CV3 Meteorite Research Articles

Search for extinct aluminum‐26 and titanium‐44 in nanodiamonds from the Allende CV3 and Murchison CM2 meteorites

Abstract– We report Mg‐Al and Ca‐Ti isotopic data for meteoritic nanodiamonds separated from the Allende CV3 and Murchison CM2 meteorites. The goal of this study was to search for excesses in 26Mg and 44Ca, which can be attributed to the in situ decay of radioactive and now extinct 26Al and 44Ti, respectively. Previous work on presolar SiC and graphite had shown that 26Al/27Al and 44Ti/48Ti ratios in presolar grains can be used to discriminate between different types of stellar sources. Aluminum and Ti concentrations are low in the meteoritic nanodiamonds of this study. Murchison nanodiamonds have higher Al and Ti concentrations than the Allende nanodiamonds. This can be attributed to contamination and the presence of presolar SiC in the Murchison nanodiamond samples. 26Mg/24Mg and 44Ca/40Ca ratios are close to normal in Allende nanodiamonds with upper limits on the initial 26Al/27Al and 44Ti/48Ti ratios of approximately 1 × 10−3. These ratios are factors of 10–1000 and, respectively, 1–1000 lower than those of presolar SiC and graphite grains from supernovae. The 26Al/27Al and 44Ti/48Ti data for nanodiamonds are compatible with an asymptotic giant branch star or solar system origin, but not with a supernova origin of a major fraction of meteoritic nanodiamonds. The latter possibility cannot be excluded, though, as the diamond separates may contain significant amounts of contaminating Al and Ti, which would lower the inferred 26Al/27Al and 44Ti/48Ti ratios considerably.

Near-infrared spectroscopy of 3:1 Kirkwood Gap Asteroids: (3760) Poutanen and (974) Lioba

A mineralogical assessment of 3:1 Kirkwood Gap Asteroids (3760) Poutanen and (974) Lioba, has been carried out from spectral data obtained May 21, 2008 (974) and May 25, 2009 (3760) UT using the NASA Infrared Telescope Facility. Results indicate (3760) Poutanen as a spinel-bearing asteroid. The spinel contained within this asteroid appears to be embedded in calcium aluminum inclusions ( CAIs) much like that of the CV3 chondrite Allende. The preservation of the CAIs implies a lack of igneous processing and places further constraints on the heating that occurred within the early Solar System. This analysis suggests (3760) Poutanen originated in the general formation zone of the CV3 meteorites, but in a sub-region depleted in iron-rich matrix material and enriched in pyroxene-bearing clasts. Analysis of (974) Lioba indicates it is an S(IV) asteroid with a surface silicate assemblage consisting of olivine and orthopyroxene, (Fs 21(±5)Wo 8(±3)), consistent with measured band centers, (0.96 ± 0.01 μm; 1.95 ± 0.02 μm), and the band area ratio (0.59 ± 0.15). The location, delivery efficiency ((24.5%) Farinella, P., Gunczi, R., Froeschlé, Ch., Froeschlé, C. [1993]. Icarus 101, 174–187), calculated mineralogy, and BAR strongly suggest (974) Lioba is a plausible parent body for the LL- or L-ordinary chondrites.

The effect of pyridine treatment on phase Q: Orgueil and Allende

Abstract– Marrocchi et al. (2005) reported that low‐temperature fractions of heavy noble gases were largely removed upon pyridine treatment of the Orgueil CI meteorite. As pyridine is known to induce the swelling of the macromolecular network of organic matter, they concluded that the low‐temperature phase Q is macromolecular organic carbon. However, Busemann et al. (2008) showed that pyridine had no significant effect on the noble gas contents for other very primitive meteorites, such as CM and CR. Therefore, we prepared an HF–HCl residue and the pyridine‐treated residue of Orgueil, and re‐examined the results of Marrocchi et al. (2005) by analyzing all noble gases. We confirmed that heavy noble gases are surely removed by the pyridine treatment, but the degree of the loss of heavy noble gases is generally small, and is even smaller for the lighter noble gases. Furthermore, we could not observe the evidence of Xe isotopic ratios by removing only phase Q after the pyridine treatment. We further prepared the HF–HCl residue and the pyridine‐treated residue of the Allende CV3 meteorite and performed noble gas analyses. For Allende, there is no significant change in the elemental abundances after the pyridine treatment. These results suggest that only Orgueil is special, and it is likely that the gas loss of the Orgueil residue is due to the loss of some kind of organic matter that was formed and that adsorbed the fractionated Q and HL gases during the aqueous alteration within the parent body of Orgueil.

