In situ cosmogenic Be-10 and Ne-21 in sanidine and in situ cosmogenic He-3 in Fe-Ti-oxide minerals

摘要

We report concentrations of in situ cosmogenic Be-10 and Ne-21 from coexisting quartz and sanidine separates and of cosmogenic He-3 in coexisting Fe-Ti-oxide minerals from ignimbritic successions of northern Chile (Oxaya and Lauca ignimbrites). New mineral-isotope pairs such as sanidine and Fe-Ti-oxide minerals are helpful in quantitative geomorphology for geological settings where the lithology lacks the commonly used minerals quartz, pyroxene or olivine. Production rates in sanidine and Fe-Ti-oxide minerals were determined by normalizing nuclide concentrations to established production rates in quartz. The experimentally determined production rates are compared to model production rates calculated with new cross-sections for He-3, Ne-21, and Be-10 production from the individual target elements. The mean experimental Ne-21 production rate for five sanidine samples is 30.4 +/- 3.7 atoms g(-1) yr(-1) (30.4 +/- 5.4 atoms g(-1) yr(-1) including the uncertainty of the Ne-21 production rate in quartz, (PNeqtz)-Ne-21). This is in excellent agreement with the modelled value of 28.3 atoms g(-1) yr(-1), which has an estimated uncertainty of 20%. The Ne-21 production rate in sanidine is thus about 50% higher than that in quartz. The cosmogenic neon in sanidine is entirely released in low temperature steps (400-600 degrees C) and no signs of an interfering nucleogenic neon component were observed. This is in stark contrast to quartz and makes sanidine an attractive mineral for terrestrial cosmogenic neon studies. He-3 diffuses out of the sanidine structure. Preliminary results also indicate that sanidine is well suited for Be-10 studies. The mean experimental Be-10 production rate from two sanidine samples is 4.45 +/- 0.38 atoms g(-1) yr(-1) (4.45 +/- 0.42 atoms g(-1) yr-1 including uncertainty of (PBeqtz)-Be-10), very close to the modelled value of 4.55 atoms g(-1) yr(-1), which has an estimated uncertainty of 20%. We emphasize that Be-10 and Ne-21 production in sanidine is composition dependent. Therefore, major element analyses should be carried out and production rates calculated on a sample by sample basis.Fe-Ti-oxide minerals retain He-3 quantitatively. Experimentally derived production rates are in excellent agreement with new values derived from physical modelling. The mean experimental He-3 production rate for five Fe-Ti-oxide minerals samples is 120 +/- 11 atoms g(-1) yr(-1) (120 +/- 12 atoms g(-1) yr(-1) including the uncertainties of (PNeqtz)-Ne-21), compared to a mean modelled value of 124 atoms g(-1) yr(-1), which has an estimated uncertainty of 20%. The variable chemical and structural composition of the solid solution lines of Fe-Ti-oxide minerals has little effect on the total He-3 production rate. CosmogeniC Ne-21 is not produced in significant quantities in Fe-Ti-oxide minerals due to the absence of suitable target elements. (c) 2005 Elsevier B.V. All rights reserved.