Carbonation of peridotites along the basal thrust of the Semail Ophiolite (OmanDP Hole BT1B): insights from Fe and Zn isotopes
Authors: Decrausaz Thierry , Godard Marguerite , Debret Baptiste , Martinez Isabelle 
Published: March 2022

&lt;p&gt;The formation of carbonated serpentinites (serpentine, Mg-Ca carbonates) and listvenites (quartz, Mg-carbonate) by reactions between exhumed mantle peridotites and percolating CO&lt;sub&gt;2&lt;/sub&gt;-bearing fluids is a major sink for carbon from spreading ridges to ophiolites and orogenic suture zones. During ICDP Oman Drilling Project, the transition from the base of the Semail Ophiolite to its metamorphic sole was drilled at Hole BT1B (Wadi Mansah), allowing to recover ~200 m of variously carbonated serpentinites and listvenites, and underlying metabasalts. Mineralogical and geochemical investigations indicate that carbonation at the expense of the Wadi Mansah peridotites was triggered by the migration of multiple fluid batches along the basal thrust at shallow depths and low temperatures (50-250 &amp;#176;C). To better constrain the impacts of fluid source(s) and protolith compositions on reaction pathways and oxidation state during carbonation, we carried out iron and zinc isotopes study of 19 variously carbonated peridotites (13 listvenites, 5 carbonated serpentinites, one serpentinized harzburgite) and of 6 underlying metamorphic samples from Wadi Mansah area (including 3 BT1B samples).&lt;/p&gt;&lt;p&gt;The partially serpentinized harzburgite and carbonated serpentinites have &amp;#948;56Fe and &amp;#948;66Zn compositions ranging between -0.05 &amp;#8211; +0.06 &amp;#8240; and -0.11 &amp;#8211; +0.15, respectively, overlapping that of previously analysed abyssal (&amp;#948;56Fe: -0.15 &amp;#8211; +0.11 &amp;#8240;; &amp;#948;66Zn: +0.12 &amp;#8211; +0.62 &amp;#8240;), ophiolitic (&amp;#948;56Fe: -0.27 &amp;#8211; +0.14 &amp;#8240;; &amp;#948;66Zn: -0.56 &amp;#8211; +0.38 &amp;#8240;), orogenic (&amp;#948;56Fe: -0.06 &amp;#8211; +0.12 &amp;#8240;; &amp;#948;66Zn: +0.03 &amp;#8211; +0.55 &amp;#8240;), and fore-arc (&amp;#948;56Fe: -0.26 &amp;#8211; +0.09 &amp;#8240;) peridotites. In contrast, listvenites display highly variable &amp;#948;56Fe and &amp;#948;66Zn values, between -0.33 &amp;#8211; +0.2 &amp;#8240; and -0.46 &amp;#8211; +0.64 &amp;#8240; respectively. Iron isotopes compositions show a positive correlation with bulk iron contents. Zinc isotope compositions are positively correlated to &amp;#948;13C&lt;sub&gt;TC&lt;/sub&gt; values, suggesting a high mobility of Zn in carbonate-bearing fluids. The lightest &amp;#948;66Zn values were measured in listvenites with minor amounts of fuchsite (Cr-mica), that often display evidences for breakdown of Cr-spinel. Metamorphic sole samples display isotopic compositions typical of mafic rocks (&amp;#948;56Fe: +0.01 &amp;#8211; +0.24 &amp;#8240;; &amp;#948;66Zn: +0.24 &amp;#8211; +0.47 &amp;#8240;), in agreement with an oceanic crust-derived protolith (MORB, &amp;#948;56Fe: +0.06 &amp;#8211; +0.18; &amp;#948;66Zn: +0.27 &amp;#8211; +0.30 &amp;#8240;).&lt;/p&gt;&lt;p&gt;Our results suggest an important control of the protolith chemistry and complexation with dissolved carbon in reactive fluids on the Fe and Zn isotopes compositions.&lt;/p&gt;