Mineralogy and K–Ar geochronology of mixed-layered illite/smectite from The Geysers coring project, California, USA
✍ Scribed by Giday WoldeGabriel; Fraser Goff; James Aronson
- Book ID
- 104333251
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 907 KB
- Volume
- 30
- Category
- Article
- ISSN
- 0375-6505
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✦ Synopsis
Clay fractions for K±Ar dating, separated from Franciscan-Assemblage (late Mesozoic) metagraywacke and argillite from The Geysers Coring Project corehole SB-15-D, and a background'' outcrop consist of illite, mixed-layer illite/smectite, mixed-layer chlorite/smectite, and chlorite with minor calcite and quartz. Except for the chlorite/smectite, these phases were formed during both subduction-related Franciscan regional metamorphism and late Cenozoic (post-1.1 Ma) hydrothermal alteration and mineralization related to The Geysers hydrothermal system. The chlorite/smectite is exclusively hydrothermal in origin. In spite of careful eorts to physically separate the metamorphic and layer silicates prior to analysis, all samples contained variable contributions from both sources. The proportion of illite plus illite/smectite clays in the <5 mm fractions of the samples ranges widely from about 41 to 97 wt.%. K 2 O contents of the dated clay fractions (43 mm) also ranged widely, from 1.66 to 8.0 wt.%. Despite having been heated to at least 300 C, matrix illite and illite/smectite do not show depth-and temperature-dependent trends of downward-increasing illite interlayers as is the case in many old sedimentary basins (e.g. Texas Gulf Coast and North Sea oil ®elds) and other geothermal systems (e.g. Valles caldera and Salton Sea). Fourteen mixed-layer illite/ smectites of dierent size fractions (0.1±0.35, 0.35±1 and 1±3 mm) from the SB-15-D metagraywackes and argillites (and a nearby background'' sample of the same rock type) yielded K±Ar dates ranging widely from 105.5 Ma (the outcrop sample) to 1.5 Ma. Although there is a general decrease in apparent age with depth, most of the clays yielded middle Tertiary dates (35.4±19.4 Ma). The outcrop age is consistent with published data for Franciscan regional Geothermics 30 (2001) 193±210 www.elsevier.com/locate/geothermics