Elemental and isotopic abundances of lithium in chondrule constituents in the Allende meteorite

Elemental and isotopic abundances of lithium in chondrule constituents in the Allende CV3 meteorite were determined using secondary ion mass spectrometry. Olivines and mesostasis dominated by a feldspathic phase are depleted in Li ( < 1 μ g g - 1 and 0.1 – 0.6 μ g g - 1 , respectively). In contrast, low-Ca pyroxenes and mesostasis dominated by a Na-rich phase are enriched in Li ( ∼ 1 – 8 μ g g - 1 and 0.4 – 3.5 μ g g - 1 , respectively) and the interchondrule matrix is generally enriched in Li ( ∼ 2.0 μ g g - 1 on average). The Li isotopic abundance of olivine ranges from δ 7 Li ∼ - 32 to 21. The spatial distributions of elemental and isotopic abundances of Li in olivines within individual chondrules exhibit no systematic pattern. This suggests that the distribution of Li in olivine was not disturbed during aqueous alteration or thermal metamorphism on the Allende meteorite parent body. Although mesostasis is the last crystallizing phase from a chondrule melt and is expected to be enriched in Li, in the Allende meteorite it is generally depleted in Li. We suggest that during aqueous alteration on the CV asteroid, Li in mesostasis was leached out by aqueous fluids. The Li-enriched Na-rich mesostasis was probably produced later by infiltration of Na-rich fluids. It seems likely that aqueous fluids sequestered alkali elements from the Allende–chondrite region in the CV parent asteroid, although significant amounts of Li are preserved in ferrous olivine in the interchondrule matrix.

Magnetic classification of stony meteorites: 2. Non‐ordinary chondrites

Abstract— A database of magnetic susceptibility (χ) measurements on different non‐ordinary chondrites (C, E, R, and ungrouped) populations is presented and compared to our previous similar work on ordinary chondrites. It provides an exhaustive study of the amount of iron‐nickel magnetic phases (essentially metal and magnetite) in these meteorites. In contrast with all the other classes, CM and CV show a wide range of magnetic mineral content, with a two orders of magnitude variation of χ. Whether this is due to primary parent body differences, metamorphism or alteration, remains unclear. C3–4 and C2 yield similar χ values to the ones shown by CK and CM, respectively. By order of increasing χ, the classes with well‐grouped χ are: R &lt;&lt; CO &lt; CK ≈ CI &lt; Kak &lt; CR &lt; E ≈ CH &lt; CB. Based on magnetism, EH and EL classes have indistinguishable metal content. Outliers that we suggest may need to have their classifications reconsidered are Acfer 202 (CO), Elephant Moraine (EET) 96026 (C4–5), Meteorite Hills (MET) 01149, and Northwest Africa (NWA) 521 (CK), Asuka (A)‐88198, LaPaz Icefield (LAP) 031156, and Sahara 98248 (R). χ values can also be used to define affinities of ungrouped chondrites, and propose pairing, particularly in the case of CM and CV meteorites.

Multi-wavelength observations of Asteroid 2100 Ra-Shalom

We observed near-Earth asteroid (NEA) 2100 Ra-Shalom over a six-year period, obtaining rotationally resolved spectra in the visible, near-infrared, thermal-infrared, and radar wavelengths. We find that Ra-Shalom has an effective diameter of D eff = 2.3 ± 0.2 km , rotation period P = 19.793 ± 0.001 h , visual albedo p v = 0.13 ± 0.03 , radar albedo σ ˆ OC = 0.36 ± 0.10 , and polarization ratio μ c = 0.25 ± 0.04 . We used our radar observations to generate a three-dimensional shape model which shows several structural features of interest. Based on our thermal observations, Ra-Shalom has a high thermal inertia of ∼10 3 J m −2 s −0.5 K −1, consistent with a coarse or rocky surface and the inferences of others [Harris, A.W., Davies, J.K., Green, S.F., 1998. Icarus 135, 441–450; Delbo, M., Harris, A.W., Binzel, R.P., Pravec, P., Davies, J.K., 2003. Icarus 166, 116–130]. Our spectral data indicate that Ra-Shalom is a K-class asteroid and we find excellent agreement between our spectra and laboratory spectra of the CV3 meteorite Grosnaja. Our spectra show rotation-dependent variations consistent with global variations in grain size. Our radar observations show rotation-dependent variations in radar albedo consistent with global variations in the thickness of a relatively thin regolith.

