Chondrite Data Research Articles

Fluid flow and chemical alteration in carbonaceous chondrite parent bodies

Carbonaceous chondrites are a class of primitive meteorites that likely represent early solar system materials, thus providing critical insights about planetary evolution. However, evidence shows that they have undergone considerable processing. Elucidating the complex formation history of aqueously altered chondrite groups has been a principal objective of cosmochemistry. Fulfilling this objective requires understanding the nature of the water–rock interactions involved. Many current alteration models are generally founded upon an assumption of uniform alteration in a closed system. In these models, different chondrite groups must originate on separate parent bodies. Contrary to this common view, we show that chondrite data are consistent with non-uniform alteration in a convecting system so that multiple chondrite groups can form within a single body.

Rhodium, gold and other highly siderophile element abundances in chondritic meteorites

The abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, Rh, Pd and Au, and 187Os/ 188Os isotope ratios have been determined for a set of carbonaceous, ordinary, enstatite and Rumuruti chondrites, using an analytical technique that permits the precise and accurate measurement of all HSE from the same digestion aliquot. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS analysis. The monoisotopic elements Rh and Au were quantified relative to the abundance of Ir. Differences in HSE abundances and ratios such as Re/Os, 187Os/ 188Os, Pd/Ir and Au/Ir between different chondrite classes are further substantiated with new data, and additional Rh and Au data, including new data for CI chondrites. Systematically different relative abundances of Rh between different chondrite classes are reminiscent of the behaviour of Re. Carbonaceous chondrites are characterized by low average Rh/Ir of 0.27 ± 0.03 (1s) which is about 20% lower than the ratio for ordinary (0.34 ± 0.02) and enstatite chondrites (EH: 0.33 ± 0.01; EL: 0.32 ± 0.01). R chondrites show higher and somewhat variable Rh/Ir of 0.37 ± 0.07. Well-defined linear correlations of HSE, in particular for bulk samples of ordinary and EL chondrites, are explained by binary mixing and/or dilution by silicates. The HSE carriers responsible for these correlations have a uniform chemical composition, indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components. Excepting Rumuruti chondrites and Au in carbonaceous chondrites, these correlations also suggest that metamorphism, alteration and igneous processes had negligible influence on the HSE distribution on the bulk sample scale. Depletion patterns for Rh, Pd and Au in carbonaceous chondrites other than CI are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites. Fractionated HSE abundance patterns of ordinary, enstatite and Rumuruti chondrites, however, are more difficult to explain. Fractional condensation combined with the removal of metal phases at various times, and later mixing of early and late formed metal phases may provide a viable explanation. Planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the HSE abundance patterns of chondrite groups. HSE abundances of some, but not all Rumuruti chondrites may be consistent with solid sulphide–liquid sulphide fractionation processes during impact induced melting.

Silver isotope variations in chondrites: Volatile depletion and the initial 107Pd abundance of the solar system

The extinct radionuclide 107Pd decays to 107Ag (half-life of 6.5 Ma) and is an early solar system chronometer with outstanding potential to study volatile depletion in the early solar system. Here, a comprehensive Ag isotope study of carbonaceous and ordinary chondrites is presented. Carbonaceous chondrites show limited variations ( ε 107Ag = −2.1 to +0.8) in Ag isotopic composition that correlate with the Pd/Ag ratios. Assuming a strictly radiogenic origin of these variations, a new initial 107Pd/ 108Pd of 5.9 (±2.2) × 10 −5 for the solar system can be deduced. Comparing the Pd–Ag and Mn–Cr data for carbonaceous chondrites suggests that Mn–Cr and Pd–Ag fractionation took place close to the time of calcium–aluminium-rich inclusion (CAI) and chondrule formation ∼4568 Ma ago. Using the new value for the initial 107Pd abundance, the revised ages for the iron-rich meteorites Gibeon (IVA, 8.5 +3.2/−4.6 Ma), Grant (IIIAB, 13.0 +3.5/−4.9 Ma) and Canyon Diablo (IA, 19.5 +24.1/−10.4 Ma) are consistent with cooling rates and the closure temperature of the Pd–Ag system. In contrast to carbonaceous chondrites, ordinary chondrites show large stable isotope fractionation of order of 1 permil for 107Ag/ 109Ag. This indicates that different mechanisms of volatile depletion were active in carbonaceous and ordinary chondrites. Nebular processes and accretion, as experienced by carbonaceous chondrites, did not led to significant Ag isotope fractionation, while the significant Ag isotope variations in ordinary chondrites are most likely inflicted by open system parent body metamorphism.

