Chemical studies of L chondrites. V: compositional patterns for 49 trace elements in 14 L4-6 and 7 LL4-6 falls

To study compositional trends associated with open-system thermal metamorphism and shock-induced collisional breakup of L4-6 chondrite parent(s), we used inductively coupled plasma mass spectrometry and radiochemical neutron activation analysis to determine 49 trace elements in 62 falls. Trends for the 49 elements, especially of the 14 rare earth elements in 5 members of a putative L/LL group (Bjurböle, Cynthiana, Holbrook, Knyahinya, Sultanpur) and 9 additional L chondrites (Aïr, Aumieres, Bachmut, Forksville, Kandahar, Kiel, Milean, Narellan, Santa Isabel) differed markedly from those in the remaining normal 46 samples. Here, we report the data for the 14 L and putative L/LL chondrites and 7 LL (Appley Bridge, Athens, Bandong, Ensisheim, Mangwendi, Olivenza, Soko-Banja), analyzed to test the affinity of the putative L/LL suite to well-characterized LL chondrites.Compositional trends of the 14 atypical L chondrites (including Aïr’s unique and possibly contaminated signature) and Mangwendi, an LL6 chondrite, indicate that each is compositionally unrepresentative of well-sampled, whole-rock chondrites. Indeed, half of the unrepresentative chondrites were ≤ 2-g samples. Compositionally, members of the putative L/LL chondrites demonstrate no affinities to normal LL chondrite falls. To establish compositional trends accompanying open-system, thermal episodes involving the L chondrite parent(s), we should ignore data for the 14 unrepresentative L chondrites reported here.     

Cumberland Falls chondritic inclusions—II. Trace element contents of forsterite chondrites and meteorites of similar redox state

Mineralogic study of black inclusions in the Cumberland Falls enstatite achondrite revealed that they constitute a highly unequilibrated chondritic suite distinct from other chondrite groups. This highly shocked suite, the forsterite (F) chondrites, exhibits mineralogic trends apparently produced during primary nebular condensation and accretion over a broad redox range. We analyzed these samples and possibly related meteorites for Ag, As, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn, trace elements known to yield important genetic information. The results demonstrate the compositional coherence and distinctiveness of the F chondrite suite relative to other chondrites. The Antarctic aubrite, ALH A78113, may include more F chondrite material. Trace element contents do not vary with mineral compositions hence do not reflect redox variations during formation of F chondrite parental matter. Trace element mobilization—during secondary heating episodes in the F chondrite parent or during its disruptive collision with the enstatite meteorite parent body—is not detectable. Chemical trends in F chondrites apparently reflect primary nebular processes. Cosmochemical fractionation of lithophiles from siderophiles and chalcophiles occurred at moderately high temperatures, certainly higher than those existing during formation of primitive carbonaceous, enstatite and ordinary chondrites of petrologic type ≤3.     

Chemical studies of L chondrites—III. Mobile trace elements and 40Ar39Ar ages

We report data for trace elements Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Se, Te, Tl and Zn determined by radiochemical neutron activation analysis in L4–6 chondrites with undisturbed ⁴⁰Ar release patterns or with patterns showing some disturbance in the 4.4–4.6 Gyr plateau indicating shock-induced loss. Mean concentrations are lower, many significantly so, in 16 chondrites with disturbed patterns than in 4 with undisturbed ones, consistent with shock-induced mobilization. Similar trends were noted earlier in L4–6 chondrites having mineralogically observable shock indicators: mean concentrations are lower in strongly shocked (i.e. > 22 GPa) than in mildly shocked (<22 GPa) samples. From trace element contents, L4–6 chondrites with undisturbed ⁴⁰Ar release patterns are mildly shocked but chondrites with disturbed patterns are more strongly heated, on average, than those of shock facies d-f (i.e. 22 to > 57 GPa). Pooling these populations, significantly lower mean concentrations of nearly all trace elements in 26 strongly shocked L4–6 chondrites than in 14 mildly shocked ones indicate loss in shock-formed FeS-Fe eutectic and/or by vaporization during cooling of shock-heated collisional debris. Two-element correlations and the pattern of them, i.e. correlation profiles, are also consistent with this picture. Trace elements can act as thermometers for collisional episodes in L4–6 chondrites but not for earlier thermal fractionations, unless compensation can be made for late shock heating.     

