Cosmogenic nuclides in recently fallen meteorites: Evidence for galactic cosmic ray variations during the period 1967–1978

Abstract

Cosmogenic radionuclides were measured on 48 fragments of 24 meteorites which fell between 1967 and 1978. Nondestructive gamma counting techniques were used to obtain data on 7Be, 46Sc, 48V, 51Cr, 54Mn, 56Co, 57Co, 58Co and 60Co on at least some of the samples. Sodium 22 and 26Al measurements are reported on all 48 samples. In addition, new rare gas data and exposure ages are reported for the meteorites Guibga, Gorlovka, Dhajala, Louisville, Acapulco, Jilin, Kabo, Alta‐Ameen, and Canon City. The cosmogenic radioisotope and rare gas data are interpreted in terms of a time dependent modulation of galactic cosmic rays spanning one full 11 year sun spot cycle. Special attention is given to the data on 22Na, 46Sc, 54Mn, and 48V with either 26Al or 22Ne/21Ne used to provide a shielding correction. The shielding normalized data using the 26Al method appear to correlate well with calculated production rates scaled against the Deep River neutron monitor. The data for the four isotopes are consistent with a production rate variation of a factor of 2.5–3 between solar maximum and solar minimum for sun spot cycle 20. These data demonstrate that the production rates of cosmic ray‐produced nuclides in meteorites vary considerably according to modulation by the 11‐year solar cycle and support the concept that variations of solar‐modulated, cosmic ray flux of similar magnitude have occurred over much longer time periods.

Terrestrial history of antarctic meteorites recorded in the cosmogenic nuclides.

“…In many cases 36Cl and 26A1 have been measured, and 7 X 105y for the A77002 (L chondrite) is the oldest age to be reported so far. The 26A1 content in this meteorite, 30dpm/kg, is about a half of the ordinary level (EVANS et al, 1979) and the 36C1, 5 ± 1 dpm/kg metal, is 1/5 of the saturation level (NISIInzuMI et al, 1979a).…”

“…All available data range between 104 and 7 X 10-'y, and the simple average is 2 ± 2 (one standard deviation) X 10'y ( (EVANS, et al, 1979: EVANS, priv. Commun.…”

Cosmogenic radionuclides in the recently fallen Kobe (CK4) meteorite.

“…2) Yoneda, S. (personal communication); 3) Komura et al (1996a); 4) Evans et al (1982), Cressy, Jr. (1970), Shedlovsky et al (1967) etc. ;5) Mason (1979 (%) 0.002 (%) } } fall) to Oct. 1st 13:18 pm (5500 min.)…”

“…For example, activity of typical neutron-capture product, 60 Co has been used as an indicator of their preatmospheric size (e.g., Eberhardt et al, 1963;Spergel et al, 1986), and those of spallation products, 22 Na, 54 Mn and 26 Al etc. reflect irradiation conditions such as shielding effect (e.g., Bhandari et al, 1993;Michel et al, 1995), exposure age (e.g., Heimann et al, 1974;Herpers and Englert, 1983) and flux of cosmic-rays (e.g., Evans et al, 1982).…”

Fall, recovery and description of the Coleman chondrite

“…The 26A1 and 2%a activities in the Coleman specimen are higher than the typical range for an L-chondrite, for which normal values fall near 59 ? 9 dpmikg for 26AI (Evans and Reeves, 1987) and 60-1 10 dpmikg for 22Na (Evans et al, 1982). Coleman fell about three years past the most recent solar maximum in 1991, during which the 22Na production rate should ., 1982) show that L-chondrites that fell during the 1969 solar maximum had 22Na activities in the range of 60-80 dpmkg; whereas, those falling during the 1976 solar minimum had activities in the range of 8&110 dpmikg.…”