Olivine Chondrules Research Articles

Isotopic evolution of the inner solar system revealed by size-dependent oxygen isotopic variations in chondrules

The systematic isotopic difference between refractory inclusions and chondrules, particularly for oxygen, has long indicated an isotopic evolution of the solar protoplanetary disk. However, it remains underconstrained whether such evolution continued during chondrule formation. Intrigued by past reports of the size-dependent oxygen isotopic compositions of chondrules in ordinary chondrites (OC), we analyzed type I olivine-rich chondrules of various sizes in two LL3 chondrites. Although most chondrules show positive Δ17O values comparable to bulk ordinary chondrites, a population of smaller (less than about 0.1 mm2 in cross-section), including many isolated olivine grains (sensu lato), are 16O-enriched (with Δ17O values down to −13.2 ‰). Literature data allow the same observation for R chondrites. All sub-TFL type I chondrules (i.e., Δ17O < 0) chondrules have Mg# > 97 mol% while the supra-TFL type I chondrule olivines extend to the formal boundary with type II chondrules (i.e., Mg# = 90 mol%). The sub-TFL chondrules are likely genetically related to isotopically similar aluminum-rich chondrules described in the literature. They therefore must have formed earlier than most OC and R chondrules when the inner disk was still sub-TFL. This interpretation is supported by the presence of similarly 16O-rich relict grains in supra-TFL OC and R chondrules that must be remains of this incompletely recycled precursor material. The non-carbonaceous reservoir was thus still evolving isotopically towards 16O-poorer composition when chondrule formation began, whether by mixing with outer disk material, late accretion streamers and/or an increase in the solid/gas ratio due to magnetothermal disk winds.

Pairing relations within CO3 chondrites recovered at the Dominion Range and Miller Range, Transantarctic mountains: Constraints from chondrule olivines, noble gas, and H, C, N bulk and isotopic compositions

AbstractThe Dominion Range (DOM) and Miller Range (MIL) dense collection areas (DCAs) have yielded more than 20 and 200 CO3 chondrites (carbonaceous chondrites of the Ornans chemical group), respectively, over multiple field seasons. Several samples have exhibited primitive characteristics and have been the focus of interest. With so many CO3s recovered from this area, a natural question is if there are multiple pairing groups (where pairing refers to two or more meteorites that are part of a single fall) and if there is additional primitive material that would interest the meteorite community. This comprehensive study looks at all samples using several approaches: field and macroscopic observations; magnetic susceptibility; Cr in ferroan olivine; bulk elemental and isotopic analysis of H, C, N, and noble gas analyses to determine cosmic ray exposure (CRE) ages. Magnetic susceptibilities (measured as logχ) for all samples correlate with their type II (i.e., FeO‐rich) olivine Cr contents, with the most primitive CO3s (3.00) have logχ values near 5, while the higher grade CO3s have logχ values as low as 4.17. Altogether, there appear to be two distinct CO3 pairing groups and five other unpaired CO3s recovered at the Dominion Range: (a) the main DOM 08004 pairing group (16 specimens with a CRE age of 10–16 Ma), (b) the DOM 08006 group (2 specimens incl. DOM 10847 with a CRE age of 25 Ma), (c) DOM 14359 with a CRE age of 6 Ma, (d) DOM 18070 with a CRE age of 8 Ma (these two samples have similar ages but distinct trapped 20Ne contents), (e) DOM 10900 with a CRE age of 5.5 Ma, (f) DOM 18286 (with a CRE age of ~59 Ma), and (g) DOM 19034 (with a CRE age of ~43 Ma). There are three distinct age groupings of 3.00–3.05 COs, highlighting the diverse pristine CO3 materials present in the DOM area. There is one large MIL pairing group (MIL 07099; n = 199; 9–14 Ma CRE age where measured) and one smaller pairing group with distinctly lower Cr2O3 in type II olivines (8 samples of unknown CRE age), and five unpaired or unique CO3s. Notably, the large DOM and MIL pairing groups have 9–16 Ma exposure ages that could have been delivered in a single large fall event spanning ~200 km, two separate falls that were ejection paired, or two separate falls from two separate ejections. Finally, we recommend reclassifying several CO3 to CM2 based on new data and that from previous studies.

