Elements In Meteorites Research Articles

Non-destructive Elemental Analysis of Lunar Materials with Negative Muon Beam at J-PARC

In the last decade, non-destructive elemental analysis using negative muon beams advanced significantly. This method can be used to determine the elemental composition of bulk materials without causing damage. In this study, we performed a negative muon irradiation experiment for a Northwest Africa 482 lunar meteorite (NWA482), which was installed in a stainless steel analysis chamber. The analysis chamber was filled with helium gas to suppress the background signals caused by air-scattering muons. The muonic X-rays from Al, Fe, Ca, Mg, Si, and O in the samples were detected using six high-purity germanium semiconductor detectors arranged around the analysis chamber. To correct the X-ray self-absorption effect of the samples, a Monte-Carlo simulation using Geant4 toolkit was used. Based on the quantitative analysis for muonic X-ray measurement with the correction application from the simulation, we successfully investigated the analytical sensitivity of each element in meteorites based on the NWA482 data.

Meteoritic materials within sulfate aerosol particles in the troposphere are detected with transmission electron microscopy

When meteorites impact the Earth’s atmosphere, they fractionalize and ablate, generating aerosol particles containing extraplanetary materials. Meteoritic materials have been identified among aerosol particles using elemental markers (iron and magnesium); however, their shapes and mixing states are largely unknown. Here we demonstrate the presence of meteoritic materials collected by a research aircraft from the troposphere over the western Pacific using transmission electron microscopy. The distribution of meteoritic elements within individual particles coincided with sulfur, indicating that they were in forms of sulfates, i.e., water-soluble. Enhanced number fractions of sulfate particles with meteoritic materials were observed during tropopause-folding events, suggesting that they originated from the stratosphere. We also estimated the potential contributions of the Chelyabinsk meteorite event, which occurred 5 months prior to the sampling and represents the largest meteorite event in the past century. This study provides unique observational evidence for the linkage between extraplanetary materials and tropospheric aerosols.

Exploring relationships between shock-induced microstructures and H2O and Cl in apatite grains from eucrite meteorites

The abundance and isotopic composition of volatile elements in meteorites is critical for understanding planetary evolution, given their importance in a variety of geochemical processes. There has been significant interest in the mineral apatite, which occurs as a minor phase in most meteorites and is known to contain appreciable amounts of volatiles (up to wt. % F, Cl, and OH). Impact-driven shock metamorphism, pervasive within many meteorites, can potentially modify the original signatures of volatiles through processes such as devolatilization and diffusion. In this study, we investigate the microstructures of apatite grains from six eucrites across a broad range of shock stages (S1–S5) using electron backscatter diffraction (EBSD) to explore shock-induced crystallographic features in apatite. New Cl and H abundance and isotopic composition data were collected on moderate to highly shocked samples (S3-S5) by Nano Secondary Ion Mass Spectrometry (NanoSIMS). Previously reported volatile data for S1 and S2 eucrites were integrated with EBSD findings in this study. Our findings indicate that apatite microstructures become increasingly more complex at higher shock stages. At low shock stages (S1–S2) samples display brecciation and fracturing of apatite. Samples in S3 and S4 display increasing crystal plastic deformation indicated by increasing spread in pole figures. At the higher shock stages (S4/S5) there is potential recrystallisation demonstrated by an increased density of subgrain boundaries. The Cl content and δ37Cl values of highly-shocked apatite grains range from ∼940 to 1410 ppm and −3.38 to +7.70‰, respectively, within the range observed in less-shocked eucrites. In contrast, H2O abundances are more variable (from 186 to ∼4010 ppm), however, the measured water content still falls within the range previously reported for low-shock eucrites. The measured δD values range from −157 to +163‰, also within the range of values from known low-shock basaltic eucrites. Weighted averages for both isotopic systems (δD −122 ± 20‰, δ37Cl + 1.76 ± 0.66‰) are consistent with the range displayed in other inner Solar System bodies. NanoSIMS isotope images of apatite grains display heterogeneity in their Cl abundance at the nanoscale which increases in complexity with shock stage. This increasing complexity, however, does not correlate with deformation microstructures observed in EBSD or with the Cl isotopic composition at either an inter-grain or intra-grain scale. These findings are similar to analyses of variably shocked lunar apatite and, therefore, apatite appears to be a robust recorder of Cl and H (at least at spatial resolution and precision currently achievable by NanoSIMS) on airless bodies, despite intensive shock.

