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Phosphate minerals in ordinary chondrites provide a record of fluids that were present during metamorphic heating of the chondrite parent asteroids. We have carried out a petrographic study of the phosphate minerals, merrillite and apatite, in metamorphosed H group ordinary chondrites of petrologic type 4-6, to understand development of phosphate minerals and associated fluid evolution during metamorphism. In unbrecciated chondrites, apatite is Cl rich and shows textural evolution from fine-grained apatite-merrillite assemblages in type 4 toward larger, uniform grains in type 6. The Cl/F ratio in apatite shows a similar degree of heterogeneity in all petrologic types, and no systematic change in compositions with metamorphic grade, which suggests that compositions in each meteorite are dictated by localized conditions, possibly because of a limited fluid/rock ratio. The development of phosphate minerals in H chondrites is similar to that of L and LL chondrites, despite the fact that feldspar equilibration resulting from albitization is complete in H4 chondrites but not in L4 or LL4 chondrites. This suggests that albitization took place during an earlier period of the metamorphic history than that recorded by preserved apatite compositions, and chemical equilibrium was not achieved throughout the H chondrite parent body or bodies during the late stages of metamorphism. A relict igneous clast in the H5 chondrite, Oro Grande has apatite rims on relict phenocrysts of (possibly) diopside that have equilibrated with the host chondrite. Apatite in the Zag H3-6 regolith breccia records a complex fluid history, which is likely related to the presence of halite in this meteorite. The porous dark H4 matrix of Zag, where halite is observed, has a high apatite/merrillite ratio, and apatite is extremely Cl rich. One light H6 clast contains similarly Cl-rich apatite. In a second light H6 clast, apatite compositions are very heterogeneous and more F-rich. Apatites in both H4 matrix and H6 clasts have very low H2O contents. Heterogeneous apatite compositions in Zag record multiple stages of regolith processing and shock at the surface of the H chondrite parent body, and apatite records either the passage of fluids of variable compositions resulting from different impact-related processes, or the passage of a single fluid whose composition evolved as it interacted with the chondrite regolith. Unraveling the history of apatite can potentially help to interpret the internal structure of chondrite parent bodies, with implications for physical and mechanical properties of chondritic asteroids. The behavior of halogens recorded by apatite is important for understanding the behavior of volatile elements in general: if impact-melt materials close to the surface of a chondritic asteroid are readily degassed, the volatile inventories of terrestrial planets could be considerably more depleted than the CI carbonaceous chondrite abundances that are commonly assumed.

This paper reports petrological data for LaPaz Icefield 02240, 03922, 031125, 031173, 031308, 04462, and 04751, which are meteoritic samples of clast‐rich impact melt rocks from the H chondrite parent asteroid. The size distribution and metallographic characteristics of Fe‐Ni metal in the melts indicate very rapid 1 to 40°C/s cooling in the temperature range between >1500 and ∼950°C when the clast‐melt mixtures were thermally equilibrating. Cooling slowed to values between 10−3 and 10−2°C/s in the temperature range between 700 and 400°C when the melt rocks were cooling to their surroundings. These data suggest that the rocks cooled near the surface of the H chondrite asteroid within suevitic impact deposits. Integrating these data with the petrologic characteristics of other H chondrite melt rocks and their radioisotopic ages indicates that the H chondrite asteroid suffered at least one large impact event while still cooling from endogenous metamorphism at ∼4500 Ma; this impact must have degraded the asteroid's integrity but did not cause shattering. Impact events in the era between ∼4100 and ∼3600 Ma produced melt volumes large enough to allow segregation of metal and troilite from silicate melts, possibly within continuous impact melt sheets contained in craters. The impact record after 3600 Ma does not display such assemblages, which suggests a decrease in the rate of large impact events or a catastrophic size reduction of the H chondrite parent asteroid at around this time.

The Moon experienced an intense period of impacts about 4 Gyr ago. This cataclysm is thought to have affected the entire inner Solar System and has been constrained by the radiometric dating of lunar samples: 40 Ar– 39 Ar ages reflect the heating and degassing of target rocks by large basin-forming impacts on the Moon. Radiometric dating of meteorites from Vesta and the H-chondrite parent body also shows numerous 40 Ar– 39 Ar ages between 3.4 and 4.1 Gyr ago, despite a different dynamical context, where impacts typically occur at velocities too low to reset geochronometers. Here we interpret the 40 Ar– 39 Ar age record in meteorites to reflect unusually high impact velocities exceeding 10 km s −1 . Compared with typical impact velocities for main-belt asteroids of about 5 km s −1 , these collisions would produce 100–1,000 times more highly heated material by volume. We propose that the 40 Ar– 39 Ar ages between 3.4 and 4.1 Gyr ago from Vesta, the H-chondrite parent body and the Moon record impacts from numerous main-belt asteroids that were driven onto high-velocity and highly eccentric orbits by the effects of the late migration of the giant planets. We suggest that the bombardment persisted for many hundreds of millions of years and affected most inner Solar System bodies. Lunar samples suggest that the inner Solar System was bombarded by asteroids about 4 Gyr ago. Radiometric ages of meteorites suggest an unusual number of high-velocity asteroids at this time, consistent with a dynamical origin of the bombardment in which the asteroids were pushed by outer planet migration onto highly eccentric orbits.

