Showing papers on "Basalt published in 1968"

The viscosity of basaltic magma; an analysis of field measurements in Makaopuhi lava lake, Hawaii

Abstract: The methods of rotation viscometry have been applied in the field to a tholeiitic lava at 1130 degrees to 1135 degrees C containing about 25 percent crystals and 2 to 5 percent gas bubbles, using a stainless steel spindle inserted through the lava crust. The results agree well with the Bingham model. Yield values in the lava are 1200 and 700 dynes cm (super -2) , and the plastic constants are 6500 and 7500 poises. Laboratory studies in silicone fluid show gas bubbles enhance plastic behavior. In general, lavas must be treated as non-Newtonian fluids; the conditions that may cause different types of fluid behavior in tholeiitic lava are outlined.

Rare Earth abundances in some basic rocks

Abstract: Rare-earth (RE) abundances are reported for several basalts, diabases, and gabbros. Compared with the RE distribution in chondritic meteorites, continental basic rocks are characterized by an enrichment of the light lanthanides. Intrusive basic rocks have lower RE contents and less fractionated RE distributions than continental basalts. Chill zones of the Stillwater and Bushveld complexes have different distributions which are Eu enriched and light RE depleted. Oceanic island basalts have RE abundances similar to those of continental basalts. Abyssal subalkaline basalts dredged from the mid-Atlantic ridge and Eastern Pacific rise have RE distributions which are nearly chondritic. There is a depletion in La, Ce, Pr, and a broad maximum from Sm to Tb. Similar patterns are found in fresh basalts, slightly altered basalts, and in greenstones. Abyssal basalts that are more alkalic are not depleted in La, Ce, and Pr. The relationship of the RE data to hypotheses for the origin of ridge basalts is considered.

Volcanism and metamorphism in the Mid-Atlantic Ridge, 22°N latitude

Abstract: The petrology of the mid-Atlantic ridge between 22° and 23°N latitude may be typical of those portions of the ridge characterized by a linear topography parallel to the axis, a well-developed median valley, and an absence of volcanic cones. Submarine basalt lavas dredged at fifteen stations on the crest of the ridge are of three eruptive facies, all derived from essentially identical magmas; (1) pillow lavas, (2) sideromelane-rich tuffs, and (3) massive, mainly holocrystalline basalts. This association is well known from continental exposures of ancient submarine lavas. Chemically, the lavas are oceanic tholeiites and thus support the view that these low-potassium olivine basalts are by far the dominant eruptive on the deep-sea floor. In the 22° area, they are probably the product of voluminous fissure eruptions. The oceanic tholeiite is evidently the counterpart of the continental flood basalts, but it differs compositionally from these, especially in a lower potassium content. As a further characterization of the basalts, seven new analyses of major, minor, and trace elements are presented. Post-cooling hydrothermal metamorphism under some overburden has transformed some of these basalts into greenschists and lower-grade metamorphic rocks. New data indicate that faulting and shearing along the median valley combined with the introduction of hot, probably saline solutions were major agents in the metamorphism.

Are Ocean Floor Basalts Primary Magma

Abstract: THE typical basalt of the ocean floor and mid-ocean ridges seems to be chemically variable, within a somewhat restricted range1–4, but most contain olivine and hypersthene in the CIPW norm, and carry sparse phenocrysts of olivine, more frequently accompanied by plagioclase than by clinopyroxene1–6. They are olivine-tholeiites7 and their tendency towards high alumina characteristics is reflected in the predominance of olivine + plagioclase phenocryst assemblages.

Some clinopyroxenes from ultramafic inclusions in Dreiser Weiher, Eifel

Abstract: Eight clinopyroxenes from wehrlites and clinopyroxenites and three clinopyroxenes of crystal lapilli in tuff of Dreiser Weiher in Eifel, Germany, have been separated and chemically analysed. One hornblende and two phlogopites from a wehrlite and clinopyroxenites have also been analysed. The rocks enclosing these inclusions are alkali basalts of basanite composition. The analysed clinopyroxenes contain considerable amounts of Al2O3 (3.87–10.84 wt%). The calculated Tschermak's component ranges from 5.9 to 18.4 mol per cent. All of the analysed clinopyroxenes are clearly different from chromian diopsides in lherzolite inclusions in basaltic rocks in Dreiser Weiher and other localities; the former has higher contents of total FeO, CaO and TiO2 and lower contents of MgO and Cr2O3 than the latter. Two clinopyroxenes separated from apatite-bearing clinopyroxenites show high contents of Fe2O3 with about 2 per cent of Na2O, indicating the presence of considerable amounts of acmite component in addition to Tschermak's component. The relative proportions of Al in the tetrahedral site and that in the octahedral site in the analysed clinopyroxenes are clearly different from those of the common igneous clinopyroxenes and eclogites, and similar to those of the clinopyroxenes from other inclusions in basaltic rocks and granulites. It is suggested that all the analysed clinopyroxenes and their host inclusions have crystallized from alkali basalt magmas in relatively deep levels of the continental crust.

