Gregory W. Kallemeyn

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.

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.

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.

The compositional classification of chondrites: II The enstatite chondrite groups

We present new data from a neutron activation analysis of four enstatite chondrites including the taxonomically important St. Sauveur, and discuss the classification of enstatite chondrites. The enstatite chondrites can be divided into two compositionally distinct sets; in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1.5–2.0× higher than in the other. A well-resolved compositional hiatus separates these two sets. The differences in composition are as great as those between the groups of ordinary chondrites, and therefore it appears best to treat these sets as separate groups. By analogy with the symbols used for ordinary chondrites we propose to designate the high-Fe, high siderophile group EH and the low-Fe, low-siderophile group EL. Known members of the EH group belong to petrologic types 4 and 5, whereas all EL members are petrologic type 6. Within the EH group no correlation is observed between petrologic type and abundance of nonrefractory siderophiles or moderately volatiles or alkalis. Two physical properties show only modest overlap between the EH and EL groups. Cosmic-ray ages for EH chondrites are 0.5–7 Ma, while those for EL chondrites are 4–18 Ma. Relative to Bjurbole, I-Xe formation intervals are −1.3 ± 0.6 Ma for EH chondrites and 2.9 ± 0.5 Ma for EL chondrites. The weight of the chemical and physical evidence indicates that the EH and EL groups formed separate bodies at similar distances from the Sun. The available evidence for Shallowater and Happy Canyon, two strongly recrystallized silicate-rich meteorites containing > 40 mg/g Fe-Ni, indicates that the former is an enstatite-clan chondrite altered by loss of sulfide- and plagioclase-rich melts, whereas the latter is intermediate in composition between EL chondrites and the chondritic silicates in the Pine River IAB-anomalous meteorite.

The compositional classification of chondrites: VII. The R chondrite group

Bulk compositional and petrographic data clearly define the new R (Rumuruti) group of chondrites consisting of Rumuruti (the only fall), ALH85151, Acfer 217, Carlisle Lakes, Dar al Gani 013, PCA91002, PCA91241, Y-75302, Y-793575, and Y-82002. Compositional, petrographic, rare-gas, and 0-isotopic data strongly suggest that PCA91002 and PCA91241 are paired. The Yamato specimens are probably not paired. The matrices of the known R chondrites have experienced similar, minor degrees of metamorphism; petrographic types are 3.8-3.9 with the exception of ALH85151, 3.6. All except Carlisle Lakes contain equilibrated (R5-R6) clasts. Petrographically, the R chondrites are characterized by a low chondrule/matrix modal abundance ratio, high states of oxidation (reflected by abundant NiO-bearing olivine with Fa37-40), relatively small chondrules (mean apparent diameters of ∼400 μ) abundant (up to ∼11 wt%) sulfides (mainly pyrrhotite and pentlandite), and negligible amounts of metallic Fe-Ni. Refractory lithophile abundances are ∼0.95 X Cl, intermediate between those in ordinary chondrites (OC) and CI chondrites. Abundances of the volatile elements Se and Zn are greatly enhanced relative to OC. The R chondrites are clearly distinguished from other chondrite groups on the basis of Al/Mn and Zn/Mn abundance ratios. The oxygen isotopic data plot roughly along a slope-12 line, with whole-rock Δ17O values higher than for any other chondrite group. Rumuruti, Acfer 217, ALH85151, PCA91002, and PCA91241 have light/ dark structures and solar-wind-implanted rare gases indicating that they are regolith breccias. The Yamato specimens also have light/dark structures and are inferred to be regolith breccias. Carlisle Lakes lacks solar rare gases and is unbrecciated. Based on similarities in refractory lithophile abundances 00.95 X CI), oxygen isotope compositions (Δ17O ≥ 0), and refractory inclusion abundances (<0.1 vol%;none have been reported), the R chondrites probably belong to a noncarbonaceous superclan of chondrites that also includes ordinary and enstatite chondrites. The high oxidation state, high matrix /chondrule modal abundance ratio, relatively low abundance of droplet chondrules, and high Δ 17O composition suggest that the R chondrites formed at greater heliocentric distances than the OC.

The compositional classification of chondrites: III. Ungrouped carbonaceous chondrites

Seven carbonaceous chondrites (Allan Hills A77307, Adelaide, Al Rais, Coolidge, Grosnaja, Karoonda and Renazzo) with uncertain classifications were analyzed by instrumental and radiochemical neutron activation analysis for 29 elements: Na, Mg, Al, K, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Se, Br, Ru, Cd, Sb, La, Sm, Eu, Yb, Lu, Os, Ir and Au. Five of these chondrites (A77307, Adelaide, Al Rais, Karoonda and Renazzo) are unique ‘grouplets’, not closely related to other groups or to each other. Only Coolidge (CV4) and Grosnaja (CV3-an) are members of previously established groups. A77307 and Adelaide have refractory lithophile abundances similar to those in the CM-CO clan; A77307 probably is a member of that clan, but Adelaide, which shows CV-like petrographic characteristics, cannot as yet be assigned to a clan. Al Rais and Renazzo have similar refractory lithophile abundances (essentially at CI levels) and probably belong to the same clan, i.e., formed in the same region of the nebula. There are insufficient data to determine whether they formed at the same general region as the CI chondrites, but separates having O-isotope compositions near the terrestrial fractionation line indicate that this is plausible. Karoonda has refractory lithophile abundances ~ 1.21 × CI and appears to belong to a new clan distinct from CM-CO (1.11 × CI) and CV (1.34×).

