¹Martin R.Lee,¹Benjamin E.Cohen,²Ashley J.King,³Richard C.Greenwood
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.027]
¹School of Geographical and Earth Sciences, University of Glasgow, G12 8QQ, U.K
²Department of Earth Science, Natural History Museum (London), Cromwell Road, London SW7 5BD, U.K
³Planetary and Space Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, U.K
Copyright Elsevier

Lewis Cliff (LEW) 85311 is classified as a Mighei-like (CM) carbonaceous chondrite, yet it has some unusual properties that highlight an unrealised diversity within the CMs, and also questions how many parent bodies are sampled by the group. This meteorite is composed of rimmed chondrules, chondrule fragments and refractory inclusions that are set in a fine-grained phyllosilicate-rich matrix. The chondrules are of a similar size to those in the CMs, and have narrow fine-grained rims. LEW 85311 has been mildly aqueously altered, as evidenced by the preservation of melilite and kamacite, and X-ray diffraction results showing a low phyllosilicate fraction and a high ratio of cronstedtite to Fe,Mg serpentine. The chemical composition of LEW 85311 matrix, fine-grained rims, tochilinite and P-rich sulphides is similar to mildly aqueously altered CMs. LEW 85311 is enriched in refractory elements and REEs such that its CI-normalised profile falls between the CMs and CVs, and its oxygen isotopic composition plots in the CV-CK-CO field. Other distinctive properties of this meteorite include the presence of abundant refractory inclusions, and hundreds of micrometer size objects composed of needle-fibre calcite. LEW 85311 could come from part of a single CM parent body that was unusually rich in refractory inclusions, but more likely samples a different parent body to most other members of the group that accreted a subtly different mixture of materials. The mineralogical and geochemical evolution of LEW 85311 during subsequent aqueous alteration was similar to other CMs and was arrested at an early stage, corresponding to a petrologic subtype of CM2.7, probably due to an unusually low proportion of accreted ice. The CM carbonaceous chondrites sample multiple parent bodies whose similar size and inventory of accreted materials, including radiogenic isotopes, led to a comparable post-accretionary evolution.

¹Z.Ren,¹P.M.Vasconcelos
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.014]
¹The University of Queensland, Brisbane, Qld 4072 Australia
Copyright Elsevier

Argon release mechanisms and diffusivity were quantified for coarsely crystalline hypogene alunite [KAl₃(SO₄)₂(OH)₆] from Marysvale, Utah, and microcrystalline supergene alunite aggregates from Coober Pedy, South Australia. Prior to diffusivity studies, all alunite samples, sorted in the 500-200, 200-100, 100-50, 50-10, and < 10 µm sieve-size ranges, were vacuum encapsulated to quantify ³⁹Ar recoil losses. Incremental-heating of single capsules inserted into Ta-crucibles and heated by a projector-lamp in a specially designed diffusion cell permits quantifying ³⁹Ar released at precisely measured temperature steps (± 1-3 °C). Incremental-heating ⁴⁰Ar/³⁹Ar analyses of the various sieve-size ranges yield reproducible ages that are indistinguishable from single grain (1-2mm) laser-heating ⁴⁰Ar/³⁹Ar dating of the same samples. The results, cast in Arrhenius plots, yield activation energies (E_a) ranging from 248.0 ± 18.5 to 281.7 ± 15.2 kJ/mol and ln(D_o/a²) from 26.2 ± 2.0 to 27.8 ± 3.2 ln(s^-1) for hypogene alunite; supergene alunite yield E_a ranging between 233.3 ± 5.4 and 293.8 ± 13.7 kJ/mol and ln(D_o/a²) between 27.3 ±1.0 and 36.8 ± 2.6 ln(s^-1). These diffusion parameters correspond to closure temperatures of 264 ± 22 °C and 246 ± 19 °C for hypogene and supergene alunite, respectively, assuming a cooling rate 100 °C·Ma^-1. In-situ TEM experiments on aliquots of alunite crystals from the same samples indicate that alunite single crystals undergo transformation to nanocrystalline aggregates at 430-460 °C, showing that alunite releases Ar by volume diffusion below ∼ 430 °C, retains a significant amount of Ar during phase transformation, and proceeds to release Ar by volume or multipath diffusion from a modified polycrystalline structure at T > 460 °C. Isothermal holding time and AGESME modelling using our calculated diffusion parameters indicate that alunite should preserve Ar quantitatively for long periods (4.0 Ga) at Earth and Mars surface conditions, and both hypogene and supergene alunite should preserve original formation ages, independently of precipitation mechanism.