OPEN-SYSTEM PROCESSES IN THE MAGMA EVOLUTION OF THE ASHI VOLCANO:EVIDENCE FROM SR ISOTOPIC COMPOSITIONS OF SINGLE-GRAIN PLAGIOCLASE
The role of open-system processes, such as magma assimilation, is controvertial 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 againgst 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 0.7139) 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 lower than the whole-rock ratios of 0.7101-0.7102.