Mafic glass compositions: a record of magma storage conditions, mixing and ascent

Abstract

The trans-crustal magma system paradigm is forcing us to re-think processes responsible for magma evolution and eruption. A key concept in petrology is the liquid line of descent (LLD), which relates a series of liquids derived from a single parent, and therefore tracks the inverse of the crystallization path. It is common practice to attribute multiple magma compositions, and/or multiple melt compositions (from melt inclusions and matrix glass), to a single LLD. However, growing evidence for rapid, and often syn-eruptive, assembly of multiple magma components (crystals and melts) from different parts of a magmatic system suggests that erupted magma and melt compositions will not necessarily represent a single LLD, but instead may reflect the multiple paths in pressure–temperature space. Here, we use examples from mafic magmatic systems in both ocean island and arc settings to illustrate the range of melt compositions present in erupted samples, and to explore how they are generated, and how they interact. We highlight processes that may be deduced from mafic melt compositions, including the mixing of heterogeneous primitive liquids from the mantle, pre-eruptive magma storage at a range of crustal and sub-Moho depths, and syn-eruptive mixing of melts generated from these storage regions. The relative dominance of these signatures in the glasses depends largely on the water content of the melts. We conclude that preserved melt compositions provide information that is complementary to that recorded by the volatile contents of crystal-hosted melt inclusions and coexisting mineral compositions, which together can be used to address questions about both the pre- and syn-eruptive state of volcanic systems.This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics’.

Architecture and dynamics of magma reservoirs

“…Other volcanoes show some of the same features, suggesting that magma mixing, and perhaps disruption of crystal mushes to entrain their more primitive crystals, is a common process beneath active volcanoes [139]. Icelandic magmas, for example, have olivine-hosted melt inclusions that are related geochemically to the liquid that carried the crystals to the surface [145], reviewed by Maclennan [12].…”

“…In fact, arc magmatic systems preserve clear evidence for extensive mixing and mingling in the form of resorption textures, sieved plagioclase, antecrystic clots and clusters, mafic enclaves and other signs of disequilibrium [132,133,[150][151][152][153][154]. Cashman & Edmonds [139] explore the heterogeneity in major elements of arc melts (glasses) inherent in magma extracted from different pressures in the arc crust, which record different liquid lines of descent owing to the influence of H 2 O on the crystallization path.…”

“…There is now abundant evidence from erupted magmas in all settings for both fractional crystallization and mixing; indeed it appears that mixing is near-ubiquitous [3,138], reviewed by Cashman & Edmonds [139]. Phenocrysts grown during fractional crystallization as grains suspended in the liquid are commonly euhedral and have rims, and perhaps cores, in equilibrium with the carrier liquid.…”

Deep Magma Storage Revealed by Multi-Method Elemental Mapping of Clinopyroxene Megacrysts at Stromboli Volcano

“…The plumbing system feeding Stromboli volcanism has been extensively studied from volcanological, petrological and geochemical perspectives (cf. reviews by Francalanci et al, 2013;Cashman and Edmonds, 2019 and references therein). The feeder system is depicted as a vertically extended mush column that, at present, has an open-conduit configuration feeding the unique, constant eruptive activity at the summit craters (e.g., Métrich et al, 2010).…”

TephraKam: Geochemical database of glass compositions in tephra and welded tuffs from the Kamchatka volcanic arc (NW Pacific)

“…The ratios between incompatible elements do not change as magma fractionates and are instructive to identify source volcano of variably fractionated melts. This information is quite unique in comparison to the systematics of major elements, which, in contrast to incompatible trace elements, is largely related to the conditions of magma storage and also to syneruptive crystallization and magma mixing (Cashman and Edmonds, 2019;Ponomareva et al, 2015a).…”