Sulfide minerals and chalcophile elements in The Pleiades Volcanic Field, Antarctica: implications for the distribution of chalcophile metals in continental intraplate alkaline magma systems

Abstract

Fractionation of sulfides during ascent of mantle-derived magmas distributes sulfur and chalcophile elements into the Earth's lithosphere. It has been suggested that continental intraplate alkaline magmas undergo early sulfide saturation. However, there is a lack of evidence linking erupted magmas to their cumulative counterparts. We examined the petrography and chalcophile and platinum group element (PGE) geochemistry of sodic and potassic differentiation lineage lavas and cumulate autoliths from The Pleiades, an intraplate alkaline volcanic field in northern Victoria Land, Antarctica. The presence of sulfide minerals and depleted PGE contents in mafic lavas show they were saturated in sulfide from the initial stages of magma differentiation. The sodic lineage lavas and their kaersutite gabbro autoliths have abundant sulfides associated with fractionation of titanomagnetite and kaersutite. Their presence is consistent with the chalcophile element geochemistry of the lavas. In the potassic lineage lavas, petrography of autoliths indicates that the sulfides are more abundant in kaersutite diorite than wehrlite, olivine essexite, olivine diorite, and syenite autoliths. Kaersutite is not a phenocryst phase in the potassic lineage but crystallized via in situ replacement of the precursor olivine and augite. During the kaersutite formation, sulfides were precipitated from the interstitial silicate melt. The melt was sulfide oversaturated due to temperature decrease, and compositional variations (mainly SiO2, & sigma;FeO, Fe3+/& sigma;Fe ratio). The mantle-like isotope compositions of Cu, Zn, and S in the potassic lineage confirms a magmatic origin of the sulfides. The magmatic sulfides from The Pleiades are characterized by depleted PGE, Au and Cu contents. Fractional crystallization models show that early sulfide saturation led to impoverishment of chalcophile metals in The Pleiades magmatic system. This phenomenon may be common in continental intraplate alkaline magmas ascending through thick crusts.