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.
Plain Language Summary
Andesite is one of the most common igneous rocks and many iconic volcanoes found at subduction zones are constructed from andesite lava. There are several theories on the origin of andesite magma (molten liquid with crystals and gases). However, well‐documented historic eruptions have highlighted the importance of mixing a low silica magma from the mantle or lower crust with a higher silica magma from the crust. Indeed, some of these eruptions appear to have been triggered this way. This raises the question whether andesite liquids (as opposed to magmas) are common. We examined andesites erupted from Ngauruhoe volcano. They lack features of mingled rocks that would be visible to the unaided eye. However, microscopically, intricate chemical and isotopic features of mixing of liquids during ascent through the crust and scavenging of crystals from deep and shallow sources can be seen. These processes may be more common than previously documented. Cryptic mixing of magmas is probably the result of thorough homogenization of the magma reservoir beneath volcanoes by the frequent input of new magmas.
Key Points
Andesites lack enclaves but microscopically record an origin via thorough mixing of contrasting magmas
Sr isotopic contrasts reveal crystal growth during liquid mixing and various cognate and non‐cognate sources for phenocrysts
Magma mixing is likely to be more prevalent at arcs than previously documented: Further evidence for the rarity of andesite liquids