THE 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 mineralogy 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 ~1.27 million years ago (core depth ~290 meters). Below 290 m, the evaporites comprise Ca-bearing sulfates (anhydrite, glauberite) and halite; fluid inclusions in the halite show that parent waters were Na⁺-Cl^--SO₄^2--rich. Above 290 m, the evaporites include sodium carbonates (trona) and halite, and fluid inclusion brines are Na⁺-K⁺-HCO₃^--CO₃^2--Cl^--SO₄^2--rich. These fluctuations in mineralogy and brine chemistry document an alkalinity spike beginning at ~1.27 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 for the last 1.27 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 suggests a causal relationship. Volcanism and hydrothermal activity provided CO₂ and elevated alkalinity to Searles lake inflow waters derived from the Owens Valley, ~0.5 m.y. before the eruptions that formed the Bishop Tuff and Long Valley Caldera.

Lawrence Hardie was a pioneer in the study of brine evolution. His synthesis, now called the principle of chemical divides, is widely used to explain how dilute waters evolve into concentrated brines. Hardie also thought that subsurface waters, such as the hydrothermal waters of the Long Valley Caldera region, were important inflow waters to many closed basins.