Search

The isotopic compositions of Mg, Ca, Ti, Cr, Zn, Sr, Ba, Nd, and Sm were measured in four relatively unaltered refractory inclusions from the Vigarano carbonaceous chondrite meteorite. Three of the inclusions (USNM 1623-2, 1623-3, and 1623-8) show similar Mg, Ca, Ti, and Cr isotopic compositions to those found in most inclusions in the Allende carbonaceous chondrite. This indicates that these Vigarano inclusions sampled the same isotopic reservoirs as the majority of the Allende inclusions that isotope signatures in the latter were not significantly modified by the secondary alteration that permeates most Allende inclusions. In contrast, inclusion 1623-5 has large deficits in Mg-26, Ca-48, and Ti-50 and small but distinct Cr-54, Zn-66, Sr-84, Ba-135, Ba-137, and Sm-144 anomalies. The magnitudes of these unusual anomalies in the refractory elements are within analytical uncertainty of those found in the Allende 'FUN" inclusion C1, yet 1623-5 has a very different bulk chemical composition from C1. The fact that 1623-5 and C1 have identical isotopic anomalies yet have significantly distinct major and trace element contents provide convincing evidence for the presence of isotopically distinct reservoirs in the early solar system.

Additional evidence for the existence in the early solar system of live Fe-60 has been found in the basaltic achondrite Juvinas. The relative abundance of Fe-60/Fe-56 of about 4 x 10 exp -10 at the time of meteorite solidification is one order of magnitude lower than that previously observed for the eucrite Chervony Kut. This difference corresponds to an about 4.7 Ma time interval between closure of the Fe-60-Ni-60 isotopic system in the two meteorites. The factor two higher initial Ni-60/Ni-58 ratio in Juvinas is consistent with this time difference and a common bulk Fe/Ni ratio in the two meteorites. Using the Fe-60-Ni-60 isotope system as a chronometer various models for the evolution of the eucrite parent body of different degrees of complexity are discussed. Regardless of the detailed differences between these models a short time interval of only a few million years between planetary differentiation and basaltic crust formation is indicated.

Isotopic analyses of nickel in samples from the differentiated meteorite Chervony Kut revealed the presence of relative excesses of Ni-60 ranging from 2.4 up to 50 parts per 10 exp 4. These isotopic excesses are from the decay of the now extinct short-lived nuclide Fe-60 and provide clear evidence for the existence of Fe-60 over large scales in the early solar system. Not only was Fe-60 present at the time of melting and differentiation (that is, Fe-Ni fractionation) of the parent body of Chervony Kut but also later at the time when basaltic magma solidified at or near the surface of the planetesimal. The inferred abundance of Fe-60 suggests that its decay alone could have provided sufficient heat to melt small (diameters of several hundred kilometers) planetary bodies shortly after their accretion.

The paper presents results of a wide-ranging isotopic investigation of the the Antarctic angrite LEW-86010 (LEW), and reassesses the type angrite Angra dos Reis (ADOR) in order to obtain precise radiometric ages and initial Sr isotopic compositions, and to search for the erstwhile presence of the short-lived nuclei Sm-146 and Al-26 via their daughter products. The isotopic compositions of Sm, U, Ca, and Ti were measured to allow a detailed appraisal to be made of the relations between, and the geneology of, these two angrites. LEW proves to be severely contaminated with modern terrestrial Pb, which is shown to result from terrestrial weathering. Concordant Pb-Pb model ages of pyroxene separates are obtained; uranium isotopic compositions are normal within error. Overall, striking age and isotopic similarities between LEW and ADOR were found, suggesting almost simultaneous production on the same asteroid, even though recent experimental studies imply that the two are not comagmatic.

The isotopic compositions of Zn, Cr, Ti, and Ca have been measured in a number of CAIs from the Allende meteorite. The aim was to test astrophysical models which predict large excesses of Zn-66 to accompany excesses in the neutron-rich isotopes of Ca, Ti, Cr, and Ni. Some of the CAIs show clearly resolved but small excesses for Zn-66 which are at least an order of magnitude smaller than predicted. This result may simply reflect the volatility and chemical behavior of Zn as compared to the other (more refractory) anomalous elements found in these samples. Alternatively, revision of parameters and assumptions used for the model calculations may be required.

High-precision nickel and chromium isotopic measurements were carried out on nine Allende inclusions. It is found that Ni-62, Ni-64, excesses are present in at least three of the samples. The results suggest that the most likely mechanism for the anomalies is a neutron-rich statistical equilibrium process. An indication of elevated Ni-60 is found in almost every inclusion measured. This effect is thought to be related to the decay of now extinct Fe-60. An upper limit of 1.6 X 10 to the -6th is calculated for the Fe-60/Fe-56 ratio at the time these Allende inclusions crystallized.

