Trace element compositions of minerals in garnet and spinel peridotite xenoliths from the Vitim volcanic field, Transbaikalia, eastern Siberia

Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volca nic province of Transbaikalia consist of garnet Iherzolite, garnet-spinel I herzolite and spinel Iherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20-23 kbar clo se to the garnet to spinel peridotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridotites, probably during formation of the lithosphere. The peridotites show petrographic and minera l chemical evidence for infiltration by an alkaline silicate melt shortly b efore their transport to the Earth's surface. The melt infiltration event i s indicated petrographically by clinopyroxenes which mimic melt morphologie s, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Eart h's surface. Single-mineral thermometry gives reasonable temperature estima tes of 1050 +/- 50 degrees C, whereas two-mineral methods involving clinopy roxene are falsified by secondary components in clinopyroxene introduced du ring the melt infiltration event. Excimer Laser-ICP-MS analysis has been pe rformed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock t ype. Orthopyroxene and garnet show only partial equilibration of trace elem ents with the infiltrating melt, whereas clinopyroxene and amphibole are cl ose to equilibration with the melt and with each other. The incompatible el ement composition of the infiltrating melt calculated from the clinopyroxen e and amphibole analyses via experimental mineral/melt partition coefficien ts is similar to the host alkali picrite, and probably represents a low mel t fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series o f events expected during the influence of an expanding rift region in the u pper mantle, namely the progressive erosion of the lithosphere and the epis odic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field. (C) 1999 Elsevier Science B.V. All rights reserv ed.