Origin of temporal - compositional variations during the eruption of Lake Purrumbete Maar, Newer Volcanics Province, southeastern Australia

Abstract

The Pleistocene Lake Purrumbete maar eruption sequence of the Newer Volcanics Province of southeastern Australia records stratigraphic, temporal and geochemical variations that cannot be explained by crustal processes, such as fractional crystallisation and crustal contamination. Samples from the Lake Purrumbete maar have distinct trace element trends that correlate with stratigraphic height, with decreasing incompatible element concentrations from the bottom of the sequence to the top, indicating that the most enriched melts were ejected during the earliest eruption stages, with the least enriched melts erupted in the final stages. Variations in rare earth element ratios indicate that these melts were formed by dynamic melting of garnet lherzolite mantle material, consistent with other volcanic centres within the basaltic cones subprovince of the Newer Volcanics Province. The geochemistry of the erupted units at Lake Purrumbete also record minor differentiation of the melts by olivine fractionation, although these processes alone cannot explain the observed geochemical variations in the eruption sequence. Further differentiation of the melts by different processes such as deep clinopyroxene fractionation or assimilation of metasomatised lithosphere may be responsible for the observed geochemical trends at Lake Purrumbete. This shows the complexity of the plumbing system of monogenetic volcanoes, but also shows the opportunities that these volcanoes provide to study these processes in detail, as the geochemical variations that are preserved in this type of volcanism by fast magma ascent through the crust, as evidenced by the presence of fresh mantle xenoliths, predominantly reflect processes that occurred in the mantle rather than in the crust.