Elsevier

Chemical Geology

Volumes 312–313, 18 June 2012, Pages 93-117
Chemical Geology

Research paper
Behavior of fluid-mobile elements in serpentines from abyssal to subduction environments: Examples from Cuba and Dominican Republic

https://doi.org/10.1016/j.chemgeo.2012.04.009Get rights and content

Abstract

Serpentinites from subduction environments represent an important sink for fluid-mobile elements. In order to constrain geochemical behavior of fluid-mobile elements hosted by serpentine phases during subduction processes, we carried out a geochemical study (trace elements and Pb isotopes) of a series of serpentinites and cumulates from the accretionary wedge of Greater Caribbean (Cuba and Dominican Republic). The trace element compositions of the primary and alteration-related phases were analyzed in situ using LA–HR-ICP-MS techniques. The studied samples represent parts of the subducted proto-Atlantic oceanic lithosphere, which has experienced low to high grade metamorphism (greenschist to eclogite facies), before being exhumed; a subset of these samples were derived from the mantle wedge. This sampling provides the opportunity to trace the chemical mobility of fluid-mobile elements during prograde metamorphism along a cold geotherm in an oceanic subduction setting.

Serpentinites display strong enrichment in fluid-mobile elements indicating extensive fluid–rock interaction. In situ analyses allow distinction of three types of serpentines related to the nature of primary minerals (olivine, ortho- or clinopyroxene). Compositions of subducted samples, especially in fluid-mobile elements, are relatively close to those of abyssal peridotites without noticeable evidence of mobility for trace elements during subduction-related prograde metamorphism, with the exception of B. This confirms that the observed enrichment results from seawater/peridotite interactions during residence time in the ocean. It also suggests that most mobile elements stored in serpentine minerals are immobile during subduction processes. A major consequence of this observation is that serpentine minerals are a good sink for mobile elements in subduction zones, until their dehydration. Additionally, Pb isotopes and over-enrichment in As–Sb in high-grade subducted serpentines (antigorite) suggest the contribution of a sedimentary component during a secondary hydration taking place at the lizardite/antigorite transition. We propose that this new serpentinization event, taking place at greater depth, results from mixing between sediments and serpentinites in the subduction channel.

Mantle wedge serpentinites present imprints of hydrothermal fluids: they are B-rich but without strong enrichment in As and Sb, and show evidence for moderate contributions of a radiogenic Pb-component. This suggests that the fluids that produced the mantle wedge serpentinites derived from the dehydration of the oceanic crust, with moderate to no contribution of sediments. We posit that mantle wedge serpentinization took place around 20–25 km depth: at such depth and temperature conditions (T > 200 °C), the subducted sediments still released their B-rich pore fluids while their structural water incorporated in hydrous minerals (phengite, lawsonite) remained stable. The existence of various potential reservoirs for fluid-mobile elements in subduction zone environments (subducted serpentinites, mantle wedge serpentinites, as well as subducted sediments and altered oceanic crust) that potentially release their fluids at different depths has strong implications for arc lava formation.

Highlights

► Mantle wedge and subducted serpentinites are both enriched in fluid-mobile elements. ► All serpentines show interaction with seawater- and sediment-derived fluids. ► Contamination by sediment-derived fluids is the strongest in high-grade subducted samples. ► This contamination occurs at the lz/atg transition in the subduction channel.

Introduction

The release of fluids from the downwelling oceanic lithosphere plays an important role during subduction; in particular, it triggers partial melting in the mantle wedge and affects the composition of the arc magmatism and the global geochemical cycles (e.g. Plank and Langmuir, 1998, Stern, 2002, Rüpke et al., 2004, Van Keken et al., 2011). Although serpentinized mantle rocks constitute a minor fraction of the downwelling slab, from a few percent (or less) of fast spread lithosphere (Iyer et al., 2010) up to 20% of slow spread lithosphere (Cannat et al., 1995, Carlson, 2001, Mével, 2003), they represent a major reservoir for fluid-mobile elements (FME) in subducting lithosphere at mantle depth (e.g. Scambelluri et al., 2001a, Scambelluri et al., 2001b, Barnes and Straub, 2010, Deschamps et al., 2010, Deschamps et al., 2011, John et al., 2011). Recent studies of serpentinites sampled at present-day and fossil convergent margins have shown that they can incorporate important quantities of FME, such as semi-volatile elements As and Sb (e.g. Hattori and Guillot, 2003, Hattori and Guillot, 2007, Deschamps et al., 2010, Deschamps et al., 2011), light elements B and Li (e.g. Bonatti et al., 1984, Scambelluri et al., 2004a, Tonarini et al., 2007, Tonarini et al., 2011, Pabst et al., 2011) and Large Ion Lithophile Elements Cs, Rb, Ba, and U (Scambelluri et al., 2001a, Scambelluri et al., 2001b, Scambelluri et al., 2004b, Tenthorey and Hermann, 2004, Garrido et al., 2005, Savov et al., 2005, Agranier et al., 2007). Further studies have demonstrated that these elements are stored in serpentine phases during prograde metamorphism (Scambelluri et al., 2001a, Scambelluri et al., 2001b, Scambelluri et al., 2004a, Scambelluri et al., 2004b, Deschamps et al., 2011, Kodolányi and Pettke, 2011, Vils et al., 2011), until their destabilization at relatively great depths (> 150 km, up to 650–700 °C; Ulmer and Trommsdorff, 1995, Wunder and Schreyer, 1997, Wunder et al., 2001). Finally, strong FME enrichments were observed in high-pressure serpentine minerals (antigorite) sampling the hydrated mantle wedge (e.g. in Himalaya, Hattori and Guillot, 2003, Hattori and Guillot, 2007, Deschamps et al., 2010; Mariana forearc, Savov et al., 2005). However, the mechanisms driving the chemical mass transfers from the downwelling slab to the mantle wedge and the sequence of mineralogical reactions controlling the dehydration–hydration processes at depth are still poorly constrained. In addition, little is known about the acquisition of the FME signature during the hydration of mantle wedge and the formation of serpentinites during subduction (e.g. Savov et al., 2005, Savov et al., 2007, Deschamps et al., 2010).

