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NASA NTRS1984

Petrologic and geophysical sources of long-wavelength crustal magnetic anomalies

The magnetic mineralogy and magnetic properties of the deep crust are studied as they pertain to the interpretation of long wavelength, or regional, crustal magnetic anomalies in satellite magnetic data and near surface magnetic data. The conclusions have relevance to the understanding of regional magnetic anomalies in magnetic field measuring satellite missions data. There are two separable studies: (1) a synthesis of available information of regional magnetic anomalies and the magnetization of metamorphic and igneous rocks, and (2) a detailed field, analytical, and experimental study of in situ and laboratory specimens from a terrain that offers exposures of high grade granlite facies rocks that have associated regional magnetic and gravity anomalies.

Marsh, B. D.↗

NASA NTRS1983

Retention of ion-implemented-xenon in olivine - Dependence on implantation dose

The diffusion of Xe in olivine, a major mineral in both meteorites and linear samples, was studied. Xe ions were implanted at 200 keV into single-crystal synthetic-forsterite targets and the depth profiles were measured by alpha particle backscattering before and after annealing for 1 hour at temperatures up to 1500 C. The fraction of implanted XE retained following annealing was strongly dependent on the implantation dose. Maximum retention of 100 percent occurred for an implanting dose of 3 x 10 to the 15th power Xe ions/sq cm. Retention was less at lower doses, with approximately more than or = 50 percent loss at one hundred trillion Xe ions/sq cm. Taking the diffusion coefficient at this dose as a lower limit, the minimum activation energy necessary for Xe retention in a 10 micrometer layer for ten million years was calculated as a function of metamorphic temperature. Previously announced in STAR as N83-18528

Melcher, C. L.↗

NASA NTRS1983

The isotopic composition of silver and lead in two iron meteorites - Cape York and Grant

Anomalies in silver isotope composition in the metal phases of the Cape York (IIIA) and Grant (IIIB) iron meteorites are studied together with the lead isotopic composition of both the metal and sulfide phases of Cape York. Following extensive surface cleaning, the Ag-107/Ag-109 ratio in the metal phases of the meteorites is found to be in excess of the terrestrial ratio, and of that found in the sulfide phases. A definite correlation between the Ag-107/Ag-109 and Pd-108/Ag-109 ratios is observed for these meteorites, indicating the in situ decay of Pd-107 and supporting the widespread presence of Pd in the early universe. Lead, determined after cleaning and with chemical separations using low blank levels, is found to exist in variable proportions in the metal and sulfide phases. The sulfides appear to be dominated by radiogenic modern lead, which may be explained by terrestrial contamination or by late metamorphism in the meteorite parent body.

Chen, J. H.↗

NASA NTRS1983

Mineralogy and petrology of chondrules and inclusions in the Mokoia CV3 chondrite

The inclusions and chondrules of the Mokoia CV3 chondrite are studied systematically and compared with those in the Allende meteorite. Five polished thin sections of Mokoia were examined by optical microscopy, backscattered scanning electron microscopy, and electronprobe microanalysis, and objects greater than about 100 microns in apparent diameter were measured and classified petrographically. Three major types of olivine chondrules are distinguished: igneous chondrules, which evidently crystallized from droplets of silicate melt; recrystallized chondrules, apparently metamorphosed at relatively high temperatures; and accretional aggregates, which are probably fragments of igneous chondrules. Refractory inclusions in Mokoia are generally similar to those found in Allende, although Mokoia inclusions include phyllosilicates rather than feldspathoids and melilite-rich Ca, Al-rich inclusions (CAIs) are more abundant in Allende. The small, fine-grained CAIs, which are more abundant than coarse-grained CAIs in both meteorites, are observed to represent aggregates of three distinct constituents: concentric objects, chaotic material and inclusion matrix. It is concluded that most of the CAIs probably formed during metamorphism, partial melting, and incomplete distillation of primitive dust aggregates heated in the solar nebula.

