Microstructure and formation mechanism of phyllosilicates in hydrothermally metamorphosed Salton Sea Shales

Conference · Tue Jan 01 00:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:5960860

McDowell, D

TEM/AEM study of authigenic phyllosilicates in a sequence of hydrothermally metamorphosed shales and siltstones from the temperature (depth) interval 115/degree/C (256 m) to 330/degree/C (1547 m) in the well IID No 2, Salton Sea Geothermal Field shows that the overall progression of mineral assemblages is consistent with that observed in the sandstone sequence of nearby wells, despite significant differences in the bulk chemistry between shale and sandstone. The phyllosilicates progress from an illite/muscovite zone at temperatures less than 260/degree/C, through a chlorite zone (220-305/degree/C) and a biotite zone (310-330/degree/C). By contrast with observations of shales in other burial metamorphic sequences, the Salton Sea samples contain significant proportions of pore space (estimated to be /approx/ 30%) even in shales. All phyllosilicates and other authigenic minerals occur as discrete euhedral crystals which partially fill pore space. The mixed-layering and structural/chemical heterogeneity, typical of phyllosilicates in shales subject to diagenesis, are generally absent. AEM analyses yield compositions of illite/muscovite as (K/sub 0.7/) (Al/sub 1.5/Mg/sub 0.2/Fe/sub 0.4/) (Si/sub 3.4/Al/sub 0.6/)O/sub 10/(OH)/sub 2/ at 220/degree/C, chlorite as (Mg/sub 2.9/Fe/sub 2/Al/sub 1/)(Al/sub 1/Si/sub 3/)O/sub 10/(OH)/sub 8/ at 300/degree/C and biotite as K(Fe/sub 1.6/Mg/sub 1.2/Al/sub 0.3/)(Al/sub 0.9/Si/sub 3.1/)O/sub 10/(OH)/sub 2/ at 330/degree/C. Texture and microstructure indicate that the mineral progression in the low grade hydrothermal metamorphic sequence involve complete dissolution of detrital phases, mass transport through interconnecting pore space, and direct crystallization of phyllosilicates from solution.

• Research Organization: Univ. of Michigan, Ann Arbor (USA)
• OSTI ID: 5960860
• Report Number(s): CONF-8510489-
• Journal Information: Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28-31 Oct 1985
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
58 GEOSCIENCES
SALTON SEA GEOTHERMAL FIELD
ROCK-FLUID INTERACTIONS
SHALES
HYDROTHERMAL ALTERATION
METAMORPHISM
SILICATE MINERALS
CHEMICAL COMPOSITION
STRUCTURAL CHEMICAL ANALYSIS
SILTSTONES
CALIFORNIA
CRYSTALLOGRAPHY
DIAGENESIS
DISSOLUTION
GEOCHEMISTRY
GEOTHERMAL FLUIDS
POROSITY
SALTON SEA
CHEMISTRY
FEDERAL REGION IX
FLUIDS
GEOTHERMAL FIELDS
LAKES
MINERALS
NORTH AMERICA
ROCKS
SEDIMENTARY ROCKS
SURFACE WATERS
USA
152003* - Geothermal Data & Theory- Rock-Water-Gas Interactions
580400 - Geochemistry- (-1989)