John T. Wasson

Ordinary chondrites: Bulk compositions, classification, lithophile-element fractionations and composition-petrographic type relationships

Concentrations of 26 elements were determined by replicate neutron-activation analysis in 66 ordinary chondrites (22 H, 20 L, 17 LL, 2 intermediate between H and L and 5 intermediate between L and LL). Olivine and kamacite compositions were determined in adjacent samples; about 20% of the chondrites contain kamacite or olivine grains with aberrant compositions > 3s from the mean. The sample set was biased in favor of the reduced, siderophile-rich and oxidized, siderophile-poor members of the groups and in favor of chondrites reported to have unusual compositional features. Several chondrites were reclassified: e.g., the photographed fall, Innisfree, is L, not LL; Albareto is LL, not L; and Xingyang and Zhovtnevyi are H6, not H5. On a plot of kamacite Co concentration versus Fa content of olivine, there is a hiatus between H and L, but no hiatus between L and LL. Five chondrites (Bjurbole, Cynthiana, Knyahinya, Qidong, Xi Ujimgin) fall between the main L and LL clusters. Cosmic-ray and U, Th-He outgassing age data do not demonstrate relationships to either group. Our siderophile data support the previous group assignments of unequilibrated chondrites in all cases but two: Bremervorde and Tieschitz have siderophile levels intermediate between H and L. Our mean group compositions are in good agreement with those previously reported. We confirm that the Co/Ni ratio decreases about 5% through the H-L-LL sequence and that Na and Mn abundances are about 7% lower in H than in L and LL. Selenium and Zn show similar abundances in the three groups; the very low (~0.1 × CI) Zn abundance is attributed to condensation as fine, ZnS aerosols that inefficiently settled to the midplane. Abundances of V and Cr decrease by only ~2% between H and LL; thus, only a small fraction was in nebular siderophile components. With the exception of highly volatile Br, no significant differences in abundance are observed among the petrographic types of each group. This conflicts with earlier conclusions that intertype differences (including a systematic increase in siderophile abundance with increasing type) are present. The small differences we observed are attributable to anomalously low or high contents of one or two phases (generally metal and/or troilite) in a few replicates. The absence of a relationship between composition and petrographic type is consistent with models calling for the progressive thermal metamorphism of primitive unequilibrated materials to produce the observed spectrum of petrographic grades, and places narrow limits on the relative accretion efficiencies of nebular components in those models calling for the sequential accretion of nebular materials.

Nebular condensation of moderately volatile elements and their abundances in ordinary chondrites

Abstract The condensation reactions of seventeen moderately volatile elements in a cooling solar nebula were investigated, and condensation temperatures reported for six previously unstudied elements (F, Li, As, Se, Sb and Te). Literature condensation temperatures were confirmed for seven elements (P, Na, S, Mn, Cu, Ga, and Au) and significant differences found for four elements (Zn, Ge, Ag and Sn). A strong correlation is observed between ordinary-chondrite/CI-chondrite abundance ratios and ideal-solution condensation temperatures, and the relationship is strengthened when condensation temperatures for seven elements are recalculated to include estimated activity coefficients. These results can be understood in terms of a model in which volatiles are lost as gases prior to condensation or as finely divided solids that are incompletely agglomerated, and the condensation/agglomeration efficiency gradually decreases as a function of time. A strong correlation between CM/CI chondrite abundance ratios and condensation temperature is also observed, but the decrease by a factor of 2 in the abundance ratios is appreciably less than the factor of 9 observed in ordinary chondrites. If the volatile-loss model is correct, the fractionation process in the nebular region where the CM chondrites formed was less efficient than that at the ordinary chondrite formation location.

Accretion Rate of Extraterrestrial Matter: Iridium Deposited 33 to 67 Million Years Ago

Iridium measured in 149 samples of a continuous 9-meter section of Pacific abyssal clay covering the time span 33 to 67 million years ago shows a well-defined peak only at the Cretaceous/Tertiary boundary. In the rest of the section iridium ranges from a minimum concentration near 0.35 nanograms per gram in the Paleocene to a maximum near 1.7 in the Eocene; between 63 and 33 million years ago the mean iridium accumulation rate is approximately 13 nanograms per square centimeter per million years. Correction for terrestrial iridium leads to an extraterrestrial flux of9 � 3 nanograms of iridium per square centimeter per million years, and an estimated annual global influx of 78 billion grams of chondritic matter, consistent with recent estimates of the influx of dust, meteorites, and crater-producing bodies with masses ranging from 10-13 to 1018 grams. Combining the recent flux of objects ranging in mass from 106 to 107 grams with the flux of 1014 - to 1015 -gram objects indicates that the number of objects is equal to 0.54 divided by the radius (in kilometers) to the 2.1 power. Periodic comet showers should increase the cometary iridium flux by a factor of 200 to 600 on a time scale of 1 to 3 million years; the predicted iridium maxima (more than 30 times background) are not present in the intervals associated with the Cretaceous/Tertiary boundary or the tektiteproducing late Eocene events.

