9615274 Mukasa Ultramafic xenolith suites from Jetty Peninsula, East Antarctica, provide an unparalleled opportunity to examine and describe the consequences of magma/rock reactions equilibrated at subcontinental mantle temperatures and pressures during the lithospheric extension that fragmented eastern Gondwanaland at the end of the Jurassic and beginning of the Cretaceous. The samples to be used in this study were collected by Soviet/Russian expeditions during 1982-83, 1983-84 and 1986-87 and have been put entirely at our disposal by Dr. Alex Andronikov of the Department of Antarctic Geology, VNII Okeangeologia, St. Petersburg, Russia (now on leave at the University of G ttingen, Germany). This award will support analytical work to determine the Nd, Sr, and Pb isotopic compositions and Rb/Sr, Sm/Nd, and U/Pb elemental ratios for clinopyroxene (cpx) and glass taken from garnet lherzolite and spinel lherzolite xenoliths, and to perform the same measurements on whole-rock samples from the host magmatic rocks which are alkaline picrite, aln ite, and nephelinite in composition. In addition, the analytical program will include concentration measurements, using secondary ionization mass spectrometry (SIMS), of the rare earth elements (REE) and Ba, Cs, Cr, K, Nb, Rb, Sr, Ti, V, Y and Zr (in different combinations) in cpx, garnet and the various glasses present in some of the xenoliths. With his recent award of a fellowship by the Alexander von Humboldt Foundation, Dr. Andronikov will carry out complementary studies on the same materials at the University of G ttingen, Germany, focusing on xenolith mineral chemistry determined by electron microprobe, and the major and trace element compositions of the host rocks measured by X-ray fluorescence spectrometry and Inductively Coupled Plasma-mass spectrometry, respectively. The combined data will allow, for a subset of the xenoliths, assessment of the degree and effects of partial melting in lherzolites derived from different depths in the upper mantle, and, for a glass-bearing subset, characterization of the compositions of the melts which intruded the mantle lithosphere and altered its composition by infiltration. Through analysis of these results, this work will lead to better understanding of the scale of chemical interactions between melts and adjacent wall rocks at mantle depths in the Earth. The principal objectives of this work are to model the magmatic processes which affect subcontinental mantle lithosphere during rifting, reconstruct the thermal evolution of the lithosphere during such deformation, determine the provenance of magmas which intrude the lithosphere at various times during its evolution, and with model ages, establish a connection between the evolution of the lithospheric mantle beneath East Antarctica and the plate tectonic history of eastern Gondwanaland. In addition to the importance of this work to regional geologic models, understanding the processes that control lithospheric evolution during extension will provide important general constraints on thermo-mechanical models of rifting, and on chemical evolution of the mantle beneath continents.