Reclassification and thermal history of Trenzano chondrite

Abstract— We present a new single‐crystal X‐ray diffraction (XRD) study performed on a suite of six orthopyroxene grains from the low‐shocked H6 Trenzano meteorite. The quenched intracrystalline Fe2+‐Mg ordering state in orthopyroxene preserves the memory of the cooling rate near closure temperature Tc, thus yielding useful constraints on the last thermal event undergone by the host rock. The orthopyroxene Tc of 522 ± 13 °C, calculated using a new calibration equation obtained by Stimpfl (2005b), is higher than in previously published H chondrite data. The orthopyroxene cooling rate at this Tc is about 100 °C/kyr. This fast rate is inconsistent with the much slower cooling rate expected for H6 in the onion shell structural and thermal model of chondrite parent bodies. A petrographic study carried out at the same time indicated that the Trenzano meteorite is an H5 chondrite and not an H6 chondrite, as it is officially classified. Furthermore, the two‐pyroxene equilibrium temperature of Trenzano (824 ± 24 °C), calculated with QUILF95, is similar to the two‐pyroxene temperature of 750–840 °C obtained for the Carcote (H5) chondrite (Kleinschrot and Okrusch 1999).

Platinum–osmium isotope evolution of the Earth’s mantle: Constraints from chondrites and Os-rich alloys

Separation of a metal-rich core strongly depleted the silicate portion of the Earth in highly siderophile elements (HSE), including Pt, Re, and Os. To address the issues of how early differentiation, partial melting, and enrichment processes may have affected the relative abundances of the HSE in the upper mantle, 187Os/ 188Os and 186Os/ 188Os data for chondrites are compared with data for Os-rich alloys from upper mantle peridotites. Given that 187Os and 186Os are decay products of 187Re and 190Pt, respectively, these ratios can be used to constrain the long-term Re/Os and Pt/Os of mantle reservoirs in comparison to chondrites. Because of isotopic homogeneity, H-group ordinary and other equilibrated chondrites may be most suitable for defining the initial 186Os/ 188Os of the solar system. The 186Os/ 188Os ratios for five H-group ordinary chondrites range only from 0.1198384 to 0.1198408, with an average of 0.1198398 ± 0.0000016 (2 σ). Using the measured Pt/Os and 186Os/ 188Os for each chondrite, the calculated initial 186Os/ 188Os at 4.567 Ga is 0.1198269 ± 0.0000014 (2 σ). This is the current best estimate for the initial 186Os/ 188Os of the bulk solar system. The mantle evolution of 186Os/ 188Os can be defined via examination of mantle-derived materials with well-constrained ages and low Pt/Os. Two types of mantle-derived materials that can be used for this task are komatiites and Os-rich alloys. The alloys are particularly valuable in that they have little or no Re or Pt, thus, when formed, evolution of both 187Os/ 188Os and 186Os/ 188Os ceases. Previously published results for an Archean komatiite and new results for Os-rich alloys indicate that the terrestrial mantle evolved with Pt–Os isotopic systematics that were indistinguishable from the H-group ordinary and some enstatite chondrites. This corresponds to a Pt/Os of 2.0 ± 0.2 for the primitive upper mantle evolution curve. This similarity is consistent with previous arguments, based on the 187Os/ 188Os systematics and HSE abundances in the mantle, for a late veneer of materials with chondritic bulk compositions controlling the HSE budget of the upper mantle. It is very unlikely that high pressure metal–silicate segregation leading to core formation can account for the elemental and isotopic compositions of HSE in the upper mantle.

Thuathe, a new H4/5 chondrite from Lesotho: History of the fall, petrography, and geochemistry