Cohen, R. E.↗

NASA NTRS1983

Pieces of the ancient lunar crust - Ages and composition of clasts in consortium breccia 67915

The composition and chronology of clasts representing three minor lithologies in consortium breccia 67915 are discussed. The lithologies studied are: sodic ferrogabbro, pristine troctolitic anorthosite (67915,26), and granulitic troctolitic anorthosite (67915,67). These lithologies were presumed to represent samples of the ancient lunar crust. The pristine troctolitic anorthosite, a typical member of the pristine anorthosite suite, contains little or no trapped liquid consistent with its two-phase mineralogy. The plagioclase separates have Ar-39-Ar-40 plateau ages of 4.10 + or - 0.06 b.y., and a final rise in the age pattern may represent a 'memory' of the earlier evolution. The granulitic troctolitic anorthosite, which has 13 times larger REE abundances, displays a continuous rise of the Ar-40/Ar-39 ratio. The last 25 percent of the Ar-39 release data establish a minimum age of over 3.5 b.y., but the Pu-244/Nd chronology suggests an age of about 4.3 b.y. The sodic ferrogabbro samples, which appear to be members of the Mg-gabbronorite group of pristine rocks, show large Ar losses but indicate plateaus in the low-temperature data, suggesting a metamorphic event more recently than 260 m.y. ago, a time that is consistent with the North Ray impact event 50 m.y. ago.

Marti, K.↗

NASA NTRS1983

Enstatite chondrites - Trace element clues to their origin

Three EH and three EL chondrites have been subjected to RNAA analysis for 20 trace elements, and interelement correlations were examined in order to assess the effects of nebular fractionation and metamorphism. The refractory siderophiles Ir, Os, and Re correlate with the normal siderophiles Ni, Pd, Au, Sb, and Ge, in ELs but not in EHs. The two element groups presumably condensed on separate phases at first, but concentrated in metal during metabolism. Volatiles are consistently more depleted in ELs than EHs. Strong correlations are found for In-Tl, Tl-Bi, and Zn-Cd-In, which are equally consistent with predicted condensation curves for the solar nebula and with volatilization curves for artificially heated Abee. The three factors accounting for 93 percent of variance reflect volatile, siderophile, and chalcophile behavior.

Hertogen, J.↗

NASA NTRS1984

Composition and origin of enstatite in E chondrites

A petrologic and electron-probe study of enstatite in six EH3, 4, and 5 chondrites, including the previously undescribed Allan Hills A77156, provides no support for Leitch and Smith's (1982) model in which crystals and liquids from two chemically distinct, partially molten planetesimals are mixed. It is found that Cr and Mn concentrations in enstatite grains in matrix and chondrules are not uniquely defined by their cathodoluminescence color. Wide variations in enstatite compositions among chondrules may partly reflect heterogeneities in the precursor material. However, correlated variations of Cr, Mn, and Fe concentrations in enstatite are probably largely due to fractionation during chondrule crystallization. The inverse correlation of petrologic type with minor element concentrations in enstatite appears to be a metamorphic effect.

Mckinley, S. G.↗

NASA NTRS1984

Chemical and physical studies of type 3 chondrites. III Chondrules from the Dhajala H3.8 chondrite

Thermoluminescence (TL) properties have been measured in 58 chondrules separated from the Dhajala H3.8 chondrite. The pyrolytic chondrules are noted to have higher mass-normalized TL values than nonpyrolytic ones. Significant correlations are noted between log(TL) and the bulk CaO, Al2O3, and MnO content of the chondrules. These, together with correlations of log(TL) with the CaO, Al2O3, SiO2 and normative anorthite content of the chondrule glass, indicate an association of the TL and the abundance and position of mesostasis. It is suggested that the TL level in a given chondrule is governed by its bulk composition and metamorphism, and it is hypothesized that the devitrification resistance of unequilibrated chondrule mesostasis explains the unequilibration of certain chondrules in type 3 ordinary chondrites.

Sears, D. W. G.↗

NASA NTRS1983

Isotopic inferences of ancient biochemistries - Carbon, sulfur, hydrogen, and nitrogen

In processes of biological incorporation and subsequent biochemical processing sizable isotope effects occur as a result of both thermodynamic and kinetic fractionations which take place during metabolic and biosynthetic reactions. In this chapter a review is provided of earlier work and recent studies on isotope fractionations in the biogeochemical cycles of carbon, sulfur, hydrogen, and nitrogen. Attention is given to the biochemistry of carbon isotope fractionation, carbon isotope fractionation in extant plants and microorganisms, isotope fractionation in the terrestrial carbon cycle, the effects of diagenesis and metamorphism on the isotopic composition of sedimentary carbon, the isotopic composition of sedimentary carbon through time, implications of the sedimentary carbon isotope record, the biochemistry of sulfur isotope fractionation, pathways of the biogeochemical cycle of nitrogen, and the D/H ratio in naturally occurring materials.

