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Identifying the ingredients of hydrous arc magmas : insights from Mt Lamington, Papua New Guinea.

Humphreys, M.C.S. and Zhang, J. and Cooper, G.F. and Macpherson, C.G. and Ottley, C.J. (2019) 'Identifying the ingredients of hydrous arc magmas : insights from Mt Lamington, Papua New Guinea.', Philosophical transactions of the Royal Society A : mathematical, physical and engineering sciences., 377 (2139). p. 20180018.


Volcanism is the surface expression of magma intrusion, crystallization, assimilation and hybridization processes operating throughout the crust over a range of time periods. Many magmas, including those erupted at subduction zones, have complex textures that reflect these processes. Here, we use textural and geochemical characteristics of calcic amphiboles to help identify multiple ingredients of subduction zone magmatism at Mt Lamington volcano, Papua New Guinea. Our approach uses existing trace element partitioning schemes to calculate the compositions of amphibole equilibrium melts (AEMs). The AEM compositions show that Mt Lamington andesites and plutonic enclaves are dominated by fractionation of amphibole + plagioclase + biotite, with assimilation of plagioclase and zircon. Magnesiohastingsite crystals in the andesite and diktytaxitic mafic enclaves reflect multiple episodes of recharge by more primitive, geochemically variable melts. The andesite also contains clots with rounded grains and melt on grain boundaries. These features indicate slow crystallization, and the retention of melt films could significantly enhance the potential for remobilization of crystals by infiltrating melts or during magma mixing. Variations in crystallization conditions could thus significantly affect the mush microstructure. We suggest that this could result in a significant bias of the volcanic record towards the preferential incorporation of more slowly cooled plutonic material from the lower crust or from more thermally mature plumbing systems.

Item Type:Article
Full text:(AM) Accepted Manuscript
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Date accepted:30 July 2018
Date deposited:09 August 2018
Date of first online publication:07 January 2019
Date first made open access:09 August 2018

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