Unravelling the complexity of magma plumbing at Mount St. Helens: a new trace element partitioning scheme for amphibole

Abstract

Volcanoes at subduction zones reside above complex magma plumbing systems, where individual magmatic components may originate and interact at a range of pressures. Because whole rock compositions of subduction zone magmas are the integrated result of processes operating throughout the entire plumbing system, processes such as mixing, homogenisation and magma assembly during shallow storage can overprint the chemical signatures of deeper crustal processes. Whereas melt inclusions provide an effective way to study the uppermost 10-15 km of the plumbing system, challenges remain in understanding magma intrusion, fractionation and hybridisation processes in the middle to lower crust (15-30 km depth), which commonly involves amphibole crystallisation. Here, we present new insights into the mid-crustal plumbing system at Mount St Helens, USA, using multiple regression methods to calculate trace element partition coefficients for amphibole 2 phenocrysts, and thus infer the trace element compositions of their equilibrium melts. The results indicate vertically distributed crystal fractionation, dominated by amphibole at higher pressures and in intermediate melts, and by plagioclase at lower pressures. Variations in Nb, Zr and REE concentrations at intermediate SiO 2 contents suggest repeated scavenging of partially remelted intrusive material in the mid-crust, and mixing with material from geochemically diverse sources. Amphibole is an effective probe for deep crustal magmatism worldwide, and this approach offers a new tool to explore the structure and chemistry of arc magmas, including those forming plutonic or cumulate materials that offer no other constraints on melt composition.

The Role of Crystal Accumulation and Cumulate Remobilization in the Formation of Large Zoned Ignimbrites: Insights From the Aso-4 Caldera-forming Eruption, Kyushu, Japan

“…The compositional variations in Aso-4 amphibole is relatively narrow compared to the compositional spread recognized in other dacitic arc volcanoes (e.g., Mt. St. Helens, USA (Humphreys et al, 2019), Ciomadul, Romania (Kiss et al, 2014); Figure 6A). However, similarly chemically restricted amphibole compositions were found in other large zoned ignimbrites like the Carpenter Ridge Tuff, USA, implying amphibole crystallization in such extensive systems from chemically less variable melts (Bachmann et al, 2014, Figures 6A,E).…”

“…the investigated pumice or scoria samples, but implies the presence of less differentiated recharge melts (available melt inclusion data is given in Supplementary Table S1). Based on this subdivision, mineral-melt equilibrium tests were conducted applying the tests of Putirka (2008Putirka ( , 2016, Zhang et al, (2017), and Humphreys et al, (2019) revealing the following mineralmelt equilibrium assemblages (Supplementary Figure S4):…”

Zircon geochronological and geochemical insights into pluton building and volcanic-hypabyssal-plutonic connections: Oki-Dōzen, Sea of Japan - a complex intraplate alkaline volcano

“…Pressure estimates are limited by available phases. Whereas composition of amphibole has been suggested to be directly related to pressure of crystallization many uncertainties exist, with large variations in modeled pressures depending on how cation distributions are calculated (cf., Humphreys et al, 2019 applying multiple thermometers to the plutonic, volcanic and hypabyssal samples we tested the consistency of the different systems. This placed further constraints on the nature of the system that fed the silicic magmatism of Oki-Dōzen from ~7-5 Ma.…”

Anorthosites in Nishiyama volcanic products from the Hachijo–jima island, Izu–Bonin arc: The direct evidence for ‘plagioclase control’ in shallow magma reservoir

“…Enclaves carried to the surface by volcanic eruption directly provide us with the petrological information on the crust and the upper mantle (e.g., Aoki, 1970Aoki, , 1971Takahashi, 1986). In particular, the petrological record of autolith with the attendant host volcanic product, is useful for revealing the magma processes in magma reservoir beneath a volcanic body (e.g., Yasui, 1997;Klaver et al, 2017;Peters et al, 2017;Humphreys et al, 2019;Aizawa et al, 2019;Yasuda et al, 2019).…”