In situ Re–Os isotopic analysis of platinum-group minerals ... et al.pdfof centimeters thick. On the other hand, only the mantle section crops out in the Mayarı´-Cristal massif

ORIGINAL PAPER

In situ Re–Os isotopic analysis of platinum-group mineralsfrom the Mayarı-Cristal ophiolitic massif (Mayarı-BaracoaOphiolitic Belt, eastern Cuba): implications for the originof Os-isotope heterogeneities in podiform chromitites

Claudio Marchesi • Jose Marıa Gonzalez-Jimenez • Fernando Gervilla •

Carlos J. Garrido • William L. Griffin • Suzanne Y. O’Reilly •

Joaquın A. Proenza • Norman J. Pearson

Received: 16 April 2010 / Accepted: 16 August 2010

� Springer-Verlag 2010

Abstract Chromitite pods in the Mayarı-Cristal ophio-

litic massif (eastern Cuba) were formed in the Late Cre-

taceous when island arc tholeiites and MORB-like back-arc

basin basalts reacted with residual mantle peridotites and

generated chromite-rich bodies enclosed in dunite enve-

lopes. Platinum-group minerals (PGM) in the podiform

chromitites exhibit important Os-isotope heterogeneities at

the kilometric, hand sample and thin section scales.187Os/188Os calculated at the time of chromitite crystalli-

zation (*90 Ma) ranges between 0.1185 and 0.1295

(cOs = -7.1 to ?1.6, relative to enstatite chondrite), and

all but one PGM have subchondritic 187Os/188Os. Grains in

a single hand sample have initial 187Os/188Os that spans

from 0.1185 to 0.1274, and in one thin section it varies

between 0.1185 and 0.1232 in two PGM included in

chromite which are only several millimeters apart. As the

Os budget of a single micrometric grain derives from a

mantle region that was at least several m3 in size, the

variable Os isotopic composition of PGM in the Mayarı-

Cristal chromitites probably reflects the heterogeneity of

their mantle sources on the 10–100 m scale. Our results

show that this heterogeneity was not erased by pooling and

mingling of individual melt batches during chromitite

crystallization but was transferred to the ore deposits on

mineral scale. The distribution of the Os model ages cal-

culated for PGM shows four main peaks, at *100, 500,

750 and 1,000 Ma. These variable Os model ages reflect

the presence of different depleted domains in the oceanic

(Pacific-related) upper mantle of the Greater Antilles

paleo-subduction zone. The concordance between the age

of crystallization of the Mayarı-Cristal chromitites and the

most recent peak of the Os model age distribution in PGM

supports that Os in several grains was derived from fertile

domains of the upper mantle, whose bulk Os isotopic

Communicated by J. Hoefs.

C. Marchesi (&)

Geosciences Montpellier, UMR 5243, CNRS-Universite

Montpellier II, Place E. Bataillon, 34095 Montpellier, France

e-mail: [email protected]

C. Marchesi � J. M. Gonzalez-Jimenez � W. L. Griffin �S. Y. O’Reilly � N. J. Pearson

GEMOC ARC National Key Centre,

Department of Earth and Planetary Sciences,

Macquarie University, Sydney, NSW 2109, Australia


W. L. Griffin


S. Y. O’Reilly


N. J. Pearson


F. Gervilla

Departamento de Mineralogıa y Petrologıa,

Facultad de Ciencias, Universidad de Granada,

Avenida Fuentenueva s/n, 18002 Granada, Spain


F. Gervilla � C. J. Garrido

Instituto Andaluz de Ciencias de la Tierra,

CSIC-Universidad de Granada, Facultad de Ciencias,

Avenida Fuentenueva s/n, 18002 Granada, Spain


J. A. Proenza

Departament de Cristal lografia Mineralogia i Diposits Minerals,

Universitat de Barcelona, Martı i Franques s/n,

08028 Barcelona, Spain


123

Contrib Mineral Petrol

DOI 10.1007/s00410-010-0575-2

composition is best approximated by that of enstatite

chondrites; on the other hand, most PGM are crystallized

by melts that tapped highly refractory mantle sources.

Keywords Caribbean �Mantle heterogeneity � Ophiolite �PGM � Podiform chromitite � Re–Os isotopes

Introduction

Os-rich platinum-group minerals (PGM) are fundamental

tools for tracing the Os isotopic evolution of the Earth. The

very low Pt/Os and Re/Os of these phases make age cor-

rections to their present-day 186Os/188Os and 187Os/188Os

generally negligible; moreover, the highly refractory nature

of these minerals limits the probability of Os exchange

with secondary reservoirs (i.e., melts/fluids in the mantle

and/or the crust) after their formation. Therefore, the Os

isotopic composition of Os-rich PGM is considered to be

highly representative of their source at the time they

formed (e.g., Hirata et al. 1998; Meibom and Frei 2002;

Malitch 2004; Meibom et al. 2004; Walker et al. 2005;

Brandon et al. 2006; Pearson et al. 2007; Shi et al. 2007).

PGM are principally associated with chromite deposits in

layered mafic intrusions and ophiolitic peridotite bodies

(e.g., Melcher et al. 1997; Garuti et al. 1999; Ahmed and

Arai 2002; Zaccarini et al. 2002). Different models have

been proposed to explain the concentration of huge

amounts of Cr in monomineralic igneous rocks (e.g., Lago

et al. 1982; Auge 1987; Arai and Yurimoto 1994; Ballhaus

1998; Matveev and Ballhaus 2002; Buchl et al. 2004a;

Rollinson 2005), and some of them invoke the participation

of subduction-related magmas with boninitic affinity (Zhou

et al. 1996, 1998; Proenza et al. 1999; Ghosh et al. 2009;

Page and Barnes 2009).

