Ping Kong

Compositional continuity of enstatite chondrites and implications for heterogeneous accretion of the enstatite chondrite parent body

Abstract Chemical compositions of eleven enstatite chondrites (1 EH5, 4 EH3, 3 EL3, I EL5, and 2 EL6) and their metal and nonmagnetic fractions determined by instrumental neutron activation analysis are reported. The abundances of nonvolatile lithophile elements, such as Al, Sc, and Mg, increase from EH to EL, while those of siderophile and moderately volatile elements decrease in the sequence of EH5, to EH3, EL3, and EL5,6. The continuity in compositions of enstatite chondrites and, in particular, the inverse variations of moderately volatile element abundances with petrographic type between EH and EL groups demonstrate that enstatite chondrites have been derived from a common parent body. The enstatite chondrite parent body formed by heterogeneous accretion of materials available in the accretional region; metal was effectively accreted into the core where EH5 chondrites were derived, and the abundances of metal decreased as accretion proceeded. In complement, silicate abundances gradually increased in the layers that accreted later. The lack of correlation between volatile element abundances and metamorphic degree demonstrates that losses of moderately volatile elements from enstatite chondrites cannot have resulted from parent body processes. Furthermore, it is expected that during accretion various components would mix and thus erase any early fractionation of moderately volatile elements of a single parent body. Variations of ambient gas temperatures during accretion are also impossible to produce the volatile element pattern of enstatite chondrites. It is suggested that moderately volatile elements in the enstatite chondrites were lost during local heating, the chondrule formation process. Formation of chondrules appears to have proceeded during the accretion of the enstatite chondrite parent body and lasted for a certain period. The mineralogical and textural features of enstatite chondrites can be explained in terms of two stages of metamorphism. The metamorphic trend from EH3 to EH5 resulted from internal heating whereas the trend from EL3 to EL6 was due to an external heat source. It seems very likely that the EH chondrites were metamorphosed during accretion, thus cooled rather rapidly. Activities of the early sun could serve as an energy source for the external heating.

The origin and nebular history of the metal phase of ordinary chondrites

Abstract We present new INAA results for bulk metal from H and LL chondrites. Both have characteristic elemental patterns similar to L chondrite metal, in particular increasing abundances of W and Ga from unequilibrated to equilibrated chondrites but a reverse variation of V and Cr abundances. These characteristics indicate that metal in ordinary chondrites formed by melting and reduction of highly oxidized material. The similarities of melting features and the complementary nature of compositions between metal and chondrules suggest that these two components were derived from a common precursor, similar to CI or CM material in redox state and compositionally related to the matrix of highly unequilibrated ordinary chondrites. Co Ni abundance ratios are similarly low for bulk metal in the least metamorphosed ordinary chondrites of all three chemical groups. This suggests that metal in ordinary chondrites initially had the same composition and formed under the same melting conditions. The chondrites that accreted earliest have preserved their melting characteristics, while those that accreted later reequilibrated with the ambient gas at different temperatures. The differences in redox state between equilibrated and unequilibrated chondrites show that formation of chondritic metal and chondrules by melting occurred during the accretion of ordinary chondrite parent bodies. The initial metal composition established during the melting stage (13 wt% Ni; Co Ni = 0.031 ) is inferred from the metal in highly unequilibrated chondrites. The accretion temperatures of about 600 k for ordinary chondrites are calculated from the reequilibration reaction. H chondrites were accreted at the highest temperature and are most reduced among the three ordinary chondrite groups. LL chondrites were accreted at the lowest temperature and are most oxidized. Compositionally, the metal component that would account for the Fe Si fractionation among H, L, and LL chondrites is different from the metal formed by melting, indicating that the different Fe S ratios of H, L, and LL chondrites were established before their accretion.