Chemical studies of L chondrites—II. Shock-induced trace element mobilization

Previous studies of chondrites heated in the laboratory for extended periods under conditions approximating those in shock-heated collisional debris indicate that Au, Co, Se, Ga, Rb, Cs, Te, Bi, In, Ag, Zn, Tl and Cd progress in mobility. We report data for these 13 trace elements in 14 L4–6 chondrites of established shock history and discuss these and 13 additional chondrites studied earlier. Trace element contents vary with petrologic type, $\text{S}\text{Fe}$ sub-group and shock history, the last dominating strongly. Absolute abundances and interelement relationships for the 6 or 7 most mobile elements vary with degree of shock-loading (i.e. residual temperatures) established from mineralogic/petrologic study. A tertiary process, shock-heating, previously known to have affected radiogenic ⁴⁰Ar and/or ⁴He in meteorites but not other elements, apparently was at least as effective as other open-system processes (secondary [parent body] and primary [nebular and/or accretionary] episodes) in establishing mobile trace element contents of L chondrites and probably others. If conditions during early genetic episodes are to be deduced from compositional information, shocked meteorites should be avoided or effects of later processes should be compensated for.     

The thermal history of equilibrated ordinary chondrites and the relationship between textural maturity and temperature

The compositions and textures of phases in eleven equilibrated ordinary chondrites from the H, L, and LL groups spanning petrographic types 4-6 were studied and used to constrain the thermal histories of their parent bodies. Based on Fe-Mg exchange between olivine and spinel, average equilibration temperatures for type 4-6 chondrites encompass a small range, 586-777 °C, relative to what is commonly assumed for peak temperatures (600-950 °C). The maximum temperatures recorded by individual chondrites, which are minima relative to peak metamorphic temperatures, increase subtly but systematically with metamorphic type and are tightly clustered for H4-6 (733-754 °C) and LL4-6 (670-777 °C). For the Ls, Ausson (L5) records a higher maximum olivine-spinel temperature (761 °C) than does the L4 chondrite Saratov (673 °C) or the L6 chondrite Glatton (712 °C). Our data combined with olivine-spinel equilibration temperatures calculated for other equilibrated ordinary chondrites using mineral compositions from the literature demonstrate that, in general, type 4 chondrites within each chemical group record temperatures lower than or equal to those of types 5-6 chondrites.For H chondrites, the olivine-spinel closure temperature is a function of spinel grain size, such that larger grains, abundant in types 5-6 chondrites, record temperatures of ∼740 °C or more while smaller grains, rare in types 5-6 but abundant in type 4 chondrites, record lower temperatures. Olivine-spinel temperatures in the type 6 chondrites Guareña and Glatton are consistent with rapid (50-100 °C/Myr) cooling from high temperatures in the ordinary chondrite parent bodies. With one exception (∼500 °C/Myr), olivine-spinel data for St.-Séverin (LL6) are consistent with similar cooling rates. Cooling rates of order 100 °C/Myr at ∼750 °C for type 6 chondrites are considerably higher than previously determined cooling rates for lower temperatures (?550 °C) based on metallography, fission tracks, and geochronology. For H chondrites, current thermal models of an “onion shell” parent body are inconsistent with a small range of peak temperatures based on olivine-spinel and two pyroxene thermometry combined with a wide dispersion of cooling rates at low temperatures. Equilibrated chondrites may have sampled regions near a major transition in physical properties such as near the base of a regolith pile.     

