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.

5Carbonate formation at hyperalkaline springs is typical of serpentinization in 6 peridotite massifs worldwide. These travertines have long been known to exhibit large 7 variations in their carbon and oxygen isotope compositions, extending from apparent 8 equilibrium values to highly depleted values. However, the exact causes of these 9 variations are not well constrained. We analyzed a suite of well-characterized fresh 10 carbonate precipitates and travertines associated with hyperalkaline springs in the 11 peridotite section of the Samail ophiolite, Sultanate of Oman, and found their clumped 12 isotope compositions vary systematically with formation environments. Based on these 13 findings, we identified four main processes controlling the stable isotope compositions of 14 these carbonates. These include hydroxylation of CO2, partial isotope equilibration of 15 dissolved inorganic carbon, mixing between isotopically distinct carbonate end-members, 16 and post-depositional recrystallization. Most notably, in fresh crystalline films on the 17 surface of hyperalkaline springs and in some fresh carbonate precipitates from the bottom 18 of hyperalkaline pools, we observed large enrichments in Δ47 (up to ~0.2‰ above 19 expected equilibrium values) which accompany depletions in δ 18 O and δ 13 C, yielding 20 about 0.01‰ increase in Δ47 and 1.1‰ decrease in δ 13 C for every 1‰ decrease in δ 18 O, 21 relative to expected equilibrium values. This disequilibrium trend, also reflected in 22 preserved travertines ranging in age from modern to ~40,000 years old, is interpreted to 23 arise mainly from the isotope effects associated with the hydroxylation of CO2 in high-24 pH fluids and agrees quantitatively with our theoretical prediction. In addition, in some 25 fresh carbonate precipitates from the bottom of hyperalkaline pools and in subsamples of 26 one preserved travertine terrace, we observed additional enrichments in Δ47 at 27 intermediate δ 13 C and δ 18 O, consistent with mixing between isotopically distinct 28 carbonate end-members. Our results suggest that carbonate clumped isotope analysis can 29 be a valuable tool for identifying and distinguishing processes not readily apparent from 30 the carbonate bulk stable isotope compositions alone, e.g., kinetic effects or mixing of 31 different carbonate end-members, which can significantly alter both the apparent 32 formation temperatures and apparent radiocarbon ages. The isotope trends observed in 33 these travertine samples could be applied more broadly to identify extinct hyperalkaline 34 springs in terrestrial and extraterrestrial environments, to better constrain the formation 35 conditions and post-depositional alteration of hyperalkaline spring carbonates, and to 36 extract potential paleoclimate information. 37 38

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