The Os isotopic composition of PGM has been generally

examined in detrital grains sampled in chromite-rich placers

(Hattori and Hart 1991; Walker et al. 1997; Hirata et al.

1998; Meibom and Frei 2002; Meibom et al. 2002, 2004;

Walker et al. 2005; Brandon et al. 2006; Pearson et al. 2007)

or separated from large amounts of chromite ore (Walker

et al. 1996; Malitch et al. 2003; Malitch 2004; Shi et al.

2007). These studies established fundamental constraints on

the Os isotopic composition and heterogeneity of the upper

mantle, which is normally considered the primary source of

Os in PGM, but they provide limited information on the Os

isotopic variability of PGM on small (\1 m) length scales

in chromitites. In order to document and interpret potential

isotopic heterogeneities at the mineral scale, in situ analysis

of primary Os-rich PGM not liberated from host chromite

(Ahmed et al. 2006) should be performed.

In this paper we examine the PGE (platinum-group

element) and Os isotopic compositions of PGM in

podiform chromitites from the Mayarı-Cristal ophiolitic

massif (eastern Cuba), using in situ electron microprobe

and laser ablation analysis. We document significant Os

isotopic heterogeneities at different length scales, from

distinct mining districts (several tens of km) to a single thin

section (several millimeters). The origin of this variability

is discussed in terms of the distribution of Os isotopic

heterogeneities in the upper mantle and their potential

homogenization by the magmatic processes that generate

the podiform chromite deposits. Further constraints are

inferred on the geochemical signature of the chromitite

parental magmas and on the nature of the PGM mantle

sources. Finally, we show that the comparison of the Os

model ages calculated for PGM with the age of crystalli-

zation of the Mayarı-Cristal chromitites supports that Os in

several PGM was derived from fertile domains of the upper

mantle, whose bulk Os isotopic composition is mostly

similar to that of enstatite chondrites.

Geological setting

In Cuba several dismembered ophiolitic massifs crop out

along an east–west trend in the northern portion of

the island and constitute the so-called Northern Cuban

Ophiolite Belt (Fig. 1a; Iturralde-Vinent 1994, 1996).

These ultramafic–mafic bodies represent pieces of oceanic

lithosphere obducted onto the North American continental

paleo-margin in Late Cretaceous to Late Eocene time,

during collision between the Florida–Bahamas platform

and the Greater Antilles paleo-island arc (Iturralde-Vinent

1994, 1996). This extinct intra-oceanic convergent margin

was defined by the relatively short-lived NE-dipping sub-

duction of the Caribbean (Pacific-Farallon) plate beneath

the Proto-Caribbean (North American-Proto Atlantic) plate

in the Early Cretaceous and by the opposite SW-dipping

subduction geometry from Aptian to Eocene time (Fig. 1b;

Pindell and Barrett 1990; Meschede and Frisch 1998;

Pindell et al. 2006; Marchesi et al. 2007; Jolly et al. 2008;

Lazaro et al. 2009).

The Moa-Baracoa and Mayarı-Cristal massifs are the

easternmost and largest Cuban ophiolites (Fig. 1a) and

jointly form the ‘‘Mayarı-Baracoa Ophiolitic Belt’’ (Pro-

enza et al. 1999). They are mostly composed of highly

depleted mantle harzburgite and subordinate dunite, locally

cut by gabbroic and pyroxenitic dykes (Proenza et al. 1999;

Marchesi et al. 2006). Up-section in the Moa-Baracoa

massif, dunites, plagioclase-rich peridotites and several

generations of gabbroic sills and dykes are increasingly

abundant and constitute the transitional zone between the

mantle and crustal sections. In this massif the crustal plu-

tonic exposures are limited to cumulate olivine gabbros that

generally make up uniform isomodal layers of several tens


123

of centimeters thick. On the other hand, only the mantle

section crops out in the Mayarı-Cristal massif. Late Creta-

ceous volcanic rocks with different geochemical signatures

are in tectonic contact with both the massifs (Fig. 1c).

Turonian-Coniacian (88–91 Ma) (Iturralde-Vinent et al.

2006) pillow lavas with back-arc geochemical affinity

probably represent the melts evolved after the crystalliza-

tion of the Moa-Baracoa cumulate gabbros from a common

parental magma (Marchesi et al. 2006). On the contrary, no

genetic relationships exist between the Turonian-Coniacian

calcalkaline arc volcanic rocks that tectonically underlie the

Mayarı-Cristal mantle peridotite and the coeval island arc

tholeiitic (IAT) dykes that intrude it (Marchesi et al. 2006,

2007). Based on the geochemical affinities of these igneous

rocks, Marchesi et al. (2006, 2007) interpreted the Moa-

Baracoa massif as a portion of MORB-like lithosphere

located near a Caribbean back-arc paleo-spreading ridge

and the Mayarı-Cristal massif as a piece of transitional

(MORB to IAT) back-arc mantle located closer to the

Greater Antilles paleo-island arc than Moa-Baracoa.

