Sector-zoned augite megacrysts in Aleutian high alumina basalts: implications for the conditions of basalt crystallization and the generation of calc-alkaline series magmas

Several high alumina basalts from the Aleutian volcanic centers of Cold Bay and Kanaga Island contain large (up to 1.5 cm diameter) megacrysts of sect or-zoned augite. The megacrysts are invariably euhedral with well developed {001}, {010} and {111} forms. All crystals display concentric bands that a re rich in mineral and glass inclusions. The sector zonation typically occu rs as well developed (010), (100), (111) and (110) sectors which grew at di fferent rates. A comparison of the width of synchronous growth bands indica tes that following relative growth rates: (111)much greater than(100) simil ar to (110) > (010). Compositionally, SiO2 and MgO abundances decrease, and TiO2, Al2O3, FeO and Na2O abundances increase in the different sectors in the order (111), (100) similar to (110), (010). This order is identical to that deduced for the relative growth rates, implying that growth rate clear ly had a role in the development of the sector zonation. Calculated pre-eru ption H2O contents of the basalts range from 1 to 3 wt% but actual (measure d) post-eruption H2O contents range from 0.01 to 0.3 wt%. Deteurium isotopi c values are heavily depleted and range from -110 to -141 parts per thousan d. Together these indicate significant vapor (H2O) exsolution prior to erup tion. Maximum H2O abundances in primitive glass inclusions, thought to be m ost representative of the host liquid reservoir at the time of melt entrapm ent, systematically decrease from the core to the rim of one augite megacry st studied in detail. We conclude that the presence of sector-zoned augite is due to augite supersaturation and rapid crystallization brought about by magma decompression and volatile (H2O) exsolution. The calculated pre-erup tion H2O contents of 1-3 wt% limit vapor exsolution and basalt crystallizat ion to depths of less than 3 and more likely 1.5 km. Very rapid crystalliza tion at very shallow depths makes it unlikely that the time scales between initial crystallization and final eruption are sufficient to permit appreci able amounts of fractional crystallization. Given that high alumina basalt fractionation is the dominant process for generating more evolved andesite, dacite and rhyolite magmas of the calc-alkaline suite, the inability of pa rental high alumina basalt to yield such derivative magmas in the low press ure environment places the likely site of fractionation in the high pressur e environment, at or near the base of the crust.