Influence of magmatic-hydrothermal activity on brine evolution in closed basins: Searles Lake, California

Evaporites in a 700-m-long core (KM-3) from Searles Lake, California, preserve a record of the chemical evolution of inflow waters. Chemical analyses of fluid inclusions in halite and evaporite minerals show that the major ion composition of inflow waters to Searles Lake was changed by distant hydrothermal activity associated with magmatism at Long Valley caldera between 1.27 and 1.0 m.y. ago. Below core depths of 291 m, the evaporites consist of Ca-bearing sulfates (anhydrite, glauberite) and halite; fluid inclusions in the halite show that parent waters were Na+-Cl--SO42--rich waters. Above 291 m, the evaporites include sodium carbonates (pirssonite, trona) and halite, and fluid inclusion brines are Na+-K+-HCO3--CO32--Cl--SO42--rich. These fluctuations in mineralogy and brine chemistry document an alkalinity spike beginning between 1.27 and 1.0 Ma, when inflow waters to Searles Lake crossed the CaCO3 chemical divide and began to produce alkaline brines that precipitated trona upon evaporation. The Owens River is a modern chemical analog for inflow water into Searles Lake beginning between 1.27 Ma and 1.0 Ma. A major contributor of solutes to the Owens River is Hot Creek in Long Valley caldera, which is fed by hydrothermal springs with high alkalinity from magmatically derived CO2. The timing of magmatism in Owens Valley and the appearance of sodium carbonate minerals in core km-3 suggest a causal relationship. Volcanism and hydrothermal activity provided CO2 and elevated alkalinity to Searles Lake inflow waters 0.5-0.2 m.y. before the eruptions that formed the Bishop Tuff and Long Valley caldera.