Isotopic and geophysical constraints on the structure and evolution of the Clear Lake volcanic system

Abstract

New Sr and Nd isotopic data are combined with available information on the composition and petrology of lavas and the thermal and seismic structure of the underlying crust to develop a detailed model for the deep structure and magmatic processes of the Clear Lake volcanic system in northern California. The isotopic data require a two-stage model for magmatic evolution. In stage I, basaltic magma ( ε Nd = + 6 to + 8; 87Sr / 86Sr = 0.703 to 0.7035; SiO 2 ≤ 50%) is fed from the mantle into the lower and middle crust and evolves through combined crustal assimilation and fractional crystallization to basaltic andesite ( ε Nd = + 5 to + 0.4; 87Sr / 86Sr = 0.70328 to 0.70485; SiO 2 ≈ 55% to 57%). In stage II, the basaltic andesite magmas are transported upward and are either erupted at the surface or stored in shallow magma chambers where they evolve by fractional crystallization to form dacitic and rhyolitic magmas (SiO 2 ≈ 65% to 70%). High-silica rhyolites (SiO 2 ≈ 75%; high 87Sr / 86Sr) show evidence that further crustal assimilation can occur where upper crustal temperatures are elevated. Calculated densities of Clear Lake lavas indicate that basalt should pond at a depth of 12–18 km where seismic data show a pronounced density boundary within the crust. Thermodynamic models of assimilation require that mid-crustal temperatures are at least 600–800 °C to allow for enough assimilation to explain the isotopic data. Both surface heat flow and thermobarometry of crustal xenoliths in andesites are consistent with these inferred high temperatures. The Clear Lake volcanic system provides an opportunity to cross-calibrate petrological, geochemical and geophysical approaches. The results confirm that magma supply, magma buoyancy, and crustal temperatures control magmatic evolution. A temporal trend of increasing ε Nd over the past 2 million years suggests that magma supply in the Clear Lake volcanic field has been increasing and is still high. This is consistent with high heat flow in the area and high 3He / 4He in thermal gasses. The current pause in eruptive activity could represent a period of silicic magma accumulation in the crust. Future eruptions in the Clear Lake area may therefore be silicic and larger in volume than those over the past 100 ky.