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At least 163 records · Page 9
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NASA NTRS1974

An atlas of volcanic ash

Volcanic ash samples collected from a variety of recent eruptions were studied, using petrography, chemical analyses, and scanning electron microscopy to characterize each ash type and to relate ash morphology to magma composition and eruption type. The ashes are best placed into two broad genetic categories: magnetic and hydrovolcanic (phreatomagmatic). Ashes from magmatic eruptions are formed when expanding gases in the magma form a froth that loses its coherence as it approaches the ground surface. During hydrovolcanic eruptions, the magma is chilled on contact with ground or surface waters, resulting in violent steam eruptions. Within these two genetic categories, ashes from different magma types can be characterized. The pigeon hole classification used here is for convenience; there are eruptions which are driven by both phreatic and magmatic gases.

Heiken, G.↗
NASA NTRS1974

Genesis of lunar regolith

From comparing the mineral and chemical composition of regoliths with the compositions of primary magmatic lunar rocks, it is shown that mare regolith is enriched in aluminum and depleted in iron, titanium, and chromium. This effect cannot be accounted for only by the admixture of highland anorthositic material. The entire array of geochemical data points to the considerable role of depth-wise differentiation of the magmatic material in forming the composition of the primary lunar rocks and, naturally, the composition of the regolith. During regolith formation, processes of the effervescing of magma as it outflowed at the lunar surface were of primary importance. Impacts by micrometeorites represent another important factor.

Vinogradov, A. P.↗
NASA NTRS1977

Riddles about the origin and thermal history of the moon

Magmatic differentiation of the moon's interior, confirmed through calculations of thermal history, was studied. It appears that differentiation was a result of the moon's initial temperature whose origin remains unknown. In solving this problem, convective models of the moon were considered as well as a two layered differentiated model of the moon, operative over the past 3.5 billion years. The high content of long lived radioactive elements present was investigated in explaining the moon's current thermal properties. The controversy concerning the true nature of magmatic differentiation continues to be unsolved.

Levin, B. Y.↗
NASA NTRS1978

Rare-earth abundances in chondritic meteorites

Fifteen chondrites, including eight carbonaceous chondrites, were analyzed for rare earth element abundances by isotope dilution. Examination of REE for a large number of individual chondrites shows that only a small proportion of the analyses have flat unfractionated REE patterns within experimental error. While some of the remaining analyses are consistent with magmatic fractionation, many patterns, in particular those with positive Ce anomalies, can not be explained by known magmatic processes. Elemental abundance anomalies are found in all major chondrite classes. The persistence of anomalies in chondritic materials relatively removed from direct condensational processes implies that anomalous components are resistant to equilibrium or were introduced at a late stage of chondrite formation. Large-scale segregation of gas and condensate is implied, and bulk variations in REE abundances between planetary bodies is possible.

Evensen, N. M.↗
NASA NTRS1979

Origin of Valles Marineris

The purpose of the paper is to propose an origin of Valles Marineris and contiguous features such as Noctis Labyrinthus, chaotic terrain, and the large channels of Mars. These are found to: (1) be continuous to each other, (2) have gradual transition between the morphologies, (3) be related by tectonic fracturing, and associated with features of possible magmatic origin. A model is presented in which the geological process is a magmatic process, and after initial uplift and fracturing, Valles Marineris, Noctis Labyrinthus and some chaotic terrain were produced by subsidence caused by lava withdrawal.

Schonfeld, E.↗
NASA NTRS1982

The 18 May 1980 eruption of Mount St. Helens: The nature of the eruption, with an atmospheric perspective

Mount St. Helens erupted somewhat less than 0.5 cu km of magma (dense rock equivalent) on May 18, 1980. The May 18 event was usually violent. As much 35% of the volume of the airfall material fell outside of the 2.5 mm isopach, which encloses about 88,000 sq km. This extraordinary dispersive power was transmitted by an eruption column which reached heights of more than 20 km. There was a lateral blast (or surge) of unusually large dimensions associated with the onset of the eruption. The magma is dacitic in composition and had a low ( 500 ppm) sulfur content. Distal ashes contain much nonmagmatic (lithic) material, but smaller ( 50 microns m) particles are mostly finely divided magmatic dacite. The grain size distributions of the ash are multimodal, frequently with peaks at 90, 25, and 10 microns. The finer populations fell out faster than their terminal velocities as simple particles would suggest. It is inferred that large proportions of the fine ash fell out as composite particles. This condition greatly reduces the atmospheric burden of silicate particles. Some of the unusual aspects (violence, intense surges, multimodal grain size distributions, lithic content of the ashes) of the eruption may be due to its phreatomagmatic character. The hydrothermal system above the magma may have infiltrated the magma body at the onset of the eruption. An "overprint" of the geochemistry of this hydrothermal system on the geochemistry of the magmatic gas system is likely. One important feature is that reduced gas species may be much more abundant than in many eruptions. Another is that fine ash may form aggregates more readily.

