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A case study of the amount and distribution of heat and fluid during metamorphism

Authors:	J M Ferry

Institution:	(1) Department of Geology, Arizona State University, 85281 Tempe, Arizona, USA

Abstract:	The volume of fluid and amount of heat involved in a portion of a metamorphic event around three synmetamorphic granitic stocks has been quantitatively estimated using mineral composition and modal data from carbonate rocks. Values of volumetric fluid-rock ratios range, with respect to a reference zoisite isograd, from 0.001 to 0.434. Amounts of heat involved range from –25 to 134 cal/cm³ rock. Contours of constant fluid-rock ratio and of constant amount of heat are generally concentric about the granitic stocks indicating that the stocks are foci of high heat and fluid fluxes during metamorphism. In addition, contours of fluid-rock ratios and amount of heat outline NE-SW-trending channelways of high fluid and heat fluxes that alternate with regions of lower fluid and heat fluxes. The NE-SW-trending vertical bedding and schistosity in the area — of premetamorphic origin — probably constrained fluid and heat transfer to occur preferentially in NE-SW directions. Large values of heat involved in metamorphism are strongly correlated with large fluid-rock ratios, suggesting that fluids are an important carrier of heat during metamorphism. Configurations of mapped isograds in the area mimic the patterns of contours of constant fluid-rock ratio and of heat content, indicating that configurations of isograds may contain useful information about regional patterns of heat and fluid transfer during metamorphism.Notation T Last temperature recorded by metacarbonate rocks (°C) - P Lithostatic pressure (bars) - Pi Partial pressure of component i (bars) - $X_{CO_2 }$ $p_{CO_2 } /(p_{CO_2 } + p_{H_2 O} )$ of last fluid in equilibrium with carbonate rocks during metamorphism - R 1.987 cal/bar-degree - K _s Activity constant for an assemblage of solid mineral phases - In Natural logarithm - c _v Volumetric heat capacity (cal/cm³-degree) - Q Heat added to or subtracted from a rock during metamorphism in the zoisite zone (kcal/100 cm³ rock; cal/cm³ rock) - Q{ibrxn} Heat added to or subtracted from a rock due to mineral reactions during metamorphism in the zoisite zone (kcal/100 cm³ rock; cal/cm³ rock) - Std. Dev. Standard Deviation - $X_{CO_2 }^ *$ Average $p_{CO_2 } /(p_{CO_2 } + p_{H_2 O} )$ of fluid in equilibrium with carbonate rocks during their metamorphism in the zoisite zone - $X_{CO_2 }^ \circ$ $p_{CO_2 } /(p_{CO_2 } + p_{H_2 O} )$ of fluid in equilibrium with carbonate rocks at the zoisite isograd - T Temperature at the zoisite isograd (°C) - X _i,j Mole fraction of component i in phase j - H _i Molar enthalpy of reaction i at 0 bars pressure - ¯V _i Change of molar volume due to reaction _ii - _i Measure of progress of reaction i - V Change in rock volume due to fluid-rock reactions - iV Initial rock volume before metamorphism within the zoisite zone - ¯V _s,i Change in molar volume of solid minerals due to reaction i Component notation an CaAl₂Si₂O₈ Phase notation Pl Plagioclase - Am Amphibole - Cc Calcite - Qz Quartz - Di Diopside - Zo Zoisite - Ga Garnet - Bi Biotite - Kf Microcline - Mu Muscovite

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