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Indares y Martignioli, 1985

Indares y Martignioli, 1985 - Canadian Mineralogist(1985...

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Canadian Mineralogist Yol. 23, pp. 187-193 (1985) BIOTITE-GARNET GEOTHERMOMETRY IN GRANULITE-FACIES ROCKS: EVALUATION OF EOUILIBRIUM CRITERIA APHRODITE INDARES AND JACQTIES MARTIGNOLE Ddportement de gdologie, Universitd de Montrdal, Case postale 6128, succursale A, Montrdal, Qudbec H3C 317 biotite and garnet can be modeledin the AKFM sys- tem by the exchange reaction: %Fe3Al2Si3O12 * 7:KMg3 AlSi3Oro(OH)2 : almandine phlogopite %Mg3Al2Si3Op+%KFe3AlSi3Oro(OH)2 (l) pyrope annite The corresponding equilibrium-constant Kis equiva- lent to a distribution coefficient Ko equal to (FelMg)[email protected]/Mg)uu provided both solid solutions are ideal. In rocks, however, incorporation of minor elements commonly results in a departure from ideal- ity in both biotite and garnet solid-solutions, so that Ko may significantly deviate from K. In the last ten years,several calibrations ofKp as a function of T have been used asgeological thermometers, each one adopting a different model for minor-element inter- action. Among these calibrations, thoseby Ferry & Spear (1978), Goldman & Albee (1978) and Thomp- son (1976) have been extensively used, but appear to give inconsistent results if applied to granulite- faciesrocks (Bohlen & Essene 1980). A significant variation in Ko may occur within a single area or even a single thin-section without significant varia- tion in concentration of minor elements @erkins el al. 1982, Indares 1982). For a given calibration, these variations in Kp lead to a large span of estimated temperature. Several authors (e.g., Tracy et al. 1976) have stressed the role of local re-equilibration during cool- ing from peak temperatures to account for suchvar- iations in Kp. Consequently,a narrow contact-zone between adjacent minerals is usually to be avoided in electron-microprobe analysis. Nevertheless, com- positional heterogeneity is not limited to mineral con- tacts, and severaltypes of Kp can be calculated, probably conesponding to compositions of quenched local equilibrium. The aim of rhis paper is to demon- strate that a judicious selection of biotite and gar- net grains, as well as the selection of targets within a given grain, provideschemical data (and thus Kp values) referring to specific temperatures attained during peak conditions or to certain stages of cool- ing. In a separate paper, which takes into account the conclusions of this contribution and the effects of nonideality in both biotite and garnet solid- solutions, a new biotite-garnet thermometric calibra- The iron-magnesium partition between coexisting tion is proposed (Indares & Martignole 1985). t87 AssrRAcr Most commonly accepted calibrations of the biotite- garnet geothermometer have yielded inconsistentestimates of the temperature of the rocks in the granulite facies. This type of inconsistency, unrelated to the calibration, can be avoided by selecting particular minerals and specific loca- tions for analysis within these minerals so that the calcu- lated Kp values may be attributed to a specific stage in the evolution of the metamorphism. Garnet cores and matrix
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