Thermomagnetic analysis of meteorites, 3. C3 and C4 chondrites

Thermomagnetic analysis was made on samples of all known C3 and C4 chondrites in a controlled oxygen atmosphere. Considerable variation was noted in the occurrence of magnetic minerals, comparable to the variation observed earlier in the C2 chondrites. Magnetite was found as the only major magnetic phase in samples of only three C3 chondrites (2–4 wt.%) and the Karoonda C4 chondrite (7.7 wt.%). The magnetite content of these three C3 chondrites is only about one-third that observed in the C1 and C2 chondrites which were found to contain magnetite as the only magnetic phase. Five C3 chondrites were observed to undergo chemical change during heating, producing magnetite: this behavior is characteristic of troilite oxidation. Upper limits on initial magnetite content of about 1–9% were established for these meteorites. Samples of the remaining five C3 chondrites and the Coolidge C4 chondrite were found to contain both magnetite and metallic iron. In two samples, iron containing ≤2% Ni was observed, while in the other four, the iron contained 6–8 wt.% Ni. In addition to containing both magnetite and iron metal, three of these samples reacted during heating to form additional magnetite. Variations in the magnetic mineralogy and, hence by inference bulk mineralogy, of C3 and C4 chondrites indicate a more complex genesis than is evident from whole-rock elemental abundance patterns.     

Deuterium in carbonaceous chondrites

Hydrogen isotopic compositions in seven carbonaceous chondrites lie in the range ?70 to +771‰ relative to SMOW. These values decrease, to a range from ?145 to +219‰, after low-temperature oxidation in an oxygen plasma. Deuterium enrichment is therefore concentrated in the organic matter, the hydrous silicates probably lying close to the terrestrial range for such material. Calculated values for δD of the organic fraction are +450 ‰ for Orgueil and Ivuna and up to +1600‰ for Renazzo. These enrichments, at least for Orgueil and Ivuna, suggest equilibration with protosolar hydrogen at very low temperatures. Assuming a value of 2.5 × 10^?5 for the protosolar D/H ratio, nominal equilibration temperatures of 230°K for silicates and 180°K for organic matter may be derived.     

The abundances of zirconium and hafnium in the solar system

The concentrations of zirconium and hafnium have been determined in Orgueil, Murchison, Allende, Bruderheim and Alais by RNAA. The mean Zr/Hf weight ratio in the first four of these meteorites is 31.3 ± 2.2 indicating no major fractionation of Zr from Hf. Alais contains anomalously high amounts of many refractory lithophile elements, including Zr and Hf. Orgueil contains 3.1 ppm Zr and 0.11 ppm Hf, corresponding to 9.0 and 0.16 atoms, respectively, relative to 10⁶ Si atoms.     

Thermomagnetic analysis of meteorites, 2. C2 chondrites

Samples of all eighteen of the known C2 chondrites have been analyzed thermomagnetically. For eleven of these, initial Fe₃O₄ content is low (generally <1%) and theJ_s-T curves are irreversible. The heating curves show variable greater (up to 10 times) than it is initially. This behavior is attributed to the production of magnetite from a thermally unstable phase — apparently FeS. Four of the remaining seven C2 chondrites contain Fe₃O₄ as the only significant magnetic phase: initial magnetite contents range from 4 to 13%. The remaining three C2 chondrites contain iron or nickel-iron in addition to Fe₃O₄. These seven C2 chondrites show little evidence of the breakdown of a thermally unstable phase.     

Zirconium and hafnium in meteorites

The abundances of zirconium and hafnium have been determined in nine stony meteorites by a new, precise neutron-activation technique. The Zr/Hf abundance ratios for the chondrites vary in a rather narrow range, consistent with previously published observations from our group. Replicate analyses of new, carefully selected clean interior samples of the C1 chondrite Orgueil yield mean zirconium and hafnium abundances of 5.2 and 0.10 ppm, respectively. These abundances are lower than we reported earlier for two C1 chondrite samples which we now suspect may have suffered contamination. The new C1 zirconium and hafnium abundances are in closer agreement with predictions based on theories of nucleosynthesis than the earlier data.     

The abundance of barium in stony meteorites

The concentration of Ba in 7 carbonaceous chondrites, 18 ordinary chondrites, 3 achondrites and 1 stony-iron meteorite has been determined by the stable isotope dilution technique using solid source mass spectrometry. Analysis of the C1 chondrite Orgueil indicates a small adjustment of the “cosmic” abundance of Ba to 4.2 on the Si=10⁶ abundance scale. The present work provides a more complete coverage of a number of meteorite classes than has so far been available for the abundance of Ba in stony meteorites.     

