Zolensky et al.: Characteristics of Chondrites, 1 Chapter 2: Physical, chemical, and petrological characteristics of chondritic materials and their relationships to small Solar System bodies Michael E. Zolensky KT Astromaterials Research and Exploration Science NASA Johnson Space Center Houston, TX 77058, USA Neyda M. Abreu The Pennsylvania State University – DuBois Campus 1 College Place DuBois, PA 15801, USA Michael A. Velbel Department of Earth and Environmental Sciences 288 Farm Lane, room 207 Natural Science Building Michigan State University East Lansing, MI 48824-1115, USA and Division of Meteorites Department of Mineral Sciences National Museum of Natural History Smithsonian Institution Washington, DC 20013-7012, USA Alan Rubin Earth, Planetary, and Space Sciences Institute of Geophysics and Planetary Physics University of California – Los Angeles Los Angeles, CA 90095-1567, USA Noel Chaumard University of Wisconsin–Madison | UW Department of Geoscience Madison, Wisconsin, USA Takaaki Noguchi Faculty of Arts and Science, Kyushu University 744, Motooka, Nishi-ku, Fukuoka 819-0395, Japan Tatsuhiro Michikami Faculty of Engineering Kinki University 1 Takaya Umenobe, Higashi-Hiroshima Hiroshima 739-2116, Japan Zolensky et al.: Characteristics of Chondrites, 2 Abstract Chondrite materials with varying abundances of volatile-bearing phases are expected at the destinations for the asteroid sample-return missions Hayabusa2 and OSIRIS-REx. The targets of the missions are 162173 (1999 JU3) Ryugu and 101955 (1999 RQ36) Bennu. Spectroscopic analyses of these asteroids suggest that their surface materials are related to types 1 and 2 carbonaceous chondrites. Some studies suggest that the parent bodies of these chondrites may have also experienced some thermal and/or shock metamorphism. The physical properties of boulders at asteroid surfaces and fine particles in asteroid regoliths are consequences of the diverse processes that fragmented them, mobilized them, and redeposited them in unique accumulations. Sample-return missions are likely to encounter a broad range of carbonaceous chondrite (CC)-like materials, to which aqueous alteration, thermal, and shock metamorphism imparted changes affecting their sub-micron- to meter-scale physical properties. Consequently, implementation of scale-dependent analytical techniques to the study of the chemical, physical, and geotechnical characteristics of these CC-like materials is fundamental to safe mission operations, sample selection, and return. However, most of the available knowledge for informing and formulating expectations about regolith processes, products, and properties on carbonaceous small bodies comes from missions that studied anhydrous (e.g., Itokawa studied by Hayabusa) and/or much larger asteroids (e.g., Vesta studied by Dawn). No previous mission is likely directly relevant to small ice-free carbonaceous NEOs 162173 Ryugu or 101955 Bennu, although the Rosetta Spaceraft performed a flyby of the large asteroid Lutetia which has variously been classified as M and C type (Pätzold et al., 2011). Carbonaceous chondrites carry the best record of the history, distribution, and activity of water in the early solar system. Ordinary and Enstatite chondrites carry only partial records, but these are still critical to understanding the full story. We will describe the records of water-rock interactions on asteroids, as recorded in these meteorites, with particular emphasis on the timing, nature, settings, and fluid compositions. An integral part of this story is the rare, but fortunate, preservation of actual early solar system water as aqueous fluid inclusions. 2.1. Introduction Our main source of extraterrestrial materials is meteorites, or the rocks that “rained” on earth, which are either observed as they fall or recognized and collected. The number of meteorites available for research has grown exponentially since American, Japanese, and other teams began organizing expeditions to retrieve these rocks from cold and hot deserts, accounting for nearly all the mass available across tens of thousands of individual samples. In addition to these rocks, extraterrestrial dust is collected via high altitude flights. Even though we are now entering the era of interplanetary sample return missions, aside from lunar samples collected in the 1960’s, spacecraft have only retrieved less than one gram of extraterrestrial mass from one asteroid and one comet. Chondrites are aggregates of materials with significantly variable histories, mineralogies, grain sizes and shapes, porosities, and physical strengths. Throughout their histories components and current lithologies of chondrites have experienced condensation from gas or liquids, heating and partial to complete vaporization, radiation