Dolomite-and magnesite-bearing pelites: poorly investigated, yet significant, sources of CO2 in collisional orogens.
Authors: Tamang Shashi , Groppo Chiara , Rolfo Franco , Girault Frédéric 
Published: March 2022

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