Pyrite growth during slab subduction: implications for S flux

Mobility of S in subduction zones is complex with far reaching impacts on the formation of large hydrothermal ore deposits in arc environments, volcanic S emissions into the atmosphere potentially impacting the Earth’s climate, S transfer into the mantle via the subducting slab and accompanying potential remodification of the S mantle budget. Sulfur flux and sulfides in subducting slabs have been extensively investigated, but most studies focus on HP metamorphism from the zone de mélanges at the mantle-crust transition, thus lacking a comprehensive overview of the slab perspective. In particular, a knowledge gap exists regarding the early stages of the S cycle within the subduction zone from sub-greenschist to epidote blueschist/eclogite facies. This study  presents new data on the control of pyrite growth during subduction-related prograde metamorphism and the implications on the flux of S as well as related elements (Ag, As, Bi, Co, Cu, Mo, Pb, Sb, Se, Te, Tl, Zn; hereafter referred to as related elements) in subduction zones.

The Cyclades, Greece, are part of the Hellenides subduction system. The Cyclades host the Cycladic Blueschist Unit (CBU), which contains a passive margin and an ophiolitic sequence hosting metasedimentary and metavolcanic rocks metamorphosed from pumpellyite to eclogite facies and are thus particularly well suited to investigate mass transfer in a subducting slab during prograde metamorphism. We selected samples from islands within the CBU to obtain insights into the S mobility along the subducting slab.

Petrological observations show that pyrite growth and abundance increase during prograde metamorphism from the pumpellyite/greenschist (~300°C, ~7 kbar) to the blueschist/ecologite facies (500–600°C, ~22 kbar). Partial resorption and hematisation/magnetisation of pyrite seems to have occurred at the transition of peak HP metamorphism to early exhumation at >500–600°C. In-situ analyses on pyrite (LA-ICP-MS) and trace element maps distribution (EPMA) within prograde pyrite shows complex concentric patterns. Based on geothermobarometric investigations of inclusions in pyrite we observed two main temperature ranges of 350–400°C and 550–600°C where metal mobilisation happenned (Au, Sb, Pb, Te, Bi, Ni, and Co). Following this, pyrite can serve as a new powerful tool to indirectly constrain metal mobility during prograde metamorphism.