the IAB iron-meteorite complex: A group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts

Abstract We present new data for iron meteorites that are members of group IAB or are closely related to this large group, and we have also reevaluated some of our earlier data for these irons. In the past it was not possible to distinguish IAB and IIICD irons on the basis of their positions on element-Ni diagrams, but we now show that plotting the new and revised data yields six sets of compact fields on element-Au diagrams, each set corresponding to a compositional group. The largest set includes the majority (≈70) of irons previously designated IA; we christened this set the IAB main group. The remaining five sets we designate “subgroups” within the IAB complex. Three of these subgroups have Au contents similar to the main group, and form parallel trends on most element-Ni diagrams. The groups originally designated IIIC and IIID are two of these subgroups; they are now well resolved from each other and from the main group. The other low-Au subgroup has Ni contents just above the main group. Two other IAB subgroups have appreciably higher Au contents than the main group and show weaker compositional links to it. We have named these five subgroups on the basis of their Au and Ni contents. The three subgroups having Au contents similar to the main group are the low-Au (L) subgroups, the two others the high-Au (H) subgroups. The Ni contents are designated high (H), medium (M), or low (L). Thus the old group IIID is now the sLH subgroup, the old group IIIC is the sLM subgroup. In addition, eight irons assigned to two grouplets plot between sLL and sLM on most element-Au diagrams. A large number (27) of related irons plot outside these compact fields but nonetheless appear to be sufficiently related to also be included in the IAB complex. Many of these irons contain coarse silicates having similar properties. Most are roughly chondritic in composition; the mafic silicates show evidence of reduction during metamorphism. In each case the silicate O-isotopic composition is within the carbonaceous chondrite range (Δ17O ≤ −0.3‰). In all but four cases these are within the so-called IAB range, −0.30 ≥ Δ17O ≥ −0.68‰. Fine silicates appear to be ubiquitous in the main group and low-Au subgroups; this requires that viscosities in the parental melt reached high values before buoyancy could separate these. The well-defined main-group trends on element-Au diagrams provide constraints for evaluating possible models; we find the evidence to be most consistent with a crystal segregation model in which solid and melt are essentially at equilibrium. The main arguments against the main group having formed by fractional crystallization are: a) the small range in Ir, and b) the evidence for rapid crystallization and a high cooling rate through the γ-iron stability field. The evidence for the latter are the small sizes of the γ-iron crystals parental to the Widmanstatten pattern and the limited thermal effects recorded in the silicates (including retention of albitic plagioclase and abundant primordial rare gases). In contrast, crystal segregation in a cooling metallic melt (and related processes such as incomplete melting and melt migration) can produce the observed trends in the main group. We infer that this melt was formed by impact heating on a porous chondritic body, and that the melt was initially hotter than the combined mix of silicates and metal in the local region; the melt cooled rapidly by heat conduction into the cooler surroundings (mainly silicates). We suggest that the close compositional relationships between the main group and the low-Au subgroups are the result of similar processes instigated by independent impact events that occurred either at separate locations on the same asteroid or on separate but compositionally similar asteroids.

The compositional classification of chondrites: V. The Karoonda (CK) group of carbonaceous chondrites

Abstract Petrographie and bulk compositional data reveal the existence of a new group of carbonaceous chondrites consisting of the observed fall, Karoonda, one large find from Maralinga, Australia, and 6–11 small finds from five sites in Antarctica. Ningqiang, also a fall, is genetically related to the group. Compositional, textural, and O-isotope data show that the new group is closely related to CV and CO chondrites. In keeping with the practice of naming carbonaceous chondrite groups after a prominent member, we designate it the Karoonda or CK group. All normal CK members are metamorphosed; petrographie grades range from 4 to 6. Some contain shock veins and all exhibit various degrees of blackening due to the dispersion of fine particles of sulfides and magnetite in silicates. Only one other group (EL) has no unequilibrated members. The unequilibrated Ningqiang chondrite is more similar to CK than to CV or CO chondrites, but differs significantly (e.g., low refractory lithophiles, high Mn and Na) in detailed composition. Elemental abundance patterns in CK chondrites are similar to those in CO chondrites, and even more similar to those in CV chondrites. Refractory lithophile abundances are about 1.21× greater than in CI chondrites, a level intermediate between those in CO and CV chondrites; CK refractory siderophile abundances are also intermediate between CV and CO levels. The CK volatile abundance pattern is quite similar to the CV pattern, with CK abundances of most volatiles 10–20% lower than CV values. It appears that nebular conditions and processes were closely similar at the CK and CV formation locations. Although precise probability calculations are difficult because of uncertainties regarding pairing and because so few samples are known, the exceptional abundance of CK chondrites in Antarctica requires an explanation. We suggest that compared to other groups, such as CO or CV, the fragmentation of the CK parent object(s) produced a substantially larger proportion of small meteoroids.

