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Possibly the most He-3 ever observed in a terrestrial sample

September 1, 2016

In a previous post I pointed out that we (that is, the noble gas mass spectrometry facility at BGC) might have recently analysed the sample with the highest concentration of cosmogenic Ne-21 ever observed in a terrestrial sample. By ‘terrestrial sample,’ of course, I mean a rock that has accumulated its entire cosmogenic-nuclide inventory on Earth — because cosmogenic-nuclide production rates are so much higher outside the Earth’s atmosphere, meteorites have orders of magnitude higher concentrations of cosmic-ray-produced radionuclides than possible on Earth. So meteorites, or extraterrestrial-dust-laden ocean-floor sediments from the central Pacific, don’t count. In any case, no one has shown up to tell me about anything with a higher Ne-21 concentration, so the claim may be true.

The point of this post is that it now appears that we may also have analysed the sample with the highest concentration of in-situ-produced cosmogenic helium-3 ever observed in a terrestrial rock. This sample is near 2300 m elevation at Roberts Massif, a nunatak at the head of the Shackleton Glacier in the Transantarctic Mountains at 85.5° S latitude. In fieldwork last year, Gordon Bromley and his colleagues collected this sample from that site; the sample is a loose clast, coarsely faceted and therefore possibly glacially transported, of the local bedrock, a Jurassic mafic intrusive rock known as the Ferrar Dolerite. Here is a picture (photo credit: Gordon Bromley).

15-ROB-028_2_small

Two separate analyses at BGC of pyroxene separated from this sample yielded 1.39 x 10^10 and 1.35 x 10^10 atoms/g He-3. Two adjacent smaller loose rocks — this one and this one — had slightly lower, but still pretty unreasonable, He-3 concentrations near 1.25 x 10^10 and 1.07 x 10^10 atoms/g, respectively. As surface rocks go, that’s a lot of helium-3. Note, however, that even though He-3 is extremely expensive at present, you would still have to mine something on the order of 1000 metric tons of this pyroxene to have $1 worth of He-3. It’s not ore-grade.

Pyroxene in this lithology is known to contain only about 6 x 10^6 atoms/g of non-cosmogenic He-3, presumably inherited from initial mineral crystallization, so the measured concentration in these samples is essentially all cosmic-ray-produced. 1.39 x 10^10 atoms/g cosmogenic He-3 at this site implies an apparent exposure age of approximately 13 Ma. As discussed in the previous post, this is not the oldest exposure age ever measured; that record still belongs to samples from the Atacama Desert, analysed by Tibor Dunai and colleagues, with apparent exposure ages exceeding 30 Ma. However, those samples are at much lower elevation where production rates are also much lower, so even though they are older, the total cosmic-ray dose they have experienced is smaller.

In any case, a relatively slapdash literature search reveals that the nearest competitor appears to be another sample of pyroxene from the Ferrar at Mt. Fleming, also a high-elevation site in the Transantarctic Mountains. Joerg Schaefer and colleagues found 6.9 x 10^9 atoms/g cosmogenic He-3 in this sample, which is still plenty but is almost a factor of two less than in the Roberts Massif sample. Are there any other contenders?

The point, once again, is that these high-elevation rock surfaces in Antarctica have enjoyed a stunning and expansive view of nearly the entire late Cenozoic evolution of the Antarctic ice sheets. Frankly, a lot of that was probably pretty boring, but if the East Antarctic Ice Sheet really collapsed in past warm-climate periods, they watched.

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