Variable extent of dehydration of clinopyroxene megacrysts from Gezi volcano of Inner Mongolia

Abstract Mineral phenocrysts/megacrysts convey important information about deep magmatic evolution and volcanic eruption especially with respect to magmatic water content. Clinopyroxene (cpx) is generally considered to be less susceptible to diffusive hydrogen loss than olivine and orthopyroxene. We analyzed hydrogen concentration profiles alongside major and trace element compositions of a suite of cpx megacrysts (1–4 cm in size) hosted in basanitic scoria from the Gezi volcano, central Inner Mongolia of north China. The anhydrous composition within each megacryst is homogeneous, but comparison between different grains shows clear elemental covariation, suggesting that these cpx megacrysts progressively crystallized from a common parental magma. The cpx megacrysts contain hydrogen contents in the range of 0–410 wt ppm H2O at their cores, and some of them show a smooth decrease of hydrogen from the core to the rim. Those with the highest hydrogen contents (290–410 wt ppm H2O) have long plateau at the core, in contrast with long dehydration profiles and no plateau in some low-hydrogen megacrysts. These observations suggest that the cpx megacrysts experienced variable extent of dehydration during volcanic eruption, arising from variable H diffusivity probably associated with differences in the composition and microstructure of cpx megacrysts. Cpx megacrysts with the lowest degree of dehydration and good diffusion profiles are the most informative. Their plateau hydrogen contents at the core are used to infer ~2 wt% H2O in the parental magma. Quantitative modeling of the diffusion profiles at the rim yields a dehydration duration of 3–28 h, suggesting an ascent rate of 1–6 m/s for the host basanitic magma.