Dolomite-and magnesite-bearing pelites: poorly investigated, yet significant, sources of CO2 in collisional orogens.

<p>Calcite-bearing sediments (calcareous pelites, marls, impure limestones) are among the most investigated sources of carbon in collisional settings (e.g. Groppo et al., 2017, 2021, 2022; Rapa et al., 2017). Dolomite- and magnesite-bearing sediments, however, can also be important constituents of evaporitic sequences deposited along passive margins and involved in collisional orogenic processes. So far, decarbonation reactions in dolomite- and magnesite-bearing rocks have been rarely investigated, and their contribution to the orogenic carbon cycle substantially neglected.&#160;&#160;&#160; &#160;&#160;&#160;&#160;&#160;&#160;</p><p>As a contribution to the understanding of the influence of dolomite- and magnesite-bearing lithologies on the global Earth's carbon cycle, a petrologic study was focused on the Lesser Himalayan Sequence (LHS) in central Nepal. The LHS is a thick Proterozoic sedimentary sequence originally deposited on the northern margin of the Indian plate, metamorphosed during the Himalayan orogeny. Abundant dolomite- and magnesite&#8211;bearing lithologies occur in the Upper-LHS, whose protoliths can be grouped in: (1) a dolomitic series (dolostones, dolomitic marls, dolomitic pelites), and (2) a magnesitic series (sparry magnesite ores, magnesitic pelites). The magnesite deposits associated to dolomitic lithologies are interpreted as the evidence of evaporitic environments during the Proterozoic.</p><p>The schists derived from dolomitic pelites show mineral assemblages similar to those of normal metapelites, but with significant amounts of Ca-rich minerals (e.g. plagioclase) and with biotite anomalously enriched in Mg. The schists derived from magnesitic pelites are, instead, characterized by uncommon assemblages such as orthoamphibole + kyanite + garnet + phlogopite. Thermodynamic forward modelling (P/T-X(CO₂) pseudosections) applied to these schists allowed to: (1) understand the nature of the main decarbonation reactions; (2) constrain the P-T conditions at which these reactions occurred, and (3) estimate the amounts of dolomite/magnesite consumed during prograde metamorphism, and the correspondent amounts of released CO₂. The main results are:</p><ul><li>the observed assemblages formed during a heating decompression stage, at P-T conditions of 620 &#177; 20&#176;C, 8.5 &#177; 0.2 kbar, consistent with those registered by the associated metapelites;</li> <li>the observed peak assemblages are predicted to be stable in equilibrium with a CO₂-bearing fluid, even in those samples where carbonates are no more preserved;</li> <li>the overall results point to an internally buffered P/T-X(CO₂) evolution. The amount of carbonates consumed during prograde metamorphism varies in the range 7-20 vol%, corresponding to 3-10 wt% of CO₂ These CO₂ amounts are nearly double the CO₂ released by calcareous pelites (Groppo et al., 2021).</li> </ul><p>The main consequence of this study is that the CO₂ productivity of dolomitic and magnesitic pelites is significant and that these lithologies could be relevant sources of CO₂, possibly contributing to the diffuse Himalayan CO₂ degassing (e.g. Girault et al., 2014, 2018).</p><p>&#160;</p><p>References</p><p>Girault et al. (2014). Geoph. Res. Lett. 41, 6358&#8211;6366</p><p>Girault et al. (2018). Nat. Comm. 9, 2956</p><p>Groppo et al. (2017). J. Petrol. 58, 53-83.</p><p>Groppo et al. (2021). J. metam. Geol. 39, 181-207.</p><p>Groppo et al. (2022). Comm. Earth Environ, doi: 10.1038/s43247-022-00340-w</p><p>Rapa et al. (2017). Lithos, 292&#8211;293, 364&#8211;378.</p>