Abstract
Constraining the pre-eruptive processes that modulate the chemical evolution of erupted magmas is a challenge. An opportunity to investigate this issue is offered by the interrogation of the crystals carried in lavas. Here, we employ clinopyroxene crystals from back-arc lavas in order to identify the processes driving basalt to andesite magma evolution within a transcrustal plumbing system. The assembled clinopyroxene archive reveals that mantle melts injected at the crust-mantle transition cool and crystalize, generating a clinopyroxene-dominated mush capped by a melt-rich domain. Magma extracted from this deep storage zone fed the eruption of basalt to basaltic andesite lavas. In addition, chemically evolved melts rapidly rising from this zone briefly stalled at shallow crustal levels, sourcing crystal-poor andesite lavas. Over time, hot ascending primitive magmas intercepted and mixed with shallower cooling magma bodies forming hybrid basic lavas. The blended clinopyroxene cargoes of these lavas provide evidence for the hybridization, which is undetectable from a whole-rock chemical perspective, as mixing involved chemically similar basic magmas. The heterogeneity we found within the clinopyroxene archive is unusual since it provides, for the first time, a complete set of mush-related scenarios by which mantle melts evolve from basalt to andesite compositions. Neither the whole-rock chemistry alone nor the record of the mineral phases crystallizing subsequent to clinopyroxene can provide insights on such early magmatic processes. The obtained clinopyroxene archive can be used as a template for interpretation of the record preserved in the clinopyroxene cargoes of basalt to andesite lavas elsewhere, giving insights into the magma dynamics of the feeding plumbing system that are lost when using whole-rock chemistry.
Highlights
The definition of the crustal processes by which mantle-derived melts evolve in arc/back-arc volcanoes is important in terms of magma genesis [1,2,3] and volcanic system architecture [4] and has implications for continental crust formation [5]
Marsili Volcano (MV) pyroxene crystals range from diopside to augite clinopyroxene; few crystals classify as enstatite orthopyroxene; see Supplementary Materials (Figure S6)
Evidence is found for two subgroups of clinopyroxene cargoes: one recording melt evolution by crystallization stages in a mush setting and the other representing the products of magma mixing associated with the injection of primitive magma into pre-existing storage zones
Summary
The definition of the crustal processes by which mantle-derived melts evolve in arc/back-arc volcanoes is important in terms of magma genesis [1,2,3] and volcanic system architecture [4] and has implications for continental crust formation [5]. A limitation of these studies, though, is the inability to fully capture the magmatic dynamics that lead to crystals with disequilibrium textures and compositions frequently found in these lava suites [8,11,12,13,14,15] They provide strong evidence that both closed- and open-system processes govern the chemical evolution of erupted magmas. The lavas have a back-arc/arc geochemical affinity [17] that is observed in the Marsili Basin oceanic-type crust [26] This implies that crustal influence in the genesis of the erupted MV lavas is circumscribed to interactions with co-genetic igneous components. The minerals crystallized from MV magmas are ideal for investigating the processing that modulates the basalt to andesite chemical evolution of magmas in a back-arc setting
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