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Composition and structural state of coexisting feldspars, Salton Sea geothermal field

Published online by Cambridge University Press:  05 July 2018

S. Douglas McDowell
S. Douglas McDowell*
Affiliation:
Department of Geology and Geological Engineering, Michigan Technological University, Houghton, MI 49931, USA
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Abstract

Active metamorphism of fine grained sandstone in the c.16000 year old Salton Sea geothermal system has produced a suite of chemically equilibrated coexisting authigenic alkali feldspars and re-equilibrated detrital feldspars in the 250–360°C temperature range. At c.335°C the average compositions, 2 Vs, and (t1o+t1m) and Z ordering parameters of coexisting authigenic feldspars are [Or0.52Ab97.40An2.08, 2Vx = 91.3±4.8, (t1o + t1m) = 0.89±0.05, Z = 0.79±0.09], and [Or94.42 Ab5.10An0.48, 2Vx = 70, (t1o + t1m) = 0.90, Z = 0.81]. At c.360°C authigenic albite becomes more An-rich and less ordered [Or1.21Ab92.83An5.97, 2Vx = 87.5±3.4, (t1o + t1m) = 0.85±0.03, Z = 0.70±0.07] and K-feldspar is no longer stable. Detrital plagioclase (An up to 40%) is preserved metastably to temperatures up to c.190°C in strongly carbonate-cemented sandstone which forms part of a geothermally produced permeability cap. It undergoes rapid alkali exchange at temperatures near 200°C, and by 250°C no plagioclase with An-content over 12% is observed. At > 250°C authigenic and most detrital alkali feldspar compositions are in excellent agreement with the Bachinski and Muller (1971) microcline-low-albite solvus.

Keywords

feldsparsandstonemetamorphismSalton SeaCaliforniaUSA
Type
Mineralogy
Information
Mineralogical Magazine , Volume 50 , Issue 355 , March 1986 , pp. 75 - 84
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1986

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