Engineering topics

Wasson, John T.

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

At least 37 records · Page 2

NASA NTRS1999

Trapped Melt in IIIAB Irons: Solid/Liquid Elemental Partitioning During the Fractionation of the IIIAB Magma

Group IIIAB, the largest iron-meteorite group, shows compositional trends (including a three-order-of-magnitude It concentration range) indicating that it formed by fractional crystallization of a metallic magma. Because about 200 irons are available, and all degrees of crystallization are well represented, IIIAB offers an excellent set of samples for the study of crystallization at all depths of the asteroidal core. On log-log Ir-Au, and Ir-As diagrams IIIAB forms a broad band; the breadth represents real meteorite-to-meteorite variations, far outside experimental or sampling uncertainties. A successful model must explain the width of this band; I suggest that it mainly resulted from the trapping of parental magma within the crystallizing solid. Because S is essentially insoluble in metal, the abundance of FeS is a measure of the fraction of trapped liquid. The trapped-melt model is supported by the observation that irons having higher S contents plot closer to the inferred composition of the magmatic parental liquid. The lowest S values are found in the irons occupying the left envelope of the IIIAB Ir-Au or Ir-As compositional fields, thus it is this set of irons that should be interpreted as the solid products of a fractionating magma. This simplifies the modeling of the crystallization process and allows inferences regarding the distribution ratios for other elements in the evolved IIIAB system. The large (multiton) Cape York irons show wide variations in their trapped-melt fractions; their compositions seem best understood in terms of a low initial S content of the IIIAB magma, about 20 mg/g. The inferred initial IIIAB distribution coefficient for Ir, 4.6, is much higher than published values based on laboratory studies of low-S systems; I suggest that low-S (and low-P) partition-ratio measurements tend to err in the direction of unity. In IIIAB distribution coefficients for Au, As, and Ni were still < 1 when the most evolved IIIAB irons formed, another indication of a low initial S content.

Wasson, John T.↗

NASA NTRS1999

Massive Chromite in the Brenham Pallasite and the Fractionation of Cr During the Crystallization of Asteroidal Cores

Large (greater than or equal to 2 mm) chromite grains are present in IIIAB iron meteorites and in the main-group a pallasites (PMG), closely related to high-Au IIIAB irons, Pallasites seem to have formed by the intrusion of a highly evolved metallic magma from a IIIAB-like core into fragmented olivine of the overlying dunite mantle. High Cr contents are commonly encountered during the analyses of metallic samples of high-Au IIIAB irons and main-group pallasites, an indication that Cr contents were high in the intruding liquid and that Cr behaved as an incompatible element during the crystallization of the IIIAB magma, contrary to expectations based on the negative IIIAB Cr-Ni and Cr-Au trends among low-Au IIIAB irons. In a region about 10 cm across in the Brenham main-group pallasite massive chromite fills the interstices between olivine grains, the site normally occupied by metal in Brenham and other pallasites. The massive chromite may have formed as a late cumulus phase; because Fe-Ni was also crystallizing, its absence in the chromite-rich region suggests a separation associated with differences in liquid buoyancy. The coexisting chromite and olivine are zoned; in the olivine FeO is highest in pallasitic (olivine-metal) regions, lowest in rims adjacent to chromite, and intermediate in the cores of these olivines. Chromite shows the opposite zoning, with the highest FeO contents at grain edges adjacent to olivine. The observed gradients are those expected to form by Fe-Mg exchange between olivine and chromite during slow cooling at subsolidus temperatures. Compared to normal Brenham, contents of phosphoran olivine and phosphates are higher in the chromitic pallasitic region. We also report data for large-to-massive chromites present in PMG Molong and in high-Au IIIAB Bear Creek that, like Brenham, formed from a highly evolved magma. The Bear Creek chromite has a much lower Mg content than that in the pallasites, implying that, in the PMG, the Mg was extracted from the olivine during high-temperature reaction with the precipitating chromite. There are other circumstantial arguments indicating that Cr was incompatible in the metal during the crystallization of the IIIAB magma, with the concentration in die residual magma rising from an initial value of about 300 micrograms/g to a value around 700 micrograms/g when Bear Creek and Brenham were formed. We consider possible explanations for these negative Cr-Au and Cr-Ni trends and find the most probable one to be that they reflect sampling artefacts resulting from analysts avoiding visible chromite (and the commonly associated phase FeS) when choosing metal samples.

