Ordinary Chondrites Research Articles

Hydrogen in acapulcoites and lodranites: A unique source of water for planetesimals in the inner Solar System

Acapulcoites and lodranites are primitive achondrites, sampling a common planetesimal formed a few million years after calcium-aluminium rich inclusions in the inner Solar System, that provides information into melting and differentiation processes in the early inner Solar System. The chemistry and mineralogy of their chondritic parent body lies in between enstatite and ordinary chondrites. As they record a range in planetary differentiation degree, from 1% up to 20% partial melting, with lodranites experiencing the most melt extraction, we investigate (i) the behaviour of hydrogen in terms of abundance and isotopic composition during the early stages of planetary differentiation and (ii) the source(s) of hydrogen for the acapulcoite-lodranite parent body in order to place it in our current understanding of the source(s) of water in the inner Solar System. In this study, we analysed water content and hydrogen isotopic composition of phosphates and nominally anhydrous minerals in a range of acapulcoite and lodranite meteorite samples. While apatites seemed to have recorded a degassing signature, no such variations were observed in the H2O–δD systematics of the nominally anhydrous minerals suggesting that subsequent to their crystallisation, acapulcoites and lodranites experienced minimal modifications to their volatile composition during thermal metamorphism and partial melting. The low abundance of water in acapulcoite and lodranite nominally anhydrous minerals (i.e., average 5.2±1.6 μg/g H2O) suggests that their parent body was much drier than what has been estimated for enstatite and ordinary chondrite parent bodies. We estimated a bulk water content for the acapulcoite-lodranite parent body of 3 to 19 μg/g H2O, similar to the ureilite parent body. The hydrogen isotopic composition of nominally anhydrous minerals in acapulcoites and lodranites (–211±145‰), and in particular for the two falls Acapulco and Lodran (–239±149‰), matches with the hydrogen isotopic composition recorded by nominally anhydrous minerals in ordinary chondrite falls, eucrites, S–type asteroid Itokawa, consistent with a common source of water for the inner Solar System planetesimals, isotopically distinctive to bulk carbonaceous chondrites.

Petrography, mineral chemistry, metamorphism and Raman spectroscopic studies of Mahadevpur fall meteorite, India

The Mahadevpur (27°39′48″ N, 95°51′32″ E; Arunachal Pradesh, India) meteorite is a fall‐type, coarse‐grained rock consisting of a heterogeneous mixture of chondrules of varying types occurring in a matrix comprising mostly chondrule fragments. It was earlier classified as poorly shocked (S1, unshocked) ordinary chondrite (H4/5). The minerals observed in the Mahadevpur meteorite include olivine, orthopyroxene, clinopyroxene, albitic‐plagioclase, Fe‐Ni alloy and troilite with minor amounts of chromite and phosphate. Olivine and orthopyroxene are almost equilibrated. The textures and compositional features of minerals and chondrule margins indicate a limited degree of recrystallization. The equilibration temperature (700–800°C) for the Mahadevpur chondrite is consistent with the computed temperature (752–902°C) derived from the coexisting orthopyroxene‐clinopyroxene pairs. In the light of new evidences observed during the present study, such as olivine exhibiting undulose extinction and planar to sub‐planar fractures, olivine and orthopyroxene showing strong mosaicism, the shock‐melted plagioclase glasses, the presence of high‐pressure mineral phases and silicate darkening, the earlier proposed shock stage pronouncement has been modified to S4–S5 with supporting petrographic, sub‐microscopic and Raman spectroscopic data. This is the first comprehensive account of the Mahadevpur chondrite with an upgraded shock stage classification.

