Meteoritic Component Research Articles

Origin of the hydrocarbon component of carbonaceous chondrites: the star-meteorite connection.

We report the results of an experiment that produced a residue which closely matches the hydrocarbon component of the Murchison carbonaceous chondrite. This experiment suggests that the parent material of the meteoritic component originated as polycyclic aromatic hydrocarbon species in carbon stars during their later stages of evolution. The experiments also indicate that the pathway from those formation sites to eventual incorporation into the meteorite parent body involved hydrogenation in a plasma in the solar nebula or in H II regions prior to the solar nebula. This model is consistent with what is known about the meteoritic hydrocarbon component including deuterium abundance, the observation of cosmic infrared emission bands best attributed to polycyclic aromatic hydrocarbon molecules, and the inherent stability of these molecules that allows their formation in stars and subsequent survival in the interstellar medium.

Geochemistry and mineralogy of Early Archean spherule beds, Barberton Mountain Land, South Africa: evidence for origin by impact doubtful

Spherule layers in the ∼ 3.4 Ga Barberton Greenstone Belt, South Africa, have been interpreted as being the result of large asteroid or comet impacts on the early earth. This interpretation was based, among other arguments, on the enrichment of siderophile elements, especially the platinum group elements. We made a detailed mineralogical, petrological and geochemical study of spherule bed samples taken from drill cores and underground exposures at the Princeton, Mt. Morgan and Sheba gold mines, as well as surface localities. The macrostructure of each sample (from within different spherule layer units) shows evidence for multiple (more than five) events over about 30 cm. This would require multiple impacts within a few million years, which is unlikely. The mineral phases are almost exclusively of secondary origin. The mineralogy provides evidence for extensive hydrothermal and metasomatic alterations of the spherule beds. Geochemical analyses of alternating spherule, shale and chert layers show no correlation between the siderophile elements (e.g., Ir, Co, Ni and Au), contrary to that which would be expected if the siderophile elements had an extraterrestrial source. Furthermore, no significant variation in the content of the siderophile elements was detected between spherule layers and shale layers; however, siderophile element contents are high only in layers containing abundant sulphide minerals and having high As, Sb, Se and Cr contents. We suggest that complex mineralizations, similar to those that have formed the Barberton Archean gold deposits or the Bon Accord deposit, were responsible for the siderophile element enrichments in the spherule beds. The mineralogical and geochemical data provide no direct evidence in favour of their initial origin by impact. Nowhere else in the world have such multiple (or even single) spherule beds been observed, and none of the numerous known impact crates (or the Cretaceous-Tertiary boundary) is associated with comparable spherule beds. Known impact debris usually contains < 1% meteoritic component, if any at all, while the Barberton spherules are anomalous in being extremely enriched compared to any known impact deposits.

Detection of a Meteoritic Component in Ivory Coast Tektites with Rhenium-Osmium Isotopes

Measurement of rhenium (Re) and osmium (Os) concentrations and Os isotopic compositions in Ivory Coast tektites (natural glasses with upper crustal compositions that are ejected great distances during meteorite impact) and rocks from the inferred source crater, Lake Bosumtwi, Ghana, show that these tektites incorporate about 0.6 percent of a meteoritic component. Analysis of elemental abundances of noble metals alone gives equivocal results in the detection of meteoritic components because the target rocks already have relatively large amounts of noble metals. The Re-Os system is ideally suited for the study of meteorite impacts on old continental crust for three reasons. (i) The isotopic compositions of the target rocks and the meteoritic impactor are significantly different. (ii) Closed-system mixing of target rocks and meteorites is linear on Re-Os isochron diagrams, which thus permits identification of the loss of Re or Os. (iii) Osmium isotopic compositions are not likely to be altered during meteorite impact even if Re and Os are lost.

Modeling of S-Type Asteroid Spectra Using Primitive Achondrites and Iron Meteorites

The stony iron model for the S asteroids has been tested using observed reflectance spectra of the S asteroids and modeled spectra of meteoritic components. Observed reflectance spectra of 40 S asteroids were fit with linear combinations of laboratory reflectance spectra of 6 representative primitive achondrites and an iron meteorite. Specular and diffuse reflection components were taken into account for metallic-iron reflectance. Even with this limited set of meteorite spectra, 16 of those 40 S asteroids were fit fairly well in terms of absorption-band features and spectral profiles. The number of well-fit S asteroids may be greater if the number and variety of metallic and stony domains are expanded. This initial result suggests that many of the S-asteroidal surfaces may be various kinds of regional mixtures of primitive achondrites and metallic iron. If true, the S asteroids could have been originally formed of chondrite-like source materials because mineral assemblages of primitive achondrites seem to have recrystallized from chondrites by segregation of metallic components and loss of partial melts.

