced by eruption in sea-water, such as littoral cones, typically show elevated 87Sr/86Sr ratiosresulting from the presence of seawater-derived Sr (e.g., White et al.,

1993). Typically, the seawater-derived Sr can be partly removed by acidleaching. In the case of both M36 and M37 the leached sample and theleachate solution (Table 3) have indistinguishable Sr isotopic composi-tions that are both well within the range of Older Series lavas, stronglysuggesting they were not produced by eruption into seawater. The87Sr/86Sr of M39 is also quite typical of Older Series compositions,suggesting it too was not seawater-contaminated. We suggest insteadthat they resulted from eruption into a body of freshwater. Brecciasoccur over a wide area (Perroud, 1982; Giorgi et al., 1999), suggesting alarge body of water, such as a caldera lake, existed at that time.

Giorgi et al. (1999) published a geologic map that shows largeoutcrops of Intermediate Series over much of the island, whereasprevious maps of Simpson (1950) and Baxter (1972) had shown theIntermediate Series confined to the Southwest. Giorgi et al. (1999)also mapped the many young cones in the central chain asIntermediate Series. In contrast, our dating reveals that all eruptivecenters in the central chain except Trou de Madame Bouchet (M20),which we interpret as intermediate series based on Pb isotopic com-position (see subsequent discussion), are part of the Younger Series.Furthermore, every dated outcrop sample and shallow drill coresample but one from outside of the southwestern region belongs tothe Younger Series. Thus the map produced by Giorgi et al. (1999) andthose based on it, such as Paul et al. (2005, 2007), are substantiallyin error. Fig. 1 is based on Giorgi et al. (1999) with revisions basedon our geochronology (as well as that McDougall and Chamalaun,1969 and Nohda et al., 2005).

CIR M

ORB

Reunion

CIR MORB

CIR MORB

Reunion

Older Series

Intermediate Series

Younger Series

Older Series (others)

Intermediate Series (others)

Younger Series (others)

Gasitao Ridge

0.7028

0.7032

0.7036

0.7040

0.7044

18.0 18.5 19.0 19.5

2

3

4

5

6

7

8

9

10

18.0 18.5 19.0 19.5

εNd

87S

r/86

Sr

206Pb/204Pb

b

a

Fig. 7. 87Sr/86Sr vs. 206Pb/204Pb (a) and εNd vs. 206Pb/204Pb (b) in Mauritius lavas. Fieldsfor CIRMORB and Réunion shown for comparison. Data sources are the same as in Fig. 5.Lines are plume–depleted mantle mixing lines discussed in Section 4.4. New data showless scatter, particularly on the 87Sr/86Sr vs. 206Pb/204Pb plot than previously publisheddata. The rejuvenescent lavas plot between the CIR MORB field and the Older Serieslavas.

59J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

Although our results confirm that the Intermediate Series outcroponly in the Southwest, Intermediate Series lavas were indeed eruptedoutside this region but were subsequently buried by Younger Seriesflows. In the center of the island, borehole sample B19-3 showsthat the Intermediate Series lies beneath a relatively thin veneer ofYounger Series flows only 35 m deep, while borehole sample B20-7,5 km southwest of borehole B19, shows that the Intermediate Seriesextends to depths of 170 m in this region. Boreholes B12 and B18in the northern center show that Intermediate Series lavas are alsopresent beneath Younger Series lavas in this area.

Our data further show that the rejuvenescent lava pile can be locallyquite thick: core B1 in the west-center of the island, bottoms in YoungerSeries lava at a depth of 150 m. In core B-2 and B-18 in the central region,the total thickness of Intermediate and Younger Series exceeds 200 m.Only 2 of the 8 boreholes from which we collected samples bottom inOlder Series. Fiveof thepurported “Older Primitive Series” samples of Paulet al.'s (2007) have isotopic compositions typical of rejuvenescent lavasand quite different from all other Older Series samples. Based on this, weconclude that all samplesof Paul et al.'s (2007) “Older Primitive Series” arein fact rejuvenescent lavas. We have classified 3 of these samples asYounger Series and 2 as Intermediate Series based on location, our datingof samples from the same boreholes, and Pb isotopic compositions.

Mauritius rejuvenescent magmatism is peculiar in having beeninterrupted by a long hiatus, although long hiatuses interruptrejuvenescent volcanism in the Canary Is. as well (e.g., Geldmacheret al., 2005). In initiating the present study, we wondered whetherthis hiatus was real. Although our new ages narrow it from 1.12 Ma to

0.66 Ma, the hiatus between the Intermediate and Younger Seriesremains. Younger Series lavas appear to have been erupted exclu-sively from a chain of vents that crosses the island from Southwestto Northeast (Fig. 1). Only a couple of Intermediate Series ventsare preserved in the Southwest, so one can only speculate thatIntermediate Series vents were positioned along the same lineamentas the Younger Series vents. Since these vents, and the flows theyproduced, are now largely buried under Younger Series flows, it ispossible that the duration of Intermediate Series activity wassomewhat longer than it appears now based on available dating.Certainly, this might be the case if volcanism progressed from south tonorth or vice versa along the lineament, as the Intermediate Series isprincipally exposed in the South. However, there is no evidence ofa progression: there is no systematic difference in ages betweenIntermediate samples from cores from the central and northern partsof the island and those from outcrops in the South. In the YoungerSeries, where far more ages are available, geographic progression iseasily ruled out: eruptions appear to have occurred randomly in timealong the entire length of the chain of vents. Spatial patterns are alsoabsent in rejuvenescent volcanism on Kaua'i and Oahu (Ozawa et al.,2005; Garcia et al., 2010).

