Kohn y Spear, 2000

Kohn y Spear, 2000 - Retrograde net transfer reaction...

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INTRODUCTION Over the past 20 yr, thermobarometry has become a common petro- logic tool. For example, in one international journal during the past decade, more than 200 articles appeared that include an estimate of pressure- temperature ( P-T ) conditions based on mineral compositions and thermo- dynamically calibrated equilibria. Most workers apply standardized criteria for selecting mineral compositions, especially for garnet, that assume that reequilibration during cooling has occurred only via retrograde exchange reactions (ReERs)—i.e., reactions that exchange Fe, Mg, or Mn among minerals without significantly affecting modal abundances. However, most compositional zoning systematics also indicate the importance of retrograde net transfer reactions (ReNTRs)—reactions that cause net growth and dis- solution of minerals. As demonstrated herein, P-T results for middle and upper amphibolite facies rocks that have undergone ReNTR may be precise, yet grossly incorrect by many kilobars and hundreds of degrees. Accurate thermobarometry requires consideration of possible ReNTRs, as well as design and application of chemical and textural tests to accommodate such reactions. Data-processing techniques are described here that maximize the likelihood of identifying peak equilibrium compositions and estimating accurate P-T conditions, i.e., to ensure against ReNTRs. Metapelites with garnet + biotite metapelites are emphasized, because these are most com- monly used for thermobarometry. REACTIONS Exchange Exchange reactions involve the exchange of two elements between two minerals and form the basis for many thermometers, such as the Fe-Mg exchange reaction between garnet and biotite (almandine + phlogopite = annite + pyrope). Retrograde exchange reactions do not significantly change mineral modes and cause divergence of mineral compositions (e.g., biotite becomes more Mg rich while garnet becomes more Fe rich). If only retro- grade exchange reactions occur, calculated temperatures will be below peak temperatures. Net Transfer Net-transfer reactions involve the production and consumption of min- erals. Kinetics may subordinate net-transfer reactions to exchange reactions during cooling (e.g., Frost and Chacko, 1989), but unlike retrograde exchange reactions, ReNTRs cause mineral compositions to shift in the same direc- tion (e.g., biotite and garnet both become more Fe rich). This fact has been discussed (e.g., Robinson, 1991; Spear, 1991; Spear and Florence, 1992; Spear and Parrish, 1996) because if ReNTRs do occur, calculated tempera- tures may be higher than peak temperatures. However, no previous study has shown how to quantitatively determine the extent of reaction or correct compositions to retrieve peak P-T conditions. As described in the following, these goals can be achieved via quantitative X-ray maps.
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Kohn y Spear, 2000 - Retrograde net transfer reaction...

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