Chromite deposits and PGM mineralogy

Chromitite bodies in the mantle sections of the Moa-Bara-

coa and Mayarı-Cristal massifs are generally included in

dunite pods (from some centimeters to 3 m thick) that are

concordant with the foliation of the host tectonite and are

cut by gabbroic and pyroxenitic dykes (Proenza et al. 1999;

Gervilla et al. 2005). Chromite ore-rich lenses are increas-

ingly abundant toward the transition zone with the oceanic

crust in the Moa-Baracoa massif and in the lower portion of

the mantle section in the Mayarı-Cristal massif. Three main

chromite mining districts have been differentiated from the

east to the west in the study region according to their

location and chromite ore composition (Proenza et al. 1999;

Gervilla et al. 2005): the Moa-Baracoa, Sagua de Tanamo

and Mayarı districts (Fig. 1c). Chromite in Moa-Baracoa is

relatively Al-rich (Cr# = [Cr/(Cr ? Al)] = 0.41–0.54),

has highly variable TiO2 (0.05–0.52 wt%) and forms tabu-

lar to lens-shaped ore bodies enclosed in dunite envelopes

and that occasionally include concordant gabbros and minor

dunite layers (Proenza et al. 1999; Gervilla et al. 2005).

Chromitite in the Sagua de Tanamo district, which is

located in the easternmost area of the Mayarı-Cristal massif

(Fig. 1c), has TiO2 contents in chromite (0.10–0.33 wt%)

similar to Moa-Baracoa but more variable chromite Cr#

(0.46–0.72). The Mayarı district has relatively Cr-rich

(Cr# = 0.69–0.83) and Ti-poor (TiO2 = 0.10–0.20 wt%)

chromite that forms pod-like ore bodies frequently cut

by dykes of olivine websterite; chromitite in these deposits

has variable microstructures from massive in the center

Atlantic Ocean

Paleogene-Quaternary rocks

Ophiolitic mantleperidotites

Cretaceous volcanicand minor plutonic rocks

Ophioliticgabbros

Microgabbrodykes

Metamorphicmelanges

Cretaceous meta-igneousrocks (Purial Complex)

Faults

Sample locations

20° 40

20° 30

75°30 75°10 74°50 74°3075°50

0 5 10 km

a

a)CUBA

Ophiolitic massifs

Mayarí-Cristal massif

Moa-Baracoa massif

study area

SouthAmerica

Caribbean Plate

Proto-

Caribbean

NorthAmerica

90Ma

b

c

GreaterAntilles paleo-arc

Atlantic Ocean

N

N

Sagua deTánamo district

Mayarí district Moa-Baracoa district

N

5 6

123

4

1

Fig. 1 a Geographic location of

ophiolitic massifs in Cuba

(Iturralde-Vinent 1994); blackbox indicates the study area.

b Middle-Late Cretaceous

paleotectonic reconstruction of

the Caribbean realm, modified

from Pindell and Kennan

(2001). c Geological map of

eastern Cuba with the location

of the chromite deposits

sampled in this study (blackcircles): 1 Tre Amigos, 2 Negro

Viejo, 3 Caridad, 4 Monte

Bueno, 5 Casimba, 6 Estrella


123

to nodular or disseminated at the rims of the podiform

body.

Gervilla et al. (2005) identified 44 grains of platinum-

group minerals in 19 out of 56 polished thin sections of

massive chromitite from the Moa-Baracoa and Mayarı-

Cristal ophiolitic massifs. Additional PGM from the Sagua

de Tanamo mining district were recognized by Gonzalez-

Jimenez et al. (2009a). For this study we selected 16 of

these sections from the Mayarı-Cristal massif as PGM are

generally more abundant in Cr-rich (Cr# [ 0.6) than in Al-

rich chromitites (Gervilla et al. 2005). We obtained precise

and accurate in situ Re–Os isotopic analyses in 13 sections

from 6 different mines whose locations are shown in

Fig. 1c. A total of 27 PGM were analyzed, 24 from the

Sagua de Tanamo and 3 from the Mayarı district. They

form single or polyphase inclusions in unaltered chromite

(Fig. 2a), occasionally connected to cracks; fewer grains

are located in the silicate (olivine, serpentine and chlorite)

matrix between strongly fractured chromite crystals

(Fig. 2b–d). The grain size of PGM varies from \5 to

50 lm, and they are mostly members of the laurite–erli-

chmanite (RuS2–OsS2) solid solution series and Ru–Os–Ir–

Fe–Ni–(Rh) oxides/alloys (Table 1). The minerals of the

laurite–erlichmanite series occur as single isolated crystals

(Fig. 2c) or in association with Os–Ir alloys (Fig. 2d),

(PGE-rich) Ni–Fe–Cu sulfides (Fig. 2b) [millerite (NiS),

pentlandite (Ni,Fe)9S8, chalcocite (CuS2), Ru-rich pent-

landite (Ru,Ni,Fe)9S8 and cuproiridsite (CuIr2S4)] or

silicates (clinopyroxene or amphibole) (Fig. 2a). Ru–Os–

Ir–Fe–Ni–(Rh) oxides/alloys form rounded subhedral to

irregularly shaped grains generally connected to fractures

in chromite or embedded in the silicate matrix; they

commonly exhibit inner spongy texture and are occasion-

ally associated with irarsite (IrAsS) and Os–Ir alloys.

Fewer grains of PGE-rich Ni–Fe–Cu sulfides have been

rarely observed coupled to millerite, Ru-rich pentlandite

and clinopyroxene. More detailed descriptions of the

chromitite field occurrence and PGM inclusion assem-

blages in chromitites from the Mayarı-Baracoa Ophiolitic

Belt are given by Proenza et al. (1999), Gervilla et al.

(2005) and Gonzalez-Jimenez et al. (2009a).

Analytical techniques

Polished thin sections of chromitites were carefully studied

by ore microscope, SEM and FE-SEM in an effort to

localize and identify the PGM grains and/or assemblages.

PGM were analyzed by electron microprobe at the Serveis

Cientificotecnics of the University of Barcelona (Spain).

