Edward R. D. Scott

Chondrites and the Protoplanetary Disk

AbstractMajor advances in deciphering the record of nebula processes in chondrites can be attributed to analytical improvements that allow coordinated isotopic and mineralogical studies of components in chondrites and to a wealth of new meteorites from hot and cold deserts. These studies have identified a few rare pristine chondrites that largely escaped heating and alteration in asteroids, which have matrices composed of submicrometer-sized grains of enstatite and forsterite and amorphous silicates, as found in comets. Isotopic analyses of components in pristine chondrites using short-lived nuclide chronometers, Pb-Pb dating, and oxygen isotopes aided by laboratory and theoretical studies of chondrites and differentiated meteorites have provided key constraints on the processes that shaped the early solar system. These processes were once thought to have followed one another sequentially over a period of several million years: chondrule formation; planetesimal accretion; alteration, metamorphism, and mel...

Chemical fractionation in iron meteorites and its interpretation

Abstract Published analyses of trace and minor elements in iron meteorites have been compiled and the distributions interpreted with the chemical groups defined by Wasson. When each element is plotted against Ni on log scales, groups are often clearly resolved with all the members of a group falling within the limits of sampling and analytical error on a straight line. The lines for groups IIIa,b and IVa are generally parallel with IIa,b plotting on a steeper gradient. In contrast to Ga and Qe, many elements show variations within a group which may approach that shown by all the iron meteorites. Group I members have a fairly uniform concentration of elements which are severely fractionated in the other major groups. There are also fewer correlations of elements in group I. Thus the genetic significance of the chemical classification is strengthened by the addition of more parameters which may be used in the definition of the groups. Relationships between groups lia and lib and between Ilia and Illb are confirmed. More important is the record of events during the formation of iron meteorites that the fractionations of elements provide. Two fractionations have occurred, a primary event which established the bulk composition of each group and a secondary event which fractionated the elements within each group. Group I appears to have escaped the secondary fractionation. An examination of possible fractionation mechanisms suggests that the secondary process took place during solidification of iron cores in the parent bodies. The elements It, Os, Pt, Bu and Rh would be enriched in the early Ni-poor solid whilst As, Au, Co, Mo, P, Pd and Sb would concentrate in later solid and produce the observed positive correlations with Ni. These trends are largely those predicted from the binary phase diagrams of Fe, although Cr and Ge behave otherwise. The magnitude of the fractionations could be predicted from the distribution coefficients of elements between solid and liquid Fe-Ni. Unfortunately these can only be guessed and confirmation awaits experiments to indicate their magnitude. The primary fractionation which established the chemical differences between the groups might have occurred during condensation.

Shock metamorphism of carbonaceous chondrites

Abstract We have studied shock effects in carbonaceous chondrites using optical microscopy of thin sections and find that our petrographic classification of progressive shock metamorphism in ordinary chondrites can also be applied to carbonaceous chondrites. We find that sixty-nine carbonaceous chondrites can be assigned to four shock stages, SI to S4, largely on the basis of shock effects in olivine. The least shocked chondrite groups are CM2 and CO3: thirty-six out of thirty-eight members are classified as shock stage S1 ( Differences between the mean shock levels of certain groups of chondrites, e.g., CO3 and CV3, are probably due to stochastic differences in the sampling of their parent bodies. However, there is an overall tendency for the mean shock level of carbonaceous chondrites (and ordinary chondrites) to increase with increasing petrologic type that may reflect real differences in the shock level of near-surface materials on their parent bodies caused by intrinsic variations in the chemical and physical properties of these materials. Petrologic type 2 and 3 chondrites are more porous and richer in volatiles than types 4 to 6. The greater porosity of types 2–3 causes higher post-shock temperatures and melting at lower pressures, and the higher volatile contents ensure that strongly shocked material is more readily dispersed on release from high pressure. We suggest that type 2–3 material shocked above 20–30 GPa normally escapes from the parent asteroids and forms particles that are too small to survive as meteorites. In the CV3 group, there is a correlation between the degree of chondrule flattening and the intensity of shock metamorphism, analogous to that discovered in ordinary chondrites by Sneyd et al. (1988), suggesting that shock rather than static overburden pressure is responsible for chondrule flattening. We infer that the correlations are due to collapse of pores under shock pressures of at least 5–10 GPa and that shock is an important process affecting many physical properties of type 2–4 chondrites.

