Showing papers on "Basalt published in 1970"

Water Content of Basalt Erupted on the ocean floor

Abstract: Deep sea pillow basalts dredged from the ocean floor show that vesicularity changes with composition as well as with depth. Alkalic basalts are more vesicular than tholeiitic basalts erupted at the same depth. The vesicularity data, when related to experimentally determined solubility of water in basalt, indicate that K-poor oceanic tholeiites originally contained about 0.25 percent water, Hawaiian tholeiites of intermediate K-content, about 0.5 percent water, and alkali-rich basalts, about 0.9 percent water. Analyses of fresh basalt pillows show a systematic increase of H2O+ as the rocks become more alkalic. K-poor oceanic tholeiites contain 0.06–0.42 percent H2O+, Hawaiian tholeiites, 0.31–0.60 percent H2O+, and alkali rich basalts 0.49–0.98 percent H2O+. The contents of K2O, P2O5, F, and Cl increase directly with an increase in H2O+ content such that at 1.0 weight percent H2O+, K2O is 1.58 percent, P2O5 is 0.55 percent, F is 0.07 percent, and Cl is 0.1 percent. The measured weight percent of deuterium on the rim of one Hawaiian pillow is −6.0 (relative to SMOW); this value, which is similar to other indications of magmatic water, suggests that no appreciable sea water was absorbed by the pillow during or subsequent to eruption on the ocean floor.

An igneous plagioclase thermometer

Abstract: An empirical approach has been taken to develop a geothermometer based on plagioclase-magmatic liquid equilibrium. Compositions of coexisting plagioclase and liquid (glass) obtained by electron microprobe analysis of quenched samples from equilibrium melting experiments of natural granitic rocks at water pressures of 0.5 and 1.0 kilobars have been used along with data from the equilibrium experiments of Bowen (1913, 1915), Prince (1943) and Yoder et al. (1957) to calibrate this geothermometer. Applications of this geothermometer to natural occurrences demonstrate that it can provide useful information on temperature of equilibration of coexisting plagioclase and liquid in rocks ranging in composition from basalt to rhyolite. The plagioclase geothermometer is in good general agreement with other geothermometers wherever these are applicable. Where temperatures are known from other sources it can be used to predict the equilibrium compositions of plagioclase in magmas as well as to provide a rough estimate of water pressure.

Petrologic history of the moon inferred from petrography, mineralogy, and petrogenesis of Apollo 11 rocks

Abstract: The mineralogy and petrology of the Apollo 11 rocks are consistent with impact melting of ilmenite and pyroxene crystals plus liquid derived from fractional crystallization of basaltic magama. The complementary plagioclase accumulate should exist in the highlands. The ferrobasaltic magma is derived from a differentiated hot moon of modified chondritic composition. The one-sided distribution of large irregular seas is explained by tidal attraction of the last fraction of liquid to the near side of the moon and release of liquid by meteorite impact. The vesicular ferrobasalt lavas contain ilmenite, clinopyroxene and plagioclase and have textures similar to some terrestrial basalts. Delayed appearance of plagioclase indicates an unusual source of the magma. The iron-enrichment of the coarser microgabbros is extreme at the end of crystallization resulting in a new iron metasilicate, pyroxferroite. The oxygen fugacity at 1050°C of these rocks indicated by the composition of opaque oxides and troilite-iron intergrowths is 10^(-14)-10^(-16) compared to 10^(- 11) for terrestrial basalts and about 10^(16) for ordinary chondrites. Absence of hydrous minerals indicates loss of volatiles at some stage in the moon's history. The breccias and soil have the same mineralogy modified by shock. Plagioclase vitrophyres are probably melted cumulates and may derive from the highlands. Lithification of the breccia results principally from welding of debris discharged from a hot gas cloud created by meteorite impact. Small glass spheres have surface features consistent with a fiery rain of boiling silicate liquid rounded by surface tension and later impacted by high-velocity micrometeorites. Melting experiments of synthetic material of mean Apollo 11 rock composition revealed low liquidus temperature, delayed appearance of plagioclase and narrow crystal-liquid interval. These support the concept of advanced fractional crystallization of a basaltic liquid under reducing conditions leading to high iron enrichment. Flotation of plagioclase and sin king of ilmenite and pyroxene should occur in the liquid basalt. We propose that meteorite impact blasted away a plagioclase-rich crust, melting a mixture of fractionated basalt liquid and ilmenite plus pyroxene crystals. The new liquid formed the Sea of Tranquillity and yielded the near-surface lava flows represented by the ferrobasalts and microgabbros. This new liquid could yield plagioclase only after the extra ilmenite and pyroxene had crystallized. We propose that the original basalt liquid was derived by fractional crystallization of a molten moon of modified chondritic composition yielding a metallic core surrounded by pressure-stable Mg-rich o li vine and pyroxene. The fractionated liquid became Fe-rich resulting in an inverse density stratification. Primitive ultra basic crust should occur in the highlands, along with dominant plagio clase- rich cumulates. The temperature-time relations of the model are discussed qualitatively. Key factors are early removal of radioactive material from the center by fractional crystallization and possible enhancement of radiative heat transfer in volatile-free silicates. The moon rocks should be more refractory and rigid because of the low volatile content.

