Evidence from the New Caledonia peridotites for contrasting behavior of highly siderophile and chalcophile elements in supra-subduction zone and normal upper mantle

The New Caledonia Peridotitic Nappe represents one of the largest exposures of mantle rocks worldwide and provides unique insight on upper mantle processes. The Peridotitic Nappe is dominated by a harzburgite-dunite association forming the Massif du Sud, but also includes minor lherzolites in the northern part of the island. The geochemical features of the harzburgites require a complex history, including episodes of melting and local overprint by fluids or hydrous melts in a supra-subduction zone setting, while lherzolites are relatively homogeneous, abyssal-type peridotites (Secchiari, 2016). In order to examine how these mantle processes affected the behavior of highly siderophile (HSE: Os, Ir, Ru, Rh, Pt, Pd, Au, Re) and chalcophile elements (S, Se, Te), a set of fully characterized (i.e., major and trace elements; Sr, Nd and Pb isotopes) peridotites has been studied. The lherzolites display homogeneous 187Os/188Osi (0.1273-0.1329), slightly subchondritic to chondritic Re/Os (0.04-0.11) and PGE, Au, Se and Te abundances, similar to other lherzolites from normal upper mantle. Sulfur contents are variable (202-1268 mg/g) and were likely increased by serpentinization. Harzburgites can be grouped in two sub-types. Type-A harzburgites (epsilonNdi=9.3-13.3) have subchondritic 187Os/188Osi (0.1203-0.1254) coupled with low Os (0.554-1.513 ppb) and Re contents (0.017-0.021 ppb) and somewhat fractionated HSE patterns. The HSE display lower concentrations than in lherzolites, with Os-Ru enrichments over Ir (OsN/IrN = 1.6-1.9; RuN/IrN = 1.5-1.7) and positive anomalies of Pt and Au. S-Se-Te have low contents (< 0.1 x primitive mantle) and approximately flat patterns, likely reflecting the presence of small amounts of sulphides. These features are consistent with high melting degrees that led to sulphide exhaustion and Os-Ru and Pt alloy stabilization. Type-B harzburgites (-0.8 ≤ εNdi ≤ +4.0) are fresh rocks showing chondritic to suprachondritic measured 187Os/188Os (0.1273-0.1524) coupled with very low Os concentrations (0.003-0.277 ppb) and high 187Re/188Os ratios of up to 30. The HSE have melt-like patterns with low concentrations of Os and Ir, strong negative Pt anomalies and positive Au spikes. Sulfur, Se and Te concentrations are remarkably low (e.g., a few mg/g S). HSE patterns are interpreted to reflect localized interaction and modification of type A harzburgites with oxidized (?) subduction-related fluids or melts, which destabilized residual mantle sulphides and PGE alloys, causing strong depletion of compatible PGE and S, Se, Te. The complex element patterns of these peridotites are unknown from mantle rocks that were exhumed in extensional environments. The data suggest that HSE fractionations (e.g., Pt anomalies and Au