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Origin and significance of mobile trace elements in volcanic lakes – a case study at Poás, Costa Rica

By M.J. Keizer


Many active volcanoes host a crater lake that receives input from magmatic-hydrothermal fluids. The chemical properties of such a lake are to a large extent determined by the injection of these fluids. The chemistry of the water can therefore provide insight in the transport of chemical elements from the sources to the surface. Monitoring the changing conditions can be an important tool in assessing the dynamic changes in subsurface parts of the system. Interpretations of lake compositions are not straightforward and rely on detailed insights into the processes involved in controlling the presence and concentration of the various chemical species involved. This study focuses on a group of trace elements that are known to be highly mobile in volcanic environments, but have rarely been investigated as yet. Based on available analytical data, it uses the active crater lake of Poás volcano in Costa Rica as an example to explore the behavior of B, Zn, As, Mo, Sn, Sb, Tl and Pb, and to identify possible sources and sinks in volcanic lake systems. Each of these elements show enrichment relative to magnesium, arguing against congruent rock dissolution, but consistent with input through subaqueous fumarolic gas. Time-series trends indicate increased concentrations when the lake system was most active. There is a general correlation between concentration levels and lake temperature, and an inverse correlation with pH. This points to an enhanced input of the elements during intervals of increased volcanic activity. Additionally, during periods of lower volcanic activity, when conduits below the lake were presumably blocked, lower concentration levels were found. Still, this group of very mobile elements remains relatively more enriched compared to elements that predominantly partition into brine water, suggesting that in these low-activity intervals gas/vapor transport was less affected by reduced permeability than the input of hydrous fluid. Though the time-series trends for the investigated elements are roughly similar, they are not identical in detail, which points to differences in behavior and in the controlling processes. Similar trends for lead and tin indicate that the input of this element pair is controlled by the same process, possibly transport by supercritical water in the form of trichloride complexes. Deviating behavior in As, Mo and Sb can be explained by a sudden event of sulfide precipitation, in response to increasing temperatures at depth. The results of this study demonstrate the potential of monitoring this group of mobile elements in crater lakes, and associated surface manifestations. They provide insights into he nature and conditions of processes in magmatic-hydrothermal system that can be different and complementary to those delivered by the more traditional group of monitored elements

Topics: Volcanic lakes, Magmatic-hydrothermal system, Poás, Costa Rica, Trace elements, Volcanic enrichment, Sulfides
Year: 2015
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