Cadmium isotopes in chondrites and acid leachates: Nucleosynthetic homogeneity and a monitor for thermal neutron capture effects

Nucleosynthetic isotope variations are well documented for refractory elements in meteorites and the Earth, while moderately volatile elements generally display homogeneous compositions. Cadmium is a moderately volatile element with eight stable isotopes generated by a variety of nucleosynthetic processes. To address the extent of the nucleosynthetic variability in moderately volatile elements, new high precision Cd isotope data are presented for bulk samples of six carbonaceous and one enstatite chondrite. In addition, we report the first Cd isotope results of sequential acid leachates for the CM2 chondrite Jbilet Winselwan. Our new Cd data displays nucleosynthetic homogeneity for bulk chondrites and acid leachates within analytical uncertainties, in agreement with results for other moderately volatile elements. This implies that Cd isotopes were efficiently homogenised prior to incorporation into planetary bodies. We propose that Cd never significantly condensed into dust in stellar environments, or alternatively that such Cd-bearing dust was efficiently destroyed and recycled in the interstellar medium. Our leachate data provides evidence for further homogenisation during thermal processing in the protoplanetary disk including parent body processing. The data shows that Cd in carbonaceous chondrites mainly resides in the more easily dissolved phases, most likely sulphides that were affected by aqueous alteration. Less than 1% of the total Cd was recovered in the final leach fractions that employed HF and mainly dissolve silicates and refractory oxides.Cadmium is susceptible to thermal neutron-capture effects due to the large neutron capture cross-section of 113Cd (∼20,000 barns). We report variations of up to −0.6 ± 0.3 for ε113Cd (internally normalised to 116Cd/111Cd) in bulk chondrites, which renders Cd a potential thermal neutron-capture monitor. Most neutron dosimeters, such as Pt, Os and Hf, are sensitive to neutron capture in the epithermal energy range and have applications mainly limited to lunar samples or iron and stony-iron meteorites. The additional use of Cd, susceptible to neutron capture in the thermal energy range, therefore provides a new tool to determine the exposure histories of stony meteorites in more detail. Our study demonstrates that thermal neutron-capture effects in carbonaceous and enstatite chondrites can produce resolvable effects and require attention when assessing nucleosynthetic isotope variations.

Migration of D-type asteroids from the outer Solar System inferred from carbonate in meteorites

Recent dynamical models of Solar System evolution and isotope studies of rock-forming elements in meteorites have suggested that volatile-rich asteroids formed in the outer Solar System beyond Jupiter's orbit, despite being currently located in the main asteroid belt. The ambient temperature under which asteroids formed is a crucial diagnostic to pinpoint the original location of asteroids and is potentially determined by the abundance of volatiles they contain. In particular, abundances and 13C/12C ratios of carbonates in meteorites record the abundances of carbon-bearing volatile species in their parent asteroids. However, the sources of carbon for these carbonates remain poorly understood. Here we show that the Tagish Lake meteorite contains abundant carbonates with consistently high 13C/12C ratios. The high abundance of 13C-rich carbonates in Tagish Lake excludes organic matter as their main carbon source. Therefore, the Tagish Lake parent body, presumably a D-type asteroid, must have accreted a large amount of 13C-rich CO2 ice. The estimated 13C/12C and CO2/H2O ratios of ice in Tagish Lake are similar to those of cometary ice. Thus, we infer that at least some D-type asteroids formed in the cold outer Solar System and were subsequently transported into the inner Solar System owing to an orbital instability of the giant planets.