Excitation voltage was 25 kV, sample current 20 nA and

beam diameter 2 lm. Pure metals were used as standards

for Os, Ir, Ru, Rh, Pt, Pd, Co and Ni; as well as Cr2O3 for

Cr; FeS2 for Fe and S; Cu2S for Cu; and GaAs for As. The

X-ray lines used were Ka for S, Fe, Co, Ni and Cr; Kb for

Cu; and La for As, Os, Ir, Ru, Rh, Pt and Pd. Online

corrections were performed for the interferences involving

Ru–Rh, Ir–Cu, Rh–Pd, Ru–Pd, Cu–Os and Rh–Pt.

Re–Os in situ isotopic analyses of platinum-group

minerals were performed at the Geochemical Analysis Unit

of GEMOC (Macquarie University, Sydney, Australia)

using the technique described in detail by Pearson et al.

(2002) and Griffin et al. (2002). A New Wave/Merchantek

UP 213 laser microprobe with a modified ablation cell was

coupled with a Nu Plasma Multicollector ICP-MS. The

laser was fired at a frequency of 4 Hz, with energies of

1–2 mJ/pulse and using a spot size of 30 lm. Several tests

were carried out to verify the negligible contents of Re and

Os in the host chromite compared to the platinum-group

minerals; these background analyses demonstrated that the

partial inclusion of chromite in the ablated volume has a

negligible contribution to the sampled Re and Os budgets

(see also Ahmed et al. 2006). A dry aerosol of Ir was bled

into the gas line between the ablation cell and the ICP-MS

to provide a mass bias correction with a precision inde-

pendent of the abundance of Os in the unknown. During

ablation runs, a standard NiS bead (PGE-A) with 199 ppm

Os (Lorand and Alard 2001) and 187Os/188Os = 0.1064

Fig. 2 Back-scattered images of representative PGM from the

Mayarı-Cristal ophiolitic massif. a Laurite associated with a silicate

grain in unaltered chromite from the Caridad chromitite (Sagua de

Tanamo district). b Laurite (hosting a base metal sulfide) in the

silicate matrix between strongly fractured chromite crystals in the

Caridad chromitite (Sagua de Tanamo district). c Euhedral laurite

grain at the contact between partly dissolved chromite and the

interstitial (serpentinized) silicate matrix in the Tres Amigos chro-

mitite (Sagua de Tanamo district). d Partly desulfurized laurite with a

rim of Os–Ir alloy in the silicate matrix of the Estrella chromitite

(Mayarı district)


123

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0.0

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36

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90

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Fe

(at%

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70

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71

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91

6.6

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7.5

32

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Co

0.0

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30

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10

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60

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Ni

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0.1

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8.8

77

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11

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91

2.2

71

4.3

71

1.4

11

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0

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.66

11

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31

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41

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04

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40

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0.0

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0.0

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0.0

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S6

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16

6.1

36

6.2

96

5.6

90

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0.0

40

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0.1

3

As

0.3

80

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1.0

70

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0.1

00

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0.0

00

.00

0.0

00

.00

Ru

/(O

s?

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Ru

)0

.70

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0.7

10

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0.8

90

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0.7

60

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3

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(0.0

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t%)


123

(Pearson et al. 2002) was analyzed between samples to

monitor and correct any drift in the ion counters.

These corrections typically were less than 2% over a

day’s analytical session. The overlap of 187Re on 187Os

was corrected by measuring the 185Re peak and using187Re/185Re = 1.6742. All the analyzed grains have187Re/188Os much lower than 0.5, thus ensuring that the

isobaric interference of 187Re on 187Os was precisely cor-

rected (c.f., Nowell et al. 2008). The data were collected

using the Nu Plasma time-resolved software, which allows

the selection of the most stable intervals of the signal for

integration. The selected interval was divided into 40

replicates to provide a measure of the standard error. An

internal precision for 187Os/188Os of 0.2–2% (2 standard

errors) was obtained. The Re–Os isotopic data for the

platinum-group minerals are reported in Table 2. cOs and

model ages have been calculated by comparison with the

Os-isotope evolution of enstatite chondrite (present-day187Os/188Os = 0.1281, 187Re/188Os = 0.421, Walker et al.

2002). The uncertainties on TMA model ages include the

uncertainties in the measured 187Os/188Os and 187Re/188Os

according to the equation of Sambridge and Lambert (1997).

Results

Laurite, with compositions variable from Os-poor [(Ru0.91

Os0.06 Ir0.04 Fe0.01) R=1.02 S1.98] to Os-rich [(Ru0.56 Os0.29

Ir0.11 Ni0.03) R=0.99 S1.01], is the most common PGM ana-

lyzed for Re–Os isotopes. Most of the laurite grains are

compositionally homogeneous, but some of them have

oscillatory zoning patterns characterized by variable Os

contents in different areas of the grain (Gervilla et al. 2005;

Gonzalez-Jimenez et al. 2009b). Ru–Os–Ir–Fe–Ni–(Rh)

alloys/oxides are micro-intergrown of metallic Ru–Os–Ir

and Fe-oxyhydroxide; they commonly have Ru–Os–Ir

atomic proportions similar to partly desulfurized laurite

connected to fractures in chromite or located in the silicate

matrix (Gonzalez-Jimenez et al. 2009a). The Ni–Fe–Cu

sulfide grain analyzed by electron microprobe is associated

with millerite and has a composition close to that of a

monosulfide solid solution rich in PGE [(Ni0.41 Ir0.23 Fe0.13

Cu0.12 Rh0.05 Pt0.01)R=0.95S1.05]. All these grains have

chondrite-normalized PGE patterns characterized by rela-

tively flat segments from Os to Ru, a slightly negative Ir

anomaly and a steep negative slope from Ru to Pd (Fig. 3).

