John A. Wood

A chemical-petrologic classification for the chondritic meteorites

Abstract A two-dimensional classification grid based on chemical and petrologic subdivisions of the chondritic meteorites is proposed. This grid extends the current chemical subdivisions to account for varying petrologic (implied metamorphic) properties of chondrites within the primary chemical groups. Six petrologic types are recognized, which, together with the five major chemical groups, produce 30 possible chondritic types. Representatives of 20 of the types are known. An Appendix giving the extended classification for 460 chondrites is included.

Chondrites: Their metallic minerals, thermal histories, and parent planets

Abstract Metal grains in 35 chondrites were studied microscopically and by electron microprobe analysis. Taenite and kamacite crystals in ordinary, unequilibrated ordinary, and type III carbonaceous chondrites exhibit the same compositional inhomogeneities previously observed in octahedrites. It is shown that the metal grains evolved in situ to their present form during cooling from metamorphic temperature. Correlations exist between the dimension and central Ni content of taenite and kamacite crystals in many chondrites, making it possible to deduce the rates at which the chondrites cooled by the same technique previously applied to octahedrites. Ordinary chondrites cooled through 500°C at 2–10°/million years, the other types noted above at 0.2–2°/m.y. Heat flow calculations show that these cooling rates would have obtained in sites respectively 20–150 km deep in planets of ⩾90 km radius, and 40–150 km deep in ⩾150 km radius planets. Some of the processes that would have occurred during metamorphism are discussed, especially losses of gas and certain trace elements. Type II carbonaceous chondrites appear to contain no high-Ni metal grains. Their properties are consistent with the theory that they represent unmetamorphosed, primordial planetary matter. Distinct differences are shown to exist between metal grains in the light and dark components of Pantar-type (gas-rich) chondrites, evidence that they evolved in widely separated sites. Several chondrites that appear to have been reheated (as, for example, by shock compression during interplanetary collisions) are described, and the metallographic changes wrought by various degrees of reheating are noted. A model for chondrite formation and evolution that embraces all these effects is presented (Sec. 7).

On the origin of chondrules and chondrites

Abstract The calculated ferrous iron content of a silicate liquid at equilibrium with solar gas approximately equals the Fe 2+ content of chondrules in a primitive chondrite (Renazzo). This points to a relationship between chondrules and the primordial solar nebula, and supports the proposition that chondrules are original (liquid) condensations from the nebula, predating the planets. Basic silicate liquids would be stable in an undifferentiated solar gas only at 10 2 −10 4 atmospheres total pressure and ∼2000°K. Recent theoretical studies of the temperature-hydrostatic pressure history of a contracting model protosun do not yield such a combination of high pressure and moderate temperature. However, chemical fractionation of the nebular gas or transient pressures associated with T Tauri-like mass ejection could make the condensation of liquid droplets (chondrules) possible. The postulated sequence of events by which chondrules and the various classes of chondrites were formed is summarized in Section 12. By the time planets accreted, a number of processes would probably have acted to fractionate Fe, O, and S with respect to Si: chemical fractionation prior to condensation; chondrule-dust, metal-silicate, and solid-gas fractionations after condensation; and incomplete reaction of solid with gas phase at the time of accretion. These effects can account for Priors law and for chemical differences between the various chondrite classes.

