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At least 667 records · Page 37

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NASA NTRS1988

Reappraisal of the geology of Kangimut Sammisoq, Ameralik Fjord, Southern West Greenland: Crustal structure and interpretation of isotopic data

In Godthabsfjord, in the North Atlantic Archean Craton, early Archean Amitsoq gneisses are found in association with the mid to late Archean Nuk gneisses. The Amitsoq gneisses contain the Ameralik dykes, now subconcordant strips of amphibolite. The Nuk and Amitsoq gneisses have amphibolite facies polymetamorphic assemblages. To the southeast of Godthabsfjord, there is an extensive region affected by granulite facies metamorphism at 2800 Ma. Early models of crustal evolution for Godthabsfjord and the region to the SE are discussed. Recent detailed mapping combined with U-Pb zircon dating is examined.

Nutman, Allen P.↗

NASA NTRS1988

Archean crustal evolution of the northern North China Craton

The Archean granultie facies rocks of the North China (Sino-Korean) Craton mostly occur inside the northern boundary forming a unique and spectacular granulite belt trending roughly E-W from eastern Hebei, North China in the east to Mt. Daqinchan, western Inner Mongolia in the west, ranging about 1,000 km long. Over the years in the middle portion of this Archean high-grade metamorphic belt a stratigraphic unconformity between the khondalite rock assemblage and the medium in composition granulite assemblage in Datong-Xinghe area is determined. The geological structural properties of the North China Craton are discussed.

Qian, Xianglin↗

NASA NTRS1988

Growth of the lower continental crust

One of the largest uncertainties in crustal composition and growth models is the nature of the lower continental crust. Specifically, by what processes is it formed and modified, and when is it formed, particularly in reference to the upper crust? The main reason for this lack of information is the scarcity of lower crustal rock samples. These are restricted to two types: rocks which outcrop in granulite facies terrains and granulite facies xenoliths which are transported to the earth's surface by young volcanics. The important conclusions arising from the xenolith studies are: the majority of mafic lower crustal xenoliths formed through cumulate process, resitic xenoliths are rare; and formation and metamorphism of the deep crust is intimately linked to igneous activity and/or orogeny which are manifest in one form or another at the earth's surface. Therefore, estimates of crustal growth based on surface exposures is representative, although the proportion of remobilized pre-existing crust may be significantly greater at the surface than in the deep crust.

Rudnick, Roberta L.↗

NASA NTRS1988

Exotic minerals in 3,500 million year old rocks: Evidence for large meteorite impacts

A relatively small area of mountainous terrain in southern Africa provides scientists from all over the world a look at what the surface of the earth was like three and a half billion years ago. The Barberton Mountains lie astride the borders of the Republic of South Africa, Mozambique, and the Kingdom of Swaziland. The discovery of several widely distributed deposits that were likely formed by major terrestrial impacts of large extraterrestrial bodies during this early period of earth's history is reported. The Barberton impact deposits are being studied by electron microscopy. The impact deposits were examined for minerals that show the effects of shock metamorphism or compositions unusual in terrestrial rocks.

Byerly, G. R.↗

NASA NTRS1987

Size-frequency distributions of EH3 chondrules

Size-frequency distributions of chondrules in three EH3 chondrites, namely Qingzhen, Kota-Kota and ALHA77156, are presently determined. Radial pyroxene chondrules are larger than cryptocrystalline chondrules, while nonporphyritic chondrules have a broader size-frequency distribution than the porphyritic, and porphyritic olivine-pyroxene chondrules are larger than porphyritic pyroxene chondrules. This last result is due to a reaction of fine-grained olivine with free silica to form pyroxene during mild thermal metamorphism of the whole-rocks.

Rubin, Alan E.↗

NASA NTRS1988

The role of presolar dust in the formation of the solar system

The characteristics of presolar dust are discussed and evidence is provided to support the possibility that presolar dust was the primary building material for the meteorites and terrestrial planets. The material in the sun's parent molecular cloud is divided into eight reservoirs. The results show that presolar dust and objects made from processed dust make up the vast majority of the material in primitive chondrites. The present theory is able to account for the chemical and oxygen isotopic variations between meteorite classes, the formation of chondrules, the accretion of chondrites, and parent body metamorphism.

