Thermal Alteration of Labile Elements in Carbonaceous Chondrites

Carbonaceous chondrite meteorites are some of the oldest Solar System planetary materials available for study. The CI group has bulk abundances of elements similar to those of the solar photosphere. Of particular interest in carbonaceous chondrite compositions are labile elements, which vaporize and mobilize efficiently during post-accretionary parent-body heating events. Thus, they can record low-temperature alteration events throughout asteroid evolution. However, the precise nature of labile-element mobilization in planetary materials is unknown. Here we characterize the thermally induced movements of the labile elements S, As, Se, Te, Cd, Sb, and Hg in carbonaceous chondrites by conducting experimental simulations of volatile-element mobilization during thermal metamorphism. This process results in appreciable loss of some elements at temperatures as low as 500 K. This work builds on previous laboratory heating experiments on primitive meteorites and shows the sensitivity of chondrite compositions to excursions in temperature. Elements such as S and Hg have the most active response to temperature across different meteorite groups. Labile element mobilization in primitive meteorites is essential for quantifying elemental fractionation that occurred on asteroids early in Solar System history. This work is relevant to maintaining a pristine sample from asteroid (101955) Bennu from the OSIRIS-REx mission and constraining the past orbital history of Bennu. Additionally, we discuss thermal effects on surface processes of near-Earth asteroids, including the thermal history of "rock comets" such as (3200) Phaethon. This work is also critical for constraining the concentrations of contaminants in vaporized water extracted from asteroid regolith as part of future in situ resource utilization for sustained robotic and human space exploration.Comment: 12 pages of text, 3 tables, 7 figures, accepted by Icaru

Petrology and geochemistry of the Northwest Africa 3368 eucrite

Analysis of the mineralogy, isotopic, and bulk compositions of the eucrite meteorites is imperative for understanding their origin on the asteroid 4 Vesta, the proposed parent body of the HED meteorites. We present here the petrology, mineral compositions, and bulk chemistry of several lithic components of the new brecciated basaltic eucrite Northwest Africa (NWA) 3368 to determine if all the lithologies reflect formation from one rock type or many rock types. The meteorite has three main lithologies: coarse- and fine-grained clasts surrounded by a fine-grained recrystallized silicate matrix. Silicate compositions are homogeneous, and the average rare earth element pattern for NWA 3368 is approximately 10× CI chondrites with a slight negative Eu anomaly. Major and trace element data place NWA 3368 with the Main Group-Nuevo Laredo trend. High-Ti chromites with ilmenite exsolution lamellae provide evidence of NWA 3368’s history of intense metamorphism. We suggest that this meteorite underwent several episodes of brecciation and metamorphism, similar to that proposed by Metzler et al. (1995). We conclude that NWA 3368 is a monomict basaltic eucrite breccia related to known eucrites in texture and in mineral, bulk, and oxygen isotopic composition

Taphonomy of the fossil insects of the middle Eocene Kishenehn Formation

The lacustrine oil shales of the Coal Creek Member of the Kishenehn Formation in northwestern Montana comprise a relatively unstudied middle Eocene fossil insect locality. Herein, we detail the stratigraphic position of the fossiliferous unit, describe the insect fauna of the Coal Creek locality and document its bias towards very small but remarkably pre-served insects. In addition, the depositional environment is examined and the mineral constituents of the laminations that comprise the varves of the Kishenehn oil shale are defined. Fifteen orders of insects have been recorded with the majority of all insects identified as aquatic with the families Chironomidae (Diptera) and Corixidae (Hemiptera) dominant. The presence of small aquatic insects, many of which are immature, the intact nature of >90% of the fossil insects and the presence of Daphnia ephippia, all indicate that the depositional environment was the shallow margin of a large freshwater lake. The fossil insects occur within fossilized microbial mat layers that comprise the bedding planes of the oil shale. Unlike the fossiliferous shales of the Florissant and Okanagan Highlands, the mats are not a product of diatomaceous algae nor are diatom frustules a component of the sediments or the varve structure. Instead, the varves are composed of very fine eolian siliciclastic silt grains overlaid with non-diatomaceous, possibly cyanobacteria-derived microbial mats which contain distinct traces of polyaromatic hydrocarbons. A distinct third layer composed of essentially pure calcite is present in the shale of some exposures and is presumably derived from the seasonal warming-induced precipitation of carbonate from the lake’s waters. The Coal Creek locality presents a unique opportunity to study both very small middle Eocene insects not often preserved as compression fossils in most Konservat-Lagerstätte and the processes that led to their preservation

Perseverance rover notional caches for Mars Sample Return

The NASA Mars 2020 Perseverance rover plans to collect a suite of scientifically compelling samples for return to Earth [1–3]. Strategic planning by the Mars 2020 Science Team has identified notional sample caches within the framework of the geology of Jezero crater and its surroundings [2]. Locations of interest were identified by considering remotely sensed data and traversability constraints [1]. Notional sample caches have been defined for the prime mission within Jezero crater, and an extended mission outside Jezero crater.Prime mission notional cache. Samples of interest include: deltaic, crater floor, crater rim, and regolith materials. Lithologies with high habitability and biosignature preservation potential, such as carbonates and/or chemical deposits, will also be targeted. Such samples would address several questions:What habitable niches were present at Jezero? Are biosignatures and/or prebiotic organics preserved?What was the timing of fluviolacustrine activity?How can Jezero lithologies facilitate absolute crater chronology calibration?What insights do these lithologies provide into Mars climate evolution?What is the origin and alteration history of regional Noachian crust?Extended mission notional cache. Nili Planum is geologically distinct from Jezero, containing diverse Early or Pre-Noachian rocks [4]. Samples of interest include: basement rocks; megabreccias; fractures cross-cutting basement; olivine- and carbonate-bearing rocks; and mafic cap rock. These samples could answer questions 1, 4 and 5 above, together with:What characteristics defined the early planetary evolution and habitability of Mars?How long did the Martian dynamo persist?How do outside-Jezero surfaces, including bedrock and ejecta enable crater chronology calibration?What were the local and regional effects of the Isidis impact?The samples to be collected by Perseverance align with community priorities for Mars exploration [3,5], addressing geological diversity and potential biological activity (iMOST Objectives 1 and 2), long-term planetary evolution, e.g., magnetic field, atmosphere and climate (Objective 3), volatiles (Objective 4) and hazards to human exploration (Objective 6).[1] Farley et al. (2021), LII LPSC #1317. [2] Stack et al. (2020) SpaceSci.Rev. 216, 127. [3] Beaty et al. (2019) Meteorit.Planet.Sci. 54, S3-S152. [4] Simon et al. (2021) LII LPSC #1515. [5] Mustard et al. (2013) MEPAG Report