Fluid inclusions as microchemical systems: evidence and modelling of fluid–host interactions in plagioclase

• Published in: Journal of Metamorphic Geology
• Main Authors: Kleinefeld, B., Bakker, R. J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2002
• Subjects: East Antarctica; Dronning Maud Land; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1046/j.1525-1314.2002.00411.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1046%2Fj.1525-1314.2002.00411.x; https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1525-1314.2002.00411.x

Abstract Dense, CO 2 ‐rich fluid inclusions hosted by plagioclases, An 45 to An 54 , of the O.‐v.‐Gruber‐ Anorthosite body, central Dronning Maud Land, East Antarctica, contain varying amounts of small calcite, paragonite and pyrophyllite crystals detected by Raman microspectroscopy. These crystals are reaction products that have formed during cooling of the host and the original CO 2 ‐rich H 2 O‐bearing enclosed fluid. Variable amounts of these reaction products illustrates that the reaction did not take place uniformly in all fluid inclusions, possibly due to differences in kinetics as caused by differences in shape and size, or due to compositional variation in the originally trapped fluid. The reaction albite + 2anorthite + 2H 2 O + 2CO 2 = pyrophyllite + paragonite + 2calcite was thermodynamically modelled with consideration of different original fluid compositions. Although free H 2 O is not detectable in most fluid inclusions, the occurrence of OH‐bearing sheet silicates indicates that the original fluid was not pure CO 2 , but contained significant amounts of H 2 O. Compared to an actual fluid inclusion it is obvious, that volume estimations of solid phases can be used as a starting point to reverse the retrograde reaction and recalculate the compositional and volumetrical properties of the original fluid. Isochores for an unmodified inclusion can thus be reconstructed, leading to a more realistic estimation of P–T conditions during earlier metamorphic stages or fluid capturing.