Evidence for a ‘third’ endmember of the Unzen 1991–1995 eruption from amphibole thermometry and crystal clots

Abstract

Amphibole phenocrysts and crystal clots composed of amphibole, plagioclase, and interstitial glass were observed in dacitic products of the Unzen 1991–1995 eruption and analyzed in order to understand pre-eruptive magmatic processes. Amphibole phenocrysts in the samples erupted at different times are classified by composition as either magnesiohornblendes or tschermakites. Temperatures of 770–830 °C are estimated from magnesiohornblende phenocrysts and amphiboles in the crystal clots, which are consistent with the proposed temperature of the silicic endmember magma. The average approximate composition of the interstitial melt is 67–73 wt% SiO2, ~0.25 wt% TiO2, ~12 wt% Al2O3, ~1.1 wt% FeO, 0.14 wt% MgO, 0.80 wt% CaO, 3.0 wt% Na2O, and 4.0 wt% K2O. The SiO2 plus H2O content (~8.8 wt%) of the melt estimated from these amphiboles (SiO2melt = 71–72 wt%) is consistent with the compositions of interstitial melts observed in the crystal clots. The H2O saturation depth estimated for the interstitial melt and plagioclase compositions is consistent with that of the magma chamber where the silicic endmember magma was likely stored (11–15 km). These results suggest that interstitial melts in crystal clots represent the silicic endmember melt. Temperatures estimated from tschermakite phenocrysts are 870–950 °C, which is lower than the proposed temperature of the mafic endmember magma. In addition, a temperature gap is observed between the tschermakites and magnesiohornblendes. These results suggest the contribution of a previously unrecognized third endmember magma to the 1991–1995 magma, which we term the mid-temperature magma (MT magma).