Effects of the s-process on Fe-group elements in meteorites

In the present paper we investigate the possible connection between s-process nucleosynthesis occurring during the asymptotic giant branch (AGB) phase of low-mass stars (LMS) and the isotopic anomalies of the “Fe-group” elements observed in several macroscopic samples of meteorites or in grains formed as circumstellar condensates (hereafter CIRCONs). The available measurements of chromium, iron, and nickel are well reproduced by stellar models, which account for the largest shifts in the heaviest isotopes of each element: in particular 54Cr, 58Fe, and 64Ni. Moreover, many circumstellar condensates reflect 50Ti excesses and some production of 46, 47, 49Ti, as predicted by slow-neutron captures in AGB stars. Nevertheless, some difficulties are found in comparing theoretical calculations of s-process nucleosynthesis with calcium, silicon, and zinc isotopic anomalies.

Nucleosynthetic Isotope Variations of Siderophile and Chalcophile Elements in the Solar System.

Numerous investigations have been devoted to understanding how the materials that contributed to the Solar System formed, were incorporated into the precursor molecular cloud and the protoplanetary disk, and ultimately evolved into the building blocks of planetesimals and planets. Chemical and isotopic analyses of extraterrestrial materials have played a central role in decoding the signatures of individual processes that led to their formation. Among the elements studied, the siderophile and chalcophile elements are crucial for considering a range of formational and evolutionary processes. Consequently, over the past 60 years, considerable effort has been focused on the development of abundance and isotopic analyses of these elements in terrestrial and extraterrestrial materials (e.g., Shirey and Walker 1995; Birck et al. 1997; Reisberg and Meisel 2002; Meisel and Horan 2016, this volume). In this review, we consider nucleosynthetic isotopic variability of siderophile and chalcophile elements in meteorites. Chapter 4 provides a review for siderophile and chalcophile elements in planetary materials in general (Day et al. 2016, this volume). In many cases, such variability is denoted as an “isotopic anomaly”; however, the term can be ambiguous because several pre- and post- Solar System formation processes can lead to variability of isotopic compositions as recorded in meteorites. Here we strictly define the term “isotopic anomaly” as referring to an isotopic deviation from the terrestrial composition resulting from the incorporation of varying proportions of elements with diverse nucleosynthetic origins into a meteorite component or parent body. The term will not be used here to refer to isotopic variations that result from mass-dependent isotopic fractionation, radioactive decay in the Solar System, or spallation effects. Based on astronomical observations and physical modelling, the formation of the Solar System has generally been thought to have initiated by the collapse of a dense molecular cloud …

Observations of the chemical composition of stratospheric aerosol particles

AbstractRecent instrumentation can distinguish various types of aerosol particles in the stratosphere and determine their relative abundance. In the background lower stratosphere between major volcanic eruptions, most particles are either relatively pure sulphuric acid, sulphuric acid with material from ablated meteoroids or mixed organic‐sulphate particles that originated in the troposphere. The meteoritic iron and magnesium appear to be dissolved whereas the aluminum and silicon appear to be inclusions. Most stratospheric aerosol mass is liquid sulphuric acid and associated water, but a large fraction of particles contain either inclusions of meteoritic elements such as silicon or organic material that is probably effloresced or glassy. These solid phases could have large but unknown implications for the ability of particles to act as freezing nuclei for polar stratospheric clouds. Internally mixed black carbon is a measurable but very small component of the stratospheric aerosol by mass. Despite their importance for heterogeneous chemistry, there are few quantitative measurements of halogens in stratospheric particles.

Nuclear field shift in natural environments

The nuclear field shift (NFS) is an isotope shift in atomic energy levels caused by a combination of differences in nuclear size and shape and electron densities at the nucleus. The effect of NFS in isotope fractionation was theoretically established by Bigeleisen in 1996 [Bigeleisen J. (1996) J. Am. Chem. Soc. 118:3676–3680] and has been analytically measured in laboratory chemical exchange reactions. More recently, some isotopic variations of heavy elements (Hg, Tl, U) measured in natural systems as well as isotopic anomalies measured for lower-mass elements in meteorites have been attributed to the NFS effect. These isotopic variations open up new and exciting fields of investigations in Earth sciences. In this paper, we review the different natural systems in which NFS has been proposed to be the origin of isotopic variations.

