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NASA NTRS1989

Iridium in the Vredefort Bronzite Granophyre - Impact melting and limits on a possible extraterrestrial component

The Bronzite Granophyre was analyzed for Ir and other elements to examine whether its origin was due to impact melting or conventional igneous assimilation and to search for a possible extraterrestrial component in the melt. No such component is identified. Ir values range from 50-130 pg/g, equivalent to less than 0.05 percent C1 material. Further evidence against an extraterrestrial source of siderophiles are the low Ir/Au ratios and the absence of systematic correlations between Ir and other elements. However, the Granophyre is significantly enriched in Ir over other Vredefort granitic rocks, implying a fundamental difference between them. Geochemical mixing models indicate that the Ir observed in the Granophyre can be supplied from high-Ir local shales and quartzites during an impact melting event. The results are consistent with the view that the Granophyre is an impact melt and that Vredefort is an impact structure.

French, B. M.↗

NASA NTRS1988

Stability of genthelvite, Zn4(BeSiO4)3S - An exercise in chalcophilicity using exchange operators

Phase relations involving genthelvite are studied on the basis of physical-chemical principles and natural occurrences. Stability diagrams and exchange operators are considered as well as an extended system, natural associations, and solid state relations. It is noted that the extremely high chalcophilicity of Zn explains, in part, why genthelvite is such as rare mineral. It is only somewhat more stable than willemite and is therefore restricted to the extremely low-S environments provided by alkaline granites and syenites.

Burt, Donald M.↗

NASA NTRS1990

Payload vibration isolation in a microgravity environment

Many in-space research experiments require the microgravity environment attainable near the center of mass of the Space Station. Disturbances to the structure surrounding an experiment may lead to vibration levels that will degrade the microgravity environment and undermine the experiment's validity. In-flight disturbances will include vibration transmission from nearby equipment and excitation from crew activity. Isolation of these vibration-sensitive experiments is required. Analytical and experimental work accomplished to develop a payload (experiment) isolation system for use in space is described. The isolation scheme allows the payload to float freely within a prescribed boundary while being kept centered with forces generated by small jets of air. The vibration criterion was a maximum payload acceleration of 10 micro-g's (9.81x10(exp -5)m/s(exp 2), independent of frequency. An experimental setup, composed of a cart supported by air bearings on a flat granite slab, was designed and constructed to simulate the microgravity environment in the horizontal plane. Experimental results demonstrate that the air jet control system can effectively manage payload oscillatory response. An analytical model was developed and verified by comparing predicted and measured payload response. The mathematical model, which includes payload dynamics, control logic, and air jet forces, is used to investigate payload response to disturbances likely to be present in the Space Station.

Alexander, Richard M.↗

NASA NTRS1989

Re-Os, Rb-Sr, and O isotopic systematics of the Archean Kolar schist belt, Karnataka, India

The Re-Os, Rb-Sr, and O isotopic compositions of mafic and ultramafic amphibolites, gold ores, and granitic gneisses of the circa 2700 Ma Kolar schist belt are reported which reveal at least two episodes of postmagmatic alteration that affected these systems. The Re-Os isotopic systematics of many of the belt rocks indicate that Os was introduced to the area via fluids that carried very radiogenic Os, probably from ancient crust. The Rb-Sr systematics of most of these rocks indicate that the alteration event probably occurred no later than the early Proterozoic. Samples of several komatiitic amphibolites have very Os-187-depleted compositions, indicating that open-system behavior also occurred at a much later time than the late Archean or early Proterozoic. The results suggest that the Re-Os system may have only limited utility for geochronologic applications in regions for which postcrystallization noble metal mineralization is evident.

Walker, Richard J.↗

NASA NTRS1989

Cosmic ray production rates of Be-10 and Al-26 in quartz from glacially polished rocks

The concentrations of Be-10 and Al-26 in quartz crystals extracted from glacially polished granitic surfaces from the Sierra Nevada range are studied. These surfaces are identified with the glacial advance during the Tioga period about 11,000 yr ago. The measurements yield the most accurate estimates to date for the absolute production rates of three nuclides in SiO2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43.8-44.6 N and altitudes 2.1-3.6 km.

