The Chemical Composition and Evolution of Volcanic Lakes

Abstract

Volcanic lakes carry fluids that range from ultra acid, high TDS brines to largely meteoric fluids. Their water compositions are governed by volcanic fluid inputs, which range from almost raw, cooled volcanic gases (largely S–Cl–F–CO2 rich fluid) to more mature solutions that result from interaction of these acid fluids with volcanic rocks. Volcanic inputs can have reacted with mature protoliths (low degrees of neutralization) or with freshly intruded magma (high degrees of water rock interaction), the latter often resulting in the precipitation of secondary minerals such as alunite. The detailed chemical lake water composition is a reflection of mineral precipitation, fractionation of trace elements in the precipitated phases (such as the rare earth elements and metals in sulfides) and element vapor phase transport by the volcanic gases. Variations in lake composition result from a changing volcanic input composition or magnitude and thus are important in volcano monitoring. The lake water dynamics also impact the lake water composition over time, and variations in evaporation rate, meteoric water input and dramatic changes in input may cause changes in element concentrations and ratios related to non steady state effects. The recovery time to steady state differs strongly between open and closed lake systems. The stable isotope compositions of volcanic lake waters reflect W/R interaction (higher δ18O), degree of volcanic gas input (higher δ18O and δD), and evaporation at elevated temperatures (flat evaporation lines). Isotope ratios of other elements usually reflect the nature of the volcanic inputs or dissolved rock. Volcanic lakes can be charged with toxic elements that upon release may impact local ecosystems and agricultural land or drinking water downstream. Volcanic lakes charged with poorly soluble gases such as CO2 and methane may represent hazards of limnic eruptions. Failure of retaining walls or dams of acid lakes may cause acid floods with damage to land and livestock downstream.