Schidlowski, M.↗

NASA NTRS1984

Structural investigation of the Canadian Shield by orbital radar and LANDSAT

Canadian Shield were studied by orbital radar. The primary objective of the study is scientific: to investigate and clarify the tectonic relationships of the Churchill, Superior, and Grenville Provinces, concentrating on their geologic boundaries, the Nelson and Grenvill Fronts. Theories about its origin range from in-situ regional metamorphism to tectonic sutures resulting from terrain accretion. The SIR-B investigation clarifies this problem. Secondary objectives are technique development, and include: (1) evaluation of the use of orbital radar in high altitude Precambrian terrains; (2) evaluation of look-azimuth biasing in radar and LANDSAT imagery; and (3) investigation of the synergistic use of radar, LANDSAT, and geophysical data in Precambrian studies.

Lowman, P. D., Jr.↗

NASA NTRS1984

Structural investigation of the Grenville Province by radar and other imaging and nonimaging sensors

The structural investigation of the Canadian Shield by orbital radar and LANDSAT, is outlined. The area includes parts of the central metasedimentary belt and the Ontario gneiss belt, and major structures as well-expressed topographically. The primary objective is to apply SIR-B data to the mapping of this key part of the Grenville orogen, specifically ductile fold structures and associated features, and igneous, metamorphic, and sedimentary rock (including glacial and recent sediments). Secondary objectives are to support the Canadian RADARSAT project by evaluating the baseline parameters of a Canadian imaging radar satellite planned for late in the decade. The baseline parameters include optimum incidence and azimuth angles. The experiment is to develop techniques for the use of multiple data sets.

Lowman, P. D., Jr.↗

NASA NTRS1985

LANDSAT 4 investigations of Thematic Mapper and Multispectral Scanner applications

Structural and stratigraphic characteristics of the Drum Mountains in Utah were obtained using LANDSAT 5 TM data that were digitally enhanced using band ratioing, principal components analysis, and spatial filtering techniques. Color ratioing composite images proved most useful for distinguishing between and identifying exposure of hyroxly bearing hydrothermally altered inclusive rocks and bleached contact metamorphic rocks dominated by calc-silicate mineral assemblages. The amount and detail of geologic information interpreted from TM images is more significant than that from MSS and in geologic maps. Enhanced LANDSAT 5 TM data also identified and mapped hydrothermal alteration and distinguished between altered and unaltered limonitic assemblages in the Tonopath, Nevada quadrangle.

Lauer, D. T.↗

NASA NTRS1982

Asteroid compositional rings: Clues to the compositions of primordial planetesimals in the middle solar system

The distribution of asteroid taxonomic types at distances between 2.1 and 5.3 AU is discussed. There are four major, overlapping but compositionally distinct rings of asteroids present within this range of heliocentric distance. The rings, within which 80% of each of 4 major taxonomic types (S, C, P, and D) fall, are centered at 2.6 (0.7), 2.9 (0.8), 3.4 (0.7), and 4.6 (1.5) AU respectively, where the numbers in parentheses are the ring widths in AU. The overall physical resemblence between the asteroid rings and planetary rings is poor; physically the asteroid belt more closely resembles a debris strewn satellite system. Once the mineralogy of these taxonomic types is established, and complications arising from postaccretionary metamorphism are dealt with, they may be used as probes of physical conditions in the early solar system. The identification of primordial planetesimals can be used in studying the siblings of the planetesimals responsible for the final stage of planetary accretion.

Tedesco, E. F.↗

NASA NTRS1985

The south-central United States magnetic anomaly

A positive magnetic anomaly, which dominates the MAGSAT scalar field over the south-central United States, results from the superposition of magnetic effects from several geologic sources and tectonic structures in the crust. The highly magnetic basement rocks of this region show good correlation with increased crustal thickness, above average crustal velocity and predominantly negative free-air gravity anomalies, all of which are useful constraints for modeling the magnetic sources. The positive anomaly is composed of two primary elements. The western-most segment is related to middle Proterozoic granite intrusions, rhyolite flows and interspersed metamorphic basement rocks in the Texas panhandle and eastern New Mexico. The anomaly and the magnetic crust are bounded to the west by the north-south striking Rio Grande Rift. The anomaly extends eastward over the Grenville age basement rocks of central Texas, and is terminated to the south and east by the buried extension of the Ouachita System. The northern segment of the anomaly extends eastward across Oklahoma and Arkansas to the Mississippi Embayment. It corresponds to a general positive magnetic region associated with the Wichita Mountains igneous complex in south-central Oklahoma and 1.2 to 1.5 Ga. felsic terrane to the north.

