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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
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

Origin of the atmosphere and hydrosphere of the terrestrial planets

An early thermal evolution of a planet growing by planetesimal impacts was studied. An evolution of an impact induced atmosphere was considered. It is known that the surface of a growing planet is heated due to the blanketing effect of the atmosphere and exceeds the melting temperature, which means that the surface of a growing planet was entirely covered by a magma ocean. The amount of water in a proto-atmosphere is influenced by the formation of a magma ocean. It is suggested the solubility of water in silicate melt controls the water content in a proto-atmsphere. It is noted that irrespective of difference in initial water content of planetesimals the final water content in the atmosphere becomes almost constant and is about 10 to the 21st power kg which is almost identical with the present amount of the ocean. It is also shown that the water in a proto-atmosphere can be liquid for the Earth and becomes to be an ocean but this does not happen on Venus.

Matsui, T.↗
NASA NTRS1985

Basaltic volcanism on the eucrite parent body - Petrology and chemistry of the polymict eucrite ALHA80102

The polymict eucrite meteorite ALHA80102 is an unequilibrated breccia of basaltic and gabbroic clasts in a fragmental matrix. Clasts include basalts of many textural types, cumulate gabbro, black 'glass', and ferroan troctolite (plagioclase, silica, Fe-rich olivine, ilmenite, mesostasis). Ferroan troctolite has not been previously reported from eucrites or howardites; it is interpreted as the end-product of fractional crystallization of eucritic magmas. Bulk and trace element compositions (by electron microprobe and INAA) of clasts and matrix from ALHA80102 are similar to those of other eucrites; the meteorite contains clasts similar to Juvinas and to Stannern. A clast of cumulate eucrite gabbro is enriched in the light rare earths (La/Lu = 2XCI). This clast is interpreted as an unrepresentative sample of metamorphically equilibrated gabbro; LREE-enriched magmas need not be invoked. ALHA80102 is similar to other polymict eucrites from the Allan Hills and may be paired with ALHA76005, ALHA77302, and ALHA78040.

Treiman, A. H.↗
NASA NTRS1985

Amphibole and hercynite spinel in Shergotty and Zagami Magmatic water, depth of crystallization, and metasomatism

Amphibole and spinel occur in the Shergotty and Zagami meteorites only in magmatic inclusions in pigeonite. The trapped magma is essentially identical to the parental magmas for Shergotty and Zagami. The amphibole is a kaersutite with minimal halogen content; by inference, it must have been hydrous. If so, the Shergotty and Zagami melts contained at least 0.2 wt pct H2O and were probably H2O-undersaturated. Pressures in excess of 1 kilobar seem necessary for the formation of amphibole. Spinel replaces magnetite in the inclusions, and olivine replaces magnetite elsewhere in the meteorites. To stabilize spinel, the melt in the inclusions must have become enriched in Al during fractionation, possibly because the small volume of the inclusions made nucleation of plagioclase unlikely. Pervasive replacement of magnetite through reduction reactions suggests that Shergotty and Zagami interacted with hydrogen-rich fluids during their cooling.

Treiman, A. H.↗
NASA NTRS1986

Volcanological constraints of Archaean tectonics

Volcanological and trace element geochemical data can be integrated to place some constraints upon the size, character and evolutionary history of Archean volcanic plumbing, and hence indirectly, Archean tectonics. The earliest volcanism in any greenhouse belt is almost universally tholeitic basalt. Archean mafic magma chambers were usually the site of low pressure fractionation of olivine, plagioclase and later Cpx + or - an oxide phase during evolution of tholeitic liquids. Several models suggest basalt becoming more contaminated by sial with time. Data in the Uchi Subprovince shows early felsic volcanics to have fractionated REE patterns followed by flat REE pattern rhyolites. This is interpreted as initial felsic liquids produced by melting of a garnetiferous mafic source followed by large scale melting of LIL-rich sial. Rare andesites in the Uchi Subprovince are produced by basalt fractionation, direct mantle melts and mixing of basaltic and tonalitic liquids. Composite dikes in the Abitibi Subprovince have a basaltic edge with a chill margin, a rhyolitic interior with no basalt-rhyolite chill margin and partially melted sialic inclusions. Ignimbrites in the Uchi and Abitibi Subprovinces have mafic pumice toward the top. Integration of these data suggest initial mantle-derived basaltic liquids pond in a sialic crust, fractionate and melt sial. The inirial melts low in heavy REE are melts of mafic material, subsequently melting of adjacent sial produces a chamber with a felsic upper part underlain by mafic magma.