The ultraviolet spectra of thin evaporated films of carbonaceous chondrite

Absorption spectra in the visible to the near ultraviolet were measured on the Belgica chondrite B-7904 in a form of thin solid film made by the vacuum evaporation. The spectra obtained exhibit a sharp peak at 226 nm and a broad bump around 280nm. These features were found arising from the meteorite component FeS (troilite). The peak at 226 nm shows a doublet structure with the band-width considerably narrower than the 217.5 nm feature in the interstellar extinction. The absorption spectra obtained previously with the pulverized chondrites suspended in a liquid were also found reproducible by the pulverized FeS.

Evidence for mantle heterogeneity from platinum-group-element abundances in Indian Ocean basalts

Oceanic-island tholeiitic basalts recovered from four sunken oceanic islands along the Reunion hot-spot trace show trace-element and mineralogical characteristics ranging from typical oceanic-island tholeiites to incompatible-element-depleted tholeiites resembling mid-ocean-ridge basalts. There are also variable degrees of magma evolution at each island. Noble metal (Au, Pd, Pt, Rh, Ru, Ir) abundances tend to decrease with magma evolution and with magma "alkalinity", indicating that the metals behave as compatible elements during crystal fractionation processes and during mantle melting processes. Palladium-to-iridium ratios also decrease with increasing alkalinity. Absolute abundances of elements such as Pd are higher than those in typical mid-ocean-ridge basalts, by factors up to 30, despite many major-element similarities with the latter. Comparison with other types of mafic rocks shows that Pd/Ir ratios increase with decreasing alkalinity in basaltic rocks but plunge to alkali-basalt values in komatiites. A model involving retention of low-melting-point Au, Pd, and Rh in mantle sulphides, which completely dissolve by intermediate percentages of melting, and the high-melting-point metals Ir and Ru in late-melting mantle alloys explains increasing Pd/Ir ratios with decreasing alkalinity (increasing melting percentages) in oceanic basalts and the low Pd/Ir ratios of high-percentage melt komatiites.The high noble metal concentrations in Indian Ocean basalts compared with basalts from many other ocean basins are most easily explained by higher concentrations in their source regions. This may be related to incomplete mixing of a post-core-formation meteoritic component of the upper mantle, or deep mantle plume-derived blebs of core material that either failed to reach the core, during core–mantle differentiation, or were plucked from the core by a convecting lower mantle. The latter is tentatively favoured due to the apparently higher noble metal concentrations in oceanic-island (plume) basalts.

Dissemination and fractionation of projectile materials in the impact melts from Wabar Crater, Saudi Arabia

Abstract— We have analyzed small, ballistically dispersed melt samples in the form of aerodynamically shaped spheres, dumbbells, teardrops, etc., from Wabar Crater, Saudi Arabia, and have compared these to our previous study of the more massive, black and white melt specimens. The smaller melt samples differ from the more massive melts in that they are petrographically and chemically more homogeneous, possess fewer, more diffuse schlieren and contain much less clastic detritus. These observations suggest higher peak temperatures for the smaller melt samples than for the massive black and white melts which represent Wabar's major melt‐zone.Analyses of the Wabar and Nejed (paired with Wabar) meteorites permit detailed comparison of the unaltered projectile with impactor residues in the melts. Siderophile element concentrations indicate that the small glass beads commonly contain &gt; 10% meteoritic component, compared to &lt; 5% for the massive black and white melts. One glass bead was found to contain ∼ 17% meteoritic component. Based on models for melt production during cratering, we deduce that more meteoritic material was mixed with the upper stratigraphic horizons of Wabar's melt zone than with the lower parts. Siderophile elements in all Wabar melt specimens are fractionated relative to the Wabar‐Nejed meteorite and have Fe/Ni ratios up to ∼ 1.8 times that of Wabar‐Nejed for the most siderophile element‐rich glasses. The abundance sequence of siderophiles in the melts relative to the projectile is Fe ≅ Co &gt; Ni ≅ Ir ≅ As » Au. Although this sequence seems incompatible with simple vapor fractionation of either elements or oxides, we believe that a complex vapor fractionation process most likely produced the observed siderophile element abundances.Our sample suite should be representative of all materials found in and around the Wabar structure, and we conclude that substantial quantities of the projectile were lost to the atmosphere, most likely as vapor. No fractionation of lithophile elements is observed in the glasses relative to the target rocks. Although fractionation of the impactor must have occurred prior to intimate mixing of projectile and target, details of the actual fractionation mechanism(s) remain poorly understood. The results of this study indicate that caution is necessary when attempting to define impactor types and masses from compositional data for impact melts from other craters.