Calcium–aluminum‐rich inclusions in non‐carbonaceous chondrites: Abundances, sizes, and mineralogy

AbstractAs the Sun was forming, calcium–aluminum‐rich inclusions (CAIs) were the first rocks to have condensed in the hottest regions of the solar nebula disk. Carbonaceous chondrites (CCs) contain abundant CAIs but are thought to have accreted in the outer Solar System, requiring that CAIs must have been transported outward. Curiously, CAIs are rare in ordinary, enstatite, rumuruti, and kakangari chondrites, non‐carbonaceous chondrites (NCs), that likely formed in the inner Solar System. Thus, CAI abundances and characteristics can provide constraints on the early dynamical evolution of the disk. In this work, we address whether the hypothesis of an early‐formed proto‐Jupiter “opening a gap” in the disk can explain the dichotomy in the relative abundance of CAIs in CC and NC chondrites. We searched 76 NC meteorite sections to find 232 CAIs which have an average apparent diameter of 46 μm and comprise 0.01 area%, about half the size of and ~200 times less abundant than CC CAIs on average. Unlike CC CAIs, only 4% of the NC CAIs contain melilite and most contain alteration features suggesting that NC CAIs underwent pervasive fluid‐assisted thermal metamorphism on asteroidal parent bodies. However, based on NC CAI populations correlating with meteorite metamorphic grade, we argue that disk dynamics is likely the primary reason behind the existence of small (<100 μm) and rare NC CAIs. Our data support astrophysical models which suggest that, after outward transport of CAIs, formation of a gap in the disk trapped CAIs in the outer Solar System.

Mineralogical Characterization and Phase Angle Study of Two Binary Near-Earth Asteroids, Potential Targets for NASA’s Janus Mission

Ground-based characterization of spacecraft targets prior to mission operations is critical to properly plan and execute measurements. Understanding surface properties, such as mineralogical composition and phase curves (expected brightness at different viewing geometries), informs data acquisition during the flybys. Binary near-Earth asteroids (NEAs) (35107) 1991 VH and (175706) 1996 FG3 were selected as potential targets of the National Aeronautics and Space Administration’s (NASA) dual spacecraft Janus mission. We observed 1991 VH using the 3 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, on 2008 July 26. 1996 FG3 was observed with the IRTF for seven nights during the spring of 2022. Compositional analysis of 1991 VH revealed that this NEA is classified as an Sq-type in the Bus–DeMeo taxonomy classification, with a composition consistent with LL ordinary chondrites. Using thermal modeling, we computed the thermally corrected spectra for 1996 FG3 and the corresponding best-fit albedo of about 2%–3% for the best spectra averaged for each night. Our spectral analysis indicates that this NEA is a Ch-type. The best possible meteorite analogs for 1996 FG3, based on curve matching, are two carbonaceous chondrites, Y-86789 and Murchison. No rotational variation was detected in the spectra of 1996 FG3, which means there may not be any heterogeneities on the surface of the primary. However, a clear phase reddening effect was observed in our data, confirming findings from previous ground-based studies.

Geochemistry of Trace Elements in Minerals of Porphyritic and Nonporphyritic Chondrules from Equilibrated Ordinary Chondrites

The paper presents the results of SIMS and EPMA studies of silicate minerals and bulk compositions (SEM-EDS) of porphyritic and nonporphyritic chondrules from equilibrated ordinary chondrites. The trace-element distribution in olivine, low-Ca pyroxene, and mesostasis of porphyritic and nonporphyritic chondrules in the equilibrated ordinary chondrites reflects heterogeneous conditions of chondrule-forming processes in the protoplanetary disk and provides insight in their evolution. The porphyritic chondrules are composed of olivine with the low content of Y and Yb relative to nonporphyritic chondrules. The enrichment of low-Ca pyroxene from porphyritic chondrules in trace elements correlates with the amount of pyroxene in the chondrule. Olivine composition in granular chondrules is close to that of porphyritic chondrules, but low-Ca pyroxene is distinguished by the high content of Y, Ti, Sr, Ba, V, and REE compared to all other chondrules. Barred chondrules are characterized by olivine enriched in trace elements (Zr, Y, Ti, Ba, Cr, and HREE) and low-Ca pyroxene highly depleted in trace elements (Zr, Y, Nb). The pyroxene of radial chondrules is enriched in Nb, Sr, and Ba. Trace elements in olivine and low-Ca pyroxene indicate the formation of the porphyritic and granular chondrules in a stable region of the protoplanetary disk, which is responsible for low-temperature heating and slow cooling of a chondrule. Trace element enrichment of olivine from the barred chondrules testifies strong heating of a precursor material and rapid cooling of the chondrule melt. Mineral composition of radial chondrules demonstrate rapid cooling of a low-temperature melt depleted in Mg and trace elements. High melt temperatures of barred chondrules and rapid cooling of nonporphyritic chondrules indicate their formation in an unstable region of the protoplanetary disk. Trace elements in silicate minerals of porphyritic and nonporphyritic chondrules reflect their formation as a result of melting of precursor minerals.

