Unequilibrated Ordinary Chondrites Research Articles

Space weathering, grain size, and metamorphic heating effects on ordinary chondrite spectral reflectance parameters

AbstractThe exposure to irradiation from high‐energy particles alters the reflectance properties of asteroid surfaces and is referred to as space weathering. This process leads to an increase in spectral slope in visible and near‐infrared wavelengths. However, changes in the regolith particle size, which can vary dramatically among the asteroid population, are known to influence the spectral properties of meteorites and asteroids. In this context, we investigate the changes in spectral slope and absorption band depths of fresh and irradiated ordinary chondrite meteorites to quantitatively compare the effects of space weathering and grain size variations. To do so, we develop and employ the Spectral Analysis for Asteroid Reflectance Investigation routine that calculates the band parameters of reflectance spectra. We then formulate a parameter called the Space Weathering Index (SWI) that is designed to encapsulate spectral changes due to space weathering. We find that the SWI is strongly dependent on the spectral slope which complicates the interpretation of asteroid spectra in the context of grain size variations and space weathering. We also show that a second parameter, the Band Depth Index, is indicative of petrologic type. Finally, we use a linear discriminant analysis to classify asteroid reflectance spectra into H, L, LL, and unequilibrated ordinary chondrites.

Physical properties and average atomic numbers of chondrules using computed tomography

Micro-computed tomography is a fast and essentially non-destructive technique for studying 3D properties of solid objects. This study explores the use of a micro-CT technique to determine the physical properties and average atomic numbers of 44 chondrules from unequilibrated (petrologic type 3.00 to 3.6) ordinary, carbonaceous, and enstatite chondrites. Many chondrules deviate from a spherical geometry, implying that they were affected by strain during cooling and prior to complete solidification. The porosity of the studied chondrules ranges from 0.04 vol% to 5.3 vol%. Chondrules from carbonaceous chondrites show the highest porosity and the largest voids. The high porosity could be caused by the presence of oxidized precursors in the chondrule melt that escaped as a gas during high temperature processing and crystallization of the melt. In some chondrules, pores are associated with opaque phases, suggesting their formation either during solidification of metal phases and/or during aqueous alteration. The average atomic numbers of chondrules range from 35 ± 4 to 22 ± 2, independently of porosity and opaque content and is likely controlled by the variation of Mg/Fe in chondrule silicates. The absence of a consistent variation between the degree of deformation, chondrule diameter, and porosity among the studied chondrules from different groups, suggests that the processes responsible for the different physical properties of the chondrules are decoupled from each other and are likely universal to all chondrules.

Bulk mineralogy, water abundance, and hydrogen isotope composition of unequilibrated ordinary chondrites

AbstractThe origin and transport of water in the early Solar System is an important topic in both astrophysics and planetary science, with applications to protosolar disk evolution, planetary formation, and astrobiology. Of particular interest for understanding primordial water transport are the unequilibrated ordinary chondrites (UOCs), which have been affected by very limited alteration since their formation. Using X‐ray diffraction and isotope ratio mass spectrometry, we determined the bulk mineralogy, H2O content, and D/H ratios of 21 UOCs spanning from petrologic subtypes 3.00–3.9. The studied UOC falls of the lowest subtypes contain approximately 1 wt% H2O, and water abundance globally decreases with increasing thermal metamorphism. In addition, UOC falls of the lowest subtypes have elevated D/H ratios as high as those determined for some outer Solar System comets. This does not easily fit with existing models of water in the protoplanetary disk, which suggest D/H ratios were low in the warm inner Solar System and increased radially. These new analyses confirm that OC parent bodies accreted a D‐rich component, possibly originating from either the outer protosolar nebula or from injection of molecular cloud streamers. The sharp decrease of D/H ratios with increasing metamorphism suggests that the phase(s) hosting this D‐rich component is readily destroyed through thermal alteration.