A compositional classification scheme for meteoritic chondrules

A taxonomic scheme is proposed for the main component of the primitive chondrite meteorites—the chondrules. The scheme provides insight into the variety of chondrules originally produced in the primordial solar nebula, and the effects on them of secondary processes on meteorite parent bodies. It may also contribute to understanding the origin of compositional diversity in the chondrites.

Mars volatile evolution: Evidence from stable isotopes

Constraints on the history of the Martian volatile system can be determined by using observations of the ratios 18O/ 16O, 17O/ 16O, 13C/ 12C, and D/H in atmospheric species and in various components of the SNC meteorites which are thought to be pieces of the Martian surface. These observations are compared with the variations expected from processes which can fractionate the different isotopes—thermal escape to space, nonthermal escape to space, formation of carbonate mineral deposits from atmospheric CO 2, and condensation of CO 2 or H 2O ice from the atmosphere. All of the available observations and models are synthesized for the first time, along with other observations of the Martian surface and atmosphere, in order to provide a coherent view of the evolution of both CO 2 and H 2O over geologic time. While no scenarios for atmospheric evolution can be absolutely and uniquely ruled out, a strongly favored scenario does exist. In this favored model, fractionation of oxygen by escape to space is diluted by the exchange with a nonatmospheric reservoir; it is unlikely that there is sufficient CO 2 adsorbed in the regolith to serve this function, so the polar water-ice deposits must be exchanging with the atmospheric water over geologic time. Exchange of oxygen between atmospheric CO 2 and H 2O probably provides the observed fractionation between those species. While the observed fractionation of D/H suggests a significant loss of water to space, the timescale on which this loss occurred is uncertain; the results can be reconciled with the oxygen results by invoking time-dependent models (which are consistent with what we know about the Mars climate). Additional observations and numerical modeling efforts which would help eliminate some of the uncertainties in the results are suggested.

Terrestrial mantle siderophiles and the lunar impact record

A simple analytical fit to the lunar cratering record, scaled from final to transient crater diameters, then to impactor masses, implies a total mass ∼1.0 × 10 20kg incident on the Moon subsequent to the solidification of the lunar crust ∼4.4 Gyr ago. About half this mass would be retained, and a comparable lunar mass would be eroded. These results are in good agreement with geochemical estimates of the meteoritic component mixed into the lunar crust, which give (0.4−1.5) × 10 20 kg. Gravitationally scaling to Earth, and taking account of the statistical probability that the largest impactors incident on Earth were more massive than the largest incident on the Moon, gives an estimate of 1.5 × 10 22 kg of material accumulated by Earth subsequent to 4.4 Gyr ago. This result is in excellent accord with geochemical estimates of post-core formation meteoritic input. These estimates, based on abundances of highly siderophile elements in the terrestrial mantle, lie in the range (1–4) × 10 22 kg. The significant result is the approximate agreement of the lunar cratering record scaling with both lunar and terrestrial geochemical constraints, numerous uncertainties render exact comparisons pointless. Nevertheless, the close agreement suggests the model developed here may credibly be used to estimate exogenous volatile and prebiotic organic delivery.

The evolution of atmospheric CO2 on Mars: The perspective from carbon isotope measurements

In situ measurements of 12C/13C of atmospheric CO2 on Mars made by Viking are, within error, similar to values normally encountered on Earth. However, high precision isotopic measurements made on SNC meteorites show there to be a 40‰ fractionation between carbon associated with silicate minerals (δ13C ≈ −25‰, 12C/13C ≈ 91.3) and trapped CO2 gas or carbonate minerals (δ13C ≈ +15‰, 12C/13C ≈ 87.7). To a first approximation, it can be considered that the silicate‐sited carbon in SNC meteorites carries the isotopic signature of the element present at depth in the Martian crust; herein it is considered that this magmatic carbon is representative of bulk Mars. On the other hand, carbon in the form of carbonate most probably results from secondary processes in operation at the surface of the planet. Trapped CO2 is thought to be a sample of Martian atmospheric gas. The significant difference in isotopic composition between the different components in SNC meteorites is difficult to interpret in terms of closed‐system equilibrium or kinetic isotopic fractionation effects. Rather, it seems that the relative enrichment of 13C in surficial carbon is due to preferential removal of the lighter isotope during an atmospheric loss process. Between 30 and 70 mbar of CO2 may have been lost from the atmosphere to outer space.