To better understand these processes, we studied a series of serpentinites from Cuba and the Dominican Republic, which sample parts of the accretionary wedge of the Greater Caribbean volcanic arc. These serpentinites are parts of the subducted proto-Atlantic oceanic lithosphere (highly serpentinized peridotites and hydrothermally altered cumulates), and of the hydrated mantle wedge (Hattori and Guillot, 2007, Guillot et al., 2009, Saumur et al., 2010). The subducted and mantle wedge serpentinites are associated in the field, and represent an extinct paleo-serpentinite subduction channel. The slab preserves evidence of low- to high-grade metamorphism (greenschist to eclogite facies). The close association in the field of serpentinites from the mantle wedge and from the subducted oceanic lithosphere provides a unique opportunity to understand the chemical mobility of FME associated with fluid loss during prograde metamorphism along the subduction zone and to characterize geochemical transfers from the slab to the overlying mantle wedge peridotite.

In order to constrain the behavior of the fluid-mobile elements hosted by serpentine minerals during subduction, we carried out in situ trace element analyses on serpentine phases and associated minerals from subducted and mantle wedge serpentinites as well as measurements of Pb isotope compositions for Cuban samples. These data are used to characterize the nature of the protolith and of hydrating fluid(s) in the slab and in the mantle wedge and to discuss the role of serpentinization and dehydration in the cycling of FME during prograde metamorphism associated with intra-oceanic subduction processes.

Section snippets

Geological setting and sampling

The Dominican Republic and Cuba are part of the extinct Greater Caribbean volcanic arc, which marks the northern margin of the Caribbean plate (Fig. 1a). The Greater Caribbean arc results from the eastward subduction of the Farallon plate at the southern margin of the North American plate during Cretaceous (Pindell et al., 2005). By mid-Cretaceous, the polarity of subduction changed and caused the migration of the arc from the Pacific to the Atlantic side and the divergence between North and

Analytical procedure

A fraction of each sample was crushed first into small fragments and then reduced to powder in an agate ring mill. Bulk rock major element compositions were published in Hattori and Guillot (2007; Cuban samples), and in Saumur et al. (2010; Dominican samples) with exception of samples RD 57 and RD 62. Complementary bulk rock trace element data were acquired using the same ICP-MS instrument on which in situ trace element compositions were obtained.

Bulk rock trace element compositions

The subducted serpentinites (Group 1) have moderately depleted compositions with MREE and HREE compositions and ratios close to C1-chondrite values (e.g. YbN = 0.67–1.28; GdN/YbN = 0.60–1.38; N = C1-chondrite normalized; Fig. 2a, b). Group 1 serpentinites are LREE depleted relative to MREE and HREE (LaN/SmN  0.25), except for samples CU 54 and RD 94 which display selective enrichments in LREE (LaN/SmN = 1.33–2.45). Group 1 serpentinites are characterized by variable Eu anomalies, from slightly positive

Discussion

The Cuban and Dominican serpentinites can be divided in two types. The first type (Group 1a and 1b) comprises the less depleted serpentinites; it is characterized by high bulk rock Al/Si ratio and low to moderate Cr# in chromite (Table 1) and it overlaps in composition with abyssal peridotites as defined by Niu (2004) and with the variously altered impregnated abyssal peridotites sampled at the Mid-Atlantic Ridge (Paulick et al., 2006) (Fig. 2a, b, c). Together with the ultramafic cumulates

Conclusions

Using bulk-rock and in situ trace element analyses coupled with Pb isotopic systematics, we distinguished three stages of serpentinization in the series of serpentinites sampled in the Cuban and Dominican Republic, each stage characterized by fluids of different origins and FME content.

  • (1)

    Subducted serpentinites derive from a fertile mantle protolith and are associated in the field with altered ultramafic cumulates mainly composed of amphibole and chlorite. These samples are considered as relics

Acknowledgments

We thank Jean-Luc Devidal (Magmas et Volcans Clermont-Ferrand) for the microprobe analyses and Adeline Besnault (LGCA Grenoble) for the help in the geochemical laboratory. Simone Pourtales and Olivier Bruguier (Géosciences Montpellier) are acknowledged for their help during LA–HR-ICP-MS analyses. We are grateful to Michel Grégoire, Marco Scambelluri and Benoit-Michel Saumur for their constructive scientific discussions. This paper has been greatly improved by L. Reisberg, M. Scambelluri and an

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