Coordinated analysis of two graphite grains from the CO3.0 LAP 031117 meteorite: First identification of a CO Nova graphite and a presolar iron sulfide subgrain

Presolar grains constitute remnants of stars that existed before the formation of the solar system. In addition to providing direct information on the materials from which the solar system formed, these grains provide ground-truth information for models of stellar evolution and nucleosynthesis. Here we report the in-situ identification of two unique presolar graphite grains from the primitive meteorite LaPaz Icefield 031117. Based on these two graphite grains, we estimate a bulk presolar graphite abundance of 5-3+7 ppm in this meteorite. One of the grains (LAP-141) is characterized by an enrichment in 12C and depletions in 33,34S, and contains a small iron sulfide subgrain, representing the first unambiguous identification of presolar iron sulfide. The other grain (LAP-149) is extremely 13C-rich and 15N-poor, with one of the lowest 12C/13C ratios observed among presolar grains. Comparison of its isotopic compositions with new stellar nucleosynthesis and dust condensation models indicates an origin in the ejecta of a low-mass CO nova. Grain LAP-149 is the first putative nova grain that quantitatively best matches nova model predictions, providing the first strong evidence for graphite condensation in nova ejecta. Our discovery confirms that CO nova graphite and presolar iron sulfide contributed to the original building blocks of the solar system.Peer ReviewedPostprint (author's final draft

Simulating Space Weathering in the Transmission Electron Microscope via Dynamic in Situ Heating and Helium Irradiation of Olivine

The chemical composition, microstructure, and optical properties of grains on the surfaces of airless bodies are predominantly altered by micrometeorite impacts and solar wind irradiation. These processes drive space weathering and result in the formation of features including chemically-altered, amorphous grain rims, Fe nanoparticles (npFe), and vesiculated grain textures. These characteristics have been identified in returned samples from the surfaces of the Moon and asteroid Itokawa. In order to advance our understanding of the formation of these microstructural and chemical features in returned samples, we have simulated space weathering processes for a variety of materials via laboratory experiments. These experiments include ion irradiation to simulate solar wind exposure and laser irradiation and in situ heating to simulate micrometeorite impacts. While these experiments have provided considerable insight into the formation mechanisms of many space weathering features, they are predominantly static and typically performed separately. Here we present results from the simulated space weathering of olivine grains via He irradiation and dynamic heating, both performed in situ inside the transmission electron microscope (TEM). These experiments allow for the real-time observation of chemical and microstructural changes resulting from the superposed effects of ion irradiation and pulsed heating

Understanding the Space Weathering of Mercury via Simulation of Micrometeorite Impacts

Space weathering alters the surfaces of airless planetary bodies via irradiation from the solar wind and micrometeorite impacts. These processes modify the microstructure, chemical composition, and spectral properties of surface materials, typically resulting in the reddening (increasing reflectance with increasing wavelength), darkening (reducing albedo), and attenuation of characteristic absorption features in reflectance spectra. In lunar samples, these changes in optical properties are driven by the production of reduced nanophase Fe particles (npFe). Our understanding of space weathering has largely been based on data from the Moon and, more recently, near-Earth S-type asteroids. However, the environment at Mercury is significantly different, with the surface experiencing intense solar wind irradiation and higher velocity micrometeorite impacts. Additionally, the composition of Mercurys surface varies significantly from that of the Moon, including a component with very low albedo known as low reflectance material (LRM) which is enriched with up to 4 wt.% carbon over the local mean. Our understanding of how carbon phases, including graphite, are altered as a result of these processes is limited

Binary systems and their nuclear explosions

Peer ReviewedPreprin

The presolar grain inventory of fine-grained chondrule rims in the Mighei-type (CM) chondrites

We investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains of fine-grained chondrule rims in six Mighei-type (CM) carbonaceous chondrites (Banten, Jbilet Winselwan, Maribo, Murchison, Murray and Yamato 791198), and the CM-related carbonaceous chondrite Sutter's Mill. Sixteen O-anomalous grains (nine silicates, six oxides) were detected, corresponding to a combined matrix-normalized abundance of ~18 ppm, together with 21 presolar SiC grains (~42 ppm). Twelve of the O-rich grains are enriched in 17O, and could originate from low-mass asymptotic giant branch stars. One grain is enriched in 17O and significantly depleted in 18O, indicative of additional cool bottom processing or hot bottom burning in its stellar parent, and three grains are of likely core-collapse supernova origin showing enhanced 18O/16O ratios relative to the solar system ratio. We find a presolar silicate/oxide ratio of 1.5, significantly lower than the ratios typically observed for chondritic meteorites. This may indicate a higher degree of aqueous alteration in the studied meteorites, or hint at a heterogeneous distribution of presolar silicates and oxides in the solar nebula. Nevertheless, the low O-anomalous grain abundance is consistent with aqueous alteration occurring in the protosolar nebula and/or on the respective parent bodies. Six O-rich presolar grains were studied by Auger Electron Spectroscopy, revealing two Fe-rich silicates, one forsterite-like Mg-rich silicate, two Al-oxides with spinel-like compositions, and one Fe-(Mg-)oxide. Scanning electron and transmission electron microscopic investigation of a relatively large silicate grain (490 nm x 735 nm) revealed that it was crystalline akermanite (Ca2Mg[Si2O7]) or a an akermanite-diopside (MgCaSi2O6) intergrowth

