IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1D-P3

Date/Time: July 24 Poster

Uranium-series timescale constraints on recent changes in the eruptive behaviour ofMerapi Volcano, Java, Indonesia
Heather Handley1 , Ralf Gertisser2 , Mark Reagan3 , Katie Preece4 , Kim Berlo5 , Jenni Barclay4 , Richard
Herd4
1

Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia,
School of Physical and Geographical Sciences, Keele University, Keele, ST5 5BG, UK, 3 Department ofGeoscience, The University of Iowa, Iowa City, IA 52242, USA, 4 School of Environmental Sciences,
University of East Anglia, Norwich, NR4 7TJ, UK, 5 Department of Earth Sciences, University of Oxford,Oxford, OX1 3AN, UK

2E-mail: heather.handley@mq.edu.au

The 2010 eruption Merapi volcano (Central Java, Indonesia) was the volcano’s largest eruption in 140 years
and was much more violent than expected. Prior to 2010, for example in 2006, volcanic activity at Merapi wascharacterised by several months of slow dome growth punctuated by gravitational dome failures, generatingsmall-volume pyroclastic density currents. We present new uranium-series (U-Th-Ra-Pb) isotope data for thevolcanic products of both the 2006 and 2010 eruptions at Merapi to investigate the driving forces behind the recent
unusual explosive behaviour and their timescales. The 2006 and 2010 volcanic rocks display 238 U excess andsuggest variable enrichment by a U-rich fluid or melt (e.g., subducted slab or crustal assimilant) within the last 380
kyr. The 2010 deposits display a slightly wider range in (238 U/232 Th) (0.671 to 0.723) compared to those of 2006
(0.702 to 0.720). A negative correlation is observed between (226 Ra/230 Th) and Th (as index of differentiation).
Assuming that the degree of disequilibria in the sample with the highest (226 Ra/230 Th) (3.28) represents that at theonset of differentiation, 226 Ra excesses implicate short timescales for magmatic differentiation at Merapi of < 750
years for the 2006 and < 300 years for the 2010 volcanic rocks, noting that apart from one 2006 sample, the volcanic
rocks of both eruptions have similar (226 Ra/230 Th) activity ratios. However, it is unlikely that the U-series data at
Merapi can be interpreted within the context of simple closed-system evolution due to the previous recognition of
open-system processes, such as magma recharge, magma mixing and mingling processes and assimilation ofcrustal carbonates. The 2006 and 2010 volcanic rocks have initial (210 Pb) values of 2.66 to 3.04 dpm/g and 2.22 to
3.08 dpm/g and (210 Pb/226 Ra)0 ratios of 0.81 to 0.95 and 0.72 to 0.96, respectively and may suggest persistent andcomplete 222 Rn degassing over a few years to a decade prior to eruption possibly related to the time for magma
ascent from a deeper crustal reservoir. The 2010 volcanic rocks show the largest 210 Pb deficits suggesting longer
timescales of degassing prior to eruption compared to the 2006 eruption. However, volcanic rocks of both eruptions
display relatively similar range in (210 Pb/226 Ra)0 ratios.

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