Abstract
Orogenic degassing is emerging as a potentially relevant source of carbon dioxide (CO
2
) from the continental crust. However, the processes of carbon mobilization are still poorly explored. ...Here, we use thermodynamic modeling to investigate the decarbonation of sediments metamorphosed under high geothermal gradients. Our modeling shows that immiscible CO
2
-rich vapors and hydrosaline brines are generated at these conditions, with different properties and mobility through the crust. The CO
2
-rich fluid fraction could rapidly rise toward the surface without interacting with the host rocks by carbo-fracturing the host rocks or through deep faults. The denser hydrosaline brines likely permeate the source rocks. When applied to the active Himalayan orogen, these observations reconcile measured CO
2
fluxes at the surface and positive conductivity anomalies associated with micro-seismicity at depth. Our modeling shows that the continental crust represents a relevant reservoir of CO
2
that can be efficiently degassed during hot collisions.
The Main Central Thrust Zone (MCTZ) is a key tectonic feature in the architecture of the Himalayan chain. In the Arun valley of the eastern Nepal Himalaya, the MCTZ is a strongly deformed package of ...amphibolite- to granulite-facies metapelitic schist and granitic orthogneiss. This package is tectonically interposed between the underlying, low-grade, Lesser Himalaya sequences and the overlying, high-grade and locally anatectic, Higher Himalayan Crystallines (HHC). The MCTZ is characterized by a well documented inverted metamorphism from the Grt–Bt zone, across the Ky-in, St-in and -out, Kfs-in, Ms-out and Sil-in isograds. Partial melting with local occurrence of migmatitic segregations has been rarely reported from the highest structural levels of the MCTZ.
While it is widely accepted that thrusting along the MCT occurred during the Miocene, geochronological data constraining the timing of crustal anatexis in the upper portion of the MCTZ are still lacking. In order to understand the link between partial melting in the MCTZ and the Miocene activation of the MCT, we present the
P–
T–time evolution of a kyanite-bearing anatectic gneiss occurring at the highest structural levels of the MCTZ, along the Arun–Makalu transect (eastern Nepal).
Microstructural observations combined with
P–
T pseudosection analysis show that dehydration partial melting occurred in the kyanite-field. After reaching peak conditions at about 820
°C, 13
kbar, the studied sample experienced decompression accompanied by cooling down to 805
°C, 10
kbar, which caused
in situ melt crystallization. SHRIMP monazite and zircon geochronology provides evidence that the anatexis affecting the upper portion of the MCTZ occurred during Early Oligocene (∼
31
Ma). These results demonstrate that in the upper MCTZ, at least in the eastern Himalaya, crustal anatexis was earlier than, and not a consequence of, decompression linked to exhumation along the MCT.
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 established
Despite different carbonate species in protoliths, both the calcite‐rich and dolomite‐rich samples show nearly identical correlations
Steady state box‐model on the carbonate preservation efficiency of continent indicates it is a notable force to regulate atmospheric CO2
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•A structural and petrological study in the Rasuwa district of Nepal is given.•Petrological and structural analysis are adequate methods to detect discontinuities.•Pseudosection ...approach and AvPT methods give T, P and T/P ratio consistent results.•T/P ratio populations highlight the existence of different tectonometamorphic units.
A detailed structural, lithological and petrological study of different transects in the Rasuwa district of central Nepal Himalaya allows the characterization of the tectonostratigraphic architecture of the area. It also facilitates constraining the P-T evolution of the different units within the Lesser (LHS) and Greater (GHS) Himalayan Sequences. Peak P–T conditions obtained for the studied metapelite samples using the pseudosection approach and the Average PT method highlight the existence of four different T/P ratio populations in different tectonometamorphic units: 80 ± 11 °C/kbar (LHS), 66 ± 7 °C/kbar (RTS), 73 ± 1 °C/kbar (Lower-GHS) and 101 ± 12 °C/kbar (Upper-GHS). Integration of structural and petrological data emphasizes the existence of three tectonometamorphic discontinuities bounding these units, characterized by top-to-the-south sense of shear: the Ramgarh Thrust, which separates the LHS (peak metamorphism at ∼600 °C, 7.5 kbar) from the overlying RTS (peak metamorphism at ∼635 °C, 10 kbar); the Main Central Thrust, which separates the RTS from the Lower-GHS (peak at 700–740 °C, 9.5–10.5 kbar with a prograde increase in both P and T in the kyanite stability field), and the Langtang Thrust, which juxtaposes the Upper-GHS (peak at 780–800 °C, 7.5–8.0 kbar with a nearly isobaric heating in the sillimanite stability field) onto the Lower-GHS. An increase in the intensity of deformation, with development of pervasive mylonitic fabrics and/or shear zones, is generally observed approaching the discontinuities from either side.