The youngest reliable ages for Mauritius are 31 ka and 40 ka(samples MR95-09 of Nohda et al. and our sample B6-3). These agesare only slightly greater than the average interval between the 48available ages of Younger Series flows, which is 21 k.y. Furthermore, ahistogram of Younger Series ages (Fig. 8) suggests that eruptionfrequency over the last 50 ka has been rather typical of Younger Seriesvolcanism. Eruption frequency appears to be rather uniform overthe last 400 ka, with possible pulses between 100 ka and 150 kaand 300 ka and 350 ka. The fewer ages older than 400 ka may simplyreflect the fact that many of these lavas are now buried beneathyounger flows (indeed, the average age of core samples is about100 k.y. older than the average age of outcrop samples). Theimportant point here is that the distribution of ages suggests thatfuture volcanic eruptions on Mauritius are possible. Many of the ventsin the center of the island produced substantial cones or gentlysloping shields. Eruptions from these vents occasionally producedquite large lava flows that reached the sea. While the probability of afuture eruption over the next century is very small, it is not negligible.Such an eruption could be quite disruptive to the island populationand economy.

Petrographically, the Intermediate and Younger Series are essen-tially impossible to distinguish in the field, except for the tendency forthe Intermediate Series to be more weathered. The chemical and traceelement compositions of the two rejuvenescent series are also nearlyindistinguishable (there are, however, some differences, which wediscuss below). Given the similarities in composition and eruptivestyle, it is more appropriate to consider the Intermediate and YoungerSeries as a single, long episode of rejuvenescent volcanism interruptedby a comparatively brief hiatus rather than two distinct episodes.

4.2. Rejuvenescent volumes

Much of the surface of Mauritius is now covered with rejuvenes-cent lava flows.We found that the total thickness of the rejuvenescentlavas, previously believed to only a few 10's of meters (e.g., Paul et al.,2007) can locally exceed 215 m (core B2; Appendix A), althoughindividual flows can be quite thin. The Intermediate Series lava pilecommonly exceeds 50 m thickness, and approaches or exceeds 100 min multiple cores. Total thicknesses of Younger Series lavas encoun-tered in cores are typically at least 15 m and can locally exceed 150 m(core B1). This suggests substantial volumes of magma were eruptedin the rejuvenescent period. But how large is this volume, how does itcompare with the volume of Older Series, and how does the ratio ofrejuvenescent-to-shield volume in Mauritius compare with Hawaii?To answer these questions, we attempted to estimate volumes of

Table 440Ar–39Ar radiometric ages for Mauritius samples.

Sample Location/depth Total fusion age(Ma)

2σ Plateau age(Ma)

2σ n MSWD Isochron age(Ma)

2σ 40Ar/36Arinitial

2σ J Sampletype

Older SeriesB14-1 141.7 11.19 0.22 – – – – 7.48 0.21 311.7 2.6 0.0017670 WRB14-2 119.5 9.06 0.35 8.89 0.17 8/10 0.29 8.98 0.54 294.3 6.7 0.0018611 WRB18-1 219.8 8.41 0.08 – – – – – – – – 0.0027507 WRM1 Port Louis 7.76 0.05 7.83 0.05 4/13 0.46 7.77 0.16 322.9 72.7 0.0017205 WRM12 Tamarin Mtn 6.55 0.07 6.83 0.05 7/14 0.14 6.83 0.05 295.2 10.7 0.0017133 WRM30 Motte A Therese 7.42 0.08 7.60 0.06 8/14 1.27 7.58 0.08 296.7 2.5 0.0016889 WRM35 Trois Mammelle 8.10 0.11 7.78 0.06 11/13 1.40 7.79 0.06 294.5 1.8 0.0016810 WRM37 Trois Mammelle 7.88 0.18 8.00 0.07 8/10 0.55 8.00 0.07 295.3 2.4 0.0026437 WRM39 Long Mountain 8.17 0.10 8.03 0.09 8/11 0.54 8.04 0.09 292.8 5.5 0.0015799 G

Intermediate SeriesB2-1 219.8 3.67 0.26 3.32 0.15 7/10 0.08 3.30 0.18 296.1 2.9 0.0015595 GB2-3 150.6 2.68 0.09 2.69 0.06 9/10 0.07 2.69 0.07 295.8 3.2 0.0016161 GB2-5 98.5 2.74 0.12 2.73 0.07 9/10 0.04 2.73 0.08 295.5 2.9 0.0016688 GB6-1 164.6 2.43 0.48 2.07 0.19 8/9 0.21 2.12 0.25 294.8 2.1 0.0027238 WRB12-1 70.1 2.21 1.15 1.66 0.42 10/10 0.44 1.49 0.53 296.2 1.3 0.0017727 WRB18-2 177.7 3.06 0.15 2.55 0.06 8/9 0.18 2.55 0.07 295.6 1.9 0.0025934 WRB18-5 99.1 2.29 0.05 2.08 0.02 5/13 1.23 2.10 0.04 284.8 15.6 0.0017473 WRB19-1 89.9 2.44 0.23 2.04 0.12 7/8 0.36 2.07 0.18 294.8 2.7 0.0027733 WRB19-2 71.6 3.41 0.72 2.77 0.50 6/8 0.03 2.69 1.28 296.0 8.2 0.0018820 WRB19-3 35.1 2.90 0.06 2.29 0.05 8/13 1.10 2.23 0.13 298.2 5.9 0.0017392 WRB20-7 169.5 2.21 0.12 2.22 0.06 9/9 0.34 2.22 0.07 295.8 2.2 0.0015112 G

