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Mesosiderite meteorites consist of a mixture of crustal basaltic or gabbroic material and metal. Their formation process is still debated due to their unexpected combination of crust and core materials, possibly derived from the same planetesimal parent body, and lacking an intervening mantle component. Mesosiderites have experienced an extremely slow cooling rate from ca. 550 °C, as recorded in the metal (0.25-0.5 °C/Ma). Here we present a detailed investigation of exsolution features in pyroxene from the Antarctic mesosiderite Asuka (A) 09545. Geothermobarometry calculations, lattice parameters, lamellae orientation, and the presence of clinoenstatite as the host were used in an attempt to constrain the evolution of pyroxene from 1150 to 570 °C and the formation of two generations of exsolution lamellae. After pigeonite crystallization at ca. 1150 °C, the first exsolution process generated the thick augite lamellae along (100) in the temperature interval 1000-900 °C. By further cooling, a second order of exsolution lamellae formed within augite along (001), consisting of monoclinic low-Ca pyroxene, equilibrated in the temperature range 900-800 °C. The last process, occurring in the 600-500 °C temperature range, was likely the inversion of high to low pigeonite in the host crystal, lacking evidence for nucleation of orthopyroxene. The formation of two generations of exsolution lamellae, as well as of likely metastable pigeonite, suggest non-equilibrium conditions. Cooling was sufficiently slow to allow the formation of the lamellae, their preservation, and the transition from high to low pigeonite. In addition, the preservation of such fine-grained lamellae limits long-lasting, impact reheating to a peak temperature lower than 570 °C. These features, including the presence of monoclinic low-Ca pyroxene as the host, are reported in only a few mesosiderites. This suggests a possibly different origin and thermal history from most mesosiderites and that the crystallography (i.e. space group) of low-Ca pyroxene could be used as parameter to distinguish mesosiderite populations based on their cooling history., SCOPUS: ar.j, info:eu-repo/semantics/published

Significance Niobium-92 is a short-lived p -process isotope that decays to 92 Zr with a half-life of 37 Ma. The initial 92 Nb/ 93 Nb ratio of the Solar System is crucial for utilizing the 92 Nb– 92 Zr chronometer, as it has the potential to provide information on early Solar System evolution and insights into the debated p -process nucleosynthesis. Herein, we precisely determine the initial 92 Nb/ 93 Nb ratio of the Solar System using rare minerals in meteorites. This significantly improved precision makes the 92 Nb– 92 Zr chronometer a powerful tool for providing precise ages of accretion, differentiation, and collision for asteroids and planets. Additionally, the initial ratio reveals that both type Ia supernovae and core-collapse supernovae contributed to the nucleosynthesis of the p -process isotope 92 Nb in our Solar System.

Collision and disruption processes of proto-planetary bodies in the early solar system are key to understanding the genesis of diverse types of main-belt asteroids. Mesosiderites are stony-iron meteorites that formed by mixing of howardite-eucrite-diogenite-like crust and molten core materials and provide unique insights into the catastrophic break-up of differentiated asteroids. However, the enigmatic formation process and the poorly constrained timing of metal-silicate mixing complicate the assignment to potential parent bodies. Here we report high-precision uranium-lead dating of mesosiderite zircons by isotope dilution thermal ionization mass spectrometry, revealing initial crust formation 4,558.5 +/- 2.1 million years ago and metal-silicate mixing at 4,525.39 +/- 0.85 million years. The two distinct ages coincide with the timing of crust formation and a large-scale reheating event on the eucrite parent body, likely the asteroid Vesta. This chronological coincidence corroborates that Vesta is the parent body of mesosiderite silicates. Mesosiderite formation on Vesta can be explained by a hit-and-run collision 4,525.4 million years ago that caused the thick crust observed by NASA's Dawn mission and explains the missing olivine in mesosiderites, howardite-eucrite-diogenite meteorites, and vestoids., Comment: 45 pages, 4 figures, Supplementary Information

Mesosiderites are a group of stony-iron meteorites, which are thought to be the result of mixing of silicates with Fe-Ni metal. In this study, we combined textural observations with geochemical and chronological studies of two zircon grains found in the Estherville mesosiderite. One of the zircons (Zrc1) occurs with pyroxene, plagioclase, troilite, and silica, and the other (Zrc2) is located at a boundary between Fe-Ni metal and a silicate part mainly composed of pyroxene and plagioclase. The textural observations demonstrate that Zrc1 is relatively homogenous, whereas Zrc2 is composed of at least two chemically distinct domains. Trace element analyses of Zrc2 resolve large concentration gradients within this single grain with variations that are an order of magnitude for rare earth elements (REE) and two orders of magnitude for U and Th. The lowest trace element concentration in Zrc2 is more than an order of magnitude lower than those of lunar and eucritic zircons. However, it is similar to those of Zrc1 and a zircon from the Vaca Muerta mesosiderite. The calculated REE composition of the melt in equilibrium with Zrc2 shows that Zrc2 and perhaps also Zrc1 did not crystallize from a melt that was produced by fractional crystallization of the primary magmatic mineral assemblages. The zircons with low REE, U, and Th concentrations can be interpreted to have formed in a residual melt after incorporation of large amounts of REE, U, and Th into secondary phosphate minerals, which formed during the metal-silicate mixing event. The large concentration gradients observed in Zrc2 suggest significant heterogeneities in the melt from which the zircon crystallized. Alternatively, either mixing or diffusion between a relict zircon and a newly formed zircon could explain the observed concentration gradients. However, the REE patterns of Zrc2 cannot be explained by mixing or diffusion between the two distinct generations of zircons. These considerations suggest that Zrc1 and Zrc2 formed during a high-temperature reheating event, which is probably related to the metal-silicate mixing event. The weighted average 207 Pb- 206 Pb age obtained by SIMS from both zircons is 4521 ± 26 Ma (2σ). This age is younger than that of a primary magmatic zircon from Vaca Muerta (4563 ± 15 Ma) and probably corresponds to the timing of the metal-silicate mixing event or a later impact event.

Mesosiderites are unique stony-iron meteorites, composed of eucrite-like silicates and Fe–Ni metals. Their formation, including silicate–metal mixing and metamorphisms, provide important insights into early planetary processes in the inner solar system. This report describes the first in-situ U–Pb and Hf–W dating of zircon in a mesosiderite Asuka 882023. The U–Pb (4502 ± 75 Ma) and Hf–W (4532.8 + 5.7/-10.5 Ma) ages may represent timing of the zircon formation, which is considerably younger than crustal differentiation of the parent body. This evidence, combined with earlier studies of chronology, implies that mesosiderites were reheated at 4530–4520 Ma, clearly after the silicate–metal mixing.