Starich, P. J.↗

NASA NTRS1985

Workshop on the Early Earth: The Interval from Accretion to the Older Archean

Presentation abstracts are compiled which address various issues in Earth developmental processes in the first one hundred million years. The session topics included: accretion of the Earth (processes accompanying immediately following the accretion, including core formation); impact records and other information from planets and the Moon relevant to early Earth history; isotopic patterns of the oldest rocks; and igneous, sedimentary, and metamorphic petrology of the oldest rocks.

Burke, K.↗

NASA NTRS1985

The 3.5 b.y. old Onverwacht Group: A remnant of ancient oceanic crust

The interaction between seawater and submarine volcanic rock has had important consequences for the chemistry of the ocean during the Phanerozoic. Most extant terranes have been regionally metamorphosed to the amphibolite and granulite facies, so that their precursor lithologies and structures are not readily determinable. However, the 3.5 b.y. old supracrustal rocks of the Barberton Mountain Land, South Africa, have not been subjected to high grade regional metamorphism, and therefore there was reason to hope that a laboratory investigation might reveal the extent to which these rocks had been exposed to subseafloor hydrothermal activity. Hart and de Wit describe bulk geochemical evidence from the entire suite as well as field evidence which support the concept of hydrothermal activity in the Barberton Mountain Land. Mineralogical and textural features which unequivocally mark it as a submarine sequence emplaced in a midocean ridge/fracture zone or back arc/fracture zone environment are briefly discussed.

Hoffman, S.↗

NASA NTRS1985

Chemical constraints on the evolution of Archean continental crust

One of the challenges of Archean geochronology is to find isotopic systems that preserve an indication of a rock's primary age in spite of the effects of later metamorphism. Zircon dating has been used widely with considerable success but not without difficulty, especially in polymetamorphic terrains. Zircons in such cases commonly are found to have lost radiogenic Pb, and despite fractionizing the zircons or abrading them to remove disturbed portions; it often is not possible to define a pattern of Pb loss from which the original age can confidently be inferred. The refinement of techniques to enable extremely small samples, or even single crystals, to be analyzed has contributed greatly to solving the problem but even those techniques cannot resolve the micron scale isotopic heterogeneities within single zircons in which much of their history is recorded. That can only be done by ion microprobe. Progress reports on studies of four Archean rocks, each of which illustrates the power and potential of ion microprobe analysis in solving problems of Archean geochronology are discussed.

Weaver, B. L.↗

NASA NTRS1984

Achondrite ALHA77005 - Alteration of chromite and olivine

The olivine crystals of the 77005 achondrite are brown except for colorless shock lamellae, mottled patches, and grains adjacent to pools of impact melt. Sporadic dark alteration patches in brown olivine and Cr-rich spinel gave the following average electron-microprobe analyses: (olivine) P2O5 0.9, SiO2 57.9, TiO2 0, Al2O3 0.7, Cr2O3 0.4, V2O3 0, Fe2O3 (assumed oxidation state) 17.0, MgO 1.6, CaO 0.2, Na2O 0, K2O 1.8, SO3 (assumed oxidation state) 9.2, Cl 0.1, sum 89.8 wt pct; (spinel) P2O5 3.5, SiO2 2.1, TiO2 2.2, Al2O3 2.1, Cr2O3 13.4, V2O3 0.8, Fe2O3 40.7, MgO 0.9, CaO 0.1, Na2O 0, K2O 2.0, SO3 11.1, Cl 0.1, sum 79.0 wt pct. Ion-microprobe analyses revealed H in both. Rare orange patches in brown olivine from another area gave SiO2 33-35, FeO 30-28, MgO 28-32, sum 93 wt pct. Thermal metamorphism under dry oxidizing conditions is discussed as a possible alternative to shock-induced oxidation for generation of the brown olivine (McSween and Stoeffler, 1980). Because alteration patches transgress shock lamellae, and because sulfatic alteration occurs in fusion crusts of Antarctic meteorites (Gibson et al., 1983), alteration of the 77005 achondrite at the Antarctic surface is preferred to a complex series of processes needed for preterrestrial alteration.

Smith, J. V.↗