Murchison CM2 Research Articles

Structural clues to the origin of refractory metal alloys as condensates of the solar nebula

Abstract– Alloys of the refractory metals Re, Os, W, Ir, Ru, Mo, Pt, and Rh with small amounts of Fe and Ni are predicted to be one of the very first high‐temperature condensates in a cooling gas of solar composition. Recently, such alloy grains were found in acid‐resistant residues of the Murchison CM2 chondrite. We used focused ion beam (FIB) preparation to obtain electron‐transparent sections of 15 submicrometer‐sized refractory metal nuggets (RMNs) from the original Murchison residue. We studied their crystallography, microstructures, and internal compositional variations using transmission electron microscopy (TEM). Our results show that all RMNs studied have hexagonal close‐packed (hcp) crystal structures despite considerable variations of their bulk compositions. Crystallographic superstructures or signs of spinodal decomposition are absent and defect microstructures are scarce. Internally, RMNs are compositionally homogeneous, with no evidence for zoning patterns or heterogeneities due to exsolution. Many RMNs show well‐defined euhedral crystal shapes and all are nearly perfect single crystal. Our findings are consistent with a direct (near‐) equilibrium condensation of refractory metals into a single alloy at high temperature in the solar nebula as predicted by current condensation models. We suggest that this alloy is generally hcp structured due to an extended ε‐phase field in the relevant multicomponent alloy system. The high degree of structural perfection and compositional homogeneity is attributed to high defect energies, high formation temperatures, slow cooling rates, small grain sizes, and rapid internal diffusion.

Delivery of dark material to Vesta via carbonaceous chondritic impacts

NASA’s Dawn spacecraft observations of Asteroid (4) Vesta reveal a surface with the highest albedo and color variation of any asteroid we have observed so far. Terrains rich in low albedo dark material (DM) have been identified using Dawn Framing Camera (FC) 0.75μm filter images in several geologic settings: associated with impact craters (in the ejecta blanket material and/or on the crater walls and rims); as flow-like deposits or rays commonly associated with topographic highs; and as dark spots (likely secondary impacts) nearby impact craters. This DM could be a relic of ancient volcanic activity or exogenic in origin. We report that the majority of the spectra of DM are similar to carbonaceous chondrite meteorites mixed with materials indigenous to Vesta. Using high-resolution seven color images we compared DM color properties (albedo, band depth) with laboratory measurements of possible analog materials. Band depth and albedo of DM are identical to those of carbonaceous chondrite xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance in DM (1–6vol.%) is consistent with howardite meteorites. We find no evidence for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of DM. Our modeling efforts using impact crater scaling laws and numerical models of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta during the formation of the ∼400km Veneneia basin by a low-velocity (<2km/s) carbonaceous impactor. This discovery is important because it strengthens the long-held idea that primitive bodies are the source of carbon and probably volatiles in the early Solar System.

SULFUR MOLECULE CHEMISTRY IN SUPERNOVA EJECTA RECORDED BY SILICON CARBIDE STARDUST

We studied about 3400 presolar silicon carbide (SiC) grains from the Murchison CM2 meteorite for C- and Si-isotopic compositions. Among these grains we identified 7 unusual or type C SiC (U/C) grains, characterized by isotopically heavy Si, and 36 supernova type X SiC grains, characterized by isotopically light Si. Selected U/C and X grains were also measured for S-, Mg-Al-, and Ca-Ti-isotopic compositions. We show that the U/C grains incorporated radioactive 44Ti, which is evidence that they formed in the ejecta of Type II supernova (SNII) explosions. Abundances of radioactive 26Al and 44Ti are compatible with those observed in X grains. U/C and X grains carry light S with enrichments in 32S of up to a factor of 2.7. The combination of heavy Si and light S observed in U/C grains is not consistent with abundance predictions of simple supernova models. The isotope data suggest preferential trapping of S from the innermost supernova zones, the production site of radioactive 44Ti, by the growing silicon carbide particles. A way to achieve this is by sulfur molecule chemistry in the still unmixed ejecta. This confirms model predictions of molecule formation in SNII ejecta and shows that sulfur molecule chemistry operates in the harsh and hot environments of stellar explosions.

