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NASA NTRS1995

Lithospheric bending at subduction zones based on depth soundings and satellite gravity

A global study of trench flexure was performed by simultaneously modeling 117 bathymetric profiles (original depth soundings) and satellite-derived gravity profiles. A thin, elastic plate flexure model was fit to each bathymetry/gravity profile by minimization of the L(sub 1) norm. The six model parameters were regional depth, regional gravity, trench axis location, flexural wavelength, flexural amplitude, and lithospheric density. A regional tilt parameter was not required after correcting for age-related trend using a new high-resolution age map. Estimates of the density parameter confirm that most outer rises are uncompensated. We find that flexural wavelength is not an accurate estimate of plate thickness because of the high curvatures observed at a majority of trenches. As in previous studies, we find that the gravity data favor a longer-wavelength flexure than the bathymetry data. A joint topography-gravity modeling scheme and fit criteria are used to limit acceptable parameter values to models for which topography and gravity yield consistent results. Even after the elastic thicknesses are converted to mechanical thicknesses using the yield strength envelope model, residual scatter obscures the systematic increase of mechanical thickness with age; perhaps this reflects the combination of uncertainties inherent in estimating flexural wavelength, such as extreme inelastic bending and accumulated thermoelastic stress. The bending moment needed to support the trench and outer rise topography increases by a factor of 10 as lithospheric age increases from 20 to 150 Ma; this reflects the increase in saturation bending moment that the lithosphere can maintain. Using a stiff, dry-olivine rheology, we find that the lithosphere of the GDH1 thermal model (Stein and Stein, 1992) is too hot and thin to maintain the observed bending moments. Moreover, the regional depth seaward of the oldest trenches (approximately 150 Ma) exceeds the GDH1 model depths by about 400 m.

Levitt, Daniel A.↗
NASA NTRS1995

Silica-rich orthopyroxenite in the Bovedy chondrite

A large (greater than 4.5 x 7 x 4 mm), igneous-textured clast in the Bovedy (L3) chondrite is notable for its high bulk SiO2 content (is approximately equal to 57.5 wt%). The clast consists of normally zoned orthopyroxene (83.8 vol%), tridymite (6.2%), an intergrowth of feldspar (5.8%) and sodic glass (3.1%), pigeonite (1.0%), and small amounts of chromite (0.2%), augite, and Fe,Ni-metal; it is best described as a silica-rich orthopyroxenite. The oxygen-isotopic composition of the clast is similar, but not identical, to Bovedy and other ordinary chondrites. The clast has a superchondritic Si/Mg ratio, but has Mg/(Mg + Fe) and Fe/Mn ratios that are similar to ordinary chondrite silicate. The closest chemical analogues to the clast are radial-pyroxene chondrules, diogenites, pyroxene-silica objects in ordinary chondrites, and silicates in the IIE iron meteorite Weekeroo Station. The clast crystallized from a siliceous melt that cooled fast enough to prevent complete attainment of equilibrium but slow enough to allow nearly complete crystallization. The texture, form, size and composition of the clast suggestion that it is an igneous differentiate from an asteroid or planetesimal that formed in the vicinity of ordinary chondrites. The melt probably cooled in the near-surface region of the parent object. It appears that in the source region of the clast, metallic and silicate partial melt were largely-to-completely lost during a relatively low degree of melting, and that during a higher degree of melting, olivine and low-Ca pyroxene separated from the remaining liquid, which ultimately solidified to form the clast. While these fractionation steps could not have all occurred at the same temperature, they could have been accomplished in a single melting episode, possibly as a result of heating by radionuclides or by electromagnetic induction. Fractionated magmas can also account for other Si-rich objects in chondrites.

