Insights into multi-stage fluid-rock-interaction processes in HP metamorphic ocean floor basalts from the Tianshan, NW China

Slab fluids, which are released by the subducting oceanic lithosphere through compaction and dehydration processes, are an essential mechanism for the transfer of volatiles from the slab to the mantle wedge. However, migration processes of slab dehydration fluids are not well understood yet, particularly with regards to transport mechanisms of redox sensitive elements such as sulfur.

In this study we investigated an eclogite-facies metabasalt from the South Tianshan Orogen, NW China, which contains several omphacite-dominated HP veins. Using mineral chemical analyses combined with in situ δ³⁴S measurements of pyrite, as well as isotope analyses of C, O, Sr and Pb in mineral and vein separates, we determined the metamorphic evolution of the studied sample and the speciation of sulfur during fluid infiltration and transfer.

Mineral chemical and isotopic compositions reveal seafloor alteration, affecting the protolith pillow basalt prior to subduction. This was followed by a two stage intra-slab fluid-flow under peak to prograde metamorphic conditions, forming the omphacite-dominated HP-veins. The first HP fluid originated from dehydrating ocean floor basalts, as documented by MORB-like pyrite δ³⁴S signatures. The second HP fluid composition suggests, instead, an origin from the basalt-sediment transition with negative pyrite δ³⁴S values of about -10‰. Pathways formed by the first fluid were reused and enlarged by the second fluid, which however also formed new pathways. This sample provides detailed insights into intra-slab fluid flow and fluid-rock-interaction processes at HP/LT metamorphic conditions and allows a better understanding of fluid transfer and sulfur speciation in subduction zones.