Wasson, John T.↗

NASA NTRS1998

Final Report

Our accomplishments during the tenure of the grant include the chemical-petrographic characterization of chondrules from the highly unequilibrated ungrouped carbonaceous chondrite LEW85332, the examination of matrix materials in several unequilibrated ordinary chondrites, the exploration of the possibility that amoeboid olivine inclusions in CO chondrites were formed by the chondrule forming process at high nebular temperatures, and the interpretation of our results and those of others in terms of nebular agglomeration and parent-body compaction.

Wasson, John T.↗

NASA NTRS1994

Equilibration temperatures of EL chondrites: A major downward revision in the ferrosilite contents of enstatite

A study of enstatite grains separated from EL6 chondrites shows ferrosilite contents to be approx. 0.02-0.04 mol%, about an order of magnitude lower than reported in the literature. The higher values reported earlier can probably be attributed to the excitation of Fe atoms in Fe-Ni and FeS, in part from micrometer-sized blebs of FeS and Fe-Ni commonly present in enstatite grains but mainly from large neighboring opaque grains excited by bremsstrahlung or by doubly backscattered electrons. Our EL6 ferrosilite contents are similar to those observed in coarse enstatite grains in some aubrites. The equilibrium between ferrosilite and Si-bearing kamacite is highly temperature dependent. At constant Si concentration in kamacite, the equilibrium ferrosilite mole fraction drops by a factor of roughly 2.5 for each 100 K decrease in the equilibrium temperature. As a result, the compositions of these two most common EL6 phases can be used to estimate metamorphic equilibration temperatures. The inferred temperatures are near 1200 K.

Wasson, John T.↗

NASA NTRS1994

Silica-merrihueite/roedderite-bearing chondrules and clasts in ordinary chondrites: New occurrences and possible origin

Merrihueite (K,Na)2(Fe,Mg)5Si12O30 (na less than 0.5, fe greater than 0.5, where na = Na/(Na + K), fe = Fe/(Fe + Mg) in atomic ratio) is a rare mineral described only in several chondrules and irregularly-shaped fragments in the Mezo-Madaras L3 chondrite (Dodd et al., 1965; Wood and Holmberg, 1994). Roedderite (Na,K)2(Mg,Fe)5Si12O30 (na greater than 0.5, fe less than 0.5) has been found only in enstatite chondrites and in the reduced, subchondritic silicate inclusions in IAB irons (Fuchs, 1966; Rambaldi et al., 1984; Olsen, 1967). We described silica-roedderite-bearing clasts in L/LL3.5 ALHA77011 and LL3.7 ALHA77278, a silica-roedderite-bearing chondrule in L3 Mezo-Madaras, and a silica-merrihueite-bearing chondrule in L/LL3.5 ALHA77115. The findings of merrihueite and roedderite in ALHA77011, ALHA77115, ALHA77278 and Mezo-Madaras fill the compositional gap betweeen previously described roedderite in enstatite chondrites and silicate inclusions in IAB irons and merrihueite in Mezo-Madaras, suggesting that there is a complete solid solution of roedderite and merrihueite in meteorites. We infer that the silica- and merrihueite/roedderite-bearing chondrules and clasts experienced a complex formational history including: (a) fractional condensation in the solar nebular that produced Si-rich and Al-poor precursors, (b) melting of fractionated nebular solids resulting in the formation of silica-pyroxene chondrules, (c) in some cases, fragmentation in the nebula or on a parent body, (d) reaction of silica with alkali-rich gas that formed merrihueite/roedderite on a parent body, (e) formation of fayalitic olivine and feerosilite-rich pyroxene due to reaction of silica with oxidized Fe on a parent body, and (f) minor thermal metamorphism, possibly generated by impacts.