Siderophile element and Hf-W isotope characteristics of the metal-rich chondrite NWA 12273 – Implication for its origin and chondrite metal formation

Metal phases in primitive chondrites contain important information for understanding early Solar System processes. Northwest Africa (NWA) 12273 is an ungrouped chondrite and, due to its preternatural high modal abundance of metal grains (>60%), provides a unique perspective on the formation and evolution of primitive planetesimals. To investigate the formation of metal phases in early solar system materials we report detailed petrography, siderophile geochemistry and Hf-W isotope geochronology for NWA 12273. The primitive textures and heterogeneous mineral chemistry of chondrules, as well as siderophile element compositions of Ni-rich metal, all indicate an affinity of NWA 12273 to unequilibrated (type 3.8) ordinary chondrites that experienced limited thermal metamorphism. Modeling shows that crucial inter-element fractionations among refractory siderophile elements for kamacite in NWA 12273, were most likely caused by solid metal-liquid metal partitioning, for example during partial melting of Fe-Ni metal containing ∼3.9 wt.% of S. Analysis of the metal fractions within NWA 12273 gives a Hf-W model age of ∼2.4 Ma after CAI formation (ε182W6/4=−3.29±0.11), older than metal formation in most of H chondrites and IIE irons. In comparison with published data for chondrites and ‘non-magmatic’ iron meteorites, siderophile element data suggest that the metal phases in NWA 12273 may have originated from similar precursor materials and/or accreted in adjacent nebular locations to IIE irons and H chondrites. These primary planetesimals experienced impact-related evaporation and mixing followed by rebuilding of the secondary body. This scenario is consistent with that proposed for IIE and other silicate-bearing iron meteorites, indicating that the building blocks of some chondrite parent bodies were not entirely primitive (undifferentiated) materials.

Complex mixture of organic matter in a xenolithic clast from the Zag meteorite revealed by coordinated analyses using AFM-IR, NanoSIMS and STXM/XANES

Primitive carbonaceous xenolithic clasts found in sturdy metamorphosed meteorites often provide opportunities to reach labile volatile-rich materials which are easily destroyed during atmospheric entry and materials which we do not have sampled as individual meteorites. Among them, a xenolithic carbonaceous clast in the Zag H3–6 ordinary chondrite has been providing us with the opportunity to analyze a possible sample from D/P-type asteroids. Here we performed a new suite of coordinated analyses of organic matter in the Zag clast using the atomic force microscope infrared spectroscopy (AFM-IR) combined with nanoscale secondary ion mass spectrometer (NanoSIMS), X-ray absorption near-edge spectroscopy (XANES) coupled with scanning transmission X-ray macroscope (STXM), Raman, and (scanning) transmission electron microscopy [(S)TEM] on adjacent ultramicrotomed thin sections from a single sample grain. We successfully demonstrated the practicality of coordinated analyses using AFM-IR, Raman and NanoSIMS on the same sample area, as well as STXM/XANES on adjacent (and nearly identical) thin sections to those used for AFM-IR. The AFM-IR map and STXM maps provided consistent and complementary results. We found that at least two types of organics were closely mixed in this specimen. One was deuterium-rich, CO rich organics with likely smaller aromatic domains, possibly originating in relatively oxidized environments from D-rich precursors. The other type was less deuterium-rich, but aromatic-rich organics, possibly produced in relatively reduced and higher temperature environments with less deuterium-rich precursors. These characteristics point to complex mixtures of materials with different origins and sampling a wide heliocentric range of the Solar System before accretion in the parent body of the clast.

