Rapid assembly of high-Mg andesites and dacites by magma mixing at a continental arc stratovolcano

Chris E Conway,Daniel J Morgan,Colin J.N Wilson,Yumiko Harigane,Katy J Chamberlain

doi:10.1130/g47614.1

Abstract

Abstract Studies of pre-eruptive processes at active volcanoes require precise petrochronological constraints if they are to contribute to hazard assessment during future eruption events. We present petrological and geochemical data and orthopyroxene diffusion time scales for samples from Late Pleistocene high-Mg andesite-dacite lavas (Mg# 53–69) at Ruapehu volcano, New Zealand, as a case study of rapid magma genesis and eruption at a continental arc stratovolcano. Assembly of Ruapehu high-Mg magmas involved the mixing of primitive magmas plus entrained mantle-equilibrated olivines with mid-crustal felsic mush bodies, yielding hybridized magmas with ubiquitous pyroxene reverse-zoning patterns. Orthopyroxene Fe-Mg interdiffusion time scales linked to quantitative crystal orientation data show that most lavas erupted &lt;10 days after resumption of crystal growth following magma mixing events. The eruption of lavas within days of mixing events implies that pre-eruptive warnings may be correspondingly short.

Highlights

Arc stratovolcanoes are active foci for research on the generation of intermediate-composition magmas and their frequent, commonly hazardous eruptions
While some examples of high-Mg andesite and dacite (HMAD) volcanism are attributed to a primary mantle origin (e.g., Bryant et al, 2011), petrographic studies highlight the role of mafic-felsic magma mixing accompanied by olivine accumulation in disproportionately raising the bulk-rock Mg abundance in other cases (e.g., Streck et al, 2007)
We present mineralogical characteristics and orthopyroxene Fe-Mg interdiffusion time scales for a suite of HMAD lavas from Ruapehu volcano, New Zealand, a large andesite-dacite stratovolcano of the Taupo Volcanic Zone with a 200 k.y. history of effusive eruptions (Fig. 1; Conway et al, 2016)

Summary

Introduction

Arc stratovolcanoes are active foci for research on the generation of intermediate-composition magmas and their frequent, commonly hazardous eruptions. We present mineralogical characteristics and orthopyroxene Fe-Mg interdiffusion time scales for a suite of HMAD lavas from Ruapehu volcano, New Zealand, a large andesite-dacite stratovolcano of the Taupo Volcanic Zone with a 200 k.y. history of effusive eruptions (Fig. 1; Conway et al, 2016). Reverse-zoned orthopyroxene and clinopyroxene are ubiquitous in Ruapehu HMAD lavas

Results

Conclusion