Abstract— On July 21, 2002, a meteorite fall occurred over the Thuathe plateau of western Lesotho. The well‐defined strewn field covers an area of 1.9 times 7.4 km. Many of the recovered specimens display a brecciated texture with leucocratic, angular to subrounded clasts in a somewhat darker groundmass. Mineralogical and chemical data, as well as oxygen isotopic analysis, indicate that Thuathe is an H4/5, S2/3 meteorite, with local H3 or H6 character. A number of anomalous features include somewhat high Co contents of kamacite and taenite relative to normal H‐group chondrites. Oxygen isotopic data plot at the edge of the normal H chondrite data field. Variable contents of metallic mineral phases and troilite result in a heterogeneous bulk composition (e.g., with regard to Si, Fe, and Mg), resulting in a spread of major element ratios that is not consistent with previously accepted H‐group composition. Trace element abundances are generally consistent with H chondritic composition, and Kr and Xe isotopic data agree with an H4 classification for this meteorite. Noble gas analysis gave U, Th‐4He gas retention and K‐Ar ages typical for H chondrites; no major thermal event affected this material since ∼3.7 Ga. The exposure age for Thuathe is 5 Ma, somewhat lower than for other H chondrites. Cosmogenic nuclide analysis indicates a pre‐atmospheric radius of this meteorite between 35 and 40 cm. In the absence of evidence for solar gases, we classify Thuathe as a fragmental breccia.Numerous narrow, black veins cut across samples of Thuathe and are the result of a brittle deformation event that also caused local melting, especially in portions rich in sulfide. The formation of these veinlets is not the result of locally enhanced shock pressures (i.e., of shock melting) but rather of shearing under brittle conditions with local, friction‐related temperature excursions causing melting mostly of Fe‐sulfide and FeNi‐metal but also, locally, of silicate minerals. Frictional temperature excursions must have attained values in excess of 1500 °C to permit complete melting of forsteritic olivine.

Carbon and nitrogen isotope ratios in meteoritic organic matter: indicators of alteration processes on the parent asteroid

Macromolecular organic materials in chondrites display significant variations in carbon and nitrogen stable isotopes. In recent years, these variations have been interpreted as a record of aqueous and thermal processing on asteroids shortly after the birth of the Solar System. In this paper we review and summarize the key data and main interpretative approaches related to this study area. Armed with these methods we attempt to reinterpret the whole rock chondrite data set in the literature.

182Hf-182W isotope systematics of chondrites, eucrites, and martian meteorites: Chronology of core formation and early mantle differentiation in Vesta and Mars

Abstract The timescale of accretion and differentiation of asteroids and the terrestrial planets can be constrained using the extinct 182Hf-182W isotope system. We present new Hf-W data for seven carbonaceous chondrites, five eucrites, and three shergottites. The W isotope data for the carbonaceous chondrites agree with the previously revised 182W/184W of chondrites, and the combined chondrite data yield an improved ϵW value for chondrites of −1.9 ± 0.1 relative to the terrestrial standard. New Hf-W data for the eucrites, in combination with published results, indicate that mantle differentiation in the eucrite parent body (Vesta) occurred at 4563.2 ± 1.4 Ma and suggest that core formation took place 0.9 ± 0.3 Myr before mantle differentiation. Core formation in asteroids within the first ∼5 Myr of the solar system is consistent with the timescales deduced from W isotope data of iron meteorites. New W isotope data for the three basaltic shergottites EETA 79001, DaG 476, and SAU 051, in combination with published 182W and 142Nd data for Martian meteorites reveal the preservation of three early formed mantle reservoirs in Mars. One reservoir (Shergottite group), represented by Zagami, ALH77005, Shergotty, EETA 79001, and possibly SAU 051, is characterized by chondritic 142Nd abundances and elevated ϵW values of ∼0.4. The 182W excess of this mantle reservoir results from core formation. Another mantle reservoir (NC group) is sampled by Nakhla, Lafayette, and Chassigny and shows coupled 142Nd-182W excesses of 0.5–1 and 2–3 ϵ units, respectively. Formation of this mantle reservoir occurred 10–20 Myr after CAI condensation. Since the end of core formation is constrained to 7–15 Myr, a time difference between early silicate mantle differentiation and core formation is not resolvable for Mars. A third early formed mantle reservoir (DaG group) is represented by DaG 476 (and possibly SAU 051) and shows elevated 142Nd/144Nd ratios of 0.5–0.7 ϵ units and ϵW values that are indistinguishable from the Shergottite group. The time of separation of this third reservoir can be constrained to 50–150 Myr after the start of the solar system. Preservation of these early formed mantle reservoirs indicates limited convective mixing in the Martian mantle as early as ∼15 Myr after CAI condensation and suggests that since this time no giant impact occurred on Mars that could have led to mantle homogenization. Given that core formation in planetesimals was completed within the first ∼5 Myr of the solar system, it is most likely that Mars and Earth accreted from pre-differentiated planetesimals. The metal cores of Mars and Earth, however, cannot have formed by simply combining cores from these pre-differentiated planetesimals. The 182W/184W ratios of the Martian and terrestrial mantles require late effective removal of radiogenic 182W, strongly suggesting the existence of magma oceans on both planets. Large impacts were probably the main heat source that generated magma oceans and led to the formation metallic cores in the terrestrial planets. In contrast, decay of short-lived 26Al and 60Fe were important heat sources for melting and core formation in asteroids.