Upper limits of water contents in olivine and orthopyroxene of equilibrated chondrites and several achondrites

AbstractHydroxyl defects in nominally anhydrous minerals (NAMs) were potential carriers of water in the early Solar System and might have contributed to the accretion of terrestrial water. To better understand this, we have conducted a nanoscale secondary ion mass spectrometry survey of water contents in olivine and orthopyroxene from a set of equilibrated ordinary chondrites of the L and LL groups (Baszkówka, Bensour, Kheneg Ljouâd, and Tuxtuac) and several ultramafic achondrites (Zakłodzie, Dhofar 125, Northwest Africa [NWA] 4969, NWA 6693, and NWA 7317). For calibration, we used terrestrial olivine and orthopyroxene with H2O contents determined by Fourier transform infrared. Our 99.7% (~3SD) detection limits are 3.6–5.4 ppmw H2O for olivine and 7.7–10.9 ppmw H2O for orthopyroxene. None of the meteoritic samples studied consistently shows water contents above the detection limits. A few exceptions slightly above the detection limits are suspected of terrestrial contamination by ferric oxyhydroxides. If the meteorite samples investigated accreted in the presence of small amounts of water ice, the upper limits of water contents provided by our survey suggest that the retention of hydrogen during thermal metamorphism and differentiation was ineffective. We suggest that loss occurred through combinations of low internal pressures, high permeability along grain boundaries, and speciation of hydrogen into reduced compounds such as H2 and methane, which are less soluble in NAMs than in water.

The Santa Filomena meteorite shower: Trajectory, classification, and opaque phases as indicators of metamorphic conditions

AbstractOn August 19, 2020, at 13:18—UTC, a meteor event ended as a meteorite shower in Santa Filomena, a city in the Pernambuco State, northeast Brazil. The heliocentric orbital parameters resulting from images by cameras of the weather broadcasting system were semimajor axis a = 2.1 ± 0.1 au, eccentricity e = 0.55 ± 0.03, and inclination i = 0.15o ± 0.05. The data identified the body as an Apollo object, an Earth‐crossing object with a pericenter interior to the Earth's orbit. The chemical, mineralogical, and petrological evaluations, as well as the physical analysis, followed several traditional techniques. The meteorite was identified as a H5‐6 S4 W0 ordinary chondrite genomict breccia. The large amount of metal in the meteorite made a metallographic evaluation based on the opaque phases possible. The monocrystalline kamacite crystals suggest a higher petrological type and the distorted Neumann lines imply at least two different shock events. The absence of the plessite phase shows that the meteorite did not reach the highest shock levels S5 and S6. The well‐defined polycrystalline taenite is indicative of petrologic types 4 and 5 due to the conserved internal tetrataenite rim at the boundaries. The presence of polycrystalline taenites and the characteristics of the Agrell Effect suggest that the Santa Filomena meteorite did not reheat above 700°C. The absence of martensite confirms reheating temperatures <800°C and a slow cooling rate. The Ni contents and sizes of the zoned taenite particles indicate a slow cooling rate ranging from 1 to 10 K Myr−1.

Shock-induced potassium and zinc isotope fractionation in ordinary chondrites and its implications

To constrain how impacts influence the behavior of moderately volatile elements (MVEs), we report potassium (K) and zinc (Zn) contents and isotopic compositions of shock melt pockets (SMPs) and unmelted parts of three heavily shocked ordinary chondrites and bulk rocks of Chelyabinsk meteorite. All SMPs are enriched in K content and have lower isotopic values (δ41K = −1.99‰, −1.22‰ and −1.40‰) while the adjacent unmelted parts are enriched in heavy K isotopes (δ41K = −0.41‰, −0.01‰ and 0.04‰) compared to the bulk rocks of Chelyabinsk meteorite (δ41K = −0.77‰ and −0.73‰). By contrast, Zn is depleted in SMPs and the isotopic compositions are heavier (δ66Zn = −0.19‰, 2.42‰, 1.74‰) in SMPs than that in unmelted parts (δ66Zn = −0.65‰, 1.76‰, −0.97‰). Our results indicate a decoupling between the two MVEs that Zn is lost from shock melts while K is dramatically enriched in shock melts during impacts. The isotope fractionation of Zn is probably caused by evaporation of shock melts, while K isotope fractionation is most likely caused by solid-melt diffusion which is controlled by its incompatibility. The isotopic decoupling of K from Zn during major impacts further enhances our understanding of high temperature elemental and isotopic behavior of MVEs and may shed new light on the variously heterogeneous distribution of MVEs in solar system.