Siderophile Elements in Martian Meteorites and Implications for Core Formation in Mars

Noble metals, Mo, W, and 24 other elements were determined in six SNC meteorites of presumably Martian origin. Based on element correlations, representative siderophile element concentrations for the silicate mantle of Mars were inferred. From a comparison with experimentally determined metal/silicate partition coefficients of the moderately siderophile elements: Fe, Ni, Co, W, Mo, and Ga, it is concluded that equilibrium between core forming metal and silicates in Mars has occurred at high temperatures (around 2200°C) and low pressures (<1 GPa). This suggests that metal segregation occurred concurrently with rapid accretion of Mars, which is consistent with the inference from excess 182W in Martian meteorites (Lee and Halliday, 1997). Concentrations of Ir, Os, Ru, Pt, and Au in the analyzed Martian meteorites, except ALH84001, are at a level of approximately 10−2–10−3 × CI. The comparatively high abundances of noble metals in Martian meteorites require the addition of chondritic material after core formation. The similarity in Au/La and Pt/Ca ratios between ALH84001 and the other Martian meteorites suggests crystallization of ALH84001 after complete accretion of Mars.

A possible causal relationship between extinction of dinosaurs and K/T iridium enrichment in the Nanxiong Basin, South China: evidence from dinosaur eggshells

Multiple distinct iridium anomalies have been observed in dinosaur eggshells in the K/T boundary sections of the Nanxiong Basin, Guangdong Province, South China. The eggshells collected at and near the fossil-pollen-defined K/T boundary interval show iridium increases of about 19 and 28 times, respectively, above the background level. The enrichment of other trace elements in the eggshells occurs mostly at and near the interval. The distribution of Ir-bearing levels proves the existence of at least six Ir-delivering events from the latest Cretaceous into the earliest Paleocene. The enrichment of Ir and other trace elements in eggshells may have been caused by the assimilation of these elements into the dinosaur body through food, and then into the eggs laid by them. Two types of pathological development, i.e. variation in eggshell thickness and eggshell microstructure, have been observed from the basin. It seems that they occur frequently during the K/T transition. The physiological mechanisms producing pathologic dinosaur eggshells are evaluated in the light of homologous phenomena occurring in living birds. On this basis, it is concluded that the formation of pathologic dinosaur eggshells was caused by anomalous trace element concentrations. The extinction of the dinosaurs in the Nanxiong Basin did not occur instantaneously, but spread out within 250 ka with major extinction beginning at the boundary interval. The cause may have been environmental poisoning and adverse changes in climatic conditions as indicated by the geochemical analyses of the dinosaur eggshells. A repeating short- and long-term geochemically induced environmental stress adversely affected the reproductive process and contributed to the extinction of the dinosaurs

Compositional and genetic relationship between chondrules, chondrule rims, metal, and matrix in the Renazzo chondrite

Individual chondrules, coarse and fine-grained metal, chondrule rims, and matrix samples were separated from the Renazzo chondrite (CR2) and studied by instrumental neutron activation analysis. Both Renazzo chondrule and matrix fractions have CI Cr/Mg concentration ratios, unlike chondrules from other classes of chondrites, which have fractionated Cr/Mg ratios. This and other arguments suggest that Renazzo chondrules and matrix formed in the same nebular region. In Renazzo, fine-grained metal—the dominant metal component, is mostly located inside chondrules or in chondrule rims. Matrix contains few coarse metal grains. Both types of metal, fine and coarse, exhibit similar chemical signatures, comparatively high in Cr and low in Ni, suggesting a genetic relationship. Although metal is mostly contained in chondrules, chondrules and matrix of Renazzo have similar common siderophile/lithophile abundance ratios. This may imply that Renazzo chondrules and metal were formed by reduction of oxidized precursors compositionally similar to CI chondrites. The CI-like concentrations of Sc, Mg, Se, and Zn in Renazzo matrix are consistent with this inference. The differences in siderophile element pattern between chondrules, chondrule metal, and matrix indicate evaporation and recondensation of volatile elements during chondrule formation. Recondensation of evaporated elements was incomplete and the degree of recondensation correlates with element volatility. The higher-than-CI K/Mg, Na/Mg, and Ga/Fe ratios of Renazzo matrix may reflect recondensation of K, Na, and Ga into the matrix. The CI Se/Sc and Zn/Sc concentration ratios of the matrix, however, reflect very limited degrees of recondensation of Se and Zn into matrix. The presence of abundant phyllosilicates and organic material in Renazzo matrix requires a low background temperature (<200 K) during formation of the Renazzo meteorite. Partial recondensation of volatile elements in a cold nebular environment implies instantaneous agglomeration of the Renazzo chondrite following chondrule formation.