Barred olivine chondrules in ordinary chondrites: Constraints on chondrule formation

AbstractIn general, barred olivine (BO) chondrules formed from completely melted precursors. Among BO chondrules in unequilibrated ordinary chondrites, there are significant positive correlations among chondrule diameter, bar thickness, and rim thickness. In the nebula, smaller BO precursor droplets cooled faster than larger droplets (due to their higher surface area/volume ratios) and grew thinner bars and rims. There is a bimodal distribution in the olivine FeO content in BO chondrules, with a hiatus between 11 and 19 wt% FeO. The ratio of (FeO rich)/(FeO poor) BO chondrules decreases from 12.0 in H to 1.6 in L to 1.3 in LL. This is the opposite of the case for porphyritic chondrules: the mean (FeO rich)/(FeO poor) modal ratio increases from 0.8 in H to 1.8 in L to 2.8 in LL. During H chondrite agglomeration, most precursor dustballs were small with low bulk FeO/(FeO + MgO) ratios and moderately high melting temperatures. The energy available for chondrule melting from flash heating was relatively low, capable of completely melting many ferroan dusty precursors (to form FeO‐rich BO chondrules), but incapable of completely melting many magnesian dusty precursors (to form FeO‐poor BO chondrules). When L and LL chondrites agglomerated somewhat later, significant proportions of precursor dustballs were relatively large and had moderately high bulk FeO/(FeO + MgO) ratios. The energy available from flash heating was higher, capable of completely melting higher proportions of magnesian dusty precursors to form FeO‐poor BO chondrules. These differences may have resulted from an increase in the amplitude of lightning discharges in the nebula caused by enhanced charge separation.

An in situ study of presolar grains and the fine‐grained matrices of the Meteorite Hills 00526 and Queen Alexandra Range 97008 unequilibrated ordinary chondrites

AbstractHere we report in situ structural and chemical analyses of four presolar grains and the matrices of the Meteorite Hills (MET) 00526 L3.05 and Queen Alexandra Range (QUE) 97008 L3.05 unequilibrated ordinary chondrites (UOCs). The presolar grains in MET 00526 include one Fe‐rich single crystal olivine, and one olivine grain that contains both amorphous and polycrystalline material. The single crystal olivine likely has origins in the circumstellar envelope (CSE) of a red giant branch (RGB) or asymptotic giant branch (AGB) star, and the amorphous/polycrystalline olivine has an O‐isotopic composition consistent with origins in a type II supernova. The presolar grains from QUE 97008 are Fe rich and include one crystalline, stoichiometric olivine that contains a Ca‐rich core and one crystalline, stoichiometric pyroxene grain, both of which have O‐isotopic compositions consistent with origins in the CSEs of low‐mass AGB/RGB stars. The matrices of both UOCs are mineralogically diverse with evidence for unaltered material in the form of amorphous silicates and a C‐rich nanoglobule and altered material in the form of Ni‐rich sulfides, abundant Fe‐rich olivine, and Fe‐Mg zoning in matrix silicates. No phyllosilicates were observed. The Fe‐rich olivine grains are the dominant alteration phase in both UOCs and likely replaced primary amorphous silicates in the presence of an Fe‐rich fluid during parent body alteration. Our work suggests that the ordinary and carbonaceous chondrites received a similar inventory of dust with comparable structures and chemistries.

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.

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.

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.

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.

Abundances of siderophile elements in H-chondrite metal grains: Implications for the origin of metal in unequilibrated ordinary chondrites

Understanding the evolution of metal in the protoplanetary disk is necessary to constrain the first steps of metal-silicate segregation and the early stages of the evolution of the protoplanetary disk. We measured the siderophile elemental compositions (PGE, Ni, Co, Fe, Cu, Ga, Ge) of individual metal grains in H ordinary chondrites by laser ablation inductively coupled plasma mass spectrometry to investigate their formation. We analyzed unequilibrated ordinary chondrites (H3) to constrain processes affecting the metal before accretion, and inferred the effects of metamorphism by comparing their elemental compositions to those of equilibrated chondrites (H4–H6). Our results highlight large variations of refractory (Re, Os, W, Ir, Ru, Mo, Pt) and moderately volatile siderophile element (Pd, Au, Ga, Ge) concentrations among metal grains in H3 samples that permit to classify them according to their Ge/Ir ratios and HSE contents. These intergrain variations are progressively homogenized in H4–H6 samples due to their increasing degrees of metamorphism. To constrain the origin of the metal, we modeled its evolution during melting and crystallization. Our melting model of a single metallic precursor containing 1.5 wt% C and up to 12 wt% S reproduces well the observed range of siderophile element compositions in the metal. Metal grains show a range of W, Mo, and Ga compositions that we interpret to reflect various local (grain-scale) oxidation states during the melting event(s) due to the heterogeneous distribution of various oxidizing components within the precursors. The very similar HSE compositions of H and L/LL metal grains suggests that the variations of bulk metal abundance and HSE concentrations observed among the different classes of ordinary chondrites (H, L, LL) result from the heterogeneous physical distribution of a relatively chemically homogeneous metal component among OC parent bodies, and not from a chemical (sensu lato) gradient between H and LL chondrites.