The White Angel: A unique wollastonite‐bearing, mass‐fractionated refractory inclusion from the Leoville CV3 carbonaceous chondrite

Abstract— We report the study of an unusual compact type A refractory inclusion, named the White Angel, from the Leoville CV3 meteorite. The petrologic, mineral chemical, isotopic, and trace‐element signatures of this once‐molten Ca‐Al‐rich inclusion (CAI), which contains large, equant wollastonite crystals, indicate a short multistage history that occurred very early, before substantial decay of 26Al. Magnesium in the inclusion is isotopically heavy, with FMg reaching 18‰/amu, in the range of fractionated and with unidentified nuclear effects (FUN) inclusions. However, the absence of any nuclear anomalies in Ca and Ti and an inferred 26Al/27Al ratio of (5.5 ± 0.9) × 10−5 indicate that the White Angel belongs to the F inclusions. Silicon and oxygen are also mass fractionated in favor of the heavy isotopes, but to a lesser extent. The O isotopes show a range in 16O excesses. On an O three‐isotope plot, data points lie on a line parallel and to the right of the carbonaceous chondrite anhydrous mineral mixing line, with wollastonite being the most 16O‐rich phase. The chondrite‐normalized rare earth and trace‐element pattern of the whole inclusion is the complement of an ultrarefractory pattern indicating that precursor phases of the CAI must have condensed in an Al‐, heavy rare earth element (HREE)‐depleted reservoir. Melting of those precursor phases in an 16O‐rich environment and evaporation led to mass‐dependent isotopic fractionation of Mg, Si, and O. Partial isotopic exchange with a reservoir containing unfractionated Mg took place at a later stage but before any measurable decay of 26Al. Some minerals (melilite and perovskite) in the White Angel equilibrated oxygen isotopes with a relatively 16O‐poor reservoir that was also mass‐fractionated toward the heavy isotopes, different from that with which the normal or FUN inclusions interacted.

Supra-Canonical 26 Al/ 27 Al and the Residence Time of CAIs in the Solar Protoplanetary Disk

The canonical initial 26Al/27Al ratio of 4.5 x 10(-5) has been a fiducial marker for the beginning of the solar system. Laser ablation and whole-rock multiple-collector inductively coupled plasma-source mass spectrometry magnesium isotope analyses of calcium- and aluminum-rich inclusions (CAIs) from CV3 meteorites demonstrate that some CAIs had initial 26Al/27Al values at least 25% greater than canonical and that the canonical initial 26Al/27Al cannot mark the beginning of solar system formation. Using rates of Mg diffusion in minerals, we find that the canonical initial 26Al/27Al is instead the culmination of thousands of brief high-temperature events incurred by CAIs during a 10(5)-year residence time in the solar protoplanetary disk.