Nishiizumi, K.↗

NASA NTRS1990

Silicate liquid immiscibility in isothermal crystallization experiments

The role of silicate liquid immiscibility (SLI) in the petrogenesis of lunar granites was investigated in experiments in which four glasses were synthesized from reagent-grade oxides and carbonates with the compositions of two of the sets of coexisting liquids reported by Hess et al. (1975): a KREEP basalt derivative and a mare basalt derivative. Isothermal crystallization experiments showed that SLI is a stable phenomenon in residual lunar liquids saturated with plagioclase, and is likely to produce large compositional separations. The results indicate that controlled-cooling-rate experiments of Rutherford et al. (1974), and Hess et al. (1975, 1978) were substantially correct analogs of the natural process of liquid immiscibility.

Longhi, J.↗

NASA NTRS1991

Impact processes in the Solar System: New understandings through numerical modeling

A collision of two rocky objects circling the sun in space, each roughly the size and mass of a large mountain range, was modeled. A fragmentation hydrocode was developed to perform dynamical computations of collisional outcomes. Explosive framentation and fluid dynamics were used and drawn together into a single application. To model a solid, certain material parameters, such as density, elasticity, rigidity, and energies of melting and vaporization were input. These parameters are well-known for a variety of important materials, such as ice, iron, granite, and basalt. Another important parameter used is the distribution of initial flaws within the material.

Asphaug, E.↗

NASA NTRS1991

Possible lunar ores

Despite the conventional wisdom that there are no lunar ores, geochemical considerations suggest that local concentrations of useful rare elements exist on the Moon in spite of its extreme dryness. The Moon underwent protracted igneous activity in its history, and certain magmatic processes can concentrate incompatible elements even if anhydrous. Such processes include: (1) separation of a magma into immiscible liquid phases (depending on composition, these could be silicate-silicate, silicate-oxide, silicate-sulfide, or silicate-salt); (2) cumulate deposits in layered igneous intrusions; and (3) concentrations of rare, refractory, lithophile elements (e.g., Be, Li, Zr) in highly differentiated, silica-rich magmas, as in the lunar granites. Terrestrial mining experience indicates that the single most important characteristic of a potential ore is its concentration of the desire element. The utility of a planet as a resource base is that the welter of interacting processes over geologic time can concentrate rare element automatically. This advantage is squandered if adequate exploration for ores is not first carried out.

Gillett, Stephen L.↗

NASA NTRS1991

Evolution of ore deposits on terrestrial planets

Ore deposits on terrestrial planets materialized after core formation, mantle evolution, crustal development, interactions of surface rocks with the hydrosphere and atmosphere, and, where life exists on a planet, the involvement of biological activity. Core formation removed most of the siderophilic and chalcophilic elements, leaving mantles depleted in many of the strategic and noble metals relative to their chondritic abundances. Basaltic magma derived from partial melting of the mantle transported to the surface several metals contained in immiscible silicate and sulfide melts. Magmatic ore deposits were formed during cooling, fractional crystallization and density stratification from the basaltic melts. Such ore deposits found in earth's Archean rocks were probably generated during early histories of all terrestrial planets and may be the only types of igneous ores on Mars. Where plate tectonic activity was prevalent on a terrestrial planet, temporal evolution of ore deposits took place. Repetitive episodes of subduction modified the chemical compositions of the crust and upper mantles, leading to porphyry copper and molybdenum ores in calc-alkaline igneous rocks and granite-hosted tin and tungsten deposits. Such plate tectonic-induced mineralization in relatively young igneous rocks on earth may also have produced hydrothermal ore deposits on Venus in addition to the massive sulfide and cumulate chromite ores associated with Venusian mafic igneous rock. Sedimentary ore deposits resulting from mechanical and chemical weathering in reducing atmospheres in Archean earth included placer deposits (e.g., uraninite, gold, pyrite ores). Chromite, ilmenite, and other dense unreactive minerals could also be present on channel floors and in valley networks on Mars, while banded iron formations might underlie the Martian northern plains regions. As oxygen evolved in earth's atmosphere, so too did oxide ores. By analogy, gossans above sulfide ores probably occur on Mars, but not submarine ferromanganese nodules and crusts which have precipitated in oxygenated seawater on earth.

