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Calculated fluid evolution path versus fluid inclusion data in the COHN system as exemplified by metamorphic rocks from Rogaland, south‐west Norway
Author(s) -
BAKKER R. J.,
JANSEN J. B. H.
Publication year - 1993
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1993.tb00153.x
Subject(s) - isograd , metamorphic rock , geology , fluid inclusions , granulite , anorthosite , geochemistry , massif , mineral redox buffer , mineralogy , mineral , metamorphism , chemistry , metamorphic facies , geomorphology , plagioclase , mantle (geology) , paleontology , quartz , organic chemistry , facies , structural basin
Fluid evolution paths in the COHN system can be calculated for metamorphic rocks if there are relevant data regarding the mineral assemblages present, and regarding the oxidation and nitrodation states throughout the entire P‐T loop. The compositions of fluid inclusions observed in granulitic rocks from Rogaland (south‐west Norway) are compared with theoretical fluid compositions and molar volumes. The fluid parameters are calculated using a P‐T path based on mineral assemblages, which are represented by rocks within the pigeonite‐in isograd and by rocks near the orthopyroxene‐in isograd surrounding an intrusive anorthosite massif. The oxygen and nitrogen fugacities are assumed to be buffered by the coexisting Fe‐Ti oxides and Cr‐carlsbergite, respectively. Many features of the natural fluid inclusions, including (1) the occurrence of CO 2 ‐N 2 ‐rich graphite‐absent fluid inclusions near peak M2 metamorphic conditions (927° C and 400 MPa), (2) the non‐existence of intermediate ternary CO 2 ‐CH 4 ‐N 2 compositions and (3) the low‐molar‐volume CO 2 ‐rich fluid inclusions (36–42 cm 3 mol −1 ), are reproduced in the calculated fluid system. The observed CO 2 ‐CH 4 ‐rich inclusions with minor N 2 (5 mol%) should also include a large proportion of H 2 O according to the calculations. The absence of H 2 O from these natural high‐molar‐volume CO 2 ‐CH 4 ‐rich inclusions and the occurrence of natural CH 4 ‐N 2 ‐rich inclusions are both assumed to result from preferential leakage of H 2 O. This has been previously experimentally demonstrated for H 2 O‐CO 2 ‐rich fluid inclusions, and has also been theoretically predicted. Fluid‐deficient conditions may explain the relatively high molar volumes, but cannot be used to explain the occurrence of CH 4 ‐N 2 ‐rich inclusions and the absence of H 2 O.