Petrological, mineralogical and geochemical studies were performed on five brecciated HED meteorites (ALH 76005, EET 92003, LEW 85300, LEW 87026 and GRO 95633) in order to elucidate the nature of impactors on the HED parent body. Some brecciated HED meteorites contain exotic materials such as FeNi-metal grains with low Co/Ni ratios (ALH 76005, EET 92003 and GRO 95633) and carbonaceous chondrite clasts (LEW 85300) in a clastic and/or impact melt matrix. Such exotic materials were incorporated during brecciation. Platinum group element (PGE) abundances vary significantly (CI × 0.002–0.05), but are higher than those of pristine rocks from the HED parent body. The PGE ratios for the five HED meteorites are inconsistent with each other, implying that the impactor components of each HED meteorites are different from each other. The various PGE ratios are consistent with those for metals from chondrites and iron meteorites, and carbonaceous chondrites. This study provides the evidence that IAB and IVA iron meteorites, and carbonaceous chondrites (CM, CO, CV, CK, CB and CR), ordinary chondrites (L and H) and enstatite chondrite (EL) are candidates of the impactor materials on the HED parent body. It is highly probable that significant amounts of siderophile elements were incorporated into the inner solar system objects like the HED parent body from both chondritic materials and differentiated materials like iron meteorites during heavy bombardment. The HED meteorites in this study and metals from mesosiderite have different Pd/Ir ratios, probably implying that HED meteorites and mesosiderites formed either at distinct settings on one common parent body or on similar parent bodies.

Stanfieldite, natural Ca-Mg-phosphate, is a typical constituent of phosphate-phosphide assemblages in pallasite and mesosiderite meteorites. The synthetic analogue of stanfieldite is used as a crystal matrix of luminophores and frequently encountered in phosphate bioceramics. However, the crystal structure of natural stanfieldite has never been reported in detail, and the data available so far relate to its synthetic counterpart. We herein provide the results of a study of stanfieldite from the Brahin meteorite (main group pallasite). The empirical formula of the mineral is Ca8.04Mg9.25Fe0.72Mn0.07P11.97O48. Its crystal structure has been solved and refined to R1 = 0.034. Stanfieldite from Brahin is monoclinic, C2/c, a 22.7973(4), b 9.9833(2), c 17.0522(3) &Aring, &beta, 99.954(2)&deg, V 3822.5(1)&Aring, 3. The general formula of the mineral can be expressed as Ca7M2Mg7(PO4)12 (Z = 4), where the M = Ca, Mg, Fe2+. Stanfieldite from Brahin and a majority of other meteorites correspond to a composition with an intermediate Ca&asymp, Mg occupancy of the M5A site, leading to the overall formula ~Ca7(CaMg)Mg9(PO4)12 &equiv, Ca4Mg5(PO4)6. The mineral from the Lunar sample &ldquo, rusty rock&rdquo, 66095 approaches the M = Mg end member, Ca7Mg2Mg9(PO4)12. In lieu of any supporting analytical data, there is no evidence that the phosphor base with the formula Ca3Mg3(PO4)4 does exist.

Asteroid (16) Psyche is the target of the NASA Psyche mission. It is considered one of the few main-belt bodies that could be an exposed proto-planetary metallic core and that would thus be related to iron meteorites. Such an association is however challenged by both its near- and mid-infrared spectral properties and the reported estimates of its density. Here, we aim to refine the density of (16) Psyche to set further constraints on its bulk composition and determine its potential meteoritic analog. We observed (16) Psyche with ESO VLT/SPHERE/ZIMPOL as part of our large program (ID 199.C-0074). We used the high angular resolution of these observations to refine Psyche's three-dimensional (3D) shape model and subsequently its density when combined with the most recent mass estimates. In addition, we searched for potential companions around the asteroid. We derived a bulk density of 3.99\,$\pm$\,0.26\,g$\cdot$cm$^{-3}$ for Psyche. While such density is incompatible at the 3-sigma level with any iron meteorites ($\sim$7.8\,g$\cdot$cm$^{-3}$), it appears fully consistent with that of stony-iron meteorites such as mesosiderites (density $\sim$4.25\,$\cdot$cm$^{-3}$). In addition, we found no satellite in our images and set an upper limit on the diameter of any non-detected satellite of 1460\,$\pm$\,200}\,m at 150\,km from Psyche (0.2\%\,$\times$\,R$_{Hill}$, the Hill radius) and 800\,$\pm$\,200\,m at 2,000\,km (3\%\,$\times$\,$R_{Hill}$). Considering that the visible and near-infrared spectral properties of mesosiderites are similar to those of Psyche, there is merit to a long-published initial hypothesis that Psyche could be a plausible candidate parent body for mesosiderites., 16 pages

The Dalgaranga meteorite crater, 100 km northeast of Yalgoo, Western Australia, was one of the first impact structures identified in Australia, the smallest isolated crater found in Australia, and the only confirmed crater in the world associated with a mesosiderite projectile. Seventeen years passed before the Dalgaranga meteorites were described in the scientific literature, and nearly 40 years passed before a survey of the structure was published. The reasons for the time gap were never explained and a number of factual errors about the discovery and early history remain uncorrected in the scientific literature. Using historical and archival documents, and discussions with people involved in Dalgaranga research, the reasons for this time gap are explained by a series of minor misidentifications and coincidences. The age of the crater has yet to be determined, but using published data, we estimate the projectile mass to be 500–1000 kg.

Mid-infrared (5 micron to 25 micron) transmission/absorption spectra of differentiated meteorites (achondrites) were measured to permit comparison with astronomical observations of dust in different stages of evolution of young stellar objects. In contrast to primitive chondrites, achondrites underwent heavy metamorphism and/or extensive melting and represent more advanced stages of planetesimal evolution. Spectra were obtained from primitive achondrites (acapulcoite, winonaite, ureilite, and brachinite) and differentiated achondrites (eucrite, diogenite, aubrite, and mesosiderite silicates). The ureilite and brachinite show spectra dominated by olivine features, and the diogenite and aubrite by pyroxene features. The acapulcoite, winonaite, eucrite, and mesosiderite silicates exhibit more complex spectra, reflecting their multi-phase bulk mineralogy. Mixtures of spectra of the primitive achondrites and differentiated achondrites in various proportions show good similarities to the spectra of the few Myr old protoplanetary disks HD104237A and V410 Anon 13. A spectrum of the differentiated mesosiderite silicates is similar to the spectra of the mature debris disks HD172555 and HD165014. A mixture of spectra of the primitive ureilite and brachinite is similar to the spectrum of the debris disk HD113766. The results raise the possibility that materials produced in the early stage of planetesimal differentiation occur in the protoplanetary and debris disks.

The Sm and Gd isotopic compositions of silicates from six mesosiderites (Dalgaranga, Estherville, Morristown, Northwest Africa (NWA) 1242, NWA 2932, and Vaca Muerta) and one iron meteorite (Udei Station) were determined to elucidate the cosmic-ray exposure records. All seven samples showed significant 150 Sm/ 149 Sm and 158 Gd/ 157 Gd isotopic shifts from neutron capture reactions corresponding to neutron fluences of (1.3–21.8) × 10 15 n cm −2 . In particular, Vaca Muerta showed a significantly higher neutron fluences than the other six samples. The parameter for the degree of neutron thermalization ( e Sm / e Gd ) also showed a significant difference between Vaca Muerta (0.76) and the other samples (0.93–1.20). These results suggest a two-stage irradiation of the Vaca Muerta silicates in the parent body (>50 Ma) before formation of the mesosiderite and during its transit to Earth (138 Ma). This is consistent with the 81 Kr–Kr cosmic-ray exposure age data of a Vaca Muerta pebble from a previous noble gas isotopic study.

– Noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate–metal mixture from the Vaca Muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods. Trapped noble gases were degassed completely by a high-temperature thermal event, probably at the formation of the Vaca Muerta parent body (VMPB). The presence of fissiogenic Xe isotopes from extinct 244Pu in the bulk samples might be a result of rapid cooling from an early high-temperature metamorphism. High concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic Kr and Xe. Spallogenic Ne from Na and unique Ar isotopic compositions were observed. The 81Kr-Kr exposure age of 168 ± 8 Myr for the silicate pebble is distinctly longer than the age of 139 ± 8 Myr for the bulk samples. The precursor of the pebble had been irradiated on the surface of the VMPB for more than 60 Myr (first stage irradiation), with subsequent incorporation into bulk materials approximately 4 Gyr ago. The Vaca Muerta meteorite was excavated from the VMPB 140 Myr ago (second stage irradiation). Relative diffusion rates among the cosmogenic Ar, Kr, and Xe based on data obtained by stepwise heating indicate that Kr and Xe can be partially retained in pyroxene and plagioclase under the condition that resets the K-Ar system. This result supports the presence of fission Xe and of excess concentration of cosmogenic Kr, which could have survived the thermal event approximately 3.8 Gyr ago.

We present a mineralogical assessment of near-Earth Asteroid, (1036) Ganymed, using data obtained May 18, 2006 UT combined with 24 Color Asteroid Survey data to cover the spectral interval of 0.3–2.45 μm. Results of the analysis indicate (1036) Ganymed is an S (VI) asteroid with a surface silicate assemblage consisting primarily of orthopyroxene, (Fs 23(±5) Wo 3(±3) ), consistent with calculated band centers and band area ratios (BAR). (1036) Ganymed appears to be once part of a large mesosiderite containing howardite, eucrite, and diogenite (HED) pyroxenes mixed with metal that was broken apart and dispersed. The calculated composition of the average pyroxenes in the surface material of (1036) Ganymed is consistent with mesosiderite pyroxenes, in particular the diogenites. A second possibility could be (1036) Ganymed is not yet represented in the meteorite collection. Our investigation has confirmed Ganymed is not a parent body of the ordinary chondrites and is not genetically related to (433) Eros.

Mesosiderites are thermal metamorphic breccias consisting of fragments of pyroxene-plagioclase rocks and FeNi metal. The silicate constituent of mesosiderites has a chemical and oxygen isotopic composition analogous to those of meteorites of the HED group: howardites, eucrites, and diogenites. The hypothesis currently most widely accepted for the genesis of mesosiderites is the impact mixing of the material of a differentiated asteroid and an iron meteorite. In contrast to many other classes of meteorites, mesosiderites exhibit no traces of metasomatic processes. The Budulan mesosiderite is the first meteorite of this type in which traces of metasomatism under the effect of an anhydrous fluid were detected. The metasomatic alterations are manifested as chemical zoning of olivine, aggregates of secondary minerals, and the mobilization and redeposition of iron and nickel in the form of metals and sulfides. These alterations were most probably caused by a reaction of olivine with S- and/or CO-bearing gases of endogenic or supergenic provenance. Traces of such metasomatic alterations were previously found in some meteorites and lunar rocks, and these processes could likely play a certain role in the differentiation of chondritic bodies.

We present high-precision Mg isotope data for most classes of basaltic meteorites including eucrites, mesosiderite silicate clasts, angrites and the ungrouped Northwest Africa (NWA) 2976 measured by pseudo-high-resolution multiple-collector inductively coupled plasma mass spectrometry and utilising improved techniques for chemical purification of Mg. With the exception of the angrites Angra dos Reis, Lewis Cliff (LEW) 86010, NWA 1296 and NWA 2999 and the diogenite Bilanga, which have either been shown to have young ages by other dating techniques or have low Al/Mg ratios, all bulk samples of basaltic meteorites have 26Mg excesses ( δ 26 Mg ∗ = + 0.0135 to +0.0392‰). The 26Mg excesses cannot be explained by analytical artefacts, cosmogenic effects or heterogeneity of initial 26Al/27Al, Al/Mg ratios or Mg isotopes in asteroidal parent bodies as compared to Earth or chondrites. The 26Mg excesses record asteroidal melting and formation of basaltic magmas with super-chondritic Al/Mg and confirm that radioactive decay of short-lived 26Al was the primary heat source that melted planetesimals. Model 26Al–26Mg ages for magmatism on the eucrite/mesosiderite, angrite and NWA 2976 parent bodies are 2.6–3.2, 3.9–4.1 and 3.5 Myr, respectively, after formation of calcium–aluminium-rich inclusions (CAIs). However, the validity of these model ages depends on whether the elevated Al/Mg ratios of basaltic meteorites result from magma ocean evolution on asteroids through fractional crystallisation or directly during partial melting. Mineral isochrons for the angrites Sahara (Sah) 99555 and D’Orbigny, and NWA 2976, yield ages of 5.06 - 0.05 + 0.06 Myr and 4.86 - 0.09 + 0.10 Myr , respectively, after CAI formation. Both isochrons have elevated initial δ 26 Mg ∗ values. Given the brecciated and equilibrated texture of NWA 2976 it is probable that its isochron age and elevated initial δ 26 Mg ∗ ( + 0.0175 ± 0.0034 ‰ ) reflects thermal resetting during an impact event and slow cooling on its parent body. However, in the case of the angrites the marginally elevated initial δ 26 Mg ∗ ( + 0.0068 ± 0.0058 ‰ ) may reflect either δ 26 Mg ∗ ingrowth in a magma ocean prior to eruption and crystallisation or in an older igneous protolith with super-chondritic Al/Mg prior to impact melting and crystallisation of these angrites, or partial internal re-equilibration of Mg isotopes after crystallisation. 26Al–26Mg model ages and an olivine + pyroxene + whole rock isochron for the angrites Sah 99555 and D’Orbigny are in good agreement with age constraints from 53Mn–53Cr and 182Hf–182W short-lived chronometers, suggesting that the 26Al–26Mg feldspar-controlled isochron ages for these angrites may be compromised by the partial resetting of feldspar Mg isotope systematics. Even when age constraints from the 26Al–26Mg angrite model ages or the mafic mineral + whole rock isochron are considered, the relative time difference between Sah 99555/D’Orbigny crystallisation and CAI formation cannot be reconciled with Pb–Pb ages for Sah 99555/D’Orbigny and CAIs, which are ca. 1.0 Myr too old (angrites) or too young (CAIs) for reasons that are not clear. This discrepancy might indicate that 26Al was markedly lower (ca. 40%) in the planetesimal- and planet-forming regions of the proto-planetary disc as compared to CAIs, or that CAI Pb–Pb ages may not accurately date CAI formation, which might be better dated by the 182Hf–182W and 26Al–26Mg chronometers as 4568.3 ± 0.7 (Burkhardt et al., 2008) and 4568.5 ± 0.3 Ma (herein), respectively, when mapped onto an absolute timescale using Pb–Pb ages for angrites.