Murchisite, Cr5S6, a new mineral from the Murchison meteorite

Murchisite (IMA 2010-003), Cr_5S_6, is a new chromium sulfide mineral, discovered in the Murchison CM2 meteorite. The type material occurs as one subhedral crystal (1.3 × 4 μm in size) in contact with low-Ni iron (“kamacite”), martensitic iron, schreibersite, and a Ca-,Al-rich glass, all of which are included in an isolated forsteritic olivine grain in the meteorite’s matrix. The mean chemical composition determined by electron microprobe analysis of the type material is (wt%) Cr 53.32, S 42.87, V 1.44, Fe 1.14, P 0.10, Ni 0.10, sum 98.97. The empirical formula calculated on the basis of 6 S atoms is (Cr_(4.60)V_(0.13)Fe_(0.09)Ni_(0.01))_(∑4.83)(S_(6.00)P_(0.01))_(∑6.01). Murchisite was also identified in another isolated olivine grain from the same meteorite. These crystals are subhedral to round in shape, 300 nm to 1 μm in size, and occur in association with tochilinite and serpentine, within which it is included, chromite, and eskolaite. Its electron backscatter diffraction patterns are an excellent match to that of synthetic Cr_5S_6 with the P31c structure, showing a = 5.982, c = 11.509 A, V = 356.67 A^3, and Z = 2, based on previously published data from synthetic material. Murchisite is named for the locality (the Murchison meteorite). It is a low-temperature phase (~327 °C in the Cr-S system), probably formed from higher temperature Cr_(1−x)S exsolved or expelled from a Cr-,S-bearing, metal-rich spherule included in forsteritic olivine grains that were probably derived from chondrule fragments. The formation of this phase reflects sulfur fugacities intermediate between those that would have led to troilite and those that would have led to no sulfide at all. The high-temperature Cr_(1−x)S phase equilibrated with coexisting alloys to ~600 °C. Murchisite formed from this precursor at low temperatures (<~300 °C) through ordering of S vacancies.

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.

HINTS FOR NEUTRINO-PROCESS BORON IN PRESOLAR SILICON CARBIDE GRAINS FROM SUPERNOVAE

We have studied more than 1000 presolar silicon carbide (SiC) grains from the Murchison CM2 chondrite for Cand Si-isotopic compositions. Twelve SiC X grains, characterized by strong enrichments in 28 Si and believed to originate from Type II Supernovae (SNeII), were also measured for Li- and B-isotopic compositions. None of these grains show resolvable isotope anomalies in Li or B. For the seven X grains without Li and B contributions from nearby or attached SiC grains of distinct origins we find on average 7 Li/ 6 Li = 11.83 ± 0.29 (solar system: 12.06) and 11 B/ 10 B = 4.68 ± 0.31 (solar system: 4.03). The average 7 Li/ 6 Li is compatible with the solar system ratio and the lithium in the X grains is likely largely dominated by contaminating Li of laboratory or meteoritic origin. Also, most of the boron in X grains appears to be contamination but the small 11 B excess of ∼16%, significant at the 2σ level, can be considered a hint for the presence of boron produced by the neutrino process in the parent SNeII. Despite this finding, a quantitative comparison of the B isotope and abundance data of X grains with model predictions reveals deficiencies in our current understanding of the details of B production in SNeII as well as on B chemistry and condensation in SNII ejecta.

In-situ chromium isotope measurement of chromium-rich fine grains in the Murchison CM2 chondrite

Cr anomaly is Cr-rich, this result implies a low abundance and a huge isotopic anomaly of the carrier, and/or its ex-tremely fine grain size. In the former case, the maximum abundance of the presolar Cr-rich grains is roughly estimated tobe ~1 ppm. In that case the minimum isotopic anomaly would be ~90‰.

