NEW TH-230 U AND C-14 AGES FROM LAKE LAHONTAN CARBONATES, NEVADA, USA, AND A DISCUSSION OF THE ORIGIN OF INITIAL THORIUM/

Five sets of coeval lacustrine carbonate samples from Pleistocene Lake Lahontan in western Nevada were dated by both the AMS C-14 and Th-230 /U isochron methods. All five groups of samples were analyzed for U-Th isotopes by alpha spectrometry and one of the groups was additionally measured by thermal and secondary ionization mass spectrometry (TIMS and SIMS) for comparison. The C-14 ages were corrected to calendar yea rs using the calibration curve recommended by Bard et al, (1992). With out local reservoir correction on the C-14 ages, mean Th-230/U isochro n ages of some sets are apparently older than their calendar-corrected C-14 ages by up to 2300 years, Modern carbon contamination of these c arbonate samples through recrystallization or deposition of secondary calcite is likely to be responsible for part of the age discrepancies. We explored additional biases associated with the isochron ages, mayb e produced by the presence of initial Th coprecipitated from the lake water. It can be shown that if dissolved (hydrogenous) Th is directly incorporated into the pure carbonates, then the three-component mixing among (1) detrital Th, (2) hydrogenous Th adsorbed on detritus, and ( 3) hydrogenous Th incorporated by the carbonate can introduce a positi ve age bias. We have developed an approach to estimate the magnitude o f this bias of the Lake Lahontan carbonates. The preliminary estimates suggest a positive age bias of 1000 to 2000 years for two sets of the samples.