Amphibole megacrysts as a probe into the deep plumbing system of Merapi volcano, Central Java, Indonesia

S.T.M. Peters, V.R. Troll, F.A. Weis, L. Dallai, J.P. Chadwick, B. Schulz

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

Amphibole has been discussed to potentially represent an important phase during early chemical evolution of arc magmas, but is not commonly observed in eruptive arc rocks. Here, we present an in-depth study of metastable calcic amphibole megacrysts in basaltic andesites of Merapi volcano, Indonesia. Radiogenic Sr and Nd isotope compositions of the amphibole megacrysts overlap with the host rock range, indicating that they represent antecrysts to the host magmas rather than xenocrysts. Amphibole-based barometry suggests that the megacrysts crystallised at pressures of >500 MPa, i.e., in the mid- to lower crust beneath Merapi. Rare-earth element concentrations, in turn, require the absence of magmatic garnet in the Merapi feeding system and, therefore, place an uppermost limit for the pressure of amphibole crystallisation at ca. 800 MPa. The host magmas of the megacrysts seem to have fractionated significant amounts of amphibole and/or clinopyroxene, because of their low Dy/Yb ratios relative to the estimated compositions of the parent magmas to the megacrysts. The megacrysts’ parent magmas at depth may thus have evolved by amphibole fractionation, in line with apparently coupled variations of trace element ratios in the megacrysts, such as e.g., decreasing Zr/Hf with Dy/Yb. Moreover, the Th/U ratios of the amphibole megacrysts decrease with increasing Dy/Yb and are lower than Th/U ratios in the basaltic andesite host rocks. Uranium in the megacrysts’ parent magmas, therefore, may have occurred predominantly in the tetravalent state, suggesting that magmatic fO2 in the Merapi plumbing system increased from below the FMQ buffer in the mid-to-lower crust to 0.6–2.2 log units above it in the near surface environment. In addition, some of the amphibole megacrysts experienced dehydrogenation (H2 loss) and/or dehydration (H2O loss), as recorded by their variable H2O contents and D/H and Fe3+/Fe2+ ratios, and the release of these volatile species into the shallow plumbing system may facilitate Merapi’s often erratic eruptive behaviour.

Original languageEnglish
Article number16
JournalContributions to Mineralogy and Petrology
Volume172
Issue number16
DOIs
Publication statusPublished - 2017

Funding

We are grateful for the insightful comments by Othmar Müntener, Jon Davidson, Szabolcs Harangi, and two anonymous reviewers that helped to significantly improve the manuscript. This work furthermore benefited from discussions with John Hora, Pieter Vroon, Gareth Davies, Hans Annersten, Frances Deegan, Carsten Münker, and Sophie Omidian. We thank Henrik Skogby for his help with the IR and Mössbauer analysis. Support by the petrology department of VU University Amsterdam to S.P., the department of applied mineralogy at Erlangen to B.S., and the Swedish Science Foundation to V.R.T. (field sampling, sample preparation and D analyses) are gratefully acknowledged. In addition, S.P. and V.R.T. acknowledge financial support by the “U4” collaboration network of the German Academic Exchange Service (DAAD), in which Göttingen and Uppsala universities are active partners.

FundersFunder number
Swedish Science Foundation
Deutscher Akademischer Austauschdienst

    Keywords

    • Arc magmas
    • Barometry
    • Dehydration
    • Dehydrogenation
    • Magmatic differentiation
    • Rare-earth elements

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