Composition of clinoamphibole as a function of fluid composition during metamorphism of basic rocks

Abstract

Mineral assemblages and their constituents serve as the basis for reconstructing the formation conditions of magmatic and metamorphic rocks. The amphibole group, in view of its wide abundance and high sensitivity to the environmental changes, is of undoubted interest in this relation [1]. Numerous previous works showed that the variations in the partial pressure of fluid components, mainly c 2 e and ee 2 , determine the composition of mineral phases and scales of their distribution in the rock [2]. It was also established by theoretical and experimental works that the fluid composition significantly deviates from a binary H 2 O‐CO 2 mixture. With increasing T and P , fluids acquire a highly reducing character and can contain significant amounts of ec 4 , c 2 , and CO [3]. Of special importance in this context is to estimate the influence of the C‐O‐H fluid composition on the character of endogenous processes and the composition of mineral assemblages. Using physicochemical modeling [4], we estimated the effect of fluid on the end-member composition of clinoamphibole during metamorphism of basic rocks. The model is based on a ten-component system (Si‐ Al‐Fe‐Mg‐Ca‐Na‐K‐C‐O‐H) and involves 70 potentially possible phases, including a gas and aqueous electrolyte solution. For condensed phases (minerals), data were taken from the internally consistent dataset of Holland and Powel [5]. The clinoamphibole compositional space is described by seven dependent components (end members): tremolite (Tr) � Ca 2 Mg 5 Si 8 O 22 (OH) 2 , ferroactinolite (Fact) � Ca 2 Fe 5 Si 8 O 22 (OH) 2 , pargasite (Parg) NaCa 2 Mg 4 Al 3 Si 6 O 22 (OH) 2 , tschermakite (Tsch) � Ca 2 Mg 3 Al 4 Si 6 O 22 (OH) 2 , riebeckite (Rieb) � Na 2 Fe 5 Si 8 O 22 (OH) 2 , glaucophane (Gl) � Na 2 Mg 3 Al 2 Si 8 O 22 (OH) 2 , and Fe-glaucophane (Fe-Gl) � Na 2 Fe 3 Al 2 Si 8 O 22 (OH) 2 . This end-member composition makes it possible to take into account isomorphic substitutions along the most typical exchange vectors: FeMg − 1 , Al 2 Mg –1 Si –1 , NaAl � –1 Si –1 , NaFe 3+ Ca − 1 , NaAlCa − 1 Mg –1 . The thermodynamic properties of components of gas phase and aqueous solutions were calculated using data from the Selector-C program complex [6, 7]. The composition of starting rocks was approximated by average basalt [8] recalculated for 1 kg of dry residue (in moles): Si 8.5447, Al 3.3934; Fe 1.4839; Mg 1.6674; Ca 1.9464; Na 1.0136, K 0.0310, O 27.8272. A fluid was added to this composition, the amount of which was determined by taking weight fluid/rock ratio at 0.1. All possible compositions of fluid in the C‐O‐H system are plotted in the narrow band between the carbon saturation curve and the tie line connecting c 2 e and ee 2 (Fig. 1). Therefore, two extreme cases were