Lu–Hf and Sm–Nd isotopic systematics in chondrites and their constraints on the Lu–Hf properties of the Earth

Sm–Nd and Lu–Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm–Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean 147Sm/ 144Nd of 0.1960±0.0005, and a mean 143Nd/ 144Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm–Nd reference parameters, 147Sm/ 144Nd=0.1966 and 143Nd/ 144Nd=0.512638 (when fractionation-corrected to 146Nd/ 144Nd=0.7219).Lu–Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of 176Lu/ 177Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10 −11 year −1 is used for the decay constant of 176Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311–324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with 176Lu/ 177Hf less than 0.030 are employed, no combination of the main population of chondrite Lu–Hf data yields a regression with sufficiently low error to constrain the decay constant of 176Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu–Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu–Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average 176Lu/ 177Hf and 176Hf/ 177Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu–Hf characteristics results in a BSE Hf–Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial ε Hf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu–Hf.

Ordinary Chondrites: 2. Diffusion Losses of Rare Gases

Data on the isotopic abundances and ratios of light rare gases (He and Ne) in 600 ordinary chondrites are analyzed. The ratio of cosmogenic isotopes (3He/21Ne) c in ∼ 20% of the ordinary chondrites has been found to lie well below the correlation line that represents the dependence of (3He/21Ne) c on (22Ne/21Ne) c . This effect shows up most clearly in 4He r -chondrites, particularly in meteorites with diffusion losses of radiogenic 21Ne c , and is most likely attributable to the predominant (compared to 3He c ) diffusion losses of cosmogenic 3Hecthrough the solar heating of meteorites in orbits with small perihelion distances. This effect is enhanced by periodic variations in orbital parameters (including the perihelion distance) of meteorites throughout their exposure histories. Thermoluminescence data for ordinary chondrites confirm this scenario. The (3He/21Ne) c ratio for ∼15% of the chondrites was significantly overestimated, which may stem from the fact that such meteorites were heavily shielded in preatmospheric bodies.

Phosphate control on the thorium/uranium variations in ordinary chondrites: Improving solar system abundances

Abstract— Isotope dilution thorium and uranium analyses by inductively‐coupled plasma mass spectrometry of 12 samples of Harleton (L6) show a much larger scatter than was previously observed in equilibrated ordinary chondrites. Th/U linearly correlates with 1/U in Harleton and in the total equilibrated ordinary chondrite data set as well. Such a correlation suggests a two component mixture and this trend can be quantitatively modeled as reflecting variations in the mixing ratio between two phosphate phases: chlorapatite and merrillite. The major effect is due to apatite variations, which strongly control the whole rock U concentrations. Phosphorous variations will tend to destroy the Th/U vs. 1/U correlation, and measured P concentrations on exactly the same samples as U and Th show a factor of 3 range. It appears that the P variations are compensated by inverse variations in U (a dilution effect) to preserve the Th/U vs. 1/U correlation. Because variations in whole rock Th/U are consequences of phosphate sampling, a weighted average of high accuracy Th/U measurements in equilibrated ordinary chondrites should converge to a significantly improved average solar system Th/U. Our best estimate of this ratio is 3.53 with σmean = 0.10.

Niobium and tantalum in carbonaceous chondrites: Constraints on the solar system and primitive mantle niobium/tantalum, zirconium/niobium, and niobium/uranium ratio

