Cosmic-Ray Records and Reaction Rates in Iron Meteorites

Details Cosmic-Ray Records and Reaction Rates in Iron Meteorites Cosmic-Ray Records and Reaction Rates in Iron Meteorites

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.533l&link_type=abstract

Meteoritics, vol. 30, no. 5, page 533

Computer Science

Exposure Ages, Isotopes, Cosmogenic, Meteorites, Iron, Noble Gases

Scientific paper

We present a new approach to the study of exposure histories of iron meteorites based on calibrations of the noble gas production rates on the 40K / 41K data base of Voshage [1,2]. Our approach also modifies some of the basic assumptions made by Voshage, and it permits the calculation of exposure ages for a larger number of iron meteorites, including those with shorter exposure ages. Our model adopts an exponential dependence of the production rates with shielding depth and we use the equality of noble gas exposure ages and 40K - 41K ages for samples from the central locations. We only make the restricting assumption of a long-term constancy of the cosmic radiation. The response of the production ratio P(40K) / P(41K) to varying Fe / Ni ratios is calculated from spallation systematics. The production rate P38(So) at the centers of assumed spherical irons depends on meteorite size and, following the initial build-up of secondaries is expected to decrease exponentially with the shielding parameter So. In order to select an appropriate data set for the 38Ar- 40K- 41K calibration we classify the available data base, based on the availability of more than one independent data set with documented shielding parameter and C38 and M parameter measurements. This allows us to exclude cases with complex exposure histories. The quality classes a and b provide a total of 15 data sets which permits a calibration. A few iterations provides fits to the P(So) and the Mo(So) parameters simultaneously. The resulting coefficients a1 to a7 show variations corresponding to 5% changes in the calculated exposure ages. There is one important exception: meteorites with very old potassium ages can not be fitted with the same parameter sets; this may indicate a variation of the cosmic ray intensity over a > 1 Ga time- scale. The calculated production rates P38(So) of 38Ar in iron meteorites is shown in Figure 1 for varying S=4He/21Ne values and for values So= 252, 348, and 433. Corrections to the production rates for off-center locations are derived from P(So) by using an exponential factor with 2 parameters: a8 and a9. These parameters are calculated by fitting production rate profiles of documented iron meteorites such as Carbo or Grant, which have distinct So values. References: [1] Voshage H. (1967) Z. Naturforsch., 22, 477-506 [2] Voshage H. (1979) EPSL, 45, 293-308. Fig. 1. Production rate P38 of 38Ar in iron meteorites versus the ratio S=4He/21Ne used as shielding monitor. P(So) represents the production rate curve at the centers of meteorites. The depth dependance is shown for 3 meteorites of different sizes, A77283, Grant and Carbo.

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