We attempt to develop a possible theory of chemical fractionations in chondrites, that is consistent with various features of chondritic components and current observation of protoplanetary disks (PPD). Combining the 3+2 component fitting calculation that simulates chondrule formation process proposed in paper (I) with additional mixing procedures, we investigate essential causes that made various types of chondrites evolve from the uniform solar system composition, the CI-chondritic composition. Seven chemical types of chondrites (CM, CV, CO, E, LL, L and H) are examined, for which reliable chemical compositions for both bulk chondrites and chondrules therein are known. High vaporization degree of the primordial dust aggregates (dustons) required by the calculation vindicates that the chondrule formation was the driving force for the chemical fractionations in all chondrites examined. Various initial redox states in dustons and different timings of CAIs’ invasion to the chondrule formation zone are identified for different chondrite types. These results, together with a good correlation with the D/H ratios of chondrites measured previously, lead us to the notion that PPD evolved from reducing to oxidizing. We explore the heating mechanism for the chondrule formation and the place it occurred. Only heat source being consistent with our chondrule formation model is lightning discharge. We postulate that large vortices encompassing the snow-line are ideal places for large charge separation to occur between dustons and small ice particles, and that direct strikes on dustons should make them boil for ten seconds and longer and allow a swarm of chondrules released from their surfaces. Chemical fractionations are completed by an aerodynamic separation of dustons from chondrules inside the vortex, in such a way that the dustons fall fast into the vortex center and form a planetesimal immediately, while chondrules with dust mantles fall slow and form a thin veneer on the planetesimal surface. During collisional episodes, the veneers are preferentially fragmented and reassemble themselves by a weak self-gravity to form a rubble-piled chondritic asteroid, i.e. chondrite.

The timescales and mechanisms for the formation and chemical differentiation of the planets can be quantified using the radioactive decay of short-lived isotopes 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 . Of these, the 182 Hf-to- 182 W decay is ideally suited for dating core formation in planetary bodies 1 , 2 , 3 , 4 , 5 . In an earlier study, the W isotope composition 1 of the Earth's mantle was used to infer that core formation was late 1 (≥60 million years after the beginning of the Solar System) and that accretion was a protracted process 11 , 12 . The correct interpretation of Hf–W data depends, however, on accurate knowledge of the initial abundance of 182 Hf in the Solar System and the W isotope composition of chondritic meteorites. Here we report Hf–W data for carbonaceous and H chondrite meteorites that lead to timescales of accretion and core formation significantly different from those calculated previously 1 , 3 , 5 , 11 , 12 . The revised ages for Vesta, Mars and Earth indicate rapid accretion, and show that the timescale for core formation decreases with decreasing size of the planet. We conclude that core formation in the terrestrial planets and the formation of the Moon must have occurred during the first ∼30 million years of the life of the Solar System.

The Jilin H5 chondrite, the largest known stony meteorite in the world, with its No.1 fragment weighing 1770 kg. It contains submillimeter- to centimeter-sized FeNi metal particles/nodules. Our optical microscopic and electron microprobe analyses revealed that the formation of metal nodules in this meteorite is a complex and long-term process, The early stage is the thermal diffusion-caused migration and concentration of dispersed metallic material along fractures to form root-hair shaped metal grains during thermal metamorphism of this meteorite. The later two collision events experienced by this meteorite led to the further migration and aggregation of metallic material into the shock-produced cracks and openings to form larger-sized metal grains. The shock-produced shear movement and frictional heating occurred in this meteorite greatly enhanced the migration and aggregation of metallic material to form the large-sized nodules. It was revealed that the metal nodule formation process in the Jilin H5 chondrite might perform in the solid or subsolidus state, and neither melting of chondritic metal grains nor shock-induced vaporization of bulk chondrite material are related with this process.

Our Solar System formed approximately 4.6 billion years ago from the collapse of a dense core inside an interstellar molecular cloud. The subsequent formation of solid bodies took place rapidly. The period of &<10 million years over which planetesimals were assembled can be investigated through the study of meteorites. Although some planetesimals differentiated and formed metallic cores like the larger terrestrial planets, the parent bodies of undifferentiated chondritic meteorites experienced comparatively mild thermal metamorphism that was insufficient to separate metal from silicate. There is debate about the nature of the heat source as well as the structure and cooling history of the parent bodies. Here we report a study of 244Pu fission-track and 40Ar-39Ar thermochronologies of unshocked H chondrites, which are presumed to have a common, single, parent body. We show that, after fast accretion, an internal heating source (most probably 26Al decay) resulted in a layered parent body that cooled relatively undisturbed: rocks in the outer shells reached lower maximum metamorphic temperatures and cooled faster than the more recrystallized and chemically equilibrated rocks from the centre, which needed approximately 160 Myr to reach 390K.

