Applications and limitations of U–Th disequilibria systematics for determining ages of carbonate alteration minerals in peridotite

Abstract

238 U-234 U-230 Th dating was conducted on carbonate alteration minerals in the peridotite layer of the Samail Ophiolite, Sultanate of Oman, in order to assess the applicability of U-series dating techniques to these types of Quaternary terrestrial carbonates and also to further constrain natural rates of carbonation of the peridotite. Due to their low U concentrations and relatively high Th/U ratios, Samail carbonates are challenging to date with the 230 Th technique because of the sensitivity of ages to corrections for initial 230 Th. Uncorrected 230 Th ages for Ca-rich travertines are consistently older than previously obtained 14 C ages. However, geologically reasonable initial 230 Th corrections bring the two sets of ages into concordance. This age concordance suggests that the travertines are generally closed systems, adding a level of credence to the reliability of previously obtained 14 C ages. In contrast, uncorrected 230 Th ages for Mg-rich carbonate veins are generally younger than previously obtained 14 C ages. These young ages are interpreted in terms of remobilization of hexavalent U, which is subsequently deposited as tetravalent U by reduced serpentinization fluids. Two Mg-rich carbonate veins sampled at a roadcut have near-equilibrium ( 230 Th/ 238 U) and ( 234 U/ 238 U) values, which indicate that these veins are >375,000 years in age, consistent with their -14 C dead‖ (>50,000 yr BP) ages. The variable young and old ages for these Mg-rich carbonate veins indicate that carbonation of the peridotite layer of the Samail Ophiolite is an ongoing process and that there have been multiple generations of subsurface carbonate vein formation. Overall, this study provides insights into some of the challenges associated with applying U-series dating methods to Quaternary terrestrial carbonates, in particular carbonate alteration minerals in peridotites, and highlights some areas where there is room for improvement, such as obtaining better constraints on the isotopic composition of admixed detritus, and also some advantages, such as the ability to identify open system behavior not apparent from 14 C dating and stable C and O isotopic analysis alone.

Formation of Magnesium Carbonates on Earth and Implications for Mars

“…Mervine et al. (2015) confirmed these quaternary formation ages with U‐series disequilibrium dating. Radiocarbon in magnesium carbonates can also be used to calculate carbonation rates (Oskierski et al., 2013) and tracing atmospheric carbon sources (Power et al., 2019).…”

“…Such putative antiquity was contradicted by the discovery of magnesite containing live 14 C and dates spanning ∼8-45 Ka (e.g., Kelemen & Matter, 2008;Kelemen et al, 2011;Mervine et al, 2014Mervine et al, , 2015. Mervine et al (2015) confirmed these quaternary formation ages with U-series disequilibrium dating. Radiocarbon in magnesium carbonates can also be used to calculate carbonation rates (Oskierski et al, 2013) and tracing atmospheric carbon sources (Power et al, 2019).…”

“…Sm and Nd concentrations in magnesite samples from the Budd ultramafic complex are at sub‐ppm levels (Toulkeridis et al., 2010); similar Sm and Nd concentrations in Martian Mg‐rich carbonates would likely preclude successful application of the Sm‐Nd method. While sedimentary crystalline magnesite from the Satka Formation exhibits relatively high U concentrations (0.94–2.1 ppm; Ovchinnikova et al., 2014), U concentrations in Mg‐rich carbonate veins from the Semail Ophiolite have orders of magnitude lower (0.03–14.8 ppb; Mervine et al., 2015). If U concentrations in magnesium carbonates from Jezero crater are similar to those of the Semail Ophiolite, the application of the isochron technique (e.g., Ovchinnikova et al., 2014; Toulkeridis et al., 2010) may not be feasible for Martian samples.…”

“…Before young radiocarbon dates were reported by Kelemen and Matter (2008), magnesium carbonate veins associated with the Semail Ophiolite were thought to have formed >60 Ma during its emplacement in the Late Cretaceous to Early Tertiary (e.g., Nasir et al., 2007). Such putative antiquity was contradicted by the discovery of magnesite containing live 14 C and dates spanning ∼8–45 Ka (e.g., Kelemen & Matter, 2008; Kelemen et al., 2011; Mervine et al., 2014, 2015). Mervine et al.…”

Timing of Magnetite Growth Associated With Peridotite‐Hosted Carbonate Veins in the SE Samail Ophiolite, Wadi Fins, Oman

“…Other work applied 14 C and U‐series to more than 40 samples from peridotite hosted travertine deposits and carbonate veins from across the ophiolite, finding that vein deposition occurred in many locations over the past ~350 ky, especially the past ~50 ky, related to Quaternary groundwater cycling (Clark & Fontes, 1990; Kelemen et al, 2011; Kelemen & Matter, 2008; Mervine et al, 2014). Though a large number of peridotite‐hosted carbonate veins formed in the Quaternary, some veins were too old for 14 C and U‐series (Mervine et al, 2015), indicating a more ancient history that has yet to be constrained. The present study investigates the timing of alteration in a ‘fossil' carbonate‐veined peridotite system in Wadi Fins, Oman.…”