CLASSIFICATION AND ORIGIN OF IAB AND IIICD IRON METEORITES

Abstract We have analyzed, by duplicate neutron-activation analysis, 125 members of iron meteorite groups IAB and IIICD. These data show no hiatus between the groups, and we recommend that the two sets be treated as a single group until data are obtained that require their separation. In cases where there is no ambiguity, we will use IAB to designate the combined group. Our data allow properties of group IAB to be more tightly constrained than heretofore. We examined the properties of ungrouped irons occupying the same region of GaNi and GeNi space as IAB and found that our more precise data did not add any to the group. Based primarily on CoNi, AuNi, and to a lesser extent, CuNi and AsNi trends we find that three irons (Hassi-Jekna, Magnesia and Qarat al Hanash) previously assigned to IIICD are better designated ungrouped, primarily because their Au contents diverge from those of IAB; six irons (EET 84300, Mertzon, Misteca, Persimmon Creek, Yongning, and Zacatecas (1792)) have also been removed from IAB on the basis of the new data. Fractional crystallization models of magmas saturated in troilite yield trends for several elements quite similar to those observed in IAB and, with moderate modification of the distribution coefficients, these can also account for the IIICD trend. However, this model requires metal segregation and convective stirring at very low temperatures, and predicts a much lower abundance of low-Ni irons than observed. It also fails to account for the ubiquitous presence of trapped melt and chondritic (or subchondritic) silicates in IAB irons. The weight of the evidence supports an impact-melt model in which individual IAB irons are interpreted as melt pools produced by impacts into a chondritic megaregolith. The lower the melt temperature of the melt, the larger the fraction of Ga, Ge, and Ir that remained sequestered in unmelted solids. The increasing range in Ga, Ge, and Ir with increasing Ni content can be explained by mixing between different primary melt pools having compositions along the lower (IIICD) envelope of IAB-IIICD. And we suggest that some high-Ni melt pools having low cotectic temperatures experienced crystal settling on a scale of cm to m; specifically, this process may be responsible for producing the most Ni-rich members of the group. We infer that the IAB precursor materials had properties (high porosity, fine grain size) that made them susceptible to impact melting. A high FeS would also favor melt generation. The oxygen isotope composition of IAB silicates is 0.45%o below the terrestrial-fractionation line in a region occupied by CR chondrites and other carbonaceous-chondrite-related meteorites. This suggests that the chondritic precursor was a metal- and FeS-rich carbonaceous chondrite. Cl-normalized element/Ni ratios in low-Ni (65–68 mg/g) TAB are ≥1.5 for Co, Ga, Ge, and Au. Although the high Ga and Ge abundance ratio could indicate fractional crystallization of elements having high solid/melt distribution coefficients, this explanation cannot account for the Co and Au enrichments. We interpret the high Ga, Ge, and Au ratios in terms of a nebular condensation-accretion model; we suggest that these elements condensed into a late-formed, fine nebular component having an enhanced abundance in the chondritic materials parental to group IAB. Because it is more refractory than Fe and Co, the Ni content of this fine metal was appreciably lower than that of the early formed coarse metal.

Composition of the metal phases in ordinary chondrites: implications regarding classification and metamorphism