Elemental and triple oxygen isotope geochemistry of new Northwest Africa (NWA) ordinary chondrites (L5, L6): Implications for onion-shell to rubble-pile OC parent body structure

The new L-type ordinary chondrites (OCs) are characterized by fayalite (25.7–26.1 mol%) and ferrosillite (21.5–22.2 mol%) contents in olivine and low-Ca pyroxene, respectively. The Fe/Mg and Fe/Mn ratios in olivine, as well as oxygen isotope compositions in bulk samples, compared to those of other impact-related brecciated OCs (e.g., NWA 869, L3-6, Ghubara, L5) and olivine from Itokawa dust particles. The data demonstrate a correlation between Fe/Mg and Fe/Mn ratios in olivine, and comparability in oxygen isotope compositions between the newly analyzed L-type meteorites and various clasts of NWA 869 used as a proxy for rubble-pile asteroid. Furthermore, these ratios from bulk and olivine of equilibrated ordinary chondrites (EOC) derived from the literature data fall close to the co-variation line (1:1). However, the ratios of different fractions in unequilibrated ordinary chondrites (UOC) such as chondrules, clasts, and matrix, do not fall close to the co-variation line. The coupled anti-correlation trend observed between the bulk and olivine ratios in EOC (i.e., H, L, and LL) is primarily due to variations in the total Fe content caused by redox reactions of silicate to metal in the nebula and metal to silicate in the parent body. The proposed scenario for the formation and evolution of larger asteroids in the early solar system suggests that the they accreted and evolved as an onion-shell structure, and over time, they may have undergone internal heating due to radioactive decay. The internal heating may have caused metal-silicate segregation and thermal metamorphism. Eventually, catastrophic impacts may have caused the fragmentation of the asteroid into smaller pieces. Later, gravitational collapse reassembled the parent asteroid's fragments into several daughter asteroids that resembled rubble-piles, with the H-chondrites representing the deepest part and the L- and LL-chondrites representing the outer parts of the larger asteroid(s). Additionally, the oxygen isotope compositions of new L-chondrites are similar to those of the impact-related clasts in brecciated chondrites NWA 869, indicating that they have a shared L-type parent asteroid that was once part of a larger asteroid.

Titanium and chromium isotopic compositions of calcium-aluminum-rich inclusions: Implications for the sources of isotopic anomalies and the formation of distinct isotopic reservoirs in the early Solar System

As the earliest-dated solids in our Solar System, calcium-aluminum-rich inclusions (CAIs) provide a record of their formation environment near the young Sun and hold clues to the formation of planetary-scale isotopic reservoirs in the solar protoplanetary disk. Although CAIs from several CV, CK, CM, CO, and ordinary chondrites have been analyzed previously for their Ti isotopic compositions, CAIs from just three CV chondrites have been analyzed for their Cr isotopic compositions, and only a handful of CAIs have been measured for both their Ti and Cr isotopic compositions. We report mass-independent Ti and Cr isotopic anomalies in several CAIs from CV and CK chondrites; this is the first report of the Cr isotopic composition of a CAI from a CK chondrite. With these additional data, we aim to better constrain the compositional range of CAIs in ε50Ti versus ε54Cr space, thereby facilitating the isotopic characterization of the material inherited by the solar protoplanetary disk and the role of CAIs in the formation of distinct planetary-scale isotopic reservoirs in our early Solar System. The narrow range in isotopic anomalies in CAIs when compared to other early-formed refractory inclusions such as platy hibonite crystals (PLACs) and spinel-hibonite inclusions (SHIBs) suggests that CAIs record the mixing of these precursor materials and the averaging of their larger isotopic anomalies. The isotopic composition of CAIs is therefore likely the result of a combination of factors, including mixing of material inherited from their formation region, heterogeneous carrier phase distribution, and thermal processing in the disk. The ε50Ti and ε54Cr isotopic compositions of CAIs are not correlated, further demonstrating that these isotopic anomalies have different carrier phases. The Ti and Cr isotopic compositions of CAIs additionally show that CAIs alone cannot be responsible for the compositional difference between the non-carbonaceous chondritic (NC) and carbonaceous chondritic (CC) isotopic reservoirs but nevertheless do play a role in the formation of these large-scale isotopic reservoirs in the early Solar System.