Chemical compositions of Fe‐rich relict olivines from cosmic spherules, understanding their links with ordinary and carbonaceous chondrites

AbstractFe‐rich relict olivine grains are found in a small percentage of cosmic spherules, which are studied here to determine the nature of their precursors. We examined 128 Fe‐rich relict olivine grains with Fa &gt;10 mol% from 53 cosmic spherules of different types collected from Antarctica (Antarctica micrometeorites [AMM]) and deep‐sea sediments (DSS) of the Indian Ocean. Fe‐rich olivines identified in cosmic spherules are close analogs of type II chondrule olivines formed in the early solar system. The olivine analysis shows well‐defined trends in molar Fe/Mn versus Fe/Mg with an affinity for ordinary and carbonaceous chondrites. The minor oxides in olivine are in ranges such as MnO ~0.1–0.8 wt%, Cr2O3 ~0–0.7 wt%, CaO ~0–0.6 wt%, and Al2O3 ~0–0.2 wt%, respectively. The chemical composition suggests that the precursors for these Fe‐rich olivine‐bearing cosmic spherules consist of ordinary chondrites (~21%–23%, AMM‐DSS), carbonaceous chondrites (~17%–36%, AMM‐DSS), and a large fraction overlapping both carbonaceous and ordinary chondrites (~41%–62% AMM‐DSS). The elemental ratios Fe/Si/CI and Mg/Si/CI for the Fe‐rich relict olivines ranging between the values 0.5–1.0 and 1.1–1.7 are compatible with IDPs, Comet 81P/Wild 2 as well as the Asteroid Itokawa and Ryugu, which are indistinguishable from carbonaceous and ordinary chondrites. In addition, pyroxene and olivine assemblages in their Fa versus Fs mol% show strong similarities to EOC chondrites. Our results on Fe‐rich relict olivines show that these grains in cosmic spherules are less common than Mg‐rich olivines, which show a narrow range of chemical compositions identical to those from ordinary chondrites and carbonaceous chondrites, indicating a supplementary contribution of an ordinary chondritic component to the micrometeorite source of dust.

Mineralogy, petrology, and oxygen isotopic compositions of chondritic and achondritic lithologies in the anomalous CB carbonaceous chondrites Sierra Gorda 013 and Fountain Hills

AbstractThe CB (Bencubbin‐like) metal‐rich carbonaceous chondrites are subdivided into the CBa and CBb subgroups. The CBa chondrites are composed predominantly of ~cm‐sized skeletal olivine chondrules and unzoned Fe,Ni‐metal ± troilite nodules. The CBb chondrites are finer grained than the CBas and consist of chemically zoned and unzoned Fe,Ni‐metal grains, Fe,Ni‐metal ± troilite nodules, cryptocrystalline and skeletal olivine chondrules, and rare refractory inclusions. Both subgroups contain exceptionally rare porphyritic chondrules and no interchondrule fine‐grained matrix, and are interpreted as the products of a gas–melt impact plume formed by a high‐velocity collision between differentiated planetesimals about 4562 Ma. The anomalous metal‐rich carbonaceous chondrites, Fountain Hills and Sierra Gorda 013 (SG 013), have bulk oxygen isotopic compositions similar to those of other CBs but contain coarse‐grained igneous clasts/porphyritic chondrule‐like objects composed of olivine, low‐Ca‐pyroxene, and minor plagioclase and high‐Ca pyroxene as well as barred olivine and skeletal olivine chondrules. Cryptocrystalline chondrules, zoned Fe,Ni‐metal grains, and interchondrule fine‐grained matrix are absent. In SG 013, Fe,Ni‐metal (~80 vol%) occurs as several mm‐sized nodules; magnesiochromite (Mg‐chromite) is accessory; daubréelite and schreibersite are minor; troilite is absent. In Fountain Hills, Fe,Ni‐metal (~25 vol%) is dispersed between chondrules and silicate clasts; chromite and sulfides are absent. In addition to a dominant chondritic lithology, SG 013 contains a chondrule‐free lithology composed of Fe,Ni‐metal nodules (~25 vol%), coarse‐grained olivine and low‐Ca pyroxene, interstitial high‐Ca pyroxene and anorthitic plagioclase, and Mg‐chromite. Here, we report on oxygen isotopic compositions of olivine, low‐Ca pyroxene, and ±Mg‐chromite in Fountain Hills and both lithologies of SG 013 measured in situ using an ion microprobe. Oxygen isotope compositions of olivine, low‐Ca pyroxene, and Mg‐chromite in these meteorites are similar to those of magnesian non‐porphyritic chondrules in CBa and CBb chondrites: on a three‐isotope oxygen diagram (δ17O vs. δ18O), they plot close to a slope‐1 (primitive chondrule mineral) line and have a very narrow range of Δ17O (=δ17O–0.52 × δ18O) values, −2.5 ± 0.9‰ (avr ± 2SD). No isotopically distinct relict grains have been identified in porphyritic chondrule‐like objects. We suggest that magnesian non‐porphyritic (barred olivine, skeletal olivine, cryptocrystalline) chondrules in the CBas, CBbs, and porphyritic chondrule‐like objects in SG 013 and Fountain Hills formed in different zones of the CB impact plume characterized by variable pressure, temperature, cooling rates, and redox conditions. The achondritic lithology in SG 013 represents fragments of one of the colliding bodies and therefore one of the CB chondrule precursors. Fountain Hills was subsequently modified by impact melting; Fe,Ni‐metal and sulfides were partially lost during this process.