These geometries are similar to the bulk rock PGE patterns

of Cr-rich chromitites from the Sagua de Tanamo and

Mayarı districts (Fig. 3), thus strongly suggesting that the

bulk rock PGE contents in these deposits are controlled by

the abundance of micrometric PGM in the chromitite

samples.

Initial 187Os/188Os has been calculated at 90 Ma, which

is the estimated age of the ophiolite formation inferred by

paleontological dating of sedimentary rocks intercalated

in the Mayarı-Baracoa crustal sections (Iturralde-Vinent

et al. 2006). However, the very low 187Re/188Os in PGM

(from \0.001 to 0.067) leads the correction for the in situ187Re decay generally negligible. In Sagua de Tanamo,

initial 187Os/188Os spans from 0.1185 to 0.1295 with an

average of 0.1236 ± 0.0045 (2r). These values corre-

spond to cOs (90 Ma) = -7.1 to ?1.6 (average = -3.0),

and all but one PGM have subchondritic cOs (90 Ma) (up

to -0.1). 187Re/188Os and 187Os/188Os are not clearly

correlated at the scale of the mining district, nor even for

grains in the same hand sample or thin section (Fig. 4).

Considering all the grains analyzed in Sagua de Tanamo,187Os/188Os variations appear to be unrelated to the

microstructural setting of the PGM (i.e., included in

unaltered chromite, associated with chromite fractures or

interstitial in the silicate matrix), but in a single hand

sample from the Caridad mine the grains included in

chromite have less radiogenic values (0.1185–0.1245)

than the two analyzed PGM embedded in the silicate

matrix (0.1263–0.1274, Table 2). This suggests a slight

contribution of radiogenic Os from crustal hydrothermal

fluids in the matrix-embedded grains. 187Os/188Os (90 Ma)

in this sample spans from 0.1185 to 0.1274, which

overlaps almost the entire range of values measured in the

whole Sagua de Tanamo district, and in a single thin

section of this sample it varies between 0.1185 and

0.1232 in two PGM included in chromite which are only

several millimeters apart. On the other hand, PGM in one

section from the Tres Amigos mine are more homoge-

neous in terms of Os isotopes (187Os/188Os (90 Ma) =

0.1210–0.1228). The few analyzed grains from the

Mayarı district have 187Os/188Os (90 Ma) = 0.1271–0.1272

(cOs (90 Ma) from -0.3 to -0.2) that overlap with the

values found in Sagua de Tanamo, but are mostly higher.

These analyses confirm the subchondritic Os-isotope

composition of chromitites in the Mayarı-Baracoa

Ophiolitic Belt (Gervilla et al. 2005; Frei et al. 2006), but

in the two samples for which both bulk chromitite and in

situ Os-isotope analyses of PGM are available (NV-100

and CS-100), the latter are more radiogenic (Fig. 4). This

discrepancy has been observed also in chromitites from

the ophiolites of eastern Egypt and Oman (Ahmed et al.

2006) and is probably due to the small number of PGM

analyzed in these samples and to their highly variable

Os-isotope compositions. Meaningful calculated TMA vary

between 0.1 and 1.4 Ga in Sagua de Tanamo and are

equal to 0.1 Ga in Mayarı. One future age reflects deri-

vation of Os from a source more radiogenic than enstatite

chondrites.


123

Ta

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123

Discussion

Geochemical signature of chromitite parental magmas

Podiform chromitites in ophiolitic sections are crystalli-

zation products of mantle-derived melts (e.g., Lago et al.

1982; Auge 1987; Leblanc and Ceuleneer 1992; Zhou et al.

1996; Rollinson 2005). Valuable information on the geo-

chemical signature of the chromite parental magmas and

the tectonic setting of their genesis can be obtained by

plotting their TiO2 contents versus Cr# (Fig. 5). The Sagua

de Tanamo district has highly variable chromite composi-

tion that overlaps with that in equilibrium with MORB (or

back-arc basin basalts) and island arc basalts. On the other

hand, chromite in Mayarı has more homogeneous high Cr#

and relatively low TiO2 compositions that coincide with

those of Cr-rich spinel in island arc basalts and boninites

(Fig. 5). This confirms that chromitite in the Mayarı-Bara-

coa Ophiolitic Belt is crystallized from different subduc-

tion-related magma types, as already proposed by Proenza

et al. (1999) and Gervilla et al. (2005).

Podiform chromitites in the Mayarı-Baracoa Ophiolitic

Belt, as well as in many other ophiolitic mantle sections (e.g.,

Zhou et al. 1996; Buchl et al. 2004a; Yumul 2004; Rollinson

2005), are normally enclosed in dunite envelopes generated

by reaction between low-silica olivine-saturated melts and

residual peridotites (Proenza et al. 1999). These reactions

normally cause dissolution of pyroxene and precipitation of

olivine in focused melt channels in the upper mantle (Quick

1981; Kelemen 1990; Kelemen et al. 1995; Suhr et al. 2003).

