Abstract
Decades of geochemical monitoring at active crater lakes worldwide have confirmed that variations in major elements and physico-chemical parameters are useful to detect changes in volcanic activity. However, it is still arduous to identify precursors of single phreatic eruptions. During the unrest phase of 2009–2016, at least 679 phreatic eruptions occurred at the hyperacid and hypersaline crater lake Laguna Caliente of Poás volcano (Costa Rica). In this study, we investigate the temporal variations of Rare Earth Elements (REE) dissolved in Laguna Caliente in order to 1) scrutinize if they can be used as a new geochemical tool to monitor changes of phreatic activity at hyperacid crater lakes and 2) identify the geochemical processes responsible for the variations of REE concentrations in the lake. The total concentration of REE varies from 950 to 2,773 μg kg−1. (La/Pr)N-local rock ratios range from 0.93 to 1.35, and Light REE over Heavy REE (LREE/HREE)N-local rock ratios vary from 0.71 to 0.95. These same parameters vary in relation to significant changes in phreatic activity; in particular, the (La/Pr)N-local rock ratio increases as phreatic activity increases, while that of (LREE/HREE)N-local rock decreases when phreatic activity increases. REE concentrations and their ratios were compared with the variations of major elements and physico-chemical parameters of the lake. Calcium versus (La/Pr)N-local rock and versus (LREE/HREE)N-local rock ratios show different trends compared to the other major elements (Na, K, Mg, Al, Fe, SO4, and Cl). Moreover, a higher loss of Ca (up to 2,835 ppm) in lake water was found with respect to the loss of Al, K, and Na. This loss of Ca is argued to be due to gypsum precipitation, a process corroborated by the mass balance calculation simulating the precipitation of gypsum and the contemporaneous removal of REE from the lake water. The observed relations between REE, changes in phreatic activity, and the parameters commonly used for the monitoring of hyperacid volcanic lakes encourage investigating more on the temporal and cause-effect relationship between REE dynamics and changes in phreatic activity at crater lake-bearing volcanoes.
Highlights
Volcanic areas are often characterized by the presence of lakes that fill the summit crater area (Manville, 2015)
We investigated if the variations of REE in Laguna Caliente could be connected to the main changes of phreatic activity, here represented by the occurrence of 679 phreatic eruptions during the period 2009–2016, and if they could be connected with the temporal variations of the geochemical indicators previously used to monitor Laguna Caliente volcanic activity (Rouwet et al, 2017; Rouwet et al, 2019)
Almost 700 phreatic eruptions were observed and reported at Laguna Caliente between June 2008 and June 2016; it is likely that the real number of phreatic eruptions that occurred during this period is even higher
Summary
Volcanic areas are often characterized by the presence of lakes that fill the summit crater area (Manville, 2015). The chemical composition of volcanic lakes mainly depends on 1) the fluid input from the underlying magmatic-hydrothermal system, 2) evaporation, meteoric precipitation, and infiltration, 3) dissolution of volcanic rocks, and 4) the precipitation of secondary minerals (Delmelle and Bernard, 1994; van Hinsberg et al, 2010; Varekamp, 2015). Poás (10°11′26′′N 84°13′56′′W) is a stratovolcano in Costa Rica that belongs to the Central Volcanic Range (CVR; Ruíz et al, 2019) It is composed of various volcanic structures, such as the composite cones at its top, Von Frantzius, Botos, and the Main Crater, that represents the spot of the historical activity (Prosser and Carr, 1987; Mora-Amador et al, 2019a). The lavas have a geochemical composition resulting from a fractional crystallization that generated products ranging from basalts to dacites; andesites and basaltic andesites are the most common products (Ruíz et al, 2010, 2019)
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