EGU22-3929, updated on 16 Jun 2022
https://doi.org/10.5194/egusphere-egu22-3929
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Carbonation of peridotites along the basal thrust of the Semail Ophiolite (OmanDP Hole BT1B): insights from Fe and Zn isotopes

Thierry Decrausaz1, Marguerite Godard1, Baptiste Debret2, and Isabelle Martinez2
Thierry Decrausaz et al.
  • 1Géosciences Montpellier, Université de Montpellier, CNRS, 34095 Montpellier, France
  • 2Institut de physique du globe de Paris, Université de Paris, CNRS, Paris, France

The formation of carbonated serpentinites (serpentine, Mg-Ca carbonates) and listvenites (quartz, Mg-carbonate) by reactions between exhumed mantle peridotites and percolating CO2-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 °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).

The partially serpentinized harzburgite and carbonated serpentinites have δ56Fe and δ66Zn compositions ranging between -0.05 – +0.06 ‰ and -0.11 – +0.15, respectively, overlapping that of previously analysed abyssal (δ56Fe: -0.15 – +0.11 ‰; δ66Zn: +0.12 – +0.62 ‰), ophiolitic (δ56Fe: -0.27 – +0.14 ‰; δ66Zn: -0.56 – +0.38 ‰), orogenic (δ56Fe: -0.06 – +0.12 ‰; δ66Zn: +0.03 – +0.55 ‰), and fore-arc (δ56Fe: -0.26 – +0.09 ‰) peridotites. In contrast, listvenites display highly variable δ56Fe and δ66Zn values, between -0.33 – +0.2 ‰ and -0.46 – +0.64 ‰ respectively. Iron isotopes compositions show a positive correlation with bulk iron contents. Zinc isotope compositions are positively correlated to δ13CTC values, suggesting a high mobility of Zn in carbonate-bearing fluids. The lightest δ66Zn 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 (δ56Fe: +0.01 – +0.24 ‰; δ66Zn: +0.24 – +0.47 ‰), in agreement with an oceanic crust-derived protolith (MORB, δ56Fe: +0.06 – +0.18; δ66Zn: +0.27 – +0.30 ‰).

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.

How to cite: Decrausaz, T., Godard, M., Debret, B., and Martinez, I.: Carbonation of peridotites along the basal thrust of the Semail Ophiolite (OmanDP Hole BT1B): insights from Fe and Zn isotopes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3929, https://doi.org/10.5194/egusphere-egu22-3929, 2022.