The cooling rates and parent planets of several iron meteorites

Abstract The Widmanstatten structure in octahedries can be understood in detail and quantitatively as the result of solid state transformations that would occur in slowly cooling Niue5f8Fe alloys. High pressure is not needed to account for the presence of plessite instead of Widmanstatten structures in those meteorites where Ni > ∼13% Ni, or in those regions of octahedrites where ∼22% > Ni > ∼13%. Experiments show that Niue5f8Fe alloys tend to remain in the form of metastable γ phase when cooled from the γ stability field into the α + γ field, but transform promptly from the metastable α2 state into α + γ. Plessite was formed when meteoritic alloys supercooled as metastable γ phase to such an extent that they transformed to α2; thereafter they decomposed to fine, non-Widmanstatten α + γ, i.e., plessite. Electron probe profiles were made of the characteristic Ni diffusion gradients in taenite-plessite regions of 11 octahedrites. A digital computer program was set up to generate artificial diffusion gradients by solving the equation for diffusion of Ni in γ alloys under differing circumstances, i.e., for systems with various bulk Ni contents, nucleation spacings, nucleation temperatures, and cooling rates. It was found possible to artificially duplicate the measured gradients only if an α/(α + γ) phase boundary somewhat different from the generally accepted Owen and Liu (1949) version was used in the program. Results of a single annealing experiment supported use of this amended boundary. Comparisons of computed and measured structures permitted conclusions to be drawn about thermal histories of the octahedrites studied. Most cooled to ∼100°C beneath the γ/(α + γ) boundary before Widmanstatten structure was precipitated. A correlation between cooling rates, structural types, and probably Ga-Ge groups appears to exist. The medium, fine, and finest octahedrites Grant, Trenton, Duchesne, Santa Apolonia, and probably Bristol cooled at ∼10°C/million years. The coarse and coarsest octahedrites Arispe, Canon Diablo, and Odessa, and the medium octahedrite Toluca cooled at ∼1°C/million years. Anoka, a fine-finest octahedrite, cooled at an anomalous 1°C/million years. These are cooling rates appropriate to the centers of asteroids, 50–90 km in radius for the first group and 130–260 km in radius for the second. Two Ni-rich ataxites studied (Pinon, Monahans) precipitated scattered kamacite crystals under circumstances of very rapid cooling, excessive supercooling, or both.

Ca-Al rich phases in the allende meteorite

Abstract This newly fallen Type III carbonaceous chondrite contains abundant, irregular bodies of unusual bulk chemistry: rich in Ca and Al, poor in Si. Phases identified in these bodies include spinel, hercynite, gehlenite, anorthite, nepheline, diopside, fassaite, ferroaugite, perovskite and Ca-Al-rich glass. High temperatures and an effective mechanism of chemical fractionation were required to form these assemblages. We suggest that they may represent early, high temperature condensates from the solar nebula.

Metamorphism in chondrites

Abstract The concept of chondrites as metamorphic rocks is examined. Mineralogies of four chondrites whose textures indicate different degrees of thermal recrystallization are presented. It appears that relatively unmetamorphosed or “primary” chondritic material consisted of reduced chondrules (metallic iron, magnesian silicates) in an oxidized matrix (containing ultrafinegrained magnetite, no metallic iron). It is suggested that chondrules condensed as liquid droplets from cooling solar gases during the formation of the sun, later accreting into planets and asteroids. Such a process could account for the peculiar oxidation-reduction state of relatively unmetamorphosed chondrites, such as Renazzo.

Mineralogy and petrology of chondrules and inclusions in the Mokoia CV3 chondrite

All objects >100 μm in apparent diameter in five polished thin sections of the Mokoia CV3 chondrite were studied and classified. Number and volume percentages and mean apparent size of each type of chondrule and inclusion were determined. Three major types of olivine chondrules were observed: igneous chondrules, recrystallized chondrules, and chondrules that appear to be accretional aggregates. Coarse-grained CAIs have igneous textures and mineral parageneses, while fine-grained CAIs are aggregates containing varying proportions of Al-rich concentric objects, Ca-rich chaotic material, and inclusion matrix. Chondrules and refractory inclusions in Mokoia and Allende are broadly similar in texture and mineral chemistry, but Mokoia refractory inclusions contain phyllosilicates rather than feldspathoids, and melilite-rich CAIs are more abundant in Allende. n nWe think that most CAIs formed during the metamorphism, partial melting, and incomplete distillation of primitive dust aggregates when they were heated in the solar nebula. In the process, Ca-rich melt appears to have been physically separated from Al-rich residues, producing the observed fractionation of Ca from Al into distinct constituents of CAIs. Some CAIs may be aggregates of devitrified, amorphous metastable condensates. Inclusion matrix may have condensed from silicate-rich vapors produced during distillation. Mokoia inclusion matrix contains phyllosilicates that are probably primitive nebular material.

Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition

Some FeNi metal grains, similar to 150 mu m in apparent diameter, in CH carbonaceous chondrites are concentrically zoned in Ni (similar to 5-10 wt%), Co (0.2-0.4 wt%), and Cr (0.3-0.8 wt%); Silicon is present at the similar to 0.1 wt% level. These observations are consistent with predicted gas-solid condensation from a gas of solar composition at temperatures of similar to 1370-1270 K and total pressure of similar to 10(-4) bar. Estimates of FeNi metal grain growth and cooling rates in this temperature range are consistent with brief and localized thermal episodes in the solar nebula. Compositionally similar FeNi metal grains have also been reported in CR and Bencubbin-like chondrites. Because FeNi metal is highly susceptible to secondary alteration (i.e., metamorphism, melting, oxidation), the observed FeNi metal condensates in CH, Bencubbin-like, and CR chondrites indicate that these meteorites experienced no thermal processing after their lithification and thus are among the most primitive meteorites in our collections.

The mineral chemistry and origin of inclusion matrix and meteorite matrix in the Allende CV3 chondrite

The two textural varieties of olivine-rich Allende inclusions (rimmed and unrimmed olivine aggregates) consist primarily of a porous, fine-grained mafic constituent (inclusion matrix) that differs from the opaque meteorite matrix of CV3 chondrites by being relatively depleted in sulfides, metal grains, and (perhaps) carbonaceous material. Olivine is the most abundant mineral in Allende inclusion matrix; clinopyroxene, nepheline, sodalite, and Ti-Al-pyroxene occur in lesser amounts. Olivine in unrimmed olivine aggregates (Type 1A inclusions) is ferrous and has a narrow compositional range (Fo50–65). Olivine in rimmed olivine aggregates (Type 1B inclusions) is, on average, more magnesian, with a wider compositional range (Fo53–96). Olivine grains in the granular rims of Type 1B inclusions are zoned, with magnesian cores (Fo>80) and ferrous rinds (Fo<70). Ferrous olivines (Fo<65) in both varieties of inclusions commonly contain significant amounts of Al2O3 (as much as ~0.7 wt%), CaO (as much as ~0.4 wt%), and TiO2 (as much as ~0.2 wt%), refractory elements that probably occur in submicroscopic inclusions of Ca,Al,Ti-rich glass (rather than in the olivine crystal structure). Defocussed beam analyses of Allende matrix materials demonstrate that: (1) inclusion matrix in Type 1A inclusions is more enriched in olivine and FeO than inclusion matrix in the cores of Type 1B inclusions; (2) opaque matrix materials are depleted in feldspathoids and enriched in sulfides and metal grains relative to inclusion matrix; (3) the bulk compositions of Type 1A and Type 1B inclusions overlap; and (4) excluding sulfides and metal, the bulk compositions of Allende matrix materials cluster in a complementary pattern around the bulk composition of C1 chondrites. n nInclusion matrix and meteorite matrix in Allende and other CV3 chondrites are probably relatively primitive nebular material, but a careful evaluation of the equilibrium condensation model suggests that these matrix materials do not consist of crystalline phases that formed under equilibrium conditions in a relatively cool gas of solar composition. Allende inclusion matrix is interpreted as an aggregate of condensates that formed under relatively oxidizing, non-equilibrium conditions from supercooled, supersaturated vapors produced during the vaporization of interstellar dust by aerodynamic drag heating in the solar nebula; CV3 meteorite matrix contains, in addition, a proportion of interstellar material that was heated (but not vaporized) in the nebula. Granular olivine in rimmed olivine aggregates may have formed during the recrystallization and incipient melting of aggregates of inclusion matrix in the nebula. The mineral chemistry of matrix olivine in Allende seems to have been established by three different processes: non-equilibrium vapor → solid condensation; recrystallization and partial melting in the nebula; and FeMg equilibration (without textural homogenization) in the meteorite parent body.

Magellan: Initial Analysis of Venus Surface Modification

Initial Magellan observations reveal a planet with high dielectric constant materials exposed preferentially in elevated regions with high slopes, ejecta deposits extending up to 1000 kilometers to the west of several impact craters, windblown deposits and features in areas where there are both obstacles and a source of particulate material, and evidence for slow, steady degradation by atmosphere-surface interactions and mass movements.