GRV 020175 is an Antarctic mesosiderite, containing about 43 vol% silicates and 57 vol% metal. Metal occurs in a variety of textures from irregular large masses, to veins penetrating silicates, and to matrix fine grains. The metallic portion contains kamacite, troilite and minor taenite. Terrestrial weathering is evident as partial replacement of the metal and troilite veins by Fe oxides. Silicate phases exhibit a porphyritic texture with pyroxene, plagioclase, minor silica and rare olivine phenocrysts embedded in a fine-grained groundmass. The matrix is ophitic and consists mainly of pyroxene and plagioclase grains. Some orthopyroxene phenocrysts occur as euhedral crystals with chemical zoning from a magnesian core to a ferroan overgrowth; others are characterized by many fine inclusions of plagioclase composition. Pigeonite has almost inverted to its orthopyroxene host with augite lamellae, enclosed by more magnesian rims. Olivine occurs as subhedral crystals, surrounded by a necklace of tiny chromite grains (about 2-3 mu m). Plagioclase has a heterogeneous composition without zoning. Pyroxene geothermometry of GRV 020175 gives a peak metamorphic temperature (similar to 1000 degrees C) and a closure temperature (similar to 875 degrees C). Molar Fe/Mn ratios (19-32) of pyroxenes are consistent with mesosiderite pyroxenes (16-35) and most plagioclase compositions (An(87.5-96.6)) are within the range of mesosiderite plagioclase grains (An(88-95)). Olivine composition (Fo(53.8)) is only slightly lower than the range of olivine compositions in mesosiderites (Fo(55-90)). All petrographic characteristics and chemical compositions of GRV 020175 are consistent with those of mesosiderite and based on its matrix texture and relatively abundant plagioclase, it can be further classified as a type 3A mesosiderite. Mineralogical, petrological, and geochemical studies of GRV 020175 imply a complex formation history starting as rapid crystallization from a magma in a lava flow on the surface or as a shallow intrusion. Following primary igneous crystallization, the silicate underwent varying degrees of reheating. It was reheated to 1000 degrees C, followed by rapid cooling to 875 degrees C. Subsequently, metal mixed with silicate, during or after which, reduction of silicates occurred; the reducing agent is likely to have been sulfur. After redox reaction, the sample underwent thermal metamorphism, which produced the corona on the olivine, rims on the inverted pigeonite phenocrysts and overgrowths on the orthopyroxene phenocrysts, and homogenized matrix pyroxenes. Nevertheless, metamorphism was not extensive enough to completely reequilibrate the GRV 020175 materials.

The Dong Ujimqin Qi mesosiderite is the first recorded fall of a stony-iron meteorite in China. According to silicate textures and metal composition, this meteorite is classified as a member of subgroup IB. Instrumental neutron activation analyses (INAA) of metals show that the matrix metal has lower concentrations of Os, Ir, Re, and Pt, but higher concentrations of Ni and Au than the 7.5 cm metal nodule present in the meteorite. We attribute these compositional differences to fractional crystallization of molten metal. Studies of olivine clasts show that FeO contents are uniform in individual olivine crystals but are variable for different olivine clasts. Although concentrations of rare earth elements (REEs) change within olivine clasts, they all exhibit a vee-shaped pattern relative to CI chondrites. The relatively high concentrations of REEs in olivine and the shape of REE patterns require a liquid high in REEs and especially in light REEs. As such a liquid was absent from the region where basaltic and gabbroic clasts formed, mesosiderite olivine must have formed in a part of the differentiated asteroid that is different from the location where other mesosiderite silicate clasts formed.

Evidence from iron meteorites indicates that a large number of differentiated planetesimals formed early in Solar System history. These bodies should have had well-developed olivine-rich mantles and consequentially such materials ought to be abundant both as asteroids and meteorites, which they are not. To investigate this "Great Dunite Shortage" we have undertaken a geochemical and oxygen isotope study of main-group pallasites and dunitic rocks from mesosiderites.Oxygen isotope analysis of 24 main-group pallasites (103 replicates) yielded a mean Delta17O value of -0.187 ± 0.0160/00 (2sigma), which is fully resolved from the HED Delta17O value of -0.246 ± 0.014 (2sigma) obtained in our earlier study and demonstrates that both groups represent distinct populations and were derived from separate parent bodies. Our results show no evidence for Delta17O bimodality within the main-group pallasites, as suggested by a number of previous studies.Olivine-rich materials from the Vaca Muerta, Mount Padbury and Lamont mesosiderites, and from two related dunites (NWA 2968 and NWA 3329), have Delta17O values within error of the mesosiderite average. This indicates that these olivine-rich materials are co-genetic with other mesosiderite clasts and are not fragments from an isotopically distinct pallasite-like impactor. Despite its extreme lithologic diversity the mesosiderite parent body was essentially homogeneous with respect to Delta17O, a feature best explained by an early phase of large-scale melting (magma ocean), followed by prolonged igneous differentiation.Based on the results of magma ocean modeling studies, we infer that Mg-rich olivines in mesosiderites formed as cumulates in high-level chambers and do not represent samples of the underlying mantle. By analogy, recently documented Mg-rich olivines in howardites may have a similar origin.Although the Dawn mission did not detect mesosiderite-like material on Vesta, evidence linking the mesosiderites and HEDs includes: (i) their nearly identical oxygen isotope compositions; (ii) the presence in both of coarse-grained Mg-rich olivines; (iii) both have synchronous Lu-Hf and Mn-Cr ages; (iv) there are compositional similarities between the metal in both; and (v) mesosiderite-like material has been identified in a howardite breccia. The source of the mesosiderites remains an outstanding question in meteorite science.The underrepresentation of olivine-rich materials amongst both asteroids and meteorites results from a range of factors. However, evidence from pallasites and mesosiderites indicates that the most important reason for this olivine shortage lies in the early, catastrophic destruction of planetesimals in the terrestrial planet-forming region and the subsequent preferential loss of their olivine-rich mantles. Evidence from iron meteorites indicates that a large number of differentiated planetesimals formed early in Solar System history. These bodies should have had well-developed olivine-rich mantles andconsequentially such materials ought to be abundant both as asteroids and meteorites, which they are not. To investigate this "Great DuniteShortage" we have undertaken a geochemical and oxygen isotope study of main-group pallasites and dunitic rocks from mesosiderites.Oxygen isotope analysis of 24 main-group pallasites (103 replicates) yielded a mean Δ17O value of -0.187 ±0.016‰ (2σ), which is fully resolved from the HED Δ17O value of -0.246 ± 0.014 (2σ) obtained in our earlier study and demonstrates that both groups represent distinct populations and were derived from separate parent bodies. Our results show no evidence for Δ17O bimodality within themain-group pallasites, as suggested by a number of previous studies.Olivine-rich materials from the Vaca Muerta, Mount Padbury and Lamont mesosiderites, and from two related dunites (NWA 2968 and NWA 3329), have Δ17O values within error of the mesosiderite average. This indicates that these olivine-rich materials are co-genetic with other mesosiderite clasts and are not fragments from anisotopically distinct pallasite-like impactor. Despite its extreme lithologic diversity the mesosiderite parent body was essentially homogeneous with respect to Δ17O, a feature best explained by an early phase of large-scale melting (magma ocean), followed by prolonged igneous differentiation. Based on the results of magma ocean modeling studies, we infer that Mg-rich olivines in mesosiderites formed as cumulates in high-levelchambers and do not represent samples of the underlying mantle. By analogy, recently documented Mg-rich olivines in howardites may have a similar origin. Although the Dawn mission did not detect mesosiderite-like material on Vesta, evidence linking the mesosiderites and HEDs includes: (i) theirnearly identical oxygen isotope compositions; (ii) the presence in both of coarse-grained Mg-rich olivines; (iii) both have synchronous Lu-Hf and Mn-Cr ages; (iv) there are compositional similarities between the metal in both; and (v) mesosiderite-like material has been identified in a howardite breccia. The source of the mesosiderites remains an outstanding question in meteorite science. The underrepresentation of olivine-rich materials amongst both asteroidsand meteorites results from a range of factors. However, evidence from pallasites and mesosiderites indicates that the most important reason for this olivine shortage lies in the early, catastrophic destruction ofplanetesimals in the terrestrial planet-forming region and the subsequent preferential loss of their olivine-rich mantles.