Rose, W. I., Jr.↗
NASA NTRS1982

A comparison of constituents of Mount St. Helens eruption clouds with those of some other volcanoes

Gases from Mount St. Helens were collected from the eruption clouds using an airplane during the period April 2 to April 8, 1980, and were analyzed for CO2, H2, CH4, and COS. The results were of similar magnitude to those obtained from magmatic eruptions in Central America. Thus, although the eruptions were evidently largely phreatic, magmatic gases may have played a larger role in these early small eruptions than has generally been believed. Electron micrographs of ash particles collected from the Mount St. Helens eruption clouds showed that the particles were for the most part much larger than those from the Central American eruptions.

Cadle, R. D.↗
NASA NTRS1983

Experimental investigations of trace element fractionation in iron meteorites. II - The influence of sulfur

The partitioning of Ir, Ge, Ga, W, Cr, Au, P, and Ni between solid metal and metallic liquid is investigated as a function of temperature and the S-concentration of the metallic liquid. The partition coefficients for siderophile elements, such as Ir, W, Ga, and Ge, are found to increase by factors of 10-100 as the S-concentration of the metallic liquid increases from 0-30 wt percent. Partition coefficients for other siderophile elements, such as Ni, Au, and P, increase by factors of only 2-3. In contrast, a decrease is seen in the partition coefficients for the more chalcophile element Cr. These experimentally determined coefficients are used in conjunction with a fractional crystallization model to reproduce the geochemical behavior of Ni, P, Au, and Ir during the magmatic evolution of groups IIAB, IIIAB, IVA and IVB iron meteorites. The mean S-concentration for each group increases in the order IVB, IVA, IIIAB, IIAB, which is in accordance with cosmochemical prediction. However, the geochemical behavior of Ge, Ga, W, and Cr cannot be reproduced in an internally consistent way. It is concluded that the magmatic histories of these iron meteorite groups are more complex than has been generally assumed.

Jones, J. H.↗
NASA NTRS1985

Hydrovolcanism

Hydrovolcanism is a common phenomena produced by the interaction of magma or magmatic heat with an external source of water, such as a surface body, an aquifer, or a glacier. The effects include hydrofracture of existing rock units in the subsurface and the formation of hyaloclastites in a subaqueous environment. Hydroexplosions originate within a few kilometers of the surface. They may be relatively small, phreatic events or devastating complex blasts. Large-scale experiments determined that the optimal mixing ratio of water to basaltic melt (thermite plus silicates) for efficient conversion of thermal energy into mechanical energy is in the range of 0.1 to 0.3. Based on experimental results, eruptions can be classified as dominantly magmatic if the ratio of external water to magma is less than 0.2. Eruptions with water/melt ratios in the range of 0.2 to 1.0 are highly explosive and carry tephra in a hot vapor that contains dominantly superheated (dry) steam.

Sheridan, M. F.↗
NASA NTRS1985

Early evolution of the crust-mantle system

Nd isotopic data indicate that most Archean igneous rocks including compositions ranging from komatiite to tonalite are derived from undepleted or depleted upper mantle sources. If sampling is representative, only a few require enriched sources. A major unresolved question is the fate of the material removed from the upper mantle leaving early depleted sources as residue. One possibility is that widespread depletion of the early mantle resulted from a period of early degassing and magmatism. Rare gas isotopic data, in particular 129Xe/130Xe ratios, seem to demand that the upper mantle was extensively degassed at or before 4.4 b.y. and this led to rapid growth of the atmosphere and oceans. The lower mantle, however, was not significantly degassed during this event. It is likely that such widespread degassing and magmatism of the upper mantle transferred significant quantities of incompatible elements into the uppermost mantle or crust. Once formed, such an enriched fraction should resist recycling into the mantle and collect at or near the Earth's surface. One possibility is that it collects chiefly in a zone of partial melting, analogous to the present low-velocity zone at the base of the lithosphere.

Condie, K. C.↗
NASA NTRS1986

Open system magnetic evolution of the taos plateau volcanic field, Northern New Mexico. I - The petrology and geochemistry of the servilleta basalt

MULTIFIT, an embodiment of the conceptual structure needed in modeling multisource and multiprocess magmatic systems, is described. This program, which uses familiar materials balance methodology and the equilibrium form of the Rayleigh equations, links evolutionary arrays, which is turn collectively relate the starting and final compositions of a given magmatic system. Moreover, MULTIFIT incorporates variations within major element data arrays; the linkage between them can be tested using an extension of the least squares algorithm, which selects the best branch point according to the minimum-sum-of-squared-residuals criterion. Advantages and disadvantages of the materials balance approach used in this program are discussed, an example is provided, and equations utilized by MULTIFIT are summarized. While MULTIFIT may not be the best approach for poorly constrained models involving partial melting for complex mixing, it may ultimately prove useful for ascertaining trace element partition coefficients in magnetic systems.

