The inception and progression of melting in a monogenetic eruption: Motukorea Volcano, the Auckland Volcanic Field, New Zealand

Abstract

Compositional variation through basaltic monogenetic eruptive sequences provides a unique view into the processes and source heterogeneity of small-scale magmatic systems. A well-exposed, continuous sequence on Motukorea volcano in the Auckland Volcanic Field, New Zealand, consists of an early tuff ring, scoriaceous deposits and late lava flows which allow the evolution of the eruption to be studied at very high resolution. The deposits show a spectrum of basaltic compositions from Mg# 60 nephelinite (early tuff ring) to Mg# 70 alkalic basalt (lava). Within the deposits of each main eruptive phase (i.e. tuff, scoria and lava) very little variation is observed in major element chemistry, suggesting that fractional crystallisation has a limited effect. Systematic changes in trace element chemistry, however, are significant through the sequence. The major and trace element features observed through the sequence are inferred to be primarily due to the mixing of two magma batches, with a two-fold increase in the degree of melting between these. Variation in Pb-isotopic compositions up-sequence indicates subtle changes in mantle source with samples representing the start of the eruption displaying higher 207Pb/204Pb than the latter parts of the eruption. This chemical change coincides with a switch in the mode of eruption, with the arrival at the surface of magmas produced by larger degrees of partial melting resulting in the beginning of a more effusive eruption phase. The silica-undersaturated, high total alkali, low Al2O3 and higher 207Pb/204Pb nature of the samples from the tuff units suggests that these samples were produced by melting of relatively young eclogite domains. The lower 207Pb/204Pb, higher silica, lower total alkali nature of the samples from the scoria and lava reflects the exhaustion of these domains and the resultant melting of the surrounding garnet-peridotite matrix. This detailed study shows that the petrogenesis of small volcanic centres may be far more complex than their physical volcanology suggests.