The association of chromitite bodies and dunite suggests that

the petrogenesis of these two rock types is somehow linked

and that melt/rock interaction has an important role in the

generation of chromitites (Arai and Yurimoto 1994; Zhou

et al. 1996; Buchl et al. 2004a; Shi et al. 2007). In spite of the

boninite-like signature of the parental magmas of chromite in

Mayarı (Fig. 5), boninites probably did not form the dunite

wraps around the podiform ore bodies, as boninitic melts

have relatively high SiO2 contents and are normally satu-

rated in orthopyroxene (Bloomer and Hawkins 1987; Taylor

et al. 1994; Falloon and Danyushevsky 2000). Moreover, the

composition of spinel in dunites from Mayarı-Baracoa is not

in equilibrium with boninites (Marchesi et al. 2006). This

suggests that dunite and chromitite in the Mayarı-Baracoa

Ophiolitic Belt were derived from the interaction of mantle

rocks with percolating island arc tholeiites and/or back-arc

basin basalts similar to the dykes and sills that intrude its

mantle sections. The precipitation of monomineralic chro-

mite deposits may have been caused by the exsolution of a

fluid phase from these (olivine-saturated) subduction-related

hydrous melts (Matveev and Ballhaus 2002) or by mingling

of primitive melt batches with relatively viscous melts whose

silica content and Cr# may have been increased by pro-

gressive melt/rock reaction, thus inducing a local and sec-

ondary boninitic affinity (Zhou et al. 1996; Ballhaus 1998;

Proenza et al. 1999; Rollinson 2005).

Origin of Os-isotope variability in chromite-hosted

PGM

Gervilla et al. (2005) explained the origin of the primary

PGM assemblage in the Mayarı-Baracoa Ophiolitic Belt by

Os Ir Ru Rh Pt Pd

Sam

ple/

Cho

ndrit

e

Bulk rock chromitites

107

105

103

10

10-1

10-3

Fig. 3 Chondrite-normalized PGE patterns of PGM from the Mayarı-

Cristal chromitites. White circles grains in CAR-301 D2; dark graycircles grains in CAR-301 D3; black triangles grain in MB102-A;

black diamonds grain in NV-100; black squares grains in TA-1. Field

enclosing the PGE patterns of bulk rock Cr-rich chromitites from the

Sagua de Tanamo and Mayarı districts is from Gervilla et al. (2005)

and Frei et al. (2006). Normalizing values are from McDonough and

Sun (1995)

187Re/188Os

0.00 0.02 0.04 0.06 0.08

187 O

s/18

8 Os

0.116

0.120

0.124

0.128

0.132

CAR-301 D2 CAR-301 D3

MB-11 MB102A

NV-100

TA-1 ES-4-2

CAR-301 D1

CS-100

0.10

Bulk NV100

Bulk CS-100

CAR-301 DCAR-302 B

Fig. 4 187Re/188Os versus 187Os/188Os in PGM from the Mayarı-

Cristal chromitites. Symbols as in Fig. 3. Black circle grain in CAR-

301 D1; light gray circles grains in CAR-301 D; crossed circlesgrains in CAR-302 B; white cross grain in ES-4-2; white trianglegrain in MB-11; black hexagons grains in CS-100. Data for bulk rock

NV-100 (dotted black diamonds) and CS-100 (dotted black hexagon)

chromitites are from Gervilla et al. (2005) and Frei et al. (2006)


123

the turbulent mingling of different batches of melt in

mantle conduits. In particular, the enrichment of Os, Ir and

Ru (IPGE) relative to Rh, Pt and Pd (PPGE) in the PGM

from the Mayarı-Cristal chromitites (Fig. 3) may indicate

that the crystallizing chromite concentrated submicro-

scopic metallic clusters of refractory IPGE together with

larger alloys and sulfides by physical trapping (Tredoux

et al. 1995; Ballhaus and Sylvester 2000; Matveev and

Ballhaus 2002). In this model, Os-rich laurite, the domi-

nant PGM inclusion in the Mayarı-Baracoa chromitites,

may have been generated from PGE clusters/alloys at

1,000–1,200�C as consequence of slight increases in fS2

and fO2 during mingling of relatively primitive and dif-

ferentiated melts. PGE-rich Ni–Fe–Cu sulfides probably

formed concurrently with laurite or at lower temperature

and higher-fS2 conditions, and Ru–Os–Ir–Fe–Ni (Rh) oxi-

des/alloys likely are the products of laurite desulfurization

during serpentinization (Gervilla et al. 2005; Gonzalez-

Jimenez et al. 2009a).

The subchondritic 187Os/188Os of the PGM included in

the Mayarı-Cristal chromitites indicate that their Os budget

is mostly controlled by variably depleted mantle sources

with little or no contribution of radiogenic Os from the

subducting slab or assimilated crustal gabbros (Brandon

et al. 1996). The Os isotopic composition of the upper

mantle is highly heterogeneous at different length scales

(Hattori and Hart 1991; Schiano et al. 1997; Parkinson

et al. 1998; Alard et al. 2002; Meibom and Frei 2002;

Meibom et al. 2002; Frei et al. 2006; Liu et al. 2008) as Os

is mainly partitioned in trace sulfide and alloy phases

(Alard et al. 2000; Luguet et al. 2001, 2004; Lorand et al.

2008) and its isotopic composition records different epi-

sodes of partial melting, subduction-related crustal recy-

cling and metasomatism (e.g., Griffin et al. 2004; Walker

et al. 2005; Pearson et al. 2007). PGM normally have an Os

concentration equivalent to a mantle volume of the order of

1 m3 (Meibom et al. 2002; Walker et al. 2005; Brandon

et al. 2006), thus implying that the partial melting and

melt percolation processes that generate the PGM are able

to homogenize Os isotopes at a minimum scale of several

m3 in the mantle. High degrees of partial melting and

melt production, such as those characteristic of the

Mayarı-Baracoa and other supra-subduction ophiolites, are

required to release sulfide and alloy inclusions in mantle

minerals and thus erase the Os-isotope heterogeneities

observed on mineral scale (Burton et al. 1999; Alard et al.

