Decoupling of the Sm-Nd and Re-Os Isotopic Systems in Sulphide-Saturated Magmas in the Halls Creek Orogen, Western Australia

Abstract

We have observed apparent decoupling of the Re–Os and Sm–Nd isotopic systems in sulphide-saturated magmas that suggests that bulk two-component or assimilation–fractional crystallization (AFC) mixing modelling, based on Re–Os isotopic data, is inappropriate for chalcophile isotopic systems in turbulent sulphide-saturated magmas. This behaviour is observed in three Palaeoproterozic layered mafic–ultramafic intrusions in the Halls Creek Orogen of Western Australia. All intrusions clearly have a basaltic parental magma based on primitive olivine and spinel compositions. The intrusions are light rare earth element enriched and define a narrow range of initial Nd-isotopic signatures (εNd = −0·9 to +0·70), being derived from an enriched mantle source or involving small degrees of crustal contamination. However, a wide variation in Os-isotopic compositions is observed, ranging from (1) slightly enriched for the Panton intrusion (γOs = +11), requiring an enriched-mantle source or a crustally contaminated mantle-derived magma, through (2) enriched for the McIntosh intrusion (γOs= +38 to +87), to (3) strongly enriched for the Sally Malay intrusion (γOs= +69 to +1300), requiring crustal contamination of a mantle-derived magma. AFC and two-component mixing models can explain the Os- and Nd-isotopic compositions, and in these models an upper crust or radiogenic Lewisian-type lower crust provides a good analogue for the contaminant. Furthermore, the Os isotopic data suggest a picritic parental magma for the Panton and McIntosh intrusions, whereas a basaltic parental magma is indicated for the Sally Malay intrusion. This is in contradiction with the clearly basaltic parental magma implied for all intrusions by olivine and spinel compositions. The inability of two-component mixing and AFC modelling to precisely simulate this system is the result of (1) the magma(s) attaining sulphide saturation, (2) the preferential partitioning of Os into immiscible sulphide and (3) the lowering of Os-isotopic ratios during equilibration between the host magma and immiscible sulphide at high R-factor (high mass ratio of silicate to sulphide melt). Such processes would not affect the lithophile Sm–Nd isotopic system, hence decoupling of the Sm–Nd and chalcophile Re–Os isotopic systems is implied.