The chemical classification of iron meteorites. IV - Irons with Ge concentrations greater than 190 ppm and other meteorites associated with group I
TL;DR: In this paper, the authors defined chemical group I as those iron meteorites with 190-520 ppm Ge which fall within the main-sequence fields on Ge vs. Ge and Ge vs Ni plots and defined three additional categories of anomalous irons which show evidence of a relationship to group I.
About: This article is published in Icarus.The article was published on 1970-05-01. It has received 116 citations till now. The article focuses on the topics: Kamacite & Winonaite.
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TL;DR: The Re-Os isotope sytem, based on the long-lived β− transition of 187Re to 187Os, has matured to wide use in cosmochemistry and high-temperature geochemistry as discussed by the authors.
Abstract: ▪ Abstract The Re-Os isotope sytem, based on the long-lived β− transition of 187Re to 187Os, has matured to wide use in cosmochemistry and high-temperature geochemistry. The siderophilic/chalcophilic behavior of Re and Os is different from that of the elements that comprise most other long-lived radiogenic isotope systems. Magmatic iron meteorites (IIIAB, IIAB, IVA, and IVB) have Re-Os isochrons that indicate asteroidal core crystallization within the first 10–40 million years of Solar System evolution. Rocks from Earth's convecting mantle show generally chondritic Re/Os evolution throughout Earth history that is explained by the addition of highly siderophile elements to the mantle after core formation via late accretion. Oceanic basalts have Os-isotope systematics that improve the detailed geological interpretation of extant mantle components. Some portions of ancient subcontinental lithospheric mantle are severely depleted in Re and have correspondingly subchondritic 187Os/188Os, indicating long-term i...
860 citations
01 Jan 1998
786 citations
TL;DR: In this article, the distribution of trace and minor elements in iron meteorites has been analyzed and the distributions interpreted with the chemical groups defined by Wasson, and two fractionations have occurred, a primary event which established the bulk composition of each group and a secondary event which fractionated the elements within each group.
289 citations
TL;DR: In this paper, the authors described the requirements for a genetically significant scheme for classifying iron meteorites and proposed a classification scheme based on Ga-Ni and Ge-Ni plots, taxonomic properties, and chemical properties.
Abstract: The paper describes the general requirements for a genetically significant scheme for classifying iron meteorites. Some of the properties which may be used to classify iron meteorites are reviewed, and a classification scheme based on Ga-Ni and Ge-Ni plots, taxonomic properties, and chemical properties is proposed. It is found that 95% of the irons can be assigned to a genetic group or an anomalous class.
272 citations
TL;DR: In this article, the IAB meteorites are classified into five subgroups based on their Au and Ni contents, i.e., high (H), medium (M), or low (L) and two others the high-Au (H) subgroups.
216 citations
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TL;DR: Signer and Suess as discussed by the authors brought together and evaluated all significant evidence on the origin of meteorites, and showed that the absence of a Widmanstatten pattern in meteorites with > 16% Ni cannot be attributed to high pressures, but to supercooling or an unusually fast cooling rate for these meteorites.
Abstract: This paper attempts to bring together and evaluate all significant evidence on the origin of meteorites. The iron meteorites seem to have formed at low pressures. Laboratory evidence shows that the absence of a Widmanstatten pattern in meteorites with > 16% Ni cannot be attributed to high pressures, but to supercooling or an unusually fast cooling rate for these meteorites, which prevented the development of a pattern. The presence of tridymite in the Steinbach siderophyre provides further, direct proof that the Widmanstatten pattern can form at pressures less than 3 kb. Neither diamond, nor cliftonite, nor cohenite are reliable pressure indicators in meteorites. Diamonds were formed by shock while cliftonite may have been derived from a cubic carbide such as Fe4C. Cohenite is apparently stabilized by kinetic rather than thermodynamic factors. Several lines of evidence suggest that the irons come from more than one parent body, perhaps as many as four. The frequency of pallasites is perfectly consistent with an origin in the transition zone between core and mantle of the parent body. “Hybrid” meteorites such as Brenham are not necessarily derived from the metal-silicate interface, but probably resulted from dendrite growth in the solidifying melt. Ordinary chondrites definitely are equilibrium assemblages rather than chance conglomerates. According to the best available evidence, Prior's rules seem to be valid. The metal particles in chondrites differentiated into kamacite and taenite in their present location, rather than in a remote earlier environment. Trace element abundances in ordinary and carbonaceous chondrites suggest that these meteorites accreted from two types of matter: an undepleted fraction that separated from its complement of gases at low temperatures, and a depleted fraction that lost its gases at high temperatures. These two fractions of primitive meteoritic matter are tentatively identified with the matrix and chondrules-plus-metal, respectively. New restrictive limits are placed on the iron-silicate fractionation in chondrites. No direct evolutionary path exists that connects the currently accepted solar abundances of Fe and Ni and the observed Fe/Si and Ni/Si ratios in chondrites. Apparently the solar abundance of iron is in error. The iron-silicate fractionation seems to have occurred while chondritic matter was in a more strongly reduced state than its present one. The U-He and K-Ar ages of hypersthene chondrites are systematically shorter than those of bronzite chondrites. Short ages are correlated with shock effects, and it seems that the hypersthene chondrites suffered reheating and partial-to-complete outgassing 0.4 AE ago. The cosmic-ray exposure ages of all classes of meteorites cluster distinctly, indicating that the meteorites were produced in a few discrete major collisions rather than by a quasi-continuum of smaller ones. The dates of the principal breakups