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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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