Overall, data and results presented in this paper demonstrate that petrological and structural analysis combined together, are reliable methods adequate to identify tectonometamorphic discontinuities in both the LHS and GHS. Geochronological data from the literature allow the interpretation of these discontinuities as in-sequence thrusts.
Abstract
Fluid–earthquake interplay, as evidenced by aftershock distributions or earthquake-induced effects on near-surface aquifers, has suggested that earthquakes dynamically affect permeability of ...the Earth’s crust. The connection between the mid-crust and the surface was further supported by instances of carbon dioxide (CO
2
) emissions associated with seismic activity, so far only observed in magmatic context. Here we report spectacular non-volcanic CO
2
emissions and hydrothermal disturbances at the front of the Nepal Himalayas following the deadly 25 April 2015 Gorkha earthquake (moment magnitude
M
w
= 7.8). The data show unambiguously the appearance, after the earthquake, sometimes with a delay of several months, of CO
2
emissions at several sites separated by > 10 kilometres, associated with persistent changes in hydrothermal discharges, including a complete cessation. These observations reveal that Himalayan hydrothermal systems are sensitive to co- and post- seismic deformation, leading to non-stationary release of metamorphic CO
2
from active orogens. Possible pre-seismic effects need further confirmation.
Although the Himalaya is the archetype of collisional orogens, formed as a consequence of the closure of the Neo-Tethyan ocean separating India from Asia, high-pressure metamorphic rocks are rare. ...Beside few eclogites, corresponding to the metamorphosed continental Indian crust dragged below Asia or underthrusted beneath southern Tibet, blueschists occur seldom along the Yarlung–Tsangpo Suture zone, i.e. the suture marking the India–Asia collision. These blueschists, mostly interpreted as related to paleo-accretionary prisms formed in response to the subduction of the Neo-Tethyan ocean below the Asian plate, are crucial for constraining the evolution of the India–Asia convergence zone during the closure of the Neo-Tethyan Ocean.
In the Western Himalaya, the best occurrence of blueschist is that of the Sapi–Shergol Ophiolitic Mélange in Ladakh. This unit is dominated by volcanoclastic sequences rich in mafic material with subordinate interbedding of metasediments, characterized by very fresh lawsonite blueschist-facies assemblages.
In this paper, the lawsonite blueschist-facies metasediments have been petrologically investigated with the aims of (i) constraining the P–T evolution of the Sapi–Shergol Ophiolitic Mélange, (ii) evaluating the influence of Fe2O3 and of H2O on the stability of the high-pressure mineral assemblages, (iii) understanding the processes controlling lawsonite formation and preservation, and (iv) interpreting the P–T evolution of the Sapi–Shergol blueschists in the framework of India–Asia collision.