The Early Solar System Abundance of Iron-60: New Constraints from Chondritic Silicates

The abundance of iron-60 in the early solar system is important for planetary evolution models, and has been hotly debated. To put further constraints on the initial 60Fe/56Fe ratio of the solar system, here we present new iron-nickel isotope data, measured in situ by NanoSIMS, for 14 silicate chondrules from three carbonaceous and three unequilibrated ordinary chondrites. NanoSIMS measurements were performed at high spatial resolution (200–300 nm primary beam diameter), to avoid inclusion of unwanted phases in the analysis volume. The average initial 60Fe/56Fe ratios that can be estimated from our pooled chondrule data are 2.1 (±1.3) × 10−7 and 0.8 (±1.0) × 10−7 for carbonaceous and ordinary chondrites, respectively (1σ uncertainties). The estimated average initial 60Fe/56Fe ratio of all analyzed chondrules is 1.0 (±0.7) × 10−7. These results are inconsistent with initial 60Fe/56Fe ratios >2.4 × 10−7 (2σ upper limit of our entire data set) reported in the literature for some chondrule silicates based on in situ isotope data, and agree better with our previously published in situ data on chondritic troilites (0.10 ± 0.15 × 10−7), as well as with 60Fe/56Fe ratios estimated from isotope data of bulk meteorites and chondrules (0.10–0.75 × 10−7). Our isotope data hint at a possible difference between the initial 60Fe/56Fe ratios of the early solar system’s two major isotope reservoirs, with the carbonaceous chondritic reservoir having higher iron-60 abundance than the non-carbonaceous reservoir. Nevertheless, in light of similar hints in the literature, this possibility deserves further investigation.

Presolar O- and C-anomalous grains in unequilibrated ordinary chondrite matrices

Presolar grains are trace components in chondrite matrices. Their abundances and compositions have been systematically studied in carbonaceous chondrites but rarely in situ in other major chondrite classes. We have conducted a NanoSIMS isotopic search for presolar grains with O- and C-anomalous isotopic compositions in the matrices of the unequilibrated ordinary chondrites Semarkona (LL3.00), Meteorite Hills 00526 (L/LL3.05), and Northwest Africa 8276 (L3.00). The matrices of even the most primitive ordinary chondrites have been aqueously altered and/or thermally metamorphosed, destroying their presolar grain populations to varying extents. In addition to randomly placed isotope maps, we specifically targeted recently reported, relatively pristine Semarkona matrix areas to better explore the original inventory of presolar grains in this meteorite. In all samples, we found a total of 122 O-anomalous grains (silicates + oxides), 79 SiC grains, and 22 C-anomalous carbonaceous grains (organics, graphites). Average matrix-normalized abundances with 1σ uncertainties are 151-46+50 ppm O-anomalous grains, 53-12+14 ppm SiC grains and 56-14+19 ppm carbonaceous grains in Semarkona, 55-10+11 ppm (O-anom.), 22-4+5 ppm (SiC) and 3-1+2 ppm (carb.) in MET 00526 and 12-3+6 ppm (O-anom.), 15-5+7 ppm (SiC) and 1-1+3 ppm (carb.) in NWA 8276. In relatively pristine ordinary chondrites and in primitive carbonaceous and C-ungrouped chondrites, the O and C isotopic composition of presolar grains and their matrix-normalized abundances are similar, despite the likely differences in chondrite-formation time and nebular location. These results suggest a relatively homogenous distribution of presolar dust across major chondrite-forming reservoirs in the solar nebula. Secondary asteroidal processes are mainly responsible for differences in presolar grain abundances between and within chondrites, highlighting the need to identify and target the most pristine chondrite matrices for such studies.

