Cosmogenic 3He and 21Ne dating of biotite and hornblende

Stable cosmogenic isotopes such as ³He and ²¹Ne are useful for dating of diverse lithologies, quantifying erosion rates and ages of ancient surfaces and sediments, and for assessing complex burial histories. Although many minerals are potentially suitable targets for ³He and ²¹Ne dating, complex production systematics require calibration of each mineral–isotope pair. We present new results from a drill core in a high-elevation ignimbrite surface, which demonstrates that cosmogenic ³He and ²¹Ne can be readily measured in biotite and hornblende. ²¹Ne production rates in hornblende and biotite are similar, and are higher than that in quartz due to production from light elements such as Mg and Al. We measure ²¹Ne_hbl/²¹Ne_qtz = 1.35 ± 0.03 and ²¹Ne_bio/²¹Ne_qtz = 1.3 ± 0.02, which yield production rates of 25.6 ± 3.0 and 24.7 ± 2.9 at g^? 1 yr^? 1 relative to a ²¹Ne_qtz production rate of 19.0 ± 1.8 at g^? 1 yr^? 1. We show that nucleogenic ²¹Ne concentrations produced via the reaction ¹⁸O(α,n)²¹Ne are manageably small in this setting, and we present a new approach to deconvolve nucleogenic ²¹Ne by comparison to nucleogenic ²²Ne produced from the reaction ¹⁹F(α,n)²²Ne in F-rich phases such as biotite. Our results show that hornblende is a suitable target phase for cosmogenic ³He dating, but that ³He is lost from biotite at Earth surface temperatures. Comparison of ³He concentrations in hornblende with previously measured mineral phases such as apatite and zircon provides unambiguous evidence for ³He production via the reaction ⁶Li(n,α)³H → ³He. Due to the atypically high Li content in the hornblende (~ 160 ppm) we estimate that Li-produced ³He represents ~ 40% of total ³He production in our samples, and must be considered on a sample-specific basis if ³He dating in hornblende is to be widely implemented.