Huss, Gary R.↗

NASA NTRS1988

I-Xe systematics in LL chondrites

A stepwise heating analysis of Ar and Xe data from five neutron-irradiated whole rock LL chondrites (Soko Banja, Alta Ameen, Tuxtuac, Guidder, and Olivenza) is presented, emphasizing the complicated thermal history of ordinary chondrites. None of the present meteorites show a well-defined (Ar-40)-(Ar-39) apparent age plateau comprised of more than two release fractions. Most of the samples are found to yield well-defined high-temperature correlations between Xe-129/Xe-130 and Xe-128/Xe-130, and thus determinations of I-129/I-127 and Xe-129/Xe-130 at the time of isotopic closure for Xe. As in the case of other ordinary chondrites, the I-Xe systematics for LL chondrites correlate neither with a metamorphic grade nor with chronologies based opon other methods.

Bernatowicz, T. J.↗

NASA NTRS1988

Origin of petrofabrics and magnetic anisotropy in ordinary chondrites

Three-dimensional finite strain and magnetic susceptibility anisotropy have been determined for 15 ordinary chondrites. The axes of strain and magnetic ellipsoids roughly correlate in both magnitude and orientation. The shapes of these ellipsoids are generally oblate spheroids that define a dominant foliation and a weak lineation. These characteristics suggest deformation involving uniaxial compaction. The degree of uniaxial deformation correlates with intensity of shock, as indicated by optical, TEM and chemical criteria. These data, plus the lack of a relationship between foliation and metamorphic history, indicate that dynamic processes, i.e., impacts, produced planar deformation fabrics in chondrites.

Sneyd, Deana S.↗

NASA NTRS1989

Mineralogy and composition of Archean Crust, Greenland: A pilot study

The Portable Instant Display and Analysis Spectrometer (PIDAS) was taken to southwestern Greenland to investigate in situ the potential application of AVIRIS to estimate the mineralogy and composition of rocks exposed in Archean terranes. The goal was to determine the feasibility of using a high spectral resolution scanner to find and study pristine rocks, those that have not been altered by subsequent deformation and metamorphism. The application of AVIRIS data to the problems in Greenland is logical. However, before a costly deployment of the U-2 aircraft to Greenland is proposed, this study was undertaken to acquire the spectral data necessary to verify that mineralogical mapping in the environmental conditions found there is possible. Although field conditions were far from favorable, all of the major objectives of the study were addressed. One of the major concerns was that lichens would obscure the rock surfaces. It was found that the spectral signature of the lichens was distinct from the underlying rocks. Thus, a spectrum of a rock outcrop, with its partial cover of lichens, can be un-mixed into rock and lichen components. The data acquired during the course of this study supports the conclusion that areas of pristine Archean crust can be differentiated from that which has experienced low grade alteration associated with Proterizoic faulting.

Goetz, Alexander F. H.↗

NASA NTRS1988

Multi-property modeling of ocean basin carbon fluxes

The objectives of this project were to elucidate the causal mechanisms in some of the most important features of the global ocean/atomsphere carbon system. These included the interaction of physical and biological processes in the seasonal cycle of surface water pCo2, and links between productivity, surface chlorophyll, and the carbon cycle that would aid global modeling efforts. In addition, several other areas of critical scientific interest involving links between the marine biosphere and the global carbon cycle were successfully pursued; specifically, a possible relation between phytoplankton emitted DMS and climate, and a relation between the location of calcium carbonate burial in the ocean and metamorphic source fluxes of CO2 to the atmosphere. Six published papers covering the following topics are summarized: (1) Mass extinctions, atmospheric sulphur and climatic warming at the K/T boundary; (2) Sensitivity of climate and atmospheric CO2 to deep-ocean and shallow-ocean carbonate burial; (3) Controls on CO2 sources and sinks in the earthscale surface ocean; (4) pre-anthropogenic, earthscale patterns of delta pCO2 between ocean and atmosphere; (5) Effect on atmospheric CO2 from seasonal variations in the high latitude ocean; and (6) Limitations or relating ocean surface chlorophyll to productivity.