Oxygen and Asteroids

Hundreds of thousands of asteroids have been discovered in the asteroid belt and in near-Earth space. Oxygen is an abundant element in meteorites and presumably in most asteroids. Spectral reflectance measurements of asteroids in the visible and near-infrared can identify oxygen-bearing minerals such as those found in the olivine, pyroxene, and serpentine groups due to their distinctive absorption features. Interpretation of the mineralogy of asteroids is complicated by the effects of space weathering, which tends to redden and darken the surfaces of asteroids. Asteroids are primarily classified into a number of taxonomic classes and subclasses according to their spectral properties in the visible wavelength region. However, asteroids with similar spectral properties in the visible may have different spectral properties in the near-infrared and, therefore, different interpreted mineralogies. Definite trends in the abundances of different taxonomic classes versus heliocentric distance are apparent. These trends appear to be a function of both composition differences and degree of heating. However, dynamical processes have significantly affected these trends. Close observation of asteroids by spacecraft, such as NEAR-Shoemaker to 433 Eros and Hayabusa to 25143 Itokawa, are currently the best way to discern the mineralogies of individual asteroids.

Ultra-Sensitive Elemental Analysis Using Plasmas. Age Determination of Meteorites Using Radioactive Nuclides.

Recently, the precise isotope ratios of some refractory elements in meteorites have been reported using inductively coupled plasma mass spectrometry. The in situ decay of 182Hf (T1/2=9 Myr), which was produced at the latest nucleosynthesis, is recognized in many meteorites as isotopic anomalies of its daughter isotope, 182W. The degrees of relative 182W isotopic deviation in extra-terrestrial and terrestrial silicate samples vary from +0.3 % to ?-?0 % related to the size of their parent bodies. One ready interpretation of its correlation is the difference in timing of metal-silicate separation in the parent bodies. Between the earth and meteorite parent bodies, the difference is calculated to be about four times of the half-life of182Hf, equivalent to 36 Myr.

MARTIAN VOLATILES: ISOTOPIC COMPOSITION, ORIGIN, AND EVOLUTION

Information about the composition of volatiles in the Martian atmosphere and interior derives from Viking spacecraft and ground-based measurements, and especially from measurements of volatiles trapped in Martian meteorites, which contain several distinct components. One volatile component, found in impact glass in some shergottites, gives the most precise measurement to date of the composition of Martian atmospheric Ar, Kr, and Xe, and also contains significant amounts of atmospheric nitrogen showing elevated 15N/14N. Compared to Viking analyses, the 36Ar/132Xe and 84Kr/132Xe elemental ratios are larger in shergottites, the 129Xe/132Xe ratio is similar, and the 40Ar/36Ar and 36Ar/38Ar ratios are smaller. The isotopic composition of atmospheric Kr is very similar to solar Kr, whereas the isotopes of atmospheric Xe have been strongly mass fractionated in favor of heavier isotopes. The nakhlites and ALH84001 contain an atmospheric component elementally fractionated relative to the recent atmospheric component observed in shergottites. Several Martian meteorites also contain one or more Martian interior components that do not show the mass fractionation observed in atmospheric noble gases and nitrogen. The D/H ratio in the atmosphere is strongly mass fractionated, but meteorites contain a distinct Martian interior hydrogen component. The isotopic composition of Martian atmospheric carbon and oxygen have not been precisely measured, but these elements in meteorites appear to show much less variation in isotopic composition, presumably in part because of buffering of the atmospheric component by larger condensed reservoirs. However, differences in the oxygen isotopic composition between meteorite silicate minerals (on the one hand) and water and carbonates indicate a lack of recycling of these volatiles through the interior. Many models have been presented to explain the observed isotopic fractionation in Martian atmospheric N, H, and noble gases in terms of partial loss of the planetary atmosphere, either very early in Martian history, or over extended geological time. The number of variables in these models is large, and we cannot be certain of their detailed applicability. Evolutionary data based on the radiogenic isotopes (i.e., 40Ar/36Ar, 129Xe/132Xe, and 136Xe/132Xe ratios) are potentially important, but meteorite data do not yet permit their use in detailed chronologies. The sources of Mars’ original volatiles are not well defined. Some Martian components require a solar-like isotopic composition, whereas volatiles other than the noble gases (C, N, and H2O) may have been largely contributed by a carbonaceous (or cometary) veneer late in planet formation. Also, carbonaceous material may have been the source of moderate amounts of water early in Martian history.

