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NASA NTRS1988

Geochemistry of K/T boundaries in India and contributions of Deccan volcanism

Three possible Cretaceous/Tertiary (K/T) boundary sections in the Indian subcontinent were studied for their geochemical and fossil characteristics. These include two marine sections of Meghalaya and Zanskar and one continental section of Nagpur. The Um Sohryngkew river section of Meghalaya shows a high iridium, osmium, iron, cobalt, nickel and chromium concentration in a 1.5 cm thick limonitic layer about 30 cm below the planktonic Cretaceous-Palaeocene boundary identified by the characteristic fossils. The Bottaccione and Contessa sections at Gubbio were also analyzed for these elements. The geochemical pattern at the boundary at the Um Sohryngkew river and Gubbio sections are similar but the peak concentrations and the enrichment factors are different. The biological boundary is not as sharp as the geochemical boundary and the extinction appears to be a prolonged process. The Zanskar section shows, in general, similar concentration of the siderophile, lithophile and rare earth elements but no evidence of enrichment of siderophiles has so far been observed. The Takli section is a shallow inter-trappean deposit within the Deccan province, sandwiched between flow 1 and flow 2. The geochemical stratigraphy of the inter-trappeans is presented. The various horizons of ash, clay and marl show concentration of Fe and Co, generally lower than the adjacent basalts. Two horizons of slight enrichment of iridium are found within the ash layers, one near the contact of flow 1 and other near the contact of flow 2, where iridium occurs at 170 and 260 pg/g. These levels are lower by a factor of 30 compared to Ir concentration in the K/T boundary in Meghalaya section. If the enhanced level of some elements in a few horizons of the ash layer are considered as volcanic contribution by some fractionation processes than the only elements for which it occurs are REE, Ir and possibly Cr.

Bhandari, N.↗
NASA NTRS1988

Noble metals in mid-ocean ridge volcanism: A significant fractionation of gold with respect to platinum group metals

Hydrothermal precipitates, black smoker particulate, and massive sulphide dredge samples from the Explorer Ridge on the Juan de Fuca Plate and the TAG hydrothermal area on the Mid-Atlantic Ridge were analyzed for selected noble metals including Au, Ir and Pd by radiochemical neutron activation analysis. The preliminary results indicate that gold contents may reach the ppm range although values in the neighborhood of 100 to 200 ppb are more typical. The platinum group elements (PGE) represented by Ir and Pd are typically less than 0.02 ppb and less than 2 ppb respectively. These abundances represent a significant enrichment of gold relative to the PGE in comparison with average noble metal abundances in mid-ocean ridge basalts (MORB). A partial explanation of this distinctive fractionation can be found in the concepts of sulfur-saturation of basic magma in mid-ocean ridge (MOR) settings, and the origin of MOR hydrothermal fluids. Experimental and petrological data suggest that MORBs are sulfur-saturated at the time of magma generation and that an immiscible sulfide component remains in the mantle residue. Hence, MORBs are noble metal-poor, particularly with respect to PGE. Consequently, black smoker fluids can be expected to reflect the low Ir and Pd contents of the rock column. The average Au content of MORB is 1.3 ppb, and so the rock column is not significantly enriched in Au. The generation of fluids which precipitate solids with 200 ppb Au is apparently dependent on highly efficient fluid chemistry to mobilize Au from the rock column, high Au solubility in seawater hydrothermal fluids and efficient precipitation mechanisms to coprecipitate Au on Fe, Zn and Cu sulfides. Significant differences in these parameters appear to be the ultimate cause of the strong Au-PGE fractionation in the MOR setting. It does not appear from the current data base that MOR hydrothermal fluids are significant contributors to the Ir enrichment seen in Cretaceous-Tertiary boundary sediments.

Crocket, James H.↗
NASA NTRS1988

Chemistry of the older supracrustals of Archaean age around Sargur

In the Archaeans of the Karnataka craton two stratigraphically distinct volcano-sedimentary sequences occur, namely the older supracrustals of the Sargur type and the younger Dharwar greenstones. The dividing line between these is the 3 by old component of the Peninsular gneiss. The trace and rare earth element chemistry of the Sargur metasediments show, in general, marked similarity to the Archaean sediments. The significant departures are in the nickel and chromium abundances. The REE data of the Sargur pelites of the Terakanambi region represented by Silli-gt-bio-feldspar schists and paragneisses show LREE enrichment and flat to depleted HREE pattern. Banded iron formations have very low REE abundance. They show slightly enriched LREE and flat to depleted HREE pattern. REE abundance in the Mn-horizons is comparable to that of the Archaean sediments. Mn-horizons show enriched LREE and flat HREE with anamolous Eu. REE patterns of these bands is well evolved and has similarities with PAAS.

