The deep sulfur cycle : insights from sulfide metamorphism in blueschist and eclogite, NE New Caledonia

Abstract

One aspect of sulfide petrology that has received very little attention is the prograde metamorphism of sulfide minerals in exhumed fragments of a subduction zone. 'Normal' approaches to studying sulfur transfer in subduction zones look at sulfur in arc magmas, tracing the su lfur pathway using isotopes. Sulfur is removed from the slab during the release of water-rich fluids associated with metamorphism to eclogite facies. Nevertheless, New Caledonian blucschist and eclogite preserve as inclusions in porphyroblastic minerals, a record of sulfides present during prograde subduction processes. Significantly, these inclusion sulfides have not been retrogressed by later fluids, as in the case of matrix sulfides in these rocks. This is a study of sulfides trapped by blucschist through to eclogite facies silicate minerals, providing data on the behaviour of sulfides during subduction. Most studies in regional metamorphic terranes ignore sulfide minerals. This is somewhat understandable, as they equilibrate an order of magnitude faster than silicate minerals (Barton 1970), and so may not necessarily reflect the origi nal equilibrium assemblage formed at the peak conditions of metamorphism. During retrogression, sulfide structure and composition is not only susceptible to rapid changes induced by cooling, but also by fluids, whose influx in New Caledonia has been facilitated along greenschist facies shear zones. Therefore it is likely that matrix sulfide minerals have re-equi librated. However, sulfide mineral inclusions in prograde porphyroblasts (lawsonite, spessartine garnet, and almandine garnet) are effectively armored against such retrogression or fluid influx. Cu-Fe sulfide inclusions have been found across metamorphic grade within the lawsonite, omphacite, and hornblende metamorphic zones, spanning a crustal profile of - 30km. Many of these inclusions show evidence of isochemical unmixing of original monosulfides during cooling, and bulk area scans were done to determine the original compositions. The area scans represent both binary and ternary mixing lines between exsolution products. Careful interpretation of this data enables the back-calculation of original equilibrium rnonosulftde phases. However, the interpretation of sulfide chemistry is complicated. Most inclusions are in the order of I μm in size. The determination of high P Cu-Fe-S phase relations relies not only on sulfide composition, but also on the interpretation of co-existing mineral phases identified in larger inclusions. The results differ significantly from the low pressure (1 atm) experimental topologies. In this way, high pressure mineral inclusion compositions provide insight into the effect of subduction pressure on Cu-F~S internal phase relations as well as silicate sulfide equilibrium. Despite a concentrated effort, prograde sulfide inclusions were not found in the intervening 'epidote' metamorphic zone. The absence of sulfide in the epidote zone is likely related to fluctuating fluid fs2 - f02, which precluded sulfide and garnet being stable together at that time. High pressure theoretical ca lculations of Cu-Fe-S equilibria support the assessment of natural sulfide specimens, and permit the determination of high P phase relations in this system. Chalcopyrite stability is P-dependent, and is not a stable mineral phase for much of the blu eschisteclogite facies metamorphism in New Caledonia. Common fsrfo2 buffers containing magnetite are irrelevant in blueschist eclogire terranes; garnet occurs in mafic rocks at the expense of plagioclase, accompanying the breakdown of magnetite (Green and Ringwood J 967). Magnetite-bearing redox equilibria have been extensively studied experimentally and theoretically (e.g. Shi 1992) and are widely applied to evaluate redox conditions in the crust and mantle. However, as magnetite is not present in eclogites, such equilibria are deemed metastable. Therefore, it is important to determine which phase equilibria are appropriate for subduction zone environments, on the basis of silicate-sulfide-oxide relationships. The importance of silicate-sulfide equilibria during subduction is demonstrated by garnet-forming sulfide-feldspar reactions, which contribute to the total garnet budget. For example: 2CaAl₂Si₂O₈ + 2FeS + Fe₃O₄ + 2SiO₂ = 2CaFe₂Al₂Si₃O₁₂ + 2FeS₂ Anorthile Pyrrhotite Magnetite Quartz Garnet Pyrite Such reactions can occur because of the ability of iron to behave as both a chalcophile and lithophile element. The prevalence of copper sulfides is characteristic of subducted sulfide. This is not due to high copper content in these lithologies per se, but is in fact a record of prograde sulfide-sil icate reactions in which iron behaviour changes from chalcophile to Jithophile. This results in the concentrating of copper in the remaining sulfides, as copper retains its chalcophile character during subduction.