Cosmochemical constraints on the sulfur content in the Earth's core

The density of the Earth's core (32.5% of the mass of the Earth) is about 8% less than that of pure metallic Fe under similar P, T conditions, requiring the presence of a substantial amount of a light element or light elements in the core ( McQueen and Marsh, 1960, 1966). Sulfur is a good candidate for reducing the density. On the basis of shock wave experiments, Ahrens (1979) estimated that 9–12% S in the core would be sufficient to account for the observed density difference. This corresponds to a S-content of 2.9–3.9% for the bulk Earth. Here we present an estimate of the bulk Earth S-content which is based on the volatility of S in the solar nebula and the general depletion of volatile elements in meteorites and in the Earth. It is suggested that the CI-normalized S-abundance of the bulk Earth is similar to, or even lower than, the CI-normalized abundance of Zn, an element of similar volatility as S in the reducing environment of the solar nebula. Since the Zn content of the mantle of the Earth is well known and since the core is most likely free of Zn, the bulk Earth S-content can be calculated from the Zn-abundance of the mantle. A maximum bulk Earth S-content of 0.56% and a corresponding maximum S-content of the core of 1.7% are estimated. The S-content of the mantle is so low that the contribution to the bulk Earth S-content is negligible. The upper limit of S in the core as derived from cosmochemical constraints is much too low to produce the required decrease in the core's density. At least one other light element is needed.

Use of platinum-group elements for impactor identification: Terrestrial impact craters and Cretaceous-Tertiary boundary

This study presents the abundances of Ru, Ir, Pt, Pd, Rh, and Au (platinum-group elements; PGE) in the ejecta layer at the Cretaceous-Tertiary boundary and melt rocks from East Clearwater Lake, Brent, New Quebec, and Lake Wanapitei impact craters, all believed to have been formed by the impact of chondritic projectiles. The objective was to evaluate the pure application of PGE interelement ratios (e.g., Ru Ir ) for impactor identification, to compare the utility of both melt rocks and ejecta deposits for this purpose, and to expand the PGE database for these craters. The similarity of PGE interelement ratios within the chondrite and iron meteorite groups precludes their use in the fine classification of impactors of this composition. Introduction of indigenous PGE from country rock may contaminate the original meteoritic signature. In addition, the extent of PGE fractionation during impact and melt sheet cooling and of post-sedimentary PGE remobilization must be evaluated in each case. For East Clearwater, the PGE chondrite-normalized plot is flat, indicating a chondritic impactor. Based on Ru Ir and Cr Ir ratios, a Cl- or L-chondrite is most likely. However, variation of ratios with depth in the core makes any fine classification of the projectile tenuous. For the Brent and New Quebec craters, the flat chondrite-normalized PGE plot indicates a chondritic impactor, but finer classification is precluded by possible post/syn depositional PGE fractionation. At Lake Wanapitei, Ni Cr and Cr Ir ratios indicate a LL-chondrite impactor ( Wolf et al., 1980) but sparse PGE data for this chondrite-type does not allow further classification of the projectile. A flat chondrite-normalized PGE plot indicates a chondritic rather than an iron impactor at Lake Wanapitei. The global ejecta layer at the Cretaceous-Tertiary boundary has a chondritic PGE signature, but only when the integrated flux of PGE on a global scale is considered. Individual sites may not show chondritic ratios due to differential PGE remobilization resulting from local differences in depositional environment (continental vs. marine), and to vaporization and/or condensation PGE fractionation occurring during impact. The results for melt rocks and the K-T ejecta deposit indicate that PGE can be used for impactor identification but primarily in combination with other siderophile/chalcophile meteoritic elements and their ratios.

