ALBERT

Description: Volatiles play an important role in magmatic and volcanic processes. Melt inclusions are a powerful tool to study pre-eruptive volatiles, but interpretation of their H2O and CO2 variations can be difficult. The H2O and CO2 contents of melt inclusions from nine basanites from Hut Point Peninsula, Mt Terror and Mt Bird on Ross Island, Antarctica, were studied to understand better the behavior of volatiles in the magmas and to provide insight into magma transport and storage processes. Ninety olivine-hosted (Fo78–-88) melt inclusions were examined along with the composition of the associated bulk-rock samples. The H2O (0·4–2·0 wt %) and CO2 (0·2–0·9 wt %, or 0·2–1·8 wt % after correction for vapor bubbles) variations in the melt inclusions cannot be explained by equilibrium degassing. A strong correlation between melt inclusion radius and H2O content for Hut Point samples indicates that diffusive loss of H+ has occurred. Based on vapor saturation pressure trends, it is inferred that a magma reservoir existed below Hut Point at a depth of ∼18 km, and by modeling diffusive loss of H+ for melt inclusions, it is shown that the magmas ascended from this depth in less than a year. Melt inclusions from Terror and Bird lack evidence of diffusive loss of H+ and there are no strong chemical indicators of CO2 fluxing. Compositional heterogeneities in melt inclusions indicate that magma mixing occurred, making it difficult to interpret H2O and CO2 trends. Melt inclusions from these volcanoes were entrapped polybarically, inconsistent with entrapment in a single storage region. Published analyses of 54 olivine-hosted (Fo53–83) melt inclusions in seven samples from Erebus volcano on Ross Island were re-examined for comparative purposes. Low H2O (∼0·1 wt %) and CO2 (0–0·2 wt %) contents and the evolved compositions of these indicate that Erebus magmas undergo shallow (〈6 km) crystallization before eruption, probably in a shallow storage region. Magmas from the surrounding volcanoes show no sign of shallow storage.

Description: The Iheya Graben is a back-arc spreading center in the middle part of the Okinawa Trough. It is also located in the center of an anomalous volcanic zone (volcanic arc migration phenomenon, or VAMP) and is characterized by bimodal volcanism, unusually high heat flow, and active hydrothermal circulation. The subvolcanic magma plumbing system and the magmatic processes related to the formation of rare erupted intermediate lavas in this area remain uncertain. In this study, we conducted systematic mineralogical analyses (in situ major element, trace element, and Sr isotopes) and whole rock geochemical analyses (major element, trace element, and Sr-Nd isotopes) on an andesite (T5-2; type C andesite) and a rhyolite (C11; type 2 rhyolite) and present evidence for magma mixing in the origins of these lavas. Andesite T5-2 contains a mafic mineral assemblage and a silicic mineral assemblage, which are derived from a basaltic melt and a type 2 rhyolitic melt, respectively. A 4:6 mixture of basalt and type 2 rhyolite from the Iheya Graben reproduces the whole-rock major element, trace element, and Sr-Nd isotope compositions of T5-2. Rhyolite C11 contains a group of disequilibrium minerals that crystallized from a less evolved rhyolitic melt with relatively more enriched Sr-Nd isotope compositions, suggesting mixing of this melt with a more evolved and isotopically more depleted rhyolitic melt. This mixing process could produce a series of rhyolitic melts with a negative correlation between SiO2 concentrations and 87Sr/86Sr ratios (or a positive correlation for 143Nd/144Nd ratios), which are recorded by the whole group of type 2 rhyolites. The results from mineral-based thermobarometers suggest that the premixing storage temperatures of the basaltic and rhyolitic melts are ∼1100 °C and 870–900 °C, respectively. The hybrid andesitic melt has temperatures of ∼950 to ∼980 °C. The magma storage pressures are not well constrained, ranging from ∼400 MPa to ∼100 MPa. We show that magma mixing plays a significant role in the origins of diverse volcanism in the middle Okinawa Trough; more specifically, two of the three types of andesites (types B and C) and one of the two types of rhyolites (type 2) are associated with magma mixing. We thus propose a complex magma plumbing system with multichamber magma storage and frequent magma mixing beneath the Iheya Graben.