Noble gases as tracers of volatile and metal fluxes in magmatic-hydrothermal systems and applications to volcano hazard monitoring and geothermal energy production

Projet collaboratif du Geotop

2018-2020

The historical record of volcanic systems is written in their geological deposits. These deposits are almost exclusively confined to eruptive episodes and, therefore, little to no information is usually available for the periods of quiescence that intersperse these eruptions. This is a major problem that severely limits the effectiveness of volcano hazard monitoring and forecasting efforts, especially for volcanoes that have not erupted in modern times, because these lack a historical record against which present-day monitoring data can be compared. This precludes proper assessment of the background variability in the system, thereby making identification of abnormal events, and evaluation of their magnitude, difficult. Moreover, a broad monitoring approach including many variables may be necessary, because the compositional or physical signature of a disturbance in the volcanic system may not be known. Recently, we have suggested that minerals precipitated from emissions of a magmatic-hydrothermal system can be used to provide the missing volcanic record for periods of quiescence. In particular, we showed that growth-zoned gypsum formed from hyperacid effluent of the crater lake of Kawah Ijen volcano in Java, Indonesia, provides a near continuous 80-year record of variations in lake chemistry during quiescence, which could be tied to changes in the underlying magmatic-hydrothermal system. Similar gypsum precipitates are known from other volcanic systems, including Poás, Costa Rica and Ruapehu, New Zealand, and the full breadth of precipitate minerals from aqueous volcanic emissions ranges from silicates, to sulfates, to oxides, to carbonates. In addition to these surface precipitates, an abundance of minerals form in the subsurface magmatic hydrothermal system, in particular in veins. These are of interest to further the understanding of both magmatic systems and the ore deposits that they host. Volcanogenic mineral precipitates, both at the surface and at depth, can provide information on the physical and chemical state of a magmatic system, as well as variations therein over time. However, this information is mostly qualitative at present, because minerals are only a proxy for the fluid from which they formed. To translate mineral composition to element fluxes requires: 1) precipitate age information, and 2) mineral-fluid element partition coefficients. Of these, the latter is more readily addressed. Although mineral-fluid partition coefficient data are still very limited, these can be determined experimentally . The major challenge in age dating of these volcanogenic mineral precipitates is the combination of a young age, commonly small differences in age between growth zones, and small amounts of material. This precludes dating by conventional methods. The most promising method appears to be dating using short-lived nuclides of the U-series decay chain, in particular 210Pb. As a daughter isotope of 222Rn, a noble gas efficiently mobilized during magmatic degassing, 210Pb is present at elevated levels in magmatic-hydrothermal fluids. With a half-life of 22.6 years, ages from approximately 2 to 80 years can be accessed. However, use of 210Pb activities to determine age requires knowledge of the radon flux, as well as the content of 226Ra, which as a parent to radon, can lead to ingrowth of 210Pb after deposition. Activities of 226Ra can be determined, but historical data on radon flux from volcanoes are virtually non-existent, thereby producing a seemingly insurmountable obstacle. The incentive for this study is the recent realization that concentrations of other noble gases such as krypton and xenon in the gypsum growth zones being dated can potentially be used as a proxy for radon, thus allowing for fluctuations in the radon flux to be corrected for and correct 210Pb ages to be derived. This is the first time that this relationship between Rn-flux and concentrations of the heavier noble gases Kr and Xe has been shown and its subsequent application to 210Pb age-correction is highly novel. This approach holds 3 great potential to open new avenues in dating young precipitates in hydrothermal, geothermal and other fluid environments. In this study we aim to use growth-zoned gypsum and carbonate precipitates to reconstruct a historical record of element concentrations in volcanic crater lakes and basalt-hosted geothermal systems, respectively. This study is expected to produce a fully quantitative time-series of fluid compositions for volcanic crater lakes and geothermal systems. This information will allow for assessing the compositional variability in crater lakes during quiescence and leading up to eruptions, information that is critical for meaningful monitoring of these systems. It also allows for metal fluxes to be quantified, which, in geothermal systems, has direct implications for fluid processing. Moreover, the noble gas correction method that will be developed here is applicable to any environment where the assumption of a constant Rn-flux is untenable, and will greatly enhance the applicability of 210Pb dating. This research addresses aspects of all three GEOTOP research axes. We reconstruct a historical record of fluid composition and element fluxes from progressively deposited mineral horizons in the geological record, which has direct implications for understanding the potential of geothermal resources and in enabling monitoring of volcanic hazards associated with active crater lakes.