MIL 03443, a dunite from asteroid 4 Vesta: Evidence for its classification and cumulate origin

Abstract– The absence of dunite (>90 vol% olivine) in the howardite, eucrite, and diogenite (HED) meteorite suite, when viewed with respect to spectroscopic and petrologic evidence for olivine on Vesta, is problematic. Herein, we present petrologic, geochemical, and isotopic evidence confirming that Miller Range (MIL) 03443, containing 91 vol% olivine, should be classified with the HED clan rather than with mesosiderites. Similarities in olivine and pyroxene FeO/MnO ratios, mineral compositions, and unusual mineral inclusions between MIL 03443 and the diogenites support their formation on a common parent body. This hypothesis is bolstered by oxygen isotopic and bulk geochemical data. Beyond evidence for its reclassification, we present observations and interpretations that MIL 03443 is probably a crustal cumulate rock like the diogenites, rather than a sample of the Vestan mantle.     

Cosmic‐ray exposure history of the Norton County enstatite achondrite

Abstract– We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of ³⁶Cl, ⁴¹Ca, ²⁶Al, and ¹⁰Be were measured by accelerator mass spectrometry; the concentrations of the stable isotopes of He, Ne, Ar, and Sm were measured by electron and thermal ionization mass spectrometry, respectively. Production rates for the nuclides were modeled using the LAHET and the Monte Carlo N‐Particle codes. Assuming a one‐stage irradiation of a meteoroid with a pre‐atmospheric radius of approximately 50 cm, the model satisfactorily reproduces the depth profiles of ¹⁰Be, ²⁶Al, and ⁵³Mn (<6%) but overestimates the ⁴¹Ca concentrations by about 20%. ³He, ²¹Ne, and ²⁶Al data give a one‐stage cosmic‐ray exposure (CRE) age of 115 Ma. Argon‐36 released at intermediate temperatures, ³⁶Ar_n, is attributed to production by thermal neutrons. From the values of ³⁶Ar_n, an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1–4 × 10¹⁶ neutrons cm^?2. We infer comparable values from ε¹⁴⁹Sm and ε¹⁵⁰Sm. Values calculated from ⁴¹Ca and a CRE age of 115 Ma, 0.2–1.4 × 10¹⁶ neutrons cm^?2, are lower by a factor of approximately 2.5, indicating that nearly half of the ¹⁴⁹Sm captures occurred earlier. One possible irradiation history places the center of proto‐Norton County at a depth of 88 cm in a large body for 140 Ma prior to its liberation as a meteoroid with a radius of 50 cm and further CRE for 100 Ma.