Abstract— We have determined Nb, Y, and Zr abundances in the carbonaceous chondrites Orgueil (CI), Murray (CM2), Murchison (CM2), Allende (CV3), and Karoonda (CK4), and in the eucrites, Pasamonte and Juvinas, by a recently developed spark source mass spectrometric technique using multiple ion counting (MIC‐SSMS). The abundance of Ta was determined in the same meteorites by radiochemical neutron activation analysis (RNAA). Precision of the MIC‐SSMS and RNAA techniques is ∼3% and ≤ 5%, respectively.The new abundances for CI chondrites are: Nb = 0.247, Ta = 0.0142, Zr = 3.86, Y = 1.56 μg/g; or 0.699, 0.0202, 11.2, and 4.64 atoms/106 Si atoms, respectively. The values agree with earlier compilations, but they are a factor of 2 more precise than earlier analyses. Trace element concentrations in the CM, CV, and CK chondrites are higher than in the CI chondrite Orgueil by about 37, 86, and 120%, respectively, in agreement with the variable absolute contents of refractory lithophile elements in different groups of carbonaceous chondrites.Of particular interest are the chondritic Nb/Ta, Zr/Nb, and Nb/U ratios, because these ratios are important tools for interpreting the chemical evolution of planetary bodies. We obtained Nb/Ta = 17.4 ± 0.5 for the carbonaceous chondrites and the Juvinas‐type eucrites investigated. Though this value is similar to previous estimates, it is much more precise. The same is true for Zr/Nb (15.5 ± 0.2) and Zr/Y (2.32 ± 0.12). In combination with recently published MIC‐SSMS U data for carbonaceous chondrites, we obtained a chondritic Nb/U ratio of 29 ± 2.Because Nb, Ta, Zr, Y, and U are refractory lithophile elements and presumably partitioned into the silicate phase of the Earth during core formation, the elemental ratios may also be used to constrain evolution of the Earth's primitive mantle and, with the more precise determinations fractionation of Nb and Ta during magmatic processes and mantle‐crust interactions, can now be interpreted with greater confidence.

Re-Os systematics in chondrites and the fractionation of the platinum group elements in the early solar system

We have investigated the Re-Os system for samples of whole rock, metal, and sulfide from ordinary chondrites. Using closed-system analytical techniques, we found complete exchange between sample and tracer isotopes for silicate-containing samples and obtained precise and reliable Re-Os concentration measurements. Results on two Group IVA iron meteorites and on a silicate-rich iron (Steinbach, IVA-AN) are consistent with the IVA-IVB isochron and support the previous observation that IVA-IVB irons may be slightly older than IIAB irons. Data on whole-rock fragments and metal-rich separates from the St. Séverin chondrite (LL6) show a large range in 187Re/ 188Os and in 187Os/ 188Os, which makes possible, in principle, the determination of a Re-Os internal isochron on a chondrite, for the first time. This Re-Os fractionation may be due to partial melting of FeNiS and macroscopic redistribution of metal and sulfide. The St. Séverin data show a good correlation line on a 187Re- 187Os evolution diagram. If this is considered to represent an internal isochron, it gives an age T = 4.68 ± 0.15 AE [λ( 187Re) = 1.64 × 10 −11 a −1] and an initial ( 187Os/ 188Os) 0 = 0.0953 ± 0.0013. This age is in agreement with but slightly older than the more precise 187Re- 187Os age for the IIAB irons as well as for irons from other groups (T = 4.61 ± 0.01 AE). A St. Séverin sulfide nodule has very low Re and Os concentrations and shows a young Re-Os model age (2.3 AE), indicating recent element remobilization. Whole rock and metal-rich separates of H-Group chondrites (H3 to H6) yield restricted ranges in 187Re/ 188Os (0.42–0.47) and 187Os/ 188Os (0.128–0.133). There is a systematic difference between Re/Os in the metal extracted from a chondrite and the bulk chondrite. This shows that there is a small but significant Re-Os fractionation within subsystems contained in the chondrites. From whole rock samples of H Group chondrites we calculate a mean 187Re/ 188Os = 0.423 ± 0.007 and 187Os/ 188Os = 0.12863 ± 0.00046, which may characterize the evolution of an average chondritic reservoir for Re-Os. The ordinary chondrite data plot close to the IIAB isochron, although the deviations found are larger than found for the irons. The Re-Os chronometer in iron meteorites is apparently controlled by the Re-Os fractionation due to fractional crystallization of liquid metal. Re-Os ages of iron meteorites give the time of crystallization of metal segregations and cores of early planetary bodies. In contrast, the behavior in ordinary chondrites, while also dominated by the metal phases, must reflect fractionation and transport on a local macroscopic scale within the chondrites between the metal phases after aggregation, due to partial melting of FeNiS or represent variable Re-Os fractionation of the metal phases prior to the accretion of the chondrites. However, for St. Séverin, we attribute the major Re-Os fractionation to early heating of the meteorite, above the Fe-FeS eutectic. We do not consider that the Re-Os fractionation observed in other chondrites is due to the redistribution of Re and Os during chondrite metamorphism (including shock) but it may plausibly represent earlier stages of Re-Os fractionation for the different FeNi metal constituents prior to accretion.

