Tracing halogen and B cycling in subduction zones based on obducted, subducted and forearc serpentinites of the Dominican Republic

Abstract

Serpentinites are important reservoirs of fluid-mobile elements in subduction zones, contributing to volatiles in arc magmas and their transport into the Earth’s mantle. This paper reports halogen (F, Cl, Br, I) and B abundances of serpentinites from the Dominican Republic, including obducted and subducted abyssal serpentinites and forearc mantle serpentinites. Abyssal serpentinite compositions indicate the incorporation of these elements from seawater and sediments during serpentinization on the seafloor and at slab bending. During their subduction and subsequent lizardite-antigorite transition, F and B are retained in serpentinites, whilst Cl, Br and I are expelled. Forearc mantle serpentinite compositions suggest their hydration by fluids released from subducting altered oceanic crust and abyssal serpentinites, with only minor sediment contribution. This finding is consistent with the minimal subduction of sediments in the Dominican Republic. Forearc mantle serpentinites have F/Cl and B/Cl ratios similar to arc magmas, suggesting the importance of serpentinite dehydration in the generation of arc magmatism in the mantle wedge.

Halogens and noble gases in serpentinites and secondary peridotites: Implications for seawater subduction and the origin of mantle neon

“…Serpentinites formed close to spreading centres have I/Cl similar to average crust and 40 Ar/ 36 Ar of close to the atmospheric/seawater value of 296 (Table 2 and In contrast with Cl, Br and I, which have high solubilities in seawater and sedimentary pore waters, the majority of serpentinites investigated have low concentrations of <20 ppm F ( Fig 3; Table 1) and similarly low F concentrations have been reported for oceanic serpentinites preserved in the Dominican Republic ophiolite (Pagé and Hattori, 2017). The low F content of many oceanic serpentinites is explained because seawater contains only 1.3 ppm F (Drever, 1997), and even though carbonate rich sediments can contain 1000-1500 ppm F (Rude and Aller, 1991), the low solubility of F under diagenetic conditions (Seyfried and Ding, 1995) means it retains a low seawater-like concentration in most sedimentary pore waters (Gieskes et al, 2002).…”

“…Recent studies have suggested that up to ~90% of non-radiogenic Ar, Kr and Xe and most of the heavy halogens (Cl, Br, I) have subducted origins in the Earth's mantle (Holland and Ballentine, 2006;Holland et al, 2009;Mukhopadhyay, 2012;Caracausi et al, 2016;Kendrick et al, 2017). However, relatively few studies have investigated the combined behaviour of multiple halogens and/or noble gases during prograde metamorphism (John et al, 2011;Kendrick et al, 2015;Pagé et al, 2016;Pagé and Hattori, 2017),…”

“…Scambelluri et al, 1997;2004b;Kendrick et al, 2013b), and in some cases a number of elements including I, As and Sb, which are usually associated with marine sediments (e.g. Hattori and Guillot, 2007;Deschamps et al, 2012;Kendrick et al, 2013b;Cannaò et al, 2015;Scambelluri et al, 2015;Peters et al, 2017;Pagé and Hattori, 2017). The sedimentlike signatures of some serpentinites can be explained by the involvement of sedimentary pore waters in serpentinisation reactions (John et al, 2011;Kendrick et al, 2011;2013b;Pagé and Hattori, 2017), which are likely to be particularly important at the slab bend where sediments overly deep fractures into the oceanic lithosphere (Ranero et al, 2003) and in subduction channels underlying forearc environments (Bostock et al, 2002;Deschamps et al, 2013;Scambelluri et al, 2015).…”

“…Hattori and Guillot, 2007;Deschamps et al, 2012;Kendrick et al, 2013b;Cannaò et al, 2015;Scambelluri et al, 2015;Peters et al, 2017;Pagé and Hattori, 2017). The sedimentlike signatures of some serpentinites can be explained by the involvement of sedimentary pore waters in serpentinisation reactions (John et al, 2011;Kendrick et al, 2011;2013b;Pagé and Hattori, 2017), which are likely to be particularly important at the slab bend where sediments overly deep fractures into the oceanic lithosphere (Ranero et al, 2003) and in subduction channels underlying forearc environments (Bostock et al, 2002;Deschamps et al, 2013;Scambelluri et al, 2015). An implication of subduction zone serpentinites carrying 'sedimentary' signatures is that aqueous fluids produced by serpentine breakdown in subarc environments might be able to contribute toward the 'sedimentary signature' of some arc lavas (Kendrick et al, 2014;Scambelluri et al, 2015).…”

“…Analysis of the new lithologies was required to better constrain the range of noble gas and halogen compositions in high grade metamorphosed serpentinites and related rocks (cf. John et al, 2011;Kendrick et al, 2011;Debret et al, 2014;Pagé and Hattori, 2017). The Cima di Gagnone garnet peridotite is of particular interest because it is the only known garnet peridotite with a demonstrable origin from dehydrated serpentinites and therefore provides otherwise unavailable information about the nature of dehydrated lithosphere subducted into the deep mantle (Evans and Trommsdorff, 1978;Scambelluri et al, 2014;.…”