Younger SeriesB1-4 88.4 0.535 0.103 0.455 0.056 7/8 0.02 0.458 0.098 295.2 6.0 0.0015358 GB1-5 152.7 0.668 0.256 0.627 0.117 7/7 0.10 0.614 0.185 295.7 2.4 0.0026187 WRB2-8 18.9 0.399 0.085 0.356 0.028 8/9 0.09 0.356 0.035 295.0 11.7 0.0017796 WRB6-3 16.8 0.107 0.158 0.040 0.096 7/9 0.00 0.035 0.253 295.8 13.8 0.0016541 GB12-4 16.8 0.642 0.279 0.669 0.176 9/9 0.04 0.674 0.208 295.4 2.1 0.0014972 GB14-3 63.1 1.00 0.16 0.671 0.046 7/10 0.19 0.663 0.138 295.8 5.8 0.0017612 WRB14-5 50.3 0.325 0.126 0.328 0.069 8/8 0.06 0.334 0.084 294.4 8.8 0.0015195 GB14-6 35.4 0.745 0.101 0.128 0.035 6/9 0.23 0.136 0.061 293.6 10.8 0.0017536 WRB14-7 6.1 0.374 0.180 0.121 0.128 5/8 0.03 0.136 0.217 294.6 11.1 0.0016343 GB18-8 41.8 0.795 0.147 0.795 0.107 10/10 0.31 0.788 0.151 295.6 2.3 0.0015995 GB18-9 20.7 0.666 0.015 0.671 0.041 8/8 0.03 0.674 0.048 295.1 3.1 0.0027002 WRB19-5 8.5 0.650 0.099 0.098 0.047 6/8 0.12 0.099 0.065 295.3 6.8 0.0017309 WRM15 Bassin Blanc 0.616 0.265 0.056 0.072 8/9 0.58 −0.041 0.159 298.0 3.6 0.0017060 WRM16 Grand Bassin Crater 0.404 0.066 0.250 0.034 6/10 0.04 0.255 0.068 294.2 14.7 0.0016985 WRM17 Kanaka Crater 0.145 0.113 0.138 0.059 7/7 0.01 0.136 0.067 295.8 4.6 0.0014972 GM21 Trou aux Cerfs 0.318 0.128 0.078 0.062 7/8 0.01 0.078 0.078 295.5 3.7 0.0016019 GM22 Malherbes 0.496 0.071 0.246 0.043 7/8 0.32 0.246 0.075 295.4 5.9 0.0015824 GM29 Nouvelle Decouverte 0.202 0.138 0.192 0.079 7/7 0.56 0.169 0.090 297.4 6.8 0.0026763 WRM32 Butte aux Papayes 0.492 0.405 0.523 0.084 9/9 0.04 0.524 0.104 295.5 2.0 0.0015412 GM33 Forbach Hill Crater 0.303 0.158 0.044 0.073 7/8 0.01 0.044 0.131 295.5 6.5 0.0015621 GC2 Curepipe 0.290 0.04C18 Lepetrin 0.130 0.13C46 La Brasserie Road 0.320 0.03C62 La Dagotiere 0.320 0.04C76 The Vale 1.000 0.05C78 Plaines des Payayes 0.000 0.24C82 Mount Piton 0.730 0.04

Bold values are the reported age for each sample. Ages calculated using FCT-3 (28.04 Ma) and the total decay constant λ=5.530×1010/yr. n is the number of heating steps (defiringplateau/total); MSWD is an F statistic that compares the variance within step ages with the variance about the plateau age. J combines the neutron fluence with the monitor age.Sample types are whole rock (WR) and groundmass (G).

60 J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

rejuvenescent lavas and the volume of the original shield. Full detailsof the calculations can be found in Moore (2009); they aresummarized below.

These calculations require making several assumptions: first, theshield volcano is inferred to extend to the top of the pre-volcanicocean floor at a depth of approximately 4500 m below sea level(mbsl), and to include all material above this surface. However,construction of a volcanic edifice on an elastic plate results in thedownward deflection of the lithosphere around the growing volcano(e.g., Watts and Zhong, 2000) that must be taken into account.Although the Réunion hotspot track lacks an observable longwavelength Hawaiian-style flexural moat (de Voogd et al., 1999),the oceanic crust is depressed approximately 3 km beneath theaverage depth of the undisturbed seafloor (Gallart et al., 1999; seetheir Plate 2). Consequently, we assume that the pre-volcanic ocean

floor is presently at a depth of 7500 mbsl and that all rocks above thisdepth are a product of plume-related volcanism.

