Philip A. Baedecker

Elemental fractionations among enstatite chondrites

Abstract Neutron activation data on 14 elements in eight enstatite-chondrite falls are reported. These and literature data on an additional 28 elements show that intragroup elemental fractionations generally fall into one of three basic patterns: 1. (1) siderophilic- and chalcophilic-element abundances are about 1.5 times greater in E4-5 than in E6 chondrites; 2. (2) non-volatile lithophile-element abundances in E4-5 chondrites are generally about 1.0–1.2 times those in E6 chondrites; 3. (3) highly volatile elements are higher in E4-5 chondrites than in E6 chondrites by factors of 6–50. In addition, abundances (relative to Si) of most refractory and volatile elements are lower (by factors of 0.5–0.9) in E4 chondrites than in C1 chondrites. Because of the compositional hiatus often observed between E4-5 and E6 chondrites, there exists the distinct possibility that they are separate groups which were stored in different parent bodies. However, because of their close similarity in oxidation state, it seems likely that they originated at the same nebular location, far removed from the formation locations of the other, much more oxidized groups of chondrites. The E-group fractionation patterns can be plausibly explained in terms of four fractionation processes: 1. (1) loss of oxidizing agents (i.e. H 2 O) and refractory materials from starting materials of solar composition: 2. (2) partial loss of moderately volatile elements, perhaps as a result of gradual loss of nebular gas during condensation; 3. (3) more efficient agglomeration of metal particles than silicate particles; and 4. (4) increase of nebular temperatures during agglomeration-accretion resulting in the loss of volatile-rich late condensates from E6 chondrites. The low degree of oxidation of enstatite chondrite materials is best understood in terms of a fractionated nebula. At a pressure of 10 −4 atm the Si content of E4 metal can be produced at 1350°K if the H 2 O H 2 ratio is 5 times lower than that in unfractionated solar-system material. A nebula-wide fractionation process involving radial transport of refractories and H 2 O is indicated, and a suitable model in which the nebula-wide mixing of the gas phase continues during condensation is proposed.

Allende inclusions: volatile-element distribution and evidence for incomplete volatilization of presolar solids

Abstract Neutron activation data on 14 trace elements in Allende bulk samples and in fractions of spheroidal Ca-Al-rich inclusions show several distinct distribution patterns. Refractories Ir and Sc have high inclusion/bulk ratios and show little variation with depth. Manganese, Fe, Co, Ni, Ga, Cd and In have low inclusion/bulk ratios and decrease with increasing depth; their presence in the inclusions reflects matrix contamination. Sodium and other alkalies have high inclusion/bulk ratios (near 0.5) and decrease with increasing depth; their high concentration despite moderate volatility seems related to condensation reactions in which refractory Al-bearing minerals are reactants. Chromium, Zn, Ge and Au show patterns similar to those of the alkalies; this seems to indicate that refractory minerals are reactants in their condensation reactions, but thermodynamic support for this hypothesis has not been found. We propose that the large size of Allende spheroidal inclusions indicates an origin by incomplete vaporization of presolar solid matter followed by recondensation of refractories on a limited number of condensation nuclei. The low abundance of large refractory inclusions in ordinary and enstatite chondrites reflects complete vaporization of presolar solids at their formation locations; constraints on homogeneous nucleation resulted in the simultaneous condensation of refractories and olivine at these locations. Quadruplicate analyses of the Orgueil chondrite are in good agreement with previous determinations with the exception of small systematic differences in Au and Ir.

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.

Distribution of Ni, Ga, Ge and Ir between metal and silicate portions of H-group chondrites.