2002, 2005). However, substantial Os-isotope heterogene-

ities appear to be preserved in the upper mantle on length

scales of 10–100 m even during intense melting events

(Parkinson et al. 1998; Brandon et al. 2000; Meibom et al.

2002; Kogiso et al. 2004). As the melting region under a

spreading ridge or island arc is normally several tens to

hundreds of kilometers across and may extend to depths

greater than 100 km (e.g., The MELT Seismic Team 1998;

Grove et al. 2009), individual ascending melt batches

whose Os budget derives from different portions of the

melting region may have different Os isotopic signatures.

Moreover, melt migration through mantle peridotites and

melt focusing into dunite channels may scavenge and dis-

solve different generations of sulfides and alloys (Zhou

et al. 1998; Buchl et al. 2002, 2004b), thus contributing to

the isotopic variability of the individual batches of melt.

As chromitites must have scavenged Cr from 300 to 400

times their mass in liquid (Leblanc and Ceuleneer 1992),

they are indicative of focused melt flow and very high melt/

rock ratios (Kelemen et al. 1995, 1997). In this scenario,

isotopically heterogeneous Os transported by melt batches

from a large portion of the upper mantle is pooled into a

single chromitite ore body. Hence, the Os-isotope vari-

ability observed between the PGM in the Mayarı-Cristal

chromitites supports that the upper mantle is constituted by

a ubiquitous distribution of small- to moderate-scale het-

erogeneities that form a statistical upper mantle assemblage

(SUMA, Meibom and Anderson 2003). This heterogeneity

is reflected in chromitites and is not homogenized at the

thin section scale during or after chromite crystallization,

thus corroborating that chromite crystals nucleated by

mingling of different batches of melt with distinct

Cr#

spi

nel

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6

TiO2 spinel (wt%)

Island arcbasalts

MORB

Boninites

Sagua deTánamo

Mayarí

Fig. 5 TiO2 content (wt%) versus Cr# of chromite from the Sagua de

Tanamo (gray area) and Mayarı (black area) mining districts (data

from Proenza et al. 1999). Spinel compositions in MORB (shortdashed line), island arc basalts (dotted line) and boninites (longdashed line) are from Arai (1992) and Kelemen et al. (1995)


123

Os-isotope compositions. Individual growing crystals of

chromite may act as collectors of PGE (mainly IPGE: Os,

Ir and Ru) in variably sized PGM as the presence of the

chromite surface lowers the surface energy contribution to

metal precipitation (Ballhaus et al. 2006), or owing to the

decrease in metal solubility induced by local fO2 gradients

at the chromite–melt interface (Finnigan et al. 2008).

Growth of chromite traps the PGM thus inhibiting fur-

ther isotopic equilibration with the melt, and the high

(104–106) PGM/chromite partition coefficients for Os

(Burton et al. 1999; Meibom et al. 2002) strongly coun-

teract the exchange of Os by solid-state diffusion within an

individual grain.

Interpretation of the Os-isotope signatures

and depletion model ages

The subchondritic 187Os/188Os of PGM in the Mayarı-

Cristal chromitites indicate that Os mainly records different

depletion events in the upper mantle. The absence of highly

radiogenic isotopic ratios shows that the PGM were not

significantly affected by interaction with crustal or outer

core-related (i.e., deep-rooted plumes) reservoirs (Brandon

et al. 1996, 1998). Os isotopes can thus be used to constrain

the nature and age of depletion of the PGM upper mantle

sources.

Figure 6 displays the distributions of TMA (excluding

one meaningless future age) in individual PGM and bulk

chromitites from the Mayarı-Cristal massif and exhibits a

multistage evolution of the upper mantle that extends back

to more than 1 Ga. The TMA calculated for PGM cluster

around four main peaks: *100, 500, 750 and 1,000 Ma

(Fig. 6b); on the other hand, the distribution of TMA in bulk

chromitites shows only three main peaks at *500, 750 and

1,100 Ma. This highlights the smoothing effect of bulk

chromitite data compared to the in situ analyses of indi-

vidual PGM and consequently the loss of high resolution

model age information in the former. Hence, while bulk

chromitites probably yield better regional information on

the age of the depletion events recorded in the Mayarı-

Cristal ophiolitic mantle, the in situ analyses of PGM have

resolving power to decipher more precisely individual

melting events through time. Although the total number of

analyses of PGM is too small to be statistically robust,

there is an interesting correlation with the tectonic evolu-

tion of the Caribbean. The most recent peak shown by their

TMA distribution corresponds to the Early-Late Cretaceous

boundary (*100 Ma) that, considering the uncertainties

inherent in model age calculations, matches well with the

age of the magmatic activity in the Greater Antilles paleo-

island arc and the formation of the chromite deposits in the

Mayarı-Baracoa Ophiolitic Belt (*90 Ma). The strong

resemblance of the Os isotopic composition of these PGM

with that of an enstatite chondritic reservoir at the time of

the ophiolite formation suggests that they crystallized from

melt batches that tapped fertile domains of the upper

mantle, as similar results have been obtained for PGM from

different ophiolites worldwide (Shi et al. 2007).

