Engineering topics

Wasson, J. T.

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

At least 19 records

NASA NTRS2004

Selected Papers on Protoplanetary Disks

Three papers present studies of thermal balances, dynamics, and electromagnetic spectra of protoplanetary disks, which comprise gas and dust orbiting young stars. One paper addresses the reprocessing, in a disk, of photons that originate in the disk itself in addition to photons that originate in the stellar object at the center. The shape of the disk is found to strongly affect the redistribution of energy. Another of the three papers reviews an increase in the optical luminosity of the young star FU Orionis. The increase began in the year 1936 and similar increases have since been observed in other stars. The paper summarizes astronomical, meteoric, and theoretical evidence that these increases are caused by increases in mass fluxes through the inner portions of the protoplanetary disks of these stars. The remaining paper presents a mathematical-modeling study of the structures of protostellar accretion disks, with emphasis on limits on disk flaring. Among the conclusions reached in the study are that (1) the radius at which a disk becomes shadowed from its central stellar object depends on radial mass flow and (2) most planet formation has occurred in environments unheated by stellar radiation.

Bell, K. R.↗

NASA NTRS2004

Petrographic Evidence for Rapid Heating and Cooling During Chrondrule Formation

The chondrule cooling rates used in most chondrule-formation models appear to be too low. Recent petrographic evidence indicates that the amount of crystal (especially olivine) growth that occurred after the last melting event was about 30 smaller than the grain sizes simulated in order to estimate cooling rates. The smaller amount of growth leads to an upwards revision of cooling rates by about a factor of 1000. Most chondrules are porphyritic. They consist of large and small crystals of olivine and, less commonly, pyroxene immersed in a mesostasis having a plagioclase-rich composition. In the most primitive chondrites the mesostasis is often vitreous. Because the large majority of chondrules contain FeS, it is clear that the nebula had cooled below the FeS condensation temperature (ca. 650 K) before chondrule formation occurred. The high FeO/(FeO+MgO) ratios of some chondrules require still lower nebular temperatures (less than 500 K). The traditional view has been that porphyritic chondrules formed in a single heating/cooling event and many laboratory experiments have been carried out in various kinds of kinds of furnaces to try to simulate the formation of chondrules textures in a single heating/cooling cycle. These furnace experiments have been used to infer the cooling rates of chondrules during the temperature range at which olivine crystallized from the melt. Most of these inferred values are in the range 0.01-1 K per second. These low cooling rates are problematical because there is no long-term nebular environment that yields such values. In transparent regions chondrules would cool at rates orders of magnitude higher, whereas in an opaque nebular disk the cooling rates would be many orders of magnitude lower. And these latter conditions are not suitable locations for chondrule formation because such high temperatures would cause the complete evaporation of chondrules (which have melting temperatures about 600 K higher than their evaporation temperatures). During the last several years several kinds of petrographic evidence indicating rapid chondrule cooling have been recognized. These include thin overgrowths on relict grains, clusters of small crystals that grew following the most recent melting event, and fragments preserving shardlike shapes that would have been hidden had several tens of micrometers of new growth occurred following the last melting event. And there are other indications of rapid cooling including the preservation of volatiles such as FeS and one case where the modeling of O-isotopic and FeO/(FeO+MgO) gradients indicated a high cooling rate.

Wasson, J. T.↗

NASA NTRS2004

Genetic Relationships Between Chondrules, Rims and Matrix

The most primitive chondrites are composed of chondrules and chondrule fragments, various types of inclusions, discrete mineral grains, metal, sulfides, and fine-grained materials that occur as interchondrule matrix and as chondrule/inclusion rims. Understanding how these components are related is essential for understanding how chondrites and their constituents formed and were processed in the solar nebula. For example, were the first generations of chondrules formed by melting of matrix or matrix precursors? Did chondrule formation result in appreciable transfer of chondrule material into the matrix? Here, we consider three types of data: 1) compositional data for bulk chondrites and matrix, 2) mineralogical and textural information, and 3) the abundances and characteristics of presolar materials that reside in the matrix and rims. We use these data to evaluate the roles of evaporation and condensation, chondrule formation, mixing of different nebular components, and secondary processing both in the nebula and on the parent bodies. Our goal is to identify the things that are reasonably well established and to point out the areas that need additional work.