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.

Estimating the Net Magnetic Moment of Geological Samples From Planar Field Maps Using Multipoles

AbstractRecent advances in magnetic microscopy have enabled studies of geological samples whose weak and spatially nonuniform magnetizations were previously inaccessible to standard magnetometry techniques. A quantity of central importance is the net magnetic moment, which reflects the mean direction and the intensity of the magnetization states of numerous ferromagnetic crystals within a certain volume. The planar arrangement of typical magnetic microscopy measurements, which originates from measuring the field immediately above the polished surface of a sample to maximize sensitivity and spatial resolution, makes estimating net moments considerably more challenging than with spherically distributed data. In particular, spatially extended and nonuniform magnetization distributions often cannot be adequately approximated by a single magnetic dipole. To address this limitation, we developed a multipole fitting technique that can accurately estimate net moment using spherical harmonic multipole expansions computed from planar data. Given that the optimal location for the origin of such expansions is unknown beforehand and generally unconstrained, regularization of this inverse problem is critical for obtaining accurate moment estimates from noisy experimental magnetic data. We characterized the performance of the technique using synthetic sources under different conditions (noiseless data, data corrupted with simulated white noise, and data corrupted with measured instrument noise). We then validated and demonstrated the technique using superconducting quantum interference device microscopy measurements of impact melt spherules from Lonar crater, India and dusty olivine chondrules from the CO chondrite meteorite Dominion Range 08006.

On the significance of oxygen-isotope variations in chondrules from carbonaceous chondrites

Oxygen-isotope studies of carbonaceous chondrite chondrules are of pivotal importance for understanding of the evolution of the solar protoplanetary disk. It has been previously concluded that the observed variations in Δ17O (=δ17O – 80.52 × δ18O) of chondrule olivines and pyroxenes are intimately tied to their Mg# (=MgO/(MgO + FeO) × 100, mol%) and the inferred oxygen fugacity (fO2) of gaseous reservoirs produced by evaporation of disk regions with different (silicate dust ± water ice)/gas ratios. Using high resolution cathodoluminescence and secondary ion mass spectrometry, we show, in contrast to host chondrule data, that Δ17O of chondrule olivines from the Yamato 81020 CO3.05 carbonaceous chondrite is independent of their Mg# and of the imposed fO2. Instead, there is a Δ17O bimodal distribution of Mg-rich olivines that gradually turns into a unimodal distribution as Mg# decreases. We suggest that chondrules recorded an evolution of an isotopically heterogeneous vapor plume resulting from a high temperature mixing of the 16O-enriched (Δ17O ≈ −6 ± 2‰) and 16O-depleted (Δ17O ≈ −2.5 ± 1‰) reservoirs. Drop in the vapor plume temperature under unbuffered redox conditions favors the dissolution of Fe,Ni-metal of chondrules and the subsequent crystallization of FeO-rich olivines at saturation; the 16O-depleted signature being the most resilient at lower temperature. Chondrules are thus inferred to have formed in a same turbulent heterogeneous environment in which locally high temperatures and reducing conditions prevailed (Type I), adjacent in space and/or in time to areas submitted to lower temperatures and more oxidizing conditions (Type II). Subtracting of chondrules at different stages of the gaseous plume evolution by fast cooling rates give rise to the chemical and isotopic diversity of chondrules. No extrinsic oxidizing agents (e.g., water ice) are needed in this scenario.