Study of melt produced bodies observed at Henbury crater region

During the impact of the Henbury meteorite with ground rock a shower of molten droplets were produced which quickly solidified in oxide phases of meteorite and rock components. A study of these has provided information on relative volatilities of various oxides during the impact process. Spherical and non spherical bodies derived from target rock and meteorite components were found in samples of soil in the Henbury crater region. From a study of the composition of these bodies, the compositions of the meteorite and the target rock, it has been possible to separate the bodies into three separate groups. A major finding was the depletion of SiO 2 in the production of both Group 1 and Group 3 bodies. Group 3 bodies are not fractionated to the same extent as Group 1. Comparing the compositions with the composition of the target rock, the general order of fractionation of the Group 3 melt is SiO 2 > Na 2O > K 2O and MgO > Al 2O 3 and TiO 2 and for Group 1 Na 2O > SiO 2 > MgO and K 2O > Al 2O 3 and TiO 2. Comparing the compositions of Group 1 and Group 3 bodies with no reference to the target rock we get the general order of volatility to be Na 2O > MgO > K 2O > TiO 2, SiO 2 and Al 2O 3, the significant change being the place of SiO 2. Group 2 bodies, in general, showed similar degrees of fractionation to Group 1.

Distribution of sulphides and oxidised sulphur components in SNC meteorites

Stepped combustion analysis of sulphur-bearing phases in SNC meteorites indicates that sulphates and sulphides are widespread at low abundance in these meteorites. The amount of sulphate sulphur is highest in shergottites: Shergotty (190 ppm) and ALHA 77005 (90 ppm). Nakhla and Chassigny contain lower sulphate sulphur contents (80–85 ppm) compared with shergottites, but their oxidised/reduced sulphur ratios are significantly greater ( ⩾ 1.5) which indicates higher net oxidation states. Nakhla and Chassigny also contain elemental sulphur as an additional oxidised component. An origin of the sulphates (and elemental sulphur) by terrestrial weathering processes cannot be completely rejected, but it is considered more likely that the minerals were incorporated into the host rocks as relict grains of weathering or alteration products formed on the surface of the SNC parent body.

Origin of opaque assemblages in C3V meteorites: Implications for nebular and planetary processes

Mineral phases from opaque assemblages (OAs) in Ca, Al-rich refractory inclusions (CAIs), chondrules and matrix in C3V meteoites were chemically analyzed and compared with experimentally determined phase equilibria and partitioning data in the NiFeS, NiFeS and NiFeO systems to estimate the temperature, sulfur fugacity ( f S 2 ) and oxygen ( f O 2 ) of OA formation. The kinetics of dissolution and exsolution of metallic phases in the NiFeRu system were used to constrain the thermal history of OAss that occur in CAIs. Based on this work, we suggest that OAs formed after the crystallization of host CAIs by exsolution, sulfidation and oxidation of precursor alloys at low temperatures (≈ 770 K) and higher than solar gas f S 2 and f S 2 . Our model contrasts with previous models that call upon the formation of CAI OAs by aggregation of previously formed phases in the solar nebula prior to the crystallization of CAIs. Opaque assemblages in CAIs and chondrules probably originated as homogeneous alloys during melting of the silicate protions of CAIs and chondrules. The compositions of these precursor alloys reflect high-temperature and low- f O 2 conditions in the early solar nebula. The similarities in the temperature, f S 2 and f O 2 of equilibrium for OAs that occur in CAIs, chondrules and matrix suggest that these three components of C3V meteorites share a common, late low-temperature history. The mineral phases in OAs do not preserve an independent history prior to CAI and chondrule melting and crystallization, but instead provide important information on the post-accretionary history of C3V meteorites and allow us to quantify the temperature, f S 2 and f O 2 of cooling planetary environments.

Interstellar Molecules in Meteorites

Substantial enrichment of deuterium, D, in certain components of chondritic meteorites is interpreted as a record of isotopic fractionation during ion-molecule reactions at the very low temperatures characteristic of dense interstellar clouds. Whether those meteorites still contain the actual molecules that were synthesised in the presolar interstellar medium, or whether the interstellar material was recycled into a later generation of molecules within the early solar system is not known.

Abundance and distribution of gallium in some spinel and garnet lherzolites

The abundance and distribution of the moderately siderophile element gallium has been determined by radiochemical neutron activation analysis in spinel lherzolites from Mt. Porndon (Australia) and garnet lherzolites from Bultfontein (S. Africa). Minerals in the lherzolite concentrate Ga in the sequence olivine < orthopyroxene < clinopyroxene < garnet < spinel. Whole rock Ga abundances calculated from the modal mineralogy are equal to those actually determined, thus demonstrating that intergranular Ga-bearing phases are unimportant. Spinel lherzolites are richer in Ga than garnet lherzolites. Revised values for the Ga content of the mantle and garnet lherzolite-based pyrolite suggest that previous estimates are too high. The Ga content of the mantle may have been controlled by equilibrium core-mantle separation and does not require the presence of a meteoritic component.