NanoSIMS STUDIES OF SMALL PRESOLAR SiC GRAINS: NEW INSIGHTS INTO SUPERNOVA NUCLEOSYNTHESIS, CHEMISTRY, AND DUST FORMATION

We have studied more than 2000 presolar silicon carbide (SiC) grains from the Murchison CM2 chondrite in the size range 0.2-0.5 μm for C- and Si-isotopic compositions. In a subset of these grains, we also measured N-, Mg-Al-, S-, and Ca-Ti-isotopic compositions as well as trace element concentrations. The overall picture emerging from the isotope data is quite comparable with that of larger grains, except for the abundances of grains from Type II supernovae (SNeII) and low-metallicity asymptotic giant branch (AGB) stars. Especially, the latter are much more abundant among submicrometer-sized grains than among micrometer-sized grains. This implies that SiC grains from lower-than-solar-metallicity AGB stars are on average smaller than those from solar metallicity AGB stars which provided the majority of presolar SiC grains. We identified five grains with large enrichments in 29Si (up to 3.5× solar) and 30Si (up to 3.9× solar in three of these grains). These grains are most likely from SNeII. The isotopically light S (32S/34S of 2× solar) together with the heavy Si in one of these grains suggests that molecule formation precedes macroscopic mixing and dust formation in SNII ejecta. This adds to the complexity of SN mixing calculations and should be considered in future studies. In total, about 2% of the presolar SiC grains in the size range 0.2-0.5 μm appear to come from SNeII. This is about a factor of 2 higher than for micrometer-sized grains and suggests that SNeII, on average, produce smaller SiC grains than solar metallicity AGB stars. The high 29Si/30Si ratio in one of the SN grains suggests that current SN models underestimate the 29Si production in the C- and Ne-burning regions by about a factor of 2. It is shown that with this adjustment the solar 29Si/28Si ratio can be well reproduced in Galactic chemical evolution models and that a merger of our Galaxy with a low-metallicity satellite some 1.5 Gyr before solar system formation could account for the slope 1.3 of the Si mainstream line.

Dhofar 225 and Dhofar 735: Relationship to CM2 chondrites and metamorphosed carbonaceous chondrites, Belgica‐7904 and Yamato‐86720

Abstract– Dhofar (Dho) 225 and Dho 735 are carbonaceous chondrites found in a hot desert and having affinities to Belgica‐like Antarctic chondrites (Belgica [B‐] 7904 and Yamato [Y‐] 86720). Texturally they resemble CM2 chondrites, but differ in mineralogy, bulk chemistry and oxygen isotopic compositions. The texture and main mineralogy of Dho 225 and Dho 735 are similar to the CM2 chondrites, but unlike CM2 chondrites they do not contain any (P, Cr)‐sulfides, nor tochilinite 6Fe0.9S*5(Fe,Mg)(OH)2. H2O‐contents of Dho 225 and Dho 735 (1.76 and 1.06 wt%) are lower than those of CM2 chondrites (2–18 wt%), but similar to those in the metamorphosed carbonaceous chondrites of the Belgica‐like group. Bulk compositions of Dho 225 and Dho 735, as well as their matrices, have low Fe and S and low Fe/Si ratios relative to CM2 chondrites. X‐ray powder diffraction patterns of the Dho 225 and Dho 735 matrices showed similarities to laboratory‐heated Murchison CM2 chondrite and the transformation of serpentine to olivine. Dho 225 and 735’s oxygen isotopic compositions are in the high 18O range on the oxygen diagram, close to the Belgica‐like meteorites. This differs from the oxygen isotopic compositions of typical CM2 chondrites. Experimental results showed that the oxygen isotopic compositions of Dho 225 and Dhofar 725, could not be derived from those of typical CM2 chondrites via dehydration caused by thermal metamorphism. Dho 225 and Dho 735 may represent a group of chondrites whose primary material was different from typical CM2 chondrites and the Belgica‐like meteorites, but they formed in an oxygen reservoir similar to that of the Belgica‐like meteorites.

