Thermal History of the Felsite Unit, Geysers Geothermal Field, From Thermal Modeling of 40Ar/39Ar Incremental Heating Data

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An Ar-40/Ar-39 and U-Pb study was performed of the Geysers plutonic complex of the Geysers Geothermal Field in California. Sixty-nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the Geysers geothermal field yielded Pb-207/Pb-206 vs. U-238/Pb-206 concordia ages ranging from 1.13 {+-} 0.04 Ma to 1.25 {+-} 0.04 Ma. The U-Pb ages coincide closely with Ar-40/Ar-39 age spectrum plateau and ''terminal'' ages from coexisting K-feldspars and with the eruption ages of overlying volcanic rocks. The data indicate that the granite crystallized at 1.18 Ma and had cooled below 350 C by {approximately}0.9-1.0 ... continued below

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35 p.

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California), T. M. Harrison (U of; U), G. B. Dalrymple (Oregon State; Utah), J. B. Hulen (U of; Lanphere, M. A.; Grove, M. & Lovera, O. M. August 19, 1999.

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Description

An Ar-40/Ar-39 and U-Pb study was performed of the Geysers plutonic complex of the Geysers Geothermal Field in California. Sixty-nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the Geysers geothermal field yielded Pb-207/Pb-206 vs. U-238/Pb-206 concordia ages ranging from 1.13 {+-} 0.04 Ma to 1.25 {+-} 0.04 Ma. The U-Pb ages coincide closely with Ar-40/Ar-39 age spectrum plateau and ''terminal'' ages from coexisting K-feldspars and with the eruption ages of overlying volcanic rocks. The data indicate that the granite crystallized at 1.18 Ma and had cooled below 350 C by {approximately}0.9-1.0 Ma. Interpretation of the feldspar Ar-40/Ar-39 age data using multi-diffusion domain theory indicates that post-emplacement rapid cooling was succeeded either by slower cooling from 350-300 C between 1.0 and 0.4 Ma or transitory reheating to 300-350 C at about 0.4-0.6 Ma. Heat flow calculations constrained with K-feldspar thermal histories and the pre sent elevated regional heal flow anomaly demonstrate that appreciable heat input from sources external to the known Geysers plutonic complex is required to maintain the geothermal system. This requirement is satisfied by either a large, underlying, convecting magma chamber (now solidified) emplaced at 1.2 Ma or episodic intrusion of smaller bodies from 1.2-0.6 Ma.

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35 p.

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OSTI as DE00014680

Medium: P; Size: 35 pages

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  • Other Information: PBD: 19 Aug 1999

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  • Report No.: DOE/ID/13543
  • Grant Number: FG07-97ID13543
  • DOI: 10.2172/14680 | External Link
  • Office of Scientific & Technical Information Report Number: 14680
  • Archival Resource Key: ark:/67531/metadc620053

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  • August 19, 1999

Added to The UNT Digital Library

  • June 16, 2015, 7:43 a.m.

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  • April 12, 2017, 1:48 p.m.

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California), T. M. Harrison (U of; U), G. B. Dalrymple (Oregon State; Utah), J. B. Hulen (U of; Lanphere, M. A.; Grove, M. & Lovera, O. M. Thermal History of the Felsite Unit, Geysers Geothermal Field, From Thermal Modeling of 40Ar/39Ar Incremental Heating Data, report, August 19, 1999; United States. (digital.library.unt.edu/ark:/67531/metadc620053/: accessed December 17, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.