Neon produced by solar cosmic rays in ordinary chondrites

Abstract

Solar-cosmic-ray-produced Ne (SCR-Ne), in the form of low cosmogenic 21 Ne/ 22 Ne ratios ( 21 Ne/ 22 Ne cos <0.8), is more likely to be found in rare meteorite classes, like Martian meteorites, than in ordinary chondrites. This may be the result of a sampling bias: SCR-Ne is better preserved in meteorites with small preatmospheric radii and these specimens are often only studied if they belong to unusual or rare classes. We measured He and Ne isotopic concentrations and nuclear tracks in 25 small unpaired ordinary chondrites from Oman. Most chondrites have been intensively heated during atmospheric entry as evidenced by the disturbed track records, the low 3 He/ 21 Ne ratios, the low 4 He concentrations, and the high peak release temperatures. Concentration depth profiles indicate significant degassing; however, the Ne isotopes are mainly undisturbed. Remarkably, six chondrites have low 21 Ne/ 22 Ne cos in the range 0.711-0.805. Using a new physical model for the calculation of SCR production rates, we show that four of the chondrites contain up to~20% of SCR-Ne; they are analyzed in terms of preatmospheric sizes, cosmic ray exposure ages, mass ablation losses, and orbits. We conclude that SCR-Ne is preserved, regardless of the meteorite class, in specimens with small preatmospheric radii. Sampling bias explains the predominance of SCR-Ne in rare meteorites, although we cannot exclude that SCR-Ne is more common in Martian meteorites than it is in small ordinary chondrites.

Chronology of martian breccia NWA 7034 and the formation of the martian crustal dichotomy

“…The measured 21 Ne/ 22 Ne ratio in NWA 7034 is at the lower limit of the range observed in stony meteorites ( 24 ). Combined GCR and SCR irradiation, as has been observed in some shergottites and ordinary chondrites with small recovery masses ( 39 , 40 ), could explain the disparate results obtained from 3 He, 21 Ne, 22 Ne, and 38 Ar. However, NWA 7034 would be required to come from the surface of a sufficiently large meteoroid such that it is irradiated by SCRs on the surface and receives secondary neutrons at epithermal or thermal energies from the back side to produce 80 Kr and 82 Kr by neutron capture on 79 Br and 81 Br, respectively.…”

“…Although the old U-Pb and Sm-Nd ages are statistically indistinguishable, the young zircon and phosphate ages indicate that one or more metamorphic events occurred at ~1700 to 1300 Ma, resulting in partial resetting of the U-Pb system. K-Ar and 40 Ar/ 39 Ar ages of 1560 Ma (10) and 1285 ± 8 Ma (11), respectively, further attest to thermal processing at that time (Fig. 1).…”

“…Here, we report 40 Ar/ 39 Ar, U-Th-Sm/He, and U-Pu/Xe ages obtained from whole-rock fragments and mineral separates to better constrain the impact and metamorphic history of NWA 7034. The chronometers span a wide range of closure temperatures and are therefore sensitive to different metamorphic conditions (13)(14)(15).…”

“…The chronometers span a wide range of closure temperatures and are therefore sensitive to different metamorphic conditions (13)(14)(15). To explore clast-to-clast variations in metamorphic ages, we selected a total of 4 feldspar mineral separates and 13 individual matrix/clast chips from visibly monomict fragments for 40 Ar/ 39 Ar measurements. In addition, we measured He, Ne, Ar, Kr, and Xe isotopes to better constrain the conditions of brecciation and ejection from Mars.…”

Protracted storage of CR chondrules in a region of the disk transparent to galactic cosmic rays

“…Helium and Ne isotopic concentrations were measured at the University of Bern using a MAP 215‐50 magnetic sector field mass spectrometer with an improved detection system connected to a low‐volume–low‐blank extraction line; the noble gases were extracted by laser melting (Roth et al. , ).…”