The second assumption concerns the location of the contactbetween Mauritius and earlier volcanics of the Mascarene Plateau.Following Bargar and Jackson (1974), we assume this contact isvertical and occurs at a saddle point roughly 30 km north of the island(the true contact is almost certainly not vertical (e.g., DePaolo andStolper, 1996), however, neither the location nor the nature of thiscontact is well known). Lastly, the Older Series is substantially eroded,which means the lost volume must somehow be taken into account.Perroud (1982) argued that the flattening and shallowing of dippinglava beds in the stratigraphically uppermost lavas of the Older Seriesmassifs implied that it is unlikely themaximum elevation of the islandwas more than 300 to 350 m higher than at present day, around800 m. We arbitrarily chose a maximum elevation of 1000 m within

0

1

2

3

4

5

6

7

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Age, Ma

freq

uen

cy

Fig. 8. Histogram of K-Ar and 40Ar/39Ar ages of the Younger Series. Drill core samplefrequencies are in light red, outcrop sample frequencies (from this paper, Nohda et al.,2005, and McDougall and Chamalaun, 1969) in dark blue.

61J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

the limits of the central caldera complex and extrapolated slopesaccordingly. This elevation is much lower than the 3000 m presentmaximum elevation of Réunion. However, Réunion has a much largercross-sectional area on the seafloor and consists of two volcanoes, soMauritius may never have been as large as Réunion. Regardless, theseassumptions more likely lead to an underestimate of shield volumethan an overestimate. Using this approach, we calculated a volume forMauritius of approximately 75,000 km3 using topographic profilescollected from GeoMapApp (a free program available from ColumbiaUniversity at http://www.geomapapp.org).

Just as in calculating the volume of shield lavas, certain assump-tions are required in order to calculate the volume of the rejuvenes-cent lavas as well. For the rejuvenescent lavas, it is important toprovide constraints on the thicknesses of the combined lavas. Fromcross-sections constructed from drill core data by (Giorgi et al., 1999)and results presented here, the Intermediate and Younger Series lavasare commonly 100 m or more in thickness. However, the regularity ofthe caldera collapse surface is not known, and lava thicknesses cannotbe reliably extended beyond established areas. Therefore, the meanthickness of the rejuvenescent suite within the caldera complex wasassumed to be 50 m. From field observations and similar arguments,the mean lava thickness outside of the caldera is assumed to be 10 m.By determining the area of each covered by rejuvenescent lavasin GeoMapApp and applying the assumed thicknesses, a volume of~35 km3 is calculated for the rejuvenescent lavas, and is likely a ratherconservative value. Thus, the rejuvenescent lavas of Mauritiusrepresent approximately 0.05% of the total volume of the island.Garcia et al. (2010) estimated a volume of approximately 58 km3

for the rejuvenescent Ko'olau Series on Kaua'i and a volume fractionof about 0.1%. Given the uncertainties in these volume estimates, theirsimilarity is remarkable. Clearly, in both Hawaii and Mauritius,rejuvenescent volumes constitute only very small factions of thevolcanic edifice.

4.3. Geochemical changes and progressions

The rejuvenescent Intermediate and Younger Series volcanics arechemically and isotopically distinct from the shield-building OlderSeries, as has beenwell established by earlier studies. The former havemore “depleted” trace element compositions and isotopic signatures.The change in isotopic composition is similar to that observed inHawaii and hot spots that have experienced rejuvenescent volcanism(e.g., Samoa, Society Islands, and the Canaries), but the change in

chemical composition is different. In the Hawaiian case, rejuvenescentvolcanics are distinctly silica-undersaturated and incompatibleelement-enriched, both of which suggest they are products of quitesmall extents of melting (e.g., Clague and Frey, 1982; Yang et al., 2003;Garcia et al., 2010). In contrast, Mauritian rejuvenescent volcanics,with the exception of a few basanites from the Intermediates Series,are transitional rather than strongly alkalic and on average onlyslightly more silica-undersaturated than the Older Series (Fig. 3). Forthe most part they probably are not particularly small degree melts.

As pointed out earlier, compositions of the Intermediate and YoungerSeries arenearly identical. There are, however, somedifferences. Althoughthere is no significant difference between mean Sr and Nd isotoperatios of these twoseries,mean 208Pb/204Pb, 206Pb/204Pb, 208Pb/206Pb, and207Pb/206Pb ratios of the Intermediate Series are higher than those of theYounger Series, and this difference is statistically significant at the 1%level (based on the t-test assuming unequal variances). In addition,mean La/Sm and Nb/Y ratios of the Younger Series are significantlylower, while Zr/Nb ratios are significantly higher, than the correspond-ing means of the Intermediate Series (also based on the t-test). Theseratios provide a consistent indication that the Younger Series sourceis slightly more depleted than that of the Intermediate Series. Weemphasize, however, that this is a considerable overlap in trace elementcomposition between the two series and that these distinctions applyonly in aggregate.

There are also differences in variance between the various series.Intermediate Series lavas are highly variable in their incompatibleelement content despite having a limited range of MgO content(Fig. 9). Statistically, Intermediate Series lavas show significantlygreater variance in La/Sm than found in either the Older or YoungerSeries, and significantly greater variance in Zr/Nb than the OlderSeries (based on the F test for variance at the 5% level). On the otherhand, isotopic compositions of the rejuvenescent lavas are notmore variable than those of the Older Series (based on the F test forvariance using our new as well as published data), indicating sourceheterogeneity is unlikely to be the cause of the trace elementheterogeneity. Since incompatible element ratios are not particularlysensitive to fractional crystallization, we conclude through thisprocess of elimination thatmore variablemelt generation, particularlyextent of melting is primarily responsible for the greater chemicalvariability of the Intermediate Series.