Ruzicka, Alex↗
NASA NTRS1993

The effect of Na vapor on the Na content of chondrules

Chondrules contain higher concentrations of volatiles (Na) than expected for melt droplets in the solar nebula. Recent studies have proposed that chondrules may have formed under non-canonical nebular conditions such as in particle/gas-rich clumps. Such chondrule formation areas may have contained significant Na vapor. To test the hypothesis of whether a Na-rich vapor would minimize Na volatilization reaction rates in a chondrule analog and maintain the Na value of the melt, experiments were designed where a Na-rich vapor could be maintained around the sample. A starting material with a melting point lower that typical chondrules was required to keep the logistics of working with Na volatilization from NaCl within the realm of feasibility. The Knippa basalt, a MgO-rich alkali olivine basalt with a melting temperature of 1325 +/- 5 C and a Na2O content of 3.05 wt%, was used as the chondrule analog. Experiments were conducted in a 1 atm, gas-mixing furnace with the fO2 controlled by a CO/CO2 gas mixture and fixed at the I-W buffer curve. To determine the extent of Na loss from the sample, initial experiments were conducted at high temperatures (1300 C - 1350 C) for duration of up to 72 h without a Na-rich vapor present. Almost all (up to 98%) Na was volatilized in runs of 72 h. Subsequent trials were conducted at 1330 C for 16 h in the presence of a Na-rich vapor, supplied by a NaCl-filled crucible placed in the bottom of the furnace. Succeeding Knudsen cell weight-loss mass-spectrometry analysis of NaCl determined the P(sub Na) for these experimental conditions to be in the 10(exp -6) atm range. This value is considered high for nebula conditions but is still plausible for non-canonical environments. In these trials the Na2O content of the glass was maintained or in some cases increased; Na2O values ranged from 2.62% wt to 4.37% wt. The Na content of chondrules may be controlled by the Na vapor pressure in the chondrule formation region. Most heating events capable of producing chondrules are sufficient to volatile Na. Sodium volatilization reaction rates will be reduced to varying degrees from melt droplets, depending on the magnitude of the P(sub Na) generated. A combination of Na vapor during, and Na diffusion back into chondrules after, formation could maintain and/or enrich Na concentrations in chondrules.

Lewis, R. Dean↗
NASA NTRS1995

10 micron spectroscopy of younger stars in the rho Ophiuchi cloud

Spectra in the 10 micrometer region were obtained of 14 young stars associated with the central core of the rho Oph dark cloud complex. Silicate dust emission and absorption features can be fairly well reproduced with simple models using the emissivity of the silicates in the Orion Trapezium region, believed to be typical of molecular cloud dust. A spectrum of the Trapezium star theta(sup 1) Ori D was obtained to define the emissivity more precisely. The emissivity of silicate dust around the late-type giant Mu Cep does not improve the fits to the absorption features and provides a poorer match to the emission features. None of the sources display a strong 11.2 micrometer peak like that seen in comet Halley and attributed to crystalline olivine. A broad weak feature near 11.2 micrometer, possibly related to the comet feature, may be present in the emission spectrum of the Herbig Ae star HD 150193. Absorption features toward two of the objects are narrower than would be expected from Trapezium-like silicates, suggesting differences in the composition of the silicates. The relation between the silicate extinction band depth and H2O ice band depths is determined for the deeply embedded objects. One late-type object, Elias 14, clearly shows the 11.25 micrometer aromatic hydrocarbon emission feature, possibly excited by the nearby B star, HD 147889, though the latter does not exhibit the feature.

Hanner, M. S.↗
NASA NTRS1994

The Meteoritical Bulletin, no. 76, 1994 January: The U.S. Antarctic Meteoritie Collection

The Meteoritical Bulletin number 76 contains a list of all classified meteorites collected by the United States' Antarctic meteroite program as of the summer of 1993. The compilation includes available information on classification, mass, weathering, olivine and pyroxene composition, Al-26 activities, natural thermoluminescence levels, pairing, collection location and references to published descriptions for 5537 meteorite specimens. A list of the properties of metallic meteorites and a list of the most significant meteorites by class are also included.

