Primordial noble gases in chondrites: the abundance pattern was established in the solar nebula

Ordinary chondrites, like carbonaceous chondrites, contain primordial noble gases mainly in a minor phase comprising 相似文献   

Rhenium-osmium isotope systematics of carbonaceous chondrites

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

Neutron activation analysis of a particle returned from asteroid Itokawa

A single grain (~3 micrograms) returned by the Hayabusa spacecraft was analyzed by neutron activation analysis. This grain is mainly composed of olivine with minor amounts of plagioclase, troilite, and metal. Our results establish that the Itokawa sample has similar chemical characteristics (iron/scandium and nickel/cobalt ratios) to chondrites, confirming that this grain is extraterrestrial in origin and has primitive chemical compositions. Estimated iridium/nickel and iridium/cobalt ratios for metal in the Itokawa samples are about five times lower than CI carbonaceous chondrite values. A similar depletion of iridium was observed in chondrule metals of ordinary chondrites. These metals must have condensed from the nebular where refractory siderophile elements already condensed and were segregated.     

Chainpur-like Chondrites: Primitive Precursors of Ordinary Chondrites?

Chainpur and similar, apparently primitive, chondritic meteorites may be precursors of ordinary chondrites; a variety of evidence supports this working hypothesis. In general, carbonaceous chondrites seem to be related collaterally to this genetic sequence rather than being direct ancestors of ordinary chondrites. Metamorphic processes may be responsible for fractionations of elements such as indium and iodine, and type-II carbonaceous chondrites seem to be more primitive than types I or IIIA.     

PLANETARY SCIENCE: Most-Common Meteorites Find a Home Among the Asteroids

Meteoriticists have long been puzzled by the fact that the most common meteorites, so-called ordinary chondrites, don't appear to have come from the most common asteroids, the S-types. Last week, however, a group of researchers attending the spring meeting of the American Geophysical Union here announced that the 31-kilometer-long S-type asteroid Eros now being orbited by the NEAR Shoemaker spacecraft is made of the same stuff as ordinary chondrites. That conclusion comes from NEAR Shoemaker's first-ever analysis of the elemental composition of an asteroid.     

Isotopic composition of rare gases in lunar samples

Data from total melt and step-by-step heating experiments on the Apollo 11 lunar samples suggest a close affinity between lunar and meteoritic rare gases. Trapped neon-20/neon-22 ratios range from 11.5 to approximately 15, resembling those for the gas-rich meteorites. Trapped krypton and xenon in the lunar fines and in the carbonaceous chondrites are similar except for an interesting underabundance of the heavy isotopes in both lunar gases which suggests that the fission component found in carbonaceous chondrites is depleted in lunar material. Spallation gases are in most cases quite close to meteoritic spallation gases in isotopic composition.     

Itokawa dust particles: a direct link between S-type asteroids and ordinary chondrites

The Hayabusa spacecraft successfully recovered dust particles from the surface of near-Earth asteroid 25143 Itokawa. Synchrotron-radiation x-ray diffraction and transmission and scanning electron microscope analyses indicate that the mineralogy and mineral chemistry of the Itokawa dust particles are identical to those of thermally metamorphosed LL chondrites, consistent with spectroscopic observations made from Earth and by the Hayabusa spacecraft. Our results directly demonstrate that ordinary chondrites, the most abundant meteorites found on Earth, come from S-type asteroids. Mineral chemistry indicates that the majority of regolith surface particles suffered long-term thermal annealing and subsequent impact shock, suggesting that Itokawa is an asteroid made of reassembled pieces of the interior portions of a once larger asteroid.     

Oxygen Isotope Fractionation between Minerals and an Estimate of the Temperature of Formation

Oxygen isotopic compositions of separated minerals from three type A and four type B rocks are very uniform. The delta(18)O values are: plagioclase, 6.20; clinopyroxene, 5.75; ilmenite, 4.45 (parts per thousand relative to Standard Mean Ocean Water). The isotopic distribution corresponds to equilibrium at 1120 degrees C. The isotopic composition of lunar pyroxenes falls within the range for pyroxenes of terrestrial mafic and ultramafic rocks, ordinary chondrites, enstatite chondrites, and enstatite achondrites, but above the range for basaltic achondrites, hypersthene achondrites, and mesosiderites. Glass isolated from the lunar soil has a delta(18)O value of 6.2, significantly richer in (18)O than the crystalline rock fragments in the soil.     