Fig. 10 shows that K2O in the Older Series volcanics increased asthe shield-building phase ended. This change is not accompanied bychanges in isotope ratios or other parameters such as La/Sm, and ismostlikely a result of increasing fractional crystallization that culminatedin trachytes and trachyandesites that are the youngest Older Serieseruptives (McDougall and Chamalaun, 1969). Baxter (1972) recognizeda slight erosional unconformity separating these felsic rocks and theearlier more mafic lavas of the Older Series. These late lavas andintrusive bodies are fairly rare, but othersmay have been lost to erosionor buried beneath rejuvenescent lavas. They may be analogous to thepost-shield Differentiated Series of Piton des Neiges on Réunion andperhaps to the post-shield phase of Hawaiian volcanoes.

K2O decreased with time in the Intermediate Series (thiscorrelation is also statistically significant at the 1% level). The oldestdated lavas of the Intermediate Series, samples MU69 and MU65of McDougall and Chamalaun (1969), both have over 1.5% K2O,suggesting this period may been initiated by eruption of highlyundersaturated basanites (complete major element chemistry wasnot reported for these samples). There are no basanites among thelater lavas dated by Nohda et al. (2005) or us, so it may be that thebasanites were restricted to the earliest phase of rejuvenescentvolcanism (three samples collected by Baxter, B5, B33, and B34 arebasanites and strongly incompatible element enriched, but thesesamples have not been dated). Thus Intermediate Series volcanismmay have begun with the eruption of incompatible element-enrichedsmall degree melts. K2O appears to continue to decline with time in

Reunion

CIR MORB

Reunion

CIR MORB

1

2

3

4

5

6

0 5 10 15 20

MgO, %

La/S

m

2

6

10

14

18

0 5 10 15 20

MgO

Zr/

Nb

1

2

3

4

5

6

3 4 5 6εNd

La/S

m0.0

0.5

1.0

1.5

2.0

0 5 10 15 20

MgO

Nb/

Y

1

2

3

4

5

6

2 4 6 8 10 12 14 16

Zr/Nb

La/S

m

0

10

20

30

40

50

0 5 10 15 20

MgO

La, p

pm

Fig. 9. Trace element abundances and trace element ratios in Mauritius lavas. Fields for CIR MORB and Réunion shown for comparison. Trace element variation in Older Series lavas isconsistent with fractional crystallization of parental magmas of relatively uniform composition. Rejuvenescent lavas show generally less incompatible element enrichment but alsoexhibit considerably more scatter, particularly those of the Intermediate Series, suggesting derivation under more variable melting conditions.

62 J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

the Younger Series, albeit onlymodestly. The correlation between K2Oand age is statistically significant only at the 2.5% level in the YoungerSeries.

Both 87Sr/86Sr and 206Pb/204Pb are lower in the rejuvenescentvolcanics than in the Older Series of the shield-building phase. Beyondthat, the data hint at a decline in 206Pb/204Pb during the rejuvenescentphase (Fig. 10), particularly as the Intermediate Series was erupted(the correlation between age and 206Pb/204Pb among the Intermedi-ate Series is statistically significant at the 1% level). There appears tobe no systematic change in isotopic composition with time duringthe shield-building phase, in contrast to Hawaii, where changes inisotopic composition occur during the late phases of the shield-building and immediate post-shield phase.

4.4. Mantle source of rejuvenescent magmas

The difference in isotopic composition between main stageand rejuvenescent volcanism is particularly problematic because itindicates a change inmantle source. Several ideas have been proposedto explain this change. In one class of models, rejuvenescent volcanicsderive from an enriched mantle source (e.g., the mantle plume)

metasomatized to various degrees by melts derived from depletedupper mantle (e.g., Roden et al., 1984; Chen and Frey, 1985; Reinersand Nelson, 1998; Sheth et al., 2003). Yang et al. (2003) found thatsuch a model could not explain the trace element and isotopiccompositions observed in the Honolulu Series volcanics, and proposeda variant in which the rejuvenescent magmas derive from a depletedsource (e.g., oceanic lithosphere) metasomatized by small degreemelts from the plume. In another class of models, rejuvenescentmagmas are derived from a mixture of the mantle plume and adepleted component consisting of deeper mantle that had beenentrained by the plume (White and Duncan, 1996; Fekiacova et al.,2007). In a third class of models, both enriched and depletedcomponents are intrinsic to the plume. Bianco et al. (2005) proposedthat shield and rejuvenescent stage magmatism in Hawaii tappedlithologically distinct components within the Hawaiian plume. In theirmodel, lithospheric flexural uplift caused by a growing upstreamvolcano results in melting of a depleted peridotitic component in theplume, whereas the shield stage magmas result from melting ofpyroxenitic and enriched peridotite components in the plume. Paulet al. (2005) proposed a similar model for Mauritius. Although neitherRéunion nor Mauritius is surrounded by a flexural ‘moat’, Fisher et al.

Older this studyOlder literatureIntermediate this study

Younger literatureYounger this studyIntermediate literature

87S

r/86

Sr

Age, Ma

206 P

b/20

4 Pb

0

1

2

3

4

5K

2O

0.7035

0.7037

0.7039

0.7041

0.7043

18.6

18.7

18.8

18.9

19.0

19.1

0 2 4 6 8 10

Piton de la Fournaise Shield

Piton des Neiges Shield

Piton des Neiges Post-shield

Fig. 10. K2O, 87Sr/86Sr, and 206Pb/204Pb vs. K-Ar or 40Ar/39Ar age. Literature data arefrom McDougall and Chamalaun (1969) and Nohda et al. (2005). There is a statisticallysignificant increase in K2O in the Older Series with time, probably due to increasingfractional crystallization, while isotope ratios show no systematic change. K2O shows astatistically significant decrease with time in the rejuvenescent lavas. 206Pb/204Pb ratiosin Intermediate Series lavas also show a statistically significant decrease with time. Alsoshown are the age ranges for volcanism on Réunion based on dating of McDougall(1971), Gillot and Nativel (1982), and Gillot et al. (1990).

