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Wasson, J. T.

Publications and source records attributed to Wasson, J. T..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Sixth foray for pristine nonmare rocks and an assessment of the diversity of lunar anorthosites

A compositional and petrographic characterization of 11 nonmare samples shows five to be pristine, and four probably pristine. Five of the 11 are unique anorthosites that considerably extend the diversity of lunar anorthosites. Alkali anorthosites are rich in incompatible elements. The nature of the correlations between longitude and such parameters as Eu and Sm concentrations, and Sc/Sm, Ti/Sm, and Ca/Na ratios, has been elucidated for the case of the pristine samples because five are fro a western site (Apollo 14). The ratios determined for the ferroan anorthosite 14312c are close to normal, but significantly displaced in the direction of the nonferroan, western pristine rocks. This suggests that the composition-longitude relationships are as old as the late stages of the crystallization of the putative magma ocean.

Warren, P. H.↗
NASA NTRS1983

The compositions of chondrules in unequilibrated chondrites An evaluation of models for the formation of chondrules and their precursor materials

Recent progress toward the resolution of the problem of chondrule composition is reviewed. Special attention is given to studies of both the chemical and textural properties of chondrules. The characteristic variations of the composition of carbonaceous chondrite chrondules are discussed within the framework of current theories of chondrule evolution. Consideration is also given to the possible mechanisms for chondrule melts, and the relationship between chondrules and other chondrite components.

Grossman, J. N.↗
NASA NTRS1982

The role of S in the evolution of the parental cores of the iron meteorites

The S contents of iron meteorite parent bodies are estimated on the basis of cosmochemical relationships to undifferentiated meteorites, the results are compared to observations on the major magmatic iron meteorite groups, and evidence is presented that S/Ni ratios in their parent melts were much lower than those inferred for the parent body. Several alternative models to account for the discrepancy are offered, including volatilization of S from the IIAB parent body, liquid immiscibility, and metastable liquid layers produced by episodic melting. Finally, the fate of the S-rich meteoroidal material is discussed, as well as the question of why it seems to be missing from meteorite collections.

Kracher, A.↗
NASA NTRS1982

Fine, nickel-poor Fe-Ni grains in the olivine of unequilibrated ordinary chondrites

Nickel-poor Fe-Ni grains smaller than 2.0 microns are common inclusions in ordinary, unequilibrated chondrites' porphyritic chondrule olivine, where the olivine grains seem to be relicts that survived chondrule formation without melting. This 'dusty' metal, whose most common occurrence is in the core of olivine grains having clear, Fe-poor rims, appears to be the product of the in situ reduction of FeO from the host olivine, with H2 or carbonaceous matter being the most likely reductants. H2 may have been implanted by solar wind or solar flare irradiation, but this requires the dissipation of nebular gas before the end of the chondrule formation process. Carbonaceous matter may have been implanted by shock. The large relict olivine grains may be nebular condensates or fragments broken from earlier chondrule generations.

Rambaldi, E. R.↗
NASA NTRS1982

Evidence for primitive nebular components in chondrules from the Chainpur chondrite

In view of the fact that the least equilibrated ordinary chondrites contain chondrules that have changed little since the time of their formation in the early solar system, and are therefore excellent indicators of the physical and chemical nature of the solar nebula, 36 chondrules were separated from the Chainpur chondrite and analyzed for 20 elements and petrographic properties. The dominant nebular components found are: (1) a mixture of metal and sulfide whose composition is similar to whole rock metal and sulfide, (2) Ir-rich metal, (3) olivine-rich silicates, (4) pyroxene-rich silicates, and possibly (5) a component containing the more volatile lithophiles. Although etching experiments confirm that chondrule rims are enriched in metal, troilite and moderately volatile elements relative to the bulk chondrules, a large fraction of the volatiles remains in the unetched interior.

Grossman, J. N.↗
NASA NTRS1982

Mechanism for the extrusion of KREEP

The evolution of the final residue of the magma ocean from the time it entered the anorthositic crust until it was extruded is modeled. The magma density first increased as anorthite and orthopyroxene crystallized, but once ilmenite precipitation began the density dropped. The KREEPy magma collected beneath the thinnest parts of the anorthitic crust, and intruded the crust when the density dropped below that of the adjacent crustal materials. The nearly constant degree of incompatible element fractionation moon-wide was maintained because the rapid drop in density during ilmenite crystallization occurred after about the same degree of crystallization at all locations. The high viscosity of the final residue of the magma ocean and its low density contrast with the crust initially resulted in very slow migration through the crust.