Mesosiderites are breccias consisting of a mixture of metal, chemically similar to IIIAB iron meteorites, and silicates, which resemble HED achondrites [1] [2]. From microscale metallographic textures and geochemistry, the cooling of the metal fraction from about 550°C appears to be uniquely slow (cooling rate of 0.25-0.5°C/My [3]) despite some debate about systematic error in these determinations [4]. On the other hand, the silicate are believed to have cooled quite quickly (e.g., [5]). The mesosiderite A 09545 [6] contains large grains of pyroxene exhibiting two generations of exsolution lamellae, which have been here investigated with electron microscopy (SEM, TEM, and EPMA) and used for constraining the thermal evolution of the silicate fraction in the sample. The host pyroxene is clino-enstatite (Monoclinic, P21/c) with composition Wo3En59Fs38 and contains thick (up to 25 µm) lamellae of augite (Monoclinic, C2/c, Wo42En41Fs17) along cleavage. A second generation of exsolution lamellae occurs within the augite lamellae, with thin (ca. 300 nm) lamellae, apparently parallel to [001], and composition Wo1En56Fs43. Geothermobarometry calculation [7] [8], lattice parameters [9], lamellae orientation [10], and the inversion of ortho- to clinopyroxene [11] have suggested a slow cooling rate of the pyroxene from the crystallization of pigeonite (ca. 1150°C) to the inversion into clinoenstatite (ca. 570°C), followed by the slow cooling recorded in the metal (e.g., [3]).

Petrological and bulk geochemical studies were performed on a large silicate clast from the Mount Padbury mesosiderite. The silicate clast is composed mainly of pyroxene and plagioclase with minor amounts of ilmenite, spinel, and other accessory minerals, and it shows subophitic texture. Pyroxenes in the clast are similar to those in type 5 eucrites and could have experienced prolonged thermal metamorphism after rapid crystallization from a near-surface melt. Ilmenite and spinel vary chemically, indicating growth under disequilibrium conditions. The clast seems to have experienced an episode of rapid reheating and cooling, possibly as a result of metal-silicate mixing. Abundances of siderophile elements are obviously higher than in eucrites, although the clast is also extremely depleted in highly siderophile elements. The fractionated pattern can be explained by injection of Fe- FeS melts generated by partial melting of metallic portions during metal-silicate mixing. The silicate clast had a complex petrogenesis that could have included: 1) rapid crystallization from magma in a lava flow or a shallow intrusion; 2) prolonged thermal metamorphism to equilibrate the mineral compositions of pyroxene and plagioclase after primary crystallization; 3) metal-silicate mixing probably caused by the impact of solid metal bodies on the surface of the mesosiderite parent body; and 4) partial melting of metal and sulfide portions (and silicate in some cases) caused by the collisional heating, which produced Fe-FeS melts with highly fractionated siderophile elements that were injected into silicate portions along cracks and fractures.

We studied the texture, mineralogy, and bulk chemical composition of Dhofar 007, a basaltic achondrite. Dhofar 007 is a polymict breccia that is mostly composed of coarse-grained granular (CG) clasts with a minor amount of xenolithic components, such as a fragment of Mg-rich pyroxene. The coarse-grained, relict gabbroic texture, mineral chemistry, and bulk chemical data of the coarse-grained clast indicate that the CG clasts were originally a cumulate rock crystallized in a crust of the parent body. However, in contrast to monomict eucrites, the siderophile elements are highly enriched and could have been introduced by impact events. Dhofar 007 appears to have experienced a two-stage postcrystallization thermal history: rapid cooling at high temperatures and slow cooling at lower temperatures. The presence of pigeonite with closely spaced, fine augite lamellae suggests that this rock was cooled rapidly from higher temperatures (>0.5 °C/yr at ~1000 °C) than typical cumulate eucrites. However, the presence of the cloudy zone in taenite and the Ni profile across the kamacite-taenite boundaries indicates that the cooling rate was very slow at lower temperatures (~1-10 °C/Myr at

Constraining the timescales for the assembly and differentiation of planetary bodies in our young solar system is essential for a complete understanding of planet-forming processes. This is best achieved through the study of the daughter products of extinct radionuclides with short half-lives, as they provide unsurpassed time resolution as compared to long-lived chronometers. Here we report high-precision Mg isotope measurements of bulk samples of basalt, gabbro, and pyroxenite meteorites obtained by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). All samples from the eucrite and mesosiderite parent bodies (EPB and MPB) with suprachondritic Al/Mg ratios have resolvable 26Mg excesses compared to matrix-matched samples from the Earth, the Moon, Mars, and chondrites. Basaltic magmatism on the EPB and MPB thus occurred during the life span of the now-extinct 26Al nuclide. Initial 26Al/27Al values range from (1.26 ± 0.37) × 10-6 to (5.12 ± 0.81) × 10-6 at the time of magmatism on the EPB and MPB, and are among the highest 26Al abundances reported for igneous meteorites. These results indicate that widespread silicate melting and differentiation of rocky bodies occurred within 3 million years of solar system formation, when 26Al and 60Fe were extant enough to induce planetesimal melting. Finally, thermal modeling constrains the accretion of these differentiated asteroids to within 1 million years of solar system formation, that is, prior to the accretion of chondrite parent bodies.

I have done a detailed petrologic study of Ibitira, a meteorite that has been classified as a basaltic eucrite since 1957. The mean Fe/Mn ratio of pyroxenes in Ibitira with

The global composition of the early solar system is thought to be roughly chondritic in terms of refractory components, and this means that metal and silicate should be present together in early planetesimals. To fully understand the metal-silicate differentiation process within the eucrite parent body (EPB), it is important to try and identify the metal reservoir that is complementary to the silicate part. The isotope 182 of tungsten (W), a siderophile element, is partly formed from the decay of 182 Hf, and W isotopes are useful for examining metal-silicate segregation. The W isotopic composition expected for the metal that is complementary to eucrites falls in the range of iron meteorites. However, mesosiderites seem to be genetically linked to eucrites based on petrologic and oxygen isotopic similarities. Therefore, we undertook the analysis of the metal phase of these stony-irons. Here we present tungsten isotopic data for mesosiderite and pallasite metal to characterize their parent body (bodies) and to assess possible relationships with eucrites. All stony-iron metals are depleted in radiogenic tungsten by −1.3 to −4.2 e units, relative to the terrestrial standard, while chondrites, for comparison, are depleted by −1.9 e units. In addition to W isotopic heterogeneity from one stony-iron to another, there is also W isotopic heterogeneity within individual meteorites. A formation model is tentatively proposed, where we show that mesosiderites, pallasites, and eucrites could possibly come from the same parent body. Several hypotheses are discussed to explain the isotopic heterogeneity: the production of cosmogenic tungsten, the in situ decay of hafnium present in inclusions, and tungsten diffusion processes after metal-silicate mixing during the cooling of the meteorites. The two latter hypotheses provide the best explanation of our data.

