A test of the cosmogenic ³He production rate in the south‐west Pacific (39°S)

Abstract

ABSTRACT Calculation of surface exposure ages, using in situ cosmogenic nuclides, requires accurate knowledge of local production rates. Here, we report the first attempt to calibrate cosmogenic 3 He production in the south‐west Pacific region. We present a new radiocarbon chronology that precisely dates the emplacement of the Murimotu Formation, a large debris avalanche deposit in central North Island, New Zealand (ca. 830 m asl; 39°S), which occurred 10.4–10.6 cal ka BP. Measurements of cosmogenic 3 He in pyroxene separated from large andesitic blocks exposed during this event yield a sea‐level high‐latitude production rate of 120 ± 12 atoms g −1 a −1 (‘Lm’ scaling). This is consistent with a recent global compilation, comprised predominantly of calibration sites located in the Northern Hemisphere. Thus, we conclude that the globally compiled cosmogenic 3 He production rate is valid in the south‐west Pacific. Using independent, proximal calibrations of cosmogenic isotopes 10 Be and 14 C from quartz in New Zealand, we derive cosmogenic 3 He/ 10 Be and 3 He/ 14 C production ratios of 32.2 ± 3.2 and 10.6 ± 1.6, respectively.