Mineralogy, petrology, and oxygen isotopic compositions of aluminum-rich chondrules from unequilibrated ordinary and the Dar al Gani 083 (CO3.1) chondrite

Understanding the genetic relationship between different chondritic components will help to decipher their origin and dynamical evolution within the protoplanetary disk. Here, we obtain insight into these processes by acquiring O-isotope data from 17 Al-rich chondrules from unequilibrated ordinary chondrites (OCs, petrologic type ≤ 3.2) and four Al-rich chondrules from the CO3.1 carbonaceous chondrite Dar al Gani (DaG) 083. These particular kinds of chondrules are of special interest, as it is suggested that their precursors may have contained refractory material related to Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs).The four investigated Al-rich chondrules from the CO3.1 chondrite Dar al Gani 083 consist of olivine, low-Ca pyroxene, Ca pyroxene, and spinel phenocrysts embedded in mostly Na-rich glassy mesostasis. Two chondrules have a homogeneous O-isotopic composition and two are heterogeneous in their O-isotopic composition. One chondrule contains relict spinel grains with a Δ17O value of −24.3 ± 1.3‰, indicative of 16O-rich precursor refractory material, similar to constituents of CAIs and AOAs. The presence of CAI-like precursors for the Al-rich chondrules from CO chondrites is consistent with their previously reported presence of 50Ti excesses (Ebert et al., 2018).The Al-rich chondrules in the ordinary chondrites studied consist of olivine, low-Ca pyroxene, Ca pyroxene, and, occasionally, spinel phenocrysts embedded in mostly Na-rich glassy mesostasis. Hibonite is present in one Al-rich chondrule. The vast majority of these chondrules have heterogeneous O-isotopic compositions: Chondrule glasses are 16O-depleted compared to chondrule phenocrysts; the Δ17O values of the former approach those of aqueously formed fayalite and magnetite grains in type 3 OCs, ∼ +5‰. We infer that the chondrule glasses experienced O-isotope exchange with an aqueous fluid on the OC parent asteroids.Chondrule phenocrysts, like spinel, olivine, low-Ca pyroxene, and Ca pyroxene, were not affected by this isotope exchange and preserved their initial O-isotope compositions. The phenocrysts within individual chondrules have similar Δ17O, whereas the inter-chondrule Δ17O values range from −4.5 to +1.4‰, i.e., they are in general 16O enriched relative to the majority of ferromagnesian type I and type II porphyritic chondrules in OCs having Δ17O of ∼ +1‰. Because no relict grains were identified in the Al-rich chondrules from ordinary chondrites, the original O-isotopic composition of the refractory precursor material remains unknown.Additional detailed Na measurements within olivine grains show no major changes in the Na content of the chondrule melt during their crystallization. This implies either that the Na was part of the precursor material or that the Na was enriched in the chondrule melt/glass after crystallization of the olivines.

TEM analyses of in situ presolar grains from unequilibrated ordinary chondrite LL3.0 Semarkona

We investigated six presolar grains from very primitive regions of the matrix in the unequilibrated ordinary chondrite Semarkona with transmission electron microscopy (TEM). These grains include one SiC, one oxide (Mg-Al spinel), and four silicates. This is the first TEM investigation of presolar grains within an ordinary chondrite host (in situ) and the first TEM study to report on any presolar silicates (in or ex situ) from an ordinary chondrite. Structural and elemental compositional studies of presolar grains located within their meteorite hosts have the potential to provide information on conditions and processes throughout the grains’ histories.Our analyses show that the SiC and spinel grains are stoichiometric and well crystallized. In contrast, the majority of the silicate grains are non-stoichiometric and poorly crystallized. These findings are consistent with previous TEM studies of presolar grains from interplanetary dust particles and chondritic meteorites. The individual silicates have Mg#’s ranging from 15 to 98. Internal compositional heterogeneities were observed in several grains, including Al in the SiC, Mg and Al in the spinel, and Mg, Si, Al, and/or Cr in two silicates. We interpret the poorly crystalline nature, non-stoichiometry, more Fe- rather than Mg-rich compositions, and/or compositional heterogeneities as features of the formation by condensation under non-equilibrium conditions.Evidence for parent body alteration includes rims with mobile elements (S or Fe) on the SiC grain and one silicate grain. Other features characteristic of secondary processing in the interstellar medium, the solar nebula, and/or on parent bodies, were not observed or are better explained by processes operating in circumstellar envelopes. In general, there was very little overprinting of primary features of the presolar grains by secondary processes (e.g., ion irradiation, grain-grain collisions, thermal metamorphism, aqueous alteration). This finding underlines the need for additional TEM studies of presolar grains located in the primitive matrix regions of Semarkona, to address gaps in our knowledge of presolar grain populations accreted to ordinary chondrites.