Abstract Concentrations of Ni, Ga, Ge and Ir have been determined in the metal and silicate portions of 21 chondrites, including 15 H chondrites. The H-group metal shows the following concentration ranges: 7.2–9.4 per cent Ni, 2.4–18 ppm Ga, 61–70 ppm Ge and 1.6–4.6 ppm Ir, and concentrations in H-group silicates are 94–380 ppm Ni, 3.0–9.2 ppm Ga, 0.06–0.66 ppm Ge and 0.03–0.12 ppm Ir. The Ni, Ge and Ir contents in the metal are positively correlated with each other and with the Fe content of olivine, as expected from oxidation-reduction or combined metal-silicate-fractionation/oxidation-reduction models. Metal/silicate concentration ratios for Ir are lower than for Ge and Ni, despite the fact that Ir is more easily reduced to the elemental form. This may indicate that at the formation location of the H-group chondrites in the solar nebula, substantial amounts of Ir were present in condensed form at temperatures which were so high that Fe and Ni were present mainly as vapor. The metal/silicate concentration ratios of Ga and Ge are lower in type-3 ordinary chondrites than in types 4–6. Apparently appreciable fractions of these elements condensed from the nebula in oxidized form and entered the metal during later thermal events. That Ga and Ge were redistributed during recrystallization, whereas appreciable Ir remained in the silicate fraction, probably indicates that Ir faced a substantially greater diffusional barrier than did Ga and Ge.

Relationship between siderophilic-element content and oxidation state of ordinary chondrites

Abstract The concentrations of Ni and Ir have been determined by neutron activation in a suite of ordinary chondrites for which accurate ferromagnesian-mineral compositional data were available. Although hiatus between the individual groups exist, the trends within the groups are in keeping with the hypothesis that the ordinary chondrites form a continuous fractionation sequence. A significant negative correlation is observed between the abundance of Ni or Ir and the Fe content of the ferromagnesian minerals in the H and L groups, and for Ir in the LL group, as expected if the metal-silicate fractionation and the variation in oxidation states were produced by the same or related processes. The Ir Ni ratio decreases by a factor of 1.2 between the H and LL groups. This fractionation must have occurred at an early stage in the condensation phase of the solar nebula.

Siderophiles and volatiles in Apollo-16 rocks and soils

Abstract The concentration of the extralunar component in mature Apollo-16 soils is about 3.8%, substantially higher than values observed at Apollo-14 (2.7%) or mare landing sites (1.2–1.3%). It appears that a large amount of local pre-Imbrium regolith was incorporated into Apollo-16 soils: the amount incorporated into Apollo-14 soils was smaller, probably as a result of the combination of two factors: 1. (1) a larger dilution by subregolith materials, and 2. (2) a lower concentration of siderophiles in the ancient Fra Mauro regolith relative to that of the Southern Highlands. Volatiles, siderophiles and cosmogenic isotopes are intercorrelated in Apollo-16 soils, mainly as a result of mixing of mature soils having high concentrations with North Ray Crater ejecta having low concentrations. The distributions of Zn, Cd and In in soils and soil breccias are largely controlled by volatility; a positive correlation with Zn suggests that volatility also affects the distribution of Ga. The concentrations of Zn, Ga and Cd are higher and those of Sc and Fe lower in light relative to dark mature and submature soils. Volatile concentrations appear to be slightly higher in Apollo-14 soils than in Apollo-16 soils.

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 .

Gallium, germanium, indium, and iridium in lunar samples.

Neutron activation analyses of gallium, germanium, indium, and iridium in eight lunar samples and in meteorites and rocks (including four calciulnrich achondrites and five terrestrial basalts) with similar bulk compositions are reported. Lunar gallium concentrations are remarkably constant at about 5 parts per million, three times higher and four times lower than those in eucritic (calcium-rich) achondrites and terrestrial basalts, respectively. Lunar germanium concentrations range from ≤ 0.04 to 0.4 part per million. Basaltic achondrites have similar germanium concentrations, whereas those in terrestrial basalts are uniformly higher at about 1.4 parts per million. Concentrations of indium in Ilunar samples range from 3 to 60 parts per billion, whereas those in terrestrial basalts are 80 to 100 parts per billion, and those in basaltic achondrites 0.4 to 3 parts per billion. Lunar iridium concentrations appear to be in the 0.1 to 10 parts per billion range. The Tranquillitatis samples are distinctly different from calcium-rich achondrites and terrestrial basalts.

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.