Fluid Mobility in Metamorphic Rocks During Deformation: Controls on Fluid Inclusion Distribution
and Mineral/Fluid Equilibria

Eric Lee Johnson Geology Dept. Central Michigan University. Mt. Pleasant, MI 48859, USA


Lincoln Hollister Dept. Geology and Geophysics, Princeton Univ. Princeton, NJ 08544, USA

Neil Mancktelow Geologisches Institut, ETH-Zentrum, CH-8092 Zurich, Switzerland

Bernardo Cesare Dipartimento di Mineralogia e Petrologia, Universita di Padova, I-35137 Padova, Italy

The widespread observation of carbonic-rich fluid inclusions in greenschist- amphibolite and granulite facies rock has led to the formulation of elaborate schemes for the formation of these inclusions. This is especially true for carbonic rich fluid inclusions in host rocks with mineral assemblages buffered by aqueous-rich fluid compositions. The observed lack of coherence between the inclusion fluid and calculated fluid composition is typically thought to be the result of the infiltration of carbonic-rich fluids late in the history of the host. Studies on the wetting behavior of carbonic-rich fluids, however, make it seem unlikely that infiltration is a viable mechanism for the formation of these carbonic-rich inclusions. The alternative solution is that mineral equilibria calculations of fluid composition are in some way flawed. While these calculations, and the data and equations used , carry significant errors, they do produce reasonable results when carefully applied to well constrained rock-fluid systems. The occurrence of carbonic fluid inclusions in high grade rocks from many locals, therefore, must reflect some fundamental principle about fluid-rock interactions during or after deformation and metamorphism. We suggest that the formation and preservation of carbonic-rich fluid inclusions is the inescapable by-product of deformation and/or post-deformation annealing. In a deforming aggregate, carbonic fluid inclusions that meet all criteria for primary in origin are produced by grain boundary migration. In order for this process to be effective, H2O-CO2-salt fluids present along or intersected by grain boundaries must be un-mixed (immiscibile). This condition will be met on the retrograde P-T path. Saline H2O-CO2-NaCl fluids will unmix at temperatures ranging from 250- 600°C. Once unmixed, the distribution of fluid inclusion types in the deforming aggregate (i.e. CO2-rich vs. aqueous rich) will be a function of the relative mobility of these fluid types. The abundance of texturally primary CO2-rich fluid inclusions will increase rapidly with annealing or retrograde deformation as these low mobility fluids are localized along grain boundaries or stranded within re crystallized grains. The more mobile H2O-salt fluid will resist entrapment in the deforming aggregate until late in the rock's history when these fluids can be trapped along healed microfractures. This process is clearly shown in samples taken from the footwall of the Simplon Fault Zone (Central Alps: Italy/Switzerland) where the successive entrapment of an evolving and eventually immiscible H2O-CO2-NaCl fluid(s) under conditions of falling pressure and temperature during deformation has led to the complete segregation of carbonic-rich and aqueous-rich fluid inclusions. Retrograde mineral assemblages in these samples (chlorite-bearing) clearly indicate the importance of H2O-rich fluids in the mineral equilibria yet nearly all texturally primary fluid inclusions are CO2-rich. The more mobile aqueous-rich end member fluid is clearly controlling mineral equilibria whereas the CO2-rich fluid became isolated in the rock and passive in terms of mineral/fluid equilibrium.