天然冲击球粒陨石的化学组成及冲击效应

本文运用电子探针、ＩＮＡＡ方法研究了两块强烈冲击的中国普通球粒陨石的矿物组成、化学组成、冲击熔融相，非溶融相和磁性金属相微量元素丰度，结合稀有气体含量和母体冲击特征，讨论了它们的冲击效应和母体热历史，证明了母体热变质作用叠加了冲击效应，冲击效应增加了陨石矿物组成平衡程度，提高了母体的岩石类型，但冲击热效应对陨石中非气体挥发性元素含量及化学组成没有明显的影响，说明冲击热效应对陨石中非气体挥发性元素含     

An Fe isotope study of ordinary chondrites

The Fe isotope composition of ordinary chondrites and their constituent chondrules, metal and sulphide grains have been systematically investigated. Bulk chondrites fall within a restricted isotopic range of <0.2‰ δ⁵⁶Fe, and chondrules define a larger range of >1‰ (−0.84‰ to 0.21‰ relative to the IRMM-14 Fe standard). Fe isotope compositions do not vary systematically with the very large differences in total Fe concentration, or oxidation state, of the H, L, and LL chondrite classes. Similarly, the Fe isotope compositions of chondrules do not appear to be determined by the H, L or LL classification of their host chondrite. This may support an origin of the three ordinary chondrite groups from variable accretion of identical Fe-bearing precursors.A close relationship between isotopic composition and redistribution of Fe during metamorphism on ordinary chondrite parent bodies was identified; the largest variations in chondrule compositions were found in chondrites of the lowest petrologic types. The clear link between element redistribution and isotopic composition has implications for many other non-traditional isotope systems (e.g. Mg, Si, Ca, Cr). Isotopic compositions of chondrules may also be determined by their melting history; porphyritic chondrules exhibit a wide range in isotope compositions whereas barred olivine and radial pyroxene chondrules are generally isotopically heavier than the ordinary chondrite mean. Very large chondrules preserve the greatest heterogeneity of Fe isotopes.The mean Fe isotope composition of bulk ordinary chondrites was found to be −0.06‰ (±0.12‰ 2 SD); this is isotopically lighter than the terrestrial mean composition and all other published non-chondritic meteorite suites e.g. lunar and Martian samples, eucrites, pallasites, and irons. Ordinary chondrites, though the most common meteorites found on Earth today, were not the sole building blocks of the terrestrial planets.     

Impact features of enstatite-rich meteorites

Enstatite-rich meteorites include EH and EL chondrites, rare ungrouped enstatite chondrites, aubrites, a few metal-rich meteorites (possibly derived from the mantle of the aubrite parent body), various impact-melt breccias and impact-melt rocks, and a few samples that may be partial-melt residues ultimately derived from enstatite chondrites. Members of these sets of rocks exhibit a wide range of impact features including mineral-lattice deformation, whole-rock brecciation, petrofabrics, opaque veins, rare high-pressure phases, silicate darkening, silicate-rich melt veins and melt pockets, shock-produced diamonds, euhedral enstatite grains, nucleation of enstatite on relict grains and chondrules, low MnO in enstatite, high Mn in troilite and oldhamite, grains of keilite, abundant silica, euhedral graphite, euhedral sinoite, F-rich amphibole and mica, and impact-melt globules and spherules. No single meteorite possesses all of these features, although many possess several. Impacts can also cause bulk REE fractionations due to melting and loss of oldhamite (CaS) – the main REE carrier in enstatite meteorites. The Shallowater aubrite can be modeled as an impact-melt rock derived from a large cratering event on a porous enstatite chondritic asteroid; it may have been shock melted at depth, slowly cooled and then excavated and quenched. Mount Egerton may share a broadly similar shock and thermal history; it could be from the same parent body as Shallowater. Many aubrites contain large pyroxene grains that exhibit weak mosaic extinction, consistent with shock-stage S4; in contrast, small olivine grains in some of these same aubrites have sharp or undulose extinction, consistent with shock stage S1 to S2. Because elemental diffusion is much faster in olivine than pyroxene, it seems likely that these aubrites experienced mild post-shock annealing, perhaps due to relatively shallow burial after an energetic impact event. There are correlations among EH and EL chondrites between petrologic type and the degree of shock, consistent with the hypothesis that collisional heating is mainly responsible for enstatite-chondrite thermal metamorphism. Nevertheless, the apparent shock stages of EL6 and EH6 chondrites tend to be lower than EL3-5 and EH3-5 chondrites, suggesting that the type-6 enstatite chondrites (many of which possess impact-produced features) were shocked and annealed. The relatively young Ar–Ar ages of enstatite chondrites record heating events that occurred long after any ²⁶Al that may have been present initially had decayed away. Impacts remain the only plausible heat source at these late dates. Some enstatite meteorites accreted to other celestial bodies: Hadley Rille (EH) was partly melted when it struck the Moon; Galim (b), also an EH chondrite, was shocked and partly oxidized when it accreted to the LL parent asteroid. EH, EL and aubrite-like clasts also occur in the polymict breccias Kaidun (a carbonaceous chondrite) and Almahata Sitta (an anomalous ureilite). The EH and EL clasts in Kaidun appear unshocked; some clasts in Almahata Sitta may have been extensively shocked on their parent bodies prior to being incorporated into the Almahata Sitta host.     

