Primary aqueous fluids in rhyolitic magmas: Melt inclusion evidence for pre- and post-trapping exsolution

Abstract

This study examines melt inclusions containing bubbles of aqueous fluid (L, V, and L + V), occurring in rhyolites from the Okataina Volcanic Centre, New Zealand, and the Rio Blanco Cu-Mo deposit, Chile. We demonstrate that these aqueous fluids coexisted with silicate melts (magmas) and represent either post-trapping exsolution (in the case of Okataina), or co-trapping of phases already coexisting in the magma (in the case of Rio Blanco). Microthermometry proves that some of the bubbles are a single-phase aqueous liquid, and all are shown by PIXE analysis to be metal rich saline solutions. As such, these aqueous fluids provide the closest approximation to direct testing of the proposition that cooling magmas exsolve metal-rich aqueous fluid. In the case of pre-trapping exsolution at Rio Blanco we show that some inclusions record and preserve magmatic emulsions (melt + aqueous fluid) that are the first stage in the evolution of hydrothermal fluids. We demonstrate that heating experiments on volatilerich melt inclusions can produce in-situ exsolution of hypersaline metal-rich aqueous fluid bubbles, potentially permitting magmachamber processes to be experimentally modelled.