Isotopic anomalies in organic nanoglobules from Comet 81P/Wild 2: Comparison to Murchison nanoglobules and isotopic anomalies induced in terrestrial organics by electron irradiation

Nanoglobules are a form of organic matter found in interplanetary dust particles and primitive meteorites and are commonly associated with 15N and D isotopic anomalies that are suggestive of interstellar processes. We report the discovery of two isotopically-anomalous organic globules from the Stardust collection of particles from Comet 81P/Wild 2 and compare them with nanoglobules from the Murchison CM2 meteorite. One globule from Stardust Cometary Track 80 contains highly aromatic organic matter and a large 15N anomaly (δ 15N = 1120‰). Associated, non-globular, organic matter from this track is less enriched in 15N and contains a mixture of aromatic and oxidized carbon similar to bulk insoluble organic material (IOM) from primitive meteorites. The second globule, from Cometary Track 2, contains non-aromatic organic matter with abundant nitrile ( C N) and carboxyl ( COOH) functional groups. It is significantly enriched in D (δD = 1000‰) but has a terrestrial 15N/ 14N ratio. Experiments indicate that similar D enrichments, unaccompanied by 15N fractionation, can be reproduced in the laboratory by electron irradiation of epoxy or cyanoacrylate. Thus, a terrestrial origin for this globule cannot be ruled out, and, conversely, exposure to high-energy electron irradiation in space may be an important factor in producing D anomalies in organic materials. For comparison, we report two Murchison globules: one with a large 15N enrichment and highly aromatic chemistry analogous to the Track 80 globule and the other only moderately enriched in 15N with IOM-like chemistry. The observation of organic globules in Comet 81P/Wild 2 indicates that comets likely sampled the same reservoirs of organic matter as did the chondrite parent bodies. The observed isotopic anomalies in the globules are most likely preserved signatures of low temperature (<10 K) chemistry in the interstellar medium or perhaps the outer regions of the solar nebula. In other extraterrestrial samples, D isotopic anomalies, but not those of 15N, may be explained in part by exposure to ionizing electron radiation.

Mid-infrared spectra of the shocked Murchison CM chondrite: Comparison with astronomical observations of dust in debris disks

We present laboratory mid-infrared transmission/absorption spectra obtained from matrix of the hydrated Murchison CM meteorite experimentally shocked at peak pressures of 10–49 GPa, and compare them to astronomical observations of circumstellar dust in different stages of the formation of planetary systems. The laboratory spectra of the Murchison samples exhibit characteristic changes in the infrared features. A weakly shocked sample (shocked at 10 GPa) shows almost no changes from the unshocked sample dominated by hydrous silicate (serpentine). Moderately shocked samples (21–34 GPa) have typical serpentine features gradually replaced by bands of amorphous material and olivine with increasing shock pressure. A strongly shocked sample (36 GPa) shows major changes due to decomposition of the serpentine and due to devolatilization. A shock melted sample (49 GPa) shows features of olivine recrystallized from melted material. The infrared spectra of the shocked Murchison samples show similarities to astronomical spectra of dust in various young stellar objects and debris disks. The spectra of highly shocked Murchison samples (36 and 49 GPa) are similar to those of dust in the debris disks of HD113766 and HD69830, and the transitional disk of HD100546. The moderately shocked samples (21–34 GPa) exhibit spectra similar to those of dust in the debris disks of Beta Pictoris and BD+20307, and the transitional disk of GM Aur. An average of the spectra of all Murchison samples (0–49 GPa) has a similarity to the spectrum of the older protoplanetary disk of SU Auriga. In the gas-rich transitional and protoplanetary disks, the abundances of amorphous silicates and gases have widely been considered to be a primary property. However, our study suggests that impact processing may play a significant role in generating secondary amorphous silicates and gases in those disks. Infrared spectra of the shocked Murchison samples also show similarities to the dust from comets (C/2002 V1, C/2001 RX14, 9P/Tempel 1, and Hale Bopp), suggesting that the comets also contain shocked Murchison-like material.

