J.F. Lovering

Minor and trace element abundances in chondritic meteorites.

Abstract The abundances of the elements Cl, F, P, Mn, Ti, Zn, Ge, Co, Ni, Fe, V, Cu, Sr, Cr, Ba and Sc have been determined in a total of fifty meteorites involving representatives of all chondrite groups. These analyses are discussed in terms of the observed co-variations among the various elements and the chemical fractionation which distinguishes the individual chondrite groups from each other. The results indicate: 1. (1) The trace elements Cu, Zn, Ge and Cl are fractionated among the carbonaceous, enstatite and ordinary chondrites similar to the so-called “fractionated chalcophile” elements; 2. (2) A fractionation of silicate plus sulphide phase from metal phase within both the bronzite and hypersthene chondrite groups; 3. (3) Priors Rule is not strictly valid in any of the chondrite groups; 4. (4) A fractionation of cobalt from nickel such that the metal-rich chondrites in both the hypersthene and bronzite groups are enriched in cobalt relative to nickel; 5. (5) The chemical differences between the hypersthene and bronzite chondrite groups can be attributed almost entirely to a relatively minor transfer of cobalt-rich metal from the hypersthene to the bronzite chondritess followed by further reduction.

The rubidium-strontium age of the Bishopville aubrite and its component enstatite and feldspar

Abstract Replicate measurements have been made of the Sr and Rb concentrations and Sr isotopic compositions of two total-meteorite samples of the Bishopville aubrite, and of the component enstatite and plagioclase from a third sample. These data provide an “internal” age for the meteorite. They apparently define an isochron whose equivalent age lies between 3·5 × 10 9 and 3·9 × 10 9 years at the 95 per cent confidence level, suggesting that this aubrite may have formed at a significantly later time than the major period of meteoritic fractionation some 4·5 to 4·6 × 10 9 years ago. Other interpretations for the apparent isochron are also discussed. Comparative data are also given for the Moore County eucrite which agree well with analyses reported elsewhere after the necessary normalization of isotopic ratios is carried out.

The abundance of mercury in meteorites and rocks by neutron activation analysis

Abstract Mercury abundances have been determined in all classes of chondritic meteorites, the principle classes of achondritic meteorites, alkali and tholeiitic basalts, differentiated tholeiitic basalt intrusions, possible lower crustal and upper mantle rock inclusions, and a collection of six standard crustal rocks. The analytical method used was based on thermal neutron activation. Relatively high mercury abundances in the carbonaceous chondrites, previously reported by Reed, Kigoshi and Turkevich, have been confirmed. Abundance data among the various classes of chondrites and achondrites suggest a more complex behavior for mercury in the evolution of the meteorites than had been previously proposed. Possible lower crustal or upper mantle materials exhibit high mercury abundances in the range of the basic achondrites while six standard crustal rocks have abundances in the range of only 0·004–0·04 ppm. Implications of the data with respect to the estimation of cosmic abundances, and the evolution of the meteorites and the earth are discussed.

Pressures and temperatures within a typical parent meteorite body

Abstract Revised estimates have been made of the metal/silicate and metal/sulphide fractionations for nickel in the stony-iron and iron meteorites, respectively. Assuming equilibrium fractionations at temperatures of about 2000°K, pressures of between 10 4 and 10 5 atm must have been operating within the typical parent meteorite body. Revised estimates of the rate of depression of the γ → α transformation temperatures of nickel-iron alloys with pressure ( Uhig , 1954) suggest iron meteorites have transformed under pressures of approximately 7 × 10 4 atm. Furthermore, if about 10 8 years were available from the time of crystallization of octahedrites, through cooling into transformation regions and to the disruption of the body, the core of the parent body must have cooled to a minimum temperature of 700°K. If graphite was transforming to diamond in the Canyon Diablo octahedrite at 700°K in the core of the parent body, pressures of between 2 × 10 4 and 3 × 10 4 atm must have been operating in the core. This evidence indicates that pressures seem to have a maximum value of between 2 × 10 4 and 10 5 atm while temperatures of approximately 200°K are assumed to have been operating at the time when the mantle of the parent body was crystallized or crystallizing and when the outer metal core at least was still molten. There are indications that the crystallized core had cooled to about 700°K before it was disrupted. The volume of the typical parent meteorite body is likely to have approached that of the moon.