Chemical studies of H chondrites. II: Weathering effects in the Victoria Land,Antarctic population and comparison of two Antarctic populations with non-Antarctic falls

We report RNAA data for 14 siderophile, lithophile and chalcophile volatile/mobile trace elements (Ag, Au, Bi, Cd, Co, Cs, Ga, In, Sb, Se, Te, Tl, U, Zn) in interior portions of 45 different H4–6 chondrites (49 samples) from Victoria Land, Antarctica and 5 H5 chondrites from the Yamato Mts., Antarctica.Relative to H5 chondrites of weathering types A and B, all elements are depleted (10 at statistically significant levels) in extensively weathered (types B/C and C) samples, probably by leaching on the ice sheet surface. Contents of 8 elements in extensively weathered samples may provide a qualitative ranking for terrestrial surface residence. Chondrites of weathering types A and B seem compositionally uncompromised and as useful as contemporary falls for trace-element studies. When data distributions for these 14 trace elements in non-Antarctic H chondrite falls and unpaired samples from Victoria Land and from the Yamato Mts. (Queen Maud Land) are compared statistically, numerous significant differences are apparent. Concentrations of 8 elements differ significantly in the Victoria Land and non-Antarctic H5 chondrite populations. Essentially the same compositional differences and ⁵³Mn content and shock history differences appear when H4–6 populations are compared. Contents of 8 elements differ when Queen Maud Land and Victoria Land populations are compared and 5 for the Queen Maud Land/non-Antarctic comparison.These and other differences give ample cause to doubt that the various sample populations derive from the same parent population. The observed differences do not reflect weathering, chance or other trivial causes: a preterrestrial source must be responsible. The least unlikely of these involves a temporal variation in the source regions from which meteoroids derive.     

“Mysterite” : a late condensate from the solar nebula

We have attempted to clarify the nature of “mysterite”, a material that had been postulated to explain the overabundance of Tl, Bi and Ag in certain chondrites. Four dark clasts and a vein sample from the H6 chondrite Supuhee were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Os, Rb, Re, Sb, Se, Te, Tl and Zn. One of the clasts is enriched in all volatile elements, while the other 4 samples are enriched only in the siderophile volatiles Ag, Bi and Tl. The enrichments range up to 100 times typical H6 chondrite abundances. The proportions of Ag, Bi, Tl suggest the presence of at least two, Tl-rich and Tl-poor, varieties of mysterite ($\text{Tl}\text{Bi}\text{= 7.2}$ and <0.1). The former seems to dominate in Supuhee and Krymka, and the latter in Mezö-Madaras. Apparently mysterite is a late condensate from the solar nebula that collected volatiles left behind by earlier generations of chondrites. It was incorporated in Supuhee and perhaps in other chondrites (mainly of low petrologic types) during brecciation events.     