マーチソン CM隕石の衝撃実験回収試料中の割れ目：衝撃圧との定量的関係

マーチソン CM隕石の衝撃実験回収試料中の割れ目：衝撃圧との定量的関係

Dust production from the hypervelocity impact disruption of the Murchison hydrous CM2 meteorite: Implications for the disruption of hydrous asteroids and the production of interplanetary dust

More than half of the C-type asteroids, the dominant type of asteroid in the outer half of the main-belt, show evidence of hydration in their reflectance spectra. In order to understand the collisional evolution of asteroids and the production of interplanetary dust and to model the infrared signature of small particles in the Solar System it is important to characterize the dust production from primary impact disruption events, and compare the disruption of hydrous and anhydrous targets. We performed a hypervelocity impact disruption experiment on an ∼30 g target of the Murchison CM2 hydrated carbonaceous chondrite meteorite, and compared the results with our previous disruption experiments on anhydrous meteorites including Allende, a CV3 carbonaceous chondrite, and nine ordinary chondrites. Murchison is significantly more friable than the ordinary chondrites or Allende. Nonetheless, on a plot of mass of the largest fragment versus specific impact energy, the Murchison disruption plots within the field of the anhydrous meteorites points, suggesting that Murchison is at least as resistant to impact disruption as the anhydrous meteorites, which require about twice the energy for disruption as terrestrial anhydrous basalt targets. We determined the mass–frequency distribution of the debris from the Murchison disruption over a nine order-of-magnitude mass range, from ∼10 −9 g to the mass of the largest fragment produced in the disruption. The cumulative mass–frequency distribution from the Murchison disruption is fit by three power-law segments. For masses >10 −2 g the slope is only slightly steeper than that of the corresponding segment from the disruption of most anhydrous meteorites. Over the range from ∼10 −6 to 10 −2 g the slope is significantly steeper than that for the anhydrous meteorites. For masses <10 −6 g the slopes of both the Murchison and the anhydrous meteorites are almost flat. Thus the Murchison disruption significantly over-produced small fragments (10 −6–10 −3 g) compared to anhydrous meteorite targets. If the Murchison results are representative of hydrous asteroids, the hydrous asteroids may dominate over anhydrous asteroids in the production of interplanetary dust >100 μm in size, the size of micrometeorites recovered from the polar ices, while both types of asteroids might produce comparable amounts of ∼10 μm interplanetary dust. This would explain the puzzle that polar micrometeorites (>100 μm in size) are similar to hydrous meteorites, while the majority of the ∼10 μm interplanetary dust particles are anhydrous.

Evaluation of dehydration mechanism during heating of hydrous asteroids based on mineralogical and chemical analysis of naturally and experimentally heated CM chondrites

Abstract Based on the evidence derived from spectroscopic observation and meteorite analysis, some hydrous asteroids were heated and dehydrated for a certain period of time after aqueous alteration. In order to reproduce the dehydration processes, we experimentally heated Murchison CM chondrite at 600°C for 1 h (600°C/1 h), 600°C/96 h, 900°C/1 h, and 900°C/96 h under controlled oxygen partial pressures. The experimental products were compared with Belgica (B-)7904 CM chondrite, a meteorite from a dehydrated asteroid in terms of characteristic mineralogical and compositional properties. B-7904 shows properties intermediate between the two experimental products heated at 900°C/1 h and 900°C/96 h. In addition, the presence or the absence of some temperature-sensitive minerals in B-7904 suggests that it experienced heating at a temperature higher than 700°C but lower than 890°C. The duration of heating, based on the diffusion time needed to achieve the Fe-Mg zoning profile in olivine in B-7904, was estimated to be between 10 and 103 days at 700°C and between 1 to 102 h at 890°C. The obtained durations are much shorter than those expected from the internal heating model which requires prolonged heating over million years. Therefore, it is unlikely that the short-lived radionuclide of 26Al is a heat source for the dehydration of B-7904. Instead, short-duration local heating, such as that from impacts or solar radiation, is a more promising heat source.