Formation of Interstellar C60 from Silicon Carbide Circumstellar Grains

We have conducted laboratory experiments with analog crystalline silicon carbide (SiC) grains using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The 3C polytype of SiC was used—the type commonly produced in the envelopes of asymptotic giant branch (AGB) stars. We rapidly heated small (~50 nm) synthetic SiC crystals under vacuum to ~1300 K and bombarded them with 150 keV Xe ions. TEM imaging and EELS spectroscopic mapping show that such heating and bombardment leaches silicon from the SiC surface, creating layered graphitic sheets. Surface defects in the crystals were found to distort the six-membered rings characteristic of graphite, creating hemispherical structures with diameters matching that of C60. Such nonplanar features require the formation of five-membered rings. We also identified a circumstellar grain, preserved inside the Murchison meteorite, that contains the remnant of an SiC core almost fully encased by graphite, contradicting long-standing thermodynamic predictions of material condensation. Our combined laboratory data suggest that C60 can undergo facile formation from shock heating and ion bombardment of circumstellar SiC grains. Such heating/bombardment could occur in the protoplanetary nebula phase, accounting for the observation of C60 in these objects, in planetary nebulae (PNs) and other interstellar sources receiving PN ejecta. The synthesis of C60 in astronomical sources poses challenges, as the assembly of 60 pure carbon atoms in an H-rich environment is difficult. The formation of C60 from the surface decomposition of SiC grains is a viable mechanism that could readily occur in the heterogeneous, hydrogen-dominated gas of evolved circumstellar shells.NSFNational Science Foundation (NSF) [AST-1515568, 1531243, AST-1907910]; NASANational Aeronautics & Space Administration (NASA) [NNX15AD94G, NNX15AJ22G, NNX16A31G, NNX12AL47G, 80NSSC19K0509]; DOEUnited States Department of Energy (DOE) [DE-AC07-051D14517]; Sloan Foundation Baseline Scholars Program; NIHUnited States Department of Health & Human ServicesNational Institutes of Health (NIH) - USA [R25GM062584]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Stagnant-lid tectonics in early Earth revealed by ¹⁴²Nd variations in late Archean rocks

The progressive μ¹⁴²Nd decrease in early Archean rocks from +20 to 0 between 3.9 to 3.6 billions years (Gyr), with rocks younger than 3.5 Gyr showing no μ¹⁴²Nd anomalies, is thought to indicate the efficient remixing of the first primitive crust into the Archaean convecting mantle that ultimately produce a well-mixed present-day convecting mantle with μ¹⁴²Nd = 0. The implied long mixing time of approximately 1 Gyr from the Hadean to Archaean for the whole mantle is paradoxical on several levels. This is much longer than the rapid mixing time (<100 Myr) inferred for the Archean due to vigorous mantle convection related to Earth's hotter thermal regime, and similar to the mixing time inferred for the present-day Earth's mantle. Here we report a resolvable positive ¹⁴²Nd anomaly of μ¹⁴²Nd = +7 ± 3 ppm relative to the modern convecting mantle in a 2.7 Gyr old tholeiitic lava flow from the Abitibi Greenstone Belt in the Canadian Craton. Our result effectively extends the early Archean convective mixing time to approximately 1.8 Gyr, i.e. even longer than present-day mantle mixing timescale, despite a more vigorous convection expected in the Archean. Different hypotheses have been examined to explain such a protracted mixing in the Archean, such as mantle overturn, two-layer convection or the existence of a dense layer at the bottom of the mantle. We postulate that the requirement of a delayed mixing in a strongly convective mantle is best explained by long periods of stasis in the global plate system, with scarce episodes of subduction throughout the Hadean and Archean. Our numerical model confirms that in absence of continuous plate tectonics, the convective mantle mixing is relatively inefficient in erasing the chemical heterogeneities inherited from the primordial differentiation of the early Earth. This constrains the tectonic regime of the Hadean and Archean to a stagnant-lid regime with episodic subduction. In this case, the timing for the onset of continuous modern plate tectonics can only occur shortly before or after 2.7 Gyr.1 page(s

How to preserve a chemically heterogeneous martian mantle? A Plate tectonics point of view

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