Shock metamorphism of enstatite chondrites

Abstract We have extended the shock-metamorphism classification scheme of Stoffler et al. (1991) to account for shock effects in orthopyroxene and applied it to sixty enstatite chondrites. Orthopyroxene exhibits the following sequence of progressive shock effects: shock stage S1 (unshocked), sharp optical extinction; S2 (very weakly shocked), undulose extinction; S3 (weakly shocked), development of clinoenstatite lamellae parallel to (100); S4 (moderately shocked), weak mosaicism; S5 (strongly shocked), strong mosaicism. As in the Stoffler et al. (1991) scheme, stage S5 is characterized principally by the solid-state transformation of crystalline plagioclase into maskelynite. Most EL3 chondrites exhibit foliations caused by impact deformation of chondrules and metal particles. EL5 and EL6 chondrites are all shock stage S2. Because opaque veins are rare in S2 ordinary chondrites, it is plausible that the centimeter-size kamacite veins in Atlanta and Blithfield and the 1.6-cm-long oldhamite-rich vein in Jajh deh Kot Lalu (all EL6) formed when their hosts were shocked to S3–S5 levels. Because removal of shock-stage S3–S5 features (including repair of shock-damaged orthopyroxene) requires levels of metamorphism comparable to those experienced by petrologic type-5 to -6 chondrites, we infer that proto-EL6 material was shocked to S3–S5 levels prior to peak metamorphism and shocked again to stage S2 after metamorphism. Overall, enstatite chondrites appear to have suffered greater shock damage than ordinary and carbonaceous chondrites.

Catastrophic fragmentation of asteroids: Evidence from meteorites

Abstract Meteorites are impact-derived fragments from ≈ 85 parent bodies. For seven of these bodies, the meteorites record evidence suggesting that they may have been catastrophically fragmented. We identify three types of catastrophic events: (a) impact and reassembly events > 4.4 Gy ago, involving molten or very hot parent bodies(> 1200°C); this affected the parent bodies of the ureilites, Shallowater, and the mesosiderites. In each case, the fragments cooled rapidly (≈ 1–1000°C day −1 ) and then reassembled, (b) Later impacts involving cold bodies which, in some cases, reassembled; this occurred on the H and L ordinary chondrite parent bodies. The L parent body probably suffered another catastrophic event about 500 My ago. (c) Recent impacts of cold, multi-kilometer-sized bodies that generated meter-sized meteoroids; this occurred on the parent bodies of the IIIAB irons (650 My ago), the IVA irons (400 My ago), and the H ordinary chondrite (7 My ago).

39Ar40Ar age and petrology of Chico: Large-scale impact melting on the L chondrite parent body

