Enrichment of upper mantle peridotite: petrological, trace element and isotopic evidence in xenoliths from SE China

Trace element patterns of spinel- and garnet peridotite xenoliths from the Nushan and Mingxi basalts (SE China) reflect the interplay between partial melting and subsequent enrichment by metasomatism. Trace element patterns indicate the common addition of Th, Sr and LREE, but not of significant Ti and Zr, which suggests a metasomatising agent (or agents) with characteristics of H2O-rich and CO2-rich fluids. Laser Ablation Microprobe linked to an Inductively Coupled Plasma Mass Spectrometer (LAM-ICPMS) analyses of volatile-bearing minerals (including amphibole, phlogopite and apatite) and glasses show that they are important hosts for K, Rb, Sr, Ba, U and rare earth elements (REE) in the mantle, and play an important role in the fractionation of these elements in mantle processes. However, mass-balance calculations require that significant proportions of LREE, Th and Sr are situated in intergranular phases. Sr-87/Sr-86 ratios of whole-rock samples from each locality are higher, and Nd-143/Nd-144 lower, than those of the corresponding acid-leached clinopyroxenes (cpx), suggesting that highly radiogenic Sr and relatively unradiogenic Nd reside in these intergranular phases. The Sr/Nd isotopic compositions of clinopyroxenes separated from the peridotites are correlated with metasomatic style, as reflected in the trace element compositions of the clinopyroxenes. Clinopyroxenes in fertile peridotites from Nushan and Mingxi have high Nd-143/Nd-144 and low Sr-87/Sr-86, reflecting long-term depletion of the mantle. Clinopyroxenes from metasomatised peridotites define trends toward lower Nd-143/Nd-144 and higher Sr-87/Sr-86, consistent with a metasomatic fluid having a Sr/Nd isotopic composition near Bulk Earth, and Sm-147/Nd-144 less than or equal to 0.09. Metasomatism by a similar type of fluid can explain the isotopic systematics of clinopyroxenes from most eastern China localities reported in the literature, but a few localities interpreted as on-craton would require fluids with still lower Nd-143/Nd-144 and higher Sr-87/Sr-86. The minimum age of the major element depletion may be given by a Sm/Nd regression line of 733 +/- 340 Ma for cpx from unmetasomatised peridotites at Nushan. The age of the metasomatic event(s) cannot be constrained, but a few metasomatised samples with high Nd-143/Nd-144 indicate that a metasomatic event must have occurred shortly before eruption. (C) 2003 Elsevier Science B.V. All rights reserved.