The Mid-Atlantic Ridge at 45° N. XIV. Oxidation and magnetic properties of basalt; review and discussion

Abstract: The magnetic properties of submarine basalt procured in a traverse across the accreting margin of a lithospheric plate are reviewed. These properties are attributed to two processes. Firstly, the rapid quenching of lava in the axial zone of accretion produces dispersed titanomagnetite of very small grain-size, ranging from about 5 μm down to submicroscopic sizes. During quenching the pillow lava acquires a very intense (0.1 c.g.s. units per cm3) and stable (mean coercivity of remanence 240 Oe) remanent magnetization, with Q of the order 100. The oxidation state is initially low (FeO/Fe2O3 averages about 7) but, presumably because of rapid quenching, it is chemically unstable, and undergoes secondary oxidation, so that at a distance of 10 km in from the accreting margin the FeO/FenO3 ratio has fallen to about unity, the remanence has decreased by a factor 10, and the Curie and blocking temperatures have risen from the range 100–300 °C to the range 300–500 °C. It is suggested that these magnetic changes are...

Tertiary basalts of Baffin Bay: Possible primary magma from the Mantle

Abstract: New analyses show that Tertiary tholeiitic basalts from the Baffin Bay area have generally primitive characteristics. Comparison of the bulk chemical compositions of the basalts with material of known phase relations suggests that these basalts have properties intermediate between investigated natural and synthetic compositions. A discrete group of these basalts coincides in projection with the composition of the liquid produced by the partial melting of garnet periodotite at 30 kb. The compositions in this group are believed to be parental to the other rocks in the province principally through a mechanism of olivine fractionation. Eclogite fractionation is invoked to account for the chemical differences between the two suites of basalts in Baffin Bay.

Elasticity of Mafic Rocks from the Mid-Atlantic Ridge

Abstract: Summary Compressional wave velocities are reported to pressures of 10 kb fOf 57 cores of rock dredged from the Mid-Atlantic Ridge at 22° N and 4° S latitude. Shear wave velocities are reported to pressures of 10 kb for 9 rock cores. The rocks studied consist of basalt, altered basalt, dolerite and chlorite-rich greenstones. Compressional wave velocities for all of the rocks, with the possible exception of the dolerites, are similar to reported velocities of the oceanic crust over the Mid-Atlantic Ridge crest. None of the rocks appear to be abundant constituents of the oceanic layer (Layer 3). Relationships between bulk density and velocity are given at pressures of 0.4, 1, 2, 6 and 10 kb. The least squares solution at 10 kb, Yp= 2.53p - O'76, falls between Birch's solutions for mean atomic weights 21 and 22.

