Dating "Dirty" Carbonates by U-series Isochrones: Insight Into Why Leaching Methods Don't Work

While accurate dating of "dirty" carbonates (carbonates having abundant detritus) using U-series methods would greatly impact the geochronology of difficult to date quaternary samples, robust methods are few in number. We set up a series of experiments using nearly pure carbonates (a calcitic stalagmite and a modern coral) that we physically mixed with variable amounts of USGS standard SDC-1 (mica schist representative of detritus). We dissolved each dirty carbonate mixture with acetic acid followed by leaching the residue with EDTA (to attempt to eliminate sorption). After completely dissolving the residue, the two solutions were spiked and measured for U- Th-Pa isotopes by MC-ICP-MS. Results show that significant amounts of 238U(235U), 234U and 232Th are removed from the mica schist despite the gentle dissolution. The concentrations of all nuclides in both solutions and residues show a linear relationship as a function of detritus/carbonate ratio. However, the linear relationship for isotope activity ratios, except (232Th/238U)A, is much weaker, especially for the residues. The migration direction of 231Pa and 230Th are consistently from the high concentration end member to the lower one; migration of these nuclides is not always the same direction as indicated by the long-lived isotopes (235U, 232Th). In summary, only total sample dissolution (TSD) methods can give an accurate age while leaching methods always are biased by nuclide migration out of the silicate detritus.