Relations among net-transfer reaction progress, 18O?13C depletion, and fluid infiltration in a clinohumite-bearing marble

Abstract

A siliceous dolomitic marble xenolith within a mid-crustal Jurassic diorite exhibits mineralogical and stable isotopic evidence for infiltration of water-rich fluid. Adjacent to endoskarn which bounds the xenolith, forsterite has been replaced by clinohumite as a result of a devolatilization reaction driven by addition of aqueous fluid and extraction of heat. Isotopic compositions of calcite also record infiltration of aqueous fluid concentrated near contacts with endoskarn. Marble calcite δ18OSMOW values range from +19.5‰ at the center of the xenolith to +12.6‰ adjacent to endoskarn. Calcite δ13CPDB decreases sympathetically from +3.0 to +1.4‰. The calculated equilibrium composition of coexisting C−O−H−F fluid and a quantitative characterization of the whole-rock reaction which produced clinohumite and consumed graphite are used to place inequality contraints on the composition of the infiltrated fluid. Continnum mechanical transport models based on the resulting fluid compositions suggest that a small time-integrated Darey flux of 44 cm3/cm2 coupled with diffusive transport in the fluid was sufficient to produce both the isotopic shifts and the net-transfer reactions evidenced in the xenolith. The calculations demonstrate the importance of graphite as an indicator of time-integrated flux. The maximum possible CH4 content of the infiltrated fluid is sufficiently high to impart a 2.0‰ uncertainty in the δ13C of the fluid. The isotopic composition of the fluid is consistent with a magmatic origin when this uncertainty is taken into account.