Title:

NEW CONSTRAINTS ON THE TIMING OF MAGMATISM, VOLCANISM, AND THE ONSET OF VAPOR-DOMINATED CONDITIONS AT THE GEYSERS STEAM FIELD, CALIFORNIA

Authors:

Jeffrey B. Hulen, Matthew T. Heizler, James A. Stimac, Joseph N. Moore, Jeffrey C. Quick

Geo Location:

The Geysers, California

Conference:

Stanford Geothermal Workshop

Year:

1997

Language:

English

File Size:

215KB

View File:

Abstract:

40Ar/39Ar incremental-heating age spectra for two plutonic rocks and a vein adularia from The Geysers steam field, numerically modeled in the light of geologic constraints, indicate that: (1) The major granodiorite phase of the ìfelsiteî pluton underlying the steam field, at least in one location, has an absolute age of 1.09 + 0.04 Ma; it is almost certainly the crystallized magmatic equivalent of a dacite in the overlying Cobb Mountain volcanic center; (2) A microgranite porphyry dike, with a minimum age of 1.1 Ma, is permissively a feeder for an older Cobb Mountain unit, the 1.2 Ma rhyolite of Alder Creek; (3) The adularia, from Geysers Coring Project corehole SB- 15-D,precipitated at 0.57 + 0.03 Ma in the probable temperature range 300-330?C. The feldspar may have remained near this temperature until 0.28-0.25 Ma, during which interval it rapidly cooled to 260?C, and after which it continued cooling gradually to the modern 230?C.

In conjunction with the adulariaís modeled age spectrum, trapping temperatures and pressures of C02-rich fluid inclusions in coexisting quartz suggest that The Geysers in this region may already have been vapor-dominated at about 0.26 Ma and 290?C. The rapid temperature drop recorded by the adularia may be linked to the onset of vapor-dominated conditions. Pruess (1985) and Shook (1995) have shown that boiling and large-scale venting of a high-temperature liquid-dominated system like the one which existed once at The Geysers would result in just such a steep temperature decline. We speculate that the caprock on this portion of the hot-water system ruptured catastrophically at about 0.26 Ma during major dislocation along the nearby Big Sulphur Creek strike-slip fault zone. Deeply-penetrating dilational jogs created steam reservoir.


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