Thermal conductivity of particulate basalt as a function of density in simulated lunar and Martian environments

Abstract: Thermal conductivity of particulate basalt as function of density in simulated lunar and Martian environments, noting temperature and pressure effects

Petrogenesis of Apollo 11 basalts, internal constitution and origin of the moon

Abstract: Apollo 11 lunar basalt petrogenesis, examining internal constitution and origin by high pressure

Water in the Earth's Mantle: Melting Curves of Basalt-Water and Basalt-Water-Carbon Dioxide

Abstract: At depths below about 45 kilometers in the earth, the presence of high proportions of carbon dioxide does not significantly affect the temperature of the beginning of melting of basalt-water compositions. Water must be present only in trace amounts in the solid part of the silicate mantle if it contains appreciable proportions of basaltic composition.

The flow of basalt lava from air into water, its structural expression and stratigraphic significance

Abstract: When basalt lava flows from air into water it leaves a distinctive record of the waterlevel of the time in the form of lava sheets overlying and passing down into vitric breccia and/or pillow lava. Relative movements of waterlevel and a volcanic pile or terrain over a period of time may be readily deciphered from such records.

Idade do vulcanismo no Oceano Atlântico Sul

Abstract: About eighty K-Ar age determinations were carried out on samples from the Brazilian South Atlantic islands, and on samples of magmatic rocks associated to post-palaeozoic fractures of the Brazilian coast. 37 analyses were obtained from 27 rock samples of Trindade island. The results show that most of the intrusive rocks of the Trindade Complex (basic and ultrabasic dikes, and phonolitic plugs) were formed during a volcanic cycle between 2.9 and 2.3 m. y. Some older results, up to 3.6 m. y. were also found. The lava flows of the Desejado Sequence exibited ages close to 2.5 m. y. A maximum age of 0.17 m. y. was postulated for the Morro Vermelho Formation, but no significant results could be obtained for the younger formations. Two samples of rocks from Martin Vaz were analysed. One of them showed a 60 m. y. apparent age, regarded as anomalous. 28 determinations were carried out on 23 rocks from Fernando de Noronha Archipelago. The phonolitic rocks, intrusives into the Remedios Formation, exibited ages close to 9 m. y. Other intrusive bodies resulted older, up to 12 m. y. Apparently, the nepheline basanite of the Sao Jose island is synchronous with the Remedios volcanism, and the ankaratritic lava flows of the Quixaba Formation are younger, with results between 6.3 and 1.7 m. y. The islands of Abrolhos are the remnants above sea level of a volcanic cone on the continental shelf. Some of their basaltic rocks were dated, with results between 52 and 42 m. y. The analysed samples belong to the last phases of the volcano, which started at least in the Upper Cretaceous, based on fossil evidences . Post-palaeozoic anorogenic magmatic activities on the continental cratonic margin were also considered. Basaltic volcanism of the Parana and Parnaiba basins took place in Lower Cretaceous times, but started somewhat earlier. The alkalic rocks of Southern Brazil, the igneous suite of Cabo, Pernambuco, and several intrusives of northeastern Brazil indicate that magmatic events were going on along the Brazilian coast until at least Miocene times. The oldest ages found for the volcanic cones of the South Atlantic are always consistent with the hypothesis' of ocean £oor spreading. The data here assembled suggest that the opening of the South Atlantic rift started in the Jurassic.

