CHEMICAL VARIATIONS IN PERIDOTITE XENOLITHS FROM VITIM, SIBERIA: INFERENCES FOR REE AND HF BEHAVIOUR IN THE GARNET-FACIES UPPER MANTLE

Abstract

Peridotite xenoliths in a Miocene picrite tuff from the Vitim volcanic province east of Lake Baikal, Siberia, are samples of the off-craton lithospheric mantle that span a depth range from the spinel to garnet facies in a mainly fertile domain. Their major and trace element compositions show some scatter (unrelated to sampling or analytical problems), which is not consistent with different degrees of partial melting or metasomatism. Some spinel peridotites and, to a lesser degree, garnet-bearing peridotites are depleted in heavy rare earth elements (HREE) relative to middle REE (MREE), whereas some garnet peridotites are enriched in HREE relative to MREE, with Lu abundances much higher than in primitive mantle estimates. Clinopyroxenes from several spinel peridotites have HREE-depleted patterns, which are normally seen only in clinopyroxenes coexisting with garnet. Garnets in peridotites with similar modal and major element compositions have a broad range of Lu and Yb abundances. Overall, HREE are decoupled from MREE and Hf and are poorly correlated with partial melting indices. It appears that elements with high affinity to garnet were partially redistributed in the Vitim peridotite series following partial melting, with few effects for other elements. The Lu–Hf decoupling may disturb Hf-isotope depletion ages and their correlations with melting indices.