Deep-seated fractionation during the rise of a small-volume basalt magma batch: Crater Hill, Auckland, New Zealand

Abstract

Crater Hill is a small volume alkali olivine basalt volcano in the Auckland volcanic field. Crater Hill consists of a sequence of pyroclastic and effusive eruptive units of which the earliest have low silica, ferromagnesian elements and Mg/Fe ratios, high incompatible elements and are more silica undersaturated while the last material to be erupted has higher silica, ferromagnesian elements and Mg/Fe ratios but relatively low incompatible elements. Through the sequence, Mg-number changes from 59 to 67 and LaN/LuN decreases by a factor of 3. This systematic compositional variation is interpreted to be the result of clinopyroxene ± spinel fractionation at pressures of at least 1.4–1.9 GPa, from a primary magma generated by small-degree partial melting in the garnet peridotite stability field (>2.5 GPa). Fractionation occurred where early crystals grew and accumulated along the conduit walls. The rising magma evolved along a polybaric liquid line of descent until it encountered lithosphere cold enough to chill the dike margin. Above this depth, further cooling resulted only in the growth of suspended phenocrysts in a magma separated from the country rock by a chilled margin. This process is observed in the Auckland volcanic field because the rate of magma production is very small allowing compositional features to be preserved that would be overwhelmed in a larger scale magmatic system.