Huss, G. R.↗

NASA NTRS2003

Milton: A New, Unique Pallasite

The Milton pallasite was found in Missouri, U.S.A. in October, 2000. It consists of a single stone that originally weighed approximately 2040 g. The chemistry of the olivine and metal phases, plus the oxygen isotope ratios of the olivines, differ significantly from other pallasites, making Milton unique. Unfortunately, the meteorite is heavily fractured and weathered.

Jones, R. H.↗

NASA NTRS2002

The IAB Iron-Meteorite Complex: A Group, Five Subgroups, Numerous Grouplets, Closely Related, Mainly Formed by Crystal Segregation in Rapidly Cooling Melts

We present new data or 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. We now find 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 (approximately equal to 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 in 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.

Wasson, J. T.↗

NASA NTRS2001

The FU Orionis Phenomenon and Solar Nebula Material

We summarize astronomical, meteoritic, and theoretical evidence relating to the FU Orionis phenomenon. This evidence suggests that at early times (the first few 10(exp 5) yr), the solar nebula experienced a hot phase characterized by high accretion rates (the "FU Ori epoch"), punctuated by episodic outbursts of enhanced mass flow through the inner part of the disk (less than or equal to 0.3 AU). Throughout this epoch, disk midplane temperatures exceeded 1000 K at 1 AU. Diminishing infall from the cloud core led to decreasing mass flux throughout the disk. When mass flow de creased below the value critical for outburst (5 x 10(exp -7) solar mass/yr, as suggested by thermal ionization instability models), outbursts ceased and the T Tauri epoch began. Outburst timescales are too long to explain calcium- and aluminum-rich inclusion (CAI) and chondrule formation. Volatility-dependent fractionation patterns seen in meteoritic materials suggest that solids formed beginning during a hot epoch when temperatures exceeded 1400 K, and the presence of volatiles in chondrites argues that this process continued until the nebula had cooled to below 400 K. The thermal ionization instability model for FU Ori outbursts is in quantitative agreement with astronomical observations. Its results imply that the terrestrial region of the nebula reached the hot end of this range only during a time when mass flow through the disk was high enough to trigger outbursts (i.e., the FU Ori epoch) and reached the cool end of this range only during the later T Tauri epoch. According to the models, heating of material in the terrestrial planet region during individual FU Ori outbursts would be limited to surface layers of the nebula, leaving midplane materials (which are at greater than or equal to 1000 K) largely unaffected. Alternative FU Ori models should be developed, particularly if compositional differences among chondrite clans are attributable to episodic heating.

Bell, K. R.↗

NASA NTRS2001

Fractionation Trends Among IVA Iron Meteorites: Contrasts with IIIAB Trends

A neutron-activation study of 49 group-IVA irons shows much lower negative slopes on Ir-Au and Ir-As diagrams than observed in the larger magmatic group IIIAB. This difference seems to reflect the tendency of D(sub Ir), D(sub Au), and D(sub As) to increase with increasing S content. Contents of S and other volatiles are much lower in IVA irons than IIIAB irons. We show that both groups can be fit with Dx values that depend quadratically on S, with initial IVA S contents about 6X lower than those in IIIAB. The IVA scatter fields show a spread in Au or As at constant Ir that appears to reflect variations in the fraction of trapped melt between 0% and 30%. Copper shows an S-shaped trend that may reflect moderate positive and negative changes in D(sub Cu) as the magma evolved or, less likely, sampling variations in a broad band reflecting fractionation and trapping of melt. Gibeon, the largest IVA iron with a mass greater than 30 tons, shows an appreciable range in compositions consistent either with differences in the degree of magma crystallization or with differences in the content of trapped melt. A striking difference between IVA and IIIAB is observed in the Ir/Au ratios in the most Ir-rich irons in the groups; that in IVA is 40% lower than the IIIAB ratio, and lower than those in other iron-meteorite groups, The IVA Ir/Au ratio is about half the ratios in the chondrite groups. We examined three possible explanations of this anomaly: (1) the high-Ir irons contain large amounts of trapped melt: or (2) half of the IVA core (i.e., the first 50% to crystallize) is missing from the terrestrial set of IVA irons; or (3) the IVA magma formed by incomplete melting of the metal in the chondritic precursor material, with the metal that remained in the mantle having high Ir and low Au contents. Plausibility arguments favor the third possibility. The third scenario is the most plausible, but the second cannot be ruled out. We review recent evidence regarding the cooling rates in group IVA. In contrast to recent interpretations, we note several lines of evidence that indicate constant cooling rates independent of composition, as expected if all IVA meteorites were in the same core when cooling between 900 and 650 K occurred.