Trapped Xe and I-Xe ages in aqueously altered CV3 meteorites

Twenty-two dark inclusions (DIs) from Allende (18), Leoville (2), Vigarano (1) and Efremovka (1) were studied by the I-Xe method. All except two of these DIs (Vigarano 2226 and Leoville LV2) produce well-defined isochrons, and precise I-Xe ages. The Allende DIs formed a tight group about 1.6 Ma older than Shallowater (4.566 ± 0.002 Ga), about 5 Ma older than four previously studied Allende CAIs. Most of the dark inclusions require trapped Xe with less 129Xe (or more 128Xe) than conventional planetary Xe (well restricted in composition by Q-Xe or OC-Xe). Studies of an irradiated/unirradiated DI pair from Allende demonstrate that the 128Xe/ 132Xe ratio in trapped is normal planetary, so that a 129Xe/ 132Xe ratio below planetary seems to be required. Yet, this is not possible given constraints on 129Xe evolution in the early solar system. Trends among all of the Allende DIs suggest that an intimate mixture of partially decayed iodine and Xe formed a pseudo trapped Xe component enriched in both 129Xe and 127I, and subsequently in 128Xe after n-capture during reactor irradiation. Enrichment in radiogenic 129Xe, but with a 129Xe/ 127I ratio less than that observed in the iodine host phase, places closure of this trapped mixture ≥13 Ma after precipitation of the major iodine-bearing phase. Because the I-Xe isochron is a mixing line between iodine-derived and trapped Xe (pseudo or not), I-Xe ages, given by the slope of this mixing line, are not compromised by the presence of pseudo trapped Xe, and the precision of the I-Xe ages is given by the statistics of the line fit.

Nitrogen microanalysis of glass inclusions in chondritic olivines by nuclear reaction

Measured were the abundance and distribution of nitrogen in glasses of glass inclusions in olivines of CV3, CO3, CR, C4, CH3, and LL chondritic meteorites by means of the 14N(d, p) 15N nuclear reaction. Similar to what was observed with carbon, nitrogen is present in low concentrations (<20 ppm) in the structure of olivines but can by stored in variable amounts in glasses of glass inclusions. These primitive glasses, characterized by a Si-Al-Ca-rich composition, have highly variable nitrogen contents (30 to 1500 ppm) and highly inhomogeneous nitrogen distribution. Nitrogen contents are independent of the chemical composition of the glasses. The heterogeneous distribution is a common feature of all studied inclusions, as is evidenced by the variable contents of nitrogen in glass inclusions occurring in the same olivine grain. Nitrogen heterogeneity is suggestive of trapping of solid nitrogen carrier phases during formation of the constituents of chondrules. However, part of the originally trapped nitrogen appears to have been lost, possibly, by ulterior oxidation and subsequent transformation into volatile species.

Extinct Radioactivities and Protosolar Cosmic Rays: Self‐Shielding and Light Elements

We study the effects of self-shielding in the X-wind model of protosolar cosmic-ray irradiation of early solar-system rocks. We adopt a two-component picture of protoCAIs consisting of cores with the elemental abundances of type B1 CAIs (calcium-aluminum-rich inclusions) and mantles of less refractory material. The cores have a power-law distribution of sizes between Rmin and Rmax. The mantles have a uniform thickness, whose value is chosen to bring the total inventory of elements at least as refractory as sulfur to cosmic abundances for the entire population of protoCAIs. Each object is irradiated with a fluence consistent with the product of their residence time in the reconnection ring and the flux of solar cosmic rays obtained by a scaling of impulsive flares from the hard X-rays observed from low-mass protostars. For Rmin in the 50 μm regime and Rmax in the few centimeter regime, which corresponds to the range of sizes of observed CAIs in micrometeorites and chondrites, we recover approximately the canonical values quoted for the ratios 26Al/27Al, 53Mn/55Mn, and 41Ca/40Ca in CV3 meteorites. Moreover, the excess 138La (denoted as 138La*) produced by proton bombardment of 138Ba lies within the CAI range obtained in the experiments of Shen et al. When we include fragmentation reactions that produce 10Be from the impact of protons, alphas, and 3He on the 16O that is bound up in rocks, we further obtain a level of 10Be/9Be that agrees approximately with the report of McKeegan et al. for a CAI from the Allende meteorite. Similar calculations for the expected anomalies in the stable isotopes of lithium show rough consistency with the measured values and further support our interpretation. The value for 10Be/9Be is particularly difficult to produce by any other astrophysical mechanism. Thus, the 10Be discovery greatly strengthens the case for an origin in early solar-system irradiation, rather than external stellar seeding, for the shortest-lived radionuclides inferred from CAIs in chondritic meteorites.

