11B-rich fluids in subduction zones: The role of antigorite dehydration in subducting slabs and boron isotope heterogeneity in the mantle
Introduction
Nominally anhydrous, melt-depleted upper oceanic mantle accommodates very low abundances of highly incompatible elements and volatiles (Salters and Stracke, 2004). Serpentinization of refractory ultramafic lithologies hydrates peridotite (Seyfried and Dibble, 1980) and dramatically increases its cargo of fluid-mobile elements (As, Sb, B, Cs, Li, Pb, U, Ba; e.g. Thompson and Melson, 1970, Bonatti et al., 1984, Benton et al., 2001, Savov et al., 2005, Savov et al., 2007, Deschamps et al., 2011). For example, boron is easily incorporated into serpentine phyllosilicates during serpentinization (Pabst et al., 2011) with boron concentrations increasing by up to four orders of magnitude compared to anhydrous peridotite (cf. Chaussidon and Jambon, 1994, Vils et al., 2008). Ultimately, serpentinites may be subducted at convergent margins, potentially transporting their fluid-mobile element-, halogen-, and noble gas-rich contents to sub-arc depths and beyond, hence introducing volatile-rich fluid into arc magma sources upon dehydration and potentially generating chemical and isotopic heterogeneity in the deeper convecting mantle (Scambelluri et al., 1995, Benton et al., 2001, Savov et al., 2005, Savov et al., 2007, Sumino et al., 2010, Kendrick et al., 2011, Kendrick et al., 2012).
Progressive dehydration of serpentinite, altered oceanic crust and pelagic sediment, caused by compaction of pore space and low temperature dehydration reactions, releases large volumes of pore fluid and structurally-bound water. An example of the consequences of this early slab-fluid out-flux can be seen in the serpentinite mud volcanoes of the modern Izu–Bonin–Mariana forearc (Fryer et al., 1985, Mottl, 1992, Fryer, 2011). Subduction-related volcanism is controlled by deeper dehydration reactions which trigger and contribute to flux-related melt generation in the overlying mantle wedge (e.g., Arculus and Powell, 1986, Hattori and Guillot, 2003). Although this melting may be initiated by the introduction of slab-derived fluids released by high P–T metamorphic devolatilization reactions (Schmidt and Poli, 1998), there is increasing evidence for fluid being generated by forearc serpentinite dragged down to sub-arc depths during subduction erosion (Savov et al., 2005, Tonarini et al., 2011, Marschall and Schumacher, 2012). Similarly, it is presently unclear whether the fluid is released continuously over a discrete interval (Schmidt and Poli, 1998, Kerrick and Connolly, 2001) or if it is released spasmodically in a series of pulses (Padrón-Navarta et al., 2010, Padrón-Navarta et al., 2011, Dragovic et al., 2012, John et al., 2012, Baxter and Caddick, 2013).
Irrespective of the mechanism governing the release of fluid, or its rate of release during dehydration, boron abundances and isotopes have been established as an excellent tracer for processes that involve flux-related melting at convergent margins (e.g. Bebout et al., 1993, Ryan and Langmuir, 1993, Tonarini et al., 2001). During dehydration reactions boron preferentially partitions into the fluid phase (Seyfried et al., 1984). Because of prior seawater–pelagic sediment interactions, boron is abundant in all of the lithologies that enter the trenches at subduction zones. However, how pelagic sediment, altered oceanic crust, and serpentinized peridotite (serpentinites) interact to produce the heterogeneous and somewhat 11B-rich isotope signatures of arc volcanics remains equivocal. This 11B-rich isotope signature of island arc volcanics, which extends to δ11B = + 18‰ (where δ11B refers to parts per thousand deviation in 11B/10B from NIST951 boric acid; Catanzaro et al., 1970) (Palmer, 1991, Ishikawa and Nakamura, 1994, Ishikawa et al., 2001, Tonarini et al., 2007, Tonarini et al., 2011), cannot be accounted for by fluid released from pelagic sediments (δ11B of < 0‰; [B] > 100 ppm; Ishikawa and Nakamura, 1993) or altered oceanic crust (mean δ11B + 3.4 ± 1‰; mean [B] = 15 ppm; Smith et al., 1995). Although fluid–mineral fractionation releases fluids with 11B-rich isotope signatures from these reservoirs at very shallow depths, deeper dehydration will only yield fluids with 11B-poor isotope ratios (i.e. δ11B < 0‰; You et al., 1995, Peacock and Hervig, 1999, Benton et al., 2001, Marschall et al., 2006). Straub and Layne (2003) suggest that a combination of altered oceanic crust and sediments is unlikely to produce a fluid, at sub-arc depths, with δ11B of > + 1‰. This is supported by the observation that not only is 11B-rich fluid released by serpentinites in the forearc (Mottl, 1992, Savov et al., 2005, Savov et al., 2007), but also that a distinctly 11B-poor signature remains in residual slab-hosted phengite, amphibole and epidote (Pabst et al., 2012). Seawater has a distinctive, 11B-rich isotope signature (δ11B c. + 39.61 ± 0.04‰; Foster et al., 2010) which is at least partially transferred to oceanic peridotite during the process of serpentinization at mid-ocean ridges (Boschi et al., 2008, Vils et al., 2009, Harvey et al., 2014), through fluid infiltration during slab bending at the outer rise of convergent margins (Ranero et al., 2003, Ranero and Sallares, 2004, Faccenda et al., 2009), or through the hydration of forearc mantle overlying a slab undergoing subduction. This forearc serpentinite may be subsequently transported to sub-arc depths by subduction erosion (Hyndman and Peacock, 2003, Hattori and Guillot, 2007, Savov et al., 2007, Scambelluri and Tonarini, 2012). Even accounting for fluid–mineral fractionation under varying conditions of serpentinization, a range of 11B-rich isotope ratios are preserved in serpentinized abyssal peridotite (δ11B = + 11.4 to + 40.7‰; Boschi et al., 2008, Vils et al., 2009, Harvey et al., 2014), which potentially constitutes the largest component of the boron feedstock to the subduction factory.
