Geochemistry of subduction zone serpentinites: A review

Highlights

• Review of > 900 geochemical analyses of serpentinites worldwide

• Identification of geochemical criteria to distinguish serpentinite protoliths

• Discussion of the possible origins of subducted serpentinites

• Serpentinites are one of the major components of the Earth's budget for fluid-mobile elements.

Abstract

Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zone geodynamics. Their presence and role in subduction environments are recognized through geophysical, geochemical and field observations of modern and ancient subduction zones and large amounts of geochemical database of serpentinites have been created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical data of abyssal, mantle wedge and exhumed serpentinites after subduction. The aim was to better understand the geochemical evolution of these rocks during their subduction as well as their impact in the global geochemical cycle.

When studying serpentinites, it is essential to determine their protoliths and their geological history before serpentinization. The geochemical data of serpentinites shows little mobility of compatible and rare earth elements (REE) at the scale of hand-specimen during their serpentinization. Thus, REE abundance can be used to identify the protolith for serpentinites, as well as magmatic processes such as melt/rock interactions before serpentinization. In the case of subducted serpentinites, the interpretation of trace element data is difficult due to the enrichments of light REE, independent of the nature of the protolith. We propose that enrichments are probably not related to serpentinization itself, but mostly due to (sedimentary-derived) fluid/rock interactions within the subduction channel after the serpentinization. It is also possible that the enrichment reflects the geochemical signature of the mantle protolith itself which could derive from the less refractory continental lithosphere exhumed at the ocean–continent transition.

Additionally, during the last ten years, numerous analyses have been carried out, notably using in situ approaches, to better constrain the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) incorporated in serpentine phases. The abundance of these elements provides information related to the fluid/rock interactions during serpentinization and the behavior of FME, from their incorporation to their gradual release during subduction. Serpentinites are considered as a reservoir of the FME in subduction zones and their role, notably on arc magma composition, is underestimated presently in the global geochemical cycle.

Introduction

Serpentinites (hydrated ultramafic rocks) and the processes of serpentinization have attracted much attention over the last two decades, and interest in these rocks continues unabated. It has recently been posited that the occurrence of serpentinites, specifically in subduction zones, could have important implications for the Earth's dynamic and global geochemical cycle (e.g. Hattori and Guillot, 2003, Hilairet et al., 2007). However, deciphering the origin of the serpentinites and the causalities of serpentinization remains a challenge as the onset of this particular process mostly occurs at the seafloor (near mid-ocean ridges, MOR) and continues during subduction of abyssal serpentinites and peridotites; moreover mantle wedge serpentinites are produced by fluids released from the subducting slab. A large number of high quality bulk rock compositions, as well as in situ geochemical data on serpentine phases, have now become available and hence correspond to a fully representative set of serpentinite compositions worldwide (see references through the text; Section 2.4). In this context, the present manuscript attempts to review the available geochemical data of abyssal peridotites and subduction zone-related serpentinites, including subducted and metamorphosed serpentinites and mantle wedge–forearc serpentinites. This review paper will emphasize the role of serpentinites on chemical cycling in subduction zones, and in doing so perhaps broach new concerns for the forthcoming studies.

This paper aims: (i) to review and to provide comprehensive geochemical compositions of serpentinites, as well as serpentine phases, in order to depict the influence and significance of protolith; (ii) to evaluate the role of fluid-mobile element (FME) compositions as tracers of fluid/rock interactions in geodynamic contexts and processes (temperature (T), pressure (P), redox conditions); and finally, (iii) to discuss the active role of serpentinites upon the global geochemical cycle in subduction zones. Notably, we summarize observations about interactions between various lithologies in the subducted slab and serpentinites into the subduction channel, and the fluids and fluid-mobile elements released during their metamorphism. We are fully aware about the non-exhaustive review of this synthesis and we refer the reader to the many outstanding studies cited below.

