Geochemistry and petrology of listvenite in the Samail ophiolite, Sultanate of Oman: Complete carbonation of peridotite during ophiolite emplacement

Abstract

Extensive outcrops of listvenite-fully carbonated peridotite, with all Mg in carbonate minerals and all Si in quartz-occur along the basal thrust of the Samail Ophiolite in Oman. These rocks can provide insight into processes including (a) carbon fluxes at the-leading edge of the mantle wedge‖ in subduction zones and (b) enhanced mineral carbonation of peridotite as a means of carbon storage. Here we examine mineralogical, chemical and isotopic evidence on the temperatures, timing, and fluid compositions involved in the formation of this listvenite. The listvenites are composed primarily of magnesite and/or dolomite + quartz + relict Cr-spinel. In some instances the conversion of peridotite to listvenite is nearly isochemical except for the addition of CO 2 , while other samples have also seen significant calcium addition and/or variable, minor addition of K and Mn. Along margins where listvenite bodies are in contact with serpentinized peridotite, talc and antigorite are present in addition to carbonate and quartz. The presence of antigorite + quartz + talc in these samples implies temperatures of 80-130°C. This range of temperature is consistent with dolomite and magnesite clumped isotope thermometry in listvenite (average T = 90 ± 15°C) and with conventional mineral-water oxygen isotope exchange thermometry (assuming fluid δ 18 O near zero). CO 2-bearing fluids responsible for the formation of listvenite were likely derived from underlying calcite-bearing metasediment during emplacement of the ophiolite. An internal Rb-Sr isochron from one listvenite sample yields an age of 97 ± 29 Ma, consistent with the timing of emplacement of the ophiolite over allochthonous sediments of the Hawasina group, and autochthonous sediments of the Arabian continental margin. Most of the initial 87 Sr/ 86 Sr values in the listvenite, ranging from 0.7085 to 0.7135, are significantly higher than seawater values and consistent with values measured in the underlying metasediments. While constraints on the pressure of listvenite formation are lacking, the moderate temperatures suggest that listvenites formed at relatively shallow depths in the subduction zone, making release of carbonate-saturated pore-water due to compaction of subducted sediment or low-pressure phase transitions of hydrous minerals, such as clays, probable sources of the CO 2-bearing fluid. Carbonate dissolution from subducted sediments and transfer of CO 2 to the mantle wedge to form listvenites may be an important process in forearc hydrothermal systems. Additionally, the presence of listvenites demonstrate that peridotite carbonation reactions can proceed to completion on large scales, suggesting that in situ mineral carbonation of peridotite may offer a viable solution for carbon storage.

Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up

“…The thickest mapped layer contains ∼1 billion tons of CO 2 . Samail listvenites have higher 87 Sr/ 86 Sr than Cretaceous to modern seawater, implying that Sr was derived from underlying sediments containing radiogenic 87 Sr/ 86 Sr, and we infer that sediment-derived fluid was also the source of the CO 2 and Ca in the listvenites (36). At ∼100°C along a shallow subduction zone geotherm of 5-15°C/km, listvenite formation occurred at 7-20 km (0.2-0.6 GPa) based on thermal models of subduction zones (e.g., refs.…”

“…1, 8, and 35). However, the presence of fully carbonated peridotites composed of magnesite + quartz and dolomite + quartz ("listvenites") in ophiolites thrust over metasediments suggests substantial carbon transfer at low temperatures (T) (36). For ∼15-20 million years starting at ∼96 Ma, the Samail ophiolite of Oman was thrust over oceanic crust and sediments (37,38).…”

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain)

“…Metamorphic devolatilization reactions of altered oceanic crust and sediments in the slab generate C‐ and Mg/Ca‐rich fluids (Caciagli & Manning, ; Facq et al., ; Tiraboschi et al., ). At different arc depths, reaction of these fluids with slab and mantle wedge serpentinites may produce ophimagnesite (magnesite–antigorite rocks), talc–magnesite rocks and listvenites (Falk & Kelemen, ; Menzel et al., ; Sieber, Hermann, & Yaxley, ), HP‐ophicarbonate (e.g. Scambelluri et al., ) and carbonate–enstatite rocks (Tumiati et al., ).…”

Dissolution‐Assisted Pattern Formation During Olivine Carbonation

“…These reactions are exothermic and can proceed vigorously and irreversibly. Where ultramafic rocks have been tectonically juxtaposed against metamorphosed carbonate‐bearing sediments, olivine and pyroxene can be entirely converted to carbonate and quartz (Beinlich et al, ; Falk & Kelemen, ). As such, they have been suggested as a reservoir for long‐term geological carbon sequestration (Seifritz, ; Kelemen & Matter, ).…”