Grossman, Jeffrey N.↗
NASA NTRS1993

Attenuation of Glass Dissolution in the Presence of Natural Additives

The study described here explored the dissolution kinetics of glasses in aqueous environments in systems which included a variety of natural crystalline solids in addition to the glass itself and the aqueous phase. The results demonstrated the possibility of a dramatic decrease in the rate of dissolution of silicate glass in the presence of certain varieties of olivine-based materials. This decrease in dissolution rate was shown to be due to the fact that these additives consist mostly of Mg-based material but also contain minor amounts of Al and Ca. The combined presence of Mg with these minor species affected the corrosion rate of the glass as a whole, including its most soluble components such as boron. The study has potentially important implications to the durability of glasses exposed to natural environments. The results may be relevant to the use of active backfill materials in burial sites for nuclear waste glasses as well as to better understanding of the environmental degradation of natural and ancient glasses.

Sang, Jing C.↗
NASA NTRS1996

Regional and Localized Deposits on the Moon

Earth-based telescopic remote sensing studies have provided important information concerning lunar pyroclastic deposits. Combined with the returned lunar sample studies and analyses of lunar photography, we have learned a great deal about the nature and origin of these explosive volcanic materials. Lunar pyroclastic deposits are more numerous, extensive, and widely distributed than previously thought. Two generic classes of lunar pyroclastics have been identified, regional and localized. From the former, two separate spectral compositional groups have been identified; one is dominated by Fe(2+)-bearing glasses, the other is composed of ilmenite-rich black spheres. Comparatively, three separate spectral groups have been identified among the localized deposits: highlands-rich, olivine-rich, and mare-rich. Returned sample studies and the recently collected Galileo and Clementine data also corroborate these findings. Albedo data and multispectral imagery suggest that the thicker core deposits of the regional dark mantle deposits (RDMD) are surrounded by pyroclastic debris and subjacent highlands material. The presence of a major component of pyroclastic debris in the regolith surrounding the core regional deposits has important implications for the resource potential of these materials. Both telescopic and orbital spectra indicate that the regional pyroclastic deposits are rich in iron, titanium and oxygen-bearing minerals. Particle shapes vary from simple glass spheres to compound droplets with quench crystallized textures. Their small grain size and friability make them ideal indigenous feedstock. Compared to other resource feedstock sources on the Moon, these pyroclastic materials may be the best oxygen resource on the Moon.

Coombs, Cassandra R.↗
NASA NTRS1995

Chondrule formation, metamorphism, brecciation, an important new primary chondrule group, and the classification of chondrules

The recently proposed compositional classification scheme for meteoritic chondrules divides the chondrules into groups depending on the composition of their two major phases, olivine (or pyroxene) and the mesostasis, both of which are genetically important. The scheme is here applied to discussions of three topics: the petrographic classification of Roosevelt County 075 (the least-metamorphosed H chondrite known), brecciation (an extremely important and ubiquitous process probably experienced by greater than 40% of all unequilibrated ordinary chondrites), and the group A5 chondrules in the least metamorphosed ordinary chondrites which have many similarities to chondrules in the highly metamorphosed 'equilibrated' chondrites. Since composition provides insights into both primary formation properties of the chondruies and the effects of metamorphism on the entire assemblage it is possible to determine the petrographic type of RC075 as 3.1 with unique certainty. Similarly, the near scheme can be applied to individual chondrules without knowledge of the petrographic type of the host chondrite, which makes it especially suitable for studying breccias. Finally, the new scheme has revealed the existence of chondrules not identified by previous techniques and which appear to be extremely important. Like group A1 and A2 chondrules (but unlike group B1 chondrules) the primitive group A5 chondruies did not supercool during formation, but unlike group A1 and A2 chondrules (and like group B1 chondrules) they did not suffer volatile loss and reduction during formation. It is concluded that the compositional classification scheme provides important new insights into the formation and history of chondrules and chondrites which would be overlooked by previous schemes.