An inner solar system origin of volatile elements in Mars

The origin of volatile elements in Mars and whether these elements derive from the inner or outer solar system is unclear. Here we show that Mars exhibits nucleosynthetic zinc (Zn) isotope anomalies similar to those of non‑carbonaceous (NC) but distinct from carbonaceous (CC) meteorites. Like for non-volatile elements, Mars' Zn isotope composition is intermediate between those of enstatite and ordinary chondrites, demonstrating that Mars acquired volatile elements predominantly from its inner solar system building blocks. The Zn isotope data limit the contribution of CI chondrite-like material to Mars to 4% by mass at most and show that Mars accreted less CC material than Earth. The origin of these disparate CC fractions is unclear, but can place constraints on how and when CC-type material was delivered to the inner solar system.

In-situ formation of halite in the Sidi El Habib 001 (H5) ordinary chondrite: Implications for hydrothermal alteration in ordinary chondrite parent bodies

The microstructures and chemistry of secondary feldspars and phosphates in equilibrated ordinary chondrites (OCs) suggest that fluids were involved in the formation of these phases, challenging the conventional view that secondary alteration of equilibrated OCs occur under water-absent conditions. The newly discovered Sidi El Habib 001 (SEH 001), a halite-bearing H5 OC, provides a unique opportunity to further probe the role of fluids during thermal metamorphism on the OC parent bodies. Here we report a petrographic and mineralogic study of SEH 001, with the aim of understanding the origins of halite grains and their implications for the alteration histories of equilibrated OCs. Our investigation reveals a main halite-bearing lithology and a halite-free lithology, both of which show equilibrated textures. Except for halides, no significant textural or compositional differences were observed between halite-bearing and -free lithologies. Halite occurs at all spatial scales in the main lithology and shows clear textures of replacing albitic plagioclase and Cl-apatite. Chlorapatite grains in SEH 001 are Cl-rich and many of them contain elevated amounts of “other” anions.Our observations suggest that halite grains in SEH 001 formed in situ on the parent body via precipitation from an aqueous fluid. The replacement of plagioclase and Cl-apatite by halite and the equilibrated textures of halite-bearing and halite-free lithologies point to a hydrothermal alteration history where halite formed during advanced thermal metamorphism before the fluid was completely lost. The two lithologies were likely affected by fluids with different Cl concentrations that resulted from heterogeneous distribution of HCl hydrate. Based on comparison to experimental data, halite in SEH 001 could have survived peak metamorphism because of its relatively high thermal stability. Collisional disruption of its original parent body could also facilitate the preservation of halite via release of heat. In the rubble pile model of the OC parent body formation, subsequent accretion of hot fragments into a rubble pile body could have resulted in the blurred boundaries between halite-free and -bearing lithologies now observed in our sample. The occurrence of halite in SEH 001 is clear evidence that aqueous fluids were involved in the alteration of equilibrated OCs.Combined with previous reports of hydrous minerals (such as phyllosilicates) and other related aqueous products in unequilibrated OCs, our study further suggests that S-type asteroids, the parent bodies of OCs, could be more hydrated than previously thought and might serve as a potential source of water for terrestrial planets in the inner solar system. Nevertheless, whether the proposed hydrothermal history of SEH 001 can be extrapolated to other equilibrated OCs needs to be tested. The in-situ formation origin of halite in SEH 001 contrasts with the exogeneous origin of halite in Monahans (1998) and Zag, suggesting that halites with different origins occurred on the OC parent bodies. The rarity of halite in OCs could be attributed to the heterogeneous distribution of HCl hydrate in the OC parent bodies, although the fragile nature of halite in terrestrial and laboratory environments also increases the likelihood of halite being destroyed in OC samples.

Trace Elements in Silicate Minerals of the Kargapole Meteorite

The aim of the present contribution was to evaluate the trace element mobility in olivine, low-Ca pyroxene and plagioclase of the Kargapole meteorite under thermal or impact metamorphic conditions and recognition of the properties of chondrule-forming events. The compositions of the minerals were analyzed using an electron probe microanalyser (EPMA) and a secondary ion mass spectrometer (SIMS). No considerable deviations in the trace element concentrations of olivine and pyroxene from unequilibrated ordinary chondrites in the Kargapole meteorite were revealed. This points to minor effects of impact metamorphism and terrestrial weathering on trace element mobilization. Olivine and low-Ca pyroxene of porphyritic olivine-pyroxene chondrule POP-0 contain higher trace element concentrations than minerals in porphyritic olivine PO-2 and olivine-pyroxene POP-4 chondrules and in the meteorite matrix.