Carbon in glass inclusions of Allende, Vigarano, Bali, and Kaba (CV3) olivines

Carbon contents and distribution have been measured in glass inclusions in olivines of CV3 carbonaceous chondrites by using the 12C(d,p) 13C nuclear reaction. All olivines from the four studied meteorites had low carbon contents (<70 ppm). Conversely, glasses of glass inclusions in the same olivines had highly variable carbon contents, all above 100 ppm. Glass inclusions in olivines from meteorites of the oxidized group (Allende, Kaba, and Bali) had carbon contents that varied from 100 to about 2000 ppm, whereas those from the reduced group member Vigarano were surprisingly poor in C (averaging 300 ppm). These relative abundances of carbon in these glasses of reduced and oxidized CV3 meteorites are in contrast to the abundances of interstellar SiC in these meteorites. This indicates that glass inclusions in olivines could have behaved, with respect to carbon, as closed systems that have escaped elemental exchange processes. The carbon content of the glasses is, therefore, likely to be primary and reflect the physico– chemical conditions during the formation of the host olivine and glass inclusions. The redox conditions prevailing during secondary processing of the olivines (e.g., metasomatic Fe–Mg exchange, Ca–Na exchange) appear not to have influenced the carbon distribution. Carbon could have been trapped initially as a refractory carbon species (e.g., carbide) by clear glass inclusions. Despite the fact that the nature of neither the primary nor that of the secondary C species was established, the commonly heterogeneous distribution of C in glass inclusions in olivines suggests entrapment of a solid C-bearing precursor.

Carbon in olivines of chondritic meteorites

Carbon concentrations have been measured, using the 12C(d,p) 13C nuclear reaction, in olivines from different chondritic meteorites: CV3 (Allende and Ningqiang), CH (Acfer214), CM (Mighei and Murchison), LL3.1 (Bishunpur) and from a dark inclusion of the Allende CV3 meteorite. Their chemical compositions vary from Mg-rich (primitive olivines grown under reducing conditions) to FeO-rich (olivines that have been affected by elemental exchange reactions in a more oxygen-rich environment), with variable contents of MnO and Cr 2O 3. The ways these olivines occur in the chondrites (e.g., as isolated olivines or olivines in aggregates or in a barred chondrule) have also been considered as they can indicate different processes involved in their genesis. Nevertheless, no correlation can be established between the chemical composition of olivines, their possible genesis and their carbon contents. Contrary to previous measurements of carbon contents in chondrules, Nuclear Reaction Analyses (NRA) show that olivines have carbon contents of less than 120 ppm. Notwithstanding the different physico-chemical conditions under which olivines could have been formed or been altered in meteorites (from highly reducing to oxidising conditions) carbon does not enter the structure of olivine. This widespread feature appears as an intrinsic characteristic of olivines regardless of their origin, either on Earth or in the Solar Nebula.

Resemblance and difference between the constitution of the Moon and Io

The Moon and Io are comparable in the mean density, the mass and moment of inertia in spite of the principle differences in bulk compositions and masses of central planets. The Moon is the only extraterrestrial body for which we have information about the mantle seismic velocities as a function of depth. Volcanic activity on the Moon was completed in the early stage of its evolution. Io is the most active of all known satellites and it is much richer in volatiles and iron than the Moon. In this work, internally consistent models of the constitution of the Moon and Io based on the geophysical (the seismic velocities, the mass and moment of inertia) and geochemical (chemical and phase composition) constraints are compared. Thermodynamic modeling of the phase relations and physical properties in the CaO–FeO–MgO–Al 2O 3–SiO 2–Fe–FeS system was used to estimate the bulk composition, the density distribution in the mantle and radius of a core from the geophysical data. The solution of the inverse problem is based on the minimization of the deviations between the calculated and observed geophysical parameters. The smallest and largest core radii are estimated to be: 500–655 km for an Fe core (6–10%) and 810–1080 km for an FeS core (12–28% of total mass) for Io; 290–345 km for an Fe core (1.1–1.9%) and 450–530 km for an FeS core (2.4–4% of total mass) for the Moon. The atomic ratios of total iron to silicon for Io are estimated to be (Fe/Si)=0.40–0.66 in comparison with 0.22–0.24 for the Moon, 0.51–0.59 for the LL and L chondrites, and 0.74–0.8 for CV3 and CM2 meteorites. The current results show that the proposed Io’s compositions similar to that of carbonaceous chondrites do not satisfy to bulk Fe/Si ratios inferred from the geophysical constraints. The bulk composition of Io is estimated to be close to that of the L and LL chondrites having 8–13 wt.% of iron and iron sulfide; a change of the core composition from pure iron to pure iron sulfide leads to a change of the core radius from 600 to 625 km (8–9%) to 930–965 km (18–20%); the radius of the eutectic Fe–FeS-core is estimated to be 840–870 km (14–16% of Io’s mass; (Fe/Si) at=0.55–0.57). If Io does have such a composition, then the core is probably Fe or Fe-rich, whereas a large FeS core is excluded by the composition of the L and LL chondrites. A comparison of estimated bulk chemistry of Io with much more reliable composition of the Moon shows that Earth’s and Jupiter’s satellites have absolutely different bulk compositions.