Krot, Alexander N.↗

NASA NTRS1994

Fayalite-silica association in unequilibrated ordinary chondrites: Evidence for aqueous alteration on a parent body

We report ten occurrences of high-fayalite (Fa56-99 mol%; four with Fa greater than 82 mol%) olivine in association with silica in type-3 ordinary chondrites. Pyroxene with high Fs contents is much less common; Fs contents do not exceed 66 mol%, and most maxima are less than 50 mol%. In those cases where the amount of fayalite is minor relative to that of silica, the fayalite forms a layer on the silica, and shows textural evidence of formation by reaction of silica with oxidized Fe; the latter seems to have resulted from reaction of metallic Fe-Ni with an oxidant, most likely H2O vapor. The fayalite is generally in contact with pyroxene (and, in one case, olivine) having a much lower Fe/(Fe + Mg) ratios, indicating that lattice diffusion has been minimal. Formation of fayalite from SiO2 explains the low Mg content of this olivine; the Mg was sequestered inside the lattices of mafic minerals and was thus inaccessible. In contrast, the moderately high Mn contents of the fayalite indicate that an appreciable fraction of the Mn in the precursor assemblage was accessible; it was probably sited in the matrix in the form of tiny, poorly crystallized oxide grains produced by nebular condensation at temperatures too low to permit diffusion into forsterite or enstatite. The reaction of SiO2 with FeO produced by oxidation (during metamorphism) of Fe-Ni can also account for fayalitic olivine associated with SiO2 microspherules in the fine-grained matrices of type-3 ordinary chondrites and, because matrix is SiO2 normative, for other occurrences of fayalite in matrix. The presence of Mn in the fayalitic rims on the olivine of carbonaceous chondrites does not require a nebular sign.

Wasson, John T.↗

NASA NTRS1993

The origin of chromitic chondrules and the volatility of Cr under a range of nebular conditions

We characterize ten chromatic chondrules, two spinelian chondrules andd one spinel-bearing chondrule and summarize data for 120 chromitic inclusions discovered in an extensive survey of ordinary chondrites. Compositional and petrographic evidence suggests that chromitic chondrules and inclusions are closely related. The Cr/(Cr + Al) ratios in the spinal of these objects range from 0.5 to 0.9 and bulk Al2O3 contents are uniformly high (greater than 10 wt%, except for one with 8 wt%). No other elements having comparable solar abundances are so stongly enriched, and alkali feldspar and merrillite are more common than in normal chondrules. The Cr/Mg ratios in chromitic chondrules are 180-750 times the ratios in the bulk chondrite. With the possible exception of magnetic clumping of chromite in the presolar cloud, mechanical processes cannot account for this enrichment. Examination of nebular equilibrium processes shows that 50%-condensation temperatures of Cr at pH2/pH2O of 1500 are several tens of degrees below those of Mg as Mg2SiO4; the condensation of Cr is primarily as MgCr2O4 dissolved in MgAl2O4 at nebular pressures of 10(exp -4) atm or below. At pH2 = 10(exp -3) atm condesation as Cr in Fe-Ni is favored. Making the nebula much more oxidizing reduces the difference in condensation temperatures but Mg remains more refractory. We conclude that nebular equilibrium processes are not responsible for the enhanced Cr/Mg ratios. We propose that both Cr and Al became enriched in residues formed by incomplete evaporation of presolar lumps. We suggest that spinals remained as solid phases when the bulk of the silicates were incorporated into the evaporating melt; vaporization of Al and Cr were inhibited by the slow kinetics of diffusion. Subsequent melting and crystallization of these residues fractionated Cr from Al. The resulting materials constituted major components in the precursors of chromitic chondrules. Our model implies that chromitic chondrules and inclusions preserve the Cr isotopic record of presolar sources.

Krot, Alexander↗

NASA NTRS1993

Constraints on chondrule origins

Textures and composition variations were studied in a group of compound chondrules and constraints on chondrule formation were derived from the results obtained. Common chondrule textures and petrological-chemical studies of separated chondrules are discussed. Relic grains and cooling rates are considered and observations of compound chondrules are described. Results indicate that the size range of chondrules is limited and that while chondrules show well resolved differences in chemical and isotopic composition, almost all contain relict grains. Chondrule composition is distinct from igneous rock and volatile phases are common in their interiors. It is suggested that chondrules are formed by turbulence generated lightning at the interface between the dusty midplane and the gas-rich portion of the nebula.