Lunar highland meteorites and the composition of the lunar crust

Major, minor, and trace element data obtained by neutron activation techniques and by spark source mass spectrometry (SSMS) on two lunar meteorites MAC88104 and MAC88105 are reported. Both MAC samples were also analysed for their contents and isotopic compositions of rare gases. Additional SSMS-data were obtained on four lunar highland meteorites previously found in Antarctica: ALHA81005, Y791197, Y82192, and Y86032. MAC88104 and MAC88105 are very similar in chemistry, suggesting that they are pieces of a single fall event. The bulk chemical composition of MAC88104/5 is not very different from the other lunar highland meteorites: highly aluminous with relatively low contents of REE and siderophile element concentrations slightly above 1% of a CI-chondritic level. However, mafic element concentrations (Mg, Cr, Mn, etc.) are slightly lower in MAC88104/5 than in the other lunar highland meteorites. The contents of solar rare gases in the two MAC samples are low, indicating only a small regolith contribution in agreement with rare petrographically identifiable regolith components. The MAC samples and also Y82192 and Y86032 are classified as fragmental breccias with negligible regolith components, in contrast to ALHA81005 and Y791197 which are regolith breccias with high solar wind derived rare gas contents. There is no correlation among lunar meteorites between peak shock pressures and solar gas contents, indicating that peak shock pressures of up to 25 GPa do not lead to gas loss. A low 26Al activity ( Vogt et al., 1990) and high contents of cosmogenic rare gases in MAC88104/5 suggest a long exposure (400,000 years) in the lunar sub-surface. K-Ar ages are in excess of 3.9 by. Lunar highland meteorites and compositionally similar granulitic rocks from the Apollo 16 and 17 landing sites contain about 1% of a CI-chondritic component, according to siderophile and volatile element contents, but independent of the amount of regolith components. Apparently, the major fraction of meteoritic elements in these rocks was not provided by micrometeorites impacting the regolith. The abundances of siderophile (e.g., Ir) and volatile elements may therefore reflect the last spike of accretion of the Moon after the formation of the anorthositic crust. Lunar meteorites of highland origin are chemically different from the bulk of the Apollo 16 highland samples in having higher Fe Mg ratios and lower contents and less fractionated patterns of incompatible and siderophile elements. Since lunar highland meteorites are associated with at least three but probably four different fall events, and since they are not derived from chemically exotic front-side terranes, they may represent a better sampling of the average chemical composition of the lunar crust than previous estimates based on returned lunar samples and remote sensing data. A comparison between an average highland composition derived by Taylor (1982) and an estimate based on lunar highland meteorites shows that the Taylor composition contains higher concentrations and more fractionated incompatible elements mainly because of a substantial amount of KREEP (a trace element rich, highly fractionated component from the front side of the Moon not present at the sites from which the lunar meteorites come).

Tungsten isotope ratio determinations by negative thermal ionization mass spectrometry

A precise determination of the isotopic abundances of tungsten with natural isotopic composition is presented. WO − 3 ions are generated by negative thermal ionization (NTI) in a double-filament ion source. La 2O 3 is used as a chemical substance to reduce the electron work function of the rhenium filament material. An ionization efficiency of 1% is obtained for sample loadings of 100 ng. The isotopic abundances are measured with relative standard deviations of 0.2% for the least abundant 180W isotope and 0.02–0.004% for the other tungsten isotopes. These improved isotopic data are used to recalculate the atomic weight of tungsten as 183.8417 ± 0.0001. The new NTI technique is an ideal tool for the application of isotope dilution mass spectrometry to analyse tungsten traces and for the measurement of isotopic shifts of this element in meteorites produced by the decay of 182Hf.