Our studies of the 105 kg Chico L chondrite show that it contains ∼60% impact melt and the largest volume of impact melt recognized in stony meteorites. We suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large, if not catastrophic, impact on the L chondrite parent body at about 0.5 Ga. Petrologic and 39Ar40Ar dating studies were made on several lithologies, including the massive melt zone, host chondrite, and melt-chondrite boundaries, for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the KAr chronometer. The chondritic host is shocked to stage S6 and contains pockets and veins of melt. There are no unmelted clasts in the interior of the melt; coalesced metal-troilite nodules reach up to 2 cm in size. Melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly. Metal-troilite textures suggest cooling rates of ∼0.1°C/s in the interior of the melt dike during crystallization. Secondary kamacite rims indicate cooling at 0.01–1°C/y over the range of 700-500°C, consistent with an impact-heated volume of up to a kilometer in thickness. Compositions of olivines and pyroxenes are generally similar in melt and chondritic host, reflecting rapid crystallization, not metamorphic equilibration. The interior melt shows an overall depletion in K, whereas the melt near the boundary is enriched in K. The 39Ar40Ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts, based on Ar diffusion properties and K/Ca ratios. The low-temperature, high K/Ca phase of both melt and host chondrite show ages of 0.54–0.78 Ga. Ages of the high-temperature, low K/Ca phase of the melt are comparable or higher, 0.61–1.35 Ga, whereas those of the host chondrite are even higher, 0.87–1.86 Ga, due to lesser degrees of degassing. Isochron plots for several melt samples suggest an age of ∼0.53 Ga and the presence of variable amounts of excess 40Ar not completely degassed by the impact. Even this age, however, is significantly higher than the previously reported RbSr isochron age of 0.467 ± .015 Ga. The apparent retention of radiogenic 40Ar in the Chico impact melt, in spite of its relatively large size, absence of clasts, and moderately slow cooling rate below 700°C, raises questions as to the reliability of using melts for 39Ar40Ar dating of meteoritic impact events.

Origin of rapidly solidified metal-troilite grains in chondrites and iron meteorites

Inclusions of troilite and metallic Fe,Ni 0.2–4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites. Cooling rates in the interval 1400−950°C calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10−7 to 104°C/sec. In 8 of these chondrites, which are breccias containing some normal slow-cooled metal grains, the inclusions solidified before they were incorporated into the breccias. Their cooling rates of 1–300 °C/sec indicate cooling by radiation, or by conduction in contact with cold silicate or hot silicate volumes only 6–40 mm in size. This is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body (by impact), and were not formed at depth from an internally derived melt. In Ramsdorf, Rose City and Shaw, which show extensive reheating to ⩾ 1000°C, Fe-FeS textures in melted areas are coarser and indicate cooling rates of 10−1 to 10−4°C/sec during solidification. This metal may have solidified inside hot silicate volumes that were 10–300 cm in size. As Shaw and Rose City are breccias of unmelted and melted material, their melted metal did not necessarily cool through 1000°C within a few m of the surface. Shock-melted, fine-grained, irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of ⩾ 105°C/sec. Their kamacite and taenite compositions may result from annealing at ~250°C of metallic glass or exceedingly fine-grained quench products.

Chemical classification of iron meteorites. VIII - Groups IC, IIE, IIIF and 97 other irons

Abstract Concentrations of Ni, Ga, Ge and Ir in 106 iron meteorites are reported. Three new groups are defined: IC, IIE and IIIF containing 10, 12 and 5 members, respectively, raising the number of independent groups to 12. Group IC is a cohenite-rich group distantly related to IA. Group IIE consists of those irons previously designated Weekeroo Station type and five others having similar compositions though diverse structures. The IIE irons are compositionally similar to the mesosiderites and pallasites, and the three groups probably formed at similar heliocentric distances. The mixing of the globular IIE silicates with the metal probably occurred during shock events. Group IIIF is a well-defined group of low-Ni and low-Ge irons. The compositions of these groups are summarized as follows: Group Ni (%) Ga (ppm) Ge (ppm) Ir (ppm) IC 6.1–6.8 42–54 85–250 0.07–10 IIE 7.5–9.7 21–28 62–75 0.5–8 IIIF 6.8–7.8 6.3–7.2 0.7–1.1 1.3–7.9 Data are reported on a number of anomalous irons including an interesting cluster of 5 plessitic octahedrites and ataxites with Ge/Ga atomic ratios between 10 and 16, among the highest values known in iron meteorites. Dermbach, at 42%, has the second highest Ni concentration known in an iron meteorite. The composition of metal and slicates in Mundrabilla indicate that it and its close relative. Waterville, are anomalous members of group IA. Several additional irons which appear to be mislabelled fragments of Canyon Diablo and Toluca are discussed.