K, Mg, Ti and Ca isotopic compositions and refractory trace element abundances in hibonites from CM and CV meteorites: implications for early solar system processes

Potassium and magnesium isotopic compositions of hibonites from the Murchison (CM) and Allende (CV) meteorites are determined by an ion microprobe to look for possible presence of the short-lived nuclides 41Ca and 26Al at the time of their formation. Abundance anomalies in the neutron-rich isotopes 50Ti and 48Ca as well as REE and additional refractory trace element abundances have also been determined to infer a plausible formation environment of the hibonites in the early solar system. The results obtained in this study suggest a widespread distribution of the short-lived nuclide 41Ca in the early solar system. Observation of correlated presence of the two short-lived nuclides 41Ca and 26Al on a microscopic scale in refractory hibonites and silicates from CM and CV meteorites suggests these nuclides to be cogenetic and support a stellar source for these and several other short-lived nuclides in the early solar system. Moderate to high enrichments in the neutron-rich isotopes 50Ti and 48Ca are seen in some of the analyzed hibonites. Our data for radiogenic and stable isotopic anomalies as well as refractory trace element abundance patterns in isolated CM hibonites with platelet morphology show trends similar to those reported earlier. They are devoid of 41Ca and 26Al, show large enrichment in 50Ti and 48Ca and have typical Group III REE patterns. However, hibonites present within hibonite-spinel inclusions show variations in their REE patterns. One of them shows the normally expected Group II REE pattern, while another has an ultrarefractory REE pattern and a third has affinities towards platelet hibonites that appear to suggest a link between these two groups of hibonites with distinct morphological characteristics. We propose a new model to explain the absence of the short-lived nuclides 41Ca and 26Al in some of the hibonites, whose REE abundance patterns and stable isotopic anomalies suggest that they are some of the first solar system solids, assuming a stellar origin for these nuclides. We attribute this absence to the very early formation of these hibonites near the central region of the collapsing protosolar cloud prior to the arrival of the short-lived nuclides injected into the cloud from a stellar source.

Melting temperatures of the Allende meteorite: implications for a Hadean magma ocean

Melting temperatures of the silicate fraction of the Allende CV3 meteorite, at upper mantle pressures, are several hundred degrees lower than that of fertile peridotite xenoliths or ‘pyrolite’. If the Earth accreted from material similar to chondrites, then deep mantle melting could have occurred with a relatively modest heat budget. It is concluded that initial chemical composition is an important variable in realistic magma ocean models.

Models for the fractionation of moderately volatile elements in the solar nebula

Abstract— We test the validity of the idea that the abundances of moderately volatile elements in chondritic meteorites reflect global condensation and coagulation in an evolving solar nebula and explore what constraints these abundances might place on nebular parameters. The abundances of moderately volatile elements were identified as particularly suitable for modeling the nebula because the data represent a simple pattern explicable in terms of a straightforward hypothesis which has implications for global evolution. The models incorporate a correspondingly simple prescription, derived from theory, for the rate at which condensible material is decoupled from the evolving nebular gas to become part of the surviving planetary system. It is concluded from model results that cooling, diminishing nebular mass and coagulation of solids, through their mutual dependences on accretion rate and opacity, lead naturally to the chondritic depletion patterns of moderately volatile elements. In particular, the trends found in CO and CV meteorites are readily and accurately produced; those of the CM meteorites are less so. The CM meteorites and ordinary chondrite patterns may require a more complicated model, but the full range of possible results from even the simplest model has yet to be completely determined.

