Search

The diversity of lithologies is an important proxy of internal evolution in differentiated planets and asteroids. The major lithologies in Vesta, based on the howardite-eucrite-diogenite clan meteorites, include basalt, gabbro, noritic orthopyroxenite, orthopyroxenite, dunite, harzburgite, and dacite. No other lithology has been reported up to date. In this study, we report a new occurrence of corundum in eucrite meteorite Northwest Africa (NWA) 8647. Three-dimensional petrographic observations reveal that the corundum grain occurs as a mineral inclusion in a highly deformed pyroxene fragment. The texture indicates that the corundum is not a contaminant. The corundum-associated pyroxenes have Fe-Mn compositions consistent with typical pyroxenes from howardite-eucrite-diogenite meteorites. We suggest that the corundum grain could be a xenocryst incorporated during the ascent of a basaltic magma. The results might indicate the presence of an Al-rich, Si-poor region, probably lithology in the interior of Vesta, implying that the evolution and internal structure should be much more complex than previously thought.

New samples returned by China Chang’e-5 (CE-5) mission offer an opportunity for studying the lunar geologic longevity, space weathering, and regolith evolution. The age determination of the CE-5 samples was among the first scientific questions to be answered. However, the precious samples, most in the micrometer size range, challenge many traditional analyses on large single crystals of zircon developed for massive bulk samples. Here, we developed a non-destructive rapid screening of individual zirconium-containing particle for isotope geochronology based on a Micro X-ray fluorescence analysis (µXRF). The selected particles were verified via scanning electron microscopy (SEM), 3D X-ray microscopy (XRM), and focused ion beam scanning electron microscopy (FIB-SEM) techniques, which showed that zirconium-bearing minerals with several microns were precisely positioned and readily suitable for site-specific isotopic dating by second ion mass spectrometry (SIMS). Such protocol could be also applicable in non-destructively screening other types of particles for different scientific purposes. We therefore proposed a correlative workflow for comprehensively studying the CE-5 lunar samples from single particles on nanometer to atomic scales. Linking various microscopic and spectromicroscopic instruments together, this workflow consists of six steps: (1) single-particle selection with non-destructive µXRF technique, (2) 2D/3D morphological and structural characterization with a correlative submicron 3D XRM and nanoscale resolution FIB-SEM imaging methods, (3) SEM analysis of the surface morphology and chemistry of the selected particle, (4) a series of microscopic and microbeam analyses (e.g., SEM, electron probe microanalysis, and SIMS) on the cross-section of the selected particle to obtain structural, mineralogical, chemical, and isotopic features from the micron to nanometer scale, (5) advanced 2D/3D characterization and site-specific sample preparation of thin foil/tip specimens on a microregion of interest in the selected particle with FIB-SEM technique, and (6) comprehensive analyses on the FIB-milled specimens at nanometer to atomic scale with synchrotron-based scanning transmission X-ray microscopy, analytic transmission electron microscopy, and atom probe tomography. Following this technical roadmap, one can integrate multiple modalities into a uniform frame of multimodal and multiscale correlated datasets to acquire high-throughput information on the limited or precious terrestrial and extraterrestrial samples.

Phosphorus-rich olivine (P 2 O 5 > 1 wt%) is a mineral that has been reported only in a few terrestrial and extraterrestrial occurrences. Previous investigations suggest that P-rich olivine mainly forms through rapid crystallization from high-temperature P-rich melts. Here, we report a new occurrence of P-rich olivine in an ungrouped carbonaceous chondrite Dar al Gani (DaG) 978. The P-rich olivine in DaG 978 occurs as lath-shaped grains surrounding low-Ca pyroxene and olivine grains. The lath-shaped olivine shows a large variation in P 2 O 5 (0–5.5 wt%). The P-rich olivine grains occur in a chondrule fragment and is closely associated with chlorapatite, merrillite, FeNi metal, and troilite. Tiny Cr-rich hercynite is present as inclusions within the P-rich olivine. The lath-shaped texture and the association with Cr-rich hercynite indicates that the P-rich olivine in DaG 978 formed by replacing low-Ca pyroxene precursor by a P-rich fluid during a thermal event, rather than by crystallization from a high-temperature melt. The large variation of P 2 O 5 within olivine grains on micrometer-scale indicates a disequilibrium formation process of the P-rich olivine. The occurrence of P-rich olivine in DaG 978 reveals a new formation mechanism of P-rich olivine.

Northwest Africa (NWA) 7755 is a newly found enriched lherzolitic shergottite. Here, we report its detailed petrography and mineralogy. NWA 7755 contains both poikilitic and non-poikilitic lithologies. Olivine has different compositional ranges in the poikilitic and non-poikilitic lithologies, Fa30–39 and Fa37–40, respectively. Pyroxene in the non-poikilitic lithology is systematically Fe-richer than that in the poikilitic lithology. The chromite grains in non-poikilitic lithology are highly Ti-richer than those in the poikilitic lithology. The chemical variations of olivine, pyroxene, and chromite between the poikilitic and non-poikilitic lithologies support a two-stage formation model of lherzolitic shergottites. Besides planar fractures and strong mosaicism in olivine and pyroxene, shock-induced melt veins and pockets are observed in NWA 7755. Olivine grains within and adjacent to melt veins and/or pockets have either transformed to ringwoodite, amorphous phase, or dissociated to bridgmanite plus magnesiowustite. Merrillite in melt veins has completely transformed to tuite; however, apatite only has partially transformed to tuite, indicating a relatively sluggish transformation rate. The partial transformation from apatite to tuite resulted in fractional devolatilization of Cl and F in apatite. The fine-grained mineral assemblage in melt veins consists mainly of bridgmanite, minor magnesiowustite, Fe-sulfide, Fe-phosphide, and Ca-phosphate minerals. The coexistence of bridgmanite and magnesiowustite in these veins indicates a shock pressure of >~24 GPa and a temperature of 1800–2000 °C. Coesite and seifertite are probably present in NWA 7755. The presence of these high-pressure minerals indicates that NWA 7755 has experienced a more intense shock metamorphism than other enriched lherzolitic shergottites.