Thurston, P. C.↗
NASA NTRS1985

Very high potassium (VHK) basalt - Complications in mare basalt petrogenesis

The first comprehensive report on the petrology and geochemistry of Apollo 14 VHK (Very High Potassium) basalts and their implications for lunar evolution is presented. The reported data are most consistent with the hypothesis that VHK basalts formed through the partial assimilation of granite by a normal low-Ti, high-Al mare basalt magma. Assimilation was preceded by the diffusion-controlled exchange of alkalis and Ba between basalt magma and the low-temperature melt fraction of the granite. Hypotheses involving volatile/nonvolatile fractionations or long-term enrichment of the source regions in K are inconsistent with the suprachondritic Ba/La ratios and low initial Sr-87/Sr-86 ratios of VHK basalt. An important implication of this conclusion is that granite should be a significant component of the lunar crust at the Apollo 14 site.

Shervais, J. W.↗
NASA NTRS1986

Pristine lunar glasses - Criteria, data, and implications

The examination of glasses from all of the Apollo landing sites has led to the identification of 25 groups of pristine glass. The nickel found in these glasses is shown to be indigenous, not meteoritic contamination, and to be correlated with Mg. Chemical data indicate that these glasses are more suitable for primary magma than most crystalline mare basalts. In addition, these pristine glasses support the view that assimilative processes involving two cumulative systems in the differentiated mantle operated during mare petrogenesis. Two linear arrays found among the chemistries of the glasses attest to the existence of these interactions. Data suggest that these cumulate components in the mantle continue for 1000 km (laterally) and therefore are likely to be products of a magma ocean that existed early in lunar history.

Delano, J. W.↗
NASA NTRS1986

Abundances of cobalt, nickel, and volatiles in the silicate portion of the moon

The trace abundances of Ni have been determined in 20 high-Mg magmas produced by partial melting of the lunar mantle. Associated with the eruption of the high-Mg magmas was the release of indigenous volatile elements. Data suggest that these volatiles were derived from primordial blocks of debris from the outer solar system that became entrained in the circumterrestrial accretion disk. The survival of these reservoirs within the lunar interior places constraints on the moon's thermal history.

Delano, John W.↗
NASA NTRS1987

Petrology of gabbroic rocks from the Mid-Cayman rise spreading center

Mineral analyses of oxide and silicate phases from a suite (collected with the DSRV Alvin in January 1976 and July 1977) of 48 gabbroic rocks collected from the vicinity of the Mid-Cayman Rise spreading center are reported. Mineral compositions of these anorthosites, leuco-troctolite, leuco-olivine gabbros, olivine gabbros, leuco-gabbros, and gabbros indicate the cumulate rocks have been produced by the crystal fractionation of basaltic liquids. Certain features of these rocks are inconsistent with the occurrence of this fractionation at low pressures (1 atm to 2 kbar). Although the experimental data are not available to conclusively demonstrate that the effects seen were produced at moderate pressures (5-10 kbar), the effects are similar to those predicted thermodynamically and to those observed in limited experimental studies within this pressure range. It is therefore suggested that the most likely scenario for the production of these gabbroic rocks is the moderate-pressure crystallization of basaltic magmas within deep-seated magma chambers underneath this slow-spreading center.