Janardhan, A. S.↗
NASA NTRS1989

Isotopic study of oxygen diffusion in oxide coatings

Diffusion of oxygen in thin films of silicon dioxide was studied using oxygen isotopically enriched in oxygen of atomic mass 18 (O-18). This subject is of interest because thin films of dielectrics such as SiO2 are proposed for use as a protective coatings for solar mirrors in low Earth orbit, which is a strongly oxidizing environment. Films of this material were prepared with a direct current magnetron using reactive sputtering techniques. To produce (O-18)- enriched SiO2, a standard 3.5-in.-diameter silicon wafer was reactively sputtered using (O-18)-enriched (95 percent) oxygen as the plasma feed gas. The films were characterized using Rutherford backscattering and Secondary Ion Mass Spectrometer (SIMS) to establish stoichiometry and purity. Subsequently, the films were exposed to an air-derived oxygen plasma in a standard laboratory plasma reactor for durations of up to 10 hr. The concentration ratio of O-16 as a function of depth was determined using SIMS profiling and compared to a baseline, nonplasma exposed sample. A value for the diffusivity of oxygen near the surface of these films was obtained and found to be about 10(-15)sq cm/sec.

Gulino, Daniel A.↗
NASA NTRS1989

Basalts and gabbros from Mare Crisium - Evidence for extreme fractional crystallization

Petrographic and electron microprobe techniques were used to examine igneous fragments in Luna 24 samples 24088 and 24105. It is the complex chemistry of the pyroxenes that distinguishes the different rock types. Basaltic pyroxenes exhibit an Fe-enrichment trend; the evolutionary trends are more complex in the gabbros, with enrichments in both Fe and Ti and a depletion in Cr. These chemical evolutionary trends are displayed by a progressive variation in rock types from Mg-rich olivine-gabbro to olivine-gabbro, and to ferrogabbro and ferrotroctolite. The low TiO2 content of the primary melt, possibly represented by the least-evolved Mg-rich olivine-gabbro, retarded the formation of early ilmenite and spinel, such that 'Fenner Trend' Fe enrichment occurred. The ferrotroctolite is probably the end product of chemical evolution by extreme fractional crystallization, controlled primarily by olivine and pyroxene crystallization.

Lu, F.↗
NASA NTRS1989

An isotopic study of biogeochemical relationships between carbonates and organic carbon in the Greenhorn Formation

Carbon-isotopic compositions of total carbonate, inoceramid carbonate, micritic carbonate, secondary cements, total organic carbon, and geoporphyrins have been measured in 76 different beds within a 17-m interval of a core through the Greenhorn Formation, an interbedded limestone and calcareous shale unit of Cretaceous age from the Western Interior Seaway of North America. Results are considered in terms of variations in the processes of primary production and in secondary processes. It is shown that the porphyrin isotopic record reflects primary isotopic variations more closely than the TOC isotopic record and that, in these sediments, TOC is enriched in C-13 relative to its primary precursor by 0.6 to 2.8 percent. This enrichment is attributed to isotope effects within the consumer foodweb and is associated with respiratory heterotrophy. Variation in this secondary enrichment are correlated with variations in the isotopic composition of marine carbonate.

Hayes, J. M.↗
NASA NTRS1991

Early mantle differentiation and its thermal consequences

This paper discusses early events (i.e., pre-3.8 Ga) leading to the mantle differentiation. Arguments are presented in favor of a pre-3.8 Ga basaltic crust covering the earth to balance the depletion of the mantle source of Archean continental crust. It is shown that isotopic and chemical considerations of surviving Archean rocks require the existence of a region enriched in Nd throughout much of the early and middle Archean, which was distinct from the continental crust. Data are presented on the essential mass balance for Nd between the depleted mantle and the enriched reservoir, showing that the enriched reservoir must have contained a large (about 30-70 percent) fraction of the total Nd inventory of the differentiated crust-mantle system at 3.8 Ga.