In Situ Argon Isotope Analyses of Chondrule-Forming Materials in the Allende Meteorite: A Preliminary Study for 40Ar/39Ar Dating Based on Cosmogenic 39Ar

The argon isotopic compositions of chondrule-forming minerals of the Allende (CV3) meteorite were examined to evaluate the possibility of in situ 40Ar/39Ar dating of planetary surface rocks based on cosmogenic 39Ar without neutron irradiation in a reactor. The investigated Allende meteorite sample (ME-247H: 50 mm × 45 mm × 5 mm; 28.85 g) contains at least three textural types of chondrules: barred olivine chondrule (BOC), porphyritic olivine chondrule (POC), and unclassified chondrule (UC). Most chondrules contain olivine, low-Ca pyroxene, clinopyroxene, and plagioclase as primary phases, with minor amounts of nepheline and sodalite formed during aqueous alteration of the CV3 parent body of the early solar system. In situ argon isotope analyses on selected chondrule-forming minerals in petrographic sections of two BOCs, two POCs, and one UC using a Nd:YAG pulse laser confirmed a significant amount of cosmogenic 39Ar that formed by a 39K (n, p) 39Ar reaction in an extraterrestrial environment. Laser step-heating analyses of five bulk chondrules irradiated in a reactor revealed a plateau age (3.32 ± 0.06 Ga) from one of the five chondrules. The age spectra of all chondrules show the younger age in the low-temperature fractions, resulting in the integrated ages from 2.7 to 3.2 Ga. These results suggest that the Allende meteorite experienced argon isotopic homogenization at 3.3 Ga and the argon loss in part after the 3.3 Ga. Apparent ages of chondrule-forming minerals that were calculated using the J values of nephelines in one BOC and two POCs do not show any consistent relationship among the three types of chondrules (BOC, POC, and UC). This might be attributed to the fact that the isotopic heterogeneity among minerals took place during the heterogeneous argon loss stage after the 3.3 Ga event.

Lithium and boron isotopic compositions of olivine in chondrules from carbonaceous and ordinary chondrite meteorites: Implications for the origin of solar 11B/10B ratio

The origin of solar 11B/10B ratio ∼4 remains an open question. It has been thought that a significant portion of boron in the Solar System was derived from continuous spallation nucleosynthesis during interactions between Galactic Cosmic Rays and CNO nuclei in the interstellar medium. However, because GCR-produced boron is characterized by 11B/10B ∼2.5, an endmember with 11B/10B > 4 is required to account for the solar 11B/10B ratio. Two leading hypotheses for the sources of 11B-rich components include low energy spallation in the Sun’s parental molecular cloud and the neutrino process during the supernova explosions. In this study, lithium and boron elemental and isotopic compositions of seven porphyritic olivine chondrules and one isolated olivine crystal from four meteorites (Allende, Yamato 81020, Asuka 12236, and QUE 97008) were determined in-situ to help constrain which of the two nucleosynthetic mechanisms has most likely supplied the forming Solar System with extra 11B. Apparent isotopic variations in Li/Si, B/Si, δ7Li and δ11B were found in chondrule olivine crystals, but only three chondrules exhibit statistically resolved δ11B heterogeneities. Using these three chondrules as a basis for discussion, we evaluated the processes that could potentially cause elemental and isotopic variations of Li and B. We argue that the data can be best understood in the context of condensation of lithium-boron-rich material onto chondrule precursors in (an) initially heterogeneous gaseous reservoir(s), which could be realized if the Solar System derived 11B-rich components from a supernova or supernovae.

Features of olivine crystallization in ordinary chondrites (Saratov meteorite): geochemistry of trace and rare earth elements

The paper discusses the geochemistry of major (EPMA) and trace (SIMS) elements in olivine of porphyritic, nonporphyritic chondrules, and the matrix of equilibrated ordinary chondrite Saratov (L4). Olivine corresponds to forsterite and is rather heterogeneous (Fo 73-77). No differences in the content of the major elements in the olivine of the chondrule and the matrix of the meteorite were found. However, the content of major and trace elements in olivine within chondrules varies considerably; high values found in olivine from barred chondrules. Olivine from porphyritic chondrules and the matrix of the Saratov meteorite have similar concentrations of trace elements. High concentrations of refractory (Zr, Y, Al) and moderately volatile (Sr and Ba) trace elements in barred olivine chondrule indicate the chondrule melt formation due to the melting of precursor minerals and its rapid cooling in the protoplanetary disk, which is consistent with the experimental data. The olivine of the chondrules center of the Saratov meteorite differs from the olivine of the chondrules rims and meteorite matrix by the increased values of the Yb/La ratio. No relict grains and magnesian cores of olivine were found in meteorite chondrules. Individual grains in the chondrules are distinguished by their enrichment in trace elements relative to the rest of the olivine grains in the chondrule.

