Plume-Ridge Interaction in the Galápagos III: The …dgeist/Chapman/Schlitzer... · Genovesa, 150...

1 1, 3 2 3 4

Nd 3 4 a

3 4

1. Colgate University, Hamilton, NY 2. WHOI, Woods Hole, MA 3. University of Idaho, Moscow, ID

Plume-Ridge Interaction in the Galápagos III: The Origins of Pinta, Marchena, and Genovesa Islands

The Galápagos Archipelago differs from many other ocean island chains in that it is underlain by a submarine platform. A submarine appendage extends ~50 km NNW includes three islands, Pinta, Genovesa, and Marchena. The flanks of these volcanoes were surveyed by EM122 bathymetry and MR1 sid-escan sonar, and dredged in 7 locations during our 2010 cruise. All islands have elongate footprints and have erupted plagioclase ultraphyric rocks. Major submarine ridges extend NW of Pinta and NE of Genovesa; a flat-topped shoal reaches ~30 km SE from Marchena (Fig. 3).

Figure 1: A map of the Galápagos region, including the submarine platform (in box).

Figure 2: The EM122 bathymetric data from the MV1007 and DRIFT04 cruises, with dredge locations. Figure 3: The shoal south of Marchena.

Whole rock and glass samples compositions of submarine samples resemble those of adjacent islands, and major element variations reflect extensive plagioclase fractionation compared lavas of the main Galápagos Archipelago (Figures 4,5).

The most enriched signatures are from near Pinta. Genovesa, 150 km to the east, produces depleted, MORB-like lavas. Marchena and the southern shoal are built of lavas with intermediate compositions (Fig. 7 a, b, c). Kurz and Geist (1999) determined that ε correlates with La/Nb in the Galápagos; La/Nb is thus a proxy for plume contribution. Intriguingly, the entire isotopic and trace element composition range of the Galápagos region is observed within our small study area (Figures 8,9). Despite plume-like compositions at Pinta, He/ He values are lower than typical MORB (6.9 R ; Kurz and Geist, 1999, Figure 6).

Mixing Models

Lavas erupted in our study area (Figure 10) do not require the most enriched mantle endmember invoked in the main archipelago (FLO; Harpp and White, 2001). Variations across the platform extension can be explained by variable mixing only two sources, plume (Pinta) and depleted mantle (Genovesa) (Fig. 11,12,13,14).

We propose the following to explain observations in our study area, invoking a heterogeneous plume with dis-tinct enriched and depleted components (e.g., Ito and Mahoney, 2005): a) Upwelling plume beneath Fernandina melts partially at the wet solidus, losing some of its volatiles (He) and high He/ He (Kurz and Geist, 1999; Graham et al., 1993). This is consistent with recent tomographic data from Villagomez et al. (2011) that reveal a low-velocity anomaly 100-300 km beneath the western Galapagos. b) The high-velocity lid formed from melt extraction and dehydration causes plume material to spread laterally north and NE, where thinner lithosphere initiates further melting. Enriched plume components dominate melts produced beneath Pinta. c) As plume material migrates further east, it experiences more melting beneath the younger, thinner litho-sphere east of the 90º50’W transform fault discontinuity (Figure 16). More depleted material melts, yielding progressively depleted signatures (Figure 17) generated at shallower depths (Figure 18) toward Genovesa.

Introduction

Geochemical Data

Conclusions

W. Schlitzer , K. S. Harpp , M. D. Kurz , E. L. Mittelstaedt , D. J. Fornari

Figure 10: Harpp & White, 2001

Figure 15: Tomographic data from Villago-mez et al. (2011)

−92˚ −91˚ −90˚ −89˚ −88˚

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Fig. 1 200 meter contour interval

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G en o v esa Fig. 2

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G en o v esa R idge

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Fig. 5

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Fig. 7a 1

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Fig. 7c

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Fig. 9 PLUME

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87 Sr/ 86 Sr

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Fig. 10 (Depleted Upper Mantle)

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Fig. 13

Pinta Genovesa

Fig. 15

P i n ta M a r chena G en o v esa

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Fig. 18