Burns, R. G.↗

NASA NTRS1990

Chondrule-like objects and brown glasses in howardites

Chondrulelike objects and brown glasses were analyzed in the howardites, Bununu, Malvern, Monticello, Pavlovka, and Yamato 7308. The objects are very similar to chondrules in ordinary and carbonaceous chondrites. Like the brown glasses, the chondrulelike objects could have been produced by impact melting that left some crystalline nuclei, followed by a slower cooling rate than for the glasses. Alternatively, these objects are chondrules implanted from chondrite impactors. They are, however, without rims or any adhering matrix. The brown glasses appear to represent melting of average regolithic surface material, except for Monticello and Y7308, both of which have some siliceous glasses. The siliceous glasses could not have been produced by vapor fractionation but by melting of differentiated lithologies such as fayalitic granites. Impact mechanics indicates that howardites with abundant brown glasses came from an asteroid larger than Vesta (greater than 400 km radius), upon which impacts occurred at relative velocities of up to 5 km/s. Howardites with little or no brown glasses came from a smaller parent body. It is concluded that at least two parent bodies are likely sources for the basaltic achondrites.

Olsen, Edward J.↗

NASA NTRS1990

Geologic lithologic mapping in NW Ontario - Remote sensing approaches and caveats

A previously noted higher proportion of coniferous vegetation over metavolcanic formations than over metasedimentary ones, in which deciduous vegetation predominates, has been quantified and extended to encompass granites, migmatites, and ultramafic bodies typical of the NW Ontario shield area. Preliminary analyses of TM data suggests that there are spectral differences associated with these vegetation parameters; more importantly, a species assemblage pattern associated with ultramafic units was found at five different locations.

Bell, R.↗

NASA NTRS1990

Cosmogenic Be-10, Al-26, and He-3 in olivine from Maui lavas

This paper reports the presence of cosmogenic Be-10 and Al-26 in olivines from the Maui Haleakala basalts in which cosmogenic He-3 and Ne-21 were discovered. Based on production-rate calibrations in quartz crystals from late Pleistocene granite exposures in the Sierra Nevada, the Haleakala crater erosion rates derived from Be-10 and Al-26 concentrations agree within 10 percent and give a mean rate of 12 m/Myr, about 40 percent greater than the rate of 8.5 m/Myr previously derived from He-3 measurements. These results establish the feasibility of the simultaneous use of radioactive and stable cosmogenic nuclides for studies of exposure-ages and erosion-rates in basaltic and andesitic terrains.

Nishiizumi, K.↗

NASA NTRS1991

Fragments of quartz monzodiorite and felsite in Apollo 14 soil particles

Samples of 'evolved' lithologies, felsite, quartz monzodiorite (QMD), and whitlockite-rich quartz monzodiorite, were identified compositionally and petrographically among 2-4-mm soil particles from Apollo 14. Fragments of QMD were found to be extremely rare in the Apollo 14 samples. Felsite is similar to previously reported samples. QMD 14161,7069 is similar to 15405 QMD and has ITE concentrations in KREEP-like concentration ratios of about twice the ITE concentrations of average high-K KREEP. QMD cumulate has the highest measured REE concentrations of any lunar sample to date with the exception of individual whitlockite grains. Felsite and whitlockite-rich lithologies appear to be petrogenetically related and have complementary compositions representing separated fractions of the QMD or KREEP-like parental melt. Felsite is a silica-rich fraction of the residual liquid or it is a derivative of the silica-rich fraction. Felsite or lunar granite of this type results from residual liquid separation following crystal-liquid separation of a QMD-like parent melt with concentration ratios of ITEs similar to those of KREEP.

Jolliff, B. L.↗

NASA NTRS1991

The history of lunar volcanism

The timing and extent of volcanism must have reflected the internal thermal evolution of the moon and a knowledge of the history of lunar volcanism is important for understanding the global evolution of the moon. Recent studies show that lunar volcanism began prior to formation of at least some of the major lunar basins, although the exact provenance of the earliest volcanism remains somewhat obscure. It was shown that the ferroan anorthosites are the best candidates to be remnants of the postulated lunar magma ocean. The Mg-suite rocks are the second major category of highland rocks, and highly evolved granites and quartz-monzodiorites comprise a third suite of lunar highland rocks. Basalts form the Apollo 12, 15, and 17 landing sites demonstrated that the lunar mantle is greatly depleted in LREE beneath the widely separated landing sites, an observation supportive of a global process of differentiation. Also, KREEP volcanism apparently predated basin formation in the Serenitatis region, but appears to have been contemporaneous with basin formation in the Imbrium region. Recent studies showed that the earliest lunar basaltic volcanism was of mare-like composition.

