5/17/2023 0 Comments Jmicrovision 2.0Studies have suggested that magmas derived from low buoyancy flux plumes such as the Canary Islands ( Hoernle & Schmincke, 1993) are dominated by lower crustal storage regions with long repose periods and residence times of thousands of years at near-solidus conditions ( Longpré et al., 2008 Longpré et al., 2014 Klügel et al., 2015 Mutch et al., 2019a Gleeson et al., 2020). These plutonic components have advanced our understanding of melt differentiation and storage in long-lived mush reservoirs ( Solano et al., 2012 Cooper et al., 2016 Melekhova et al., 2017 Jackson et al., 2018 Klaver et al., 2018 Yanagida et al., 2018 Mutch et al., 2019b Gleeson et al., 2020). Examination of plutonic material in the form of cumulate nodules, enclaves and xenoliths has been key to constraining the spatial extent and compositional characteristics of these mush systems, providing textural and geochemical evidence for high degrees of chemical heterogeneity, open system processes and multiple melt source depths ( Bachmann & Bergantz, 2008 Cooper et al., 2016 Jackson et al., 2018 Holness et al., 2019 Stock et al., 2020). However, the Fasnia eruption simultaneously fragmented and removed material from both reservoirs, implying the mafic system was subjacent to the felsic, but they did not form a contiguous body.ĭetailed petrological, geochemical and geophysical studies of individual magmatic systems have shown that long-lived storage regions are dominantly composed of ‘mushes’, frameworks of crystals and domains of melt-rich pockets or sills ( Wager et al., 1960 Cashman et al., 2017 Edmonds et al., 2019 Sparks et al., 2019). ![]() This observation implies that the mafic–felsic cumulate mush and the phonolite did not experience significant two-way mixing and existed as separate crustal reservoirs. Nevertheless, all melts are of different basanite-intermediate composition to the juvenile phonolitic pumice ejected during the same eruption. Relative homogeneity of melt compositions through the mafic and felsic lithologies testifies to melt mobility through the cumulates. In addition, the cumulus phases record complex interactions between felsic and mafic magmas throughout their development, providing evidence for mush remobilization and disequilibrium. ![]() Their textures record a range of crystallization environments, including both crystal- and melt-rich groundmass domains, and invasion of near-solidus domains by ascending reactive melts. These cumulates span a diverse range of alkaline plutonic lithologies, from wehrlite and pyroxenite, through hornblende gabbros, to monzodiorite and syenite. We have analysed >100 of the mush nodules from the massive lithic breccia facies within the Fasnia Member of the Diego Hernández Formation. As such, the nodules preserve a record of the proportions and relationships between the crystal framework and pre-eruptive melt in an active magma mush reservoir, importantly, capturing a snapshot of the sub-volcanic system at a single point in time. ![]() We find no surficial or intrinsic evidence that the nodules were transported from their reservoir in a ‘carrier’ magma, and it is most likely that the mush was in situ when it was explosively fragmented and ejected during eruption. Both the microcrystalline groundmass and crystal framework are generally unaltered as this crystal ‘mush’ remained supra-solidus until the eruption. Nodules contain an average of 26% melt that is preserved as vesiculated and microcrystalline basanite in segregations, pathways and interstitial domains. This eruption ejected juvenile pyroclasts of melt-bearing, partially crystalline cumulate nodules alongside phonolitic pumice and accidental lithic clasts. To examine these relationships, we investigated material from one of the largest caldera-forming explosive eruptions on the ocean island of Tenerife, the 312-ka Fasnia event. Deciphering the dynamics of sub-volcanic magmatic processes requires a detailed understanding of the compositional and textural relationships between melt and crystals.
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