Radionuclide Studies of Stony Meteorites from Hot Deserts

Abstract

We summarize the use of radiocarbon produced by spallation in meteorites in space to determine their terrestrial age or residence time. This "age" gives us important information as it can be compared to the rates of weathering and infall of meteorites. The processes that affect the collection of meteorites in a given area can be related to the rates of infall of new meteorites, and the rate of removal by chemical weathering and physical erosion.

Current Performance and Preliminary Results of a New14C Extraction Line for Meteorites at the University of Bern

“…For calculating 14 C terrestrial ages, first the absolute 14 C/ 12 C ratio and the number of 14 C atoms (N 14 ) have to be determined using Equations (5) and (6) given by Hippe et al (2013) and Hippe and Lifton (2014): where the value of 1.1694×10 –12 corresponds to the abundance of 14 C in modern carbon, Ab12 S is the 14 C/ 12 C of the sample determined by the measured δ 13 C of the sample, F 14 C S is the fraction of modern carbon in the sample normalized to δ 13 C=–25‰ VPDB and AD 1950, and y is the year of measurement. where N A is Avogadro’s number (6.022×10 23 atoms/mol), V S is the volume of CO 2 of the sample at 20°C and 1013 mbar, V A is the volume of 1 mol CO 2 at 20°C and 1013 mbar, B is the number of 14 C atoms in the blank, and M S is the mass of the sample in g. From the concentration of 14 C we can calculate the activity (in dpm/kg) of the sample (A m ) using Equation (7) where λ 14 is the decay constant of 14 C (1.216×10 −4 yr −1 =2.314×10 −10 min −1 ). Finally, we can calculate the age via where A sat is the saturation activity of the sample (in dpm/kg), which is taken to be 51±1 dpm/kg for recent L chondrite falls based on analyses of the meteorite Bruderheim (e.g., Jull et al 2013), an L6 chondrite which fell in 1960. Since at the current status of our project we do not have 10 Be and/or shielding data, this average value for L chondrites was taken into account calculating the terrestrial age of JaH 073.…”

“…Jull et al 2013). More information on the 14 C/ 10 Be age determinations can be found in, e.g., Jull (2006) and Jull et al (2013).…”

“…Note that cosmogenic in situ 14 C is not only produced in space but also on the Earth’s surface. However, the latter production rate is about 220–1600 times lower than the production rate in space (Lifton et al 2001; Jull et al 2013), therefore terrestrial in situ 14 C contributions are usually negligible in meteorites. The 14 C activity of bulk recent ordinary chondrite falls ranges between 35–59 dpm/kg (e.g., Jull et al 1989; Jull 2006).…”

“…However, this method can only be used if 10 Be is in saturation, i.e., if the cosmic-ray exposure age is longer than about 7–8 million (e.g. Jull et al 2013). More information on the 14 C/ 10 Be age determinations can be found in, e.g., Jull (2006) and Jull et al (2013).…”

AN UPDATE ON THE PERFORMANCE OF THE IN SITU14C EXTRACTION LINE AT THE UNIVERSITY OF BERN

“…The major target element for cosmogenic 14 C production in stony meteorite and planetary surfaces is O, though spallation reactions on Mg, Al, Si, Ca, Fe, and Ni also produce in situ 14 C. Their contributions, however, are significantly smaller (Born and Begemann 1975;Leya et al 2001). Apart from the production in space, cosmogenic in situ 14 C is also produced on the Earth's surface, but in amounts approximately 220 to 1600 times smaller than in space (e.g., Lifton et al 2001;Jull et al 2013). Therefore, contributions from terrestrial production of in situ 14 C are usually considered negligible.…”