Grain Size Effects on Visible and Near-infrared (0.35–2.5 μm) Laboratory Spectra of Ordinary Chondrite and HED Meteorites

Remote spectral characterization of near-Earth asteroids (NEAs) relies on laboratory spectral calibration to constrain their surface composition, including mineral chemistry and relative mineral abundances. Often these calibrations are based on fine meteorite powders that are representative of regolith observed on large NEAs such as (433) Eros. However, spacecraft observations of smaller NEAs such as (25143) Itokawa, (101955) Bennu, and (162173) Ryugu show surfaces devoid of a thick layer of regolith and instead find variegated landscapes with millimeter-sized particles to meter-scale boulders. Here we present the results of a laboratory study to understand the effects of grain size on the spectral properties of meteorites and how this can impact ground-based characterization of NEAs. Our study focuses on ordinary chondrites (H, L, LL) and HED meteorites, as they comprise ∼90% of all meteorites that fall on Earth. Compared to ordinary chondrites, the spectral band parameters of HED meteorites are less affected by changing grain size. Among the ordinary chondrites, LL chondrites are least affected, but the spectral band parameters and mineral chemistries and abundances for H and L chondrites are most affected by changing grain size. Grain size does not seem to have any significant effect on the taxonomic classification of our meteorite spectra. We also used the Hapke model to investigate trends in single-scattering albedo as a function of grain size and present equations to recover the grain size from a spectrum.

FEATURES OF NITROGEN DEGASSING OF DHAJALA CHONDRITE (H3.8)

Samples of the Dhajala meteorite (ordinary chondrite, type H 3.8) were kept isothermically in a specially designed device in the temperature range from 300 to 800°C for 90 minutes. The composition and content of the released gases were studied on a gas chromatograph. The following were detected: CO, CO2, H2O in concentrations of 300–1000 μg/g of the sample; also H2, CH4 and H2S in concentrations of 0.1–40 μg/g. The total nitrogen content during degassing increased quasi-linearly over time from 40 to 500 μg/g at each fixed temperature. Based on experimental observations of the change in the rate of nitrogen release depending on temperature, it was concluded that the effect of phase transitions on the permeability of the mineral matrix.

Multi‐zone fusion crust formation and classification of the 2004 Auckland meteorite (L6, S5, and W0)

AbstractOn June 12, 2004, a meteorite passed through Earth's atmosphere and landed under the television in the living room of a house in Auckland, New Zealand. Textural characteristics, the chemistry of olivine (Fa23–24) and orthopyroxene (Fs20.7), and the bulk rock triple oxygen isotopes (δ17O + 3.1; δ18O + 4.2‰) from the interior of the completely unweathered (W0) 1.3 kg meteorite, hereafter referred to as Auckland, suggest it to be a strongly metamorphosed fragment from the interior of a low iron ordinary chondrite (L6) parent asteroid. The occurrence of maskelynite but shock fracturing of olivine and pyroxene indicates Auckland experienced extreme shock metamorphism (S5), likely during Ordovician fragmentation of the asteroid parent. The fusion crust consists of three zones: (1) an innermost zone containing narrow Fe‐Ni‐S‐bearing veins that migrated along pre‐existing shock fractures in olivine and pyroxene; (2) a middle zone in which the meteorite partially melted to form a silicate glass and immiscible blebs of metal and troilite, and is accompanied by unmelted silicate minerals; and (3) an approximately 0.1 mm wide vesicular‐rich outermost layer that largely melted, volatilizing sulfides, before quenching to form glass and olivine. Oxygen isotope values of the bulk rock and/or maskelynite of melted rim and modified substrate are 2–3‰ greater than the meteorite interior and indicate that up to 19% of terrestrial atmospheric O2 was incorporated into the fusion crust during the formation. The fusion crust migrated inwards as ablation occurred, enabling melting, migration, and re‐precipitation ± loss of sulfide and metal components, with the prominent glassy rim therefore forming from an already chemically modified zone.

