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Carbonate-bearing sediments, containing calcite, dolomite or magnesite as major carbonate components, are important constituents of sedimentary sequences deposited on passive margins through Earth's history. When involved in collisional orogenic processes, these sediments are metamorphosed at variable temperatures and pressures, and undergo decarbonation reactions. While the orogenic metamorphism of some of these lithologies (i.e. impure limestones and dolostones, marls sensu stricto and calcareous pelites) is relatively well understood, very little is known about the metamorphic evolution and decarbonation history of mixed carbonate–silicate rocks in which either dolomite or magnesite is the dominant carbonate component. Here we present the results of a petrologic study of representative samples of metasediments from Central Nepal, derived from Proterozoic dolomitic and magnesitic protoliths metamorphosed during the Himalayan orogeny. The main metamorphic assemblages developed in sediments originally containing different amounts of dolomite or magnesite are characterised in detail. Forward thermodynamic modelling applied to seven samples allows constraints to be placed on (i) the main decarbonation reactions, (ii) the P–T conditions under which these reactions took place, (iii) the composition of the fluids, and (iv) the amounts of CO2 released. We conclude that the CO2 productivity of dolomitic and magnesitic pelites and marls originally containing 15–40% carbonate is significant (>5.5 ± 1.0 CO2 wt% and up to 10.5 ± 1.5 CO2 wt%), whereas for carbonate contents above 60–70%, CO2 productivity is negligible unless aqueous fluids infiltrate from the outside and trigger decarbonation reactions. Since the dolomitic and magnesitic protoliths are significantly abundant in the sedimentary sequences involved in the still active Himalayan orogen, the decarbonation processes described here could contribute to the diffuse CO2 degassing currently observed at the surface. Furthermore, we propose for the first time that the peculiar magnesium-rich assemblages investigated in this study may derive from evaporitic protoliths, and that the whole Upper Lesser Himalayan Sequence may therefore represent the metamorphic product of a Proterozoic sequence consisting of alternating layers of carbonatic, evaporitic and pelitic sediments. [ABSTRACT FROM AUTHOR]

A reliable quantitative estimate of the metamorphic CO2 flux from collisional orogens is fundamental to our understanding of the deep carbon cycle, but it is still far from being constrained. Among major uncertainties are the poor knowledge of the nature of metamorphic CO2-producing processes and the amount of CO2 potentially released through these reactions. Previous studies of metamorphic decarbonation reactions in metacarbonate rocks mainly used simple model reactions between end-members in simplified model systems. However, fully quantitative modelling of calcsilicate rocks requires an investigation of very complex systems with more than six components. Moreover, scapolite solid solution has been rarely included in previous studies, although this mineral is often a major constituent of calc-silicate rocks. This study focuses on (1) the CO2-producing processes occurring in scapolite-bearing calc-silicate rocks and (2) the discussion of a methodological approach suitable to understand and quantify these processes. In this framework, phase relations and devolatilization reactions in the Na2O-K2O-CaO-(FeO)-MgO-Al2O3-SiO2-(TiO2)-H2O-CO2 [NKC(F)MAS(T)-HC] system are considered, with application to high-grade clinopyroxene+calcite+K-feldspar+scapolite+plagioclase+zoisite calc-silicate rocks from the Himalaya. The NKC(F)MAS(T)-HC equilibria involving scapolite and plagioclase solid solutions are investigated using (1) isobaric T-X(CO2) phase diagram sections and pseudosections and (2) mixed-volatile P-T phase diagram projections. This phase diagram approach allowed us to identify scapolite-bearing, CO2-producing, univariant (i.e. isobaric invariant) equilibria that have never been recognized before, and that could not be detected without considering Na-Ca solid solutions in the calculations. It is demonstrated that the investigated calc-silicate rocks behaved as a nearly closed, internally buffered, system during prograde metamorphism and that most of the observed key microstructures correspond to isobaric univariant or invariant assemblages. In such a nearly closed system, the fluid was mostly produced during prograde heating at the isobaric invariant points, where abrupt changes in mineral modes also occurred. The proposed phase diagram approach further allows quantitative estimation of the amount and composition of the fluid produced at such isobaric invariant points. On average, 2.5 mol of CO2 (110 g) per 1000 cm³ of reacting rock were produced during prograde metamorphism of this calc-silicate rock-type. Because similar scapolite bearing calc-silicate rocks are abundant in the Himalayan orogen, it is suggested that this calc-silicate rock-type might have produced large amounts of CO2-rich fluids during Himalayan metamorphism. A preliminary estimate of these amounts at the scale of the whole orogen suggests a total metamorphic CO2 production of ~(2-7) x 1017 mol, corresponding to (1-3)_1010 Mt of CO2. Integrated over ~20 Myr (i.e. the maximum duration of prograde metamorphism), the calculated metamorphic CO2 flux would be (1.1-3.4) x 1010 mol a-1, corresponding to an annual mass flux of (0.5-1.5) x 10³ Mt a-1. Nevertheless, further studies are still needed to assess more precisely the amount of CO2 released during the Himalayan orogeny. [ABSTRACT FROM AUTHOR]

Partial melting of deep continental crust may occur during either prograde heating or decompression. Although the effect of temperature on crustal melting has been widely investigated, few experimental studies have addressed the question of the influence of pressure on crustal anatexis. To understand the influence of decreasing pressure on partial melting processes, the thermodynamic approach of isochemical phase diagrams has been applied to garnet–K-feldspar–kyanite–sillimanite anatectic gneisses (Barun Gneiss) from the Higher Himalayan Crystallines (HHC) of eastern Nepal. The main melt-producing reactions, the amount of melt produced during heating vs decompression, and the effects of melt loss on the mineral assemblages and compositions have been investigated along four ideal P–T trajectories, dominated by either heating or decompression. Based on these results, the observed microstructures and mineral compositions of the Barun Gneiss have been interpreted in terms of melt-producing vs melt-consuming reactions (e.g. growth of peritectic garnet with preserved ‘nanogranite’ inclusions vs microstructures related to back-reactions between solids and melt), and used to derive the metamorphic evolution of the studied samples. The P–T pseudosection modelling predicts that at least 15–20 vol. % of melt was produced at peak P–T conditions through dehydration melting of both muscovite and biotite, and that melt production was mainly triggered by heating, with or without the combined effect of decompression. The preserved granulitic peak metamorphic assemblage, however, is consistent with a significant loss of most of this melt. The P–T evolution inferred for samples from different, strategically located, structural levels of the Barun Gneiss is consistent with the expectations of a ‘channel flow’ model, including: (1) the clockwise shape of the P–T paths; (2) the estimated P at peak T (new data: 10–8 kbar at 800°C; model: 13–7 kbar at 800°C); (3) the decreasing P structurally upward, which defines a ‘normal’ metamorphic sequence, in contrast to the inverted metamorphic sequence occurring in the lowermost Main Central Thrust Zone; (4) the nearly isothermal exhumation of the structurally lowest sample, reflecting the progressive exhumation of rocks that have been entrained in the deep, high-T region of the channel, versus the nearly isobaric heating of the structurally uppermost sample, reflecting the evolution of those rocks that flowed outwards with the underlying channel. [ABSTRACT FROM PUBLISHER]