球粒陨石的冲击作用与热释光特征研究

测定了我国部分球粒陨石的自然热释光峰温、峰半高宽度和灵敏度, 强冲击型陨石的热释光灵敏度偏低,峰温、峰半高宽温度则偏高。岩庄陨石属天然冲击球粒陨石,具有强烈的冲击特征。测定岩庄陨石两种不同颜色冲击脉体的热释光,其强度明显地不一样,并发现该陨石的热释光具有β辐照诱发热释光位移和退火热释光位移现象。冲击作用明显地改变着热释光的各种特性。     

A single asteroidal source for extraterrestrial Ordovician chromite grains from Sweden and China: High-precision oxygen three-isotope SIMS analysis

We examined oxygen three-isotope ratios of 48 extraterrestrial chromite (EC) grains extracted from mid-Ordovician sediments from two different locations in Sweden, and one location in south-central China. The ages of the sediments (∼470 Ma) coincide with the breakup event of the L chondrite parent asteroid. Elemental compositions of the chromite grains are generally consistent with their origin from L or LL chondrite parent bodies. The average Δ¹⁷O (‰-deviation from the terrestrial mass-fractionation line, measured in situ from 15 μm spots by secondary ion mass spectrometry; SIMS) of EC grains extracted from fossil meteorites from Thorsberg and Brunflo are 1.17 ± 0.09‰ (2σ) and 1.25 ± 0.16‰, respectively, and those of fossil micrometeorites from Thorsberg and Puxi River are 1.10 ± 0.09‰, and 1.11 ± 0.12‰, respectively. Within uncertainty these values are all the same and consistent with the L chondrite group average Δ¹⁷O = 1.07 ± 0.18‰, but also with the LL chondrite group average Δ¹⁷O = 1.26 ± 0.24‰ (Clayton et al., 1991). We conclude that the studied EC grains from correlated sediments from Sweden and China are related, and most likely originated in the same event, the L chondrite parent body breakup. We also analyzed chromites of modern H, L and LL chondrites and show that their Δ¹⁷O values coincide with averages of Δ¹⁷O of bulk analyses of H, L and LL chondrites. This study demonstrates that in situ oxygen isotope data measured by SIMS are accurate and precise if carefully standardized, and can be used to classify individual extraterrestrial chromite grains found in sediments.     

The nature,origin and modification of insoluble organic matter in chondrites,the major source of Earth’s C and N

All chondrites accreted ∼3.5 wt.% C in their matrices, the bulk of which was in a macromolecular solvent and acid insoluble organic material (IOM). Similar material to IOM is found in interplanetary dust particles (IDPs) and comets. The IOM accounts for almost all of the C and N in chondrites, and a significant fraction of the H. Chondrites and, to a lesser extent, comets were probably the major sources of volatiles for the Earth and the other terrestrial planets. Hence, IOM was both the major source of Earth’s volatiles and a potential source of complex prebiotic molecules.Large enrichments in D and ¹⁵N, relative to the bulk solar isotopic compositions, suggest that IOM or its precursors formed in very cold, radiation-rich environments. Whether these environments were in the interstellar medium (ISM) or the outer Solar System is unresolved. Nevertheless, the elemental and isotopic compositions and functional group chemistry of IOM provide important clues to the origin(s) of organic matter in protoplanetary disks. IOM is modified relatively easily by thermal and aqueous processes, so that it can also be used to constrain the conditions in the solar nebula prior to chondrite accretion and the conditions in the chondrite parent bodies after accretion.Here we review what is known about the abundances, compositions and physical nature of IOM in the most primitive chondrites. We also discuss how the IOM has been modified by thermal metamorphism and aqueous alteration in the chondrite parent bodies, and how these changes may be used both as petrologic indicators of the intensity of parent body processing and as tools for classification. Finally, we critically assess the various proposed mechanisms for the formation of IOM in the ISM or Solar System.     

