Résumé
A new collection of spinel-peridotite xenoliths from Quaternary basaltic centers in the Tariat region of Mongolia provide a sample of the shallow lithospheric mantle beneath this area. Ninety-seven of these xenoliths were analyzed for whole rock and mineral major element composition. Both orthopyroxenes and clinopyroxenes separated from the samples were analyzed for trace element content. For a selection of samples, whole rocks were analyzed for trace element contents and clinopyroxenes for the isotopic composition of Sr, Nd, Hf, and Pb. A number of the whole rocks also were analyzed for their Re-Os systematics.Mineral thermometry provides equilibration temperatures in the range of 858–1073 °C that places the origin of these samples in the shallow upper mantle just below the crust of this region. Over half of the samples have fertile major element compositions with Al2O3 concentrations between 3.5 and 4.5 wt%. About a third of the samples are more refractory, yet show enrichment in highly incompatible elements suggestive of recent metasomatism that is hosted both by major minerals and interstitial materials. About 10% of the samples show the compositional consequences of having experienced addition of cumulates from mafic melts in the mantle, including composite samples with pyroxenite and peridotitic portions.The fertile peridotite xenoliths from the Tariat region are samples of mantle that is compositionally and isotopically similar to that which supplies modern mid-ocean ridge basalt (MORB) volcanism. While the major element compositions of the majority of these samples approach the composition estimated for the Bulk-Silicate-Earth (BSE), the peridotites with fertile major element chemistry all show a slight degree of depletion in the more highly incompatible elements and have radiogenic isotopic compositions (87Sr/86Sr mostly from 0.7021 to 0.7035, εNd from +5.6 to +11.0 and εHf from +12.0 to +18.5) consistent with small volume melt removal in the mid-Proterozoic. The composition of these xenoliths is well-matched by models that suggest the MORB-source mantle is produced in a dynamic Earth by the many melt extraction events that have built up the continental crust and created a portion of the mantle depleted in the elements that are enriched in continental crust. For this reason, the most obvious petrogenetic explanation for this section of continental lithospheric mantle is that it is essentially undifferentiated MORB-source mantle that was accreted beneath the Paleozoic crust of this region during the ocean-closing events that formed the Central Asian Orogenic Belt. The composition of these xenoliths provides an opportunity to examine details of estimates for the composition of both the depleted mantle sources of MORB and the BSE. The Tariat xenoliths clearly support the observation that the MORB mantle is unexpectedly depleted in Nb, Ta, and possibly Ti compared to models that create the MORB mantle by extraction of continental crust. Given these characteristics, the Tariat xenoliths, though sampled in the middle of Earth’s largest continent, may provide the least altered look at the composition of the upper mantle that provides volcanism along the world’s ocean ridges.