Wasson, John T.↗

NASA NTRS1993

A 10.8-kg layered (Muong-Nong-type) tektite from Wenchang, Hainan, China

A 10.79-kg layered tektite from Hainan is the largest tektite from China, and the fifth largest reported to date. It, together with a 1.9-kg Hainan tektite described by Yuan (1981), greatly extends the area of the Australasian field within which layered tektites having masses greater than 1 kg have been found.

Futrell, Darryl S.↗

NASA NTRS1993

Iridium in sediments containing large abundances of Australasian microtektites from DSDP hole 758B in the Eastern Indian Ocean and from DSDP hole 769A in the Sulu Sea

Excess Ir found in sediments at the Cretaceous/Tertiary (K/T) boundary and in other (e.g., Pliocene) sediments from deep sea drilling cores is widely interpreted as evidence of major impact events. The Australasian tektites originated in an impact event approximately 0.77 Ma ago; microtektites have been found in deep-sea sediment cores from throughout the Indian Ocean, the Philippine Sea, and western Pacific Ocean, but Ir has not been previously reported in these horizons. The deep-sea record of tektites is of particular interest, because in contrast to most continental occurrences, the stratigraphy preserves the original depositional position. Recently several cores having exceptionally high contents of Australasian microtektites have been investigated, Glass and Wu found shocked quartz associated with the microtektites. We used neutron activation to determine concentrations of Ir and other elements in two cores bearing microtektites, one from Deep Sea Drilling Project (DSDP) hole 758B in the Eastern Indian Ocean and one from DSDP hole 769A in the Sulu Sea (near Mindanao, Philippines). The sedimentation age for the microtektite layers in core 758B lies between 0.73 - 0.78 Ma and agrees well with the mean laser-fusion Ar-40/Ar-39 age of Australasian tektites of 0.77 +/- 0.02 Ma by Izett et al. We are able to resolve a small positive Ir enhancement in 758B. Core 769A shows too much scatter to allow resolution of an Ir peak.

Schmidt, Gerhard↗

NASA NTRS1993

Multiplicity of chondrule heating events and the coarsening of chondrule textures

Our studies of compound chondrules show that most chondrules experienced at least two thermal events that produced appreciable amounts of melt. I suggest that chondrules were subjected to a hierarchy of brief heating events, the number increasing exponentially with decreasing intensity, and that some of intermediate intensity deposited enough heat to partially melt mesostasis glass and promote the growth of mafic mineral grains. This scenario can account for textures that require improbably low monotonic cooling rates in laboratory simulations, and also for the rarity of glassy chondrule textures despite the ease with which these are produced in the laboratory.

Wasson, John T.↗

NASA NTRS1992

Origin of metallic Fe-Ni in Renazzo and related chondrites

To assess the formation of metallic Fe-Ni in Renazzo and related chondrites, Ni and Co zoning profiles in metallic Fe-Ni are determined from different petrographic sites (chondrule interiors, chondrule margins, chondrule rims, and matrix) in Renazzo, Al Rais, and the related chondrite, MacAlpine Hills 87320. Metal from chondrule interiors shows flat Ni and Co concentrations and profiles, moderately large grain-to-grain compositional variations (even with chondrules), and generally high Ni and Co. Nickel concentrations extend above the kamacite stability limit; etching such 'martensite' shows high-Ni domains in some cases, but observed Ni concentrations do not exceed 190 mg/g. Metal from chondrule margins adjacent to matrix shows convex Ni and Co zoning profiles; the highest Ni and Co concentrations are at grain centers, although the mean central Ni and Co concentrations in margin grains are much lower than those from chondrule interiors; the remainder are convex. The low Co and Ni contents at the edge of grains in chondrule margins are interpreted to reflect dilution by Fe produced by FeO reduction.

Lee, Min S.↗

NASA NTRS1990

Compositional range in the Canyon Diablo meteoroid

The compositional range within the Canyon Diablo (CD) iron meteorites associated with the formation of the Meteor Crater (Arizona) was examined, using the INAA to analyze a set of CD samples consisting of nine irons collected within 7 km of the Meteor Crater, four Arizona IAB irons that were identified by Wasson (1968) as transported CD fragments, and irons from Las Vegas (Nevada) and Moab (Utah) that Buchwald (1975) suggested to be transported CD fragments. Results show that the irons named Helt Township, Idaho, Las Vegas, Mamaroneck, Moab, and Pulaski County are, most likely, mislabeled CD specimens. On the other hand, meteorites named Alexander County, Allan Hills A77283, Ashfork, Fairfield, and Rifle are identified as compositionally distinct independent falls.