Thermal alteration of hydrated minerals during hypervelocity capture to silica aerogel at the flyby speed of Stardust

Abstract— Outside the Earth's atmosphere, silica aerogel is one of the best materials to capture finegrained extraterrestrial particles in impacts at hypervelocities. Because silica aerogel is a superior insulator, captured grains are inevitably influenced by frictional heat. Therefore, we performed laboratory simulations of hypervelocity capture by using light‐gas guns to impact into aerogels finegrained powders of serpentine, cronstedtite, and Murchison CM2 meteorite. The samples were shot at &gt;6 km s−1 similar to the flyby speed at comet P/Wild‐2 in the Stardust mission. We investigated mineralogical changes of each captured particle by using synchrotron radiation X‐ray diffraction (SR‐XRD), transmission electron microscope (TEM), and field emission scanning electron microscope (FE‐SEM). SR‐XRD of each grain showed that the majority of the bulk grains keep their original mineralogy. In particular, SR‐XRD and TEM investigations clearly exemplified the presence of tochilinite whose decomposition temperature is about 300 °C in the interior of the captured Murchison powder. However, TEM study of these grains also revealed that all the samples experienced melting and vesiculation on the surface. The cronstedtite and the Murchison meteorite powder show remarkable fracturing, disaggregation, melting, and vesiculation. Steep thermal gradients, about 2500 °C/μm were estimated near the surface of the grains (&lt;2 μm thick) by TEM observation. Our data suggests that the interior of &gt;4 μm across residual grains containing abundant materials that inhibit temperature rise would have not experienced &gt;300 °C at the center.

Heating effects of the matrix of experimentally shocked Murchison CM chondrite: Comparison with micrometeorites

Abstract— Micrometeorites have been significantly altered or melted by heating, which has been mainly ascribed to aerodynamic drag during atmospheric entry. However, if a major fraction of micrometeorites are produced by impacts on porous asteroids, they may have experienced shock heating before contact with the Earth's atmosphere (Tomeoka et al. 2003). A transmission electron microscope (TEM) study of the matrix of Murchison CM chondrite experimentally shocked at pressures of 10–49 GPa shows that its mineralogy and texture change dramatically, mainly due to shock heating, with the progressive shock pressures. Tochilinite is completely decomposed to an amorphous material at 10 GPa. Fe‐Mg serpentine is partially decomposed and decreases in amount with increasing pressure from 10 to 30 GPa and is completely decomposed at 36 GPa. At 49 GPa, the matrix is extensively melted and consists mostly of aggregates of equigranular grains of Fe‐rich olivine and less abundant low‐Ca pyroxene embedded in Si‐rich glass. The mineralogy and texture of the shocked samples are similar to those of some types of micrometeorites. In particular, the samples shocked at 10 and 21 GPa are similar to the phyllosilicate (serpentine)‐rich micrometeorites, and the sample shocked at 49 GPa is similar to the olivine‐rich micrometeorites. The shock heating effects also resemble the effects of pulse‐heating experiments on the CI and CM chondrite matrices that were conducted to simulate atmospheric entry heating. We suggest that micrometeorites derived from porous asteroids are likely to go through both shock and atmospheric‐entry heating processes.

Bulk mineralogical changes of hydrous micrometeorites during heating in the upper atmosphere at temperatures below 1000 °C

