THE EFFECT OF GRAIN SIZE ON THE CLOSURE TEMPERATURE OF HE IN CALCITE: A CASE STUDY FROM THE MIOCENE BARSTOW FORMATION

Initial investigations into the suitability of calcite as a (U-Th)/He thermochronometer have shown that 1) the bulk closure temperature of He in calcite is similar to that for apatite, 2) the effective diffusion dimension is less than the smallest of grains so far analyzed (diameter 0.3 mm), 3) He from low-U calcites can be dominated by common He and therefore produce erroneously old ages (Copeland et al., 2007). We are investigating the high-U, fine-grained travertine deposits of the Miocene Barstow Formation of southern California in order to assess the effect of grain size on He retentivity in calcite. Previously published U-Pb data from these calcites indicate an age of ~15 Ma and [U]> 90 ppm (Cole et al., 2005). Helium analysis of six samples from Barstow Fm travertines yields apparent (U-Th)/He ages in the range of only ~20 to 4 ka, suggesting a much different closure temperature for these samples than coarse-grained calcite previous analyzed from limestones, carbonatites, and marbles. The Barstow ages show very little dependence on apparent grain size, which ranges from ~ 1 to 50 µm. Because these samples have much more than adequate concentrations of U to produce significant concentrations of He in 15 million years, our working hypothesis for the low He concentrations is that these samples have uniformly small effective diffusion domains, producing a closure temperature much less than previously analyzed samples. These data further support the hypothesis that fine-scale microstructure determines He retentivity in calcite and not bulk sample size.