Brittle Deformation of Carbonated Peridotite—Insights From Listvenites of the Samail Ophiolite (Oman Drilling Project Hole BT1B)

Abstract

Hole BT1B of the Oman Drilling Project provides a continuous sampling from listvenite into the metamorphic sole that preserves the deformation, hydration, and carbonation processes of oceanic mantle peridotite at the base of the Samail ophiolite, Oman. We present evidence of multistage brittle deformation in listvenites and serpentinites based on field observations, visual core logging and petrography. About 10 vol% of listvenite and serpentinite in Hole BT1B is composed of cataclasite bands. Cataclasites contain lithic clasts of listvenite with spheroidal, zoned magnesite and quartz, and fragments of chalcedony-carbonate veins that elsewhere crosscut listvenite-showing that cataclasis postdates listvenite formation. Locally the cataclasites are reworked and cut by thin, sharp faults, pointing to repeated reactivation of brittle structures. SEM-EDS mapping shows that cataclasis was related to dissolution of carbonate and/or silica cementation. Dolomite veins crosscut cataclasites and breccias, suggesting that part of the Ca gain in BT1B is related to late fluids after listvenite formation. These results indicate a multistage tectonic overprint after peridotite carbonation and listvenite formation, which may be related to the tectonic history of the deformed continental margin under the ophiolite. These relatively late brittle structures should be excluded when trying to understand the carbonation of peridotite to listvenite.

Deep sourced fluids for peridotite carbonation in the shallow mantle wedge of a fossil subduction zone: Sr and C isotope profiles of OmanDP Hole BT1B

“…As noted above, after the magnesite and dolomite listvenites formed they were cataclastically deformed, and then cut by late Ca-rich carbonate veins (Menzel et al 2020). Thus, one might expect the veins to have formed from a later, geochemically distinctive fluid as suggested by variable clumped isotope (D47) derived temperatures (Beinlich et al, 2020) and changes in geochemical signatures in dolomite-dominated intervals (Godard et al 2021).…”

Geochemical Profiles Across the Listvenite-Metamorphic Transition in the Basal Megathrust of the Semail Ophiolite: Results from Drilling at Oman DP Hole BT1B

“…The most prominent occurs at 18-35 mbg in the Listvenite domain I, and shows enrichments in Li, Mn, U, Zn and Cu, and high Fe 3+ /Fe T . These variations are not correlated to major changes in mineralogy or to geochemical domains, but they are observed in areas recording extensive fracturing and late re-cementation events, with locally the late precipitation of iron oxides (mostly hematite) [Menzel et al, 2020b]. Efficient fracturing may have favored effective and long-lasting fluid-rock interactions and elemental redistributions, possibly until the final stages of listvenitization.…”

“…The density of carbonated serpentinites and listvenites (2.52±0.05 g.cm -3 and 2.86±0.07 g.cm -3 respectively ) being relatively close to that of the variously serpentinized Semail peridotites (2.81±0.50 g.cm -3 [Dewandel, 2002]), volume expansion is of the same order of magnitude as mass changes (Supplementary Table S4). The abundant fractures and veining characterizing the listvenite series Menzel et al, 2020b] possibly accommodated these mass and volume changes at the scale of the BT1B cored section. However these results raise the question of the mechanisms that facilitate such a large and heterogeneous increase in mass and volume at the scale of the Wadi Mansah massif.…”

Tracing Carbonate Formation, Serpentinization, and Biological Materials With Micro‐/Meso‐Scale Infrared Imaging Spectroscopy in a Mars Analog System, Samail Ophiolite, Oman

“…Listvenite at and near this locality has been the subject of many studies (de Obeso et al, 2021;Falk & Kelemen, 2015;Nasir et al, 2007;Stanger, 1985;Wilde et al, 2002) and was sampled in rock core from Oman Drilling Project (OmanDP) Hole BT1B (Beinlich et al, 2020;Kelemen, Matter. et al, 2020;Menzel et al, 2020).…”