Cosmogenic effects in Bouvante Eucrite

Abstract

The Bouvante meteorite, a single stone weighing 8.3 kg, was found on 30th July 1978 and has been classified as a mononict recrystallized eucrite by Michel Levy et al. (1987). Here are the results of measurements of cosmic ray track density and radioisotopes ^26Al, ^22Na, and ^44Ti in a 700-g fragment taken from the main mass. Track Density: Four samples for track density measurements were taken from the farthest corners of the 700-g fragment and feldspars were etched in boiling 1:2 NaOH for 60 minutes. Track density in different fragments ranged between 3.1 × 10^4 to 2.7 × 10^5 tracks/cm^2 showing some gradient within the fragment. The exposure age of Bouvante has been determined from ^21,22Ne and other rare gases that yield values between 5.6 and 6.7 Ma (Weber et al., 1983). Adopting a value of 6 Ma leads to track production rate of 5 × 10^3 to 4.5 × 10^4 tracks/cm^2 Ma corresponding to ablation ranging between about 18 and 9 cm. If the mean value of 13.5 cm is taken as representative ablation then Bouvante had a pre-atmospheric radius of ~21 cm. Cosmogenic Radionuclides: Gamma ray emitters were measured by whole-rock counting with a low background gamma-ray spectrometer. A high purity, 372 cm^3, coaxial Ge diode located within a 10-cm- thick active NaI(Tl) well scintillator in a 20-cm-thick lead shield, in the underground laboratory of Monte dei Cappuccini in Torino (Bonino et al., 1991) served as the gamma ray detector. The detector was used in two modes, in coincidence and in anticoincidence with the scintillator. The counting efficiency was determined by using the inherently present ^40K as an internal standard and by calibrating the detector at different energies. K in Bouvante is known to be 610 ppm (Michel Levy et al., 1987). The sensitivity and selectivity of the detector is high and the background extremely low so that, in addition to the long lived ^26Al, 48-yr ^44Ti which is produced at very low levels and 2.6-yr ^22Na could be measured even after lapse of more than 13 years of the fall of the meteorite. The measured activities, at the time the meteorite was found are ^26Al = 91.6±0.7 dpm/kg, ^22Na = 76.0±7 dpm/kg and ^44Ti = 2.1±0.5 dpm/kg. Upper limits of 0.8 dpm/kg were obtained for ^60Co and of 0.05 dpm/kg for ^42Ar. Uranium also could be estimated via its daughter ^214Bi to be ~200 ppb. The results could be compared with activities in another eucrite Bereba where a 400-g fragment was counted in a similar manner. The track density and the low ^60Co activity suggests that the meteorite was exposed as a small body (R(sub)pre ⋍ 21 cm) in space. Measured activity of ^22Na is close to the expected value, when corrected for solar cycle modulation, and indicates that the meteorite did not fall much before the time it was found.