Fluid-mediated element cycling in subducted oceanic lithosphere: The orogenic serpentinite perspective

Abstract

Serpentinites are central to water (re)cycling in subduction zones and thus effect fluid-mediated element transfer between the hydrosphere and the Earth's mantle and back into continental crust via calcalkaline magmatism. Diverse and often controversial models exist on the relevance of various source contributions to the budget of fluid mobile elements of hydrous peridotites and how these evolve during the subduction cycle. This work offers novel constraints on ongoing debates. We present a comprehensive bulk rock and silicate mineral major to trace element study covering the antigorite dehydration reaction based on Cerro del Almirez antigorite-serpentinites and chlorite-harzburgites, including the systematics of As, Sb, B, W, Li, In, Tl, Bi, Cd, and Sn - so far unavailable for Almirez, and there exist only few such data for orogenic serpentinites in general. We integrate these with reviewed literature data and develop a general model for the geochemical systematics of subducting hydrous slab mantle covering magmatic peridotite conditioning, element enrichment upon oceanic hydration, compositional evolution with progressive subduction to peak temperature antigorite dehydration, and retrograde metasomatism upon exhumation.Pre-hydration magmatic processes produce strong compositional variations on centimetre to metre to kilometre scales. Serpentinisation via seawater and sediment-equilibrated pore fluids produces highly variable fluid-mobile element (FME) bulk rock enrichments in B, As, Sb, W, Cs, ±Li, ±Bi, ±Pb, ±U exceeding primitive mantle concentrations. Hydration enrichment numbers represent a novel concept introduced in this work to refine the extent of hydration-mediated FME enrichment. They represent the measured ratio of fluid-mobile element over a fluid-immobile element of closely comparable magmatic compatibility normalised to its corresponding primitive mantle abundance ratio. Hydration enrichment numbers are highest for Sb and As (up to 650) and lowest for Ba and Rb (down to 0.06) for Almirez data, quantifying fractions of minimal enrichment (values >1) and minimal prograde subduction loss (values <1). FME enrichment occurred primarily in ocean floor to trench to shallow forearc settings where sediment-equilibrated pore fluids are relevant, while addition from deeper sediment metamorphic dehydration fluids with progressive subduction is subordinate at best. Prominent fractions of As, Sb, B, Rb, Sr, Cs, Ba, Pb, Zn, Cl, Br, Li, Na, K, and Ca are then lost to the fluid upon serpentinite dehydration including the antigorite-out reaction. We find no evidence in support of significant fluid-mediated element addition (e.g., Th, U, Ta, Sr, Pb, Cs, Rb, Li) upon antigorite dehydration as has been postulated in literature by simple comparison between Atg-serpentinite and Chl-harzburgite coexisting at Almirez. Magmatic preconditioning prior to serpentinisation can account for the differences in HFSE between Atg-serpentinite and Chl-harzburgite, while prominent FME addition upon retrogression as measured on retrograde serpentine and talc is demonstrated for Cs, Pb, Sr and Ba, thus adulterating bulk rock systematics for certain FME.This work concludes that oceanic serpentinisation dominates the FME imprint of subducting slab serpentinites and that progressive subduction goes along with successive FME loss. We propose that serpentinites from fluid-dominated, highly hybridised and oxidised plate interface melange materials displaying spectacular FME enrichments with a sedimentary flavour are distinctly overrepresented in our sample record. Therefore, the combined data set of hydrous peridotites from Almirez and Erro Tobbio may offer a more representative compositional estimate of the bulk mass of subducted slab serpentinite, to be used in modelling of the geochemical impact of serpentinite-derived matter on fluid-mediated chemical cycling in subduction zones.