ISOTOPIC AND GEOPHYSICAL CONSTRAINTS ON THE STRUCTURE AND EVOLUTION OF THE CLEAR LAKE VOLCANIC FIELD

The Clear Lake Volcanic Field (CLVF), located about 135 km north of San Francisco has been the focus of silicic magmatism over the past 2 million years, although large cataclysmic eruptions have been absent. There has been considerable interest in the likelihood of future eruptions at Clear Lake and in particular, the potential for the system to generate a large caldera-forming eruption.

New Sr and Nd isotopic data, combined with available information on the composition and petrology of the lavas and the thermal and seismic structure of the underlying crust allow us to determine the deep structure of the Clear Lake volcanic system. The data are consistent with a two-stage model for magmatic evolution. During stage I, basaltic magma (ε_Nd=+6 – 8; ⁸⁷Sr/⁸⁶Sr=0.703-0.7035) is fed from the mantle into the base of the crust. At a depth of 12-18km, which corresponds to a pronounced density boundary within the crust, basaltic magma becomes neutrally buoyant, accumulates in chambers and evolves through combined crustal assimilation and fractional crystallization. Surface heat flow is consistent with sufficiently high crustal temperatures for significant amounts of assimilation at the depth at which mantle-derived basalt becomes neutrally buoyant. During stage II, the products of stage I (55-57% SiO2; ε_Nd=+5 – 0.4; ⁸⁷Sr/⁸⁶Sr=0.70328 – 0.70485) are transported upward through the crust and are either erupted at the surface or are stored in shallow magma chambers where they evolve by fractional crystallization to form dacitic and rhyolitic magmas. A correlation of high ⁸⁷Sr/⁸⁶Sr (0.705 – 0.707) in some rhyolites and areas of high heat flow suggest that some late-stage assimilation occurs where upper crustal temperatures are elevated.

A temporal trend of increasing ε_Nd with time suggests that magma supply in the CLVF has been increasing and is still high. This is consistent with high heat flow in the area and magmatic He isotope signatures in thermal gasses. It is possible that the current pause in activity could represent a period of magma accumulation and evolution within the crust. Future eruptions in the Clear Lake area may be more silicic in nature than the most recent, basaltic eruptions.