Sears, Derek W. G.↗
NASA NTRS1991

Constraints on the Parent Bodies of Collected Interplanetary Dust Particles

Samples of Interplanetary Dust Particles (IDPs) have now been collected from the stratosphere, from the Earth's ocean beds, and from the ice caps of Greenland and Antarctica. The most likely candidates for the sources of these particles are comets and asteroids. Comparison of the infrared spectra, elemental compositions, and mineralogy of the collected dust with atmospheric entry models and data obtained from cometary probes and telescopic observations has provided important constraints on the possible sources of the various types of collected dust. These constraints lead to the following conclusions. First, most of the deep sea, Greenland, and Antarctic spherules larger than 100 microns are derived from asteroids. Second, the stratospheric IDPs dominated by hydrated layer-lattice silicate minerals are also most likely derived from asteroids. Finally, the stratospheric IDPs dominated by the anhydrous minerals olivine and pyroxene are most likely from comets. The consequences of these parent body assignments are discussed.

Sandford, S. A.↗
NASA NTRS1996

A Glass Spherule of Questionable Impact Origin from the Apollo 15 Landing Site: Unique Target Mare Basalt

A 6 mm-diameter dark spherule, 15434,28, from the regolith on the Apennine Front at the Apollo 15 landing site has a homogeneous glass interior with a 200 microns-thick rind of devitrified or crystallized melt. The rind contains abundant small fragments of Apollo 15 olivine-normative mare basalt and rare volcanic Apollo 15 green glass. The glass interior of the spherule has the chemical composition, including a high FeO content and high CaO/Al2O3, of a mare basalt. Whereas the major element and Sc, Ni, and Co abundances are similar to those of low-Ti mare basalts, the incompatible elements and Sr abundances are similar to those of high-Ti mare basaits. The relative abundance patterns of the incompatible trace elements are distinct from any other lunar mare basalts or KREEP; among these distinctions are a much steeper slope of the heavy rare earth elements. The 15434,28 glass has abundances of the volatile element Zn consistent with both impact glasses and crystalline mare basalts, but much lower than in glasses of mare volcanic origin. The glass contains siderophile elements such as Ir in abundances only slightly higher than accepted lunar indigenous levels, and some, such as Au, are just below such upper limits. The age of the glass, determined by the Ar-40/Ar-39 laser incremental heating technique, is 1647 +/- 11 Ma (2 sigma); it is expressed as an age spectrum of seventeen steps over 96% of the Ar-38 released, unusual for an impact glass. Trapped argon is negligible. The undamaged nature of the sphere demonstrates that it must have spent most of its life buried in regolith; Ar-38 cosmic ray exposure data suggest that it was buried at less than 2m but more than a few centimeters if a single depth is appropriate. That the spherule solidified to a glass is surprising; for such a mare composition, cooling at about 50 C/s is required to avoid crystallization, and barely attainable in such a large spherule. The low volatile abundances, slightly high siderophile abundances, and the young age are perhaps all most consistent with an impact origin, but nonetheless not absolutely definitive.

Ryder, Graham↗
NASA NTRS1997

Phase Equilibrium Investigations of Planetary Materials

This grant provided funds to carry out experimental studies designed to illuminate the conditions of melting and chemical differentiation that has occurred in planetary interiors. Studies focused on the conditions of mare basalt generation in the moon's interior and on processes that led to core formation in the Shergottite Parent Body (Mars). Studies also examined physical processes that could lead to the segregation of metal-rich sulfide melts in an olivine-rich solid matrix. The major results of each paper are discussed below and copies of the papers are attached as Appendix I.

