Isotope Ratio Triangulation: A Method for Determining Uranium Isotope Ratios and Application to the Search for Uranium Isotope Anomalies in the Mineral Titanite

The U-Pb dating methods used in many geochronology laboratories take advantage of a mixed 235U-233U spike for precise uranium isotopic measurements and current data reduction algorithms assume a uniform 238U/235U value of 137.88. Recent re-evaluation of the isotope ratio of ?natural? uranium value used in geochronology has called into question both this value and its constancy in U-bearing minerals, most notably titanite, formed in high-temperature magmatic and metamorphic settings. A 233U-236U spike may be used for direct determination of the uranium isotopic composition, but this spike is not widely used and in some labs where it is, the 238U/235U ratio is not independently measured. Isotope Ratio Triangulation (IRT) is a new and effective method for determining 238U/235U values analyzed with the more common 235U-233U uranium spike. This method leverages the effect of mass-spectrometer-induced isotopic fractionation in three measured ratios to determine 238U/235U values. Graphically, this is represented by three intersecting lines for fractionation factors calculated at varying 238U/235U ratios, hence the term ?triangulation?.

The IRT method is here applied to 43 aliquots of 23 titanite samples from a wide range of geologic settings, ages and locations. Of these, five aliquots yielded anomalously high 238U/235U ratios. Three were likely to be entirely uranium blank. Two other samples yielded anomalously high 238U/235U values. Both were roughly 2.5 Ga and coincide with the Archean to early Proterozoic oxidation of the world?s oceans termed the ?Great Oxidation Event?. Low temperature redox reactions have been shown to have a similar depletion effect. These results demonstrate the accuracy of this new method, which can be used to efficiently scan large volumes of existing geochronologic data in search of anomalous U isotope ratios. In addition, results indicate that laboratory blank in the TAMU radiogenic isotope laboratory is not of ?natural? composition. Finally, these results are more consistent with a ?natural? uranium composition of 238U/235U = 137.88 than the recently suggested value of 137.818.