Elthon, Don↗
NASA NTRS1987

A vaporization model for iron/silicate fractionation in the Mercury protoplanet

A study has been carried out on the vaporization of a totally molten silicate magma of chondritic composition heated into the range 2500-3500 K. The motivation for this was to determine the changes in the composition of the mantle that would occur in the Mercury protoplanet should that body have been subjected to the high-temperature phase in the evolution of the primitive solar nebula, but the results are of more general interest. An empirical model based on ideal mixing of complex components was used to describe the nonideal magma. It is found that vaporization of about 70-80 percent of the original amount of silicate from a chondritic planet is required to produce an iron-rich body with a mean uncompressed density equal to that deduced for Mercury. At this point the silicate is depleted in the alkalis, FeO, and SiO2, and enriched in CaO, MgO, Al2O3, and TiO2 relative to chondritic material.

Fegley, Bruce, Jr.↗
NASA NTRS1987

Volcanology

Studies conducted in the period between 1983 and 1986 on the physical aspects of volcanology, including investigations of physical processes involved in magma transport, are reviewed. Eruptions that occurred during this quadrennium are discussed together with the studies of the methods of volcanic hazard assessment and prediction around active volcanoes. Consideration is given to advances made in the understanding of magma systems, including volatile components, as well as to studies on the atmospheric impact of volcanism. Tectonic controls of volcanism and the volcanic aspects of mineralization are also discussed.

Self, Stephen↗
NASA NTRS1987

Ice-covered water volcanism on Ganymede

Eruption of liquid H2O magmas along extensional fractures and graben-bounding normal faults may have played a critical role in the development of Ganymede's grooved terrain. The resurfacing potential of a water magma is dependent on a variety of factors, including the areal extent of the source region, the rate of discharge, the thickness of the flow, and the time that it takes the flow to completely freeze to its base. In this paper the thermal evolution of such a flow is considered in detail. The minimum unfrozen lifetime of a 5-m flow is approximately 12.5 days while a 10-m flow would survive for at least 50 days. Heating resulting from frictional head loss could reasonably extend these lifetimes by 50 percent or more. With a discharge rate of the order of 1-10 cu km/d, an individual volcanic water flow could flood an area of about 10,000 sq km before freezing. As the flow solidifies, its volume will increase, thus lifting and arching its protective ice cover. Extensional fractures may then develop in the ice subparallel to the graben walls. These fractures could result in grooves directly, given a sufficiently thick (1 km) flow, or they may simply act to concentrate various tectonic forces that could initiate groove-producing faults.

Allison, M. Lee↗
NASA NTRS1987

Mutual interactions of redox couples via electron exchange in silicate melts - Models for geochemical melt systems

The mutual interactions via electron exchange of redox couples in glass-forming melts were investigated both theoretically and experimentally. A thermodynamic approach for considering the mutual interactions leads to conclusion that the degree of mutual interaction in the melt should be proportional in part to the difference in relative reduction potentials of the interacting redox couples. Experimental studies verify this conclusion for numerous redox couples in several composition/temperature/oxygen fugacity regimes. Geochemical systems simultaneously possess many potentially multivalent elements; the stabilized redox states in the resulting magmas can be explained in part by mutual interactions and by redox buffering through the central Fe(III)- Fe(II) couples in the melts. The significance of these results for basaltic magmas of the earth, moon, and meteorites is addressed.

Schreiber, Henry D.↗
NASA NTRS1987

Origin of SNC kaersutitic amphibole: Experimental data

The SNC meteorites, a group of cumulus textured, fine grained diabases, pyroxenites, and dunites, appear to have crystallized at relatively shallow depths on the same SNC parent body. Hydrous minerals generally are not present among the cumulus and intercumulus minerals in these meteorites except for some iddingsite alteration of olivine. The presence of hydrous magmatic amphibole in the SNC melt inclusions indicates that crystallization of the melt inclusions had to take place at significant pressure, probably greater than 1 kb based on previous amphibole stability data. If experimental data for kaersutite amphibole were to be obtained, it should be possible to estimate this pressure more precisely then previously, and to estimate the volatile (H2O) content of the parent magma. At this point, the factors controlling the chemistry and stability of high TiO2, kaersutitic amphiboles are not known. In an attempt to determine the factors which control the stability and chemistry of TiO2-rich amphibole, data was refined and extrapolated from four experimental studies of amphibole-melt equilibria recently completed. At the same time, hypothermal experiments were performed on a composition considered to be an early melt in the Shergotty magma liquid line of descent. The latter experiments were an attempt to reproduce crystallization of the amphibole-bearing melt inclusions.