Wasson, John T.↗

NASA NTRS1990

Compositions of large metal nodules in mesosiderites - Links to iron meteorite group IIIAB and the origin of mesosiderite subgroups

The compositions of large metal nodules in 12 mesosiderites, ranging from 3 mm to 4 cm in minimum dimensions and from 10 to 200 mg/g in silicate content, were analyzed by neutron activation; also analyzed were two additional samples (Pennyweight Point and Murchison Downs) which were originally designated iron meteorites but were later classified as Mes nodules. It was found that mesosiderites subgroups classified on the basis of metal composition are virtually the same as those based on silicate composition. The general covariation of the silicate and the metal compositional characteristics suggests that these features were established in the same process or in linked processes. A mesosiderite formation model is proposed.

Hassanzadeh, Jamshid↗

NASA NTRS1990

Ungrouped iron meteorites in Antarctica - Origin of anomalously high abundance

Eighty-five percent of the iron meteorites collected outside Antarctica are assigned to 13 compositionally and structurally defined groups; the remaining 15 percent are ungrouped. Of the 31 iron meteorites recovered from Antarctica, 39 percent are ungrouped. This major difference in the two sets is almost certainly not a stochastic variation, a latitudinal effect, or an effect associated with differences in terrestrial ages. It seems to be related to the median mass of Antarctic irons, which is about 1/100 that of non-Antarctic irons. During impacts on asteroids, smaller fragments tend to be ejected into space at higher velocities than larger fragments, and, on average, small meteoroids have undergone more changes in orbital velocity than large ones. As a result, the set of asteroids that contributes small meteoroids to earth-crossing orbits is larger than the set that contributes large meteoroids. Most small iron meteorites may escape from the asteroid belt as a result of impact-induced changes in velocity that reduce their perihelia to values less than the aphelion of Mars.

Wasson, John T.↗

NASA NTRS1990

Oxygen isotopes in chondrules and coarse-grained chondrule rims from the Allende meteorite

The relationship between the porphyritic chondrules and coase-grained chondrule rims of the Allende CV chondrite are examined. The oxygen isotopic compositions of seven chondrule-rim pairs and a large rimless refractory chondrule from Allende are determined. The results suggest that, to account for the O-isotopic compositions of the CV chondrules and rims, three solid precursor components are required: a high-temperature, refractory-, alkali, and (O-16)-rich component; a low-temperature, FeO-rich, refractory, and (O-16)-poor component, and an additional component to explain the composition of BO chondrules.

Rubin, Alan E.↗

NASA NTRS1989

Chemical classification of iron meteorites. XI - Multi-element studies of 38 new irons and the high abundance of ungrouped irons from Antarctica

Concentrations of 14 elements in the metal of 38 iron meteorites and a pallasite are reported. Three samples are paired with previously classified irons, raising the number of well-classified, independent iron meteorites to 598. Several of the new irons are from Antarctica. Of 24 independent irons from Antarctica, eight are ungrouped, a much higher fraction than that among all classified irons. The difference is probably related to the fact that the median mass of Antarctic irons is about two orders of magnitude smaller than that of non-Antarctic irons. Smaller meteoroids may tend to sample a larger number of asteroidal source regions, perhaps because small meteoroids tend to have higher ejection velocities or because they have random-walked a greater increment of orbital semimajor axis away from that of the parent body.

Wasson, John T.↗

NASA NTRS1988

Compositional study of a suite of samples from the 28-t Armanty (Xinjiang) iron meteorite

Neutron activation analysis of 15 samples from widely separated locations on the surface of the 28-t Armanty (Xinjiang) group-IIIE iron meteorite showed no compositional variations attributable to magmatic fractionation processes such as fractional crystallization. The homogeneity contrasts with that observed in the Cape York IIIAB iron, in keeping with the idea that the latter is an exceptional case. From the maximum compositional gradients the radius of the IIIE core is estimated at greater than 1 km. One sample taken from a surface formed by fracture associated with its fall to Earth is slightly anomalous in composition. It is suggested that this region is near the border between two parental gamma crystals and has a high content of minor phases.

Wasson, John T.↗