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

Abstract

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 intrusionmore » of smaller bodies from 1.2-0.6 Ma.« less

Authors:
 [1];  [2];  [3]; ; ;
  1. (U of California)
  2. (Oregon State U)
  3. (U of Utah)
Publication Date:
Research Org.:
USDOE Idaho Operations Office, Idaho Falls, ID (US); Oregon State University, Corvallis, OR (US)
Sponsoring Org.:
USDOE Office of Geothermal Technologies (EE-12) (US)
OSTI Identifier:
14680
Report Number(s):
DOE/ID/13543
TRN: AH200129%%439
DOE Contract Number:  
FG07-97ID13543
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: 19 Aug 1999
Country of Publication:
United States
Language:
English
Subject:
15 GEOTHERMAL ENERGY; CALIFORNIA; FELDSPARS; GEOTHERMAL FIELDS; GEOTHERMAL SYSTEMS; GEYSERS GEOTHERMAL FIELD; GRANITES; HEAT FLUX; HEATING; MAGMA; Geothermal Legacy; ZIRCONS; PLUTONIC COMPLEX; CONCORDIA AGES; K-FELDSPARS; MULTI-DIFFUSION; DOMAIN THEORY; GEOTHERMAL SYSTEM

Citation Formats

T. M. Harrison, G. B. Dalrymple, J. B. Hulen, M. A. Lanphere, M. Grove, and O. M. Lovera. Thermal History of the Felsite Unit, Geysers Geothermal Field, From Thermal Modeling of 40Ar/39Ar Incremental Heating Data. United States: N. p., 1999. Web. doi:10.2172/14680.
T. M. Harrison, G. B. Dalrymple, J. B. Hulen, M. A. Lanphere, M. Grove, & O. M. Lovera. Thermal History of the Felsite Unit, Geysers Geothermal Field, From Thermal Modeling of 40Ar/39Ar Incremental Heating Data. United States. doi:10.2172/14680.
T. M. Harrison, G. B. Dalrymple, J. B. Hulen, M. A. Lanphere, M. Grove, and O. M. Lovera. Thu . "Thermal History of the Felsite Unit, Geysers Geothermal Field, From Thermal Modeling of 40Ar/39Ar Incremental Heating Data". United States. doi:10.2172/14680. https://www.osti.gov/servlets/purl/14680.
@article{osti_14680,
title = {Thermal History of the Felsite Unit, Geysers Geothermal Field, From Thermal Modeling of 40Ar/39Ar Incremental Heating Data},
author = {T. M. Harrison and G. B. Dalrymple and J. B. Hulen and M. A. Lanphere and M. Grove and O. M. Lovera},
abstractNote = {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.},
doi = {10.2172/14680},
journal = {},
number = ,
volume = ,
place = {United States},
year = {1999},
month = {8}
}