Wasson, J. T.↗

NASA NTRS2000

Iron Meteorites from Antarctica: More Specimens, Still 40% Ungrouped

Clarke (1986) was the first to recognize that ungrouped irons are more common in Antarctica than in the regions where most irons have been collected; his conclusion was based on the first 21 irons collected in Antarctica. Wasson et al. (1989) reported compositional data for 24 Antarctic irons and reported that 8 were ungrouped; the ungrouped fraction of 0.33 was found to be about twice that (0.153) observed in irons from the remainder of the world. Wasson (1990) reported data for 7 additional Antarctic irons, and reported that 12 of 31 were ungrouped, a fraction of 0.39. In Table 1 I summarize the data obtained to date on independent Antarctic iron meteorites by our UCLA neutron-activation laboratory. With about 5 exceptions, the listed values are the means of duplicate determinations. We have now analyzed 40 independent iron meteorites; in Table 2 I list 8 other irons that proved to be paired with meteorites listed in Table 1. Because of the close relationship between pallasites and iron meteorites, in Table 3 I also list our data for two Antarctic pallasites that were studied at UCLA. Our new results confirm the previously reached conclusion about the abundance of ungrouped irons. In fact, the ungrouped fraction has increased slightly; of the 40 irons 16 are ungrouped, a fraction of 0.40. The two meteorites with pallasite structures are both small (approx. equals 50 g); one is ungrouped, the other a high-Ir anomalous member of the main-group pallasites (PMG).

Wasson, J. T.↗

NASA NTRS1999

Iron Meteorites from Antarctica: More Specimens, Still 40% Ungrouped

Clarke was the first to recognize that ungrouped irons are more common in Antarctica than in the regions where most irons have been collected; his conclusion was based on the first 21 irons collected in Antarctica. Wasson et al. reported compositional data for 24 Antarctic irons and reported that eight were ungrouped; the ungrouped fraction of 0.33 was found to be about twice that (0.153) observed in irons from the remainder of the world. Wasson reported data for seven additional Antarctic irons, and reported that 12 of 31 were ungrouped, a fraction of 0.39. I summarize the data obtained to date on independent Antarctic iron meteorites by our UCLA neutron-activation laboratory. With about five exceptions, the listed values are the means of duplicate determinations. We have now analyzed 40 independent iron meteorites; I list eight other irons that proved to be paired with meteorites listed. Because of the close relationship between pallasites and iron meteorites, I also list our data for two Antarctic pallasites that were studied at UCLA. Our new results confirm the previously reached conclusion about the abundance of ungrouped irons. In fact, the ungrouped fraction has increased slightly; of the 40 irons 16 are ungrouped, a fraction of 0.40. The two meteorites with pallasite structures are both small (= 50 g); one is ungrouped, the other a high-Ir anomalous member of the main-group pallasites (PMG).

Wasson, J. T.↗

NASA NTRS1999

Iron Meteorites from Antarctica: More Specimens, Still 40% Ungrouped

I summarize the data obtained to date on independent Antarctic iron meteorites by our UCLA neutron-activation laboratory. With about 5 exceptions, the listed values are the means of duplicate determinations. We have now analyzed 40 independent iron meteorites; I list 8 other irons that proved to be paired with meteorites. Because of the close relationship between pallasites and iron meteorites, I also list our data for two Antarctic pallasites that were studied at UCLA. Our new results confirm the previously reached conclusion about the abundance of ungrouped irons. In fact, the ungrouped fraction has increased slightly; of the 40 irons 16 are ungrouped, a fraction of 0.40. The two meteorites with pallasite structures are both small (approx. 50 g); one is ungrouped, the other a high-Ir anomalous member of the main-group pallasites (PMG).