Grove, T. L.↗
NASA NTRS1997

Seventh Annual V. M. Goldschmidt Conference

Topics considered include: Subduction of the Aseismic Cocos Ridge Displaced Magma Sources Beneath the Cordillera de Talamanca, Costa Rica; Topography of Transition Zone Discontinuities: A Measure of 'Olivine' Content and Evidence for Deep Cratonic Roots; Uranium Enrichment in Lithospheric Mantle: Case Studies from French Massif Central; Rare-Earth-Element Anomalies in the Decollement Zone of the nankai Accretionary Prism, Japan: Evidence of Fluid Flow?; Rare Earth Elements in Japanese Mudrocks: The Influence of Provenance; The Evolution of Seawater Strontium Isotopes in the Last Hundred Million Years: Reinterpretation and Consequences for Erosion and Climate Models; From Pat to Tats: The Lead Isotope Legacy in the Studies of the Continental Crust-Upper Mantle System; Geochronology of the Jack Hills Detrital Zircons by Precise Uranium-Lead Isotope-Dilution Analysis of Crystal Fragments; Iridium in the Oceans; The Helium-Heat-Lead Paradox; Control of Distribution Patterns of Heavy Metals in Ganga Plain Around Kanpur Region, India, by Fluvial Geomorphic Domains; Geochemical and Isotopic Features of Ferrar Magmatic Provience (Victoria Land, Antarctica); Rare Earth Elements in Marine Fine-Grained Sediments from the Northwestern Portuguese Shelf (Atlantic); Aspects of Arc Fluxes; General Kinetic Model for Dolomite Precipitation Rate with Application to the Secular History of Seawater Composition; High-Precision Uranium-series Chronology from Speleothems; Trace-Element Modeling of Aqueous Fluid-Peridotite Interaction in the Mantle Wedge of Subduction Zones; Rainfall Variations in Southeastern Australia over the Last 500,000 Years from Speleothem Deposition; The Role of Water in High-Pressure Fluids; The Kinetic Conditions of Metamorphic Minearogenesis: Evidence from Minerals and Assemblages.

Source record↗
NASA NTRS1997

Moessbauer Mineralogy on the Moon: The Lunar Regolith

A first-order requirement for spacecraft missions that land on solid planetary objects is instrumentation for mineralogical analyses. For purposes of providing diagnostic information about naturally-occurring materials, the element iron is particularly important because it is abundant and multivalent. Knowledge of the oxidation state of iron and its distribution among iron-bearing mineralogies tightly constrains the types of materials present and provides information about formation and modification (weathering) processes. Because Moessbauer spectroscopy is sensitive to both the valence of iron and its local chemical environment, the technique is unique in providing information about both the relative abundance of iron-bearing phases and oxidation state of the iron. The Moessbauer mineralogy of lunar regolith samples (primarily soils from the Apollo 16 and 17 missions to the Moon) were measured in the laboratory to demonstrate the strength of the technique for in situ mineralogical exploration of the Moon. The regolith samples were modeled as mixtures of five iron-bearing phases: olivine, pyroxene, glass, ilmenite, and metal. Based on differences in relative proportions of iron associated with these phases, volcanic ash regolith can be distinguished from impact-derived regolith, impact-derived soils of different geologic affinity (e.g., highlands, maria) can be distinguished on the basis of their constituent minerals, and soil maturity can be estimated. The total resonant absorption area of the Moessbauer spectrum can be used to estimate total FeO concentrations.

Morris, Richard V.↗
NASA NTRS1998

Experimental Study of Lunar and SNC Magmas

The research described in this progress report involved the study of petrological, geochemical and volcanic processes that occur on the Moon and the SNC parent body, generally accepted to be Mars. The link between these studies is that they focus on two terrestrial-type parent bodies somewhat smaller than earth, and the fact that they focus on the role of volatiles in magmatic processes and on processes of magma evolution on these planets. The work on the lunar volcanic glasses has resulted in some exciting new discoveries over the years of this grant. We discovered small metal blebs initially in the Al5 green glass, and determined the significant importance of this metal in fixing the oxidation state of the parent magma (Fogel and Rutherford, 1995). More recently, we discovered a variety of metal blebs in the Al7 orange glass. Some of these Fe-Ni metal blebs were in the glass; others were in olivine phenocrysts. The importance of these metal spheres is that they fix the oxidation state of the parent magma during the eruption, and also indicate changes during the eruption (Weitz et al., 1997) They also yield important information about the composition of the gas phase present, the gas which drove the lunar fire-fountaining. One of the more exciting and controversial findings in our research over the past year has been the possible fractionation of H from D during shock (experimental) of hornblende bearing samples (Minitti et al., 1997). This research is directed at explaining some of the low H2O and high D/H observed in hydrous phases in the SNC meteorites.