Investigating S-type asteroid surfaces through reflectance spectra of ordinary chondrites

The search for asteroidal parent bodies of chondrites through various techniques is an ongoing endeavor. A link between ordinary chondrites (OCs) and S-type asteroids has previously been established by the sample return of the Hayabusa space mission. OCs are the class with the most abundant samples in our meteorite collection. We present an in-depth study of the reflectance spectra of 39 equilibrated and 41 unequilibrated ordinary chondrites (EOCs and UOCs). We demonstrate that consistent measuring conditions are vital for the direct comparison of spectral features between chondrites, otherwise hampering any conclusions. We include a comparison with a total of 466 S-type asteroid reflectance spectra from various databases. We analyze (i) if a difference between EOCs and UOCs as well as between H, L and LL can be seen, (ii) if it is possible to identify unequilibrated and equilibrated S-type asteroid surfaces and (iii) if we can further constrain the match between OCs and S-type asteroids all based on reflectance spectra.As a first step, we checked the classification of the 31 Antarctic UOCs analyzed in the present work, using petrography and magnetic measurements, and evidenced that 74% of them were misclassified. Reflectance spectra were compared between EOCs and UOCs as well as between H, L and LL chondrites using a set of spectral features including band depths and positions, peak reflectance values, spectral slopes and the Ol/(Ol + Px) ratio. UOCs and EOCs reflectance spectra show no clear-cut dichotomy, but a continuum with some EOCs showing stronger absorption bands and peak reflectance values, while others are comparable to UOCs. Moreover, we show by the example of 6 EOCs that their band depths decrease with decreasing grain size. Based on reflectance spectra alone, it is thus highly challenging to objectively identify an unequilibrated from an equilibrated S-type surface. There is no clear distinction of the chemical groups: only LL EOCs of petrographic type >4 can be distinguished from H and L through less deep 2000 nm band depths and 1000 nm band positions at longer wavelengths. No dichotomy of S-type asteroids can be seen based on the Ol/(Ol + Px) ratio. Their average Ol/(Ol + Px) ratio matches EOCs better than UOCs. A principal component analysis (PCA) was performed illustrating that both the unknown degree of space weathering and the unknown regolith grain size on asteroid surfaces hinder the distinction between equilibrated and unequilibrated surfaces. Lastly, an anti-correlation between the diameter of the asteroids and their 1000 nm band depth is found indicating that larger sized S-type asteroids show finer grained surfaces.

High precision 26Al-26Mg chronology of chondrules in unequilibrated ordinary chondrites: Evidence for restricted formation ages