DISENTANGLING NEBULAR AND ASTEROIDAL FEATURES OF CO3 CARBONACEOUS CHONDRITE METEORITES

Abstract Analyses of olivines and low-Ca pyroxenes in a suite of porphyritic chondrules of types IA and II in each of ten CO3 chondrites and comparisons with similar chondrules in LL chondrites suggest that the CO3 chondrites may be divided into a metamorphic sequence of subtypes 3.0 to 3.7. Chondrules in Allan Hills A77307 and Colony, which we classify as type 3.0, show only igneous zoning, whereas in the other more metamorphosed type 3.1–3.7 CO chondrites, chondrule silicates are enriched in FeO, most markedly in olivine crystals near the rims of type IA chondrules and along cracks. The FeO enrichments in metamorphosed chondrules result largely from equilibration by solid-state diffusion between chondrules and FeO-rich matrices. Type 3.1–3.7 CO chondrites were formed from chondrites like Colony and ALH A77307 in heated asteroids or planetesimals. We find no evidence for nebular condensation of fayalite rims around these chondrules, as reported for CV3 chondrites, or nebular modification of chondrule silicates, as favored by some workers. Metallic Fe,Ni in ALH A77307 contains up to 0.1–1 wt% Cr, Si, and P, as in the least metamorphosed ordinary chondrites and CM2 chondrites, and grains of cohenite, Fe3C, and haxonite, Fe23C6. These carbides, which are the first reported in carbonaceous chondrites, are associated with pentlandite and magnetite, as in certain LL3 chondrites. We assign the following petrologic subtypes to other CO chondrites: Kainsaz, 3.1; Felix, 3.2; ALH 82101 and Ornans, 3.3; Lance and ALH A77029, 3.4; ALH A77003, 3.5; Warrenton, 3.6; and Isna, 3.7.

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

A few eucrites have anomalous oxygen isotopic compositions. To help understand their origin and identify additional samples, we have analyzed the oxygen isotopic compositions of 18 eucrites and four diogenites. Except for five eucrites, these meteorites have DO values that lie within 2r of their mean value viz., 0.242 ± 0.016&, consistent with igneous isotopic homogenization of Vesta. The five exceptional eucrites—NWA 1240, Pasamonte (both clast and matrix samples), PCA 91007, A-881394, and Ibitira—have DO values that lie, respectively, 4r, 5r, 5r, 15r, and 21r away from this mean value. NWA 1240 has a dO value that is 5r below the mean eucrite value. Four of the five outliers are unbrecciated and unshocked basaltic eucrites, like NWA 011, the first eucrite found to have an anomalous oxygen isotopic composition. The fifth outlier, Pasamonte, is composed almost entirely of unequilibrated basaltic clasts. Published chemical data for the six eucrites with anomalous oxygen isotopic compositions (including NWA 011) exclude contamination by chondritic projectiles as a source of the oxygen anomalies. Only NWA 011 has an anomalous Fe/Mn ratio, but several anomalous eucrites have exceptional Na, Ti, or Cr concentrations. We infer that the six anomalous eucrites are probably derived from five distinct Vesta-like parent bodies (Pasamonte and PCA 91007 could come from one body). These anomalous eucrites, like the isotopically normal, unbrecciated eucrites with 4.48 Gyr Ar-Ar ages, are probably deficient in brecciation and shock effects because they were sequestered in small asteroids ( 10 km diameter) during the Late Heavy Bombardment following ejection from Vesta-like bodies. The preservation of Vesta’s crust and the lack of deeply buried samples from the hypothesized Vesta-like bodies are consistent with the removal of these bodies from the asteroid belt by gravitational perturbations from planets and protoplanets, rather than by collisional grinding. 2009 Elsevier Ltd. All rights reserved.