Galer, S. J. G.↗
NASA NTRS1993

Mineral equilibrium in fractionated nebular systems

We investigated the equilibrium mineral assemblages in chemically fractionated nebular systems, using a computer routine that finds the set of minerals and gases which minimizes the Gibbs free energy of a system with stipulated elemental abundances. Diagrams are presented showing the equilibrium mineralogy, as a function of temperature (400-2300 K), for unfractionated solar material and five fractionated systems. The fractionated systems were defined by mixing, in various proportions, the following four volatility components that solar material can be divided into: refractory dust, carbonaceous matter, ices, and H2 gas. Dust enrichment is seen to increase temperatures of condensation/evaporation and the Fe(2+) content of mafic minerals and to permit existence of stable melt phases. Enrichment of dust and organic matter produces mineral assemblages that are similar in many ways to those of enstatite chondrites, but with mafic minerals that are far more reduced than those in primitive enstatite chondrites. Enrichment of dust, organics, and ices leads to highly ferrous mineralogies even at the highest temperatures but does not predict the stability of hydrous phases above about 450 K.

Wood, John A.↗
NASA NTRS1993

Oxygen isotopic composition of individual olivine grains from the Allende meteorite

The oxygen isotopic composition of a variety of individual olivine grains (including refractory forsterite grains, cores of isolated olivine grains, FeO-rich rims, and individual matrix olivine grains) from the Allende CV3 meteorite was investigated by ion microprobe mass spectrometry, in order to obtain information on the formation mechanism of these samples. It was found that the most primitive (i.e., refractory) olivine in Allende is far less enriched in O-16 than are spinel and pyroxene in Ca,Al-rich inclusions, suggesting that Allende olivine must have formed in an environment that is less enriched in O-16 compared to the gas from which Ca,Al-inclusions are formed. FeO-rich (26-30 wt pct) rims of isolated olivine grains are significantly higher in delta-O-17 and delta-O-18 than forsteritic cores, suggesting that these rims formed by condensation from an oxidized gas with higher delta-O-17 than delta-O-18. Matrix olivine was found to be highest in FeO and to have the lowest enrichment in O-16.

Weinbruch, S.↗
NASA NTRS1993

Iron and nickel isotopic mass fractionation in deep-sea spherules

Magnetite-wuestite spherules collected from deep-sea sediments are thought to have originally been Fe-Ni metal particles at the top of the atmosphere that were oxidized and melted during entry into the earth's atmosphere. Some likely sources for the metal particles are Fe-Ni interplanetary dust particles (IDP's) and metal or sulfide from stony IDP's that separated after melting. Davis et al. reported that four of these spherules are enriched in the heavy isotopes of iron, with enrichments of 8-23%/amu. We have developed a technique for analysis of both iron and nickel isotopes on the same ion microprobe spot and have applied this technique to a number of deep-sea spherules in order to better understand the processes leading to isotopic mass fractionation. Eight spherules show iron and nickel isotopic mass fractionation, with iron and nickel enriched in the heavy isotopes by 10-19%/amu and 4-32%/amu, respectively. If the mass fractionations are due to Rayleigh fractionation during evaporation, these spherules lost 76-94% of their original mass. We have analyzed the four magnetite-wuestite spherules for which iron isotopic data were reported by Davis et al. as well as four new spherules.

Davis, Andrew M.↗
NASA NTRS1993

Time-dependent grid adaptation for meshes of triangles and tetrahedra

This paper presents in viewgraph form a method of optimizing grid generation for unsteady CFD flow calculations that distributes the numerical error evenly throughout the mesh. Adaptive meshing is used to locally enrich in regions of relatively large errors and to locally coarsen in regions of relatively small errors. The enrichment/coarsening procedures are robust for isotropic cells; however, enrichment of high aspect ratio cells may fail near boundary surfaces with relatively large curvature. The enrichment indicator worked well for the cases shown, but in general requires user supervision for a more efficient solution.