Chondrules in ordinary chondrites are considered to form in high density environments, likely related to the evolution of protoplanets and large planetesimals. In order to determine the timing of their formation at high time resolution (≤0.1 Ma), we conducted high precision Al-Mg chronology of 17 porphyritic chondrules from 6 different unequilibrated ordinary chondrites (UOCs) of low petrologic subtypes (3.00–3.05). Detailed petrology, mineralogy, and oxygen isotope ratios of individual chondrules were also obtained that include 10 additional chondrules without Al-Mg ages. Seventeen chondrules for Al-Mg chronology consist of 14 chondrules with plagioclase (An1-An87) and three chondrules with Na-rich glassy mesostasis, all of which have high 27Al/24Mg ratios (30–3000). The inferred initial (26Al/27Al)0 ratios range between (6.5 ± 0.6) × 10–6 to (9.5 ± 1.0) × 10–6, corresponding to chondrule formation ages of 1.74 ± 0.12/0.11 Ma to 2.13 ± 0.09 Ma after CAIs, which have a canonical (26Al/27Al)0 ratio of 5.25 × 10–5. Six albite-bearing chondrules (An<30) show a much more restricted ages range, spanning between 2.00 ± 0.11/0.10 Ma and 2.07 ± 0.11/0.10 Ma. Including 14 anorthite-bearing chondrules studied previously, chondrules in ordinary chondrites have a restricted range of formation ages from 1.8 Ma to 2.2. Ma after CAIs.Based on the newly acquired oxygen isotope data and previous high precision studies, chondrules in L and LL chondrites do not show systematic difference in their δ18O and δ17O signatures. Majority of plagioclase-bearing chondrules studied for Al-Mg chronology show similar oxygen isotope ratios to those of glass-bearing chondrules. There is no obvious difference in Al-Mg ages of chondrules between L and LL chondrites. Thus, chondrules in L and LL chondrites would have formed in common environments and processes, though they accreted to two separate parent bodies by 2.2 Ma after CAIs, which timing is consistent with the proposed thermal model for ordinary chondrite parent bodies. Onset of chondrule formation at 1.8 Ma after CAIs may be caused by the delay of Jupiter formation or the formation of protoplanets in ordinary chondrite chondrule forming regions if chondrules formed by large scale disk shock or impact jetting of protoplanets. Alternatively, early formed chondrules would not be preserved before 1.8 Ma if chondrules formed by the impacts of molten planetesimals.

Mineralogy, petrology, and oxygen‐isotope compositions of magnetite ± fayalite assemblages in CO3, CV3, and LL3 chondrites

AbstractWe report on the mineralogy, petrology, and O‐isotope compositions of magnetite and fayalite (Fa90−100) from several metasomatically altered and weakly metamorphosed carbonaceous (Y‐81020 [CO3.05], EET 90043 [CO3.1], MAC 88107 [CO3.1‐like], and Kaba [oxidized Bali‐like CV3.1]) and unequilibrated ordinary chondrites (UOCs; Semarkona [LL3.00], MET 00452 [LL3.05], MET 96503 [LL3.05], EET 910161 [LL3.05], Ngawi [LL3.0−3.6 breccia], and Vicência [LL3.2]). In MAC 88107, EET 90043, and Kaba, nearly pure fayalite (Fa98−100) associates with phyllosilicates, magnetite, Fe,Ni‐sulfides, and hedenbergite (Fs~50Wo~50), and occurs in all chondritic components—chondrules, matrices, and refractory inclusions. In UOCs, nearly pure fayalite (Fa95−98) associates with phyllosilicates and magnetite, and occurs mainly in matrices and fine‐grained chondrule rims. Oxygen‐isotope compositions of fayalite and magnetite in UOCs, COs, CVs, and MAC 88107 are in disequilibrium with those of chondrule olivine and low‐Ca pyroxene phenocrysts, and plot along mass‐dependent fractionation lines with slope of ~0.5, but different Δ17O (~+4.3 ± 1.4‰, −0.2 ± 0.6‰, −1.5 ± 1‰, and −1.8 ± 0.8‰, respectively). Based on the mineralogical observations, thermodynamic analysis, O‐isotope compositions, and recently reported experimental data, we infer that (1) fayalite and magnetite in COs, CVs, MAC 88107, and UOCs resulted from aqueous fluid–rock interaction on the chondrite parent asteroids that occurred at low local water‐to‐rock mass ratios (0.1−0.4) and elevated temperatures (~100−300 °C), and (2) Δ17O of fayalite and magnetite reflects O‐isotope compositions of aqueous fluids on the host meteorite parent bodies. The observed differences in Δ17O of fayalite–magnetite assemblages in UOCs, CVs, COs, and MAC 88107 suggest that water ices that accreted into the ordinary chondrite and carbonaceous chondrite parent asteroids had different Δ17O, implying spatial and/or temporal variations in O‐isotope compositions of water in the protoplanetary disk.

