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Pyroxene standards for SIMS oxygen isotope analysis and their application to Merapi volcano, Sunda arc, Indonesia
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. Department of Geosciences, Swedish Museum of Natural History, SE-104 05, Stockholm, Sweden.
Department of Geosciences, Swedish Museum of Natural History, SE-104 05, Stockholm, Sweden.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
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2016 (English)In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 447, p. 1-10Article in journal (Refereed) Published
Abstract [en]

Measurement of oxygen isotope ratios in common silicate minerals such as olivine, pyroxene, feldspar, garnet, and quartz is increasingly performed by Secondary Ion Mass Spectrometry (SIMS). However, certain mineral groups exhibit solid solution series, and the large compositional spectrum of these mineral phases will result in matrix effects during SIMS analysis. These matrix effects must be corrected through repeated analysis of compositionally similar standards to ensure accurate results. In order to widen the current applicability of SIMS to solid solution mineral groups in common igneous rocks, we performed SIMS homogeneity tests on new augite (NRM-AG-1) and enstatite (NRM-EN-2) reference materials sourced from Stromboli, Italy and Webster, North Carolina, respectively. Aliquots of the standard minerals were analysed by laser fluorination (LF) to establish their δ18O values. Repeated SIMS measurements were then performed on randomly oriented fragments of the same pyroxene crystals, which yielded a range in δ18O less than ± 0.42 and ± 0.58‰ (2σ) for NRM-AG-1 and NRM-EN-2, respectively. Homogeneity tests verified that NRM-AG-1 and NRM-EN-2 do not show any crystallographic orientation bias and that they are sufficiently homogeneous on the 20 μm scale to be used as routine mineral standards for SIMS δ18O analysis. We subsequently tested our new standard materials on recently erupted pyroxene crystals from Merapi volcano, Indonesia. The δ18O values for Merapi pyroxene obtained by SIMS (n = 204) agree within error with the LF-derived δ18O values for Merapi pyroxene but differ from bulk mineral and whole-rock data obtained by conventional fluorination. The bulk samples are offset to higher δ18O values as a result of incorporation of mineral and glass inclusions that in part reflects crustal contamination processes. The Merapi pyroxene SIMS data, in turn, display a frequency peak at 5.8‰, which allows us to estimate the δ18O value of the primary mafic magma at Merapi to ~ 6.1‰ when assuming closed system differentiation.

Place, publisher, year, edition, pages
2016. Vol. 447, p. 1-10
Keyword [en]
Pyroxene crystals; SIMS standardisation; δ18O analysis; Merapi volcano; Sub-Java primary δ18O
National Category
Geochemistry
Identifiers
URN: urn:nbn:se:uu:diva-309608DOI: 10.1016/j.chemgeo.2016.10.018ISI: 000390632600001OAI: oai:DiVA.org:uu-309608DiVA: diva2:1052281
Funder
Swedish Research CouncilThe Swedish Foundation for International Cooperation in Research and Higher Education (STINT)
Available from: 2016-12-06 Created: 2016-12-06 Last updated: 2017-08-08Bibliographically approved

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Deegan, Frances M.Troll, Valentin R.Budd, David A.Geiger, Harri

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