Sulfur isotopic heterogeneity in continental eclogites as a record of locally-sourced metasomatic fluids in the orogenic lithosphere (North Muya, Eastern Siberia)

The response of sulfur to subduction-zone metamorphism has been recently a matter of increasing attention because of its ability to govern the oxidation state of suprasubduction mantle and mobility of chalcophile-siderophile elements from microscale to high-level metal deposits. Along with that, little is still known about sulfur budget and its behavior in continental-collision settings with much more limited fluid transport and crustal permeability. Symplectitic eclogites, garnet amphibolites and an amphibolite from the North Muya block in Eastern Siberia bear disseminated pyrite + chalcopyrite ± pyrrhotite mineralization of metasomatic origin formed as a response of retrograde hydration. Primary (magmatic) or prograde to peak metamorphic sulfides of similar assemblage are rarely present as mineral inclusions in garnet, pointing to at least partial preservation of S budget stored in precursor mafic rocks. Trace-element contents in pyrite from symplectitic eclogites and their amphibolized analogues (garnet amphibolites and the amphibolite) indicate parental fluids, which might derive from dehydrated non-ultramafic, mainly metasedimentary sources. The ingression of S- and H₂O-bearing fluids led to precipitation of pyrite with contrasting δ³⁴S signatures (from -10‰ to +9‰), of which positive δ³⁴S values in garnet amphibolites and an amphibolite are typical of hydrothermal sulfides, while negative ones are mainly characteristic for metasediments or shallow ocean-floor volcanics. In symplectitic eclogites, within-sample δ³⁴S variations up to ～5‰ were produced by metasomatic interaction with an oxidized, SO₄^2--bearing fluid, which is supported by barite inclusions in euhedral pyrite. In turn, more strict δ³⁴S variations in garnet amphibolites might be produced by precipitation from reduced H₂S or HS^--bearing fluids with limited Δpyrite-H₂S fractionation effect. The role of open-system flow of oxidized CO₂-bearing fluids in a more fluid-rich environment is favored by high δ³⁴S and the presence of carbonate phases in the amphibolite. The data indicate contrasting sulfur reservoirs in the deep orogenic lithosphere of NMB, which might be governed by distribution, local availability and devolatilization of deeply buried and variably oxidized metasediments. Although some continental or continental-arc mafic rocks may introduce reduced S fluid species, both local devolatilization of oxidized sediments and progressive metasomatic evolution of produced C-H-O-S fluids towards SO₄^2- (HSO₄^-)- and CO₃^2- (or CO₂)-enriched compositions are possible in continental-collision zones. The latter may account for significant δ³⁴S variations even in the weakly permeable continental lithosphere.