are: irons, 0.6 and 0.9 AE; aubrites, 45 m.y.; bronzite chondrites, 4 m.y.; hypersthene chondrites, 0.025, 3, 7–13, and 16–31 m.y. All four clusters of hypersthene chondrites show evidence of severe outgassing 0.4 AE ago, which implies that most or all hypersthene chondrites come from the same parent body. As already noted by Signer and Suess, two distinct types of primordial gas occur in meteorites. Differentiated meteorites always contain unfractionated gas, while relatively undifferentiated meteorites contain fractionated gas. The former component is invariably associated with shock effects, and seems to have been derived from the solar wind. The latter component is correlated with other volatiles and seems to be a truly primitive constituent of meteoritic matter. Isotopic anomalies in the fractionated gas suggest that meteoritic matter was irradiated with ⩾ 1017 protons/cm2 at a very early stage of its history. There is very little doubt that most, if not all, meteorites come from the asteroid belt rather than from the moon. The orbits and geocentric velocities of stony meteorites resemble those of the Apollo asteroids (most of which are former members of the asteroid belt that have strayed into terrestrial space), but disagree strongly with the calculated orbits and velocities for lunar ejecta. Opik's conclusions about the difficulty of accelerating lunar debris to escape velocity represent a further argument against a lunar origin of stony meteorites. The most likely parent bodies of the meteorites are the 34 asteroids which cross the orbit of Mars. Collisional debris from these objects will remain in Mars-crossing orbits, and perturbations by Mars will inject some fraction of this material into terrestrial space. Most of the Mars asteroids, comprising 98% of the mass and 92% of the cross-section, belong to three Hirayama families (Phocaea, Desiderata, and Aethra), and an additional, previously unrecognized family. These families were apparently produced by disruption of parent asteroids ca. 104, 105, and 46 km in diameter. The size distribution and light curves of asteroids indicate that the larger asteroids are original accretions, rather than collision fragments. There is no reason to believe that the meteorites ever resided in bodies larger than Ceres (d = 770 km). Various theories on the origin of the meteorites are critically reviewed in the light of the preceding evidence. Wood's theory, which postulates a high-temperature and a low-temperature variety of primordial matter, is in best accord with the evidence. Apparently the asteroids accreted from varying proportions of these two types of material, and were then heated by extinct radioactivity produced in the early irradiation.
699 citations
TL;DR: A detailed study of the thermal evolution of interually heated planetesimals was undertaken with a view to discovering spontaneous processes that might have led to the development of the meteorites.
Abstract: It was proposed that the meteorites originated in planetesinals of asteroidal dimensions, heated by some transient internal energy source, such as extinct radioactivity. The usual objections to an origin in small bodies were examined and were shown to be unfounded. It was shown that segregation of metal and silicate phases and mineral differentiation by crystal settling will take place on reasonable time scales even in small bodies. Evidence concerning the occurrence of diamonds in meteorites was critically examined and wse found to be inconsistent with an origin in large bodies. Instead, it was decided that diamonds were formed as a metastable phase by decomposition of cohenite (Fe/sub 3/ C) under localized stresses or else upon impact with the earth (as proposed by Nininger). A detailed study of the thermal evolution of interually heated planetesimals was undertaken with a view to discovering spontaneous processes that might have led to the development of the meteorites. After the onset of melting, the equilibrium configuration of the planetesimals will comprise an inner core of metal and an outer core of silicate; a mantle of chondritic composition, compacted by sintering; and more » an unconsolidated surface layer. The expected properties of material from each of these strata agree well with those of the known classes of meteorites. Further temperature rise in the planetesimal will result in quasi-volcanic eruptions due to evolution of gases and vapors (e.g., elemental sulfur, carbon monoxide, silicon monosulfide) from the interior. This quasi-volcanic activity will cause extensive recycling of material and can lead to the development of many detailed features of the meteorites. Evidence was presented to show that the capillary veins in stone meteorites were produced by momentary action of hot, sulfur-containing gases. This was in accord with the proposed model. Another consequence of the model is the operation of a cyclic process that will deplete the chondritic mantle in some chalcophile elements, such as In, Tl, Pb, and Bi. This may account for the discrepancies between the observed and predicted abundances of these elements in chondrites. Possible energy sources were critically examined. Only extinct radioactivity seems to meet all requirements. In order for this source to have been important, it is necessary for the parent bodies of the meteorites to have accreted within 6 to 7 million years after nucleogenesis, in which case 7.3 x 10/sup 5/ year Al/sup 26/ could have provided an adequate source of heat, or that an uncharacterized nuclide with a half life of 10/sup 6/ to 10/sup 8/ years exists and was present in the early solar system. (auth) « less
221 citations
TL;DR: In this article, the concentrations of gallium, germanium, cobalt, chromium, and copper have been determined spectrographically in eighty-eight iron meteorites, including representatives from all structural classes, and in nine stony-iron meteorites.
179 citations
TL;DR: In this paper, cooling rates for Widmanstatten pattern formation in iron meteorites used to obtain data on parent meteorite bodies were investigated. And they showed that the Widman statten pattern formed in meteorites can be used to estimate the cooling rate of parent meteorites.
159 citations
TL;DR: In this paper, the concentrations of nickel, cobalt, gallium, palladium and gold have been determined in 45 iron meteorites including representatives from all structural classes including hexahedrites, nickel-poor ataxites and coarse and coarsest octahedrite.
157 citations