Two recent cm falls: new evidence for a lithologically and isotopically heterogeneous cm parent body

Introduction: 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. Materials and Methods: Two polished blocks of AZ and one polished block of MP were studied using a FEI Quanta 200 3D scanning electron microscope (SEM). Bulk mineralogy of the powders was determined using X-ray diffraction (XRD) at the Natural History Museum, London. Oxygen isotopic compositions were made using the “single shot” laser fluorination method, where only one sample and one standard are loaded per tray, thus reducing the effect of reaction with BrF5 at room temperature [9]. Powdered and homogenized chips of AZ (500 mg) and MP (200 mg) were prepared, from which a few mg were used for analysis. Several mg was also sampled from a dark, lozenged-shaped clast found in the MP hand specimen by gently scraping with a stainless steel spatula. Results and Discussion: AZ and MP are diverse regolith breccias [5]. MP appears more altered than AZ, owing to a greater abundance of phyllosilicate and poorly characterized phase (PCP) throughout its clasts [1]. Our sample contains chondrule-rich and chondrule-poor lithologies/clasts [8], exhibiting varying degrees of aqueous alteration. A large (cm-sized) clast of chondrule-free, dark, fine-grained material is also present. However, to-date only the chondrule-rich lithologies were examined by SEM. XRD patterns indicate that the bulk mineralogy of both AZ and MP are typical of CM2s, with diffraction peaks for olivine, enstatite, phyllosilicates (including Mg serpentine and Fe cronstedtite), magnetite and carbonates present (Fig. 1) [10, 11]. Our measurements of AZ and MP are similar to other CM falls having been recovered before terrestrial alteration (Fig. 1). The pattern from the dark clast in MP indicates an absence of olivine, enstatite, tochilinite and high abundance of magnetite, consistent with the high degree of aqueous alteration seen in the CM1s (Fig. 1) [13, 14]. The clast may therefore be “CM1-like”. Our oxygen isotope analyses of bulk AZ plot within the CM2 field (δO = 0.98, 0.76; δO = 7.39, 7.06) but are ~ 4‰ lighter in δO and span a smaller range than those published in the Meteoritical Bulletin [8] (Fig. 2). Further analyses of AZ on a third set of samples have intermediate compositions [12] highlighting the significant (~6‰) oxygen isotope heterogeneity in AZ specimens. This isotopic variation is likely also reflected in significant mineralogical differences, and