Volatiles and trace elements in melt inclusions from Siberian Traps

Abstract:

The eruption of the Siberian Traps Large Igneous Province (LIP) was synchronous with the largest known mass extinction, at the Permo-Triassic boundary. Understanding the volatile budget of the eruptions and hence their potential effects on climate is of critical importance. The volcanism spanned an enormous territory (5 million km²) over 0.8 Ma and the magmas feeding the eruptions were heterogeneous in their chemistry in space and time. In terms of volatiles in pre-eruptive magmas, there are multiple possible sources: the mantle (including metasomatized lithosphere) and crustal rocks and sediments. Discriminating between these sources requires not only microanalysis of volatiles in melt inclusions, but also analysis of trace elements. Crucially, the magmas sampled for this study did not intersect and assimilate evaporite deposits or brines prior to emplacement as sills or eruption as lavas, in contrast to previous studies, which might allow mantle-derived volatile heterogeneity to be preserved in the melts. We present a new dataset of clinopyroxene-hosted melt inclusion geochemistry. Crystalline inclusions in clinopyroxene with Mg# from 69.2 to 82.6 were homogenized at temperatures of 1190°C and fO₂ of FMQ_-1 in a high temperature gas mixing furnace. We show that, for this particular suite of lavas, considerable variability exists in trace and volatile element ratios (e.g. La/Yb, Nb/Y, Ba/La, F/Nd, Cl/K) that may be explained entirely by mantle heterogeneity. The most depleted melts (e.g. low La/Yb) have the highest range and values of S/Dy, Cl/K and F/Nd ratios; and the most “enriched” melts (highest La/Yb) exhibit low volatile/trace element ratios. These trends are consistent with mixing between end member sources: low degrees of melting of a volatile-poor source and high degrees of melting of a volatile-rich component with a depleted trace element signature (which might be consistent with minimally devolatilised recycled oceanic crust). There is no clear evidence of continental crust being assimilated and promoting additional volatile enrichment.