damage and implantation of solar wind gas, atoms, nuclei and electrons, accretionary and later stage shock metamorphism, dynamic and thermal metamorphism, sulfidation, and aqueous alteration (Hutchison, 2004). This record is Zolensky et al.: Characteristics of Chondrites, 3 preserved in the chemical and istopic characteristics of these samples. A myriad of stable and radiogenic isotopes have been measured for chondrites. For the purpose of this chapter, we focus on the oxygen and Al-Mg systematics of chondrites. This selection was based on understanding the history of water in chondrites and not the scientific importance of other systems, which are critical to our understanding of early solar system history. Chondrites carry the best record of the history, distribution, and activity of water in the early solar system. Chondritic meteorites, especially carbonaceous chondrites, have special significance cosmochemically. They are similar in composition to the solar photosphere, excluding a few very volatile elements (e.g., Sears and Dodd, 1988). They are samples of primitive, in other words, undifferentiated materials that are found as asteroids in the belt between Mars and Jupiter, as near earth objects (NEOs), and potentially as cometary materials. Chondrites consist of ferromagnesian igneous spherules called chondrules and refractory- element rich inclusions (i.e., Calcium Aluminum-rich Inclusions or CAIs and Amoeboid Olivine Aggregates or AOAs) embedded in a sub-micron silicate-rich matrix. It is widely accepted that processes in the solar nebula resulted in the formation of the high-temperature components, chondrules and refractory inclusions. After their formation, chondrules and CAIs accreted along with surviving interstellar grains, fine-grained matrices, and carbonaceous materials into asteroidal parent bodies. Chondrules are thought to have formed within the first few million years of solar system history. Although for a long time it was thought that chondrules formed relatively late in the evolution of the solar nebula, a more recent U-corrected Pb–Pb study indicates that at least some chondrules have formation ages ranging from 4567.32 ± 0.42 to 4564.71 ± 0.30 Myr, which overlaps with the time in which CAIs formed (Connelly et al., 2012). Mineralogically, chondrules are dominated by olivine, pyroxenes, a feldspathic mesostasis, and Fe-rich opaque minerals (e.g., Rubin 2010; Jones 2012). In terms of textures, chondrules are predominantly porphyritic (over 80% - Jones 2012), including porphyritic olivine and pyroxene (POP), porphyritic olivine (PO), and porphyritic pyroxene (PP) varieties. Non-porphytic textures include barred olivine (BO), radiating pyroxene (RP), and cryptocrystalline (CC) textural types (e.g., Rubin, 2010; Jones, 2012). Chondrules are classified based on their FeO contents, into type I chondrules (FeO-poor; Fe/(Fe + Mg) ratios <10 mole%.), type II chondrules (FeO-rich; Fe/(Fe + Mg) ratios > 50 mole%) and comprise as little as 1 vol% in some chondrite groups, and Aluminum-rich chondrules (ARCs) (e.g., Weisberg et al., 1993; Krot and Keil, 2002; Connolly et al., 2007; Connolly and Huss 2010). In a 훿17O vs. 훿18O plot, chondrules from primitive chondrites plot on a line with slope ~1 and type I chondrules are often enriched in 16O with respect to type II chondrules (e.g., Clayton, 1993; Krot et al., 2006a, 2010; Connolly and Huss, 2010; Libourel and Chaussidon, 2011; Ushikubo et al. 2012; Schrader et al., 2013, 2014; Tenner et al., 2013, 2015). Numerous experimental and petrologic studies indicate that chondrules formed by rapid (seconds to minutes), partial to complete melting of dust precursors (e.g., Hewins et al., 2005; Connolly and Jones, 2016 and references therein). However, the mechanism of chondrule formation has long been a subject of debate. Shocks, magnetized turbulence, and impacts have been considered as potential drivers for chondrule formation (see Krot and Nagashima, 2017 and references therein). In addition to condensation, chondrules record a variety of other processes occurring in the nebula, asteroids, and on earth (e.g., multiple condensation/heating events, collisions with other chondrules, elemental exchange with nebular gas, metamorphism, aqueous alteration). Therefore, the characteristics of chondrules can be used to explore a variety of environments. For example, isotopic compositions of chondrules have Zolensky et al.: Characteristics of Chondrites, 4 been used to explore the characteristics of nebular gas and dust, as well as the processes operating in different locations of the solar nebula. Authors have argued that the oxygen isotopic compositions of chondrules in different chondrite groups can be attributed to formation at different heliocentric distances