Geothermobarometry in pelitic schists: A rapidly evolving field (MSA presidential address)

Conference · Wed Jan 01 00:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5936792

Holdaway, M J ^[1]

---

1. Southern Methodist Univ., Dallas, TX (United States). Dept. of Geological Sciences

During the past decade, important developments have included new thermodynamic data bases, sophisticated crystal structure determinations, site assignments and analyses for Fe[sup 3+] and Fe[sup 2+], and light element analyses, especially H and Li, for many minerals. It is now necessary to apply these new findings to geothermobarometry in several important ways: (1) The stoichiometric basis for hydrous minerals should be revised in light of highly variable H and variable Fe[sup 3+], which can now be estimated with considerable accuracy is a function of grade and mineral assemblage. (2) Activity models should be based on the best crystal chemical considerations. For some minerals, H may be omitted from the activity model if all the substitutions involving H are coupled substitutions. (3) Thermodynamic data should be based on careful analysis of experiments and secondary comparison with natural assemblages. (4) The possibility of non-ideal solid solution should be considered, as ideality is merely a special case of non-ideality. The garnet system and the garnet-biotite geothermometer are examples of systems which have undergone great evolution, but additional improvements are still necessary. As the pressure and temperature determinations become more precise, one can better evaluate X(H[sub 2]O), X(CO[sub 2]), and X(CH[sub 4]) in the fluid phase. In all probability, patterns will develop concerning the compositions of pelitic metamorphic fluids, especially when graphite is present. In graphitic rocks at known pressure and temperature, knowledge of f(O[sub 2]) from coexisting hemoilmenite and rutile or hemoilmenite and magnetite enables one to determine the mole fraction of all fluid species.

• OSTI ID: 5936792
• Report Number(s): CONF-921058-; CODEN: GAAPBC
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
SCHISTS
GEOBAROMETRY
GEOTHERMOMETRY
BIOTITE
GARNETS
GEOTHERMOMETERS
GRAPHITE
METAMORPHISM
RESERVOIR PRESSURE
RESERVOIR TEMPERATURE
TECHNOLOGY ASSESSMENT
THERMODYNAMIC PROPERTIES
CARBON
ELEMENTAL MINERALS
ELEMENTS
MEASURING INSTRUMENTS
METAMORPHIC ROCKS
MICA
MINERALS
NONMETALS
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
THERMOMETERS
580000* - Geosciences