Geochemical Profiles Across the Listvenite‐Metamorphic Transition in the Basal Megathrust of the Semail Ophiolite: Results From Drilling at OmanDP Hole BT1B

The transition from the Semail ophiolite mantle to the underlying metamorphic sole was drilled at ICDP OmanDP Hole BT1B. We analyzed the bulk major, volatile and trace element compositions of the mantle‐derived listvenite series and metamorphic rocks, with the aim to constrain chemical transfers associated with peridotite carbonation along the ophiolite basal thrust. The listvenite series comprise variously carbonated serpentinites and (fuchsite‐bearing) listvenites. They have high CO2 (up to 43 wt.%) and variable H2O (0–12 wt.%). Yet, they have compositions close to that of the basal banded peridotites for most major and lithophile trace elements, with fuchsite‐bearing listvenites overlapping in composition with amphibole‐bearing basal lherzolites (e.g., Al2O3 = 0.1–2.2 wt.%; Yb = 0.05–1 x CI‐chondrite). The protolith of the listvenite series was likely similar in structure and composition to serpentinized banded peridotites which immediately overlie the metamorphic sole elsewhere in Oman. The listvenite series are enriched in fluid mobile elements (FME) compared to Semail peridotites (up to ∼103–104 x Primitive Mantle), with concentrations similar to the underthrusted metabasalts and/or metasediments for Cs, Sr and Ca and sometimes even higher for Pb, Li, As, and Sb (e.g., Li up to 130 μg/g; As up to 170 μg/g). We also observe a decoupling between Sr‐Ca enrichments and other FME, indicating interactions with several batches of deep CO2‐rich fluids transported along the basal thrust. These results suggest that peridotite carbonation could represent one of the major trap‐and‐release mechanisms for carbon, water and FME along convergent margins. Plain Language Summary Ophiolites are sections of oceanic lithosphere emplaced on‐land as tectonic plates converge. The faults developed at their base are analogues to plate interfaces in subduction zones, where mass transfers occur and play a key role in the global cycling of elements. A core was drilled at the base of the Semail Ophiolite, where variously hydrated and carbonated mantle rocks known as serpentinites and listvenites witnessed major fluid fluxes. Reactions with CO2‐bearing fluids (carbonation reaction) enhanced the mobility of elements during mass transfers along the basal thrust. We measured the elemental composition of 84 samples spaced along this core. Results indicate that CO2‐bearing fluids derive from at least two sources or pathways. As peridotites reacted, their volume increased, causing crack