Unravelling the complexity of magma plumbing at Mount St. Helens: a new trace element partitioning scheme for amphibole
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 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 SiO2 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.
KeywordsTrace elements Partitioning Amphibole Subduction zones Mount St. Helens Magma plumbing
Jing Zhang was supported by a Durham Doctoral Studentship (Durham University), China Scholarship Council (201206170178) and National Natural Science Foundation of China (41702362). Madeleine Humphreys was supported by a Royal Society University Research Fellowship (UF140711). AJRK and ML were supported by National Science Foundation Grants 1425491 and 1028707. GFC was supported by a NERC Grant (NE/K010824/1). We thank Mark Allen and Ed Llewellin for helpful comments on an earlier version of the manuscript, and John Pallister and Mike Clynne for additional internal reviews that significantly improved the text. We acknowledge Mike Krawczynski and an anonymous reviewer for helpful reviews. We greatly appreciate helpful discussions with Maren Wanke as well as early access to her whole-rock geochemical dataset. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
JZ, MCSH and AJRK conceived the project. JZ and GFC performed the multiple regression and ML conducted the major and trace element analysis. MCSH and GFC drafted the text and all authors contributed to discussion of the data and writing of the manuscript.
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