Hafnium–tungsten chronometry and the timing of terrestrial core formation

THE accretion of the Earth and Moon within the solar nebula is thought1–3 to have taken 50 to 100 million years. But the timing of formation of the Earth's core has been controversial, with some4,5 proposing that it took place within the first 15 Myr of Earth's accretion history and others6,7 proposing that it occurred after 50 Myr of accretion. Meteorite chronometry based on the 182Hf–182W system has the potential to resolve this debate, as segregation of a metal core from silicates should induce strong fractionation of hafnium from tungsten. Here we report tungsten isotope compositions for two iron meteorites, two carbonaceous chondrites, and a lunar mare basalt. We see clear 182W deficits in both iron meteorites, in agreement with previous results4,5. But the data for chondrites are inconsistent with the hypothesis of early core formation, suggesting that both this event and the formation of the Moon must have occurred at least 62 ± 10 Myr after the iron meteorites formed.

The Mode Tree: A Tool for Visualization of Nonparametric Density Features

Abstract Recognition and extraction of features in a nonparametric density estimate are highly dependent on correct calibration. The data-driven choice of bandwidth h in kernel density estimation is a difficult one that is compounded by the fact that the globally optimal h is not generally optimal for all values of x. In recognition of this fact a new type of graphical tool, the mode tree, is proposed. The basic mode tree plot relates the locations of modes in density estimates with the bandwidths of those estimates. Additional information can be included on the plot indicating factors such as the size of modes, how modes split, and the locations of antimodes and bumps. The use of a mode tree in adaptive multimodality investigations is proposed, and an example is given to show the value in using a normal kernel, as opposed to the biweight or other kernels, in such investigations. Examples of such investigations are provided for Ahrens's chondrite data and van Winkle's Hidalgo stamp data. Finally, the bivariate mode tree is introduced, together with an example using Scott's lipid data.

Plutonium-244 dating

Re-examination of all known xenon isotopic data for ordinary chondrites reveals that244Pu fission xenon can be resolved in about one-fourth of the meteorites of this class. The amounts of244Pu fission xenon found in these meteorites range from ca. 1–2 up to 6–8·10−12 ccSTP/g. These meteorites started to retain their xenon some 200–500 million years later than did the carbonaceous chondrites Allende, Groznaya, Mokoia, Murchison, Murray, and Renazzo which began to retain their xenon over 4800 million years ago.

Actinide abundances in ordinary chondrites

Measurements of 244Pu fission Xe, U, Th, and light REE (LREE) abundances, along with modal petrographic determinations of phosphate abundances, were carried out on equilibrated ordinary chondrites in order to define better the solar system Pu abundance and to determine the degree of variation of actinide and LREE abundances. Our data permit comparison of the directly measured Pu/ U ratio with that determined indirectly as ( Pu/ Nd) × ( Nd/ U) assuming that Pu behaves chemically as a LREE. Except for Guareña, and perhaps H chondrites in general, Pu concentrations are similar to that determined previously for St. Séverin, although less precise because of higher trapped Xe contents. Trapped 130Xe 136Xe ratios appear to vary from meteorite to meteorite, but, relative to AVCC, all are similar in the sense of having less of the interstellar heavy Xe found in carbonaceous chondrite acid residues. The Pu/U and Pu/Nd ratios are consistent with previous data for St. Séverin, but both tend to be slightly higher than those inferred from previous data on Angra dos Reis. Although significant variations exist, the distribution of our Th/U ratios, along with other precise isotope dilution data for ordinary chondrites, is rather symmetric about the CI chondrite value; however, actinide/(LREE) ratios are systematically lower than the CI value. Variations in actinide or LREE absolute and relative abundances are interpreted as reflecting differences in the proportions and/or compositions of more primitive components (chondrules and CAI materials?) incorporated into different regions of the ordinary chondrite parent bodies. The observed variations of Th/U, Nd/U, or Ce/U suggest that measurements of Pu/U on any single equilibrated ordinary chondrite specimen, such as St. Séverin, should statistically be within ±20–30% of the average solar system value, although it is also clear that anomalous samples exist.