Carbon and sulfur concentration and isotopic composition in Apollo 11 lunar samples

Abstract: Carbon and sulfur concentration and isotopic variations in Apollo 11 fines, breccias and fine- grained basalts

The sulfur isotope composition of basaltic rocks

Abstract: The sulfur isotope composition of tholeiitic basalts, olivine alkali basalts and alkalirich undersaturated basalts were investigated. A method of preparation was devised Tholeiitic and olivine alkali basalts show a predominance of sulfide-sulfur. Alkali-rich undersaturated basalts show sulfide- and sulfate-sulfur. The oxidation potential of the magma is reflected in the proportions of sulfide- and sulfate-sulfur. Differences in the conditions of oxidation are also the cause of the sulfur isotope fractionation observed. The mean in the isotope composition of the sulfur in the olivine alkali basalts (with the exception of two samples which show extreme deviation) is δ 34S= +1.3 per mil. The values for the olivine alkali basalts are concentrated around this mean in a remarkable way, showing only small deviation for the individual samples. When the tholeiitic basalts deviate from this mean, it is only with a relative enrichment in the 32S isotope. With a pronounced variation of the individual values, the mean for the sulfide-sulfur is δ 34S=−0.3 per mil. The few sulfate values of both types of basalt are without significance for the discussion of their origin. However, this does not apply to the alkali-rich undersaturated basalts. Due to the higher water content, this basaltic magma had a higher oxygen partial pressure which favoured the formation of SO2 and SO 4 2− besides H2S while pressure was released during the ascent of the magma. The sulfur isotope fractionation connected with this oxidation led to a total enrichment of 34S in the rock, (δ 34S for total sulfur: +3.1 per mil) with particular favouring the sulfate (δ 34S=+4.2 per mil). It is accepted that the sulfur of all three types of basalts derives directly from the mantle. The olivine alkali basalts show the least deviation from the mantle value, which, in the place of origin of the basalts from the region investigated, would probably have been δ 34S=+1.3(±0.5) per mil. From this it may be concluded that the olivine alkali basalts — the most frequent type of basalt in this region — had their origin in the partial melting of the mantle without further differentiation. From the sulfur isotope data we concluded that the primary isotope composition of the continental tholeiitic basalts probably corresponds to that of the olivine-alkali basalts, and to that of the mantle. However, due to degasing in the layers near to the surface, some samples lost 34S, which may be related to the formation of SO2 during the release of pressure. There is no positive indication of a differentiation in shallow depths (<15 km — in the sense of Green and Ringwood, 1967). The reason for the obvious isotopic fractionation of the alkali-rich undersaturated basalts may be seen in their higher primary water content. This is a pronounced indication of the origin of this type of magma. Bultitude and Green (1968) proved by experiment, that the formation of alkali-rich undersaturated basaltic magma is possible in the mantle in the presence of water. Only a small amount of water is available for the formation of magma in the mantle. With a water content higher than normal for basalts, only small amounts of magma can be formed, but at lower temperatures this would allow the melting of a larger fraction of mantle material. By reaction with the wall rock, these magmas could be enriched in those components of mantle minerals which have the lowest melting point. This may help to explain their geochemical characteristics.

Special Papers—Apollo 11 Symposium: Petrogenesis of Apollo 11 Basalts and Implications for Lunar Origin