Rutherford, Malcolm J.↗
NASA NTRS1997

Mineralogy of the Mafic Anomaly in the South Pole-Aitken Basin: Implications for excavation of the lunar mantle

Mineralogy of South Pole-Aitken Basin (SPA) (the largest confirmed impact basin on the Moon) is evaluated using five-color images from Clementine. Although olivine-rich material as well as basalts rich in clinopyroxene are readily identified elsewhere on the farside, the dominant rock type observed across the interior of SPA is of a very noritic composition. This mineralogy suggests that lower crust rather than the mantle is the dominant source of the mafic component at SPA. The lack of variation in observed noritic composition is probably due to basin formation processes, during which extensive melting and mixing of target materials are likely to occur.

Pieters, C. M.↗
NASA NTRS1998

Development of a High Resolution-High Sensitivity Ion Microprobe Facility for Cosmochemical Applications

NASA NAGW-4112 has supported development of the CAMECA ims 1270 ion microprobe at UCLA for applications in cosmochemistry. The instrument has been brought to an operational status and techniques developed for accurate, precise microbeam analysis of oxygen isotope ratios in polished thin-sections. We made the first oxygen isotopic (delta(18)O and delta(17)O) measurements of rare mafic silicates in the most chemically primitive meteorites, the a chondrites (Leshin et al., 1997). The results have implications for both high temperature processing in the nebula and low-T aqueous alteration on the CI asteroid. We have performed measurements of oxygen isotopic compositions of magnetite and co-existing olivine from carbonaceous (Choi et al., 1997) and unequilibrated ordinary chondrites (Choi et al., in press). This work has identified a significant new oxygen isotope reservoir in the early solar system: water characterized by a very high Delta(17)) value of approx. 5 % per thousand. We have determined the spatial distributions of oxygen isotopic anomalies in all major mineral phases of a type B CAI from Allende. We have also studied an unusual fractionated CAI from Leoville and made the first oxygen isotopic measurements in rare CAIs from ordinary chondrites.

McKeegan, Kevin D.↗
NASA NTRS1995

The Weathering of Antarctic Meteorites: Climatic Controls on Weathering Rates and Implications for Meteorite Accumulation

Weathering of meteorites includes a variety of chemical and mineralogical changes, including conversion of metal to iron oxides, or rust. Other changes include the devitrification of glass, especially in fusion crust. On a longer time scale, major minerals such as olivine, pyroxene, and feldspar are partially or wholly converted to various phyllosilicates. The degree of weathering of meteorite finds is often noted using a qualitative system based on visual inspection of hand specimens. Several quantitative weathering classification systems have been proposed or are currently under development. Wlotzka has proposed a classification system based on mineralogical changes observed in polished sections and Mossbauer properties of meteorite powders have also been used. In the current paper, we discuss induced thermoluminescence (TL) as an indicator of degree of weathering of individual meteorites. The quantitative measures of weathering, including induced TL, suffer from one major flaw, namely that their results only apply to small portions of the meteorite.

Benoit, P. H.↗
NASA NTRS1995

Congruent Melting Kinetics: Constraints on Chondrule Formation

The processes and mechanisms of melting and their applications to chondrule formation are discussed A model for the kinetics of congruent melting is developed and used to place constraints on the duration and maximum temperature experienced by the interiors of relict-bearing chondrules. Specifically, chondrules containing relict forsteritic olivine or enstatitic pyroxene cannot have been heated in excess of 1901 C or 1577 C, respectively, for more than a few seconds.

Greenwood, James P.↗