Browsing by Author "Srivastava, Yash"

Sample return missions have provided the basis for understanding the thermochemical evolution of the Moon. Mare basalt sources are likely to have originated from partial melting of lunar magma ocean cumulates after solidification from an initially molten state. Some of the Apollo mare basalts show evidence for the presence in their source of a late-stage radiogenic heat-producing incompatible element-rich layer, known for its enrichment in potassium, rare-earth elements, and phosphorus (KREEP). Here we show the most depleted lunar meteorite, Asuka-881757, and associated mare basalts, represent ancient (~3.9 Ga) partial melts of KREEP-free Fe-rich mantle. Petrological modeling demonstrates that these basalts were generated at lower temperatures and shallower depths than typical Apollo mare basalts. Calculated mantle potential temperatures of these rocks suggest a relatively cooler mantle source and lower surface heat flow than those associated with later-erupted mare basalts, suggesting a fundamental shift in melting regime in the Moon from ~3.9 to ~3.3 Ga.

Two diogenites (Johnstown and ALHA 77256) and two eucrites (Malotas (b) and Stannern) meteorites from the Howardite–Eucrite–Diogenite (HED) clan are investigated by petrography, mineral chemistry and using crystal size distribution (CSD) technique applied to pyroxene grains to demonstrate their crystallization history and post-magmatic processes. Among the dominant mineral phases, plagioclase is invariably anorthitic in all the samples. However, pyroxene has variable compositional ranges: En66-84Fs16-34Wo2-4 in Johnstown, En65-83Fs17-35Wo2-4 in ALHA 77256, En26-40Fs60-74Wo0-21 in Malotas (b), and En44-58Fs42-66Wo2-48 in Stannern, indicating diogenite pyroxenes are more Mg-rich and Ca-poor than the eucrite pyroxenes. The CSD result indicates that the diogenites show a near-linear slope with a major turning down of the curves at finer grains, which can be attributed to the onset of thermal annealing. Concave-up trends in the slopes of the diogenites are indicative of crystal accumulation at larger sizes leading to textural coarsening. The CSD plot of Malotas (b) eucrite suggests multiple mixing of magmas at a relatively deeper part of Vesta, whereas the lack of kink in the CSD pattern of Stannern eucrite indicates the crystal fractionation trend at smaller sizes above the diogenitic layer and its subsequent eruption on the surface of parent Vesta. This study suggests multiple stages of melting, crystallization, and subsequent sub-solidus recrystallization in the deep-seated diogenites, while the eucrites underwent a lesser amount of metamorphism at a shallower crustal level than the deep-seated diogenites.

Aubrites are rare meteorites from highly reduced differentiated parent bodies. The Rantila meteorite was recovered soon after falling on 17 August 2022 at Rantila and Ravel villages in Gujarat state, India. We report the petrography, mineralogy, chemical composition, oxygen- and chromium-isotope compositions, along with reflectance spectroscopy, all showing that Rantila is an aubrite. Coarse enstatite and diopside grains constitute the main mass of Rantila, while mm-wide fracture domains pervade the coarse enstatites. In the fractures, comminuted enstatite, diopside blebs, olivine, a plagioclase–silica assemblage, sulfides, and metals occur. Rantila consists of enstatite (>85 vol%), diopside (~8 vol%), forsterite, albite, and silica along with various sulfides and Fe-Ni alloys. The concentration of rare earth elements is ~1–2 × CI, consistent with main group aubrites. Noble gas and nitrogen isotopic analyses reveal young exposure ages (13.81 ± 6.47 Ma), a heterogeneous nitrogen isotopic composition, and a major K-Ar resetting event around 3.2 ± 0.4 Ga in the parent body of Rantila. The bulk oxygen isotope values are within the range of aubrites. The chromium isotopic values of Rantila are consistent with main group aubrites. The mineral assemblages, texture, and crystallization modeling suggest that Rantila had an igneous origin. The mineral assemblages in fractures indicate the involvement of external melt possibly during an impact-fracturing event, which aligns well with the heterogeneous N isotopic composition. Additionally, Rantila shows a wider range of oxygen isotopes than other aubrites suggesting some extent of O isotopic heterogeneity, likely stemming from exogenous processes. The variation in intra-sample bulk O and N isotope values implies inherent heterogeneity within the main group aubrites, potentially caused by late-stage impact contamination.