Chondritic Metal Research Articles

The metal phase in unequilibrated ordinary chondrites and its implications for calculated accretion temperatures

The abundance, composition and grain size of the metal available to volatile siderophile elements strongly affect the condensation of these elements. These parameters are redefined on the basis of published chemical analyses and new mechanical analyses of the unequilibrated ordinary chondrites. The results suggest that previous workers have seriously overestimated the amount of metal present and available during condensation, and seriously underestimated the heat of solution of Bi in chondritic metal. Correction of these parameters results in nominal accretion temperatures for Bi which are substantially (95–110°K) lower than those calculated earlier, and which are discordant with the temperatures inferred for chalcophile trace elements.

Mesosiderites—I. Compositions of their metallic portions and possible relationship to other metal-rich meteorite groups

The metal from 17 mesosiderites has been analyzed for Ni, Ga, Ge and Ir by the techniques of atomic-absorption spectrometry and neutron activation. Most mesosiderite metal samples fall in a narrow compositional range: Ni, 7·0–9·0 per cent; Ga, 13–16 ppm; Ge, 47–58 ppm; and Ir, 2·4–4·4 ppm. Most of those falling outside these ranges belong to Powell's (1971) least-metamorphosed type. Mesosiderite metal falls in the same general composition range as IIIAB irons, IIIE irons, pallasites and H-group chondrite metal. There are distinct differences in detail, however, and firm evidence for a close genetic relationship between any of these groups and the mesosiderites is lacking. Metallic portions of Weekeroo-type irons tend to have slightly higher Ni, Ga, Ge and Ir contents than found in mesosiderite metal, and the two groups tend to form a single trend on all plots. The Weekeroo-type silicates closely resemble mesosiderites in terms of orthopyroxene composition and oxygen-isotope ratio. We interpret these similarities to indicate that the silicate and metallic portions of these two groups are closely related; if the mesosiderite silicates and metal were initially formed in separate parent bodies, these were of similar composition and formed at about the same distance from the Sun.

The distribution of trace elements in chondritic meteorites. 1. Gallium, germanium and indium

A rapid neutron activation technique, using the nuclides 70Ga + 72Ga, 75Ge and 116mIn, was developed for the simultaneous determination of gallium, germanium and indium in geological materials. This method was employed to examine the fractionation of gallium, germanium and indium between ordinary, enstatite and carbonaceous chondrites and to investigate the distribution of gallium and germanium between the magnetic and non-magnetic fractions of ordinary and enstatite chondrites. A comparison of the gallium and germanium contents of the metal phases of different chondrite groups was also made. Some indium determinations on separated ordinary chondrite phases show that indium is concentrated mainly in the non-magnetic phases. Germanium is strongly siderophile in both ordinary and enstatite chondrites; gallium is strongly siderophile in enstatite chondrites, but less strongly in ordinary chondrites. Germanium results for metal phases belonging to the same chondrite groups are very similar, but there exist distinct differences between the germanium values found in the metal phases of enstatite, bronzite and hypersthene chondrites. The gallium and germanium contents of chondritic metal phases are not so variable as gallium and germanium results for iron meteorites. All three elements are shown to be fractionated among the different chondrite classes with depletion ratios in the order Ga< Ge⪡ In. Germanium, but not gallium, is enriched in bronzite relative to hypersthene chondrites. The results obtained here are in support of previous suggestions that enstatite chondrites should be subdivided into two groups. The fractionation patterns of gallium and germanium between types I, II and III carbonaceous chondrites are similar, but indium is more strongly fractionated.

The distribution of trace elements in chondritic meteorites. 2. Antimony, arsenic, gold, palladium and rhenium

A radiochemical separation scheme for the isolation of the nuclides 76As (26.8 h), 109Pd (13.6 h), 122Sb (2.7 d), 188Re (17 h) and 198Au (2.7 d) from neutron irradiated geological materials was developed and was employed to study the distribution of arsenic, palladium, antimony, rhenium and gold in chondritic meteorites. Values for all five elements were obtained in the magnetic and non-magnetic fractions of eight bronzite, nine hypersthene and four enstatite chondrites as well as in six carbonaceous chondrites representative of all three classes. The results show that all these trace elements are predominantly siderophile in chondrites; significant quantities of antimony only were detected in the non-magnetic fractions. The values found in the metal phases of chondrites are confined to a much narrower range than the results for the same elements in iron meteorites. Mean values for these trace elements in bronzite, hypersthene and enstatite chondrite metal phases are distinctly different and none of the metal phase groups shows compositional similarities with iron meteorite structural groups. All the elements are depleted in hypersthene chondrites relative to bronzite chondrites, but only antimony can possibly be included as one of the group of “fractionated elements”. The results obtained here together with observations by other workers suggest that cosmic abundance figures in the mass region A = 105–125 should be raised.

Genesis of chondritic meteorites

Genesis of chondritic meteorites