Two Recent CM Falls: New Evidence for a Lithologically and Isotopically Heterogeneous CM Parent Body

R. Findlay, R. C. Greenwood, A. J. King, M. Anand, I. A. Franchi

51st Lunar and Planetary Science Conference (2020), Abstract #2880


“The CM carbonaceous chondrites (CCs) are a diverse group of meteorites which occupy a spectrum of mineralogical variation as a result of aqueous alteration and thermal metamorphism [1]. The degree of modification is expressed as a petrologic type, ranging from partially altered CM2s to fully altered CM1s [1, 2]. The nature, or even number, of CM parent bodies remains unclear, in part due to the large variation between CM specimens [1, 3]. However, it is apparent that individual samples also exhibit strong intra-sample variation [4]. Many CMs are complex breccias containing multiple lithologies (as clasts) that reflect varying degrees of alteration [4, 5, 6]. This mineralogical heterogeneity is reflected in large oxygen isotopic variations, as a result of the very different signatures of the original anhydrous precursor silicates and the water-ice that originally accreted into these parent bodies [1, 7]. Herein lies a considerable sampling problem, as bulk analyses of the same meteorites often yield isotopic compositions far apart on the slope ≈ 0.7 CM array [8]. A further challenge is the increased susceptibility of these very fine-grained samples to modification in the terrestrial environment, prior to collection and even subsequently once the material has been curated. Interaction with meteoric water and atmospheric oxygen have the potential to significantly impact the measured oxygen isotopic signature. CM2 falls are rather rare. However, two recent CM2 falls, Aguas Zarcas (AZ) and Mukundpura (MP), including a dark clast which bears similarities to a CM1-like lithology, offer a new opportunity to better understand the isotopic variation within these samples and implications for the nature and evolution of their parent body. “