Wasson, J. T.↗

NASA NTRS1997

Origin of magnetite in oxidized CV chondrites: in situ measurement of oxygen isotope compositions of Allende magnetite and olivine

Magnetite in the oxidized CV chondrite Allende mainly occurs as spherical nodules in porphyritic-olivine (PO) chondrules, where it is associated with Ni-rich metal and/or sulfides. To help constrain the origin of the magnetite, we measured oxygen isotopic compositions of magnetite and coexisting olivine grains in PO chondrules of Allende by an in situ ion microprobe technique. Five magnetite nodules form a relatively tight cluster in oxygen isotopic composition with delta 18O values from -4.8 to -7.1% and delta 17O values from -2.9 to -6.3%. Seven coexisting olivine grains have oxygen isotopic compositions from -0.9 to -6.3% in delta 18O and from -4.6 to -7.9% in delta 17O. The delta 17O values of the magnetite and coexisting olivine do not overlap; they range from -0.4 to -2.6%, and from -4.0 to -5.7%, respectively. Thus, the magnetite is not in isotopic equilibrium with the olivine in PO chondrules, implying that it formed after the chondrule formation. The delta 17O of the magnetite is somewhat more negative than estimates for the ambient solar nebula gas. We infer that the magnetite formed on the parent asteroid by oxidation of metal by H2O which had previously experienced minor O isotope exchange with fine-grained silicates.

Iron/analysis/chemistry↗

NASA NTRS1994

Chondrule origins: Constraints from chondrule properties and cosmochemistry

The origin of chondrules is a problem for the ages. Yet even though a detailed formation mechanism remains elusive, the constraints on possible mechanisms become more stringent with each systematic examination of chondrules that involves a higher level of resolution. Constraints presented here are mainly based on studies of chondrules in unequilibrated ordinary chondrites because these have been studied much more than other primitive chondrites. These constraints are listed in six categories: the first four are constraints of chondrule origin and the last two reflect the constraints chondrule properties place on the ambient nebula.

Wasson, J. T.↗

NASA NTRS1994

The fine nebula dust component: A key to chondrule formation by lightning

Our assessment indicates that chondrule formation by lightning is indeed possible in the solar nebula. Previously the overriding objection to the lightning process of chondrule formation has been that low nebula pressures prevented the buildup of large potential differences. The breakdown potential is controlled by the mean free live distance of an electron. We calculate the mean free live distance in pure H2 gas at 2 AU to be approximately 500 m. A fine dust load constituting 4 wt% of the dust in the dusty midplane region leads to a reduced mean free live distance of only 7 m. Very conservatively we estimate the breakdown potential to be at least 10, 1.8, and 0.7 V/cm at 1, 2, and 3 AU respectively. We set the radius of the lightning bolt equal to the kinetic mean free path of the gas. Our calculations based on electron drift velocities in a fully ionized H2 gas show that first strike durations are 0.96, 3.4, and 7.0 ms at 1, 2, and 3 AU respectively, in much better accordance with the meteoritic evidence than previous estimates of 10-100 s.

Wasson, J. T.↗

NASA NTRS1993

Systematic chemical variations in large 3AB iron meteorites: Clues to core crystallization

Analysis of numerous individual iron meteorites have shown that fractional crystallization of iron cores result in variations in chemical concentration of the solid core which span several orders of magnitude. The magnitude and direction of the resulting spatial gradients in the core can provide clues to the physical nature of the core crystallization process. We have analyzed suites of samples from three large 3AB irons (Cape York, 58t; Chupaderos, 24t; Morito, 10t) in order to estimate local chemical gradients. Initial results for the concentrations of Ge, Pd, Pt (Massey group), Ir, Au, As, Co, Os, and Rh (Dalhouse group), and P (Arizona group) show significant ranges among the Cape York and Chupaderos samples and marginally significant ranges among the Morito samples. Measurements of Au, Ir, Co, Ni, Cu, Ga, As, W, Re (from UCLA) and Ni and Co (Arizona group) are in progress. We find a spatial Ir gradient in Chupaderos with a magnitude similar to the one reported for Agpalilik (Cape York iron) by Esbensen et al.