Pb-Pb dating constraints on the accretion and cooling history of chondrites

We have analyzed the Pb isotopic compositions of whole-rocks and various components (CAIs, chondrules, and/or mineral separates) of two carbonaceous chondrites, Allende (CV3) and Murchison (CM2), and nine ordinary chondrites, Sainte Marguerite (H4), Nadiabondi and Forest City (H5), Kernouvé (H6), Bjurböle (L/LL4), Elenovka and Ausson (L5), Tuxtuac (LL5), and Saint-Séverin (LL6) by MC-ICP-MS. Three CAI fractions from Allende define an isochron with an age of 4568.1 ± 9.4 Ma (MSWD = 0.08) and plot on the same isochron as fragments of the Efremovka inclusion E60 analyzed by Amelin et al. [Amelin, Y., Krot, A. N., Hutcheon, I. D., and Ulyanov, A. A. (2002a). Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions. Science297, 1679-1683]. When these two groups of samples are combined, the isochron yields an age of 4568.5 ± 0.5 (MSWD = 0.90), which is our best estimate of the age of the Solar System. Chondrules and pyroxene-olivine fractions from the ordinary chondrites yield ages that reflect the blocking of Pb isotope equilibration with the nebular gas. The combination of these ages with the corresponding metamorphic phosphate ages provides constraints on the thermal history of the different chondrite parent bodies. Among the H chondrites, Sainte Marguerite cooled to below ∼1100 K within a few My at 4565 Ma and to ∼800 K at 4563 Ma. Nadiabondi appears to have experienced a slightly more protracted cooling history with the corresponding interval lasting from 4559 to 4556 Ma. The data from Forest City and Kernouvé show evidence of late-stage perturbation with resulting U/Pb fractionation. Likewise, Pb isotopes in Tuxtuac (LL5) record a cooling history lasting from ∼4555 to 4544 Ma, which may indicate that the cooling history for the LL parent body was more prolonged than for the H parent body. We suggest a thermal evolution model for the growth of the planetary bodies based on the release of radiogenic heat from ²⁶Al and ⁶⁰Fe. This model incorporates the accretion rate, which determines the time at which the radiogenic heat becomes efficiently trapped, and the terminal size of the parent body, which controls its overall thermal inertia. The parent bodies of carbonaceous chondrites, which show little indication of metamorphic transformation, collect cooler nebular material at a relatively late stage. Small asteroids of ∼10-50 km radius accreting within 1-3 My could be the parent bodies of H and LL chondrites. The parent body of the L chondrites is likely to be a larger asteroid (r > 100 km) or possibly the product of collisions of smaller planetary bodies.     

The origin and evolution of chondrites recorded in the elemental and isotopic compositions of their macromolecular organic matter

Extraterrestrial organic matter in meteorites potentially retains a unique record of synthesis and chemical/thermal modification by parent body, nebular and even presolar processes. In a survey of the elemental and isotopic compositions of insoluble organic matter (IOM) from 75 carbonaceous, ordinary and enstatite chondrites, we find dramatic variations within and between chondrite classes. There is no evidence that these variations correlate with the time and/or location of chondrite formation, or with any primary petrologic or bulk compositional features that are associated with nebular processes (e.g., chondrule and volatile trace element abundances). Nor is there evidence for the formation of the IOM by Fischer-Tropsch-Type synthesis in the nebula or in the parent bodies. The elemental variations are consistent with thermal maturation and/or oxidation of a common precursor. For reasons that are unclear, there are large variations in isotopic composition within and between chondrite classes that do not correlate in a simple way with elemental composition or petrologic type. Nevertheless, because of the pattern of elemental variations with petrologic type and the lack of any correlation with the primary features of the chondrite classes, at present the most likely explanation is that all IOM compositional variations are the result of parent body processing of a common precursor. If correct, the range of isotopic compositions within and between chondrite classes implies that the IOM is composed of several isotopically distinct components whose relative stability varied with parent body conditions. The most primitive IOM is found in the CR chondrites and Bells (CM2). Isotopically, the IOM from these meteorites resembles the IOM in interplanetary dust particles. Chemically, their IOM resembles the CHON particles of comet Halley. Despite the large isotopic anomalies in the IOM from these meteorites, it is uncertain whether the IOM formed in the interstellar medium or the outer Solar System, although the former is preferred here.     

