Controls on the stable isotope compositions of travertine from hyperalkaline springs in Oman: Insights from clumped isotope measurements

Abstract

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

Mantle exhumation and metamorphism in the Basque‐Cantabrian Basin (NSpain): Stable and clumped isotope analysis in carbonates and comparison with ophicalcites in the North‐Pyrenean Zone (Urdach and Lherz)

“…Very low d 13 C values in carbonates could also be explained by a carbonation process driven by CO 2 uptake by hyperalkaline solutions derived from hydrating peridotite in surface conditions. In the Oman ophiolite, Falk et al [2016] characterized recent peridotite-hosted travertines which gave values of d 13 C as low as 225& with d 18 O of 220 to 0& and high D 47 values (0.675-0.883&), the d 18 O for the formational fluid ranging from 21.5 to 2.5& and spring water temperature ranging between 17.6 and 38.48C. Streit et al [2012] also studying the Oman ophiolite, estimated a range of temperatures for the carbonate samples between 15 and 508C and detected oxygen isotopic signatures similar to those of the groundwater and alkaline spring waters in peridotite catchments.…”

“…In the Oman ophiolite, Falk et al . [] characterized recent peridotite‐hosted travertines which gave values of δ 13 C as low as −25‰ with δ 18 O of −20 to 0‰ and high Δ 47 values (0.675–0.883‰), the δ 18 O for the formational fluid ranging from −1.5 to 2.5‰ and spring water temperature ranging between 17.6 and 38.4°C. Streit et al .…”

“…Moreover, the volume of mapped peridotites in Ziga is not sufficiently significant (<10 m 3 ), and typical travertine textures such as “surface films” at the surface of the springs or “bottom floc” as unconsolidated carbonate lining in the spring pools, as described by Falk et al . [] are not identified in the Ziga mélange .…”

What do we really know about early diagenesis of non-marine carbonates?

“…Falk et al (2016), for example, analysed fresh travertine carbonate precipitates from hyperalkaline springs in Oman and found systematic variations in their clumped isotope compositions with formation environments. They suggested that the isotope trends could be applied (1) to identify extinct hyperalkaline springs, (2) to constrain the formation conditions and possible post-depositional alterations, including dissolution and re-precipitation; and (3) to provide information about paleoclimate.…”

Kapcsolt izotópok (clumped isotopes) a földtudományi kutatásokban

“…Recens édesvízi mészköveket KELE et al (2015) vizsgált a kapcsolt izotópos módszer kalibrációja céljából, FALK et al (2016) pedig ománi hiperalkáli forrá -sokból kivált recens travertínókon végzett méréseket. A szárazföldi klímarekonstrukció talán legfontosabb karbonátos képződményei azonban a cseppkövek, mivel radiomet rikus módszerrel nagy pontossággal korolhatóak, finoman rétegzettek és a kiválási hőmérsékletük becsülhető a stabilizotópos összetételük alapján.…”