The Monahans chondrite and halite: Argon‐39/argon‐40 age, solar gases, cosmic‐ray exposure ages, and parent body regolith neutron flux and thickness

Abstract

Abstract-The Monahans H-chondrite is a regolith breccia containing light and dark phases and the first reported presence of small grains of halite. We made detailed noble gas analyses of each of these phases. The 39Ar-40Ar age of Monahans light is 4.533 & 0.006 Ma. Monahans dark and halite samples show greater amounts of diffusive loss of4oAr and the maximum ages are 4.50 and 4.33 Ga, respectively. Monahans dark phase contains significant concentrations of He, Ne and Ar implanted by the solar wind when this material was extant in a parent body regolith. Monahans light contains no solar gases. From the cosmogenic 3He, 2lNe, and 3*Ar in Monahans light we calculate a probable cosmic-ray, space exposure age of 6.0 & 0.5 Ma. Monahans dark contains twice as much cosmogenic 21Ne and 38Ar as does the light and indicates early near-surface exposure of 13-18 Ma in a H-chondrite regolith. The existence of fragile halite grains in H-chondrites suggests that this regolith irradiation occurred very early. Large concentrations of 36Ar in the halite were produced during regolith exposure by neutron capture on 35C1, followed by decay to 36Ar. The thermal neutron fluence seen by the halite was ( 2 4 ) x 1014 n/cm2. The thermal neutron flux during regolith exposure was -0.4-0.7 n/cm2/s. The Monahans neutron fluence is more than an order of magnitude less than that acquired during space exposure of several large meteorites and of lunar soils, but the neutron flux is lower by a factor of 1 5 . Comparison ofthe 36Arn/21Necos ratio in Monahans halite and silicate with the theoretically calculated ratio as a function of shielding depth in an H-chondrite regolith suggests that irradiation of Monahans dark occurred under low shielding in a regolith that may have been relatively shallow. Late addition of halite to the regolith can be ruled out. However, irradiation of halite and silicate for different times at different depths in an extensive regolith cannot be excluded.

The halite‐bearing Zag and Monahans (1998) meteorite breccias: Shock metamorphism, thermal metamorphism and aqueous alteration on the H‐chondrite parent body

“…However, these proportions may not be representative of the whole rock. Bogard et al (2001) Light-Colored Metamorphic Clasts-The light-colored clasts in the 1.2 kg Monahans stone range up to 1.5 em in maximum dimension and have subrounded to subangular shapes. They exhibit moderately recrystallized textures consistent with petrologic type 5; the chondrules do not contain glass, and olivine and low-Ca pyroxene compositions are relatively equilibrated.…”

Itawa Bhopji (L3–5) chondrite regolith breccia: Fall, classification, and cosmogenic records

“…(1) partial gas loss; (2) inappropriate production rate calculation; and (3) multiple irradiation ofItawa Bhopji; a space irradiation and a parent body irradiation, as happened in the case of Fayetteville (Wie1er et al, 1989) and Monahans meteorites (Bogard et al, 2001). Though the absolute cosmic-ray exposure ages might depend on the method adopted for calculating the production rates (e.g., Wie1er et al, 1996;Leya et al, 2000) the relative differences in cosmic-ray exposure ages for the two lithologies should remain the same.…”

“…7, we compare the trapped noble gas amounts for the light and dark lithologies ofItawa Bhopji with the trend shown by the gas-rich meteorite Pesyanoe (Marti, 1969). The data for Monahans, having solar gases and also bearing halites (Bogard et al, 2001), is also shown for comparison. Though the presence of solar gases is clear from the trends in the dark lithologies, the absolute amounts (of 20Ne and 36Ar, for example) in both these meteorites are about an order of magnitude lower than in Pesyanoe.…”

39Ar‐40Ar chronology of R chondrites

“…Large impact events that produce a thermal environment sufficiently hot to cause resetting of the 39 Ar-40 Ar ages should also cause pervasive loss of solar gases. Thus, in breccias that have reset Ar-Ar ages and solar gases, the solar gases must reside in the matrix grains that were not heated to high temperatures during breccia formation (Bogard, Garrison, and Masarik 2001). In contrast, breccias with reset Ar-Ar ages but little or no solar gases could be inferred either to have experienced prolonged burial and heating to relatively high temperatures at depth, or to contain grains that never resided at the regolith surface.…”