An interstellar dust component rich in 12C

Chondritic meteorites contain grains which condensed in circumstellar environments and carry unusual isotope signatures1 providing clues to the location in which grain formation took place. Isotope measurements on grains that are composed entirely of carbon or carbon compounds have, up to now, given compositions either close to typical Solar System values or enriched in 13C (refs 2–5). Despite the existence of astrophysical environments where 12C production predominates, highly 12C-enriched carbon com-ponents in meteorites are uncommon. Here we report the results of an analysis of an acid-resistant residue from the Allende (CV3) meteorite1,6, extensively combusted in oxygen at low temperatures (<500 °C) to remove isotopically normal carbon, which revealed the presence of a small amount (<0.1 p.p.m.) of carbon with a 12C/13C ratio close to 120, well above the value of 89 ± 3 found in terrestrial samples or bulk meteorites. This component is denoted Cλ. Stellar evolutionary processes favour lowering of the 12C/13C ratio; hence Cλ, which has a combustion temperature consistent with graphite, might be a primitive type of interstellar material.

δ 13C and δ 18O study of acid-soluble inorganic components in meteorites

δ 13C and δ 18O study of acid-soluble inorganic components in meteorites

General discussion after session I

D. A. Williams ( UMIST, Manchester, U. K. ) . I wish to highlight some of the interesting and possibly controversial points that were raised. Professor Tayler gave us a very good introduction to the subject and I expect we shall discuss the questions that he raised on abundance anomalies and in particular the survival of grains. There are two particular aspects that interest me. First there is deuterium fractionation in the interstellar medium and it is, of course, known that deuterium fractionation occurs in meteorite material. That seems to indicate that material was fractionated in cold conditions and that the conditions have remained cold ever since because, if the temperature gets above ca . 200 K, that fractionation will disappear. The other point that I found particularly interesting in recent literature is the detection of diamond in the carbonaceous component of certain meteorites and again this seems to indicate a low-temperature regime for that particular material; diamond not being the most stable form of carbon. In Professor Kroto’s stimulating talk there were raised a number of questions, but not so much about chemistry of the interstellar medium as on chemistry in the laboratory or possibly chemistry in circumstellar regions; the main question that I would expect to hear discussed today is that of the applicability of what he has done. Very exciting though it is, there is some uncertainty about the applicability of his work to the problems that we are considering in this particular meeting. The conditions that you might find in the circumstellar regions are obviously not going to be quite like the conditions produced in the laboratory. The second important question that I would expect to be addressed in discussion now is the following. As material moves out of the circumstellar regions into the interstellar regions are the structures that Professor Kroto was describing expected to persist or not? He mentioned that C 60 may be formed in Bunsen burners and he also said that C 60 is very stable. If that is so why is all carbon on the earth not in the form of C 60 ? There must be some destruction mechanisms applying to these structures. Actually, if one makes amorphous carbon by having a surface in a carbon rich medium then, in fact, one does not get the sort of structures that he talked about. A mixture of diamond-like and graphitic-like regions of fairly small extent, perhaps a few tens of angstroms, is found.

Highly fractionated rare-earth elements in ferromagnesian chondrules from the Felix (CO3) meteorite

Chondrules and Ca, Al-rich inclusions (CAIs) are important constituents of chondritic meteorites which preserve a record of chemical fractionation at high temperature in the early solar nebula1. Cosmochemical research on these components of meteorites shows that chondrules were formed by melting of preexisting solid precursor materials2–4. Although highly fractionated rare-earth element (REE) patterns have been known for CAIs5,6 and are considered as evidence for high-temperature fractionation in the nebula7,8, little is known about refractory precursor components of chondrules. In addition, chondrules so far studied rarely exhibit fractionated REE patterns, except in a few cases9,10, so that no single variety of refractory precursor component can be invoked as the fractionated REE carrier in chondrules. Here we describe two ferromagnesian chondrules from the Felix (Ornans-subtype) carbonaceous chondrite which carry a marker signature of REE fractionation in the nebula. Both show positive Ce and Yb anomalies and one exhibits a light/heavy REE fractionation similar to those of Group II CAI6. On the basis of the REE characteristics of these chondrules, as well as those of Allende (CV) (ref. 11), we suggest that one of the precursor materials of chondrules in CO–CV carbonaceous chondrites is a high-temperature condensate from the nebular gas.