Volk, Tyler↗

NASA NTRS1988

Ocean alkalinity and the Cretaceous/Tertiary boundary

A biogeochemical cycle model resolving ocean carbon and alkalinity content is applied to the Maestrichtian and Danian. The model computes oceanic concentrations and distributions of Ca(2+), Mg(2+), and Sigma-CO2. From these values an atmospheric pCO2 value is calculated, which is used to estimate rates of terrestrial weathering of calcite, dolomite, and calcium and magnesium silicates. Metamorphism of carbonate rocks and the subsequent outgassing of CO2 to the atmosphere are parameterized in terms of carbonate rock reservoir sizes, total land area, and a measure of overall tectonic activity, the sea-floor generation rate. The ocean carbon reservoir computed by the model is used with Deep Sea Drilling Project (DSDP) C-13 data to estimate organic detrital fluxes under a variety of ocean mixing rate assumptions. Using Redfield ratios, the biogenic detrital flux estimate is used to partition the ocean carbon and alkalinity reservoirs between the mixed layer and deep ocean. The calcite flux estimate and carbonate ion concentrations are used to determine the rate of biologically mediated CaCO3 titration. Oceanic productivity was severely limited for approximately 500 kyr following the K/T boundary resulting in significant increases in total ocean alkalinity. As productivity returned to the ocean, excess carbon and alkalinity was removed from the ocean as CaCO3. Model runs indicate that this resulted in a transient imbalance in the other direction. Ocean chemistry returned to near-equilibrium by about 64 mybp.

Caldeira, K. G.↗

NASA NTRS1988

Magnetic microspherules associated with the K/T and upper Eocene extinction events

Magnetic microspherules were identified in over 20 K/T boundary sites, and in numerous Deep Sea Drilling Project (DSDP) cores from the Caribbean and Pacific, synchronous with the extinction of several radiolarian species near the end of the Eocene. The K/T magnetic spherules are of particular interest as carriers of Ir and other siderophiles generally found in abundance in K/T boundary clay. Furthermore the textures and unusual chemistry of their component magnetic phases indicate an origin at high temperature, possibly related to (an) unusual event(s) marking the end of the Cretaceous and Eocene periods. Their origin, along with the non-magnetic (sanidine) spheules, is generally ascribed directly to megaimpact events hypothesized to have periodically disrupted life on Earth. A survey of microspherical forms associated with known meteorite and impact derived materials reveals fundamental differences from the extinction related spherules. Low temperature magnetic experiments on the K/T and Upper Eocene spheroids indicate that, unlike tektites, extremely small superparamagnetic carriers are not present in abundance. The extensive subaerial exposure of Cretaceous combustible black shale during sea level regression in the latest Cretaceous represents a potential source for the magnetic spheroids found in certain K/T boundary clays. The recent discovery of high Ir abundances distributed above and below the K/T boundary within shallow water sediments in Israel, which also contain the most extensive known zones of combustion metamorphism, the so called Mottled Zone, adds a further dramatic footnote to the proposed association between the magnetic spheroids and combustion of organic shales. Interestingly, the Mottled Zone also contains the rare mineral magnesioferrite, which was identified both within the K/T magnetic spheroids and as discrete crystals in boundary clay from marine and continental sites.

Cisowski, Stanley M.↗

NASA NTRS1988

Geological remote sensing signatures of terrestrial impact craters

Geological remote sensing techniques can be used to investigate structural, depositional, and shock metamorphic effects associated with hypervelocity impact structures, some of which may be linked to global Earth system catastrophies. Although detailed laboratory and field investigations are necessary to establish conclusive evidence of an impact origin for suspected crater landforms, the synoptic perspective provided by various remote sensing systems can often serve as a pathfinder to key deposits which can then be targetted for intensive field study. In addition, remote sensing imagery can be used as a tool in the search for impact and other catastrophic explosion landforms on the basis of localized disruption and anomaly patterns. In order to reconstruct original dimensions of large, complex impact features in isolated, inaccessible regions, remote sensing imagery can be used to make preliminary estimates in the absence of field geophysical surveys. The experienced gained from two decades of planetary remote sensing of impact craters on the terrestrial planets, as well as the techniques developed for recognizing stages of degradation and initial crater morphology, can now be applied to the problem of discovering and studying eroded impact landforms on Earth. Preliminary results of remote sensing analyses of a set of terrestrial impact features in various states of degradation, geologic settings, and for a broad range of diameters and hence energies of formation are summarized. The intention is to develop a database of remote sensing signatures for catastrophic impact landforms which can then be used in EOS-era global surveys as the basis for locating the possibly hundreds of missing impact structures. In addition, refinement of initial dimensions of extremely recent structures such as Zhamanshin and Bosumtwi is an important objective in order to permit re-evaluation of global Earth system responses associated with these types of events.

