Redox events in cratonic mantle underneath Obnazhennaya kimberlite, Yakutia &#8211; chemical records in pyroxenites
Authors: Alifirova Taisia A. , Aulbach Sonja , Korolev Nester M. , Golovin Aleksandr V. , Oleinikov Oleg B. 
Published: March 2020

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