The Rumuruti chondrite group

Since 1994, the Rumuruti (R) chondrites have been recognized as a new, well-established chondrite group differing from carbonaceous, ordinary, and enstatite chondrites. The first R chondrite, Carlisle Lakes, was found in Australia in 1977. Meanwhile, the number has increased to 107 (December, 2010). This group is named after the Rumuruti meteorite, the first and so far the only R chondrite fall. Most of the R chondrites are breccias containing a variety of different clasts embedded in a clastic matrix. Some textural and mineralogical characteristics can be summarized as follows: (a) the chondrule abundance in large fragments and in unbrecciated rocks is ∼35–50 vol%; (b) Ca,Al-rich inclusions are rare; (c) the olivine abundance is typically 65–78 vol%; (d) the mean chondrule diameter is ∼400 μm; (e) in unequilibrated R chondrites, low-Ca pyroxene is dominating, whereas in equilibrated R chondrites it is Ca-rich pyroxene; (f) the typical olivine in a metamorphosed lithology is ∼Fa_38–40; (g) matrix olivine in unequilibrated, type 3 fragments and rocks has much higher Fa (∼45–60 mol%) compared to matrix olivines in type 4–6 lithologies (∼Fa_38–41); (h) spinels have a high TiO₂ of ∼5 wt%; (i) abundant different noble metal-bearing phases (metals, sulfides, tellurides, arsenides) occur. The exception is the metamorphosed, type 5/6 R chondrite La Paz Icefield 04840 which contains hornblende, phlogopite, and Ca-poor pyroxene, the latter phase typically occurring in low-grade metamorphosed R chondrites only.In bulk composition, R chondrites have some affinity to ordinary chondrites: (a) the absence of significant depletions in Mn and Na in R chondrites and ordinary chondrites is an important feature to distinguish these groups from carbonaceous chondrites; (b) total Fe (∼24 wt%) of R chondrites is between those of H and L chondrites (27.1 and 21.6 wt%, respectively); (c) the average CI/Mg-normalized lithophile element abundances are ∼0.95 × CI, which is lower than those for carbonaceous chondrites (≥1.0 × CI) and slightly higher than those for ordinary chondrites (∼0.9 × CI); (d) trace element concentrations such as Zn (∼150 ppm) and Se (∼15 ppm) are much higher than in ordinary chondrites; (e) the whole rock Δ¹⁷O of ∼2.7 for R chondrites is the highest among all meteorite groups, and the mean oxygen isotope composition is δ¹⁷O = 5.36 ± 0.43, δ¹⁸O = 5.07 ± 0.86, Δ¹⁷O = +2.72 ± 0.31; (f) noble gas cosmic ray exposure ages of R chondrites range between ∼0.1 and ∼70 Ma. More than half of the R chondrites analyzed for noble gases contain implanted solar wind and, thus, are regolith breccias. The 43 R chondrites from Northern Africa analyzed so far for noble gases seem to represent at least 16 falls. Although the data base is still scarce, the data hint at a major collision event on the R chondrite parent body between 15 and 25 Ma ago.     

Determination of REE,Ba, Fe,Mg, Na and K in carbonaceous and ordinary chondrites

Precise determination of REE and Ba abundances in three carbonaceous (Orgueil Cl, Murchison C2 and Allende C3) and seven olivine-bronzite chondrites were carried out by mass spectrometric isotope dilution technique. Replicate analyses of standard rock and the three carbonaceous chondrites demonstrated the high quality of the analyses (accuracies for REE are ±1–2 per cent). Certain carbonaceous chondrite specimens showed small positive irregularities in Yb abundance. The Yb ‘anomaly’ (approximately + 5 per cent relative to the average of 10 ordinary chondrites) in Orgueil may relate to high temperature components. The REE pattern of Guareña (H6) exhibits comparatively extensive fractionation (about factor 2) with a negative anomaly for Eu (17 ± 1 percent) compared to the average H chondrite. This could be interpreted in terms of extensive thermal metamorphism leading to melting.Apart from absolute abundance differences, there appears to be small but recognizable fractionation among the average relative REE abundances of Cl, E, H and L chondrites. However, individual chondrites within these groups showed more or less fractionated REE patterns relative to each other. The distinction between H and L chondrites was well demonstrated in Eu-Sm correlation curves and absolute abundance differences of REE and major elements.Si-normalized atomic ratios of the REE abundances in different kinds of chondrites to those in Orgueil (Cl) chondrite were 0.58 (E), 0.75 (H), 0.81 (L), 1.07 (C2) and 1.32 (C3).     