Abstract— Small particles 200 μm in diameter from the hydrous carbonaceous chondrites Orgueil CI, Murchison CM2, and Tagish Lake were experimentally heated for short durations at subsolidus temperatures under controlled ambient pressures in order to examine the bulk mineralogical changes of hydrous micrometeorites during atmospheric entry. The three primitive meteorites consist mainly of various phyllosilicates and carbonates that are subject to decomposition at low temperatures, and thus the brief heating up to 1000 °C drastically changed the mineralogy. Changes included shrinkage of interlayer spacing of saponite due to loss of molecular water at 400–600 °C, serpentine and saponite decomposition to amorphous phases at 600 and 700 °C, respectively, decomposition of Mg‐Fe carbonate at 600 °C, recrystallization of secondary olivine and Fe oxide or metal at 700–800 °C, and recrystallization of secondary low‐Ca pyroxene at 800 °C. The ambient atmospheric pressures controlled species of secondary Fe phase: taenite at pressures lower than 10−2 torr, magnesiowüstite from 10−3 to 10−1 torr, and magnetite from 10−2 to 1 torr. The abundance of secondary low‐Ca pyroxene increases in the order of Murchison, Orgueil, and Tagish Lake, and the order corresponds to saponite abundance in samples prior to heating. Mineralogy of the three unmelted micrometeorites F96CI024, kw740052, and kw740054 were investigated in detail in order to estimate heating conditions. The results showed that they might have come from different parental objects, carbonaterich Tagish Lake type, carbonate‐poor Tagish Lake or CI type, and CM type, respectively, and experienced different peak temperatures, 600, 700, and 800˜900 °C, respectively, at 60–80 km altitude upon atmospheric entry.

TEM study of experimentally shocked Murchison CM chondrite

TEM study of experimentally shocked Murchison CM chondrite

Structural, chemical, and isotopic microanalytical investigations of graphite from supernovae

We report the results of coordinated ion microprobe and transmission electron microscope (TEM) studies of presolar graphites from the KE3 separate (1.65–1.72 g/cm3) of the Murchison CM2 meteorite. Isotopic analysis of individual graphites (1–12 μm) with the ion microprobe shows many to have large 18O excesses combined with large silicon isotopic anomalies, indicative of a supernova (SN) origin. Transmission electron microscopy (TEM) of ultramicrotome slices of these SN graphites revealed a high abundance (25–2400 ppm) of internal titanium carbides (TiCs), with a single graphite in some cases containing hundreds of TiCs. Isotopic compositions of individual TiCs by nanoscale resolution secondary ion mass spectrometry (NanoSIMS) confirmed their presolar origin. In addition to TiCs, composite TiC/Fe grains (TiCs with attached iron–nickel subgrains) and solitary kamacite internal grains were found. In the composite grains, the attached iron phase (kamacite [0–24 at. % Ni] or taenite [up to 60 at. % Ni]) was epitaxially grown onto one or more TiC faces. In contrast to the denser Murchison KFC1 graphites, no Zr-Ti-Mo carbides were observed. The average TiC diameters were quite variable among the SN graphites, from 30 to 232 nm, and were generally independent of the host graphite size. TiC grain morphologies ranged from euhedral to anhedral, with the grain surfaces exhibiting variable degrees of corrosion, and sometimes partially amorphous rims (3 to 15 nm thick). Partially amorphous rims of similar thickness were also observed on some solitary kamacite grains. We speculate that the rims on the internal grains are most plausibly the result of atom bombardment caused by drift of grains with respect to the ambient gas, requiring relative outflow speeds ∼100 km/s (i.e., a few percent of the SN mass outflow speed). Energy dispersive X-ray spectrometry (EDXS) of TiCs revealed significant V in solid solution, with an average V/Ti ratio over all TiCs of ∼83% of the solar value of 0.122. Significant variations about the mean V/Ti ratio were also seen among TiCs in the same graphite, likely indicating chemical equilibration with the surrounding gas over a range of temperatures. In general, the diversity in internal TiC properties suggests that TiCs formed first and had substantially diverse histories before incorporation into the graphite, implying some degree of turbulent mixing in the SN outflows. In most graphites, there is a decrease in the number density of TiCs as a function of increasing radial dis- tance, caused by either preferential depletion of TiCs from the gas or an acceleration of graphite growth with decreasing ambient temperature. In several graphites, TiCs showed a trend of larger V/Ti ratios with increasing distance from the graphite center, an indication of progressive equilibration with the surrounding gas before they were sequestered in the graphites. In all but one graphite, no trend was seen in the TiC size vs. distance from the graphite center, implying that appreciable TiC growth had effectively stopped before the graphites formed, or else that graphite growth was rapid compared to TiC growth. Taken together, the chemical variations among internal grains as well as the presence of partially amorphous rims and epitaxial Fe phases on some TiCs clearly indicate that the phase condensation sequence was TiC, followed by the iron phases (only found in some graphites) and finally graphite. Since graphite typically condenses at a higher temperature than iron at low pressures (<10−3 bars) in a gas with C > O and otherwise solar composition, the observed condensation sequence implies a relative iron enrichment in the gas or greater supersaturation of graphite relative to iron. The TEM observations allow inferences to be made about the physical conditions in the gas from which the grains condensed. Given the TiC sizes and abundances, the gas was evidently quite dusty. From the observed TiC size range of ∼20 nm to ∼500 nm (assuming ∼1 yr growth time and T ∼ 1800°K), we infer minimum Ti number densities in the gas to be ∼7 × 104 to ∼2 × 106 atoms/cc, respectively. Although the gas composition is clearly not solar, for scale, these number densities would correspond to a pressure range of ∼0.2 μbar to ∼5.0 μbar in a gas of solar composition. They also correspond to minimum TiC grain number densities of ∼3 × 10−4 to ∼0.2 grains/cc, assuming complete condensation of Ti in TiC. We estimate the maximum ratio of mean TiC grain separation distance in the gas to grain diameter from the Ti number densities as ∼3 × 105 to ∼1 × 106.

