Geochemistry of Northeast China Cenozoic Basalts and Experimental Study on Crystal Dissolution.

Abstract

The origin of the Cenozoic volcanism of Northeast China is under debate. Geochemical data on the major and trace element compositions of the volcanic rocks and helium isotope ratios of the mantle xenoliths are reported. The results suggest that the previously reported helium isotopic data are affected by cosmogenic production of 3He and cannot be used to support the mantle plume hypothesis. The Sr, Nd and Pb isotope data are re-interpreted to be a mixture of a FOZO and a LoMu end-member. The trace element data do not show features that are often observed in subduction-related volcanism. The variations in the major and trace elements of the volcanic rocks can be explained by correlated variation in the depth and degree of mantle partial melting, with a greater depth associated with a smaller degree of partial melting. Reactive textures between mantle xenoliths and hosting magmas are common in the basalts from Northeast China as well as other volcanic fields. Experiments were conducted at ~0.47-1.90 GPa and 1236-1517 °C to study diffusive olivine and clinopyroxene dissolution and to obtain the saturation conditions and diffusivities in the melt, which are critical parameters to model convective crystal dissolution. Olivine saturation is treated as being determined by MgO, and clinopyroxene saturation is treated as being determined by MgO and CaO. Diffusivities show Arrhenian dependence on temperature, but the pressure dependence is small and not resolved. The interface melt compositions depend on temperature and pressure, but much less on the crystal and bulk melt compositions within the composition range examined. Modeled clinopyroxene dissolution rate responsed more strongly to superheating than that of olivine, which is due to the difference in the thermodynamics of the two minerals. Applying the models to basalts from Northeast China yields eruption temperatures from 1160 to 1310 °C, depending on the assumption of the pressure of the xenoliths-melt reaction. The residence time of the xenoliths is estimated to be from a few hours to a few days, and the ascent rate is from 26 m/hour to ~6.3 km/hour. The composition of the hosting basalt was not significantly modified by the mantle xenoliths.