Abstract: The origin of Apollo 11 basalts is discussed in terms of two hypotheses: (1) formation by a small degree of partial melting in the lunar interior, and (2) formation by prolonged near-surface crystallization differentiation in a lava lake. The second hypothesis is rejected on the following grounds: Most Apollo 11 magmas are far removed from the plagioclase-pyroxene-ilmenite cotectic; fractional crystallization cannot explain the large variations in concentrations of incompatible trace elements in conjunction with the small variations in major-element compositions, particularly, Mg/Fe ratios; experimentally determined partition coefficients show that the high abundances of Cr and V in Apollo 11 rocks cannot be reconciled with the previous separation of large quantities of ore minerals and pyroxenes. On the other hand, the major-element and trace-element contents of Apollo 11 rocks can be readily explained by partial melting of source material that buffers the major-element compositions and causes enrichments of incompatible elements according to the degree of partial melting (first hypothesis). Two alternative sources have been suggested for Apollo 11 basalts formed by partial melting: (1) unfractionated pyroxenite source region at depths of 200–600 km, and (2) fractionated source region with incompatible elements (e.g. Ba, U, and rare earths) strongly enriched over chondritic abundances and containing plagioclase (approximate eucritic composition). Mass-balance calculations and plagioclase-stability conditions show that the second hypothesis requires Apollo 11 basalts to be generated by partial melting in the outer 150 km of the moon. This is very difficult to achieve one billion years after the moon's formation, since the outer 200 km will have cooled well below the solidus by conduction. Furthermore, magmas generated by partial melting of a plagioclase-bearing source region should have plagioclase on the liquidus, which is contrary to observation. The second hypothesis accordingly appears improbable. The first hypothesis is capable of explaining the major-element chemistry and the trace-element abundances (Eu; see below) in terms of a simple, single-stage model that is consistent with the moon's density, moment of inertia, and inferred thermal history. A possible explanation of the europium anomaly is suggested on the basis of the first hypothesis. It will be necessary to determine the appropriate partition coefficients in order to test this explanation. If the lunar highlands are anorthositic, extensive differentiation of the outer 150 km of the moon is required. This may have been caused by heating arising from partial conservation of gravitational potential energy during the final stage of accretion. Formation of Apollo 11 basalts by partial melting 3.7 billion years ago was probably the result of radioactive heating (U, Th) in the deep interior of the moon. A two-stage magmatic history for the moon is thus required. Similarities between compositions of Apollo 11 and terrestrial basalts and between their respective source regions are suggestive of a genetic relationship between moon and earth. Nevertheless, important differences in trace-element abundances, major-element compositions, and oxidation states exist. These abundance patterns are unfavorable to the traditional fission, binary planet, and capture hypotheses of lunar origin. However, they may be explicable in terms of the precipitation hypothesis proposed by the author. This maintains that during the later stages of accretion of the earth, a massive primitive atmosphere developed that was hot enough to evaporate selectively a substantial proportion of the silicates that were accreting on the earth. Subsequently, the atmosphere was dissipated and the relatively nonvolatile silicate components were precipitated to form a swarm of planetesimals or moonlets, from, which the moon accreted.

Relations between Tectonics and Magmatology in the Northern Danakil Depression (Ethiopia)

Abstract: Tectonics, volcanism and petrology of the northern Afar (or Danakil) Depression have been investigated during two recent successive expeditions. The tectonics is characterized by numerous NNW trending normal faults. Two important active volcanic chains, namely Erta’Ale Range and Alayta Range, are disposed in en echelon structure. They both display differentiated lavas of subcrustal origin, ranging from abundant basalts to scarce soda-rhyolites with interm ediate products, such as dark trachytes as well as oversaturated ones. 87 Sr/ 86 Sr isotopic ratios are very low and approximately constant through the whole series. This fact, and other petrological considerations, suggest that this series has not been contaminated by any crustal sialic material. In addition to these two well-defined volcanic ranges, the Depression is covered by extensive basaltic fields and significant quantities of rhyolites. The variegated petrography of these silicic lavas and ignimbrites, as well as the 87 Sr/ 85 Sr ratios, on the contrary strongly suggest that these rocks were formed by interaction between the subcrustal m agm a and the sialic crust. It can be tentatively concluded that the sialic crust is missing beneath the northern p art of the Danakil Depression only below the two en echelon elongated volcanic ranges m entioned above.

The Mid-Atlantic Ridge near 45° N. XI. Seismic velocity, density, and layering of the crust

Abstract: Compressional wave velocities at pressures to 1000 kg/cm2 and densities are given for a representative suite of rocks selected from 42 dredge hauls on the Mid-Atlantic Ridge near 45° N. The spectrum of rocks studied includes vesicular and massive basalts, metabasalts, meta-gabbros, and serpentinites. Evidence is presented for block faulting of an originally continuously layered crust of vesicular basalt and massive basalt underlain by a metamorphosed basalt and gabbro sequence. A study of the velocities of these layers at in situ pressures shows that they correlate well with the seismic layering of the oceanic crust. The velocity of the massive basalt layer is in agreement with that reported for layer 2. The underlying layer, consisting of low-to-medium grade metamorphosed basalts and gabbros (greenstones and greenschists) exhibits higher velocities. None of these exceed 6 km/s but it is suspected that these rocks at greater burial depths will exhibit velocities comparable to those of layer 3. The occurre...