Dungan, M. A.↗
NASA NTRS1987

Ru, Re, Os, Pt and Au in iron meteorites

Neutron activation analysis is used to ascertain the proportions of Ru, Re, Os, and Pt refractory siderophiles, and moderately volatile Au, in 41 iron meteorites. The Ni-element trends defined for groups IID and IIIF support a magmatic origin; in addition, the results obtained for groups IAB and IIICD contrast with trends observed in magmatic groups, where the Ru and Pt slopes are substantially less steep. In group IIIAB, at Ni concentrations above 90 mg/g, the steep decrease of Re, Os, and Ir with Ni levels off, and the interelement ratios exhibit considerable scatter. These observations may be explained in terms of the contamination of the residual molten core with small amounts of late primitive melts draining from the mantle.

Pernicka, Ernst↗
NASA NTRS1988

Metamorphic P-T paths and Precambrian crustal growth in East Antarctica

The metamorphic constraints on crustal thicknesses in Archean and post-Archean terranes are summarized along with possible implications for tectonic processes. It is important to recognize that P-T estimates represent perturbed conditions and should not be used to estimate steady state geothermal gradients or crustal thicknesses. The example is cited of the Dora Maira complex in the French Alps, where crustal rocks record conditions of 35 kbar and 800 C, implying their subduction to depths of 100 km or more, followed by subsequent uplift to the surface. Therefore such P-T estimates tell more about processes than crustal thicknesses. More importantly, according to the author, are determinations of P-T paths, particularly coupled with age measurements, because these may provide constraints on how and when perturbed conditions relax back to steady state conditions. P-T paths are illustrated that should be expected from specific tectonic processes, including Tibetan style collision, with and without subsequent extension, rifting of thin or thickened crust, and magmatic accretion. Growth of new crust, associated with magmatic accretion, for example, could possibly be monitored with these P-T paths.

Harley, S. L.↗
NASA NTRS1988

The mechanism of intrusion of the Inyo Dike, Long Valley Caldera, California

The dike geometry of the 11-km-long Inyo Dike at the margins of the Long Valley calderas in eastern California is explained here in terms of the interaction between tectonic stresses and local variations in host rock rheology. Relationships between tectonic and magmatic stresses are evaluated, and the magnitude of magmatic pressure at the depth necessary to get the dike to the surface is estimated. The application of the model to other volcanoes is also discussed.

Reches, Zeev↗
NASA NTRS1987

Experimental cosmochemistry in the Space Station

The purpose of two workshops was to identify and discuss experiments in cosmochemistry that cannot be conducted under the conditions available in terrestrial laboratories, but may be carried out successfully in the proposed Space Station. The scientific discussions focused on two general areas of research: chemical and physical processes in the earliest history of the general areas of research, and general principles of magmatic process applicable both to planetary formation and evolution, as well as present-day magmatic activity in and on terrestrial planets.

Duba, AL↗
NASA NTRS1989

C-O-H ratios of silicate melt inclusions in basalts from the Galapagos Spreading Center near 95 deg W - A laser decrepitation mass spectrometry study

Ratios of C, O, and H dissolved in silicate glass inclusions and pillow rind glasses in samples from the Galapagos Spreading Center near 95 deg W were analyzed (using laser volatilization and mass spectrometry), and the data were assessed in terms of mantle source compositions, oxygen fugacity, kinetic fractionation, and magmatic degassing. It was found that glass inclusions in the Galapagos failing rift lavas are higher and more variable in CO2/H2O (about 0.641) relative to inclusions in propagating rift lavas (about 0.245). This difference is considered to reflect different degrees of degassing during contrasting magmatic histories of the two regions.

Yonover, Robert N.↗
NASA NTRS1990

Derivation of primary magmas and melting of crustal materials on Venus - Some preliminary petrogenetic considerations

As an aid to understanding crustal formation and evolution processes on Venus, a general paradigm is developed for the derivation of primary magmas, and the range of possibilities of conditions for remelting of crustal materials and the evolution of the products of remelting. The present knowledge of the bulk and surface composition is used as a basis. A wide range of magma types is possible for the range of conditions of derivation of primary magmas and crustal remelting and no magma type can be arbitrarily excluded from consideration on Venus. The composition of Venus and the nature of source materials for melting, the melting of mantle material peridotites, and the melting of basalts including tholeiites and modified basalts are discussed. Magmatic differentiation is considered, and a comparison to terrestrial magmatic environments is conducted. It is concluded the magnetic and volcanic activity on Venus could be very similar to that on the earth, although eruption styles are expected to vary due to environmental conditions.

Hess, Paul C.↗
NASA NTRS1990

Rare earth elements in Angra dos Reis and Lewis Cliff 86010, two meteorites with similar but distinct magma evolutions

Data are presented on ion microprobe measurements of REE and selected trace element abundances in individual grains of merrillite, fassaite, olivine, kirschsteinite, and plagioclase of Lewis Cliff 86010 (LEW 86010) meteorite and in merrillite and fassaite grains of Angra dos Reis (ADOR). Results show a close relationship between the two meteorites and support a magmatic origin for LEW 86010. However, the measurements indicate that, despite numerous common characteristics, the two meteorites must have been produced in separate magmatic events involving similar but distinct processes and parent melts.

Crozaz, Ghislaine↗