Nyquist, Laurence E.↗

NASA NTRS1992

Status of the Whipple Observatory Cerenkov air shower imaging telescope array

Recently the power of the Cerenkov imaging technique in Very High Energy gamma-ray astronomy was demonstrated by the detection of the Crab nebula at high statistical significance. In order to further develop this technique to allow the detection of weaker or more distant sources a second 10 m class reflector was constructed about 120 m from the original instrument. The addition of the second reflector will allow both a reduction in the energy threshold and an improvement in the rejection of the hadronic background. The design and construction of the second reflector, Gamma Ray Astrophysics New Imaging TElescope (GRANITE) is described.

Akerlof, C. W.↗

NASA NTRS1992

The isotopic record of lunar volcanism

Some fundamental aspects of lunar history which have been established as a result of the Apollo program of lunary exploration and contemporaneous Soviet lunar missions are summarized, with emphasis on the isotopic data which record lunar global evolution most directly. It is shown that some lunar anorthosites formed about 4.45 Ga ago, consistent with the early formation of an anorthositic lunar crust. Some 'Mg-suite' norites, troctolites, and dunites also formed approximately contemporaneously with the oldest anorthosites, but others are up to 200-300 Ma younger. More evolved rocks (granites and quartz-monzodiorites) also formed between about 4.4 and 4.0 Ga ago. KREEP basalt volcanism began at least 4.1 Ga ago, approximately contemporaneously with that of the ancient, mare-like basalts, and continued during the cataclysm. The isotopic data record an early primary lunar differentiation during which mare basalt sources formed, probably by crystallization of cumulus minerals from a lunar magma ocean.

Nyquist, L. E.↗

NASA NTRS1992

Utility of imaging spectrometry for lithologic mapping in Greenland

Landsat Thematic Mapper (TM) multispectral image data and field-based spectral reflectance measurements for a portion of the island of Storo, southwestern Greenland, were used to evaluate the potential of imaging spectrometry for lithologic mapping in arctic terrains. TM data allow mapping of tundra vegetation that typically covers moraines at lower elevations, and lichen-covered bedrock exposed at higher elevations. However, the ubiquitous lichen cover, combined with the limited spectral and radiometric capabilities of TM, severely hamper mapping of the amphibolite, anorthosite, gneiss, and granite outcrops on the island. Diagnostic mineral signatures can be discerned from high spectral and radiometric resolution observations, because lichen cover is patchy at mineral and outcrop scales. Results imply that high resolution imaging spectrometer data (e.g., from the HIRIS sensor to fly on the Earth Observing System), detailed field work, and application of subpixel mixing models will dramatically improve the ability to identify and map bedrock in similar terrains.

Rivard, Benoit↗

NASA NTRS1992

A quasi-Hertzian stress field from an internal source: A possible working model for the Vredefort structure

The Vredefort structure is a large domal feature about 110 km southeast of Johannesburg, South Africa, situated within and almost central to the large intracratonic Witwatersrand Basin. This structure consists of an Archean core of ca. 45 km in diameter, consisting largely of granitic gneiss, surrounded by a collar of metasedimentary and metavolcanic supracrustal rocks of the Dominian Group, Witwatersrand and Ventersdorp Supergroups, and Transvaal Sequence. The interpretation of images of the gravity and magnetic fields over Vredefort has permitted the delineation of several important features of the structure and of its environment. The outline of the collar strata is a prominent feature of both the gravity and the magnetic fields. The Vredefort structure shares this distinctive geometry with other structures (e.g., Manicouagan, Decaturville, Sierra Madera) of debated impact origin. In all these, successively older strata with steep outward dips are encountered while traversing inward to the center of the structure. A further attribute of these structures is the shortening of the outcrop of a particular stratigraphic unit compared to the original perimeter of that unit. To account for the geometric attributes of the Vredefort structure a mechanical scheme is required where there is radial movement of horizontal strata toward, with uplift in, the center of the Vredefort structure. Two models can be proposed: (1) one in which there is a rapid rise and violent disruption of cover rocks in response to expansion of a fluid accumulation; and (2) one in which there is, in contrast, a nonexplosive, quasi-Hertzian stress field resulting from a diapiric process. Both models can accommodate the geometry and structural components of Vredefort.

Antoine, L. A. G.↗