Isotopic analysis of rare gases with a laser microprobe

A ruby-pulsed laser and high-sensitivity mass spectrometer are used to analyze isotopic abundances of rare gases from microgram samples of polished sections. The feasibility of the technique is demonstrated by the analysis of primordial helium and neon from the Kapoeta and Fayetteville meteorites.     

Application of two-step laser mass spectrometry to cosmogeochemistry: direct analysis of meteorites

Polycyclic aromatic hydrocarbons in C1, C2, and C3 carbonaceous chondrites and in some ordinary chondrites have been directly analyzed by two-step laser desorption/ laser multiphoton ionization mass spectrometry, a selective and sensitive method requiring only milligram samples. At the ionization wavelength of 266 nanometers, parent ion peaks of polycyclic aromatic hydrocarbons dominate the mass spectra. Quantitative analysis is possible; as an example, the concentration of phenanthrene in the Murchison meteorite was determined to be 5.0 parts per million.     

Graphitic carbon in the Allende meteorite: a microstructural study

High-resolution transmission electron microscopy, shows that carbon in the Allende carbonaceous chondrite meteorite is predominantly a poorly crystalline graphite. Such material is of interest as an important carrier of the isotopically anomalous noble gases found in carbonaceous chondrites.     

Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206

Measurements of the lead isotopic composition and the uranium, thorium, and lead concentrations in meteorites were made in order to obtain more precise radiometric ages of these members of the solar system. The newly determined value of the lead isotopic composition of Canyon Diablo troilite is as follows: (206)Pb/(204)Pb = 9.307, (207)Pb/(2O4)Pb = 10.294, and (208)Pb/(204)Pb = 29.476. The leads of Angra dos Reis, Sioux County, and Nuevo Laredo achondrites are very radiogenic, the (206)Pb/(204)Pb values are about 200, and the uranium-thorium-lead systems are nearly concordant. The ages of the meteorites as calculated from a single-stage (207)Pb/(206)Pb isochron based on the newly determined primordial lead value and the newly reported (235)U and (838)U decay constants, are 4.528 x 10(9) years for Sioux County and Nuevo Laredo and 4.555 x 10(9) years for Angra dos Reis. When calculated with the uranium decay constants used by Patterson, these ages are 4.593 x 10(9) years and 4.620 x 10(9) years, respectively, and are therefore 40 to 70 x 10(6) years older than the 4.55 x 10(9) years age Patterson reported. The age difference of 27 x 10(6) years between Angra dos Reis and the other two meteorites is compatible with the difference between the initial (87)Sr/(86)Sr ratio of Angra dos Reis and that of seven basaltic achondrites observed by Papanastassiou and Wasserburg. The time difference is also comparable to that determined by (129)1-(129)Xe chronology. The ages of ordinary chondrites (H5 and L6) range from 4.52 to 4.57 x 10(9) years, and, here too, time differences in the formation of the parent bodies or later metamorphic events are indicated. Carbonaceous chondrites(C2 and C3) appear to contain younger lead components.     

Origin and metamorphic redistribution of silicon, chromium, and phosphorus in the metal of chondrites

Chromium, silicon, and phosphorus concentrations of 0.1 to 1 percent by weight are common in metal grains in the least metamorphosed ordinary and carbonaceous chondrites. These concentrations are fairly uniform within single chondrules (but different from chondrule to chondrule) and are inversely correlated with the fayalite concentrations of the chondrule olivines. This relation shows that these chromium, silicon, and phosphorus concentrations could not have been established by condensation or equilibration in the solar nebula but are the result of metal-silicate equilibration within chondrules. Two generations of inclusions made by the exsolution of those elements have been identified: One formed during chondrule cooling and the other formed during metamorphism. The distribution and composition of the latter in type 3 to type 5 chondrites are consistent with increasing metamorphism relative to type 2 and type 3.0 material.     