Aqueous alteration in the matrix of the Vigarano (CV3) carbonaceous chondrite

Abstract The matrix of Vigarano, a meteorite which belongs to the reduced subgroup of the CV3 chondrites, contains small amounts (&lt;10%) of ferrihydrite and smectite. These hydrous minerals occur together as fine fibrous intergrowths between anhydrous silicate and oxide grains. Coarser crystals of ferrihydrite fill fractures that cut matrix minerals, and smectite also lines narrow channels within olivine grains. These channels may have formed by preferential alteration of olivines along (100)‐parallel defects.Formation of ferrihydrite and smectite in the matrix of Vigarano was the result of mild aqueous alteration in a low‐temperature (&lt;150 °C), oxidising parent body environment. Partial equilibration of matrix olivines indicates that alteration was followed by thermal metamorphism with a peak temperature of 400–500 °C. Mineralogically similar alteration products, which also were formed by parent body processes, have previously been described from the matrices of four CV meteorites: Bali, Grosnaja, Kaba and Mokoia, all of which belong to the oxidised subgroup. This discovery of the products of oxidative aqueous alteration in Vigarano has important consequences for understanding the chemical and thermal history of the CV class of meteorites.

Boron cosmochemistry. Part II: Boron nucleosynthesis and condensation temperature

Abstract— The new B solar‐system abundance calculated by Zhai and Shaw (1994), 16.9 atoms/106 Si (or 606 atoms/1012 H) is used to reevaluate the different possibilities of LiBeB (except 7Li) nucleosynthesis. The revised abundances support two models: (1) Light elements were formed by continual bombardment of interstellar medium (ISM) by galactic cosmic rays (GCRs), but these galactic cosmic rays should contain a very intense low‐energy component, in the form of E−5 which cannot be observed near the Earth due to solar modulation effects; (2) Light elements are a mixture of two sources. In the first source, light elements were synthesized by continual bombardment of interstellar medium by galactic cosmic rays. In the second source, they were made by the interactions of C and O nuclei ejected from supernovae with the H and He in the surrounding gas. The first source constitutes ∼46% of total B.The Si‐normalized and CI‐meteorite‐normalized abundances of common and volatile elements in carbonaceous chondrites show a linear correlation with their condensation temperatures. Using this relationship and the normalized B abundances in CM, CO, and CV meteorites, we can estimate the B condensation temperature to be ∼910 K, which is similar to Ga.

Pressure‐temperature phase diagram for the Allende meteorite

Piston cylinder and multianvil experiments from 1 to 27 GPa have been performed on the Allende CV3 meteorite to establish a pressure‐temperature phase diagram that includes major phase boundaries and the silicate‐oxide‐sulfide melting intervals. Olivine is the liquidus phase up to ∼14 GPa, followed by garnet up to ∼25 GPa. Near 26 GPa a cotectic exists where garnet and magnesiowüstite are liquidus phases. Magnesiowüstite is likely to be a lower mantle liquidus phase in both chondritic and peridotitic (see also Zhang and Herzberg, 1994) compositions. Hence element partitioning tests that neglect the role of liquidus magnesiowüstite may be incomplete for describing planetary differentiation at pressures &gt;25 GPa. Allende shows immiscibility between (Fe,Ni)‐sulfide melt and FeO‐rich silicate melt. (Fe,Ni)‐sulfide is the lower temperature melt phase and is present at all experimental pressures and temperatures investigated. It is concluded that a terrestrial planet with a radius of ∼3000 km (maximum internal pressure of ∼30 GPa), and a bulk composition of carbonaceous chondrite, will upon magmatic differentiation form an FeO‐rich silicate mantle with an Fe‐Ni‐S core. The silicate fraction of Allende in our high‐pressure experiments is too rich in FeO to be a good match for the composition of peridotite xenoliths from Earth's upper mantle. However, the major elements of a peridotite upper mantle may be derived from an Allende‐like bulk Earth by a combination of lower mantle magnesiowüstite, perovskite, and sulfide fractionation and by upper mantle olivine flotation.