We report here the results of a study of trace element microdistributions and 53Mn-53Cr systematics in several basaltic and orthopyroxenitic clasts from the Vaca Muerta mesosiderite. Ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that, following igneous crystallization, these clasts underwent extensive metamorphic equilibration that resulted in intra- and inter-grain redistribution of elements. There is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents (such as phosphorus, derived from the mesosiderite metal) at the time of metal-silicate mixing. Furthermore, our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal-silicate mixing event. After taking into account the effects of these postmagmatic secondary processes, comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites, respectively, suggests that the primary igneous petrogenesis, including parent magma and source compositions, of Vaca Muerta silicates were similar to those of achondritic meteorites of the Howardite-Eucrite-Diogenite (HED) association. Internal 53Mn-53Cr isochrons obtained for two basaltic (pebble 16 and 4679) and two orthopyroxenitic (4659 and 4670) clasts show that chromium isotopes are equilibrated within each clast. Nevertheless, just as for noncumulate eucrites and diogenites, 53Cr excesses in whole-rock samples of the basaltic clasts (∼1.01 e in pebble 16; ∼1.07 e in 4679) are significantly higher than in the orthopyroxene-rich clasts (∼0.62 e in 4659; ∼0.53 e in 4670). As in the case of the HED parent body, this suggests that Mn/Cr fractionation in the parent body of the Vaca Muerta silicate clasts occurred very early in the history of the solar system, when 53Mn was still extant. However, the slope of the 53Mn-53Cr isochron defined by the whole-rock samples of Vaca Muerta clasts (corresponding to a 53Mn/55Mn ratio of 3.3 ± 0.6 × 10−6) is distinctly lower than that defined by the HED whole-rock samples (corresponding to a 53Mn/55Mn ratio of 4.7 ± 0.5 × 10−6), indicating that the global Mn/Cr fractionation event that established mantle source reservoirs on the parent body of the Vaca Muerta silicate clasts occurred ∼2 Ma after a similar event on the HED parent body.

A possible mesosiderite meteorite was found in the area of the Putorana Plateau, Noril'sk district, Siberia, Russia. Although this rock resembles a mesosiderite in its hand-sample aspect and in having Ni-bearing iron metal, it is not a meteorite. This inference is based on the lack of a fusion crust, the lack of cosmogenic nuclides, oxygen with terrestrial isotope ratios, and several mineral chemical criteria. Most likely, the rock is from the iron-metal-bearing basalts of the Siberian Trap basalt sequence, which are mined for their base and platinum-group metals. Mesosiderite imposters like this may be recognized by: (1) the presence of Cu metal in hand sample or as microscopic blebs in the low-Ni metal (kamacite), (2) the absence of high-Ni metal (taenite), and (3) the presence of iron carbide (cohenite) enclosing the kamacite. Even if these macroscopic tests are inconclusive, isotopic and mineral chemical tests will also distinguish rocks like this from mesosiderites.

— Meteoritical Bulletin No. 86 lists information for 1154 newly classified meteorites, comprising 661 from Antarctica, 218 from Africa, 207 from Asia (203 of which are from Oman), 62 from North America, 3 from South America, and 3 from Europe. Information is provided for 5 falls (El Idrissia, Undulung, Dashoguz, El Tigre, and Yafa). Noteworthy specimens include 7 martian meteorites (Dhofar 378, Grove Mountains 99027, Northwest Africa 856, 1068, and 1110, and Sayh al Uhaymir 060 and 090); 4 lunar meteorites (Dhofar 301, 302, 303, and 489); 9 new iron meteorites; a mesosiderite (Northwest Africa 1242); an ungrouped stony-iron meteorite (Dar al Gani 962); and a wide variety of other interesting stony meteorites, including CH, CK, CM, CR, CV, R, enstatite, unequilibrated ordinary, and ungrouped chondrites, primitive achondrites, howardite-eucrite-diogenite (HED) achondrites, and ureilites.

The importance of oceanic impacts of collisional events and resulting energy release, are briefly described. Data collection methods from the Eltanin (a mesosiderite projectile) deep water impact, are presented.

A total of 1.7g of unmelted meteorite particles have been recovered from FS Polarstern piston cores collected on expedition ANT XII/4 that contain ejecta from the Eltanin impact event. Most of the mass (1.2 g) is a large, single specimen that is a polymict breccia, similar in mineralogy and chemistry to howardites or the silicate fraction of mesosiderites. Most of the remaining mass is in several large individual pieces (20-75mg each) that are polymict breccias, fragments dominated by pyroxene, and an igneous rock fragment. The latter has highly fractionated REE, similar to those reported in mafic clasts from mesosiderites. Other types of specimens identified include fragments dominated by maskelynite or olivine. These pieces of the projectile probably survived impact by being blown off the back surface of the Eltanin asteroid during its impact into the Bellingshausen Sea.

— We propose that mesosiderites formed when a 200–400 km diameter asteroid with a molten core was disrupted by a 50–150 km diameter projectile. To test whether impacts can excavate core iron and mix it with crustal material, we used a low-resolution, smoothed-particle hydrodynamics computer simulation. For 50–300 km diameter differentiated targets, we found that significant proportions of scrambled core material (and hence potential mesosiderite metal material) could be generated. For near-catastrophic impacts that reduce the target to 80% of its original diameter and about half of its original mass, the proportion of scrambled core material would be about 5 vol%, equivalent to ∼10 vol% of mesosiderite-like material. The paucity of olivine in mesosiderites and the lack of metal-poor or troilite-rich meteorites from the mesosiderite body probably reflect biased sampling. Mesosiderites may be olivine-poor because mantle material was preferentially excluded from the metal-rich regions of the reaccreted body. Molten metal globules probably crystallized around small, cool fragments of crust hindering migration of metal to the core. If mantle fragments were much hotter and larger than crustal fragments, little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally. The rapid cooling rates of mesosiderites above 850 °C can be attributed to local thermal equilibration between hot and cold ejecta. Very slow cooling below 400 °C probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris. We infer that our model is more plausible than an earlier model that invoked an impact at ∼1 km/s to mix projectile metal with target silicates. If large impacts cannot effectively strip mantles from asteroidal cores, as we infer, we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material. This may help to explain why relatively few olivine-rich (A-type) and metal-rich asteroids (M-type) are known. Some S-type asteroids may be scrambled differentiated bodies.

