Cosmogenic ^3He and ^(21)Ne dating of biotite and hornblende

Abstract

Stable cosmogenic isotopes such as ^3He and ^(21)Ne 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 ^(21)Ne 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 ^(21)Ne can be readily measured in biotite and hornblende. ^(21)Ne 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 ^(21)Ne_(hbl)/^(21)Ne_(qtz) = 1.35 ± 0.03 and ^(21)Ne_(bio)/^(21)Ne_(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 ^(21)Ne_(qtz) production rate of 19.0 ± 1.8 at g^(−1) yr^(−1). We show that nucleogenic ^(21)Ne concentrations produced via the reaction ^(18)O(α,n)^(21)Ne are manageably small in this setting, and we present a new approach to deconvolve nucleogenic ^(21)Ne by comparison to nucleogenic ^(22)Ne produced from the reaction ^(19)F(α,n)^(22)Ne 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.

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