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

February 19, 2019

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.

Publication Year 2019
Title Unravelling the complexity of magma plumbing at Mount St. Helens: A new trace element partitioning scheme for amphibole
DOI 10.1007/s00410-018-1543-5
Authors Madeleine C. S. Humphreys, George F. Cooper, Jing Zhang, Matthew W. Loewen, Adam J. R. Kent, Colin G. Macpherson, Jon P. Davidson
Publication Type Article
Publication Subtype Journal Article
Series Title Contributions to Mineralogy and Petrology
Index ID 70202242
Record Source USGS Publications Warehouse
USGS Organization Volcano Science Center