The Sn isotope composition of chondrites: Implications for volatile element depletion in the Solar System

The origin of volatile element depletion in terrestrial planets and meteorites relative to a solar composition represented by CI carbonaceous chondrites remains an unsolved problem. The isotope compositions of moderately volatile elements may offer the possibility to distinguish between the various processes that may have caused this depletion (e.g., partial condensation or partial evaporation). We report high precision Sn isotope measurements in carbonaceous chondrites and ordinary chondrites and the results are reported as δ124/116Sn. Four carbonaceous chondrites (Orgueil CI, Murchison CM2, NWA 5240 CV3 and Allende CV3) show a limited range in δ124/116Sn (–0.02‰ to 0.11‰) with an average value of 0.04 ± 0.11‰ (2 s.d.) for a wide range of Sn concentrations (0.63 ppm to 1.57 ppm). The absence of Sn isotope fractionation among carbonaceous chondrites suggests that volatile depletion may have taken place under thermodynamic equilibrium conditions between solid and vapor in the Solar Nebula. Alternatively, the mixing of two components, a volatile-free component containing no or little Sn and a volatile-rich component could explain this trend. This latter hypothesis is consistent with the overall trace element pattern found in carbonaceous chondrites, showing a constant relative abundance when normalized to CI chondrites for the most volatile elements. In contrast with carbonaceous chondrites, ordinary chondrites exhibit a larger range of Sn isotope compositions (δ124/116Sn from –2.02‰ to 0.64‰), but neither the degree of metamorphism (3–6) nor the group (H, L, LL) is correlated with Sn isotopic variations, or with the Sn contents (range 0.20 to 1.44 ppm). Nineteen out of twenty-one ordinary chondrites are enriched in light Sn isotopes compared with carbonaceous chondrites and the bulk silicate Earth. The trace element patterns of volatile elements in ordinary chondrites suggest that equilibrated ordinary chondrites have been disturbed by parent body processes related to metamorphic or shock overprinting but also inherited isotope fractionation found in unequilibrated ordinary chondrites. Last, the isotope composition of the bulk silicate Earth (BSE) indicates that the volatile element depletion observed in the Earth took place in conditions perhaps similar to those of carbonaceous chondrites, as a simple model describing the effect of Earth’s core formation on Sn isotopes shows that the Sn isotope composition of the bulk Earth is identical to that of the BSE and of carbonaceous chondrites.

Chondrule Flattening by Shock Recovery Experiments on Unequilibrated Chondrites

AbstractShock recovery experiments were conducted using ALH‐78084 H3 and Y‐793375 L3 chondrites in the shock pressure range of 11–43 GPa to reproduce shock‐induced melting and chondrule flattening. Shock‐induced melt and chondrule flattening in 15 H3, 23 L3, and 23 LL3 ordinary chondrites were also investigated for comparisons. The shock experiments proved that, at least in unequilibrated ordinary chondrites, shock‐induced melting occurred beyond 11 GPa. The melting occurs at the boundary between chondrule and matrix. The melts included fine‐grained silicate minerals, glasses, and ameba or spherical metallic Fe–Ni or metallic Fe–Ni–iron sulfide with a eutectic texture that coincided with shock‐induced melts in the investigated H/L/LL3 ordinary chondrites. Shock experiments also proved that shock‐induced flattening of chondrules occurs and the flattening degree increases with increasing shock pressure. Considering the shock experiments not only of ordinary chondrites but also of carbonaceous chondrites, the flattening degree is not significantly affected by the density, porosity, and chondrule/matrix ratio of chondrites. The long axes of chondrules in shocked ALH‐78084 H3 and Y‐793375 L3 chondrites have preferred orientations and the degree increases with increasing shock pressure. It is difficult to estimate quantitatively the shock pressure recorded in unequilibrated ordinary chondrites using the empirical formula between the aspect ratio of chondrules and shock pressure. Nevertheless, the investigated L/LL3 ordinary chondrites with shock‐induced melts had higher aspect ratios (median, 1.36) and more strongly preferred orientations than those without shock‐induced melts (median, 1.25).