C.-L. Chou

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.

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.

Mesosiderites and howardites: igneous formation and possible genetic relationships

We report neutron activation data for major, minor and trace elements determined in whole rock howardites and silicates from mesosiderites. Compositions of howardites and mesosiderites are similar, and intermediate between those of eucrites and diogenites. Relative to howardites mean mesosiderite abundances are slightly nearer the diogenites. Literature data indicate that mesosiderites have a higher normative silica component than howardites. It appears that this partly results from a higher content of a highly evolved igneous component, and partly from in situ reduction of FeO to Fe followed by magnetic separation of metal prior to analysis. Removal of a portion of the FeO in this manner yields a higher normative SiO2 component for the nonmagnetic fraction. Petrographic observations demonstrate the formation of SiO2 which may have resulted from a combination of various factors including accretion of a reducing agent together with the Fe-Ni metal, extensive reaction during the long cooling period, and catalysis by the finely divided metal. In the mesosiderites Mincy, Lowicz and Veramin the light rare earth elements (REE) are enriched. The resulting REE pattern is qualitatively similar to that in terrestrial basalts thought to have been formed by small degrees of partial melting. Of several partial melting models tested, the best match to the REE patterns is provided by one involving ~2–4% partial melting of a source containing low REE abundances. It appears that the light REE enrichment is not associated with the hypothetical silica enriched igneous phase. Since numerous properties separate mesosiderite silicates from howardites, it is clear that they are not composed of precisely the same material. Whether or not they originated on the same parent body is unresolved. If parent body regoliths were mixed vertically and horizontally on a planet-wide basis, then separate bodies would be required.

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 Apollo 16 soils and the decay rate of the extralunar flux 4.0 Gy ago.

Abstract The concentration of extralunar materials in the Apollo 16 regolith is about 3.5%, about 50% higher than that observed at the Apollo 14 site, and three times higher than values at mare landing sites. The integrated flux of extralunar materials is 2.5 times higher at the Apollo 16 than at the Apollo 14 site. These data support the hypothesis that the flux of extralunar materials decreased rapidly between 4.0 and 3.7 Gy ago, and demonstrate the presence of appreciable unaccreted materials near 1 AU 600 my after the formation of the solar system. Assumed ages of 3.91 and 4.00 Gy for the Apollo 14 and 16 sites yield a half-life of this short-lived population of about45 ± 15 my. The lunar accretion rate 4.00 Gy ago was about 2.4 × 10 −6 g cm −2 yr −1 , about 700 times greater than the current accretion rate of 3.5 × 10 −9 g cm −2 yr −1 .

Composition and classification of clasts in the St. Mesmin LL chondrite breccia

Abstract Seven samples of the unique St. Mesmin meteorite have been analyzed by instrumental and radiochemical neutron activation analysis for Na, Ca, Sc, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, Se, In, Sm, Yb, Ir and Au. St. Mesmin is unique in being the only ordinary chondrite known to contain an unmelted xenolith of another ordinary chondrite. Data for two host matrix samples and three light clasts are consistent with their classification as LL chondrite material. The composition of the large dark xenolith confirms earlier evidence that it is an H chondrite; volatile abundances are consistent with it being highly shocked, petrologic type-4 material. In an olivine microporphyry, siderophile abundances are mostly about 0.13 times LL abundances, an apparent indication of metal loss during the shock melting which produced the clast. As in other regolithic chondrites, the dark host has higher contents of highly volatile elements than do the light clasts. We suggest that this results from a combination of differences in intensity of preexisting metamorphism as well as a redistribution of volatiles during regolith gardening. The H-group xenolith in St. Mesmin is a relatively recent addition to the parent body (

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.