Engineering topics

C Crow

Publications and source records attributed to C Crow.

3 records

NASA NTRS2024

Microstructural Evolution of the Civet Cat Norite (72255)

The ‘Civet Cat Norite’ is a ~ 2.5 cm clast within the Apollo impact melt breccia sample 72255 collected during Apollo 17 EVA 2 at Boulder 1. The norite is primarily composed of plagioclase and orthopyroxene with fine-scale exsolution lamellae of augite. The plagioclase and orthopyroxene (OPX) coexist with minor quartz and augitic clinopyroxene (CPX), and accessory apatite, baddeleyite, chromite, ilmenite, loveringite, merrillite, rutile and zircon. The major minerals range from 1 to 4 mm in size and show a cataclastic texture indicated by banding of the light and dark phases]. The chemical composition of the orthopyroxene, augitic exsolution lamellae, and exsolved oxides suggest the norite crystallized in the lower crust of the Moon (10 – 70 km depth) and was subsequently excavated by impact(s). Shock pressure estimates for the norite range from 15 – 35 GPa based on maskeleynite formation within the plagioclase, kink banding in pyroxenes, and granular recrystallization in the accessory phosphates. Previous Rb-Sr and 40 Ar/ 39 Ar analyses recalculated with modern decay constants yield relatively young ages of 4.16 ± 0.05 Ga and 3.99 ± 0.03 Ga, respectively. However, recent U-Pb analyses of some zircon grains from within the Civet Cat yield an age of 4.45 ± 0.03 Ga. While this is the oldest age for any rock recovered from the Moon, the full age spread of zircon from the Civet Cat range from ca. 4.46 to 4.02 Ga, indicating variable Pb-loss and complicating interpretation of the primary age. Recent atom probe analyses indicate minimal Pb mobilization within the oldest zircon grain from Civet Cat, supporting the interpretation that these ancient zircon ages reflect the age of the norite, rather than an apparent antiquity caused by reverse discordance. To investigate the potential antiquity and deep crustal origin of the Civet Cat norite, we have undertaken high resolution microstructural analyses of the major, minor, and accessory minerals to characterize the primary igneous textures and secondary overprinting by impact and metamorphism. The employed analytical techniques include scanning electron microscope-based electron backscatter diffraction (EBSD) analyses, electron probe microanalyzer (EPMA) chemical analyses, and transmission electron microscopy (TEM) of pyroxene, plagioclase, quartz, zircon, and phosphates.

T M Erickson↗

NASA NTRSDate not supplied

The Oxidation State of Sulfur in Apollo Samples 71035 and 71055

Lunar apatites contain 100s-1000s ppm sulfur [1-3]. This was initially puzzling because lunar basalts are thought to form, including at the time of apatite crystallization, in low oxygen fugacity (fO2) conditions where sulfur exists in its reduced form (S2-), a substitution not previously observed in natural apatite. It has been recently shown, using S-XANES measurements of the oxidation state of sulfur in apatites and proximal mesostasis in the lunar basalts 12039 and 10044, that sulfur is indeed present as S2- in both the mesostasis glass and apatite when measurements were performed far from cracks or pits in the thin section [4]. This observation is consistent with other mineralogical indications of the low fO2 during formation of these samples (~IW-1) such as the presence of Fe-metal, ulvöspinel, ilmenite, fayalite, and silica in the thin sections. In the same study, in addition to clear spectral evidence for the dominance of S2-, analyses of apatite grains in both samples that occurred near cracks or pits in the thin section sometimes revealed non-negligible spectral evidence (e.g., S6+/ΣS > 0.03; [4]) for the presence of S6+. Because S6+ was not observed in the mesostasis glass near the apatite grains measured, or in any phase far from cracks or pits in the studied thin sections, this S6+ was interpreted as either primary S2- in the sulfide altered to S6+ in the thin section, or S6+ of secondary origin, deposited in the fractures of the samples [4, 5]. Whether this is of lunar or terrestrial origin is unknown but has implications for lunar petrogenesis if lunar [e.g., 5], or sample handling and curation if terrestrial. To test between lunar and terrestrial origins for observed S6+ in Apollo-era thin sections, we will measure the oxidation state of sulfur in apatite and associated phases in a specially frozen sample from a boulder sampled at Station 1A during the Apollo 17 mission as part of the NASA Apollo Next Generation Sample Analysis (ANGSA) program. Here, we present “control” measurements of samples 71035 and 71055, using Apollo era thin sections of aliquots of these rocks that were processed upon return to Earth and since stored under N2 atmospheres at room temperature. These will be compared to measurements using newly made thin sections of aliquots of the same rocks (71035 and 71055). In addition, 71036, which has been in cold storage (-20℃) since the return of the Apollo 17 mission to Earth, has been recently made available through the ANGSA program. With 71036 we can test the effect of storage temperature on potential oxidation of S2- to S6+ in the relatively oxidizing and warm conditions of Earth’s surface.

apollo↗

NASA NTRSDate not supplied

Phosphate (U-Th)/He Thermochronology of Apollo 14 Melt Breccia 14311

Our ability to confidently characterize the impact history of the inner solar system is limited by discrepancies in radiometric dates and age interpretations for lunar rocks, impact melts, and recovered meteorites. It is therefore important to explore different thermally sensitive radiometric systems to unravel the timing and extent of the long term impact flux. Low-temperature thermochronology of lunar samples has the potential to provide more complementary geochronological datasets and further test dynamical models related to the evolution of the inner solar system (e.g., [1]). (U-Th)/He dating is based on the production of 4He atoms by radioactive alpha decay of U and Th (and to a lesser extent, Sm) in a crystal and the thermally activated volumetric diffusion of those 4He nuclides. At high temperatures, the crystal is an open system from which 4He can escape; at lower temperatures, 4He may be retained. This retention temperature depends on factors such as crystal structure and volume fraction of radiation damage in the crystal (e.g., [2, 3]), but is significantly lower for phosphate minerals compared to the diffusion of the radiogenic daughter products in other widely used chronometric systems (e.g., ~75°C in terrestrial apatite (U-Th)/He vs. ~500°C in apatite U-Th-Pb vs. ~900°C in zircon U-Th-Pb chronometry). Phosphate (U-Th)/He dating has been used to decipher peak temperatures and cooling rates related to terrestrial impact events (e.g., [4]). Pairing a low-temperature thermochronometer with higher-temperature approaches (i.e., a combined 207Pb-206Pb and (U-Th)/He approach), can therefore resolve multiple impact ages within a given sample or even grain. However, despite its potential, phosphate (U-Th)/He dating has not been reported on any lunar samples. Here we present the first lunar phosphate (U-Th)/He thermochronology on an Apollo 14 impact melt-breccia.

C A Diaz↗