Anomalously high δD and micro-scale hydrogen isotope heterogeneities in the mantle: Ion microprobe analysis of amiboles from peridotite xenoliths at Nushan, eastern China

Abstract

Hydrogen isotopic compositions of four amphibole grains from three pieces of lherzolite xenoliths in Cenozoic basanites of Nushan, eastern China have been analyzed by ion microprobe. δD values of all analyzed points range from −94‰ to +46‰, some of which are much higher than the highest δD (+8‰) reported previously for mantle materials. The heterogeneities of D/H ratios within single grains have been observed, the variation of δD is up to 80‰ on the scale of less than 400 μm. No correlation between hydrogen isotopic ratios and hydrogen contents can be found, implying that the scatter of δD values could not result from a late shallow process such as hydrogen loss or hydrothermal alterations and should be considered as inherited from the source at depth. Chemical compositions of Nushan amphiboles are very homogeneous, excluding the fact that the scatter of δD values could arise from variable fractionation factors between a single fluid source and minerals. Therefore, metasomatic fluids responsible for the formation of Nushan amphiboles should be heterogeneous and result in the observed large variable and anomalously high δD values of amphiboles. We suggested that such metasomatic fluids could be related to magma degassing in the mantle source. Based on the D-H diffusion data and the scale of hydrogen isotope heterogeneities, it was inferred that the mantle metasomatism took place soon before the eruption of host magma.