Rhenium-Osmium Isotope Systematics of Carbonaceous Chondrites

Rhenium and osmium concentrations and Os isotopic compositions of eight carbonaceous chondrites, one LL3 ordinary chondrite, and two iron meteorites were determined by resonance ionization mass spectrometry. Iron meteorite (187)Re/(186)Os and (l87)Os/(l86)Os ratios plot on the previously determined iron meteorite isochron, but most chondrite data plot 1 to 2 percent above this meteorite isochron. This suggests either that irons have significantly younger Re-Os closure ages than chondrites or that chondrites were formed from precursor materials with different chemical histories from the precursors of irons. Some samples of Semarkona (LL3) and Murray (C2M) meteorites plot 4 to 6 percent above the iron meteorite isochron, well above the field delineated by other chondrites. Murray may have lost Re by aqueous leaching during its preterrestrial history. Semarkona could have experienced a similar loss of Re, but only slight aqueous alteration is evident in the meteorite. Therefore, the isotopic composition of Semarkona could reflect assembly of isotopically heterogeneous components subsequent to 4.55 billion years ago or Os isotopic heterogeneities in the primordial solar nebula.

STUDY ON CHEMICAL COMPOSITIONS OF 19 L AND LL CHONDRITES AND ITS APPLICATION IN TAXONOMY

Reported in this paper are precise INAA data on 19 L and LL chondrites (7 Chinese chondrites and 12 from other countries). The L and LL chondrite data normalized to Mg and to CI chondrite indicate that for several lithophiles (V, Al, Cr and Mn), the extent of scatter is low, similar to that observed in carbonaceous chondrites. The average concentration of siderophiles in the LL group is lower than that in the L group. On the basis of olivine composition and atomic ratio (Ir/Au), we reclassify the five previously labeled L ehondrites as LL members. The atomic ratios of Ir/Au in the LL chondrites are lower than those in the L chondrites (the division is set at an Ir/Au ratio of 2.5). The atomic ratio of Ir/Au appears to be a useful classification parameter for ordinary chondrites. Therefore, it can be considered as a new taxonomic parameter. Of the elements studied, Br, a high volatile, seems most likely to show thermometamorphic effects, depending on petrologic types. The concentration of bromine and the atomic ratio of Br/Se are0.5 μg/g and0.05 for types 3 and 4, and 0.5 μg/g and0.05 for types 5 and 6, respectively.

Sm-Nd isotopic evolution of chondrites and achondrites, II

The 147Sm 143Nd and 146Sm 142Nd isotope systematics have been investigated in five chondrites and the achondrites Moama and Angra dos Reis (ADOR). The new chondrite data and those we have reported before are all consistent with our previously reported reference values for CHUR (“chondritic uniform reservoir”) of ( 143Nd/ 144Nd) CHUR 0 = 0.511847 and ( 147Sm/ 144Nd) CHUR 0 = 0.1967 . Most of the bulk chondrites analyzed have 143Nd/ 144Nd and 147Sm/ 144Nd within 0.5 ε-units and 0.15% of the CHUR values, respectively. This strongly suggests that the CHUR evolution is now known to within these error limits throughout the history of the solar system. The St. Severin chondrite yields an Sm Nd internal isochron age of T = 4.55 ± 0.33AE and an initial ε Nd = 0.11 ∓ 0.26 . Much larger variations in Sm/Nd ratios were measured in mineral separates of the Moama and ADOR achondrites. Thus, very precise ages of 4.46 ± 0.03AE and4.564 ± 0.037AE were obtained for these meteorites, respectively. The initial ε Nd values obtained for Moama and ADOR are 0.03 ∓ 0.25and0.14 ∓ 0.20, respectively. The values obtained on these meteorites are fully consistent with the CHUR evolution curve. Initial ε Nd data on terrestrial igneous and meta-igneous rocks demonstrates that positive initial ε Nd values occur throughout the past 4 AE. This confirms our earlier report that a light rare earth element-depleted layer has existed throughout most of the Earth history and is the source of present-day mid-ocean ridge basalts. The inferred shape of the ε Nd vs. age curve for the depleted mantle suggests profound changes in tectonic regimes with time; in particular, it suggests a much higher rate of recycling of continental materials into the mantle during the Archean as compared to later time periods. 146Sm 142Nd systematics of ADOR and Moama are supportive of the hypothesis that 146Sm was present in the early solar system and suggests a 146Sm/ 144Sm ratio of about 0.01 for the solar system ∼ 4.56 AE ago. This inferred high 146Sm abundance cannot be explained as a late injection from a supernova and must be due to galactic nucleo-synthesis.