Redox events in cratonic mantle underneath Obnazhennaya kimberlite, Yakutia – chemical records in pyroxenites

<p><span>Metasomatism is omnipresent in subcontinental lithospheric mantle (SCLM). </span><span>Whatever </span><span>a distribution scale </span><span>in the SCLM</span><span>,</span> <span>it has</span><span> a strong link to changes in oxygen fugacity (&#402;O</span>_{<span>2</span>}<span>)</span> <span>[1]</span><span>. It is </span><span>also </span><span>known that &#402;O</span>_{<span>2</span>}<span> of Earth&#8217;s interior controls speciation within the C&#8211;H&#8211;O&#8211;S&#8211;</span><span>N</span><span> system and stability of C-bearing</span> <span>phases (diamond, graphite, carbonates, carbides</span><span>, </span><span>volatile-bearing fluid and melt; </span><span>[2]</span><span>). </span><span>Our new </span><span>geochemical,</span> <span>M&#246;ssbauer and Raman spectroscopic results</span><span> suggest no less than one </span><span>episode </span><span>of </span><span>mantle metasomatism </span><span>related to</span> <span>the </span><span>formation and preservation of elemental carbon minerals within Siberian SCLM, </span><span>w</span><span>hile</span><span> later events </span><span>are </span><span>connected to interaction </span><span>of rocks </span><span>with hydrous </span><span>and carbon dioxide </span><span>components</span><span>.</span></p><p><span>We studied a graphite-bearing mantle xenolith from the diamond-free Obnazhennaya kimberlite pipe, Republic of Sakha (Yakutia), Russia, that represents a garnet websterite consisting of garnet (Grt), clinopyroxene (Cpx), </span><span>o</span><span>rthopyroxene (Opx), </span><span>graphite, rutile, ilmenite, pyrrhotite, pentlandite, secondary serpentine, p</span><span>hlogopite </span><span>and carbonates (calcite)</span><span>. Cpx hosts Opx and Grt lamellae. Grt cores contain </span><span>scarce but </span><span>oriented mineral inclusions (silicates and Ti-oxides) that we interpret to be exsolved from a Si- and Ti-rich precursor. </span><span>L</span><span>inearly </span><span>distributed</span> <span>m</span><span>elt </span><span>and fluid </span><span>inclusions in </span><span>silicates </span><span>are thought </span><span>to postdate the exsolutions. </span><span>Both m</span><span>ajor and trace ele</span><span>ment</span><span>s</span><span> of rock-forming silicates match that of peridotite</span><span>s</span><span> and pyroxenite</span><span>s</span><span> with exsolutions in Grt and </span><span>pyroxenes from Obnazhennaya and</span><span> worldwide. </span><span>Pressure and temperat</span><span>ure estimates (</span><span><em>T</em></span><span> 910 &#176;C, </span><span><em>P</em></span><span> 3.5 GPa</span><span>) also fal</span><span>l</span><span> int</span><span>o</span><span> the range in which </span><span>alike</span><span> rocks have been equilibrated in the SCLM.</span></p><p> <span>Microstructural and chemical data </span><span>allowed to propose</span><span> crystallization of the garnet websterite from high-</span><span><em>T</em></span><span> Mg-rich magmas similar to komatiite </span><span>[3]</span><span>, which forms in deep </span><span>Grt-bearing depleted</span><span> mantle sources with low oxygen fugacity </span><span>[4]</span><span>. </span><span>S</span><span>ubsequent metasomatism of the reduced websterite by oxidising C-O-H fluids caused graphite precipitation through redox freezing </span><span>[5]</span><span>, and such reactions constitute an important part of Earth&#8217;</span><span>s</span><span> hidden carbon cycle</span><span>. </span><span>I</span><span>nfiltration of hydrous and CO</span>_{<span>2</span>}<span>-rich fluids </span><span>likely </span><span>p</span><span>ostdated this episode</span><span>.</span></p><p>The work was supported by the Russian Science Foundation (grant No 16-77-10062).</p><p><span>[1] </span><span>Frost D.J. & McCammon C.A. (2008) Annu Rev Earth Planet Sci 36</span><span>:</span><span> 389-420.&#160;</span><span>[2] </span><span>Stagno V. </span><span>et al</span><span>. (2013) Nature 493:84-88.&#160;</span><span>[3] </span>Spengler D. & Alifirova, T.A. (2019) Lithos 326: 384-396.&#160;<span>[4] </span><span>Berry A.J. </span><span>et al</span><span>. (2008) Nature 455: 960-</span><span>963</span><span>.&#160;</span><span>[5] </span>Rohrbach A. & Schmidt M.W. (2011) Nature 472: 209-212.</p>