Cosmic-ray-produced40K and50V in the metal phase of chondrites

Cosmic-ray-produced⁴⁰K in the metal phase of six chondrites and⁵⁰V in that of one chondrite were determined using a surface ionization mass spectrometer. The²²Ne_total/⁴⁰K_metal ratios of the chondrites are explained in part by shielding effects during cosmic-ray irradiation. The wide variation of this ratio in some groups of meteorites is explained in terms of partial loss of rare-gas nuclides. Radiation ages for the chondrites were determined using⁴⁰K measurements and production-rate estimates from thick target calculations.     

The cosmic abundance of molybdenum

Fifteen carbonaceous chondrites were analysed for Mo and Ir by neutron activation analysis combined with a metal extraction method. The results of two Orgueil analyses gave a mean concentration of 915 ppb Mo. This corresponds to 2.51 atoms Mo/10⁶ atoms Si, which is 50% lower than data reported by Case et al. [3]. The lower Mo concentration for Orgueil was predicted by Suess and Zeh [4] from semi-empirical abundance rules. A constant Mo/Ir ratio is found for C1, C2, and C3V chondrites; C3Os have variable Mo/Ir ratios. These variations are due to variable Ir concentrations. Micron-sized grains enriched in Ir but not in Mo are presumably responsible for these variations. The Mo content of Karoonda is nearly a factor of four lower than that of C3V chondrites.     

A new type of chondritic meteorite found in lunar soil

A fragment found in soil from the Apollo 12 site (12037, from the rim of Bench Crater) appears to be a unique type of chondrite, petrologically and chemically distinct from other chondrites and lunar rocks. Inclusions consisting of shocked pyroxene rimmed by euhedral troilite crystals are set in a black aphanitic matrix. Abundant magnetite in the matrix exhibits microscopic morphologies (framboids and plaquets) characteristic of C1 chondrites. The bulk composition of this sample has high Mg/Si and low Fe/Si relative to other chondrites, and P and S are strongly enriched. Most compositional differences between this meteorite and other chondrites may be explained by fractionation of Fe phases, such as magnetite and troilite. Low refractory element contents preclude mixing with lunar materials. This sample may be a surviving fragment of the meteoritic component present in the lunar regolith. Its characteristics suggest that ancient meteoritic debris sampled by the moon may be significantly different from that captured by the present-day earth.     

Boron cosmochemistry

Boron, sulfur, silicon, and sodium abundances have been measured in 50 pieces from 28 chondritic meteorites. Thirty-one of these samples were carefully selected and prepared to avoid composition changes from contamination or alteration in the terrestrial environment. Boron and sodium abundances define two domains within C2M carbonaceous chondrites: one contains8.5 B10⁶ Si and0.044 NaSi and the other25 B10⁶ Si and0.021 Na10⁶ Si. It is proposed that these differences are the result of element redistribution within C2M chondrites in response to low-temperature hydrous processes. Petrologic type 5 and 6 chondrites are also composed of two types of material, one has3.5 B10⁶ Si and the other11 B10⁶ Si. The mass weighted average of the boron concentration in high-petrologic-type ordinary chondrites is indistinguishable from the measured abundances in petrologic type 3 and 4 ordinary chondrites. We suggest that the compositional differences within petrologic types 5 and 6 are the result of high-temperature modifications of materials that had boron abundances like those now observed in the low petrologic types.We assumed that the redistribution of these elements was closed on the scale of the mass of the available meteorites, and determined the abundances in unaltered material by calculating the mass weighted average of the measured abundances in all the samples of one chondrite type. We used these averages to calculate boron depletion factors in chondrite types. The factors areC2 = 0.62, C3 = 0.31, andordinary chondrites= 0.34. These are similar to the sulfur depletion factors in the same types of meteorites. The agreement suggests that boron, like sulfur, had a nebular condensation temperature at the lower end of the range of moderately volatile elements. Within the context of this interpretation, the solar abundance of boron is21 B10⁶ Si—the concentration in the interior piece of the C-1 carbonaceous chondrite Orgueil.     

Mass spectrometric isotope dilution analyses of tin in stony meteorites and standard rocks

The abundance of tin in 21 stony meteorites and 12 standard rocks has been determined by the stable isotope dilution technique. The measurements of the C1 chondrite Orgueil gives a cosmic abundance for tin of 3.7 (with respect to Si= 10⁶ atoms) and thus substantiates the peak in the cosmic abundance curve due to the closed proton shell forZ = 50. The low abundance of tin in ordinary chondrites allows tin to be reassigned to the strongly depleted group of elements in the context of the Larimer-Anders model. Our values for the standard rocks are in reasonable agreement with the presently accepted abundances of tin, except for the diabase W-1 and tonalite T-1 for which significantly lower values were obtained.     

