Timescales of magma ascent and degassing and the role of crustal assimilation at Merapi volcano (2006–2010), Indonesia: Constraints from uranium-series and radiogenic isotopic compositions

H.K Handley; M Reagan; R Gertisser; K Preece; K Berlo; L.E McGee; J Barclay; R Herd

doi:10.1016/j.gca.2017.10.015

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Timescales of magma ascent and degassing and the role of crustal assimilation at Merapi volcano (2006–2010), Indonesia: Constraints from uranium-series and radiogenic isotopic compositions

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Timescales of magma ascent and degassing and the role of crustal assimilation at Merapi volcano (2006–2010), Indonesia: Constraints from uranium-series and radiogenic isotopic compositions

H.K Handley, M Reagan, R Gertisser, K Preece, K Berlo, L.E McGee, J Barclay and R Herd

Geochimica et cosmochimica acta, Vol.222, pp.34-52

02/01/2018

DOI: 10.1016/j.gca.2017.10.015

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Abstract

We present new 238U-230Th-226Ra-210Pb-210Po, 87Sr/86Sr and 143Nd/144Nd isotopic data of whole-rock samples and plagioclase separates from volcanic deposits of the 2006 and 2010 eruptions at Merapi volcano, Java, Indonesia. These data are combined with available eruption monitoring, petrographic, mineralogical and Pb isotopic data to assess current theories on the cause of a recent transition from effusive dome-building (2006) to explosive (2010) activity at the volcano, as well as to further investigate the petrogenetic components involved in magma genesis and evolution. Despite the significant difference in eruption style, the 2006 and 2010 volcanic rocks show no significant difference in (238U/232Th), (230Th/232Th) and (226Ra/230Th) activity ratios, with all samples displaying U and Ra excesses. The 226Ra and 210Pb excesses observed in plagioclase separates from the 2006 and 2010 eruptions indicate that a proportion of the plagioclase grew within the decades preceding eruption. The 2006 and 2010 samples were depleted in 210Po relative to 210Pb ((210Po/210Pb)i < 1) at the time of eruption but were variably degassed (69%–100%), with the degree of 210Pb degassing strongly related to sample texture and eruption phase. In good agreement with several activity monitoring parameters, 210Po ingrowth calculations suggest that initial intrusion into the shallow magma plumbing system occurred several weeks to a few months prior to the initial 2010 eruption. The 2006 and 2010 samples show a wide range in (210Pb/226Ra) activity ratio within a single eruption at Merapi and are largely characterised by 210Pb deficits ((210Pb/226Ra) < 1). Assuming a model of complete radon degassing, the 210Pb deficits in the 2006 volcanic rocks indicate relatively longer degassing timescales of ∼2–4 years than those given by the 2010 samples of ∼0–3 years. The uranium-series and radiogenic isotopic data do not support greater crustal assimilation of carbonate material as the explanation for the more explosive behaviour of Merapi in 2010 (as has been previously suggested) and instead indicate that relatively rapid ascent of a more undegassed magma was the primary difference responsible for the transition in explosive behaviour. This interpretation is in good agreement with gas monitoring data, previous petrological studies (mineral, microlite and melt inclusion work) and maximum calculated timescale estimates using Fe-Mg compositional gradients in clinopyroxene, that also suggest more rapid movement of relatively undegassed magma in 2010 relative to 2006.

Sr-Nd-Pb isotopes

Subduction zone

Explosive

Differentiation

Petrogenesis

238U-230Th-226Ra-210Pb-210Po

Details

Title: Subtitle: Timescales of magma ascent and degassing and the role of crustal assimilation at Merapi volcano (2006–2010), Indonesia: Constraints from uranium-series and radiogenic isotopic compositions
Creators: H.K Handley - Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
M Reagan - Department of Earth and Environmental Sciences, The University of Iowa, Iowa City, IA 52242, USA
R Gertisser - School of Geography, Geology and the Environment, Keele University, Keele ST5 5BG, UK
K Preece - School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
K Berlo - Department of Earth and Planetary Sciences, McGill University, Montreal H3A 0E8, Canada
L.E McGee - Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
J Barclay - School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
R Herd - School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
Resource Type: Journal article
Publication Details: Geochimica et cosmochimica acta, Vol.222, pp.34-52
DOI: 10.1016/j.gca.2017.10.015
ISSN: 0016-7037
eISSN: 1872-9533
Publisher: Elsevier Ltd
Grant note: DOI: 10.13039/501100000923, name: Australian Research Council, award: FT120100440; DOI: 10.13039/501100000270, name: Natural Environment Research Council, award: NE/I029927/1; name: DEST Systemic Infrastructure Grants; DOI: 10.13039/501100000923, name: ARC LIEF; name: NCRIS; DOI: 10.13039/501100001230, name: Macquarie University
Language: English
Date published: 02/01/2018
Academic Unit: Earth and Environmental Sciences
Record Identifier: 9983983645902771

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