A Field Test of Cosmogenic 3He Dating in Calcite

Because ³He is produced by spallation of C, O, and Ca, calcite may be a suitable target for cosmogenic ³He dating.  Samples of crystalline calcite veins in limestone cobbles were collected from three inset fill terraces in eastern Tajikistan (~3800 m), which are offset by the Karasu fault.   The upper three terrace levels have 56 ± 1.2, 40 ± 1.9, and 18 ± 0.6 Mat/g of ³He (n=3-5 samples for each level).  We believe this ³He is cosmogenic in origin because: 1) rocks from a given terrace level yield very similar concentrations, 2) these concentrations are consistent with relative terrace ages, 3) ³He/⁴He ratios of 20-350 Ra far exceed ratios expected for trapped or radiogenic components, and 4) Li contents in the calcite are extremely low (< 50 ppb) so nucleogenic ³He is negligible.    To evaluate the potential for diffusive loss of ³He from these calcites, two samples were proton irradiated to make large quantities of ³He and subjected to step-heating diffusion experiments.   Resultant Arrhenius plots show a “lightning bolt” pattern in which successive prograde or retrograde heating steps are linearly related in (1/T) vs. ln(D/a²) space, but ln(D/a²) decreases for successive isothermal steps.   Based on a mean annual temperature of ~3° C, and modeling following Wolf et al. (1998), we conclude that for 30,000 yr time scales, these calcites should be >99 % retentive to cosmogenic ³He.  The ages of the terraces are presently unknown; assuming a reasonable cosmogenic ³He production rate of 130 at/g/yr gives ages of 24, 18, and 7.5 Ky, corresponding to nearly identical strike-slip rates of ~2.75 mm/yr for both of the upper two terraces.   These preliminary results suggest that cosmogenic ³He in calcite may be a valuable dating tool in young samples found in cool climates but possibly not in hot climates.