A History of Andesite Production via Magma Mixing and Mingling Revealed Microscopically at Ngauruhoe Volcano

Abstract

Abstract Many andesites erupted at arc volcanoes are the products of magma mixing, and this process could be important in triggering their eruption. Throughout the life of Ngauruhoe volcano (∼2.5 ka), erupted andesites lack macroscopic enclaves or lithological features of mingled magma. However, intricate microscopic features of magma mixing and mingling occur. Plagioclase phenocrysts have a wide range of textures. Their diverse 87Sr/86Sr values (0.7039–0.7060) are mostly discordant with the groundmass (∼80%), and many phenocrysts (>40%) are isotopically zoned. Resorbed calcic cores overgrown by sodic rims are common, and record a gradient of increasing 87Sr/86Sr, the result of progressive mixing with a radiogenic melt. Clinopyroxenes and orthopyroxenes have relict cores that nucleated in silicic melts (∼Mg# 30–40), and their rims record cycles of growth in mafic melts (∼Mg# 50–60). Olivine‐bearing, mafic glassy blebs (mostly <500 μm) that occur in the groundmass are the relicts of the intruding magma that disintegrated during the mixing/mingling process, and liberated crystals. Thus, some andesites that lack banding or enclaves are in fact the product of thorough mixing of contrasting magmas. This may require a regime of frequent intrusion leading to hybridization with the stagnant resident magma. Thus, frequent eruptions during Ngauruhoe's historic episode ending in 1975 CE are likely to have been a manifestation of frequent replenishment. A mixing origin for intermediate rocks at volcanic arcs is likely to be more prevalent than previously documented: further evidence for the rarity of andesite liquids in such settings.

Keywords

• andesite
• magma
• plagioclase
• Sr isotopes
• subduction
• volcano