Metal phases of L chondrites : Their formation and evolution in the nebula and in the parent body

Abstract Metal phases of six (three equilibrated and three unequilibrated) L chondrites were studied by INAA, SEM, and Mossbauer spectroscopy. Characteristics retained in the bulk metals of unequilibrated chondrites (abundant carbon, high contents of Cr, V, Mn, and low contents of W, Mo, and Ga compared to metals of equilibrated chondrites, less enrichment of W than Mo, and fractionation of Co from Ni) demonstrate that chondrite metals are not nebular condensates. All those characteristics can be well explained by melting. Chondrite metals are not melting remnants of previously condensed metals, rather, they were produced by reduction of CI- or CM-like material during the melting process. The complementarity in composition and the similarity in melting feature suggest that chondritic metals and chondrules are the complementary components of the same melting event. Distribution of trace siderophile elements between taenite and bulk metal indicates that kamacite and taenite can only be the low-temperature diffusion products and must have been developed in the chondrite parent body. The difference in the taenite composition between equilibrated and unequilibrated chondrites reveals that the equilibrated chondrites were located near the surface while the unequilibrated chondrites were in the interior if they were derived from the common parent body. Thus, while the exsolution of chondrite metal into kamacite and taenite was due to the internal thermal activity, the crystallization of EOC silicates resulted from an external heating. The internal metamorphism was mild (400–600°C) and long whereas the external heating was intense (with a maximum temperature in range of 800–950°C) and short. Tetrataenite is present not only in UOCs but also in EOCs, suggesting that the external heating occurred during the internal metamorphism, i.e., within 100 myr of chondrite formation.

Distribution of siderophile elements in CR chondrites: evidence for evaporation and recondensation during chondrule formation

Abstract New data on the chemical composition of bulk samples, and of metallic and nonmagnetic fractions of six CR chondrites (Renazzo, Y793495, PCA91082, EET92042, Acfer 209 and El Djouf 001) are reported. It is shown that volatile siderophile element abundance patterns of metallic and nonmagnetic fractions of CR chondrites are complementary and volatility-dependent. In the metallic fraction CI-normalized abundances of Au, As, Sb and Ga decrease with increasing volatility, whereas in the nonmagnetic fraction abundances increase in the same sequence as in the metallic fraction. We argue that the siderophile element patterns of the metallic and nonmagnetic fractions reflect those of the chondrule and matrix fractions of CR chondrites, respectively, based on: (1) CR metal is mostly located inside chondrules and those metal grains outside chondrules probably were also derived from chondrules; (2) element partitioning within chondrules has been reset during chondrule formation; and (3) resetting of element distribution within chondrules occurred at sufficiently reducing conditions to allow partitioning of Au, As, Sb and Ga into CR metal. The complementary siderophile element patterns of CR chondrules and matrix are difficult to explain by gradual gas loss during condensation. The CI proportions of highly volatile elements in the bulk CR chondrites further argue against the possibility of loss of solids during condensation. Thus, the fractionation of volatile elements in CR chondrites is unlikely to be the result of gas-solid fractionation during condensation. The fractionation of volatile siderophile elements between CR chondrules and matrix requires evaporation of volatile elements during chondrule formation. The matrix pattern indicates recondensation of evaporated volatile elements. It appears, based on the composition of CR matrix, that the depletion and fractionation of moderately volatile elements in the bulk CR chondrites is due to formation of CR chondrites before complete recondensation of volatile elements which were evaporated during chondrule formation. This implies that agglomeration of CR chondrites proceeded simultaneously with chondrule formation.