Rhenium and osmium abundances in chondritic meteorites

Abstract The abundances of Re and Os in thirty-two chondrites representing all classes have been determined by neutron activation analysis. Some disagreement with previous determinations of Os in ordinary chondrites can be explained by the difficulties of sampling for siderophilic elements when metallic nickel-iron is present. Considerable fractionation of Re and Os between chondrite classes is apparent, and cannot be interpreted solely on the basis of transfer of metallic phase rich in these two elements. Solar atomic abundances relative to 106 atoms Si calculated from the mean of all chondrites analysed are 0.053 atoms Re and 0.61 atoms Os.

Differentiation in the iron-nickel core of a parent meteorite body

Abstract The observed range in nickel content of iron meteorites may be explained by their having differentiated during the crystallization of an originally homogeneous iron melt containing 11 % nickel which formed the core of a typical parent meteorite body. Quantitative estimates of the relative abundances of the various types of iron meteorites which can differentiate from nickel-iron melts of varying compositions are calculated. Again assuming an original melt of 11 % nickel, the agreement between the calculated relative abundances and new estimates of the observed relative abundances of iron meteorites is very striking. It is significant that the metal phase of stony-iron meteorites, which may be expected to be a sample of the undifferentiated melt, has a mean nickel content of 11 %. The well-known metallurgical phenomenon of “coring” (under conditions approximating complete mixing in the liquid), which causes segregation of solute in the solid phase during the crystallization of alloys, is probably the best analogy to the differentiation process suggested.

Fractionation of fluorine, chlorine and other trace elements during differentiation of a tholeiitic magma

Abstract The elements F, Cl, Ni, Co, Cu, Ga, Sc, Cr, V, Ba and Sr have been determined in two drill cores through a differentiated tholeiitic dolerite sill from Tasmania. A multivariate statistical technique is employed to demonstrate the occupancy of hydroxyl lattice sites by both chlorine and fluorine. Consideration of trace-element/major-element ratios suggests that, in these dolerites. Ni occupies Fe +2 sites, Co and Sc replace both Mg and Fe +2 Cr replaces Fe +3 ,V +4 replaces Ti, Ga replaces Al and Ba and Sr both replace Ca.

Temperatures and mass losses in iron meteorites during ablation in the earth's atmosphere

Abstract The results of thermomagnetic analyses of two nickel-rich ataxites (Tawallah Valley and Wedderburn) have been used to estimate the temperature gradients within their thermal alteration rims which formed during flight through the earths atmosphere. Knowing these temperature gradients it is possible to calculate temperature rises within iron meteorites during ablation as well as the average rate of ablation of the surfaces of these meteorites (i.e. about 0.18 cm/sec). If the time during which ablation takes place averages 3.3 sec, then the average thickness of material ablated from the surfaces of these iron meteorites is about 0.6 cm. These data indicate that about 27 per cent of the pre-atmospheric mass of the Tawallah Valley ataxite and about 60 per cent of the smaller Wedderburn ataxite has been ablated in the atmosphere.

Electron microprobe analysis of terrestrial and meteoritic cohenite

Abstract Cohenite (FeNi) 3 C occurs in metallic masses found in basaltic rocks at Uivfaq, Disko Island, West Greenland. Electron microprobe studies have shown that the large cohenite grains up to 0.5 mm across contain about 0.60 per cent nickel while narrow plates of cohenite around 20 μ across contain up to 3.14 per cent nickel. Cohenite from the Coolac coarse octahedrite contains 1.34 per cent nickel. The existence of terrestrial cohenite in nickel-iron metallic masses in coarse grained basaltic flows or very shallow intrusions would suggest that cohenite can form and be preserved in a relatively slowly cooling, low pressure environment. It follows that meteoritic cohenite can no longer be considered an unequivocal “high pressure indicator” in the iron meteorites.

The evolution of tektites: elemental volatilization in tektites

Abstract Zinc and copper concentrations have been determined by spectrophotometric and emission spectrographic techniques in a number of tektites (i.e. australites, javaites, phillipinites, indochinites, bediasites and moldavites). The data indicate a considerable loss of zinc and, possibly, copper from the tektite parent material by selective volatilization during the fusion process which formed the tektites. Furthermore, the observed positive correlation between mass and specific gravity in both the australites and bediasites indicates that the major element content of tektites has also been significantly altered by selective volatilization. This work suggests that acid igneous rocks are suitable parent materials for the formation of tektites.