20%

40%

60%

80%

20%

40%

60%

80%

εNd

206Pb/204Pb

87Sr/86Sr

εNd

206Pb/204Pb

87Sr/86SrCIR MORBReunion

Older SeriesIntermediate SeriesYounger Series

Gasitao Ridge

80%

20%

20%

40%

60%40%

60%

a

b

Fig. 11. Two views of a three-dimensional plot of εNd206Pb/204Pb and 87Sr/86Sr in Mauritiuslavas. Only Mauritius data from this study are shown. CIR MORB and Réunion data arefrom the Earthchem database. Gasitao Ridge data is from Nauret et al. (2006). Lines aremixing lines Both assume a CIRMORB source component with 87Sr/86Sr=0.70300, εNd=+8, 206Pb/204Pb=18.22, 207Pb/204Pb=15.475, 208Pb/204Pb=38.100, Sr/Nd=13.8 andSr/Pb=240. The solid line assumes a plume end-memberwith 87Sr/86Sr=0.70422, εNd=+4, 206Pb/204Pb=19.04, 207Pb/204Pb=15.615, 208Pb/204Pb=39.145 (the “Mauritius” endmember); dashed line assumes a plume end-member with 87Sr/86Sr=0.70415, εNd=+3.75, 206Pb/204Pb=18.95, 207Pb/204Pb=15.595, 208Pb/204Pb=39.010 (the “Réunion”end member). Both plume end-members are assumed to have Sr/Nd=15 andSr/Pb=180.

63J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

(1967) suggested the moat has been buried by slumping and turbiditycurrents as gravity surveys revealed a pronounced local compensationinterpreted by Takin (1966) as a depression of the crust by the volcanicpile.

Part of the rationale for this model was Paul et al.'s conclusionthat the rejuvenescent source could not be a mixture of a MORBsource component and the source of the Older Series lavas (i.e., theRéunion mantle plume) since the isotopic compositions of therejuvenescent lavas did not fall on a mixing line between thesetwo end-members; this was particularly evident in 3-dimensionalplots of 87Sr/86Sr-εNd–206Pb/204Pb. However, using only our newhigh quality data, this is no longer true. Fig. 11 shows two viewsof 87Sr/86Sr-εNd–206Pb/204Pb isotope space and demonstrates that it ispossible to construct mixing lines between the Central Indian Ridge(CIR) MORB field and Mauritius and Réunion shield-building fieldsthat passes through the rejuvenescent data (although the latterclearly scatter rather than plotting along mixing lines). Two mixingcurves are shown: one that uses an average Réunion composition asthe plume end-member and the other an average Mauritius OlderSeries composition. Both use the same MORB end member (details ofthe calculation are given in the caption to Fig. 11). Because Sr/Pb andNd/Pb ratios in the two end-members differ (judging from averages of

these ratios in CIR MORB and Réunion and Mauritius shield-buildinglavas), mixing lines are curved in 87Sr/86Sr-εNd–206Pb/204Pb space(but nearly straight on a 87Sr/86Sr-εNd plot). As Fig. 6 shows, bothmixing lines fall very close to a regression line through the new highquality Pb isotope data, although the model using the “Mauritius” endmember fits slightly better. However, in 3-dimensional 87Sr/86Sr-εNd–206Pb/204Pb space, the “Réunion model” fits both the Younger andOlder Series somewhat better (judging from a 12 to 19% reduction insum of squares of deviations). These two mixing lines also pass near,but not through, the Gasitao near-ridge seamount data of Nauret et al.(2006), consistent with the idea that they contain a Réunion plumecomponent. Mixing lines that match the Gasitao data more closelywould require the choice of a different depleted component fromthat shown.

Since both the Réunion plume and MORB source appear to beheterogeneous, one should not expect plume-depleted mantlemixtures to conform tightly to a single mixing line: the mixing linesshown represent only two of many plausible examples. We concludethat the source of the Mauritius rejuvenescent lavas could indeed bea mixture of Réunion mantle plume and mantle similar to the CIR-MORB-source. On the other hand, this observation alone does not ruleout other possible explanations.

In the model of Paul et al. (2005), both enriched and depletedcomponents are intrinsic to the plume with the enriched pyroxeniticcomponents melting at greatest depth and contributing primarily toshield stage magmas, while rejuvenescent magmas are derived from a

64 J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

depleted peridotitic component, which melts later at shallower depthdue to its higher solidus. However, detailed, thermodynamic-basedmodeling with Adiabat_1ph (Smith and Asimow, 2005) shows it isdifficult to simultaneously account for both the isotopic and chemicalcompositions of both shield and rejuvenescent lavas in Mauritius bydifferential melting of a lithologically heterogeneous plume (Moore,2009). While it was possible to find reasonable model parametersthat correctly predict some aspects of magma chemistry, no set ofreasonable parameters was found that simultaneously predictedmajor, trace element, and isotope geochemistry of both shield andrejuvenescent magmas. In Hawaii, Garcia et al. (2010) concluded thatthe Bianco et al. model was unable to produce the volume of magmaobserved for the rejuvenescent Koloa volcanics.