Oxygen isotopic compositions of asteroidal materials returned from Itokawa by the Hayabusa mission

Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves have not been established. We measured, using secondary ion mass spectrometry, oxygen isotopic compositions of rock particles from asteroid 25143 Itokawa returned by the Hayabusa spacecraft. Compositions of the particles are depleted in (16)O relative to terrestrial materials and indicate that Itokawa, an S-type asteroid, is one of the sources of the LL or L group of equilibrated ordinary chondrites. This is a direct oxygen-isotope link between chondrites and their parent asteroid.     

Vesicle-Specific Noble Gas Analyses of "Popping Rock": Implications for Primordial Noble Gases in Earth

Gases trapped in individual vesicles in the volatile-rich basaltic glass "popping rock" were found to have the same carbon dioxide, helium-4, and argon-40 composition, but a variable 40Ar/36Ar ratio ( approximately 4000 to >/=40,000). The argon-36 is probably surface-adsorbed atmospheric argon; any mantle argon-36 trapped in the vesicles cannot be distinguished from an atmospheric contaminant. Consequently the 40Ar/36Ar ratios and 3He/36Ar ratios (1.45) determined are minimum estimates of the upper mantle composition. Heavy noble gas relative abundances in the mantle resemble solar noble gas abundance patterns, and a solar origin may be common to all primordial mantle noble gases.     

Meteorites: optical activity in organic matter

A low- amplitude, positive, Cotton effect, centered at about 340 millimicrons, has been observed in organic extracts of samples from ordinary (noncarbonaceous) chondrites. Ancillary evidence renders it likely that this optical activity derived from contamination by biologic materials on Earth.     

Nitrogen abundances in chondritic meteorites

Carrier-gas fusion extractions of total nitrogen in 22 chondritic meteorites indicate a wide variation in total nitrogen contents, ranging from 660 parts per million for an enstatite chondrite to 18 parts per million for an ordinary chondrite. Total nitrogen and total carbon contents of individual chondrites do not show a positive correlation.     

Netschaevo: a new class of chondritic meteorite

The ratios of refractory elements to silicon and of zinc to silicon indicate that the silicate portion of the Netscha?vo meteorite is an ordinary chondrite. The scarcity of chondrules, the large dimensions (about 100 micrometers) of plagioclase grains, and the low indium content (0.09 nanogram per gram) indicate that Netscha?vo belongs to petrologic type 6. On a diagram of reduced iron versus oxidized iron, Netscha?vo lies along an extrapolation of the LL-L-H ordinary chondrite fractionation trend. The abundances of siderophile elements (nickel, germanium, iridium, and gold) are about 1.6 to 2.0 times greater than in H-group chondrites, and siderophile/nickel ratios are, with one exception, those expected from LL-L-H trends. This evidence indicates that Netscha?vo is an extremely iron-rich member of the ordinary chondrite sequence, and that plausible models to account for the ordinary chondrite sequence must produce materials having iron/silicon ratios 25 percent greater than those in CI carbonaceous chondrites. The existence of Netscha?vo emphasizes that the chondritic meteorites in terrestrial collections are a biased and incomplete selection of primitive solar system materials.     

Indium variations in a petrologic suite of L-group chondrites

Indium concentrations have been determined by neutron activation in four members of each of the L3, L4, L5, and L6 chondritic meteorite classes. The range of concentrations is found to be from 0.14 to 22 parts per billion, with the highest values in L3 chondrites and the lowest values in the L5 and L6 classes. Plots of indium concentration versus relative mean deviation of pyroxene iron content, total carbon concentration, and primordial argon-36 concentration show positive correlations to varying degrees. Indium concentration appears to be a valuable parameter relating to variable formation conditions of the chondritic meteorites during the early history of the solar system.     

Iron meteorites with low cosmic ray exposure ages

Analysis of argon-38 and argon-39 produced by cosmic rays in four iron meteorites gives normal amounts of the radioactive product argon-39 and abnormally low amounts of stable argon-38. This indicates that these meteorites were exposed to cosmic rays for unusually short periods of time. These exposure times are one or two orders of magnitude shorter than those for the average iron meteorite, and they overlap the periods found for chondrites. It is suggested that perhaps 20 percent of the iron meteorites have similarly short exposure periods.