Most PGM in the Mayarı-Cristal chromitites have Os

model ages that are older than the supposed age of for-

mation of the ophiolite (Fig. 6). These ages reflect the

coexistence of variably Re-depleted reservoirs within the

oceanic upper mantle (Parkinson et al. 1998; Brandon

et al. 2000; Harvey et al. 2006; Liu et al. 2008) and/or the

presence of ancient subcontinental lithospheric domains in

the mantle wedge beneath the Greater Antilles paleo-

island arc. Melting of highly depleted and refractory

TMA (Ga)0.0 0.5 1.0 1.5 2.0

Rel

ativ

e pr

obab

ility

n

0

2

4

6

8

10

TMA (Ga)

bTMA PGM

TMA Bulkchromitites

0 0.15 0.3 0.45 0.6 0.75 0.9 1.05 1.2 1.35 1.5

a

Fig. 6 Distribution histogram (a) and cumulative probability plot

(b) (Ludwig 2000) of Os model ages (TMA, Ga) for PGM (gray area)

and bulk chromitites (dashed line) from the Mayarı-Cristal massif;

n = number of analyses. A minimum uncertainty of 0.1 Ga was

assumed for model ages, to avoid overemphasis on single data points

determined by high internal precision (Pearson et al. 2007). Data used

to calculate the depletion model ages of bulk chromitites are from

Gervilla et al. (2005) and Frei et al. (2006)


123

domains in the upper mantle is not common beneath mid-

ocean ridges (Liu et al. 2008) but in a subduction zone

may be promoted by the flux of aqueous fluids and/or

silicic melts from the subducting slab to the mantle wedge

(e.g., Elliott 2003). Melts with a highly depleted Os-iso-

tope composition are thus expected to significantly

influence the Os budget of podiform chromitites, as these

deposits are normally generated by intense flux melting in

subduction zones (Matveev and Ballhaus 2002). In addi-

tion, the variably depleted isotopic signature of most

PGM may reflect the incorporation of isotopically heter-

ogeneous Os during the ascent of magmas. Hence, the Os-

isotope compositions of PGM in the Mayarı-Cristal

chromitites confirm the highly depleted nature of the

oceanic upper mantle sampled in this ophiolitic massif

(Marchesi et al. 2006). Moreover, our results suggest that

PGM in ophiolitic podiform chromitites retain better than

MORB (Liu et al. 2008) the Os isotopic signatures of

both fertile and refractory domains heterogeneously dis-

tributed in the upper mantle.

Conclusions

PGM in podiform chromitites from the Mayarı-Cristal

ophiolitic massif generally have subchondritic 187Os/188Os

that are highly variable within a single kilometer-sized

mining district, within a single chromitite hand sample and

within a single thin section. Chromite ores formed in the

Late Cretaceous when island arc tholeiites and MORB-like

back-arc basin basalts ascended through the oceanic lith-

osphere in the Greater Antilles paleo-subduction zone.

These olivine-saturated magmas reacted with residual

mantle peridotites causing pyroxene dissolution and the

generation of pod-shaped chromite deposits enclosed in

dunite.

Melting and melt/rock reaction may be able to homo-

genize the Os isotopic composition of the mantle on a

minimum scale of several m3, but significant isotopic

heterogeneities are likely to be preserved on larger scales.

As chromitites form by focused melt flow at high melt/rock

ratios, melt batches that generate a single chromitite body

are derived from a large portion of the mantle and have

probably different signatures in terms of Os isotopes. The

variable Os-isotope composition of PGM in the Mayarı-

Cristal chromitites shows that this heterogeneity may be

imparted to a single meter-scale chromitite pod and even to

a single chromitite thin section by mingling of different

batches of melt. Hence, the Os isotopic variability observed

between PGM on the minimum scale of several millimeters

in the Mayarı-Cristal chromitites probably reflects the

original Os isotopic heterogeneity of their mantle sources

on a minimum scale of several m3.

The subchondritic Os isotopic ratios of PGM in the Mayarı-

Cristal ophiolitic massif indicate that they were derived from

variably depleted mantle regions. The most recent peak in the

TMA distribution, calculated against the Os isotopic evolution

of enstatite chondrite, is consistent with the age of the ophi-

olite formation. This supports that a portion of the PGM is

crystallized from melts that tapped fertile domains of the

upper mantle, whose bulk Os isotopic evolution is best

approximated by that of enstatite chondrites. Most of the

analyzed PGM have Os model ages that are older than the age

of crystallization of their host chromite. The Os budget of

these grains probably derives from variably refractory

domains in the oceanic (Pacific-related) upper mantle of the

Greater Antilles paleo-subduction zone.

Acknowledgments We thank Anders Meibom and two anonymous

reviewers for their constructive remarks on the submitted version of

the manuscript. We are grateful to O. Alard for his useful comments

on a preliminary version of the manuscript. The analytical data were

obtained using instrumentation funded by ARC LIEF, and DEST,

Systemic Infrastructure Grants, industry partners and Macquarie

University. This study was financially assisted by the ‘‘International

Lithospheric Project’’ (ILP) task force CC4-MEDYNA; by the

Spanish ‘‘Ministerio de Ciencia e Innovacion’’ (MICINN) research

grants CGL2010-14848/BTE, CGL2007-61205/BTE, AI-HF2008-

073 and F.P.I. BES-2005-8328; by the ‘‘Generalitat de Catalunya’’

grant 2009 SGR 444; and by the ‘‘Junta de Andalucıa’’ research

groups and grant RNM-131, RNM-145, and ‘‘Proyecto de Excelencia-

2009-RNM-4495’’. C.M.’s research has been supported by a Marie

Curie Intra European Fellowship within the 7th European Community

Framework Programme and by a postdoctoral fellowship from the

Universidad de Granada (Spain). This is contribution 676 from the

Australian Research Council National Key Centre for the Geochemical

Evolution and Metallogeny of Continents (http://www.gemoc.mq.edu.au).

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In situ Re–Os isotopic analysis of platinum-group minerals ... et al.pdfof centimeters thick. On the other hand, only the mantle section crops out in the Mayarı´-Cristal massif