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

&amp;lt;p&amp;gt;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.&amp;amp;#160;&amp;amp;#160;&amp;amp;#160; &amp;amp;#160;&amp;amp;#160;&amp;amp;#160;&amp;amp;#160;&amp;amp;#160;&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;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&amp;amp;#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.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;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(CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;) 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 CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;. The main results are:&amp;lt;/p&amp;gt;&amp;lt;ul&amp;gt;&amp;lt;li&amp;gt;the observed assemblages formed during a heating decompression stage, at P-T conditions of 620 &amp;amp;#177; 20&amp;amp;#176;C, 8.5 &amp;amp;#177; 0.2 kbar, consistent with those registered by the associated metapelites;&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;the observed peak assemblages are predicted to be stable in equilibrium with a CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-bearing fluid, even in those samples where carbonates are no more preserved;&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;the overall results point to an internally buffered P/T-X(CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;) evolution. The amount of carbonates consumed during prograde metamorphism varies in the range 7-20 vol%, corresponding to 3-10 wt% of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; These CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; amounts are nearly double the CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; released by calcareous pelites (Groppo et al., 2021).&amp;lt;/li&amp;gt; &amp;lt;/ul&amp;gt;&amp;lt;p&amp;gt;The main consequence of this study is that the CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; productivity of dolomitic and magnesitic pelites is significant and that these lithologies could be relevant sources of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, possibly contributing to the diffuse Himalayan CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; degassing (e.g. Girault et al., 2014, 2018).&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;References&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Girault et al. (2014). Geoph. Res. Lett. 41, 6358&amp;amp;#8211;6366&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Girault et al. (2018). Nat. Comm. 9, 2956&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Groppo et al. (2017). J. Petrol. 58, 53-83.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Groppo et al. (2021). J. metam. Geol. 39, 181-207.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Groppo et al. (2022). Comm. Earth Environ, doi: 10.1038/s43247-022-00340-w&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Rapa et al. (2017). Lithos, 292&amp;amp;#8211;293, 364&amp;amp;#8211;378.&amp;lt;/p&amp;gt;