Isotopic analyses of the Rb-Sr and Sm-Nd systems in phosphates from ordinary chondritic meteorites (Bjurbole, Mocs, Menow, Elenovka, Ambapur Nagla, and St. Severin) are presented. The Sm-Nd data yield an isochron of age 4.55 + or - 0.45 Ga and an initial Nd-143/Nd-144 consistent with that of Juvinas. Initial Sr-87/Sr-86 ratios are characteristically (possibly excepting St. Severin) elevated with respect to BABI or ALL. Considering both presently and previously reported data, elevation of initial Sr-87/Sr-86 ratios corresponding to metamorphism intervals of the order of 10 to the 8th years seems quite characteristic of chondrites of metamorphic grade 4 or higher. These formation intervals, however, do not correlate with either degree of metamorphism or with I-Xe ages, hence such data for metamorphic chronologies should be interpreted with caution.

The extent to which plate tectonics in its present form operated during the Precambrian is unknown, but is a subject of considerable current interest. A remarkable succession of Precambrian rocks in Rajasthan, Northwestern India, which may help to shed more light on this question are discussed. Data indicates that the Aravalli sequence has a number of characteristics generally ascribed to active continental margins. Although much more work is required to bear this out, the evidence suggests that the processes operating in such an environment in the early Proterozoic or late Archean were not vastly different from today.

Ca-isotope measurements of Allende Ca-Al-rich inclusions (CAIs), together with those on an apatite-enriched fraction from Orgueil, indicate the existence of widespread excesses on the neutron-rich isotope Ca-48. Isotopic anomalies are noted in 7 out of 11 CAIs analyzed. This abundance of isotopic excesses places Ca alongside Ti and O, although no clear correlation has yet been found between Ca-48 and Ti-50, which are thought to be coproduced by neutron-rich nucleosynthetic processes within stars. It is suggested that the higher volatility of Ca, by comparison with Ti compounds, led to a variable dilution with isotopically normal Ca in vaporization and recondensation processes in stellar envelopes, the interstellar medium, and/or the solar nebula.

Studies of Ti isotopic compositions have shown that virtually every Ca-Al-rich Allende inclusion contains anomalous Ti. The present investigation is concerned with the results of a study of Ti isotopic compositions in meteorites. One objective of the study is to evaluate the possibility of a relation between oxygen and Ti anomalies, while another objective is to explore questions regarding the origin of the Ti anomalies. A summary of the major experimental findings of the study of Ti isotopic compositions is also presented. It is noted that an assessment of the implications of the Ti results favors a chemical memory type of model in which products from various nucleosynthetic sources survive in mineral grains. Isotopic heterogeneities are then preserved due to incomplete mixing and/or equilibriation with the bulk of solar system matter. Strong arguments are found to exist against a pure late supernova injection model.

The Sr isotopic composition and Rb-87/Sr-86 ratio have been measured in carbonates and sulfate separated from the Orgueil meteorite in order to determine the time when liquid water may have acted on the parent body. Both of the studied phases probably precipitated from aqueous solution. The results show that carbonate deposition occurred contemporaneously with parent body formation or shortly after it, probably within 100 Myr. On the other hand, at least some of the calcium sulfate seems to have been formed recently.

A carbon-chromite fraction from the Allende C3V chondrite shows strikingly large isotopic enrichments of neodymium-142 (0.47 percent) and neodymium-143 (36 percent). Both apparently formed by alpha decay of samarium-146 and samarium-147 (half-lives 1.03 x 10 to the 8th and 1.06 x 10 to the 11th years), but the isotopic enrichment was greatly magnified by recoil of residual nuclei into a carbon film surounding the samarium-bearing grains. These data provide an improved estimate of the original abundance of extinct samarium-146 in the early solar system, Sm-146/Sm-144 = (4.5 + or - 0.5) x 10 to the -3rd, higher than predicted by some models of p-process nucleosynthesis. It may be possible to use this isotopic pair as a chronometer of the early solar system.

Carbon and chromite fractions from the Allende meteorite that contain isotopically anomalous xenon-131 to xenon-136 (carbonaceous chondrite fission or CCF xenon) at up to 5 x 10 to the 11th atoms per gram show no detectable isotopic anomalies in barium-130 to barium-138. This rules out the possibility that the CCF xenon was formed by in situ fission of an extinct superheavy element. Apparently the CCF xenon and its carbonaceous carrier are relics from stellar nucleosynthesis.

A new technique for high-precision isotopic analyses of Ni is applied to terrestrial samples, Allende inclusions, and materials from other meteorites. It is noted that most of the Allende inclusions analyzed here were previously reported to contain isotopically anomalous Ti. In contrast, the Ni isotopic abundances are found to be indistinguishable from normal within presently obtainable precision, with only one possible exception. The latter inclusion has been shown by others to contain a significantly fractionated magnesium isotopic pattern of 9 parts per thousand/amu. A normal Ni isotopic pattern has also been observed for the chromite/carbon fraction of an Allende acid residue, which is known to have heavy noble gases of highly anomalous isotopic composition. All other meteoritic samples analyzed reveal normal isotopic compositions of Ni; no evidence for effects from now extinct Fe-60 could be detected. Despite ubiquitous isotopic anomalies in Ti from normal Allende inclusions there is no signature of isotopic variations in Ni from the same samples.

