Trace element fingerprints of Ni–Fe–S–As minerals in subduction channel serpentinites

Abstract

A variety of base-metal minerals (BMM) may form during hydration-dehydration of ultramafic rocks within subduction zones. However, the trace element fingerprints of these minerals and their relation to different stages of the subduction cycle are still unexplored. Here, we present the first comprehensive in situ analysis by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) of (semi)-metals (Ni, Fe, As, Sb, Co, Bi, Te, Pb, Cd, Se, Cu, Zn, and Mn) and precious metals (Os, Ir, Rh, Pt, Pd, Au, and Ag) for Ni–Fe–S–As minerals in subducted serpentinites from the La Cabaña area (South-Central Chile). The targeted rocks are medium- and high-pressure serpentinites recording the entire cycle of burial-exhumation within a subduction zone. A first stage of hydration of upper mantle peridotites within the mantle wedge led to formation of lizardite after magmatic olivine at ~300 °C. This stage was followed by prograde hydration (i.e., antigoritization at ~320–400 °C; <1GPa) and subsequent partial dehydration (formation of prograde olivine at ~600 °C and 1.1 GPa) within the subduction channel, and final exhumation of the serpentinites and incorporation in the accretionary prism still in the stability field of antigorite (>300 °C). The Ni–Fe–S–As minerals in these serpentinites include Ni-Fe-rich sulfides [pentlandite (Ni,Fe)9S8), smythite (Fe9S11), heazlewoodite (Ni3S2), millerite (NiS)], arsenides [orcelite (Ni5-xAs2), nickeline (NiAs) and maucherite (Ni11As8)] and, to a lesser extent, alloy (awaruite, Ni3Fe) and sulfarsenides (gerdorffite, NiAsS). Their abundance, morphology and chemistry strongly vary with the degree of serpentinization and style of deformation of the host rock. Thus, euhedral grains of heazlewoodite ± awaruite ± magnetite formed in equilibrium with lizardite when magmatic olivine was hydrated within the mantle wedge. Once the lizardite-olivine serpentinites were incorporated into the subduction serpentinite channel, the infiltration of hotter (S-As-Sb)-bearing fluids promoted antigoritization under static regime and precipitation of orcelite and pentlandite in equilibrium with antigorite. Channelling of fluids in zones of focussed strain enhanced further antigoritization in some schistose serpentinites at decreasing fO2 and fS2, resulting in the transformation of pentlandite into a second generation of heazlewoodite–awaruite–magnetite. After relic olivine exhaustion, fO2 and fS2 increased promoting the replacement of the secondary heazlewoodite to millerite. No significant variations in terms of trace elements were observed during these mineral replacements associated to hydration upon prograde metamorphism and/or increasing of strain. In contrast, partial dehydration of serpentinites under high-pressure conditions (> 1GPa) generated Ni-rich awaruite in equilibrium with the prograde assemblage antigorite-metamorphic olivine. These awaruites are depleted in trace elements, indicating substantial (semi)-metal and precious metal mobility during high P-T metamorphism within the subduction channel. During the final stage of deformation linked to exhumation inside the accretionary prism some orcelite grains formed during early antigoritization recrystallized without substantial change in trace element concentration. At this stage, nickeline singularly enriched in gold formed in equilibrium with the recrystallizing orcelite. These results demonstrate that Ni–Fe–S–As minerals formed or modified during the entire subduction cycle of upper mantle rocks have their own characteristic trace element signature (i.e., depletion in precious metals and enrichment in As, Sb, Te, Bi and Pb) that distinguish them from magmatic counterparts.