Magma evolution and processes of the Ashi Volcano, NW Tibet, China: Sr isotope studies of plagioclase crystals

Abstract

The Ashi Volcano is a member of the Ashikule Volcanic Field (AVF) and is located on the northwestern margin of the Tibetan Plateau. The Ashi Volcano draws much attention from volcanologists due to its recent eruptions in a high plateau (the last eruption occurred in 1951), in spite of its moderate size with a cone height of 110 meters. The composition of the Ashi volcanic rocks is trachyandesite. The role of open-system processes, such as magma assimilation, is controversial for the evolution of the Ashi Volcano. Previous studies in support of open-system processes were based on the presence of plagioclase compositional reverse zoning, whereas previous arguments against open-system processes were derived from the absence of any correlations between whole-rock Sr isotopic compositions and SiO2. However, compositional zoning of plagioclase may be derived by other factors, such as temperature, pressure and water contents. And whole-rock Sr isotopic compositions may not have the sensitivity to detect open-system processes. Here we use single-grain Sr isotopic compositions in plagioclase to evaluate the role of open-system processes in the magma evolution of the Ashi Volcano. Unlike plagioclase compositional zoning, single-grain plagioclase Sr isotopic compositions are not affected by temperature, pressure, or water content. Compared with whole-rock Sr isotope compositions, single-grain Sr isotopic compositions are more sensitive to open-system processes. Single-grain plagioclases and whole-rock samples of Ashi Volcano have been analyzed for their Sr isotopic compositions by a thermal ionization mass spectrometer at Auburn University. About 50% of the plagioclase grains have Sr isotopic compositions similar to whole-rock Sr isotopic compositions. The other 50% of the plagioclase grains have 87Sr/86Sr ratios (up to 0.7159) significantly higher than the whole-rock 87Sr/86Sr ratios (0.7101-0.7102), indicating assimilation of the Ashi magmas by crustal materials with more radiogenic Sr. Such materials with more radiogenic Sr do not represent recharge of mantle-derived magmas, because mantle-derived magmas have 87Sr/86Sr ratios significantly lower than the whole-rock ratios of 0.7101-0.7102. Using AU-EMPA the primary mineral phases were confirmed and compositional analyses of clinopyroxene, orthopyroxene, and plagioclase were conducted. Reverse compositional zonation was observed in plagioclase grains, which can be caused by changing pressure, temperature, and water content. The compositional analyses on orthopyroxene and clinopyroxene phases were used to calculate pre-eruption equilibrium pressure and temperature values. The temperature estimates range from 1081-1104 ºC and pressure estimates range from 5.6-7.1 kbars, which translates to a depth range of 18.5-23.4 km.