Primary CO2-bearing fluid inclusions in granulitic garnet usually do not survive

Abstract

CO2-bearing fluid inclusions (FI) from granulites have been the main advocate for an active role of fluids during high-temperature metamorphism, yet the fluid regime of the deep crust and the role of fluids remain largely debated topics. In this dispute, one of the most controversial issues is the timing of FI entrapment relative to peak metamorphic conditions. Here we investigate three world-renowned high- to ultra-high temperature metamorphic terranes to evaluate the fate of primary CO2 and COH fluid inclusions certainly trapped during the prograde path. Fluid inclusions coexist along with nanogranitoids in peritectic garnet. Combination of cutting-edge techniques indicates that the FI are composed of fluid and aggregates of solid phases. The latter usually comprises siderite, ferroan magnesite, pyrophyllite, calcite, corundum, quartz, and in some cases kaolinite, dolomite, biotite and muscovite. In the fluid phase, low-density CO2 is the most common component and no free H2O has been detected. Methane and N2 may be also present. The high proportion of solids in the FI with carbonates and OH-bearing phases cannot have precipitated as daughter phases directly from a simple COH-fluid as daughter phases. These minerals require additional cations (e.g. Fe, Mg, Ca, Al, Si) which may be dissolved in very small amounts in crustal fluids and yet are very abundant in the host. The solids, therefore, imply that the initial composition of high-density carbonic inclusions must have been changed by the interaction with the host garnet during cooling. Thermodynamic modelling of such fluid-garnet interaction demonstrates that the assemblages found in FI are metastable and that, whatever retrograde path is followed by the host rock, primary C-bearing FI must change their nature to a multiphase assemblage of stepdaughter minerals as a natural consequence of cooling. It follows that supposed primary unmodified FI in garnet previously reported in the literature are, in most cases, secondary, retrograde features whose low temperature of entrapment inhibited fluid-host interaction. Our finding undermines the main pillar of the theory of carbonic fluid-assisted metamorphism (the presence of superdense CO2 inclusions in granulites), but at the same time it offers a novel perspective to identify prograde COH fluids in the deep hot crust.