Volatile and Trace Element Partitioning for a Peralkaline Rhyolite Volcano: Mayor Island, New Zealand

Abstract

Mayor Island, New Zealand is a small isolated occurrence of a peralkaline rhyolite volcano 60km away from the calc-alkaline Taupo Volcanic Zone. During the volcano's 130,000 year sub-aerial history the erupted magma increases in peralkalinity, with the Agpaitic Index (AI) rising from 1 in the very oldest lavas to ~ 1.25 in the most recent eruptions, straddling the compositional divide between comendite and pantellerite. In addition to this an ~40% enrichment of REE's and certain trace elements (e.g. Zr,Y,Nb) is observed. Phenocryst phases present include alkali feldspar, quartz, aenigmatite, Na-hedenbergite, aegerine and fayalitic olivine. With the slight exception of the feldspar which has a slight bimodality in Na/K ratio the major element compositions of these phases remains stable throughout the evolution of the magma. Analysis of melt inclusions, matrix glass and phenocryst phases has allowed an estimation of (i) volatile element partitioning between melt and vapour and, (ii) non-volatile trace element partitioning between phenocrysts and melt. These data allow an assessment of the role that the phenocrysts have played in the evolving magma composition through fractional crystaUisation and they give insight into the effect of degassing on magma chemistry. Trace elements were measured using the secondary ion mass spectrometer (ion microprobe) analysis and the volatile components of both inclusion and matrix glass utilised FTIR (H20), electron microprobe (C1) and ion microprobe (H,F) techniques.