Shirley, D. N.↗
NASA NTRS1981

Metal and associated phases in Bishunpur, a highly unequilibrated ordinary chondrite

Bishunpur is one of the most unequilibrated ordinary chondrites, and preserves a relatively unaltered record of solar nebular processes. A survey of three polished thin sections of Bishunpur revealed that metal occurs in three textural domains: (1) in the matrix, (2) within chondrules and (3) in coatings on chondrules. A table provides a textural classification of the various metal types found in Bishunpur and their composition. Electron-microprobe analyses are discussed, and similarities between Bishunpur metal and opaques and those in carbonaceous chondrites are considered. Attention is given to low-Ni kamacite grains in chondrule olivine, Si-bearing chondrule metal, spheroidal metal in chondrules, metal and sulfide-bearing rims, coarse matrix metal, and fine matrix metal formation.

Rambaldi, E. R.↗
NASA NTRS1981

Foraging westward for pristine nonmare rocks - Complications for petrogenetic models

New advances are reported concerning an investigation which is being conducted to find and characterize more 'pristine' (compositionally endogenous) lunar rocks. Eleven nonmare samples are characterized petrographically and (except for two clasts that were prohibitively small) by chemical composition. The samples are assessed whether they are pristine on the basis of diagnostic features discussed by Warren and Wasson (1977). Five samples are found to be almost certainly pristine. They include the alkali-rich anorthosite 12073c, the troctolite 14179c, the anorthositic troctolite 14321c1, the troctolitic anorthosite 14321c2, and the anorthositic norite 15565c. Correlations between petrochemistry and longitude are also investigated, taking into account Eu fractionations, Sc-Ti-Sm fractionations, and Ca/Na-Mg/Fe fractionations.

Warren, P. H.↗
NASA NTRS1980

Contribution of the mantle to the lunar asymmetry

Evidence of three kinds indicates a lunar compositional asymmetry: mare basalts are much more abundant on the near side; the incompatible rich KREEP component is mainly observed in near-side soils; and materials on the far side are less dense than those of the near side, as indicated by the 2-km offset between the center of mass and center of figure. Recent models to explain the 2-km offset are based on near side-far side differences in the thickness of crustal units. The most widely discussed model calls for a thickness of anorthosite of about 24 km greater on the far side than on the near side, but no satisfactory method of generating such a large difference has been proposed. It is suggested that much of the offset reflects longitudinal differences in mantle composition, primarily resulting from earlier (or more rapid) crystallization of the magma ocean on what is not the far-side hemisphere. As a result, the far-side mantle would be more magnesian and thus less dense than the near-side mantle.

Wasson, J. T.↗
NASA NTRS1980

Si-rich Fe-Ni grains in highly unequilibrated chondrites

Consideration is given to the Si contents of Fe-Ni grains in highly unequilibrated chondrites, which have undergone little metamorphosis and thus best preserve the record of processes in the solar nebula. Electron microprobe determinations of silicon content in grains of the Bishunpur chondrite are presented for the six Si-bearing Fe-Ni grains for which data could be obtained, five of which were found to be embedded in olivine chondrules. In addition, all grains are found to be Cr-rich, with Cr increased in concentration towards the grain edge, and to be encased in FeS shells which evidently preserved the Si that entered the FeNi at higher temperatures. A mechanism for the production of Si-bearing metal during the condensation of the cooling solar nebula is proposed which considers the metal to have condensed heterogeneously while the mafic silicates condensed homogeneously with amounts of required undercooling in the low-pressure regions where ordinary and carbonaceous chondrites formed, resulting in Si mole fractions of 0.003 at nebular pressures less than 0.000001 atm.

Rambaldi, E. R.↗
NASA NTRS1980

Origin of iron meteorite groups IAB and IIICD

Several low Ni-iron meteorites previously classified with group IAB are reclassified with group IIICD because of lower Ge, Ga, W, and Ir concentrations and higher As concentrations. The low Ni extreme of IIICD is now 62 mg/g, and that of IAB is 64 mg/g. It is proposed that the meteorites of both groups formed as individual shock melts on a chondritic parent body. The differences in log element-log Ni slopes of the daughter irons demonstrate that there were detailed differences in the composition and size of phases in the parental material (e.g., more Ni in the sulfides or metal of IAB, or more Ge and Ir in the oxides of IIICD).

Wasson, J. T.↗
NASA NTRS1980

Chemical classification of iron meteorites. IX - A new group /IIF/, revision of IAB and IIICD, and data on 57 additional irons

The paper discusses the chemical classification of independent iron meteorites which include 57 meteorites based on structural observations and concentrations of Ni, Ga, Ge, and Ir. Instrumental neutron activation analysis indicates that five previously studied irons with very high Ge/Ga ratios are compositionally closely related and can be gathered together as group IIF; a previously unstudied iron, Dehesa, has the highest Ge/Ga ratios known in an iron meteorite, a ratio 18 times higher than that in CI chondrites. In terms of Ge/Ga ratios and other properties, group IIF shows genetic links to the Eagle station pallasites and CO/CV chondrites. The iron with the highest Ni concentration, Oktibbeha County, is a member of group IAB, and it extends the concentration ranges of all elements in this nonmagnetic group.