A novel technique for the high-precision isotopic analysis of titanium was applied to three terrestrial rocks and coarse- and fine-grained Allende inclusions. Repeated analyses of the three terrestrial rocks gave excellent agreement with a Ti metal standard. All seven Allende inclusions were previously determined to contain isotopically normal Nd and/or Sm, indicating that none belongs to a small group of peculiar intrusions. The discovery of widespread Ti isotopic anomalies in normal Allende inclusions establishes Ti as the first non-noble gas element studied since oxygen to show such isotopic heterogeneity. A survey of nucleosynthetic origins of Ti isotopes suggests that the dominant Ti-50 excesses in these inclusions are due to the relative enrichment of isotopes synthesized during hydrostatic burning in, or near, the core of a massive star.

Due to remaining uncertainties in model parameters and assumptions in the calculation of model ages, it is not yet possible to resolve the precise dating and duration of lunar crust-mantle differentiation phenomena. Norites 78236 and 73255,27,45 were examined utilizing the Sm-Nd radiometric system. The systematics of 78236 show post-crystallization disturbance but indicate a crystallization date 4.34 AE ago. An isochron is defined by the 73255,27,45 data, with a crystallization date of 4.23 AE. The initial Nd isotopic composition of both norites is within uncertainty limits for a 'chondritic' reference reservoir at the time of their respective crystallizations. The implications for lunar crustal formation persisting over a time span of nearly 350 million years are discussed.

The history of lunar troctolite 76535 was investigated on the basis of information from the Sm-147-Nd-143 mineral isochron age, the Nd-143 evolution, extinct Pu-224, the Kr-81-Kr exposure age and the cosmic ray exposure record. It was found that the troctolite minerals last equilibrated 4.26 plus or minus 0.06 AE ago and that the Sm-Nd system became closed at this time. The rock remained at temperatures where the loss of radiogenic Ar-40 was small.

The internal Sm-Nd systematics of lunar KREEP basalt 15386 and its implications for the evolutionary history of KREEP on the moon are reported. The age of the 15386 rocklet indicated by the measured Sm-Nd mineral isochron is consistent with the reported Rb-Sr and (Ar-39)-(Ar-40) ages of other KREEP-rich samples, which is interpreted as the time 15386 crystallized from a liquid on or near the lunar surface. This suggests that the eruption of these incompatible element-rich liquids was related to deep impact events during the final bombardment phase of the surface of the moon. It is concluded that the extremely narrow spread of Sm-Nd model ages for these samples around 4.36 AE, and the compatibility of this age with that indicated by the U-Pb and Rb-Sr systems, indicate that the source of later KREEP volcanism was produced in the closing stages of an early global scale lunar differentiation episode.

The Sm-Nd evolution of Apollo 15 green glass is discussed. The ICE age (intercept with chondritic evolution) of 3.8 + or - 0.4 eons overlaps the range of reported (Ar-39)-(Ar-40) ages and implies a distinct source region for green glass, characterized by very low and unfractionated REE abundances. Evidence is presented that LINd (lunar initial Nd) is compatible with a 'chondritic'-type Nd isotopic evolution as observed in the Juvinas meteorite. This normalization is used to study the Sm-Nd system of various lunar rock types. The results obtained from a limited number of rocks clearly indicate differential Sm-Nd evolution for the lunar crust and mantle. High-Ti basalts returned by the Apollo 11 and 17 missions were derived from distinct source regions. The Nd-143 evolution in KREEP requires a source region which is clearly distinct from any mantle reservoir.

Sm-Nd whole rock measurements on a variety of KREEP-rich samples from different landing sites are reported. Despite a variation of Nd and Sm concentrations of almost a factor of 3, the Sm-Nd ratios, as well as the Nd-143/Nd-144 values, show an extremely close grouping. No systematic differences between samples from different landing sites are resolved. These results are taken to be indicative of a moon-wide process having been responsible for the generation of the KREEP source reservoir, 4.36 plus or minus 0.06 AE ago, as estimated from model age calculation.

Isotope ratios and age estimations are presented for the ultramafic pyroxenite Angra dos Reis meteorite. A pyroxene-phosphate internal isochron age of 4.55 plus or minus 0.04 eons was obtained, in agreement with reported Pb-Pb ages. It is suggested that Sm-146 decay led to an increase in the ratio of Nd-142 to Nd-144 in pyroxene, which is greater than the ratio of the Nd isotopes in phosphates. The effect of Pu-244 on the fission xenon components in pyroxene and phosphate is examined. The ratios of Pu-244 to Nd, U-238, and Th-232 are compared for pyroxene and phosphate separates. The exposure age of the meteorite, as obtained by the Kr-81 to Kr-83 method, was found to be 55.5 plus or minus 1.2 million years.