OSMIUM AND NEODYMIUM ISOTOPIC CONSTRAINTS ON THE TEMPORAL AND SPATIALEVOLUTION OF SIBERIAN FLOOD-BASALT SOURCES

High-Mg volcanic rocks from the ca. 250 Ma old Siberian Flood Basalt P rovince (SFBP) were analyzed for their osmium and neodymium isotopic c ompositions in order to help to constrain source characteristics as th e system evolved. Picrites from the Gudchikhinsky suite, the oldest ro cks examined, have gamma(Os) of +5.3 to +6.1 and epsilon(Nd) of +3.7 t o +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 in teraction with either subcontinental lithospheric mantle (SCLM), or th e Precambrian Siberian craton through which the parental melts passed. Picrites from the stratigraphically higher Tuklonsky suite have simil ar gamma(Os) of +3.4 to +6.5, but epsilon(Nd) of -0.9 to -2.6. The sim ilar gamma(Os) but lower epsilon(Nd), for the Tuklonsky picrites as co mpared with the Gudchikhinsky picrites suggest that some magmas from t he same OIB-type, mantle source were contaminated by lithospheric comp onents. The osmium isotopic composition of the Tuklonsky picrites was not significantly affected by this interaction, possibly because Os co ncentrations in the magmas were substantially greater than those in th e contaminant. A differentiated ankaramite flow, associated with the t op of the stratigraphically higher Morongovsky suite, has gamma(Os) of +9.8 to +10.2 and epsilon(Nd) of +1.3 to +1.4. The higher gamma(Os) m ay indicate that the plume source was heterogeneous with respect to os mium isotopic composition, consistent with osmium isotopic measurement s in rocks from other plume sources. In contrast to these rocks, Mg-ri ch, alkaline rocks (meymechites) from the Gull area that erupted much nearer the end of the flood-basalt event have gamma(Os) of -1.2 to -2. 6 and epsilon(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 f lood-basalt production. The relatively low gamma(Os) and high epsilon( Nd) for the meymechites, together with a variety of trace-element char acteristics, are most consistent with derivation from a mixed source-o ne that included both the OIB-type source that fed the majority of the flood-basalt system and a major component from the SCLM underlying th e Siberian craton. These results, taken together with earlier investig ations of the Noril'sk-type ore-bearing intrusions, suggest that much of the SFBP consists dominantly of plume-derived material, until relat ively late in the magmatic event, when the SCLM became a significant s ource of material.