Russ D Rausch↗
NASA NTRS1994

Watson: A new link in the IIE iron chain

Watson, which was found in 1972 in South Australia, contains the largest single silicate rock mass seen in any known iron meteorite. A comprehensive study has been completed on this unusual meteorite: petrography, metallography, analyses of the silicate inclusion (whole rock chemical analysis, INAA, RNAA, noble gases, and oxygen isotope analysis) and mineral compositions (by electron microprobe and ion microprobe). The whole rock has a composition of an H-chondrite minus the normal H-group metal and troilite content. The oxygen isotope composition is that of the silicates in the IIE iron meteorites and lies along an oxygen isotope fractionation line with the H-group chondrites. Trace elements in the metal confirm Watson is a new IIE iron. Whole rock Watson silicate shows an enrichment in K and P (each approximately 2X H-chondrites). The silicate inclusion has a highly equilibrated igneous (peridotite-like) texture with olivine largely poikilitic within low-Ca pyroxene: olivine (Fa20), opx (Fs17Wo3), capx (Fs9Wo14)(with very fine exsolution lamellae), antiperthite feldspar (An1-3Or5) with less than 1 micron exsolution lamellae (An1-3Or greater than 40), shocked feldspar with altered stoichiometry, minor whitlockite (also a poorly characterized interstitial phosphate-rich phase) and chromite, and only traces of metal and troilite. The individual silicate minerals have normal chondritic REE patterns, but whitlockite has a remarkable REE pattern. It is very enriched in light REE (La is 720X C1, and Lu is 90X C1, as opposed to usual chonditic values of approximately 300X and 100-150X, respectively) with a negative Eu anomaly. The enrichment of whole rock K is expressed both in an unusually high mean modal Or content of the feldspar, Or13, and in the presence of antiperthite.

Olsen, Edward↗
NASA NTRS1994

Partial melting of apatite-bearing charnockite, granulite, and diorite: Melt compositions, restite mineralogy, and petrologic implications

Melting experiments (P = 6.9 kbar, T = 850-950 deg C, NNO is less than fO2 is less than HM) were done on mafic to felsic charnockites, a dioritic gneiss, and a felsic garnet granulite, all common rock types in the Grenville basement of eastern North America. A graphite-bearing granulite gneiss did not melt. Water (H2O(+) = 0.60 to 2.0 wt %) is bound in low-grade, retrograde metamorphic minerals and is consumed during the earliest stages of melting. Most melts are water-undersaturated. Melt compositions range from metaluminous, silicic granodiorite (diorite starting composition) to peraluminous or weakly metaluminous granites (all others). In general, liquids become more feldspathic, less silicic, and less peraluminous and are enriched in FeO, MgO, and TiO2 with increasing temperature. Residual feldspar mineralogy controls the CaO, K2O, and Na2O contents of the partial melts and the behavior of these elements can be used, particularly if the degree of source melting can be ascertained, to infer some aspects of the feldspar mineralogy of the source. K-feldspar, a common restite phase in the charnockite and granulite (but not the diorite) should control the behavior of Ba and, possibly, Eu in these systems and yield signatures of these elements that can distinguish source regions and, in some cases, bulk versus melt assimilation. Apatite, a common restite phase, is enriched in rare earth elements (REE), especially middle REE. Retention of apatite in the restite will result in steep, light REE-enriched patterns for melts derived from the diorite and charnockites.

Beard, James S.↗
NASA NTRS1994

Influence of plumes from biomass burning on atmospheric chemistry over the equatorial and tropical South Atlantic during CITE 3

During all eight flights conducted over the equatorial and tropical South Atlantic in the course of the Chemical Instrumentation Test and Evaluation (CITE 3) experiment, we observed haze layers with elevated concentrations of aerosols, O3, CO, and other trace gases related to biomass burning emissions. They occurred at altitudes between 1000 and 5200 m and were usually only some 100-300 m thick. These layers extended horizontally over several 100 km and were marked by the presence of visible brownish haze. Air mass trajectories indicate that these layers originate in the biomass burning regions of Africa and South America and typically have aged at least 10 days since the time of emission. In the haze layers, O3 and CO concentrations up to 90 and 210 ppb were observed, respectively. The two species were highly correlated. The ratio concentrations in plume minus background concentrations of O3/CO is typically in the range 0.2-0.7, much higher than the ratios in the less aged plumes investigated previously in Amazonia. In most cases, aerosol (0.12-3 micrometer diameter) number concentrations were also elevated by up to 400/cu cm in the layers; aerosol enrichments were also strongly correlated with elevated CO levels. Clear correlations between CO and NO(x) enrichments were not apparent due to the age of the plumes, in which most NO(x) would have already reacted away within 1-2 days. Only in some of the plumes could clear correlations between NO(y) and CO be identified; the absence of a general correlation between NO(y) and CO may be due to instrumental limitations and to variable sinks for NO(y). The average enrichment of the ratio concentrations in plume minus background concentrations of NO(y)/CO was quite high, consistent with the efficient production of ozone observed in the plumes. The chemical characteristics of the haze layers, together with remote sensing information and trajectory calculations, suggest that fire emissions (in Africa and/or South America) are the primary source of the haze layer components.

