Microscale effects of melt infiltration into the lithospheric mantle: Peridotite xenoliths from Xilong, South China

Abstract

Melting and reactions between minerals and melts are important processes in the evolution of the lithospheric mantle, and are usually inferred from their geochemical fingerprints in mantle samples. However, a suite of mantle-derived peridotite xenoliths from the Xilong area, South China, records the reaction of successive silicate melts of different compositions with mineral assemblages in the mantle, preserved by quenching during entrainment. These xenoliths form two groups and record a compositionally layered mantle. Group 1 has olivine Mg# ~91, (and is thus relatively refractory), is derived from depths of ~50–65km, and shows the trace-element geochemical signature of “old” carbonatitic metasomatism. Group 2 is more fertile with olivine Mg# mainly ~89–90, is derived from ~40 to 55km and has ubiquitous modal spinel. Xenoliths of both groups then show sequential infiltration by two compositionally distinct melts (Na-rich and K-rich) not long before eruption.The Na-rich melts are enclosed in spongy clinopyroxene and spinel rims and are inferred to have triggered the reactions that formed the spongy rims, which have lower Al2O3, Na2O and Mg#, but higher FeO, TiO2 and Cr# than the primary phases. The undersaturated Na-rich mafic melts were probably formed in the asthenosphere by low-degree melting.The K-rich melts occur mainly in reaction zones around orthopyroxene and in reaction patches containing fine-grained secondary olivine, clinopyroxene and minor spinel. The melts have high contents of SiO2, K2O (mean 14.3wt.%), Rb, Ba, and LREE but very low Na2O/K2O (0.01–0.29), positive anomalies in Eu and Sr, and variable HFSE anomalies. These compositional characteristics are consistent with an origin as low-degree partial melts of pre-existing phlogopite-bearing rocks.The K-rich melts also react with primary olivine, and the spongy-textured secondary clinopyroxene and spinel inferred to have formed by reaction with the Na-rich melts, yielding secondary olivine, vermicular clinopyroxene and spinel compositionally similar to that in the reaction patches. This observation suggests that the infiltration of K-rich melt occurred after the percolation of Na-rich melts.This study demonstrates the complexity of metasomatic processes in the lithospheric mantle, and emphasizes the caution required when interpreting the geochemical signatures of mantle-derived xenoliths in which multiple overprinting by compositionally diverse fluid and melt interactions with mantle minerals cannot be so clearly observed and characterized as in these samples.