These observations suggest other models should be investigated.We envision a particular scenario similar to the model of Yang et al.(2003) in which the plume is indeed a mixture of eclogite andperidotite as suggested by Paul et al. (2005) and Sobolev et al. (2005).The earliest melts should be rather silicic melts of the eclogite fraction.As they migrate upward into the still cold lithosphere before magmaconduits are well established, these melts could react with depletedlithospheric peridotite to produce pyroxenitic veins in a mannerconsistent with Sobolev et al. (2005). Simple thermal modelingdetailed below suggests that such veins could later melt as the deeplithosphere is conductively heated by the plume.

For modeling purposes, we assumed (a) 65 Ma old-lithospherethat was 75 km thick and cooled conductively from an initial potentialtemperature of 1300 °C, (b) the plume, with a potential temperatureof 1500 °C rises to the base of the lithosphere and heats itconductively.4 In the first case, we assume the plume cools due toconductive heat loss. Assuming a conductivity of 10−6 m2 s−1, wefind that peridotite in the lithosphere never reaches its solidus;however, pyroxenite in the lower 1 km of lithosphere would melt(assuming the pyroxenite solidus of Pertermann and Hirschmann,2003), but only barely: temperatures exceed the pyroxenite solidus byonly a few degrees. Such a model, however, underestimateslithospheric heating because it assumes the plume progressivelycools, ignoring heat advected to the base of the lithosphere by theconvectively rising and spreading plume. In a second set ofcalculations we assumed that the plume maintained constantpotential temperature at depths greater than 80 km, i.e., 5 km beneaththe base of the lithosphere. In this case, even peridotite within thelowermost 2 km of the lithosphere melts (assuming the peridotitesolidus of Herzberg et al., 2000), and does so rather quickly. Thissuggests that even shield stage magmas could be contaminated bylithosphere. The pyroxenite solidus is exceeded at progressivelyshallower depth, reaching 12 km above the base of the lithosphere in9 m.y. after the lithosphere comes in contact with the plume.

This model is overly simplistic in several respects: for one thing,the plume is unlikely to maintain strictly constant potentialtemperature below 80 km;more importantly, we have not consideredheat advected into the lithosphere by magmas rising from the plumeduring the shield building stage. The latter would lead to more rapidheating of the lithosphere. In addition, our modeling has been strictly1-dimensional; in reality some heat will be conducted laterally.

4 The modeling is based on solutions to the Fourier heat flow equation. For a planesheet initially at a temperature of T0 greater than the adjacent plane sheet,temperature evolves as:

T z; tð Þ = T0 erfz

2ffiffiffiffiffikt

p� �

where z is distance from the boundary between sheets, t is time, k is thermalconductivity, and erf is the error function. If the hot sheet is maintained at constanttemperature, then temperature in the adjacent sheet evolves as:

T z; tð Þ = T0 erfzffiffiffiffiffikt

p� �

:

Nevertheless, these simple calculations show that pyroxenite veinsformed by reaction of plume-derived melts and the deep lithospherecould remelt as a consequence of heat conducted from theplume millions of years later — over the time period correspondingto rejuvenescent volcanism. Incompatible elements in lithosphericperidotite would, to some degree, partition into melts that ultimatelyform the pyroxenite veins. Consequently, we expect the veins wouldhave something of a depleted-mantle trace element and isotopic“flavor”, as do the rejuvenescent magmas. If the veins melted to alarge extent, their major element chemical composition would not bevery different from the magmas that formed them in the first place,explaining the similarity in major element composition of shieldand rejuvenescent lavas in Mauritius.

Much more sophisticated thermal and chemical modeling wouldbe required to establish the likelihood of this hypothesis. At this point,we argue only that it appears chemically and thermally plausible.Furthermore, while this scenario explains how volcanism mightcontinue long after the volcano has drifted off the plume and how itattains a depleted-mantle “flavor”, it does not explain the hiatus involcanism following the end of the shield stage or the hiatus betweenthe Intermediate and Younger Series. As Paul et al. (2005) pointed out,the timing might be affected by the growth of the Réunion volcanoes.Ten Brink and Brocher (1987),Watts and ten Brink (1989), and Biancoet al. (2005) suggested construction of the youngest volcano at theend of the Hawaiian chain causes the lithosphere 200 kmdownstreamof the hot spot to be flexed upward by hundreds of meters. This,they argue, contributes in the rejuvenescent volcanism in Hawaii asdecompression causes or enhances melting and as the consequentextension opens pathways to the surface for melts already present atdepth. Fig. 10 illustrates how the volcanic history of Réunion relates torejuvenescent volcanism on Mauritius. The oldest dates lavas fromPiton des Neiges are about 2.1 million years old (McDougall, 1971;Gillot and Nativel, 1982), significantly younger than the oldestIntermediate Series lavas, while the oldest lavas from Piton de laFournaise are about 0.5 million years old (Gillot et al., 1990),compared to an age of 1.0 Ma for the oldest Younger Series lavas.Undoubtedly, construction of both Réunion volcanoes predate theseages, but it seems particularly unlikely that construction of Piton desNeiges began as early as the oldest Intermediate Series lavas.Furthermore, both Ozawa et al. (2005) and Garcia et al. (2010)questioned the flexure model in Hawaii, pointing out that the timingof the flexure does not match the timing of rejuvenescent volcanismon Oahu and Kaua'i. It would seem then that the question of whatcontrols the timing of rejuvenescent volcanism remains unresolved.