Kracher, A.↗
NASA NTRS1980

Composition of the metal phases in ordinary chondrites - Implications regarding classification and metamorphism

The paper examines the composition of metal phases and metamorphism in ordinary chondrites. It is shown that below 550 C increasing Co decreases the equilibrium kamacite Ni concentration of an alpha to gamma system, and that the equilibrated L chondrites have kamacite and taenite Co concentrations in the L-group range. Metal-phase studies of petrologic type-3 ordinary chondrites with highly unequilibrated silicates showed a wide range in the degree of matrix kamacite equilibration; in the three most unequilibrated chondrites most taenite is clear, and the high Ni content shows that metamorphic temperatures were lower than 400 C in these meteorites.

Afiattalab, F.↗
NASA NTRS1980

Early lunar petrogenesis, oceanic and extraoceanic

An attempt is made to ascertain which (if any) pristine nonmare rocks, other than KREEPy ones, are not cumulates from the magma ocean. It is noted that the only pristine rocks having bulk densities low enough to have formed by floating above the magma ocean are the ferroan anorthosites, which are easily recognizable as a discrete subset of pristine rocks in general, on the basis of mineral composition relationships. The other class of pristine nonmare rocks, the Mg-rich rocks, did not form from the same magma that produced the ferroan anorthosites. It is suggested that they were formed in layered noritic-troctolitic plutons. These plutons, it is noted, were apparently intruded at, or slightly above, the boundary between the floated ferroan anorthosite crust and the underlying complementary mafic cumulates. It is thought that the parental magmas of the plutons may have arisen by partial melting of either deep mafic cumulates from the magma ocean or a still deeper, undifferentiated primordial layer that was not molten during the magma ocean period.

Warren, P. H.↗
NASA NTRS1980

Further foraging for pristine nonmare rocks - Correlations between geochemistry and longitude

The most recent results from a project to find and describe pristine (that is, compositionally endogenous) nonmare rocks are reported. Sixteen nonmare samples are characterized petrographically and by composition, among them numerous key trace elements (siderophiles, incompatibles). Current knowledge about nonmare lunar rocks is surveyed, with emphasis placed on correlations between geochemistry and longitude. Several systematic differences between western ANT (that is, nonKREEPy, nonmare) rocks and the much more thoroughly studied eastern ANT rocks are noted. It is noted that western ANT rocks, whether pristine or nonpristine, tend to have higher Eu/Sm than their eastern counterparts. Pristine western ANT rocks, however, tend to have lower Sc/Sm and Ti/Sm than pristine eastern ANT rocks.

Warren, P. H.↗
NASA NTRS1979

Nebular condensation of Ga, Ge and Sb and the chemical classification of iron meteorites

The quantization of the Ga and Ge contents in iron meteorites, which is used as a key parameter in the chemical classification of iron meteorites, is discussed in terms of nebular condensation. The calculation of nebular equilibrium condensation is examined, taking into account the dependence of the activity coefficient on temperature and composition, and recent calculations of the condensation temperatures of Ga, Ge, Sb, Au, As and Cu are presented, noting that Ge is the most volatile siderophile, followed by Ga and Sb. The narrow intragroup ranges of Ga and Ge are interpreted in terms of minimal fractionation during core crystallization, while the larger ranges of Sb are attributed to its significantly smaller solid/liquid distribution coefficient in IIIAB meteorites.

Wai, C. M.↗
NASA NTRS1979

Volatiles in Chainpur chondrules

A study of volatile element concentrations in individual chondrules in the Chainpur LL3 chondrite is presented. Volatile elements differing in geochemical behavior tended to vary randomly and correlated variations were observed only for those elements inferred to occupy similar mineral sites. Variations in Co and Ni in kamacite show that metal in chondrule interiors has not approached equilibrium, and that postformational element distribution has mainly altered siderophile concentrations in surface metal grains, and has probably left ionic species unaltered. Etching to remove 3 to 5% of the chondrule dissolved siderophiles, enhanced Zn and Na volatiles relative to nonvolatile Cr in the etch, and produced a large Cd enhancement. If metamorphic redistribution of nonsiderophile volatiles was neglible, their high concentrations are inconsistent with chondrule formation by direct condensation, and models involving sudden melting of preexisting solids can explain the observations but require rapid cooling to prevent volatile loss.

Grossman, J. N.↗