Rutherford, M. J.↗
NASA NTRS1988

Recycling of lower continental crust through foundering of cumulates from contaminated mafic intrusions

A mechanism is presented for recycling of lower continental material back into the mantle. Picritic magmas, possible parental to volumious continental volcanics such as the Karoo and Deccan, became trapped at the Moho, where they interacted with and become contaminated by lower crustal materials. Upon crystallization, the magmas differentiated into lower ultramafic cumulate zones and upper gabbroic-anorthositic zones. The ultramafic cumulates are denser than underlying mantle and sink, carrying lower crustal components as trapped liquid, as xenoliths or rafts, and as constituents of cumulate minerals. This model provides a potentially significant crust-mantle differentiation mechanism, and may also represent a contributing factor in crustal recycling, possibly important in producing some OIB reservoirs.

Arndt, Nicholas T.↗
NASA NTRS1988

Rifts of deeply eroded Hawaiian basaltic shields: A structural analog for large Martian volcanoes

Recently derived morphologic evidence suggests that intrusive events have not only influenced the growth of young shield volcanoes on Mars but also the distribution of volatiles surrounding these volcanoes: in addition to rift zones and flank eruptions on Arsia Mons and Pavonis Mons, melt water channels were identified to the northwest of Hecates Tholus, to the south of Hadriaca Patera, and to the SE of Olympus Mons. Melt water release could be the surface expression of tectonic deformation of the region or, potentially, intrusive events associated with dike emplacement from each of these volcanoes. In this study the structural properties of Hawaiian shield volcanoes were studied where subaerial erosion has removed a sufficient amount of the surface to enable a direct investigation of the internal structure of the volcanoes. The field investigation of dike morphology and magma flow characteristics for several volcanoes in Hawaii is reported. A comprehensive investigation was made of the Koolau dike complex that passes through the summit caldera. A study of two other dissected Hawaiian volcanoes, namely Waianae and East Molokai, was commenced. The goal is not only to understand the emplacement process and magma flow within these terrestrial dikes, but also to explore the possible role that intrusive events may have played in volcano growth and the distribution of melt water release on Mars.

Knight, Michael D.↗
NASA NTRS1988

Mineral compositions of plutonic rocks from the Lewis Hills massif, Bay of Islands ophiolite

Mineral compositions of residual and cumulate rocks from the Lewis Hills massif of the Bay of Islands ophiolite complex are reported and interpreted in the context of magnetic processes involved in the geochemical evolution of spatially associated diabase dikes. The mineral compositions reflect greater degrees of partial melting than most abyssal peridotites do and appear to represent the most depleted end of abyssal peridotite compositions. Subsolidus equilibration between Cr-Al spinal and olivine generally has occurred at temperatures of 700 to 900 C. The spinel variations agree with the overall fractionation of basaltic magmas producing spinels with progressively lower Cr numbers. The compositions of clinopyroxenes suggest that the fractionation of two different magma series produced the various cumulate rocks.

Smith, Susan E.↗
NASA NTRS1987

Phase relations of a high-Mg basalt from the Aleutian Island arc - Implications for primary island arc basalts and high-Al basalts

An experimental investigation of a primitive high-Mg basalt, MK-15, collected from lava flows of the Unalaska Island in the Aleutian Island arc has been conducted in order to study primary and parental island arc basalts and the development of island arc magmas. The results suggest a model in which high-Al basalts are generated by moderate amounts of crystal fractionation from more primitive (high Mg/Mg + Fe, lower Al2O3) basaltic magmas near the arc crust-mantle boundary. Somewhere between 20-30 depth, significant amounts of clinopyroxene and olivine, with lesser amounts of spinel and possibly amphibole, fractionate, forming layer of olivine-clinopyroxenite at the base of the arc crust.

Gust, D. A.↗