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

We present new U-238-Th-230-Ra-226-Pb-210-Po-210, Sr-87/(86) Sr and Nd-143/(144) Nd 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 (U-238/(232) Th), (Th-230/Th-232) and ((226)-Ra/ Th-230) activity ratios, with all samples displaying U and Ra excesses. The Ra-226 and Pb-210 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 Po-210 relative to Pb-210 ((Po-210/Pb-210)(i) < 1) at the time of eruption but were variably degassed (69%-100%), with the degree of Pb-210 degassing strongly related to sample texture and eruption phase. In good agreement with several activity monitoring parameters, Po-210 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 (Pb-210/Ra-226) activity ratio within a single eruption at Merapi and are largely characterised by Pb-210 deficits ((Pb-210/Ra-226) < 1). Assuming a model of complete radon degassing, the 210 Pb deficits in the 2006 volcanic rocks indicate relatively longer degassing timescales of similar to 2-4 years than those given by the 2010 samples of similar to 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.