We measured the concentrations of the cosmogenic radionuclides 14C (half-life = 5.73 × 103 years) in the bulk and of 10Be (1.5 × 106 years), 26Al (7.05 × 105 years), 36Cl (3.01 × 105 years) and the light noble gases in metal and stone fractions of the Chinguetti meteorite to investigate the controversial claim that the 4.5 kg mesosiderite is part of a much larger mass in the Mauritanian desert. Based on the 36Cl-36Ar, 10Be-21Ne and 26Al-21Ne pairs in the metal fraction, we derive an average cosmic-ray exposure age of 66 ± 7 million years (Ma). Chinguetti is now the third out of 20 mesosiderites with an exposure age between 60 and 70 Ma. This may be the first hint of a major impact on the parent body of the mesosiderites, which show ages ranging from 10–300 Ma (Terribilini et al., 2000). From the 14C-10Be pair we derive a terrestrial age of 18 ± 1 ka, which seems too recent to be consistent with the original description of the main mass having a heavily wind eroded base, overhung by the upper part of the meteorite. Finally, from the radionuclide concentrations in combination with Monte Carlo based calculations, we conclude that our sample of Chinguetti was irradiated at a depth of ~15 cm in an object not larger than 80 cm in radius. This is the most compelling evidence against the reports that the Chinguetti mesosiderite is a small fragment of a mass 100 m long and 40 m high.

— We reviewed here the recent development on the mathematical formulation of closure temperature of a cooling geochronological system, which permits direct retrieval of cooling rate from cooling age when the diffusion parameters, grain size and initial temperature are known. This formulation is used to show how the cooling rate can be retrieved by comparing the core and bulk age of a mineral determined by a single decay system. The cooling rates of seven H chondrites of the metamorphic types H4, H5 and H6 were retrieved from the available data on the Pb-Pb model ages of the phosphates and the diffusion kinetic data of Pb in apatite. The results are in excellent agreement with the metallographic cooling rates and show an inverse relation with the metamorphic grade of these chondrites. We also addressed the problem of ∼90 Ma younger Sm-Nd mineral isochron age, defined by orthopyroxene, phosphate and plagioclase, of the Morristown mesosiderite compared to the Pb-Pb age of the Estherville mesosiderite. It is shown that this younger age could have been a consequence of resetting during cooling instead of an “impulsive heating” event, as suggested earlier.

We report measurements of 26A1, loBe, 41Ca, and 36Cl in the silicate and metal phases of 11 mesosiderites, including several specimens each of Budulan and Estherville, of the brecciated meteorite Bencubbin, and of the iron meteorite Udei Station. Average production rate ratios (atodatom) for metal phase samples from Estherville and Budulan are 26AVlOBe = 0.77 f 0.02; 36CYlOBe = 5.3 f 0.2. For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find 26AVloBe = 0.72 f 0.01 and 36CVlOBe = 4.5 f 0.2. The average 41Ca/36Cl production rate ratio is 1.10 f 0.04 for metal separates from Estherville and four small iron falls. The 41Ca activities in dpm/(kg Ca) of various silicate separates from Budulan and Estherville span nearly a factor of 4, from 1600, indicating preatmospheric radii of >30 cm. After allowance for composition, the activities of 26A1 and loge (dpmkg silicate) are similar to values measured in most ordinary chondrites and appear to depend only weakly on bulk Fe content. Unless shielding effects are larger than suggested by the 36Cl and 41Ca activities of the metal phases, matrix effects are unimportant for loge and minor for 26A1. Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of 36C1/36Ar exposure ages for the metal phases agree well with published values. Neon-2 1 production rates for mesosiderite silicates calculated from these ages and from measured 2lNe contents are consistently higher than predicted for L chondrites despite the fact that the mesosiderite silicates have lower Mg contents than L chondrites. We suggest that the elevation of the 2lNe production rate in mesosiderite silicates reflects a "matrix effect," that is, the influence of the higher Fe content of mesosiderites, which acts to enhance the flux of low-energy secondary particles and hence the 2lNe production from Mg. As lOBe production is relatively insensitive to this matrix effect, 10BePlNe ages give erroneously low production rates and high exposure ages. By coincidence, standard 22NePlNe based "shielding" corrections give fairly reliable 2lNe production rates in the mesosiderite silicates.

Two main types of material survive from the Canyon Diablo impactor, which produced Meteor Crater in Arizona: iron meteorites, which did not melt during the impact; and spheroids, which did. Ultrasensitive measurements using accelerator mass spectrometry show that the meteorites contain about seven times as much nickel-59 as the spheroids. Lower average nickel-59 contents in the spheroids indicate that they typically came from 0.5 to 1 meter deeper in the impactor than did the meteorites. Numerical modeling for an impact velocity of 20 kilometers per second shows that a shell 1.5 to 2 meters thick, corresponding to 16 percent of the projectile volume, remained solid on the rear surface; that most of the projectile melted; and that little, if any, vaporized.

The microstructure of Fe-Ni metallic phases in Dong Ujimqin mesosiderite was studied using the field emission SEM. Taenite is characterized by a zoned structure, consisting of outer clear taenite and inner cloudy zone (CZ). CZ has a typical “island-honeycomb” microstructure. The average size of the island phase is about 358 nm, suggesting a cooling rate of ∼ 0.5°C/Ma at low temperature (< 400°C). The Ni concentration profiles across kamacite and zoned taenite were also measured by electron probe microscope analysis (EPMA). Formation of the Fe-Ni metallic phases, microstructure in Dong Ujimqin mesosiderite was discussed based on the new low-temperature Fe-Ni phase diagram.

Re-Os data on pallasite metal yield a sufficient range in Re/Os to permit the determination of a whole rock isochron, if pallasites from different groups are considered together. The results are indistinguishable from the data obtained on Groups IAB, IIAB, IID, IIIAB, IVA, and IVB iron meteorites. Pallasite data were obtained on samples of the Main Group, which have been considered related to Group IIIAB irons, and on samples of the distinctive pallasites Eagle Station, Finmarken, and Marjalahti. It appears that iron meteorites and pallasites crystallized within a time interval of 20 Ma. This result is consistent with the evidence from the short-lived chronometers ^(107)Pd-^(107)Ag and ^(53)Mn-^(53)Cr. It is also consistent with the evidence from ^(182)Hf-^(182)W that the time interval over which molten FeNi metal from different iron meteorites equilibrated with and became segregated from silicates was ∼10 Ma. The well-defined ^(187)Re-^(187)Os whole-rock isochron for pallasites and iron meteorites requires that they were formed in parent bodies that had been partially molten and segregated FeNi metal and FeS and then all crystallized within a 20 Ma time interval. This conclusion appears to apply to members of distinct groups of iron meteorites and pallasites which are viewed as not being cogenetic. The short time-scale requires that the irons and pallasites were formed in small parent planets of ≲10 km radius or near the surface of larger bodies. In contrast to the pallasites, we find that FeNi samples from mesosiderites have a narrow range in Re/Os so that it is not possible to determine a whole-rock isochron for these samples. The mesosiderite data also lie somewhat displaced from the iron meteorite and pallasite isochron indicating a more complex multi-stage evolution. Based on Sm-Nd data on mesosiderite silicate clasts, the Re-Os results on mesosiderite metal are compatible with the model of melted or partially molten FeNi cores or pods still preserved in protoplanetary bodies when these bodies were disrupted up to 150 Ma after early formation and the FeNi was splashed onto the surfaces of other small, differentiated planetesimals.

