Behaviour of rare earth elements during water-rock interaction and alteration processes in volcanic lake systems
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The rare earth elements are widely used to study processes in many different geological environments. Although their behaviour is particularly suitable for research on water-rock interactions, volcanic lakes have received little attention so far. This thesis documents the results of a study on rare earth elements (REE) in active, crater-lake hosting volcanic-hydrothermal systems worldwide. Particular attention has been paid to Poás and Rincón de la Vieja, two Costa Rican volcanoes for which new analytical data are presented. A literature survey covers geochemical information available for volcanic lakes in Indonesia, Argentina, the Philippines, New Zealand, Japan, USA and Russia. Most of these lakes are highly acidic. The main objective of this research was to explore how REE that are dissolved in crater lake water can be used to gain insights into water-rock interaction processes and to monitor activity in the underlying hydrothermal system. For this purpose, the newly acquired data and those from the literature compilation were evaluated to discern systematics in the behaviour of the REE and other elements, whereby the compositions of local rocks were taken in account as well. The REE concentrations and their normalized trends show significant variations for the investigated lakes. There is a rough correlation between REE patterns of the lake waters and the local rocks, testifying that lavas and other solid volcanic products are the principal source of the REE. In general, the correspondence is best for highly acidic lakes. The observed systematics indicate that rock dissolution plays an important role but fractionation among the REE shows that complete, congruent dissolution is rare and only occurs under extreme conditions. In general, water-rock interaction in these volcanic-hydrothermal settings affects individual igneous minerals in different ways, some being more resistant than others. The collected data indicate that the REE signatures in lake waters are controlled by preferential retention or release from REE hosting phases, either primary igneous minerals or glass in volcanic materials, or secondary phases formed as alteration product. An important discovery is the apparent role of alunite in regulating the signatures of dissolved REE in lake waters. This is surprising because this mineral is an alteration product that is commonly not stable in acidic volcanic lakes. However, as could be confirmed in PHREEQC models, the saturation state of alunite in these waters is inversely correlated with temperature. This implies that alunite can be present in hotter parts of the hydrothermal system below the lake, and that the REE signatures in the lake may be inherited from inflow of water that had interacted with alunite present at depth. Changes in REE patterns in lakes that have been monitored over longer times indicate that the influence of alunite may vary with time. This suggests that a lake may signal the stability of the mineral, and its formation or dissolution in response to changing conditions. Apart from the REE data, the potential role of alunite can be confirmed by comparing the concentrations of its main cation constituents with other dissolved cations in the lake. Time-series results on REE concentrations in the highly dynamic crater lake of Poás provide evidence for an intermittent role of alunite in the subsurface system. Strong fluctuations in LREE/HREE ratios, observed over decades of monitoring, probably reflect variations in the stability of the mineral, induced by changing hydrothermal conditions. Changes in the REE concentrations sometimes coincided with phreatic eruptive events. Since alunite is a secondary, non-magmatic mineral, its presence or absence may affect the porosity and permeability of volcanic rocks and conduits that act as pathways for upflowing fluids. Evidence for Poás suggests that the formation of alunite has a sealing effect that reduces the influx of fluid and heat into its lake. It is concluded that REE in volcanic lakes are a promising new instrument to study water-rock interaction and alteration processes that are active at the very present. Monitoring the REE provides a valuable parameter in assessing changes in the state of activity for volcanic-hydrothermal systems.