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Amphibole megacrysts as a probe into the deep plumbing system of Merapi volcano, Central Java, Indonesia
Geowissenschaftliches Zentrum Georg August Univ t, Abt Isotopengeol, Goldschmidtstr 1, D-37077 Gottingen, Germany.;Vrije Univ Amsterdam, Fac Aard Levenswetenschappen, Boelelaan 1085, NL-1081 HV Amsterdam, Netherlands..
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
Swedish Museum Nat Hist, Dept Geosci, Stockholm, Sweden..
CNR, Ist Geosci & Georisorse, Via Moruzzi 1, I-56124 Pisa, Italy..
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2017 (English)In: Contributions to Mineralogy and Petrology, ISSN 0010-7999, E-ISSN 1432-0967, Vol. 172, no 4, 16Article in journal (Refereed) Published
Abstract [en]

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. Amphibolebased 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 fO(2) 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 (H-2 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.

Place, publisher, year, edition, pages
SPRINGER , 2017. Vol. 172, no 4, 16
Keyword [en]
Magmatic differentiation, Rare-earth elements, Arc magmas, Barometry, Dehydrogenation, Dehydration
National Category
Geophysics Geochemistry
Identifiers
URN: urn:nbn:se:uu:diva-321335DOI: 10.1007/s00410-017-1338-0ISI: 000397978000002OAI: oai:DiVA.org:uu-321335DiVA: diva2:1103906
Funder
Swedish Research Council
Available from: 2017-05-31 Created: 2017-05-31 Last updated: 2017-05-31Bibliographically approved

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