Formation of Metal and Silicate Globules in Gujba: A New Bencubbin-like Meteorite Fall

Gujba is a coarse-grained meteorite fall composed of 41 vol% large kamacite globules, 20 vol% large light-colored silicate globules with cryptocrystalline, barred pyroxene and barred olivine textures, 39 vol% dark-colored, silicate-rich matrix, and rare refractory inclusions. Gujba resembles Bencubbin and Weatherford in texture, oxygen-isotopic composition and in having high bulk δ15N values (∼+685‰). The 3He cosmic-ray exposure age of Gujba (26 ± 7 Ma) is essentially identical to that of Bencubbin, suggesting that they were both reduced to meter-size fragments in the same parent-body collision. The Gujba metal globules exhibit metal-troilite quench textures and vary in their abundances of troilite and volatile siderophile elements. We suggest that the metal globules formed as liquid droplets either via condensation in an impact-generated vapor plume or by evaporation of preexisting metal particles in a plume. The lower the abundance of volatile elements in the metal globules, the higher the globule quench temperature. We infer that the large silicate globules also formed from completely molten droplets; their low volatile-element abundances indicate that they also formed at high temperatures, probably by processes analogous to those that formed the metal globules. The coarse-grained Bencubbin-Weatherford-Gujba meteorites may represent a depositional component from the vapor cloud enriched in coarse and dense particles. A second class of Bencubbin-like meteorites (represented by Hammadah al Hamra 237 and QUE 94411) may be a finer fraction derived from the same vapor cloud.

Origin of planetary cores: evidence from highly siderophile elements in martian meteorites

We present new bulk compositional data for 6 martian meteorites, including highly siderophile elements Ni, Re, Os, Ir and Au. These and literature data are utilized for comparison versus the siderophile systematics of igneous rocks from Earth, the Moon, and the HED asteroid. The siderophile composition of ALH84001 is clearly anomalous. Whether this reflects a more reducing environment on primordial Mars when this ancient rock first crystallized, or secondary alteration, is unclear. QUE94201 shows remarkable similarity with EET79001-B for siderophile as well as lithophile elements; both are extraordinarily depleted in the “noblest” siderophiles (Os and Ir), to roughly 0.00001 × CI chondrites. As in terrestrial igneous rocks, among martian rocks Ni, Os and Ir show strong correlations vs. MgO. In the case of MgO vs. Ni, the martian trend is displaced toward lower Ni by a large factor (5), but the Os and Ir trends are not significantly displaced from their terrestrial counterparts. For Mars, Re shows a rough correlation with MgO, indicating compatible behavior, in contrast to its mildly incompatible behavior on Earth. Among martian MgO-rich rocks, Au shows a weak anticorrelation vs. MgO, resembling the terrestrial distribution except for a displacement toward 2–3 times lower Au. The same elements (Ni, Re, Os, Ir and Au) show similar correlations with Cr substituted for MgO. Data for lunar and HED rocks generally show less clear-cut trends (relatively few MgO-rich samples are available). These trends are exploited to infer the compositions of the primitive Earth, Mars, Moon and HED mantles, by assuming that the trend intercepts the bulk MgO or Cr content of the primitive mantle at the approximate primitive mantle concentration of the siderophile element. Results for Earth show good agreement with earlier estimates. For Mars, the implied primitive mantle composition is remarkably similar to the Earth’s, except for 5 times lower Ni. The best constrained of the extremely siderophile elements, Os and Ir, are present in the martian mantle at 0.005 times CI, in comparison to 0.007 times CI in Earth’s mantle. This similarity constitutes a key constraint on the style of core-mantle differentiation in both Mars and Earth. Successful models should predict similarly high concentrations of noble siderophile elements in both the martian and terrestrial mantles (“high” compared to the lunar and HED mantles, and to models of simple partitioning at typical low-pressure magmatic temperatures), but only predict high Ni for the Earth’s mantle. Models that engender the noble siderophile excess in Earth’s mantle through a uniquely terrestrial process, such as a Moon-forming giant impact, have difficulty explaining the similarity of outcome (except for Ni) on Mars. The high Ni content of the terrestrial mantle is probably an effect traceable to Earth’s size. For the more highly siderophile elements like Os and Ir, the simplest model consistent with available constraints is the veneer hypothesis. Core-mantle differentiation was notably inefficient on the largest terrestrial planets, because during the final ∼ 1% of accretion these bodies acquired sufficient H2O to oxidize most of the later-accreting Fe-metal, thus eliminating the carrier phase for segregation of siderophile elements into the core.

Allan Hills 85085: A subchondritic meteorite of mixed nebular and regolithic heritage

Abstract Allan Hills 85085 is a tiny (12-g) meteorite whose closest relatives are the Renazzo and Al Rais chondrites and the subchondritic host material in the Bencubbin meteorite. Its bulk composition is generally chondritic but siderophile abundances are tens of per cent higher and abundances of moderately volatile elements factors of 2–4 × lower than in known authentic chondrites. In addition to this unusual composition ALH85085 has an extremely small particle size, metal that shows no evidence of metamorphic equilibration and a very high abundance of pyroxenitic lithic particles. Chondrules are rare and small; most are pyroxenitic and chemically similar to the lithic particles. Previous workers have inferred that most or all of these strange features of ALH85085 resulted from nebular processes, but we suggest that melting, vaporization, outgassing, condensation and size-sorting in a cloud of impact ejecta offer a more viable alternative. Until additional similar materials have been discovered and characterized, it seems safest to infer nebular properties and processes on the basis of the record in chondrites regarding which there is little doubt that nebular processes were dominant.