The Maryborough meteorite is a new H5 ordinary chondrite discovered about 2 km south of Maryborough, Victoria, in May 2015. It is a single stone measuring approximately 39 × 14 × 14 cm and with a mass of 17 kg. Plentiful indistinct chondrules are up to 1 mm across in a strongly recrystallised plagioclase-bearing matrix. Olivine and orthopyroxene in both the matrix and chondrules are uniform in composition (Fo80.1Fa19.3Te0.5Ca-ol0.04 and En81.5Fs17.1Wo1.5 respectively).The main metallic phases present are kamacite, taenite and tetrataenite, often forming composite grains with troilite. There is no evidence for any shock-inducing event and the meteorite shows incipient weathering in the form of thin iron-oxide mantles around the Fe–Ni grains. A terrestrial age of less than 1000 years is estimated from C14 dating. While there are a number of historic reported meteor sightings in the Maryborough district, none can be tied to the meteorite’s find site. To date, Maryborough is the third H5 ordinary chondrite and the second largest single chondritic mass, after Kulnine (55 kg), found in Victoria.

The petrography, mineral modal data and major and trace element compositions of 15 silicate inclusions in the Elga iron meteorite (chemical group IIE) show that these inclusions represent chemically homogeneous zoned objects with highly variable structures, reflecting the sequence of crystallization of a silicate melt during cooling of the metal host. The outer zones of inclusions at the interface with their metal host have a relatively medium-grained hypocrystalline texture formed mainly by Cr-diopside and merrillite crystals embedded in high-silica glass, whereas the central zones have a fine-grained hypocrystalline texture. Merrillite appears first on the liquidus in the outer zones of the silicate inclusions. Na and REE concentrations in merrillite from the outer zones of inclusions suggest that it may have crystallized as α-merrillite in the temperature range of 1300–1700°С. Merrillite tends to preferentially accumulate Eu without Sr. Therefore, strongly fractionated REE patterns are not associated with prolonged differentiation of the silicate melt source but depend on crystallization conditions of Н-chondrite droplets in a metallic matrix. The systematic decrease in Mg# with increasing Fe/Mn in bronzite may indicate partial reduction of iron during crystallization of the inclusion melt. The modal and bulk compositions of silicate inclusions in the Elga meteorite, as well as the chemical composition of phases are consistent with the model equilibrium crystallization of a melt, corresponding to 25% partial melting of H-chondrite, and the crystallizing liquidus phase, merrillite, and subsequent quenching at about 1090°С. Despite a high alkali content of the average weighted bulk inclusion composition, La/Hf and Rb/Th fall within the field of H chondrites, suggesting their common source. Our results reveal that silicate inclusions in the Elga (IIE) iron meteorite originated by mixing of two impact melts, ordinary chondrite and Ni-rich iron with а IIE composition, which were produced by impact event under near-surface conditions on a partially differentiated parent asteroid.

We present results of an image‐based numerical model aimed at quantifying the microsegregation and flow of liquid metal in meteorites prior to the onset of silicate melting. The sample material is the H6 chondrite Kernouvé. The model utilizes the observed geometry of two distinct chondrite textures associated with grain‐scale melt segregation in the following: (1) the undeformed (natural) state and (2) during deformation and partial melting under controlled (laboratory) conditions. The numerical simulations recover liquid metal segregation rates of ∼10−6 to 10−4 m s−1 for matrix permeabilities (k) of 10−12 < k < 10−10 m2 and pressure gradients of ∼1 and 104 Pa m−1. The velocity flow field is position‐dependent across the sample, reflecting initial grain‐scale heterogeneity and anisotropy in the spatial distribution of metal prior to melting. In addition to porous flow, we use a coupled Brinkman‐Navier‐Stokes solution to quantify liquid metal segregation through deformation‐induced microscale veins. Melt flow velocities in veins are several orders faster than matrix flow, implying that a combination of porous (grain‐scale) flow feeding into a network of small‐scale cracks and veins during the initial stages of partial melting may be an extremely efficient mechanism for segregating liquid metal from silicate matrix in planetesimals undergoing deformation. This mechanism may be temporary and confined only to the earliest stages of melt microsegregation because with increasing temperature, the onset of silicate melting shuts off liquid metal segregation by creeping matrix flow. The point at which this occurs marks an important transition in the mode and style of internal differentiation. Key Points Numerical simulations on natural meteorites show rapid metallic segregation Metallic liquid segregates more quickly through cracks and veins than porous flow Physical core formation model predicts rates that support isotopic studies