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

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 3He 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 3He 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 3He production rate is valid in the south‐west Pacific. Using independent, proximal calibrations of cosmogenic isotopes 10Be and 14C from quartz in New Zealand, we derive cosmogenic 3He/10Be and 3He/14C production ratios of 32.2 ± 3.2 and 10.6 ± 1.6, respectively.