All island arc volcanics are enriched in boron compared to the primitive mantle (cf. Ryan and Langmuir, 1993, Chaussidon and Jambon, 1994), but the evidence of how sufficient boron with a 11B-rich signature is delivered to sub-arc depths to generate arc volcanics with δ11B of up to + 18‰ (Tonarini et al., 2007 and references therein) is not conclusive. This is because the series of hydration–dehydration reactions en route to the deep mantle suggests that much of the boron transported by slabs should be lost before sub-arc depths are attained (e.g. Kodolányi and Pettke, 2011). The aim of this study is to examine the boron elemental and isotopic signatures of natural samples that preserve a unique antigorite-serpentinite to prograde chlorite-harzburgite isograd in the Cerro del Almirez, Southern Spain (Trommsdorff et al., 1998, Garrido et al., 2005, Padrón-Navarta et al., 2011). This unique locality is an excellent natural laboratory for the examination of serpentinite dehydration reactions at near sub-arc depths (680 to 710 °C and 1.6 to 1.9 GPa; Padrón-Navarta et al., 2010). This study explores the significance of dehydration reactions in down-going slabs at convergent margins and, specifically, evaluates the contribution these reactions may (or may not) make to the production of 11B-rich fluids implicated in arc-related volcanism. In addition, we comment on the fate of boron retained within the prograde lithologies and the likelihood that this boron may be implicated in the distinctive boron systematics observed in ocean island basalts.
Section snippets
Geological setting and sampling
The Cerro del Almirez massif is one of the several lenses of ultramafic material within the upper sequences of the Nevado–Filábride Complex (Betic Cordillera Internal Zones, Southern Spain; Fig. 1). It comprises c. 2–3 km2 of antigorite-serpentinite and chlorite-harzburgite separated by a narrow (c. 1 m) zone of transitional lithologies (chlorite-serpentinite and antigorite–chlorite–orthopyroxene–olivine serpentinite). The Nevado–Filábride Complex experienced extensive metamorphism as a result of
Analytical methods
Boron abundances and isotopic compositions were measured at IGG (CNR-Pisa, Italy) using a VG Isomass 54E positive ion thermal ionisation mass spectrometer following boron extraction and purification procedures described by Tonarini et al., 1997, Tonarini et al., 2003. Briefly, following a K2CO3 alkali fusion, boron is extracted in ultra-pure water and purified using standard column chemistry. Boron is loaded onto Ta filaments as caesium borate prior to analysis by thermal ionisation mass
Results
Boron and strontium isotope ratios and elemental abundances were measured in representative samples of each major lithological division (Table 1). Boron abundances range from 7 to 12 μg g− 1, significantly higher than in primitive mantle estimates (0.25 μg g− 1; Chaussidon and Jambon, 1994), but much lower than in bulk-rock serpentinites from mid-ocean ridge settings (mean [B] = 49 μg g− 1; Boschi et al., 2008, Vils et al., 2009), and mantle wedge serpentinites (mean [B] c. 20 μg g− 1; Benton et al., 2001,
Discussion
Elements which preferentially partition into a fluid phase during dehydration will become strongly depleted in the prograde assemblage (Bebout et al., 1993) and, in the case of boron in particular, the potential exists for a strong fractionation of boron isotopes given favourable temperature and pH conditions. The drastic shift in δ11B observed at Cerro del Almirez during the transformation of antigorite-serpentinite, first to the transitional lithologies, and subsequently to the
Concluding remarks
The prograde lithologies on the high P–T side of the antigorite-out isograd at Cerro del Almirez are complex and likely preserve evidence not only for the dehydration of antigorite-serpentinite but also for the flux of fluids partially equilibrated with pelagic sediment. Notwithstanding the complexities of the prograde lithologies, the antigorite-serpentinite on the low P–T side of the antigorite-out isograd appears to preserve the composition of subducted serpentinite prior to dehydration at
Acknowledgements
JH was supported in this work by the University of Leeds and a research grant from the Fearnsides Fund of The Geological Society of London. CM and JAPN have been supported by an EU-FP7-funded Marie Curie postdoctoral grant under contract agreement PERG08-GA-2010-276867 and PIOF-GA-2010-273017 respectively. Grants from the Ministerio de Economía y Competitividad (CGL2009-12518/BTE, CGL2010-14848/BTE and CGL2012-32067) and Junta de Andalucía (research groups RNM-145 and RNM-131) are also
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