Richard W. Bild

Classification of and elemental fractionation among ureilites

Abstract Concentrations of Ni, Zn, Ga, Ge, Cd, In, Ir and Au in five ureilites can be combined with petrographie evidence to yield a well-defined suite extending from Goalpara (heavily shocked, low Ir concentration, low Ir/Ni ratio) through Havero, Dyalpur, Novo-Urei to Kenna (moderately shocked, high Ir concentration, high Ir/Ni ratio). Arguments are presented indicating that this suite represents the sampling of a vertical section within the ureilitic parent body. The large range in Ir/Ni and Ir/Au ratios indicates greater efficiency of extraction of primitive, refractory metal in the Goalpara region than in the Kenna region, and implies that higher maximum temperatures prevailed in the former during the production of ureilitic ultramafic silicates by a partial melting process. A major impact event injected a deposit of C-rich material into the ultramafic silicates. This C-rich material had a moderately high content of metal; there is no direct evidence that it contained volatiles other than the rare gases. High Ca contents of the ferromagnesian minerals indicate that the ultramafics were hot at the time the injection occurred; the zoning of these mineral grains also indicates high temperatures (ca. 1400 K) and low pressures (⩽S 10atm) such that reaction between C and Fe2SiO4 could occur, but that cooling occurred too quickly to allow complete equilibration. The ureilitic C-rich material appears to represent an important type of primitive material. Two siderophile-rich components are necessary to explain the relative siderophile trends in ureilites. We interpret the high-Ir component to be a refractory nebular condensate or residue that was retained during the partial melting event. The low-Ir component, which roughly resembles E4 chondrite siderophiles, is attributed to metal injected together with the vein material.

Netschaëvo: 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.

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

Abstract 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.

Structure and formation of the San Cristobal meteorite, other IB irons and group IIICD

Abstract San Cristobal is an unusual group IB ataxite with 25 per cent Ni, composed of taenite grains 2–3 cm in diameter and silicate-troilite-graphite nodules concentrated on the grain boundaries. Silicate compositions are typical of group IAB: olivine Fa 3.3 , orthopyroxene Fs 6.9 and feldspar Ab 88 . Plagioclase shows peristerite unmixing, previously unrecorded in meteorites, and occasional K-rich feldspar grains have an unusual antiperthite exsolution. Brianite Na 2 CaMg(PO 4 ) 2 and haxonite (Fe, Ni) 23 C 6 are common in nodules and matrix, respectively, while cohenite is rare. Part of the matrix contains a pearlitic kamacite precipitate instead of the usual oriented platelets. San Cristobal has extreme concentrations of many elements; e.g. the highest published Ag, Cu, In and Sb contents and the lowest Mo and Pt in irons. These data and the mineralogy show that San Cristobal has many characteristics of both groups IB and IIID, but that it fits group IB trends better. Ratios of refractory element abundances to those in Cl chondrites (both normalized to Ni) decrease through IB from l in IA to 0.03 in San Cristobal, but the other siderophilic elements have a small range of abundance ratios, 0.5–2, throughout IAB. We suggest that IB grains either formed in a part of the solar nebula where refractories had been previously removed, or else failed to equilibrate with a refractory-rich, high-temperature condensate. After condensation of the volatiles, Fe was partially removed, perhaps by oxidation. Group IIICD seems to have experienced similar fractionations. Unlike other iron meteorite groups, neither IAB nor IIICD appears to have been fully molten.

New occurrences of phosphates in iron meteorites

New occurrences of the phosphates sarcopside/graftonite (Fe, Mn)3(PO4)2, farringtonite (Mg, Fe)3(PO4)2, and brianite Na2CaMg(PO4)2 in iron meteorites are reported. The sarcopside in Duel Hill (1854) (IVA-Anom) contains less than 0.1 mole % Mn3(PO4)2 and more nearly approaches the Fe-rich end member than does any previously reported natural occurrence. This identification of farringtonite in Barranca Bianca (IIE) is the first report of this mineral in a meteorite group other than the pallasites.

Element distribution in size fractions of Apollo-16 soils: Evidence for element mobility during regolith processes

Abstract Three Apollo-16 soils, 61220, 63500 and 65500, having diverse properties were separated into six size fractions and analyzed for 8 volatiles and siderophiles. Relative concentrations of an additional 20 elements were determined in 61220 and 63500. The volatile elements Cd, Zn, In and Ga increase in concentration with decreasing grain size; in the finest fractions the increase is roughly parallel to the increase in specific surface area, and a surface correlation is inferred. The total increase from coarsest (177–500 μm) to finest ( Concentration-size distributions of siderophiles show peaks in the 80–300 μm range for each soil, independent of whether they are dominantly extralunar (Ni, Ge, Au, Ir) or lunar (Co) in origin. If this peak results from agglutinate formation, a viable mechanism must allow for incorporation of the extralunar siderophiles. Alternatively, the peak may result from a continuous growth of metal grain size during the evolution of the regolith.

Extralunar materials in Cone-Crater soil 14141

Abstract Radiochemical neutron activation analysis has been used to determine Ni, Zn, Ga, Ge, Cd, In, Ir and Au in duplicate samples of lunar soil 14141 and one additional replicate each of soils 14163 and 14259. The concentrations of extralunar trace elements Ni, Ge, Ir and Au in 14141 and 14163 are, respectively, about 69 and 82 per cent as high as those in 14259. Although most of the mass of 14141 appears to be ejecta from Cone Crater, a sizable contamination by mature Fra Mauro soil such as 14259 is also present. The siderophilic-element concentrations of the subregolith Fra Mauro materials are estimated to be 25 ± 25 per cent of those observed in 14259.