Automated isotopic measurements of micron-sized dust: application to meteoritic presolar silicon carbide

We report the development of a new analytical system allowing the fully automated measurement of isotopic ratios in micrometer-sized particles by secondary ion mass spectrometry (SIMS) in a Cameca ims-6f ion microprobe. Scanning ion images and image processing algorithms are used to locate individual particles dispersed on sample substrates. The primary ion beam is electrostatically deflected to and focused onto each particle in turn, followed by a peak-jumping isotopic measurement. Automatic measurements of terrestrial standards indicate similar analytical uncertainties to traditional manual particle analyses (e.g., ∼3‰/amu for Si isotopic ratios). We also present an initial application of the measurement system to obtain Si and C isotopic ratios for ∼3300 presolar SiC grains from the Murchison CM2 carbonaceous chondrite. Three rare presolar Si 3N 4 grains were also identified and analyzed. Most of the analyzed grains were extracted from the host meteorite using a new chemical dissolution procedure. The isotopic data are broadly consistent with previous observations of presolar SiC in the same size range (∼0.5–4 μm). Members of the previously identified SiC AB, X, Y, and Z subgroups were identified, as was a highly unusual grain with an extreme 30Si enrichment, a modest 29Si enrichment, and isotopically light C. The stellar source responsible for this grain is likely to have been a supernova. Minor differences in isotopic distributions between the present work and prior data can be partially explained by terrestrial contamination and grain aggregation on sample mounts, though some of the differences are probably intrinsic to the samples. We use the large new SiC database to explore the relationships between three previously identified isotopic subgroups—mainstream, Y, and Z grains—all believed to originate in asymptotic giant branch stars. The isotopic data for Z grains suggest that their parent stars experienced strong CNO-cycle nucleosynthesis during the early asymptotic giant branch phase, consistent with either cool bottom processing in low-mass (M < 2.3M ⊙) parent stars or hot-bottom burning in intermediate-mass stars (M > 4M ⊙). The data provide evidence for a sharp threshold in metallicity, above which SiC grains form with much higher 12C/ 13C ratios than below. Above this threshold, the fraction of grains with relatively high 12C/ 13C decreases exponentially with increasing 29Si/ 28Si ratio. This result indicates a sharp increase in the maximum mass of SiC parent stars with decreasing metallicity, in contrast to expectations from Galactic chemical evolution theory.