Mauritius and Hawaii differ in that the rejuvenescent lavas ofthe latter are often highly silica-undersaturated and incompatibleelement-enriched compositions. The Hawaiian plume rises underolder, thicker, and cooler lithosphere than does the Réunion plume. Italso appears to be a more robust and possibly hotter plume (Sleep,1990). These different circumstances might explain some ofthe differences in rejuvenescent volcanism on Hawaii and Mauritiusalthough, as Fekiacova et al. (2007) and Garcia et al. (2010)demonstrated, the depleted component in Hawaiian rejuvenescentlavas appears to be isotopically distinct from MORB.

The model of Fekiacova et al. (2007) is similar to the Paul et al.(2005) and Bianco et al. (2005) models in postulating a lithologicallyheterogeneous plume. However, their model differs in that theypropose that rejuvenescent magmas are mixtures of melts derivedfrom the peridotitic component of the plume and thermally entraineddepleted mantle that is isotopically distinct from depleted MORB. Thethermal entrainment model can also be applied to the Réunion plumeif the entrained mantle is similar to the Indian Ocean MORB source. Apossible objection to this model is numerical simulations by Farnetaniand Richards (1995) showing that melting is almost entirelyrestricted to the core of a mantle plume. Their study was not,however, meant to address the question of rejuvenescent volcanism.

65J. Moore et al. / Journal of Volcanology and Geothermal Research 207 (2011) 47–66

Furthermore, even in their model, a small amount of melting of themixed plume-entrained mantle does occur, and as we saw fromvolumetric calculations, rejuvenescent lavas constitute a quite smallfraction of the entire volcano. Finally, their model imposes a zero netradial velocity on the top of the plume, which would not be the casewhen lithosphere is moving relative to the plume, as it is in bothHawaii and Mauritius/Réunion. Thus the thermally entrained mantleis a viable alternative model. Like the veined lithosphere model,however, more detailed physical and chemical modeling are requiredto establish the likelihood of the thermal entrainment model.

5. Conclusions

Volcanism on Mauritius began earlier than previously thought – atleast 8.9 million years ago – and has continued through at least thelatest Pleistocene. Several eruptions have occurred in the last50,000 years and eruption frequencies over this period do not differfrom eruption frequencies over the last 500,000 years. Consequently,future eruptions are possible (although the probability of an eruptionin any given year century is extremely low). Volcanism wasinterrupted by two hiatuses — one following the end of the shieldbuilding stage 4.7 million years ago and lasting 1.2 million years, theother between 1.66 Ma and 1.0 Ma, separating the rejuvenescentIntermediate Series and Younger Series. Outcrops of the IntermediateSeries lavas are restricted to the southwestern portion of the island,as originally mapped by Simpson (1950) and Baxter (1972), butIntermediate Series lavas are present throughout the island buried byYounger Series flows. Hyaloclastite breccias of the Older Series, whichoutcrop extensively, are not products of submarine eruptions andmayinstead have formed when lava erupted into freshwater caldera lakes8million years ago. Borehole samples classified by Paul et al. (2007) as“Older Primitive Series” are actually Intermediate and Younger Serieslavas; in this study, Older Series lavas from boreholes are notchemically distinct from outcrop samples. Although the rejuvenescentYounger and Intermediate Series lavas cover 75% of the surface ofMauritius, we estimate that they constitute only about 0.05% of thevolume of the entire edifice. This is within the range of the estimatesof volume fraction of rejuvenescent lavas on Hawaiian volcanoes.

Like Hawaii, rejuvenescent lavas on Mauritius have more depletedisotopic signatures than do shield-stage lavas. Unlike Hawaii,however, most rejuvenescent lavas of Mauritius do not have thecharacteristics of very small degree melts, such as strong silicaundersaturation and strong enrichment in incompatible elements.Instead, the rejuvenescent lavas are generally only slightly less silicasaturated than the shield-building lavas and are generally lessincompatible element enriched.

Newhigher precision Pb isotope data show considerably less scatterthan previously published data for Mauritius. In 3-dimensional isotopespace, rejuvenescent lavas plot close to a line drawn between the OlderSeries andMORB, suggesting their source couldbe amixture of Réunionmantle plume and MORB-source depleted mantle. One possibility thissource is pyroxenite veins in the deep lithosphere created by reactionbetween plume-derived melts and cold lithosphere during the earliestpart of the shield building stage. Heat conducted from the plume beloweventually melts these veins, which erupt as the rejuvenescent lavas.Alternatively, the depleted component could be thermally entraineddepleted mantle that mixes with the plume as it rises.

Supplementary material related to this article can be found onlineat doi:10.1016/j.jvolgeores.2011.07.005. These data include Googlemap of the most important areas described in this article.

Acknowledgments

We gratefully acknowledge the support and assistance providedby the Mauritius Water Resources Unit during the field campaign. Wethank Chen Chen for carrying out some of the isotopic analyses and

John Huard for conducting the 40Ar–39Ar incremental heatingexperiments. Financial support was provided by NSF Grants EAR-0635895 to WMW and EAR-0635842 to DP. Our work benefited fromdiscussions with Jason Phipps Morgan and reviews by DominqueWeis and Michael Garcia.

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