Subduction zones are one of the most challenging geological contexts in Earth sciences. Since the first reference to these particular environments six decades ago (Amstutz, 1951), numerous studies have been undertaken in order to constrain their geophysical and geochemical signatures (see Stern, 2002, for a review). Subduction zones and oceanic convergent boundaries represent a total length of around 67,500 km (Bird, 2003, Lallemand, 1999). As a weak and buoyant mineral, and its broad P–T stability field, a serpentine mineral can play a key role in the dynamics of subduction zone, notably on the triggering of earthquakes, exhumation of HP to UHP rocks, and probably initiation of subduction itself (Hirth and Guillot, 2013). One of the most important features of subduction zones is the recycling of hydrated lithologies back into the mantle, the so called “subduction factory” (Tatsumi, 2005). This recycling mechanism has important consequences into the global geochemical cycling as well as on the dynamics of the Earth. Due to the downgoing movement of the hydrated oceanic lithosphere and its heating related to prograde metamorphism, fluids are progressively released from the slab. These fluids are considered to trigger partial melting within mantle wedge leading to arc magmatism (e.g. Green, 2007, Tatsumi et al., 1986). In this context, subduction-related metamorphism will play a major role in the dynamics, chemistry and rheology of subduction zones. Numerous studies were conducted in order to determine the water budget of the subducting lithosphere and the timing of water released (e.g. Rüpke et al., 2002, Schmidt and Poli, 1998, van Keken et al., 2011). Dehydration mostly occurs during the first 100 to 170 km of subduction, depending on the geothermal gradient, and is related to the stability of key hydrous phases such as amphiboles (Pawley and Holloway, 1993, Poli and Schmidt, 1995) and serpentines (Ulmer and Trommsdorff, 1995, Wunder and Schreyer, 1997; Fig. 1a).

Water is one of the most important components in subduction zone and its geochemical cycle. Water can be transported, and recycled, at different depths when it is stored into subducted sediments, and to a lesser extent in the oceanic lithosphere. Under the effect of subduction-related prograde metamorphism, water can also be released into the overlying mantle wedge and react with mantle peridotites to form hydrous minerals. Water is present in different forms: (1) molecular (H₂O) in magmas and/or silicate fluids released from the slab, (2) hydroxyl (OH⁻) as part of hydrous phases (e.g. chlorite, amphibole, serpentine), (3) hydrogen as point defects in nominally anhydrous minerals such as olivine, pyroxene or garnet, or (4) supercritical fluids at high pressure (HP)–high temperature (HT) conditions. Behavior of water in subduction zone is relatively well constrained, notably through studies concerning the mineralogical changes associated to HP–LT (low temperature) metamorphism characteristics of subduction context and related dehydration reactions (e.g. Peacock, 1990, Schmidt and Poli, 1998). Experimental petrology on subduction zone lithologies has contributed significantly toward clarifying the influence of water (in its different forms) in various petrological processes during subduction. It results into phase diagrams that constrained stability field and water content for most common minerals in this context (e.g. Hacker et al., 2003). Nearly all metamorphic facies in subduction zones are able to transport significant quantity of water, despite dehydration processes. Taking the example of the oceanic crust, the average amount of water varies from 7 wt.% in the zeolite facies to 0.09 wt.% in the eclogite facies (Hacker et al., 2003). Thus, it seems that a very small amount of water is recycled back into the asthenospheric mantle.

The majority of subducted water is released from the slab lithologies and percolates through the mantle wedge. The main dewatering takes place by compaction at temperatures between 300 and 600 °C and at a pressure lower than 1.5 GPa (Rüpke et al., 2004). In mantle wedges, water is hosted by serpentine minerals, chrysotile (ctl)/lizardite (lz) and antigorite (atg), which can hold on an average of 13 wt.% of water until isotherms 600–700 °C (Ulmer and Trommsdorff, 1995, Wunder and Schreyer, 1997; Fig. 1a). Then, other hydrous minerals such as amphibole, chlorite, talc, mica, or phase A can bring water deeper but in less important quantity (Hacker et al., 2003, Schmidt and Poli, 1998). Above the 800 °C isotherm and 2 GPa, mantle peridotites cannot host water under hydroxyl form anymore (Ulmer and Trommsdorff, 1995).

Above the mantle wedge serpentinite layer, water percolates into the anhydrous mantle and can trigger partial melting, a process which is at the heart of arc magmatism (e.g. Morris et al., 1990, Plank and Langmuir, 1993, Tatsumi, 1986). This theory is reinforced by observations of high concentrations of water in the arc magmas (on average 1.7 wt.%; Sobolev and Chaussidon, 1996) compared to those observed in primitive magmas from oceanic ridge (0.1–0.5 wt.%). Concerning the modality of water migration into the anhydrous mantle wedge, several possible scenarios are proposed (Stern, 2002): (1) for a cold lithosphere having a sufficient porosity, water is present under its molecular form, causing pore pressure to increase, which can subsequently trigger seismic rupture (Davies, 1999), (2) water circulation is also possible between mineral pores using an interconnected network, facilitating interaction between fluid and minerals, and (3) a third type, highlighted by numerical models, presents formation of cold plumes of serpentinites from the serpentinite layer to the mantle wedge (Gerya and Yuen, 2003).