Correlation of magnetotelluric, seismic, and temperature data from southwest Iceland

Abstract: Regional enhancement of temperatures in the crust and upper mantle are revealed by magnetotelluric measurements performed during 1969 in Thingvellir graben in southwest Iceland. An electrical interface at a depth of 15 to 20 km apparently coincides with the seismic boundary between layers having P-wave velocities of 6.5 km/sec and 7.4-km/sec, respectively. Based on the interpretation that the 6.5-km/sec material is intrusive basalt and the 7.4-km/sec material is partially fused upper-mantle material consisting of peridotite and 10% basalt magma, the electrical data have been inverted to determine a temperature range of 820°C to 1120°C at a depth of approximately 15 km. These temperatures are significantly above Green and Ringwood's oceanic geotherm but below their solidus. To be consistent with the seismic interpretation, hydrous conditions may persist to depths of 15 km, causing he solidus to be depressed below the observed temperature, so that a 10% liquid fraction of basalt can form. The data suggest that a surface geothermal gradient of 60°C/km can be linearly extrapolated to depths of 15 km, indicating that the major source of heat is below this depth. Effective viscosities based on power-law creep mechanisms may be on the order of 1022 poise at depths between 15 km and 20 km.

Chemical composition and petrogenesis of basalts from Tranquillity Base

Abstract: Apollo 11 lunar basalt chemical composition and petrogenesis, using stable isotope dilution method

Geochemistry of the volcanics of central Mexico

Abstract: The Tertiary and Recent volcanics of Mexico occur in two provinces. The Cordillera Province is made up of about 1700 m of ignimbrite sheets overlain and intercalated in the upper part by olivine basalt and basaltic andesite. The Rio Lerma Province extends transversely across Mexico and in the Valley of Mexico the lavas consist mainly of andesite and dacite, 68 % of those analysed having 62 | 4.7 % SiO2. A total of 108 chemical analyses were made for the major elements, 90 of these including determinations of Cr, Ni, Cu, Zn, Rb, Sr, Ba, Th and Pb for two areas, the Valley of Mexico in the Rio Lerma Province and the Guadalajara region which lies at the intersection of the two provinces. Computer constructions of normative components in the basalt tetrahedron and other projections support an origin of partial melting of tholeiitic to pyrolitic material for the production of andesite. The Guadalajara lavas have consistently higher K/SiO2 and K/Rb ratios and lower Mg/SiO2 ratio than the Valley of Mexico rocks suggesting generation at greater depth.

Petrology of the lunar soil and geophysical implications

Abstract: The Apollo 11 soil sample consists of particles derived from two rock suites, basaltic and anorthositic. Crystalline, glassy, and brecciated forms of each rock type are present. The basaltic suite (95% of identifiable soil particles) must be representative of mare rock; the anorthosites appear to derive from the lunar highlands. An anorthositic crust ∼10 km thick is required beneath the highlands to support their relief above the mare surfaces isostatically. To form such a crust via crystal fractionation requires that magma derived from initial partial melting of a substantial fraction of the moon (one-third the volume, for a chondritic moon) be collected at the lunar surface in early times. Such a layer could not be kept hot and molten for very long; hence the young basalts collected by the Apollo 11 and 12 missions must have been produced by subsequent melting at depth, attributable to internal radioactive heating, and are not products of the early magma system that gave rise to the anorthositic crust. Basaltic lava, being less dense than solid basalt, is capable of rising into topographic basins (maria) that are already isostatically equilibrated with their higher surroundings. A lava overfilling of this type would constitute a mascon, giving rise to a positive gravity anomaly.