— Meteoritical Bulletin No. 82 lists information for 974 new meteorites, including 521 finds from Antarctica, 401 finds from the Sahara, 21 finds from the Nullarbor region of Australia, and 7 falls (Ban Rong Du, Burnwell, Fermo, Jalanash, Juancheng, Monahans (1998), and Silao). Many rare types of meteorites are reported: counting pairing groups as one, these include one CR chondrite, two CK chondrites, two CO chondrites, four CV chondrites, one CH chondrite or Bencubbin-like, six C2 (unclassified) chondrites, two EH chondrites, two EL chondrites, three R chondrites, thirty unequilibrated ordinary chondrites, one un-grouped chondrite, three eucrites, six howardites, one diogenite, eleven ureilites, nine iron meteorites, one mesosiderite, two brachinites, one lodranite, one winonaite, and two lunar meteorites (Dar al Gani 400 and EET 96008). All italicized abbreviations refer to addresses tabulated at the end of this document.

Determinations of 39Ar-40Ar ages for plagioclase separates of three clasts from the Vaca Muerta and Mount Padbury mesosiderites give similar results. Ages increase monotonically by ∼0.2–0.3 Ga when plotted against increasing 39Ar release from the samples and give average values of ∼3.95 Ga. When combined with revised ages for mesosiderite analyses previously reported, nineteen analyses of thirteen mesosiderites give an average Ar-Ar age of 3.94 ± 0.10 Ga. The average age at 90% 39Ar release is 4.13 ±0.14 Ga, and the ages at 10% 39Ar release range over 3.32–3.97 Ga. It seems unlikely that the 39Ar-40Ar ages were produced by direct impact heating, but they probably are independent evidence for slow cooling deep within the mesosiderite parent body. We conclude that the sloped age plateaus and the significant range in Ar-Ar closure temperatures calculated for these mesosiderites (∼75–340°C) are due to significant differences in Ar diffusion properties among samples. From the Ar diffusion data, we estimate an approximate mesosiderite cooling rate of ∼0.2°C/Ma, which is similar to the preferred cooling rate of 0.1–0.5°C/Ma derived by others from Ni diffusion and metal textures. The 39Ar/40Ar ages seem incompatible with very slow cooling rates of

— We report induced thermoluminescence (TL) data for separates from three howardite, eucrite and diogenite (HED) meteorites and the Vaca Muerta mesosiderite. The results of thermal modeling of the surface of their parent body are also described. The TL sensitivities for matrix samples from the LEW 85300, 302 and 303 paired eucrites and the Bholghati howardite are lower than the TL sensitivities for the clasts, which is consistent with regolith working of the matrix in fairly mature regoliths. Within an isochemical series of HED meteorites, TL sensitivity reflects metamorphic intensity, but clast-to-clast variations in the TL sensitivities of the Vaca Muerta mesosiderite and clasts in the EET 87509, 513 and 531 paired howardite primarily reflect differences in mineralogy and petrology. Thermoluminescence peak temperatures indicate that all the components from the LEW 85300, 302 and 303 paired eucrites experienced a reheating event involving temperatures >800 °C, which is thought to have been due to impact heating, and therefore that the event was concurrent with or postdated brecciation. The Vaca Muerta clasts are essentially unmetamorphosed, but the induced TL data indicate that the remaining howardite, eucrite, dioenite and mesosiderite (HEDM) meteorites experienced metamorphism to a variety of intensities but involving temperatures 800 °C cause a change in TL peak temperature. Feldspars from a variety of terrestrial and extraterrestrial sources show this behavior, and x-ray diffraction and kinetic studies suggest that it is indirectly related to Al, Si disordering. Cooling rates are not consistent with autometamorphism following the initial igneous event or with heating by subsequent eruptions of lava onto the surface of the HED parent body. Instead, our thermal models suggest that the metamorphism occurred within a regolith ejecta blanket of up to a few kilometers thick, with different levels of metamorphism corresponding to different thicknesses of blanket, between essentially 0 and ∼2 km, rather than different burial depths in a regolith of uniform thickness. We argue that metamorphism occurred 3.9 Ga ago and was associated with the resetting of the Ar-Ar system for the HED meteorites.

We present a report on the status of ^(107)Pd (τ = 9.4 × 10^6 y) in the early solar system and the implications of its presence for protoplanet evolution. Over the last two decades we have carried out an extensive search for the evidence of presently extinct ^(107)Pd in meteorites. From these results we conclude that: 1) ^(107)Ag^* (excess ^(107)Ag) is present in a wide variety of iron and stony-iron meteorites; 2) ^(107)Ag^* is due to the in situ decay of ^(107)Pd in these meteorites; 3) Pd-Ag metal - FeS and metal whole-rock isochrons have been established for a few meteorites; 4) the correlation observed for the total metal isochrons reflects a large variation in normal silver contents (∼10^3) with much small variations in Pd contents (factors of 10); 5) there is a wide range in apparent ^(107)Pd/^(108)Pd ratios but many samples show a narrow range (^(107)Pd/^(108)Pd = 1.5 − 2.5 × 10^(−5)); and 6) there are clear cases of complex or pathologic behavior relating to some sulfides and their associated metal phases. The ^(107)Pd-^(107)Ag chronometer reflects the times of major chemical fractionation of Pd and Ag. The scenarios we have postulated to explain the Pd/Ag fractionation include two basically different fractionation processes, namely nebular and planetary. Major fractionation between Pd/Ag can only be achieved during condensation, early accretion, and metal segregation in the solar nebula. The reaction of FeNi with H_2S gas to form FeS and subsequent melting and segregation of FeNi and FeS provides a mechanism for minor (x2) fractionation. For some volatile-depleted meteorites, the processes of condensation of FeNi with isolation from later condensates effectively fractionated Pd/Ag and produced metal with a ^(108)Pd/^(109)Ag ∼ 10^4 – 10^5. For other meteorites not so extensively depleted in volatiles, a smaller degree of effective Pd/Ag fractionation is produced due to the presence of later condensates which, upon planetary melting, produced metal with a ^(108)Pd/^(109)Ag ∼ 50–100. Some fractionation of Pd and Ag occurred by metal-liquid, metal-crystal fractionation during crystallization in planets similar to the models of Scott [1972], Larimer and Anders [1967], and Wai and Wasson [1977]. If the variation in initial ^(107)Pd/^(108)Pd among meteorites indicates a time difference (ΔT) in the condensation and melting-segregation of planetesimals, the data indicate a total range of ∼12 my for many meteorites. This tight cluster includes samples of the IIAB, IIIAB, IVA, IVB, and “anomalous” groups, as well as mesosiderites and pallasites. However, some meteorites exhibit no evidence of ^(107)Pd. A comparison of ^(107)Pd and ^(53)Mn (τ = 5.3 × 10^6 y) chronometers on the same meteorites is possible for a few samples. In one case, ^(107)Pd/^(108)Pd and ^(53)Mn/^(55)Mn from Cape York (group IIIA) show values of ∼2×10^(−5). In other cases, group IIIAB irons show ^(107)Pd/^(108)Pd ∼2×10^(−5), but a much smaller ^(53)Mn/^(55)Mn ∼1×10^(−6). In addition, ^(107)Pd is absent in the metal of the Eagle Station pallasite, but has ^(53)Mn/^(55)Mn = 2.3×10^(−6) in the Eagle Station silicates. There appear to be major discrepancies between the ^(107)Pd and ^(53)Mn chronometers. The ^(53)Mn-^(53)Cr system may be affected by the formation or equilibration of the microscopic phases containing Mn and Cr and the FeNi “host” during extended cooling and exsolution reaction.