Person:

Mukhopadhyay, Sujoy

Cosmogenic He-3 was measured in apatite, titanite, and zircon and cosmogenic Ne-21 in quartz at 13 depth intervals in a 2.7-m long drill core in a Miocene ignimbrite from the Altiplano of Bolivia. All three He-3 depth profiles as well as the Ne-21 profile attenuate exponentially with depth, indicating that both of these isotopes are cosmogenic in origin with no significant contribution from other sources. The attenuation lengthscale for He-3 production of Lambda=180 +/- 11 g/cm(2) is consistent with expectations for neutron spallation, and is identical to that found for the cosmogenic Ne-21 in quartz. By normalizing the measured He-3 concentrations to Ne-21 and using the independently known cosmogenic Ne-21 production rate, the apparent cosmogenic He-3 production rates in apatite, titanite, and zircon were respectively found to be 112, 97, and 87 atoms/g/yr at sea-level and high latitude. The formal uncertainty on these estimates is similar to 20% (2 sigma), and arises in equal parts from uncertainties in the measured He-3/Ne-21 ratios and the uncertainty in the Ne-21 production rate. However an additional factor affecting the apparent He-3 production rate in these phases arises from the long stopping range of spalled He-3 and tritium (which decays to He-3). Because all three accessory phases have higher mean atomic number than major rock-forming minerals, they will have lower He-3 production rates than their surroundings. As a consequence the long stopping ranges will cause a net implantation of He-3 and therefore higher apparent production rates than would apply for purely in-situ production. Thus these apparent production rates apply only to the specific grain sizes analyzed. Analysis of sieved zircon aliquots suggests that a factor of 2 increase in grain size (from similar to 50 to similar to 100 mu m cross-section) yields a 10% decrease in apparent production rate. While this effect warrants further study, the grain sizes analyzed here are typical of the accessory phases commonly encountered, so the apparent rates provide an appropriate starting place for surface exposure dating using He-3 in these minerals.

Helium concentration and isotopic composition were measured in a suite of samples across the Permian–Triassic boundary at Opal Creek, Canada, to determine whether high extraterrestrial helium concentrations are associated with a possible extinction-inducing impact event at this time. No extraterrestrial [super]3He was detected, implying that neither fullerene-hosted nor IDP-hosted He is present at or near the boundary. This observation is consistent with similar studies of some Permian–Triassic sections, but contrasts sharply with reports of both fullerene- and IDP-hosted extraterrestrial [super]3He at other sections. Step-heat experiments indicate rapid diffusion of extraterrestrial helium from sediments heated to temperatures above ~70 °C. Given the likelihood of burial and associated heating in Permian–Triassic age rocks, the initially unexpected absence of IDP-hosted [sup]3He likely indicates thermally induced diffusive loss. Indeed one of the key sections (Graphite Peak, Antarctica) from which extraterrestrial [super]3He has been reported at and near the Permian–Triassic boundary has been sufficiently heated that the reported preservation of extraterrestrial helium, in both IDPs and fullerenes, is inexplicable. Recent contamination provides a plausible explanation for extraterrestrial [super]3He in these samples. While no extraterrestrial [sup]3He was detected at Opal Creek, there is a sharp increase in nucleogenic [super]3He very close to or at the Permian–Triassic boundary. This presumably arises from the major lithologic change at this time, from cherts in the Permian to shales and siltstones in the Triassic. Increased nucleogenic [super]3He is associated with increases in both lithium and organic carbon content into the Triassic. Either the production rate or the retention of this [super]3He is higher in the shales and siltstones than in the cherts. Care must be taken to eliminate such artifacts before interpreting changes in [super]3He concentration in terms of fluctuations in the delivery of [super]3He from space.