Andreae, M. O.↗
NASA NTRS1994

A new ejecta shell surrounding a Wolf-Rayet star in the LMC

We have obtained CCD spectra of newly discovered shell-like nebulae around the WN4 star Breysacher 13 and the WN1 star Breysacher 2 in the Large Magellanic Cloud (LMC). The shell around Br 13 shows definite signs of enrichment in both nitrogen and helium, having much stronger (N II) and He I emission lines than are seen in typical LMC H II regions. From the measured electron temperature of about 17,000 K in the shell, we derive He/H and N/O abundance ratios which are factors of 2 and more than 10 higher, respectively, than the average LMC interstellar values. The derived oxygen abundance in the Br 13 shell is down by a factor of 8 compared to the local LMC interstellar medium (ISM); however, the derived electron temperature is affected by the presence of an incomplete shock arising from the interaction of the stellar wind with photoionized material. This uncertainty does not affect the basic conclusion that the Br 13 shell is enriched by processed material from the Wolf-Rayet star. In contrast, the shell around Br 2 shows no clear evidence of enrichment. The nebular spectrum is characterized by extremely strong (O III) and He II emission and very weak (N II). We derive normal He, O, and N abundances from our spectrum. This object therefore appears to be simply a wind-blown structure associated with a relatively dense cloud near the Wolf-Rayet star, although the very high-ionization state of the gas is unusual for a nebula associated with a Wolf-Rayet star.

Garnett, Donald R.↗
NASA NTRS1994

The origin of carbon monoxide in Neptunes's atmosphere

The CO abundance in the observable atmosphere of Neptune can be plausibly explained by rapid vertical mixing from the deeper atmosphere if Neptune has a greater complement of water than Uranus. Thermochemical equilibrium and kinetic calculations reveal that Neptune must and Uranus may have about 10 times more oxygen than carbon, whereas for Jupiter and Saturn equal enrichments of carbon and oxygen are satifactory to explain the observed CO abundances by deep vertical mixing. Relative to hydrogen and solar composition, the respective enrichment factors for carbon and oxygen are 41, 440 (Neptune); 32, less than or equal 260 (Uranus); 6.6, 6.6 (Saturn); and 2.8, 2.8 (Jupiter). Because water ice is the most refractory ice among the ices assumed to be present in the outer solar nebula, the most massive H2O enrichment is expected for the outermost planet of this group. Thus, Neptune can indeed be regarded as the 'god of the seas.'

Lodders, K.↗
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 NTRS1995

Processes involved in the formation of magnesian-suite plutonic rocks from the highlands of the Earth's Moon

The earliest evolution of the Moon likely included the formation of a magma ocean and the subsequent development of anorthositic flotation cumulates. This primary anorthositic crust was then intruded by mafic magmas which crystallized to form the lunar highlands magnesian suite. The present study is a compilation of petrologic, mineral-chemical, and geochemical information on all pristine magnesian-suite plutonic rocks and the interpretation of this data in light of 18 'new' samples. Of these 18 clasts taken from Apollo 14 breccias, 12 are probably pristine and include four dunites, two norites, four troctolites, and two anorthosites. Radiogenic isotopic whole rock data also are reported for one of the 'probably pristine' anorthositic troctolites, sample 14303,347. The relatively low Rb content and high Sm and Nd abundances of 14303,347 suggest that this cumulate rock was derived from a parental magma which had these chemical characteristics. Trace element, isotopic, and mineral-chemical data are used to interpret the total highlands magnesian suite as crustal precipitates of a primitive KREEP (possessing a K-, rare earth element (REE)-, and P-enriched chemical signature) basalt magma. This KREEP basalt was created by the mixing of ascending ultramafic melts from the lunar interior with urKREEP (the late, K-, REE-, and P-enriched residuum of the lunar magma ocean). A few samples of the magnesian suite with extremely elevated large-ion lithophile elements (5-10x other magnesian-suite rocks) cannot be explained by this model or any other model of autometasomatism, equilibrium crystallization, or 'local melt-pocket equilibrium' without recourse to an extremely large-ion lithophile element-enriched parent liquid. It is difficult to generate parental liquids which are 2-4 x higher in the REE than average lunar KREEP, unless the liquids are the basic complement of a liquid-liquid pair, i.e., the so-called 'REEP-fraction,' from the silicate liquid immiscibility of urKREEP. Scarce age information on lunar rocks suggests that magnesian-suite magmatism was initiated at progressively more recent time from the northeast to the southwest on the lunar nearside from 4.45 to 4.25 Ga.

Snyder, Gregory A.↗