Mass-independent Sn isotope fractionation and radiogenic 115Sn in chondrites and terrestrial rocks

Tin has ten stable isotopes, providing the opportunity to investigate and discriminate nucleosynthetic isotope anomalies from mass-dependent and mass-independent isotope fractionation. Novel protocols for chemical separation (based on TBP-resin) and MC-ICP-MS analyses are reported here for high precision Sn isotope measurements on terrestrial rocks and chondrites. Relative to the Sn reference standard (NIST SRM 3161a), terrestrial basalts and chondrites show isotope patterns that are consistent with mass-dependent and mass-independent isotope fractionation processes as well as with 115Sn radiogenic ingrowth from 115In.Two different mass-independent isotope effects are identified, namely the nuclear volume (or nuclear field shift) and the magnetic isotope effect. The magnetic isotope effect dominates in the two measured ordinary chondrites, while repeated analyses of the carbonaceous chondrite Murchison (CM2) display a pattern consistent with a nuclear volume effect. Terrestrial basalts show patterns that are compatible with a mixture of nuclear volume and magnetic isotope effects. The ultimate origin of the isotope fractionation is unclear but a fractionation induced during sample preparation seems unlikely because different groups of chondrites show distinctly different patterns, hence pointing towards natural geo/cosmochemical processes. Only the carbonaceous chondrite Murchison (CM2) shows a Sn isotope pattern similar to what expected for nucleosynthetic variations. However, this pattern is better reproduced by nuclear volume effects. Thus, after considering mass-independent and mass-dependent effects, we find no evidence of residual nucleosynthetic anomalies, in agreement with observations for most other elements with similar half-mass condensation temperatures.Most chondrites show a deficit in 115Sn/120Sn (typically −150 to −200 ppm) relative to terrestrial samples, with the exception of one ordinary chondrite that displays an excess of about +250 ppm. The 115Sn/120Sn data correlate with In/Sn, being consistent with the β− decay of 115In over the age of the solar system. This represents the first evidence of the 115In-115Sn decay system in natural samples. The radiogenic 115Sn signature of the BSE derives from a suprachondritic In/SnBSE, which reflects preferential partitioning of Sn into the Earth’s core.

Tracing the history of an unusual compound presolar grain from progenitor star to asteroid parent body host

We conducted a transmission electron microscopy (TEM) study of an unusual oxide-silicate composite presolar grain (F2-8) from the unequilibrated ordinary chondrite Semarkona (LL3.00). The presolar composite grain is relatively large (>1 µm), has an amoeboidal shape, and contains Mg-rich olivine (forsterite), Mg-Al spinel, and Ca-rich pyroxene. The shape and phase assemblage are reminiscent of amoeboid-olivine-aggregates (AOAs) and add to the growing number of TEM observations of presolar refractory inclusion-like (CAIs and AOAs) grains. In addition to the dominant components, F2-8 also contains multiple subgrains, including an alabandite-oldhamite composite grain within the olivine and several magnetite subgrains within the Mg-Al spinel. We argue that the olivine, Mg-Al spinel, and alabandite-oldhamite formed by equilibrium condensation, whereas the Ca-rich pyroxene formed by non-equilibrium condensation, all in an M-type AGB star envelope. On the other hand, the magnetite subgrains are likely the result of aqueous alteration on the Semarkona asteroidal parent body. Additional evidence of secondary processing includes Fe-enrichment in the Mg-Al spinel and olivine, elevated Al contents in the olivine, and beam sensitivity and a modulated structure for the olivine.Compound presolar grains, in particular oxide-silicate AOA-like grains such as F2-8, record condensation conditions over a wide range of temperatures. Additionally, the presence of several different presolar phases in a composite grain can impart information on the relative rates and effects of post-condensation processing in a range of environments, including the interstellar medium, solar nebula, and the host asteroid parent body. For example, the olivine and spinel in F2-8 show evidence of fluid infiltration, but each component reacted in different ways and to different extents. The TEM observations of F2-8 provide insights across the lifetime of the grain from its formation by condensation in an M-type AGB star envelope, its transit through the interstellar medium, and aqueous alteration during its residence on Semarkona’s asteroidal parent body.