Elemental Concentrations and 210Pb Activity of a Growth-zoned Gypsum Stalactite generate Historical Volcanic Activity Record at Kawah Ijen’s Crater Lake, Indonesia
Elemental Concentrations and 210Pb Activity of a Growth-zoned Gypsum Stalactite generate Historical Volcanic Activity Record at Kawah Ijen’s Crater Lake, Indonesia
Previously Published Material: Up to 15% of the total work, such as initial ideas and interpretations on a limited number of elemental concentrations and 210Pb activity, have been reported in an oral presentation at the Cities on Volcanoes 8 Conference in Yogyakarta, Indonesia on 9 September 2014.
Abstract ID#: 34217
English Abstract:
The active Kawah Ijen stratovolcano in Indonesia hosts the world’s largest hyper-acidic crater lake, with 32 million m3 of hot SO4-HCl brine [1]. The volcano has produced phreatic and phreatomagmatic eruptions, tephra, and lahars in the past [2]. This is exemplified by its 1817 eruption, which resulted in expulsion of the entire lake [3]. At the time, the area was scarcely populated, and the overall impact from the volcano was low. As population density around the volcano has raised dramatically, the risk to nearby inhabitants have also risen markedly. Hence, Kawah Ijen requires monitoring, and monitoring depends on having a continuous record of background activity to identify signals of impending eruptions. Unfortunately, there is currently a severely limited record of volcanic activity of Kawah Ijen available. Gypsum grows continuously on Kawah Ijen’s western flanks [4], where it precipitates from crater lake seepage springs as stalactites in chemical equilibrium with its fluids. Given that the brine receives direct elemental input from the degassing magma, including elements such as Tl, As and Sb [4,5], we posit that gypsum can be used to reconstruct a continuous geochemical record of volcanic activity for Kawah Ijen. Trace elements were analysed in a cross-section of a growth-zoned stalactite, as was 210Pb activity. Initial 210Pb activity varies with that of its parent’s, 222Rn, flux from the degassing magma. When combining the timing information from growth zoning with concentrations and 210Pb activity, we can produce a continuous geochemical record of activity over time. Sharp increases in volatile element ratios above baseline concentrations likely correspond to significant elemental input from the degassing magma, and hence increased activity. The record of δ18O disequilibrium between the crystalline H2O and SO4[6] is shown to be preserved in gypsum, and thus can be used to track changes in the temperature of and flux through the hydrothermal system. Knowledge of background concentrations and how activity affects these, allows us to pinpoint geochemical signals of impending eruptions and aid the monitoring efforts at Kawah Ijen volcano.
[1] Delmelle and Bernard, 2004
[2] Mulyana et al., 2006
[3] Kemmerling, 1921
[4] van Hinsberg, 2009
[5] van Hinsberg et al., 2010
[6] Chiba and Sakai, 1985.
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