Determination of 18 siderophile elements including all platinum group elements in chondritic metals and iron meteorites by instrumental neutron activation.

An instrumental neutron activation method is developed to analyze chondritic metals and iron meteorites. By changing irradiation and decay times, and selecting suitable γ-ray and X-ray photopeaks, not only all platinum group elements (Ru, Rh, Pd, Os, Ir, Pt) but also other siderophilic elements (Fe, Co, Ni, Cu, Ga, Ge, As, Mo, Sb, W, Re, Au) can be nondestructively determined in the meteoritic metal samples. To obtain analytical data as accurate as possible, interfering reactions and neutron flux gradients during irradiation are considered. Siderophile elemental abundances measured for the Odessa iron meteorite are highly consistent with the literature values. Rh abundances for bulk H, L, LL, EH, and EL chondrites, which had been scarcely reported in the literature, are derived from Rh/Ni abundance ratios in the metal separates of the corresponding chondritic groups.

Chemical characteristics of metal phases of the Richardton H5 chondrite

Abstract The magnetic fraction was separated from the Richardton H5 chondrite with a hand magnet and was leached with concentrated HF at high temperature for different periods. The resulting metallic fractions and the untreated magnetic fraction were analyzed using Mossbauer spectroscopy and instrumental neutron activation analysis (INAA). Leaching in concentrated HF for 1.5–5 min was effective in removing silicates from the magnetic fraction while leaving the metal grains intact. Leaching with increasing time selectively dissolved kamacite, with only little attack of the taenite grains. The Mossbauer spectroscopy shows that martensite (α-phase iron) is present in the Richardton metals and is dissolved in HF less easily than kamacite, but more easily than taenite. The distributions of trace siderophile elements among the different metal phases of Richardton were obtained based on the INAA data. Siderophile elements, except for Co, are enriched in the taenite fraction but with different concentration ratios between taenite and kamacite, suggesting that kamacite and taenite were not formed by either oxidation/reduction of Fe of the Fe Ni metal, or by condensation from the nebula, or by metal-solid differentiation. A more likely explanation is that kamacite and taenite have equilibrated by low-temperature diffusion. It seems that most trace siderophile elements were once dissolved in the Fe Ni metal and later distributed among the metal phases by diffusion. However, Ir, Os and Ru may still partly exist as tiny separate grains in the FeNi metal and their distributions among the metal phases are not completely equilibrated.

Fluctuation history of the interior East Antarctic Ice Sheet since mid-Pliocene

Abstract Cosmogenic 10Be and 26Al measurements from bedrock exposures in East Antarctica provide indications of how long the rock surface has been free from glacial cover. Samples from the crests of Zakharoff Ridge and Mount Harding, two typical nunataks in the Grove Mountains, show minimum 10Be ages of 2.00 ± 0.22 and 2.30 ± 0.26 Ma, respectively. These ages suggest that the crests were above the ice sheet at least since the Plio–Pleistocene boundary. Adopting a ‘reasonable’ erosion rate of 5–10 cm Ma-1 increases the exposure ages of these two samples to extend into the mid-Pliocene. The bedrock exposure ages steadily decrease with decreasing elevation on the two nunataks, which indicates ~200 m decrease of the ice sheet in the Grove Mountains since mid-Pliocene time. Seven higher elevation samples exhibit a simple exposure history, which indicates that the ice sheet in the Grove Mountains decreased only ~100 m over a period as long as 1–2 Ma. This suggests that the East Antarctic Ice Sheet (EAIS) was relatively stable during the Pliocene warm interval. Five lower elevation samples suggest a complex exposure history, and indicate that the maximum subsequent increase of the EAIS was only 100 m higher than the present ice surface. Considering the uncertainties, their total initial exposure and subsequent burial time could be later than mid-Pliocene, which may not conflict with the stable mid-Pliocene scenario.