Evidence for244Pu fission tracks in hibonites from Murchison carbonaceous chondrite

We have developed a technique for revealing nuclear tracks in the mineral hibonite (CaAl₁₂O₁₉), found in the refractory inclusions from carbonaceous chondrites. The tracks in hibonitesfrom Murchison carbonaous chondrite are dominated by fission tracks from²⁴⁴Pu (constituting more than 90% of the total). The measured uranium contents in these crystals range from 1.2 to 62 ppb. We deduce that the average value for the²⁴⁴Pu/²³⁸U ratio in most of the Murchison hibonites at the time of track retention is0.022 ± 0.011.     

New kind of type 3 chondrite with a graphite-magnetite matrix

We have discovered four clasts in three ordinary-chondrite regolith breccias which are a new kind of type 3 chondrite. Like ordinary and carbonaceous type 3 chondrites, they have distinct chondrules, some of which contain glass, highly heterogeneous olivines and pyroxenes, and predominantly monoclinic low-Ca pyroxenes. But instead of the usual fine-grained, Fe-rich silicate matrix, the clasts have a matrix composed largely of aggregates of micron- and submicron-sized graphite and magnetite. The bulk compositions of the clasts as well as the types of chondrules (largely porphyritic) are typical of type 3 ordinary chondrites, although chondrules in the clasts are somewhat smaller (0.1–0.5 mm). A close relationship with ordinary chondrites is also indicated by the presence of similar graphite-magnetite aggregates in seven type 3 ordinary chondrites. This new kind of chondrite is probably the source of the abundant graphite-magnetite inclusions in ordinary-chondrite regolith breccias, and may be more common than indicated by the absence of whole meteorites made of chondrules and graphite-magnetite.     

Tungsten in ordinary chondrites

In ordinary chondrites tungsten displays both lithophile and siderophile characteristics. Its concentration in the metal phase is positively correlated with petrologic type, and with the distribution coefficientK_D =W in metal/W in silicates plus troilite. The oxidation-reduction reactions involved are temperature-dependent and the recrystallization temperature recorded on the basis of the partition of W between coexisting metal and silicate plus troilite fractions are950° ± 100°C for equilibrated chondrites (types 5 and 6), and800° ± 50°C for type 4, while Shaw (L7) records the highest recrystallization temperature (>1200°C).The different metallic content of the three groups of ordinary chondrites has been attributed to a metal-silicate fractionation process. Such a process appears to have fractionated W and Ir, but not W and Fe as these elements were partly oxidized when the fractionation process took place.     

吉林陨石中铁镍合金的磁学性质

铁镍合金是陨石中重要的磁性物质,其中铁纹石、镍纹石和四方镍纹石是球粒陨石中的主要铁镍合金.然而,迄今针对陨石中铁镍合金的磁学性质研究仍非常缺乏.本文研究了吉林陨石中的铁纹石、四方镍纹石、以及陨硫铁的磁学特征.实验表明,镍含量为6%~7%的铁纹石是该陨石中最主要的铁镍合金物质,它具有低矫顽力和高的热稳定性,居里温度~750 ℃.镍含量为~48%的四方镍纹石具有高矫顽力和高的热稳定性,居里温度~565 ℃,它是剩磁的主要载体.陨硫铁在室温为反铁磁性,不具有载剩磁能力,在60 K左右存在一个低温转换,在氩气中加热较稳定而在空气中加热被氧化转化为磁铁矿.这些研究结果为鉴定球粒陨石中的磁性物质提供了依据.     

87Rb87Sr dating of LL chondrites

⁸⁷Rb⁸⁷Sr analyses of LL chondrites have been made in 10 whole rock meteorites, chondrules from Chainpur (LL3) and Soko Banja (LL4), density separates and chondrules from Guidder (LL5) and density separates from Jelica (LL6) and Ensisheim (LL6). Whole rocks define an isochron of age 4.486±0.020 Ga (λ⁸⁷Rb=1.42×10^?11a^?1) and initial ratio (⁸⁷Sr/⁸⁶Sr)_I=0.69887±0.00012. This is in agreement with the results for H- and E-type chondrites. Analyses for chondrules from Soko Banja yield a very good isochron of age 4.452±0.020 Ga and strontium initial ratio 0.69954±0.00024, and give an interval for metamorphism of (37±10)×10⁶ a. A more poorly defined isochron is obtained for Jelica; the age is 4.423±0.041 Ga and the strontium initial ratio 0.69959±0.00029, indicating an interval for metamorphism of (70±60)×10⁶ a. No isochron could be obtained for Chainpur. This could be due to terrestrial alteration or to a late isotopic disturbance of the meteorite. The⁸⁷Rb-⁸⁷Sr system is also disturbed in Guidder and Ensisheim, probably as a consequence of shock. These results are discussed in comparison with our former studies, and in relation with thermal metamorphism in the LL chondrite parent body(ies).     