Cretaceous-Tertiary boundary event: Evidence for a short time scale

Three non-meteoritic trace elements (Sb, As and Zn) are strongly enriched at eleven K-T boundary sites, along with mainly or partly meteoritic elements (Ir, Ni, Cr, Fe and Co). The proportions (As, Sb, Zn/Ir) are remarkably constant over a ≈ 100-fold range in concentration. This correlation persists in sub-layers of boundary clay and even extends to soot (from burned land biomass). Apparently, all the components, despite their diverse origins, became associated in a single, global component prior to deposition. No wholly satisfactory source is available for As, Sb and Zn: the trace element pattern in volcanic gases does not match that in KT boundary clay, with ratios to Ir falling short by 1 to 2 orders of magnitude, terrestrial rocks do not reach high enough concentrations and (modern) ocean water contains too little Zn, but on balance, the latter source seems preferable—perhaps augmented by volatiles from the impact crater. Apparently, Ir-bearing ejecta and soot from forest fires coagulated in the stratosphere and then fell out together, sweeping out oceanic biomass and anoxically precipiting As, Sb and Zn. Significantly, the amounts of marine and land biomass at the KT boundary correspond to about the steady-state global inventory (1 generation), and the amounts of As, Sb and Zn are also within a factor of ≈5 of the global inventory. This is expected in a catastrophic but not a gradualist scenario.

Isotopic variations in the rock-forming elements in meteorites

Variations in isotopic abundances of the major rock-forming elements can be used as tracers for chemical processes in the solar nebula, and may also provide links to the presolar cloud from which the solar nebula was derived. Emphasis in this paper is placed on the correlation of isotopic variations between pairs of elements, both for mass-dependent fractionation effects and for nucleosynthetic effects. Variations in oxygen isotope abundances, which are ubiquitous in all Solar System matter, are decoupled from those in other elements, probably because of the effect of a large oxygen reservoir in the nebular gas. Among the metallic rock-forming elements (Mg, Si, Ca, Ti, Cr, Fe, Ni) isotopic variations are small to immeasurable in ordinary chondrites and achondrites. Large variations are observed in refractory inclusions in carbonaceous chondrites in the elements Mg, Si, Ca and Ti. Fractionation effects result from evaporation and condensation at high temperatures. The dominant nucleosynthetic effects are seen as excesses and deficiencies of the neutron-rich isotopes in the region of the iron abundance peak: 48 Ca, 49 Ti, 50 Ti, 54 Cr, 64 Ni. These effects result from mixing in different proportions of the products of different regions of a supernova. The rock-forming elements also show isotopic variations due to extinct radioactivities: 26 Mg, 53 Cr and 41 K. A local source is likely, as is heterogeneous distribution within the solar nebula.

Global trace-element biogeochemistiry at the K/T boundary: Oceanic and biotic response to a hypothetical meteorite impact

Calculations of trace-element fluxes to the ocean associated with a hypothetical 10-km-diameter Cl chondrite indicate that the mass of certain elements contained in the bolide, such as Fe, Co, Mn, Al, Zn, Ni, Ge, Cr, Sn, Cd, Pb, Ag, Cu, Hg, and Se, are comparable to or larger than the world ocean burden. Consequently, the introduction of meteoritic elements, in the form of atmospheric aerosol particles, to the surface ocean after a large meteor impact, concurrent with various element-specific solubilities, could rapidly alter the geochemical balance of some oceanic trace elements. We believe that this hypothetical pulse of trace elements would have been of sufficient magnitude to perturb the biogeochemical cycles operative at 66.4 Ma, a possible time of meteorite impact. Through direct exposure and bioaccumulation, many trophic levels of the marine food chain would have been adversely affected by these meteoritic elements. Those organisms residing in the surface ocean would be exposed to high prolonged concentrations of toxic trace elements. Preferential extinctions as driven by species-specific tolerance of elevated trace-element levels in the ocean would occur. The trace-element aerosol flux associated with K/T boundary volcanism may also have contributed to elevated trace-element concentrations in the world ocean. The trace-element toxification hypothesis postulates a mechanism that may have contributed to the selective extinction of marine organisms during the enigmatic terminal Mesozoic event.

Some parameters involved in the interpretation of meteorite ages: A review of the present status

In obtaining fission-track ages for meteorites, the effects of cosmic-ray primaries and induced fission must be taken into account. The presence of 244Pu tracks allows the cooling rates of meteorite parent bodies to be determined with good time-resolution. Chondrites and IA irons cooled at rates of a few degrees K per Myr and probably originated in bodies of ∼ 100 km in radius. The mesosiderates cooled rather more slowly. No fission-track evidence for the existence of extinct superheavy elements in meteorites has yet been obtained.