Haack, H.↗

NASA NTRS1986

Composition and formation of metal nodules and veins in ordinary chondrites

Five large metal nodules, a composite sample of five small metal nodules, one troilite nodule, and two metal veins from five ordinary chondrites were analyzed by electron microprobe and neutron activation analysis. Metal nodules and veins in H chondrites generally consist of large single crystals of kamacite, whereas L nodules contain significant taenite. Most nodules and veins are depleted by large factors ranging up to 240 in refractory siderophiles (Re, Os, Ir, Pt). Tungsten (normally a refractory siderophile) and Au, As, and Ga (volatile siderophiles) have abundance ratios similar to those of the common siderophiles Fe, Co and Ni. It is proposed that the metal with extremely low refractory-element contents was produced by shock-induced vaporization of chondritic material. The refractory elements condensed near the point of vaporization and were not transported with the vapor. Because the shock-generated gas was mildly oxidizing, W formed volatile oxides.

Widom, E.↗

NASA NTRS1986

A nonmagmatic origin of group-IIE iron meteorites

New neutron activation data on 10 elements in 12 IIE and IIE-related irons lead to a reclassification of several irons. Seymchan and Lonaconing are removed from IIE, and Leshan added. Four IIE members are designated IIE-An to call attention to some anomalous properties. The eight normal IIE members define element-Ni trends generally similar to those in the nonmagmatic group IAB; the small negative slopes on W-Ni and Ir-Ni diagrams are strongly indicative of a nonmagmatic origin of the IIE irons. It is proposed that IIE irons like IAB irons originated as individual pools of impact-produced melt in the near-surface region of a chondritic parent body. The positive As-Ni and Au-Ni trends are the only evidence suggesting fractional crystallization, but their slopes are lower than those in magmatic group IIIAB, and only slightly higher than those of Cu and Sb in IAB. It is suggested that the S and C contents of the IIE precursor materials were much lower than those of the IAB precursors, thus higher temperatures were required to generate enough metallic melt to segregate into pools. These higher temperatures are also reflected in the nonchondritic compositions of the silicate inclusions.

Wasson, J. T.↗

NASA NTRS1986

Chondrules in the Murray CM2 meteorite and compositional differences between CM-CO and ordinary chondrite chondrules

Thirteen of the least aqueously altered chondrules in Murray (CM2) were analyzed for bulk compositions, by means of a broad beam electron microprobe, to explore the compositional differences between the CM-CO, and the ordinary chondrite OC chondrules. The CO chondrules are richer in refractory lithophiles and poorer in Cr, Mn, and volatile lithophiles than the OC chondrules; much lower refractory lithophile abundances in CM chondrules resulted from aqueous alteration. Evidence is found for two important lithophile precursor components of CM-CO chondrite chondrules: (1) pyroxene- and refractory-rich, FeO-poor, and (2) olivine-rich, refractoryand FeO-poor. It is suggested that the pyroxene- and refractory-rich, FeO-poor lithophile precursor component has formed by an incomplete evaporation of presolar silicates that brought these materials into the enstatite stability field.

Rubin, A. E.↗

NASA NTRS1986

Properties of the Guin ungrouped iron meteorite - The origin of Guin and of group-IIE irons

The composition and structure of the Guin ungrouped iron meteorite inclusions have been investigated experimentally. The structural characteristics of polished and etched slabs of the meteorite were studied microscopically in reflected light. Modal abundances of troilite nodules and silicate inclusions were determined by weighing paper traces. The bulk composition of the silicate inclusions was calculated by combining modal phase abundances and mineral compositions. It is found that the largest silicate inclusion (2 x 4 cm) consists mostly of a shock-melted plagioclase-rich matrix surrounding large, partly melted augite grains. The oxygen isotopic composition of the inclusion is near that of LL chondrites. The inclusion is found to be similar in composition to selected melt pocket glasses in ordinary chondrites produced in situ by preferential melting of plagioclase rock due to shock compression. It is suggested that the Guin assemblage was formed by impact melting on a chondritic parent body. Silicate inclusions in IIE irons share many of the compositional and petrological characteristics of the Guin inclusions, indicating that IIE irons also formed by impact-melting of chondritic materials. Black and white photomicrographs of the silicate inclusions are provided.

Rubin, A. E.↗