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

TitleApplications and limitations of U–Th disequilibria systematics for determining ages of carbonate alteration minerals in peridotite
Publication TypeJournal Article
Year of Publication2015
AuthorsMervine, EM, Sims, KWW, Humphris, SE, Kelemen, PB
JournalChemical Geology
Pagination151 - 166
Date PublishedJan-09-2015

238U–234U–230Th 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 230Th technique because of the sensitivity of ages to corrections for initial 230Th. Uncorrected 230Th ages for Ca-rich travertines are consistently older than previously obtained 14C ages. However, geologically reasonable initial 230Th 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 14C ages. In contrast, uncorrected 230Th ages for Mg-rich carbonate veins are generally younger than previously obtained 14C 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 (230Th/238U) and (234U/238U) values, which indicate that these veins are > 375,000 years in age, consistent with their “14C dead” (> 50,000 years 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 14C dating and stable C and O isotopic analysis alone.