SIMS matrix effects in oxygen isotope analysis of olivine and pyroxene: Application to Acfer 094 chondrite chondrules and reconsideration of the primitive chondrule minerals (PCM) line

The instrumental bias (here expressed as bias*, the difference relative to San Carlos Olivine, SC-Ol) of oxygen isotopes in secondary ion mass spectrometry (SIMS) analyses of olivine and pyroxene were investigated. Sixteen olivine (Fo0–100), nine orthopyroxene (En70–100Wo0–3), and three clinopyroxene (En28-49Wo45–50) reference materials (RMs) were utilized. The values of bias* are nearly invariant (within 0 ± 0.5‰) among magnesian olivine (Fo ≥60) RMs but decrease systematically with increasing Fe molar fraction down to ~ −10‰ (Fo0). The session-to-session variability of bias* for Fo ≥60 olivines is ≤0.3‰ but becomes slightly larger for more Fe-rich compositions (≤1.5‰). Orthopyroxene RMs have a narrow range of bias* values (−1.7‰ to −2.6‰) that show session-to-session variability ≤0.3‰; correspondingly, the sputter rates are similarly low and resemble those of Fo ≥60 olivines. Clinopyroxene RMs bias* values (−0.2‰ to +1.2‰) are more variable and have larger session-to-session variability (≤0.8‰), which appear to be independent of sputter rates. Bias* of the same RMs change slightly from session to session with different analytical settings (≤0.6‰ for Fo ≥60 and as much as 2‰ for fayalite) so that equations of bias* as a function of Fo, En, and Wo should be determined for each SIMS session by the analyses of multiple RMs.The new suite of olivine and pyroxene RMs with expanded composition range were used to correct the instrumental biases during oxygen three-isotope analysis of olivine and low-Ca pyroxene in chondrules of the ungrouped Acfer 094 chondrite, from which the primitive chondrule minerals (PCM, δ18O vs. δ17O) line was originally derived. The PCM line is considered to be a mixing trend of two extreme primary oxygen isotope reservoirs of solids in the early solar system. However, this new dataset, with better analytical precision and instrumental bias correction, allows two oxygen isotope trends to be clearly identified. One is represented by chondrules that plot on or above the PCM line, likely linked to ordinary chondrite (OC)-like materials. A second trend is represented by the major chondrule population in this chondrite that defines a regression line of δ17O = (0.968 ± 0.022) × δ18O − (3.46 ± 0.10) (MSWD = 2.0), consistent with recent oxygen isotope data of chondrules in carbonaceous chondrites (i.e., CV, CK, CO, and CM). We propose that this regression line represents a mixing of two oxygen isotope reservoirs (possibly 16O-rich solids and 16O-poor H2O ice) in the outer solar system and likely resulting from the separate evolutions of isotope reservoirs after the “Jupiter divide” built up.

Slow cooling during crystallisation of barred olivine chondrules

Barred olivine (BO) chondrules are small ferromagnesian silicate igneous droplets with unique dendritic textures that are considered to have formed in the early solar system during one or more brief high-temperature episodes, followed by rapid cooling in a gas. Rapid cooling rates of 100–7200 °C/h during chondrule formation have been proposed based on experiments attempting to reproduce BO crystal textures. However, the BO texture has never truly been reproduced under such rapid cooling conditions. Here, we experimentally show that true BO textures can be produced either after rapid cooling (>50 °C/h) following by reheating step or by cooling rates slower than 10 °C/h. Regardless of the thermal history considered, the chemical compositions of glass inclusions trapped within olivines of BO chondrules imply a final slow cooling rate one to two orders of magnitude below previous estimates. Such slow cooling rates are consistent with those estimated for plagioclase-bearing porphyritic chondrules and magmatic type-B Ca-Al-rich inclusions, suggesting that slow cooling rates are common to all similar chondritic objects.

