Search

Investigating the carbonate preservation efficiency (CPE) of continental crust is crucial to understand the global carbon cycle, which requires constraints on initial carbonate abundances (ICAs) of crustal rocks. To link Mg isotopes to ICAs, we present elemental and Mg isotopic data for Himalayan carbonate‐bearing and carbonate‐free metasedimentary rocks. Given no evident melt extraction or external‐fluid infiltration, ICAs of these samples can be independently estimated by elemental data. Despite different carbonate species in the protoliths, all the samples show congruent relationship between their δ26Mg and ICAs, owing to the elevated carbonate δ26Mg and Mg/Ca in protoliths of calcite‐rich samples resulting from diagenetic processes. When collated with literature data, we suggest the observed correlation here can be applied to most carbonate‐bearing (meta‐)sedimentary rocks. Based on a steady state box‐model, we constrained the modern net carbonate accretion flux (9.50−5.56+9.50 ${9.50}_{-5.56}^{+9.50}$ Tmol/year) and the average time‐integrated CPE (∼80−43+20 ${80}_{-43}^{+20}$%) for continental crust. Plain Language Summary: Investigating the fate of carbonate preserved in continental crust is fundamental for understanding its role playing in the global carbon cycle, but is hindered by the lack of knowledge about the initial carbonate abundance of metasedimentary rocks prior to modification (e.g., anatexis). By analyzing Himalayan metasedimentary rocks, here we show a congruent relationship between their δ26Mg and initial carbonate abundances, irrespective of their protolith carbonate species, and suggest it is applicable to most carbonate‐bearing (meta‐)sedimentary rocks. Based on this relationship, the carbonate preservation in continental crust was simulated using a steady state box‐model. The results indicate a very high carbonate accretion influx to the continental crust, seven times higher than the C degassing flux in the mid‐ocean ridge. Considering explosive degassing of the accreted carbonates during episodic tectonomagmatic events, the continental crust could have been an important driving force for regulating the atmospheric CO2 during Earth's history. Key Points: A strong correlation between bulk δ26Mg of carbonate‐bearing metasedimentary rocks and their initial carbonate abundances was establishedDespite different carbonate species in protoliths, both the calcite‐rich and dolomite‐rich samples show nearly identical correlationsSteady state box‐model on the carbonate preservation efficiency of continent indicates it is a notable force to regulate atmospheric CO2 [ABSTRACT FROM AUTHOR]

A well‐preserved remnant of the middle crust of the former Adriatic passive margin is exposed in the Southern Alps (Italy). The Dervio–Olgiasca Zone is located south of the Insubric Line along the northern part of Como Lake and, because of the lack of Alpine overprint, provides favourable conditions to investigate the pre‐Alpine (rift‐related) history. We reconstruct the P–T–t–d evolution of the Adria middle crust through petrological (petrography, mineral chemistry, thermobarometry and thermodynamic modelling) and geochronological (Lu/Hf in garnet and U–Pb in monazite) data from pegmatites and host micaschists. These data allow reconstruction of a complex tectono‐thermal evolution of the future proximal Adriatic margin at the onset of Alpine rifting. The amphibolite‐facies Carboniferous metamorphic basement (7.6–10 kbar and 610–660°C at 318–312 Ma) was affected by pervasive extensional deformation (5.1–7.6 kbar and 580–660°C) in the Middle‐ to Late‐Permian (257.5 ± 3.8 Ma). Pegmatite intruded at 249.8 ± 1.1 Ma in an extensional phase that re‐equilibrated rocks of the basement at 3.5–4.5 kbar and 560–600°C. During the Middle‐ to Late‐Triassic (241–235 Ma), the basement experienced static thermal recrystallization (T = 689 ± 41°C and ~5.0 kbar). This Late‐Anisian to Early‐Carnian thermal event was simultaneous with the emersion of carbonate platforms, volcanism and ore deposition in the future proximal Adriatic margin. The subsequent cooling of the middle crust was synchronous with large‐scale extensional detachments developed in the upper crust (e.g., the Lugano‐Val Grande Fault), which controlled the formation of the Monte Generoso Basin. This study reveals that the local post‐Carboniferous thinning and heating events recorded in the Adriatic middle crust were interconnected to other processes occurring at different crustal levels that were, in turn, induced by crustal stretching in the early stages of the Alpine rifting. [ABSTRACT FROM AUTHOR]