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.     

Variation of the phase composition of C2 chondrites on heating

The kinetics of metamorphism of the Staroe Boriskino C2 chondrite heated at 450°C in an inert atmosphere of helium flow was investigated. After being heated at 450°C during 160 minutes one specimen was moreover heated for 10 minutes at 500°C. The phase distribution was determined by means of Mössbauer spectroscopy, X-ray diffraction analysis, and electron probe microanalysis.The material changes rapidly (1–2 minutes). As a result of dehydration, the iron of the phyllosilicate is oxidized, the charge compensation being realized through the removal of iron and magnesium cations with the formation of magnetite and forsterite. Upon 10 minutes additional heating at 500°C iron appears in the olivine structure, the degree of iron oxidation declines, and magnetite disappears. Possible trends of change of C2 chondrite material are:

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Meteorite magnetism and the early solar system magnetic field

The ferromagnetism of irons, stony-irons, E-, H-, L- and LL-chondrites and achondrites is due to a metallic phase comprising mostly Fe and Ni and small amounts of Co and P. The ferromagnetic constituent in non-metamorphosed C-chondrites is magnetite, but some metamorphosed C-chondrites contain FeNi metallic grains too.

Among the stony meteorites, the content of metals as determined by their saturation magnetization (I_S) sharply decreases in the order E → H → L → LL → achondrites, whereas the I_S value for magnetite and additional metals in C-chondrites ranges from the I_S value of achondrites to that of L-chondrites.

With an increase of Ni-content in the metallic phase in chondrites of the order E → H → L → LL → C, the relative amount of Ni-poor kamacite magnetization, I_S(), in the total I_S decreases in the same order, from I_S()/I_S 1 for E-chondrites to I_S()/I_S 0 for C-chondrites. Thus, E-, H-, L-, LL- and C-chondrites and achondrites are well separated in a diagram of I_S()/I_S versus I, which could be called a magnetic classification diagram for stony meteorites.

As the surface skin layer of all meteorites is anomalously magnetized, it must be removed and the natural remanent magnetization (NRM) of the unaltered interior only must be examined for the paleomagnetic study. The NMR of C-chondrites is highly stable and that of achondrites is reasonably stable against AF-demagnetization, whereas the NMR of E-chondrites and ordinary chondrites as well as stony-iron meteorites is not very stable in most cases. Although the NRM of iron meteorites is reasonably stable, it is not attributable to the extraterrestrial magnetic field.

The paleointensity for Allende C₃-chondrite is estimated to be about 1.0 Oe assuming that its NRM is of TRM origin. The paleointensity for other reasonably reliable C-chondrites (Orgueil, Mighei, Leoville and Karoonda) is also around 1 Oe.

The paleointensity for two achondrites has been determined to be about 0.1 Oe. The NRM of other achondrites also suggests that their paleointensity is roughly 0.1 Oe.

The NRM of ordinary chondrites is less stable than that of C-chondrites and achondrites so that the estimated paleointensity for ordinary chondrites is less reliable. The paleointensity for comparatively reliable ordinary chondrites ranges from 0.1 to 0.4 Oe.

The paleointensity values of 1 Oe for C-chondrites and 0.1 Oe for achondrites may represent the early solar nebula magnetic field about 4.5 × 10⁹ years ago. A possibility that the paleomagnetic field for achondrites was a magnetic field attributable to a dynamo within a metallic core of their parent planet may also not be rejected.     

Natural remanent magnetization of carbonaceous chondrites

We have observed natural remanent magnetizations (NRM), ranging from 10⁻³ to 10⁻⁵ Gauss cm³/g, at least two orders of magnitude greater than 10⁻⁷, the noise level of our spinner magnetometer, in the carbonaceous chondrites, Orgueil, Cold Bokkeveld, Nogoya and Mighei. Preliminary alternating field demagnetization of two samples of Orgueil showed a moderate sized stable component of 10⁻⁴ Gauss cm³/g. Microscopic study reveals that a grey spinel oxide, Ni-Fe and an Fe sulphide are the principal opaque minerals in both Cold Bokkeveld and Orgueil; the oxide phase predominates in both, while the Ni-Fe is more visible in the former. Coercive force determinations in Orgueil and Cold Bokkeveld indicate single domain particles as the NRM carrier. These meteorites are suitable for the determination of the magnetic field intensity in the primary solar nebula.