Serpentinites are hydrated ultramafic rocks (with H₂O content up to 15–16 wt.%, average of 13 wt.%) which form through the alteration of olivine- and pyroxene-dominated protoliths at temperatures lower than 650–700 °C (e.g. Evans et al., 2013, Hemley et al., 1977, Janecky and Seyfried, 1986, O'Hanley, 1996). Water-rich and stable over a relatively important P–T range as demonstrated by experimental works (Bromiley and Pawley, 2003, Ulmer and Trommsdorff, 1995, Wunder and Schreyer, 1997; Fig. 1a), serpentinites are among the most efficient lithology to carry great amount of water at relatively great depths (120 to 170 km depth). Largely underestimated in the past (compared to metasediments and eclogites) in subduction zone models, an increasing number of studies on serpentinites were conducted over the last 20 years. It appears that serpentinites are widespread on oceanic floor (e.g. Cannat et al., 2010). In this context, it appeared that subducted serpentinites, and those resulting from the hydration of the mantle wedge, represent a particularly significant water reservoir influencing arc magmatism (Hattori and Guillot, 2003, Hattori and Guillot, 2007, Savov et al., 2005a, Scambelluri et al., 2001a, Scambelluri et al., 2004a, Scambelluri et al., 2004b, Ulmer and Trommsdorff, 1995). In parallel, some studies on the trace element and isotope compositions of arc magmas have shown geochemical evidence for fluids released after dehydration of subduction-related serpentinites (e.g. Barnes et al., 2008, Singer et al., 2007, Tonarini et al., 2007). Additionally, serpentinites present strong enrichments in fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, Pb, U, Cs, Sr, Ba; e.g. Bonatti et al., 1984, Deschamps et al., 2010, Deschamps et al., 2011, Deschamps et al., 2012, Hattori and Guillot, 2003, Hattori and Guillot, 2007, Kodolányi et al., 2012, Lafay et al., 2013, O'Hanley, 1996, Scambelluri et al., 2001a, Scambelluri et al., 2001b, Scambelluri et al., 2004a, Scambelluri et al., 2004b, Tenthorey and Hermann, 2004, Vils et al., 2008, Vils et al., 2011). These enrichments result from fluid/rock interactions occurring for example at mid-ocean ridges after percolation of seawater and/or hydrothermal fluids, during subduction by percolation of fluids released from different lithologies from the slab, or also by interactions (mechanical, diffusive and/or fluid assisted) with metasediments during subduction.

During their subduction, serpentinites experience prograde metamorphism until their dehydration, the so-called antigorite breakdown (up to 600–700 °C; Fig. 1a). Fluids released from dehydrating serpentinites are rich in fluid-mobile elements as demonstrated by the experimental work of Tenthorey and Hermann (2004). These fluids are enriched in FME (such as B, Cs, As, Sb, Pb, Li, Ba); in parallel, Ryan et al. (1995) shown that FME are also enriched in arc magmas. Yet, despite considerable progress for characterizing the geochemistry of serpentinites in different geological contexts, little is known about the real impact they might have on the geochemistry of arc magmas. First, the trace-element fingerprint of the protolith, as well as primary minerals, upon the geochemistry of bulk serpentinites and mineral phases starts to become clearer. Second, the sequence of enrichment in FME and its relation to geological contexts are still unclear and need further clarifications since FME are a powerful tool to discriminate serpentinites (e.g. De Hoog et al., 2009, Deschamps et al., 2010, Deschamps et al., 2011, Deschamps et al., 2012, Kodolányi et al., 2012). Additionally, recent studies have emphasized geochemical interactions between serpentinites and metasediments in the accretionary prism and subduction channel (Deschamps et al., 2010, Deschamps et al., 2011, Deschamps et al., 2012, Lafay et al., 2013). Third, the role of serpentinite-derived fluids in subduction zone is not perfectly understood and no geochemical tracers (elements or isotopes) are discriminating enough to highlight the role of serpentinites upon arc magma geochemical signature, as it is the case for sediments.