Spinel in CV chondrules: Investigating precursor legacy and chondrule thermal histories

AbstractIn carbonaceous chondrites, Mg‐spinel (MgAl2O4) grains are ubiquitous in refractory inclusions but rarely reported in chondrules, where they may correspond to minerals either (i) inherited from chondrule precursors or (ii) crystallized from chondrule melts. Here, we report high‐current quantitative electron microprobe measurements and secondary ion mass spectrometry oxygen isotopic analyses of Mg‐spinel‐bearing chondrules in the CV3 carbonaceous chondrites Northwest Africa 10235 and Allende. Compared to spinels in refractory inclusions, chondrule spinels are characterized by higher Cr contents and 16O‐poorer oxygen isotopic signatures (∆17O ≡ δ17O−0.52 × δ18O, from −2 to −6‰). Because the similar Δ17O values of chondrule olivine and spinel crystals imply their comagmatic origin, we applied a geothermometer based on the Al‐Cr distribution between these minerals to determine their crystallization temperatures. The calculated temperatures range from 1200 to 1640 °C (mean = 1470 °C), most being lower than the estimated liquidus temperature of porphyritic chondrules (~1600 °C). Our results suggest that chondrules experienced relatively slow cooling rates (slower than a few hundreds of °C h−1), which is in good agreement with models of chondrule formation invoking nonlinear or two‐stage cooling rates.

A record of low-temperature asteroidal processes of amoeboid olivine aggregates from the Kainsaz CO3.2 chondrite

Amoeboid olivine aggregates (AOAs) from the Kainsaz CO3.2 chondrite were analyzed using transmission electron microscopy in order to gain a more complete understanding of thermal metamorphism on the parent body and the role of fluids during metamorphic heating. The Kainsaz AOAs are dominated by strongly zoned, fine-grained, olivine grains (Fa2-31) with heterogeneous Fe enrichments along the grain boundaries, which are interpreted as the result of Fe2+-Mg2+ interdiffusion with the matrix during thermal metamorphism. However, our diffusion calculations show that such AOA olivine zoning and compositions cannot be produced by a simple diffusional exchange during metamorphic heating, unlike chondrule olivine zoning and compositions. In addition, fine-grained ferroan olivine overgrowths occur heterogeneously in crystallographic continuity with olivines on the AOA exteriors. The overgrowths (Fa33-36) are compositionally distinct from the underlying AOA olivines and are not fully equilibrated with the matrix olivines (Fa∼20-55). The ferroan olivine overgrowths likely formed by precipitation from fluids in an epitaxial relationship with forsteritic olivine on the edges of AOAs. Texturally and compositionally diverse chromite grains are also observed along olivine grain boundaries, in olivine grains, and in pore spaces between olivine grains. They share a similar crystallographic orientation relationship with adjacent olivine, suggestive of their formation by exsolution and/or epitaxial growth. Collectively, these observations provide evidence for the mobilization of Fe, Mg, Si, Cr, and Al in the presence of fluids along olivine grain boundaries and into olivine grains during thermal metamorphism. We conclude that in Kainsaz AOAs, the strong zonation development in individual olivine grains and the formation of ferroan olivine overgrowths and chromite grains were a fluid-driven process that occurred at relatively low temperatures (<500 °C), during the cooling history of the CO3 chondrite parent body, following the peak of thermal metamorphism.

Oxygen isotope variations in Mg-rich olivines from type I chondrules in carbonaceous chondrites

Using high-resolution cathodoluminescence (HR-CL) panchromatic imaging for the location of high-precision oxygen three-isotope analyses by secondary ion mass-spectrometry (SIMS), this study is aimed at characterizing the oxygen-isotope variations in Mg-rich olivines (≥Fo99) of selected type I chondrules from the Yamato (Y) -81020 CO3.05 (Ornans-type) carbonaceous chondrite. Cathodoluminescence being extremely sensitive to faint changes in CL activator/quencher concentrations (Al, Cr, Mn, Fe) allows us to describe various overlooked cycles of growth and dissolution in Mg-rich olivines, which strongly suggest an intimate relationship with their gaseous environment during their formation. The present study confirms significant Δ17O variations of ten ‰ in Mg-rich olivines but does not support the relationship previously found between Mg# [MgO/(MgO + FeO) × 100, mol%] and Δ17O among type I chondrules, nor the interpretation of redox changes that has been made of it. We instead show that Mg-rich olivines in Y-81020 chondrules exhibit a prominent 16O-enriched and 16O-depleted bimodal distribution, which is considered as the most primordial signature of type I chondrules from Y-81020 and very likely other carbonaceous chondrites. This signature is interpreted as a snapshot of the early stages of a mixing occurring between two clouds/environments in which chondrules formed and evolved by gas–melt interaction and mixed according to hydrodynamical instabilities imposed by the process responsible for the mixing. As far as this study allows, O-isotope variations of Mg-rich olivines seems to account for large scale dynamical instabilities while chemical variations highlighted by HR-CL (dissolution/growth) bear witness of smaller scale instabilities very likely occurring in the immediate vicinity of the chondrules. Without being able to decide on plausible astrophysical settings yet, we note however that processes like disruptive and vaporizing collisions between planetesimals offer a range of processes and physicochemical conditions, e.g., expansion, decompression, dynamical instabilities, that deserve to be explored in more detail, some of which resembling those highlighted in this study, e.g., gas–melt interaction, partial pressure fluctuations, heterogeneous materials, gas mixing.

