VOLATILE-BEARING MINERALS AND LITHOPHILE TRACE ELEMENTS IN THE UPPER MANTLE

Abstract

Amphibole (n = 11), mica (n = 16), apatite (n = 9) and clinopyroxene (n = 19) from mantle xenoliths in alkali basalts from Siberia, Mongolia and Spitsbergen were analysed for Rb, Sr, Ba, Y, Zr, Nb, Th, U, Pb, La, Ce by proton microprobe; some mineral separates were analysed by solution ICP-MS for a larger number of elements. These results are examined together with literature data to further constrain the variation ranges and elemental ratios for these elements in the volatile-bearing minerals and the role of the latter as hosts of lithophile trace elements in metasomatised upper mantle. Mica is the dominant host of Rb and Ba and has high but variable Rb/Sr (0.13—60) and Ba/Sr ratios. Amphibole and mica are major hosts for Nb in the peridotitec upper mantle and strongly affect its Zr/Nb ratio. Apatite is a major host for Th, U, Cl, Br, LREE and Sr, but some apatites are low in Th, U, Br. Clinopyroxene contains the bulk of Sr, Zr, Y, REE in fertile spinel lherzolites with disseminated amphibole and mica and can host significant enrichments of Th, U, Pb. There are significant differences in trace element composition between vein amphibole and mica and those disseminated in peridotites; in particular, the former commonly have higher Nb and Zr contents than the latter. Trace element partition coefficients between amphibole, mica, apatite and clinopyroxene in natural ultramafic rocks from this work generally agree with results of recent experimental studies and provide important constraints on the partition coefficients between these minerals and basaltic melts. The volatile-bearing minerals may play a major role in creating geochemical and radiogenic isotope characteristics of lithospheric mantle domains and source regions of mantle-derived melts. In particular, the long-term presence of mica is required to create lithospheric mantle reservoirs enriched in radiogenic Sr as the Rb/Sr ratio in amphibole, the only other common alkali-bearing mantle mineral, is usually below bulk earth value of 0.03. The abundance and melting relationships of amphibole, mica and apatite at low degrees of partial melting of metasomatised mantle may strongly affect the absolute and relative abundances of K, Rb, Ba, HFSE and other elements in the derivative melts.