The compositional classification of chondrites: II The enstatite chondrite groups

We present new data from a neutron activation analysis of four enstatite chondrites including the taxonomically important St. Sauveur, and discuss the classification of enstatite chondrites. The enstatite chondrites can be divided into two compositionally distinct sets; in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1.5–2.0× higher than in the other. A well-resolved compositional hiatus separates these two sets. The differences in composition are as great as those between the groups of ordinary chondrites, and therefore it appears best to treat these sets as separate groups. By analogy with the symbols used for ordinary chondrites we propose to designate the high-Fe, high siderophile group EH and the low-Fe, low-siderophile group EL. Known members of the EH group belong to petrologic types 4 and 5, whereas all EL members are petrologic type 6. Within the EH group no correlation is observed between petrologic type and abundance of nonrefractory siderophiles or moderately volatiles or alkalis.Two physical properties show only modest overlap between the EH and EL groups. Cosmic-ray ages for EH chondrites are 0.5–7 Ma, while those for EL chondrites are 4–18 Ma. Relative to Bjurböle, I-Xe formation intervals are $\text{?1.3 ± 0.6}$ Ma for EH chondrites and $\text{2.9 ± 0.5}$ Ma for EL chondrites. The weight of the chemical and physical evidence indicates that the EH and EL groups formed separate bodies at similar distances from the Sun.The available evidence for Shallowater and Happy Canyon, two strongly recrystallized silicate-rich meteorites containing > 40 mg/g Fe-Ni, indicates that the former is an enstatite-clan chondrite altered by loss of sulfide- and plagioclase-rich melts, whereas the latter is intermediate in composition between EL chondrites and the chondritic silicates in the Pine River IAB-anomalous meteorite.     

Chemical characteristics and origin of H chondrite regolith breccias

We report petrologic data and contents of Ag, Bi, Cd, Co, Cs, Ga. In, Rb, Se, Te, Tl and Zn-trace elements spanning the volatility/mobility range-in light and dark portions of H chondrite regolith breccias and L chondrite fragmental breccias. The chemical/petrologic characteristics of H chondrite regolith breccias differ from those of non-brecciated chondrites or fragmentai breccias. Petrologic characteristics and at least some trace element contents of H chondrite regolith breccias reflect primary processes; contents of the most volatile/mobile elements may reflect either primary or secondary processing, possibly within layered H chondrite parent object(s). Chemical/petrologic differences existed in different regions of the parents). Regolith formation and gardening and meteoroid compaction were not so severe as to alter compositions markedly.     

The relationship between CK and CV chondrites

CK chondrites are highly oxidized meteorites containing abundant magnetite and trace amounts of Fe,Ni metal. Although the group is predominately composed of equilibrated meteorites (types 4-6), in recent years a significant number of new samples have been classified as being either CK3 or CK3-anomalous. These unequilibrated CKs often display a close affinity with members of the CV oxidized subgroup. CKs and CVs (oxidized subgroup) may therefore form a continuum and by implication could be derived from a single common parent body. To investigate the relationship between these two groups a detailed study of the oxygen isotope composition, opaque mineralogy and major and trace element geochemistry of a suite of CV and CK chondrites has been undertaken. The results of oxygen isotope analysis confirm the close affinity between CV and CK chondrites, while excluding the possibility of a linkage between the CO and CK groups. Magnetites in both CV and CK chondrites show significant compositional similarities, but high Ti contents are a diagnostic feature of the latter group. The results of major and trace element analysis demonstrate that both CV and CK chondrites show overlapping variation. Supporting evidence for a single common source for both groups comes from their similar cosmic-ray exposure age distributions. Recent reflectance spectral analysis is consistent with both the CVs and CKs being derived from Eos family asteroids, which are believed to have formed by the catastrophic disruption of a single large asteroid. Thus, a range of evidence appears to be consistent with CV and CK chondrites representing samples from a single thermally stratified parent body. In view of the close similarity between CV and CK chondrites some modification of the present classification scheme may be warranted, possibly involving integration of the two groups. One means of achieving this would be to reassigned CK chondrites to a subgroup of the oxidized CVs. It is recognized that a full evaluation of this proposal may require further study of the still poorly understood CK3 chondrites.