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.

Impact plume‐formed and protoplanetary disk high‐temperature components in CB and CH metal‐rich carbonaceous chondrites

AbstractWe report on the mineralogy, petrology, and oxygen isotopic compositions of ferroan olivine–pyroxene‐normative cryptocrystalline chondrules (Fe‐CCs) in CH chondrites and discuss their origin and the origin of other components in the genetically related CH and CB chondrites. There are two kinds of Fe‐CCs: (1) compositionally uniform (Fe/[Fe+Mg] = 0.17–0.34) chondrules with euhedral Fe, Ni‐metal grains and (2) metal‐free chemically zoned (Fe/[Fe+Mg] = 0.05–0.4) chondrules surrounded by ferroan olivine (Fa44−62) rims; the Fe/(Fe+Mg) ratio increases toward the rims. Both types contain low CaO and Al2O3 (&lt;0.04 wt%), but relatively high contents of MnO and Cr2O3 (up to 1 wt%). Compositionally uniform euhedral Fe, Ni‐metal grains are Ni‐rich (9–20 wt%) and have subsolar Co/Ni ratio. There is a positive correlation between iron content in the metal grains and Fe/(Fe+Mg) ratio in silicate portion of their host chondrules. Some Fe‐CCs experienced postcrystallization solid‐state reduction of ferroan silicates to metallic iron. Ferroan cryptocrystalline chondrules and olivine rims have similar oxygen isotopic compositions (interchondrule Δ17O ranges from ~ −2‰ to 2‰), which are slightly 16O‐depleted relative to those of magnesian olivine–pyroxene‐normative cryptocrystalline chondrules (Mg‐CCs; Δ17O ~ −2‰) commonly observed in CBs and CHs. We suggest that the Fe‐CCs and Mg‐CCs formed in the impact plume under different redox conditions (~IW−1 and ~IW−3, respectively), which may have been controlled by heterogeneous distribution of water‐bearing phases (water ice, hydrated materials) in the collided bodies and/or in the disk. We propose the following impact plume scenario for the origin of Fe‐CCs: (1) condensation of ferromagnesian silicate melt around Fe, Ni‐metal melt droplets from a highly oxidized portion of the plume; (2) crystallization of euhedral metal grains from the supercooled ferromagnesian silicate melt followed by its solidification; (3) condensation of ferroan olivine rims around solidified Fe‐CCs; (4) high‐temperature annealing of Fe‐CCs and their rims in the plume accompanied by Fe‐Mg interdiffusion between ferroan olivine rims and their host chondrules. Subsequently, some Fe‐CC experienced solid‐state reduction to various degrees, possibly in the reduced portions of gaseous plume. The impact plume‐produced or reprocessed components in CBs and CHs include Ca,Al‐poor magnesian and ferroan cryptocrystalline chondrules; Ca,Al‐rich skeletal olivine chondrules; isotopically uniform, 26Al‐poor 16O‐depleted (Δ17O ~ −15 to −5‰) igneous CAIs surrounded by igneous forsterite rims; chemically zoned and unzoned Fe,Ni‐metal grains; and metal‐sulfide nodules. These objects are dominant in CBs and abundant in CHs. The CH chondrites also contain other high‐temperature chondritic components, which avoided processing in the plume and most likely predate the plume event: 26Al‐poor and 26Al‐rich, mostly 16O‐rich CAIs (Δ17O ~ −40 to −10‰) surrounded by Wark–Lovering rims, and porphyritic chondrules (magnesian [type I], ferroan [type II], and Al‐rich) showing a range of Δ17O (from ~ −10 to ~ +5‰). Some of these components appear to have been melted in the plume. We conclude that CH and CB chondrites contain multiple generations of chondrules and refractory inclusions formed by different mechanisms at different times and different regions of the protoplanetary disk, consistent with the hypothesis of Wasson and Kallemeyn (1990).

