Osmium and neodymium isotopic constraints on the temporal and spatial evolution of Siberian flood basalt sources

Abstract

High-Mg volcanic rocks from the ca. 250 Ma old Siberian Flood Basalt Province (SFBP) were analyzed for their osmium and neodymium isotopic compositions in order to help to constrain source characteristics as the system evolved. Picrites from the Gudchikhinsky suite, the oldest rocks examined, have γ Os of +5.3 to +6.1 and ϵ Nd of +3.7 to +4.0. The osmium and neodymium isotopic compositions of these rocks are similar to some modern ocean-island basalts (OIB), consistent with their derivation from a mantle plume, and show little evidence for interaction with either subcontinental lithospheric mantle (SCLM), or the Precambrian Siberian craton through which the parental melts passed. Picrites from the stratigraphically higher Tuklonsky suite have similar γ Os of +3.4 to +6.5, but ϵ Nd Of −0.9 to −2.6. The similar γ Os but lower ϵ Nd for the Tuklonsky picrites as compared with the Gudchikhinsky picrites suggest that some magmas from the same OIB-type, mantle source were contaminated by lithospheric components. The osmium isotopic composition of the Tuklonsky picrites was not significantly affected by this interaction, possibly because Os concentrations in the magmas were substantially greater than those in the contaminant. A differentiated ankaramite flow, associated with the top of the stratigraphically higher Morongovsky suite, has γ Os of +9.8 to +10.2 and ϵ Nd of +1.3 to +1.4. The higher γ Os may indicate that the plume source was heterogeneous with respect to osmium isotopic composition, consistent with osmium isotopic measurements in rocks from other plume sources. In contrast to these rocks, Mg-rich, alkaline rocks (meymechites) from the Guli area that erupted much nearer the end of the flood-basalt event have γ Os of −1.2 to −2.6 and ϵ Nd of +3.7 to +4.9. These rocks were probably produced by low degrees of partial melting of mantle after the main stages of flood-basalt production. The relatively low γ Os and high ϵ Nd for the meymechites, together with a variety of trace-element characteristics, are most consistent with derivation from a mixed source—one that included both the 0113-type source that fed the majority of the flood-basalt system and a major component from the SCLM underlying the Siberian craton. These results, taken together with earlier investigations of the Noril'sk-type ore-bearing intrusions, suggest that much of the SFBP consists dominantly of plume-derived material, until relatively late in the magmatic event, when the SCLM became a significant source of material.