Abstract EMP determinations of Fe, Co and Ni in the metal phases of ordinary chondrites confirm the report of Sears and Axon that kamacite Co contents show restricted, nonoverlapping ranges in the three groups; ranges are 3.3–4.8 mg/g in H, 6.7–8.2 mg/g in L and 15–110 mg/g in LL. Experimental data by Widge and Goldstein show that the Ni concentration of the α (α + γ) boundary increases with increasing Co concentration: unexpectedly, we find lower kamacite Ni concentrations in unequilibrated LL chondrites (44–55 mg/g) than in H and L chondrites (57–69 mg/g). We infer that, at temperatures below 550° C increasing Co causes a decrease in the equilibrium kamacite Ni concentration of an α-γ system. Although some evidence indicates that the equilibrated L chondrites Barratta, Knyahinya and Shaw have siderophile concentrations lower than the normal L-group range, they have kamacite and taenite Co concentrations in the L-group range. Metal-phase studies of petrologic type-3 ordinary chondrites having highly unequilibrated silicates showed a wide range in the degree of matrix kamacite equilibration ranging from nearly equilibrated in Mezo-Madaras to highly unequilibrated in Bishunpur, Ngawi and Semarkona. Kamacite in chondrule interiors is highly unequilibrated in all 9 chondrites, and in each setting taenite data are consistent with the expectation that it should be less equilibrated than kamacite. Our kamacite Co data confirm that Sharps is H and Hallingeberg. Khohar and Mezo-Madaras are L chondrites. Chainpur and Parnallee have kamacite Co concentrations between the L and LL ranges: we present evidence indicating that they are truly intermediate, i.e. neither L nor LL. Highly unequilibrated Ngawi is either LL or, less likely, still more oxidized. Bishunpur and Semarkona have mean kamacite Co concentrations in the H range but too unequilibrated to be used for classification. The highly heterogeneous compositions of the metal in Bishunpur, Ngawi and Semarkona indicate that their metal partially preserves properties established during nebular processes. Most of the taenite in these chondrites has high Ni contents (>470 mg/g) and is essentially unzoned; much of the kamacite is polycrystalline with crystals ⩽5μm across. Metamorphism causes tiny grains to disappear, increases the grain size of both kamacite and taenite, tends to equilibrate metallic minerals and, during cooling, can produce zoned taenite. A petrologic type-5 clast in the Ngawi LL3 chondrite has 3 coexisting metal phases, clear taenite (540 mg/g Ni, 21 mg/g Co), kamacite (30 mg/g Ni, 120 mg/g Co) and a phase tentatively identified as ordered FeCo (8.5 mg/g Ni, 370 mg/g Co).

Ru, Re, Os, Pt and Au in iron meteorites

The refractory siderophiles Ru, Re, Os and Pt and moderately volatile Au were determined in 41 iron meteorites by neutron activation analysis. For the first time element-Ni trends are defined for groups IID and IIIF; these support a magmatic origin. Concentrations of Re and Os, like that of Ir, are lower in nonmagmatic group IIICD than in IAB at low Ni-concentrations, whereas Ru and Pt concentrations strongly overlap. In groups IAB and IIICD Ru and Pt slopes are slightly steeper than those of Re, Os and Ir on log element-log Ni diagrams; this contrasts with trends observed in magmatic groups, where the Ru and Pt slopes are much less steep. There is no significant change in the ReOs and IrOs ratios with Ni content in nonmagmatic groups, but the ReOs ratio increases by a factor of ∼3 and the OsIr ratio decreases by a factor of ∼6 as one proceeds from low-Ni to moderately high-Ni members of the large magmatic groups IIAB and IIIAB. It should be possible to determine radiometric ages by the 187Re-187Os method for individual iron meteorite groups providing precise isotopic ratios can be determined at low (0.1 μg) Os concentrations. In group IIIAB at Ni concentrations above 90 μg the steep decrease of Re, Os and Ir with Ni levels off and the interelement ratios show appreciable scatter. A plausible explanation of these observations is contamination of the residual molten core with small amounts of late primitive melts draining from the mantle.

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.

Compositions of enstatite (EH3, EH4,5 and EL6) chondrites: Implications regarding their formation

Abstract We report concentrations of 30 elements in 34 replicate samples of 17 enstatite chondrites, including 8 EH3 chondrites. Unweathered EH3 and EH4,5 chondrites appear to be compositionally indistinguishable, and, therefore, observed differences in phase compositions reflect metamorphism of EH4,5 chondrites. The system EH3 kamacite and perryite is essentially equivalent to EH4,5 kamacite, suggesting that perryite 1. (1) condensed on metal at high nebular temperatures, 2. (2) dissolved into the kamacite during the metamorphism of the EH4,5 chondrites. Our data revealed some unexpected fractionations between closely related elements in EL chondrites: the La Sm ratio is resolvably lower and the Co Ni ratio resolvably higher than in EH chondrites. The EL chondrites have a fractionated “refractory lithophile” abundance pattern relative to CI, unlike other chondrite groups. Ni-normalized refractory siderophile abundances are higher and volatile siderophile abundances lower in EL relative to EH. The Au Ni ratio is ~1.4× the CI ratio in both EL and EH. Antarctic EH chondrites show similar patterns of element loss during weathering, independent of petrologic type. Factor analysis suggests that appreciable La condensed together with sulfides that nucleated on the metal; Na and Br are also associated with this component. The remaining rare earths are in an oldhamite-rich component. Vanadium and Cr are strongly correlated. The highly reduced state of the enstatite chondrites as well as their unique EL fractionation patterns seem most consistent with formation in the innermost part of the solar nebula.