Сlassification and characteristic of inclusions in the Severny Kolchim meteorite (H3.4)

Research subject.Fragments of the Severny Kolchim meteorite.Materials and methods.The study was performed in the Geoanalyst Centre for Collective Use, Institute of Geology and Geochemistry, UB RAS. The clast and inclusions were studied using a scanning electron microscope JSM-6390LV from JEOL with an energy dispersing attachment INCA Energy 450 X-Max 80. The bulk chondrule compositions were obtained by EDS analysis of whole chondrule areas in thin sections. The composition of minerals was studied using an electron microprobe analyser Cameca SX100 equipped with five wavelength spectrometers. The concentrations of trace elements in olivine were measured using a mass spectrometer with inductively coupled plasma NexION 300S (PerkinElmer) with a laser ablation attachment NWR 213 (ESI) at a crater diameter of 25 microns.Results.According to the revealed set of petrographic and mineralogical features, the meteorite was clarified as H3.4. In addition, this meteorite can be further classified as genomict breccia. In the Severny Kolchim meteorite, a 6×6 mm clast fragment composed of chondrite H3.9 was studied. This inclusion has a slightly higher degree of S2 shock transformations compared to the host rock. Refractory forsterite-rich objects were found and studied. These inclusions are composed of low-ferroan forsterite (f = 0.004–0.2, f – ratio Fe/(Fe + Mg)mol).Conclusion.The porphyry olivine chondrules consisting of refractory forsterite and high-calcium glass in mesostasis are likely to be parental to the refractory forsterite-rich inclusions. Al-rich chondrules and pyroxene chondrule with tridymite identified in the matrix of the chondrite are likely to be xenogenic, originating from the formation area of enstatite chondrites.

Petrology and mineralogy of the Viñales meteorite, the latest fall in Cuba.

The new Cuban chondrite, Viñales, fell on February first, 2019 at Pinar del Rio, northwest of Cuba (22°37'10″N, 83°44'34″W). A total of about 50-100 kg of the meteorite were collected and the masses of individual samples are in a range 2-1100 g. Two polished thin sections were studied by optical microscope, Raman spectroscopy and electron microprobe analysis in this study. The meteorite mainly consists of olivine (Fa24.6), low-Ca pyroxene (Fs20.5), and troilite and Fe-Ni metal, with minor amounts of feldspar (Ab82.4-84.7). Three poorly metamorphosed porphyritic olivine-pyroxene and barred olivine chondrules are observed. The homogeneous chemical compositions and petrographic textures indicate that Viñales is a L6 chondrite. The Viñales has fresh black fusion crust with layered structure, indicating it experienced a high temperature of ∼1650°C during atmospheric entry. Black shock melt veins with width of 100-600 μm are pervasive in the Viñales and olivine, bronzite, and metal phases are dominate minerals of the shock melt vein. The shock features of major silicate minerals suggest a shock stage S3, partly S4, and the shock pressure could be >10 GPa.

Oxygen isotope systematics of chondrules in the Paris CM2 chondrite: Indication for a single large formation region across snow line

In-situ oxygen three-isotope analyses of chondrules and isolated olivine grains in the Paris (CM) chondrite were conducted by secondary ion mass spectrometry (SIMS). Multiple analyses of olivine and/or pyroxene in each chondrule show indistinguishable Δ17O values, except for minor occurrences of relict olivine grains (and one low-Ca pyroxene). A mean Δ17O value of these homogeneous multiple analyses was obtained for each chondrule, which represent oxygen isotope ratios of the chondrule melt. The Δ17O values of individual chondrules range from −7‰ to −2‰ and generally increase with decreasing Mg# of olivine and pyroxene in individual chondrules. Most type I (FeO-poor) chondrules have high Mg# (∼99) and variable Δ17O values from −7.0‰ to −3.3‰. Other type I chondrules (Mg# ≤97), type II (FeO-rich) chondrules, and two isolated FeO-rich olivine grains have host Δ17O values from −3‰ to −2‰. Eight chondrules contain relict grains that are either 16O-rich or 16O-poor relative to their host chondrule and show a wide range of Δ17O values from −13‰ to 0‰.The results from chondrules in the Paris meteorite are similar to those in Murchison (CM). Collectively, the Δ17O values of chondrules in CM chondrites continuously increase from −7‰ to −2‰ with decreasing Mg# from 99 to 37. The majority of type I chondrules (Mg# >98) show Δ17O values from −6‰ to −4‰, while the majority of type II chondrules (Mg# 60–70) show Δ17O values of −2.5‰. The covariation of Δ17O versus Mg# observed among chondrules in CM chondrites may suggest that most chondrules in carbonaceous chondrites formed in a single large region across the snow line where the contribution of 16O-poor ice to chondrule precursors and dust enrichment factors varied significantly.