Our results indicate that (i) the Sapi–Shergol blueschists experienced a cold subduction history along a low thermal gradient, up to peak conditions of ca. 470°C, 19kbar; furthermore, in order to preserve lawsonite in the studied lithologies, exhumation must have been coupled with significant cooling, i.e. the resulting P–T path is characterized by a clockwise hairpin loop along low thermal gradients (<8–9°C/km); (ii) the presence of ferric iron in the investigated system has a non-negligible (lowering) effect on pressure estimates, whereas temperatures estimates are not influenced by the oxidation state of the system; (iii) the observed sequence of mineral growth (i.e. simultaneous growth of lawsonite and garnet) suggests that (a) the system was initially H2O-undersaturated and lawsonite growth was triggered by a protracted H2O influx at high pressure (equilibrium approach), or (b) the system was H2O-saturated since the beginning, but lawsonite growth was delayed due to the predominance of kinetic factors over equilibrium dynamics (nonequilibrium approach); (iv) the inferred P–T evolution is consistent with a cold subduction zone system in an intra-oceanic subduction setting. Moreover, the estimated peak P–T conditions roughly coincide with the maximum P–T estimates predicted by thermo-mechanical models for the metasediments exhumed in accretionary wedges, and with the maximum P–T conditions recorded by natural occurrences of blueschist accretionary complexes worldwide.
•The Sapi–Shergol lawsonite-blueschist metasediments are petrologically investigated.•They experienced cold subduction history (7–8°C/km gradient) up to 470°C, 19kbar.•The presence of Fe+3 has a non-negligible effect on P estimates.•Simultaneous growth of Grt and Lws implies discussion on equilibrium/nonequilibrium.•Peak PT estimates are close to those of blueschist accretionary complexes worldwide.
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•For the first time, a detailed fluid inclusions study on Himalayan blueschists was carried out.•There are aqueous-carbonic and aqueous fluids during peak P-T metamorphism.•A steep ...nearly isothermal decompression P-T path is compatible with the fluid inclusions evolution.•Inclusions textures record decompression following the peak metamorphism.•Dominantly aqueous fluids were most likely produced through metamorphic devolatilization reactions in the subducting slab.
The best occurrence of blueschist-facies lithologies in Himalaya is that of the Shergol Ophiolitic Mélange along the Indus suture zone in Ladakh region of north-western India. These lithologies are characterized by well preserved lawsonite-glaucophane-garnet-quartz assemblages. This paper presents for the first time the results of a detailed fluid inclusion study on these lithologies, in order to understand the fluid P-T evolution and its tectonic implications.
The blueschist rocks from Shergol Ophiolitic Mélange record metamorphic peak conditions at ∼19kbar, 470°C. Several types of fluid inclusions are trapped in quartz and garnet, most of them being two-phase at room temperature. Three types of fluid inclusions have been recognised, basing on microtextures and fluid composition: Type-I are primary two-phase carbonic-aqueous fluid inclusions (VCO2−LH2O); Type-II are two-phase (LH2O−VH2O) aqueous fluid inclusions, either primary (Type-IIa) or secondary (Type-IIb); Type-III are re-equilibrated fluid inclusions. In the Type-I primary carbonic-aqueous inclusions, H2O is strongly predominant with respect to CO2; the homogenization temperature of CO2 range from −7 to −2°C. The clathrate melting temperature in such inclusions varies in between +7.1 and +8.6°C. Type-II two-phase aqueous fluid inclusions show a wide range of salinity, from 7.8–14wt.% NaCleq (Type-IIa) to 1.65–6.37wt.% NaCleq (Type-IIb) with accuracy ±0.4wt.% NaCleq.
Type-I and Type-IIa primary fluid inclusions are hosted in peak minerals (garnet and quartz included in garnet), therefore they were likely entrapped at, or near to, peak P-T conditions. The dominantly aqueous fluid of both Type-I and Type-IIa inclusions was most likely produced through metamorphic devolatilization reactions occurring in the subducting slab. Despite their primary nature, the isochores of Type-I and Type-IIa inclusions do not intersect the peak metamorphic conditions of the blueschist mineral assemblage, suggesting that these inclusions stretched or re-equilibrated during nearly isothermal decompression from 19kbar to 3kbar or less, at T=290°C. This conclusion is further supported by their large variability in shapes and sizes which range from irregular inclusions (‘C’/arc shaped, hook shape and satellite type). This decompression stage was followed by nearly isobaric cooling, testified by the occurrence of dendritic networks of decrepitated and ‘imploded’ fluid inclusions.
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