OBSERVATIONS OF APPARENT CRUSTAL 3HE/4HE RATIOS IN SHALLOW AQUIFERS FROM AROUND THE WORLD

The ³He/⁴He ratio produced in crustal materials can be theoretically calculated based on their U, Th, and Li content. However, several studies have shown that production ratios can differ from the ratios released into the water flowing through these rocks. The objective of this research was to evaluate apparent ³He/⁴He ratios released from crustal materials by evaluating the He and Ne composition of shallow aquifers throughout the world. In aquifers without mantle He, the ³He/⁴He ratio will evolve from that of air saturated water to the ratio being released from crustal materials. On a plot of Ne/He versus ³He/⁴He this evolutionary path will be linear, and the crustal ³He/⁴He ratio will be the y intercept of a linear regression through the data. If mantle He enters the aquifer at some point, then the evolutionary path will be non-linear. We analyzed data for apparent crustal ³He/⁴He ratios from a large noble gas data set for basins throughout California, as well as previously published data from around the world. For the California data, there were groundwater basins along active fault zones or volcanism where the presence of mantle He was apparent, and a linear trend was not discernable. However, in other basins an evolutionary trend was linear, and the apparent crustal of ³He/⁴He ratios ranged from 4X10^-8 to 6X10^-7. The data from published manuscripts was divided into two sets, one where the authors had reported a crustal ratio, and another where we calculated a ratio based on plots of the data in the manuscript. Crustal ratios reported by other authors ranged from 6X10^-9 to 4X10^-7. For published data which we plotted, crustal ratios ranged from 8X10^-9 to 7X10^-7. For all the data, we noted that ratios from sediments were consistently higher than those from consolidated rocks. This analysis suggests that apparent crustal ³He/⁴He release ratios range over 2 orders of magnitude, however, it does not shed light on the mechanisms responsible for these variations. We suggest that rather than an assumed value, crustal ratios should be calculated for individual basins/aquifers. The potential impacts of an assumed versus calculated ratio include determinations of the percent mantle He in an aquifer and on corrections applied to ³H/³He ages where excess ⁴He is present in the sample.