Garvin, J. B.↗

NASA NTRS1988

Mineralogy and phase-chemistry of the Cretaceous/Tertiary section in the Lattengebirge, Bavarian Alps

The Lattengebirge K/T section reveals three distinct Ir spikes. Two of them are contained in the K/T transition zone sensu-strictu termed clayey interval, with 4.4 ppb Ir at the actual K/T boundary, and 2.8 ppb Ir 10 cm above the boundary. The highest Ir enrichment of 9 ppb, however, was detected in semi-cleaned organic material from a thin sandstone layer of Upper Maastrichtian age at 16 cm below the boundary. In this layer various discernible phases are preserved, contrasting with the worldwide observed K/T transition zones which are generally entirely composed of diagenetically altered materials. Given that, important clues to understanding the Cretaceous terminal events may be provided. The phases of the Cretaceous Ir bearing layer at Lattengebirge consist of: sandstone fragmental minerals in a carbonate matrix, coal which is partly burnt, melt glasses presumably of combustion-metamorphic origin, and sulfides, mainly chalcopyrite, contained in the coal. Like many known K/T sections and the Lattengebirge boundary sensu-strictu, the Cretaceous horizon is enriched in Ir and chalcophile elements as well. Although the Lattengebirge section offers the freshest materials, including melt glasses, of all K/T localities investigated, no unequivocal evidence of formation by impact has been found there.

Graup, G.↗

NASA NTRS1988

Inducement of heterochronic variation in a species of planktic foraminifera by a Late Eocene impact event

While it is well known that the cosmic impact event at or near the Cretaceous-Tertiary boundary coincides with an interval of mass extinction, a similar impact (or series of impacts) near the Eocene-Oligocene boundary presents a more complex picture, in terms of associated fluctuations in marine biotic diversity. Tektites, microtektites, and mineral grains exhibiting features of shock metamorphism found in Eocene sediments of the western N. Atlantic, Caribbean, and Gulf of Mexico (comprising the North American microtektite strewn field) offer compelling evidence for a catastrophic impact event. Despite the magnitude of this event, however, few extinctions in the planktic marine fauna are known to have occurred coincident with this event. Instead, changes in relative abundance, morphology, and development occurred. Cosmic impacts generally have been interpreted as influencing the course of evolution through the wholesale elimination of significant portions of standing biotic diversity. Indeed, extinction traditionally has been viewed as the negative side of evolution. In some instances, it is suggested such impact events can serve instead to increase, rather than decrease, morphological and ecological diversity, by altering the developmental programs within species at the level of the local population.

Macleod, N.↗

NASA NTRS1988

The Calvin 28 cryptoexplosive disturbance, Cass County, Michigan: Evidence for impact origin

The Calvin 28 cryptoexplosive disturbance is an isolated, nearly circular subsurface structure of Late Ordovician age in southwestern Michigan. The structure is defined by 107 wells, is about 7.24 km in diameter and consists of a central dome, an annular depression and an encircling anticlinal rim. Seismic and geophysical well log data confirm that an intricate system of faults and structural derangement exists within the structure. Deformation decreases with depth and distance from the structure. U.S.G.S. topographic maps and aerial imagery show the structure is reflected as a subtle surface topographic rise controlling local drainage. Igneous or diapiric intrusion and solution collapse are rejected as possible origins for Calvin 28 on the basis of stratigraphic, structural and geophysical evidence. A volcanic origin is inconsistent with calculated energy requirements and an absence of igneous material. Although shock-metamorphic features are unidentified, microbreccias occur in deep wells that penetrate the structure. Morphology and structural parameters support an impact origin.

Milstein, Randall L.↗

NASA NTRS1988

Workshop on the Deep Continental Crust of South India

The various theories in the field on the Southern Indian granulite formation are discussed in this workshop proceeding. That some widely accepted ideas about the origin of granulites, particularly relating to the role of metamorphic fluids, will have to be modified, is one outcome of the workshop. Extended abstracts of the papers presented, summaries of the attendant discussions, up-to-date accounts of the geology of the South Indian Precambrian Shield, and detailed field trip guides to all areas visited are presented. It should serve as a convenient source of information and reference to those interested in the classic Precambrian high grade terrains on Earth.

Ashwal, Lewis D.↗

NASA NTRS1988

Water activities in the Kerala Khondalite Belt

The author and colleagues presented their determinations of water activities in various granulite-facies rocks of the Kerala Khondalite Belt. Using mineral equilibria, thermodynamic data, and assumed geopressure-geotemperature conditions of 5.5 kbar and 750 C, they calculated uniformly low a(H2O) values of about 0.27 over a large geographic region. They suggested that these conditions were produced by the presence of abundant CO2-rich fluids, derived either from deeper levels or from metamorphic reactions involving graphite.

Chacko, Thomas↗