Recent studies have demonstrated that kinetic factors can significantly influence garnet nucleation, delaying its appearance with respect to the equilibrium predictions. Overstepping of garnet nucleation occurs in both contact and regional metamorphic settings, with extremely variable degrees: The factors controlling such highly variable degrees of overstepping are still unclear. This study focuses on garnet nucleation and growth in aluminous metapelites from the Barrovian inverted metamorphic pile of the upper portion of the Lesser Himalayan Sequence (Upper‐LHS; central Nepal), with the aim of (i) investigating if (and how) the bulk‐rock composition can influence overstepping of garnet nucleation and (ii) which are the implications of garnet nucleation overstepping for the P–T evolution of a Barrovian metamorphic sequence. Detailed petrographic, microstructural and compositional data are presented for six phyllitic schists, containing porphyroblasts of garnet, staurolite and/or kyanite. Their P–T evolution is constrained through thermodynamic forward modelling (i.e., isochemical phase diagrams combined with isopleth thermobarometry), assuming that equilibrium was attained at every stage of their metamorphic evolution. Comparison between the P–T conditions inferred for the growth of garnet core, the assemblages predicted to be stable at these P–T conditions and the modelled garnet‐in reaction boundary suggests that the studied samples have experienced different degrees of apparent thermal (ΔT) and/or baric (ΔP) overstepping of garnet nucleation. We suggest that the bulk‐rock MnO and CaO amounts might have influenced the apparent ΔT and ΔP overstepping of the garnet‐in reaction: more specifically, the higher the bulk‐rock MnO content, the more pronounced the apparent ΔT overstepping, whereas the lower the bulk‐rock CaO content, the greater the ΔP overstepping. However, rather than an effective delay of garnet appearance with respect to equilibrium predictions, the apparent ΔT overstepping of garnet nucleation could reflect the attainment of the critical 0.5% threshold of garnet abundance, below which garnet is not readily detected in thin section. Kinetic factors seem much less critical in controlling the growth of the garnet rim at peak P–T conditions, confirming that peak metamorphic conditions constrained through equilibrium approaches based on the composition of garnet rim and of the matrix assemblage can be considered as reliable. Overall, the P–T paths of the studied samples are characterized by prograde heating coupled with tectonic overload (peak‐P conditions of 9.5–10.5 kbar, 580–590°C), followed by heating during exhumation (peak‐T conditions of 8.2–8.9 kbar, 610–630°C), supporting those thermo‐mechanical models that predict a period of slowdown (or quiescence) of the Main Central Thrust activity. [ABSTRACT FROM AUTHOR]

The type and kinetics of metamorphic C[O.sub.2]-producing processes in metacarbonate rocks is of importance to understand the nature and magnitude of orogenic C[O.sub.2] cycle. This paper focuses on C[O.sub.2] production by garnet-forming reactions occurring in calc-silicate rocks. Phase equilibria in the CaO-FeO-[Al.sub.2][O.sub.3]-Si[O.sub.2]-C[O.sub.2] -[H.sub.2]O (CFAS-C[O.sub.2]-[H.sub.2]O) system are investigated using P-T phase diagrams at fixed fluid composition, isobaric T-X(C[O.sub.2]) phase diagram sections and phase diagram projections in which fluid composition is unconstrained. The relevance of the CFAS-C[O.sub.2]-[H.sub.2]O garnet-bearing equilibria during metamorphic evolution of calc-silicate rocks is discussed in the light of the observed microstructures and measured mineral compositions in two representative samples of calc-silicate rocks from eastern Nepal Himalaya. The results of this study demonstrate that calc-silicate rocks may act as a significant C[O.sub.2] source during prograde heating and/or early decompression. However, if the system remains closed, fluid-rock interactions may induce hydration of the calc-silicate assemblages and the in situ precipitation of graphite. The interplay between these two contrasting processes (production of C[O.sub.2]-rich fluids vs. carbon sequestration through graphite precipitation) must be considered when dealing with a global estimate of the role exerted by decarbonation processes on the orogenic C[O.sub.2] cycle. Keywords Orogenic C[O.sub.2] cycle * Decarbonation processes * Calc-silicate rocks * Mixed-volatile phase diagram projections * Graphite * Himalaya, Introduction Metamorphic degassing from active collisional orogens supplies a significant amount of C[O.sub.2] to the atmosphere, playing a fundamental role in the long-term (>1 Ma) global carbon cycle (e.g. Kerrick [...]