Intraglacial volcanoes of the Laugarvatn region, south-west Iceland—I

Abstract: The shape and structure of a group of basaltic volcanoes is described. It is concluded that these volcanoes grew within bodies of melt-water in an ice-sheet. The activity of some was limited to effusion in water, resulting in steep-sided piles of pillow lava. In others which mounted to a water surface activity culminated in explosions which mantled the pillow lava pile with vitric tuff. With still others explosive emergence was superseded by quiet effusion of lava in air. Flows advanced outwards into an encircling lake on deltas of breccia formed at their fronts, mantling and obscuring the products of the earlier phases of aquatic effusion and explosive emergence. Details of the structure and structural interrelationships of the volcanoes are explained in terms of eruptions within an ice-sheet of continually increasing thickness. Attention is drawn to the morphological contrast between intraglacial and extraglacial basaltic volcanoes.

Origin of andesite and its bearing on the Island arc structure

Abstract: The hypothesis that andesite magmas originate from basalt magmas through fractionation is supported for the following reasons: 1) A close association of andesite and dacite with basalt in many volcanoes and a complete gradation in chemistry and mineralogy throughout this suite. 2) Formation of andesite magmas from basalt magmas by differentiation in situ of some intrusive and extrusive bodies. 3) Agreement between the calculated compositions of solid materials to be subtracted from basalt magmas to yield andesite magmas and the observed mineralogy of phenocrysts in these rocks. 4) Higher alkali contents in andesite and dacite associated with high-alumina basalt than in those associated with tholeiite. 5) A complete gradation from the high iron concentration trend of basalt magma fractionation (Skaergaard) to the low or noniron concentration trend (the calc-alkali series) which can be ascribed to the difference of the stage of magnetite crystallization. 6) Similarity between the orogenic rock suite and plateau basalts in the preferential eruption of magmas of middle fractionation stage, givin rise to the great volume of andesite in the orogenic belts and iron-rich basalt in the plateau lavas. Petrological and seismic refraction studies suggest that a great volume of gabbroic materials are present in the lower crust underneath the volcanic belts as a complementary material for the andesite lavas. The island arc structure would develop by repeated eruption of andesite on the surface and by thickening of the oceanic crust underneath the arc due to the addition of gabbroic materials. The suitable portion of the lower crust may be subjected to partial melting to produce granitic magma in the later stage of development of the arc, successively changing it to a part of the adjacent continent.

Mineralogy as a function of depth in the prehistoric Makaopuhi tholeiitic lava lake, Hawaii

Abstract: The electron probe X-ray microanalyzer has been used to determine the compositional variability of the groundmass minerals and glass in 10 specimens from a complete 225-foot section of the prehistoric tholeiitic lava lake of Makaopuhi Crater, Hawaii. The order of beginning of crystallization was: (1) chromite, (2) olivine, (3) augite, (4) plagioclase, (5) pigeonite, (6) iron-titanium oxides and orthopyroxene, (7) alkali feldspar and apatite, and (8) glass. Although the lake is chemically tholeiitic throughout, the occurrence of ferromagnesian minerals is as though there were a gradation from alkali olivine basalt in the upper chill downwards to olivine tholeiite. Groundmass olivine decreases downwards and disappears at about 20 feet. Pigeonite is absent in the uppermost 5±2 feet, then increases in amount down to 20 feet, below which augite and pigeonite coexist in constant 2∶1 proportions. Strong zoning and metastable compositions characterize the pyroxenes of the chilled zones, but these features gradually disappear towards the interior of the lake to give way to equilibrium pyroxenes. Relatively homogeneous poikilitic orthopyroxene (≈ Ca4Mg70Fe26) occurs in the olivine cumulate zone, having formed partly at the expense of pre-existing olivine, augite, and pigeonite (≈ Ca8Mg66Fe26). The growth of orthopyroxene is believed to have been facilitated by the slower cooling rate and higher volatile pressure at depth, and by the rise in Mg/Fe ratio of the liquid due to the partial dissolution of settled olivine. Unlike olivine and pyroxene, feldspar is least zoned in the upper and lower chilled regions. The greatest range of compositional zoning in feldspar occurs at 160 to 190 feet, where it extends continuously from Or1.0Ab22An77 to Or64Ab33An3. The feldspar fractionation trend in the An-Ab-Or triangle gradually shifts with depth toward more “equilibrium” trends, even though the zoning becomes more extreme. The variation with depth in the initial (core) composition of the plagioclase suggests the influence of either slow nucleation and growth (undercooling) or slow diffusion in the liquid, relative to the rate of cooling. Idiomorphic opaque inclusions in olivine phenocrysts are chrome-spinels showing continuous variation from 60 percent chromite to 85 percent ulvospinel and to magnetite-rich spinel. A pre-eruption trend of increasing Al with decreasing Cr can be recognized in chromites from the upper chill. Most of the inclusions show a trend of falling Cr and Al, toward an ulvospinelmagnetite solid solution which is progressively poorer in Usp with depth. This trend was produced by solid state alteration of the chromite inclusions during cooling in the lava lake. Ilmenite (average Ilm91Hm9) coexists with variably oxidized titaniferous magnetite in the basalt groundmass. Estimated oxygen fugacities agree well with other independent determinations in tholeiitic basalt. No sulfide phase has been detected. Fractional crystallization produced a groundmass glass of granitic composition. Average, in percent, is: SiO2, 75.5; Al2O3, 12.5; K2O, 5.7; Na2O, 3.1; CaO, 0.3; MgO, 0.05; total FeO, 1.2; and TiO2, 0.8. Normative Or> Ab. Minor changes in glass composition with depth are consistent with a greater approach towards the granite minimum. Incipient devitrification precluded reliable analysis of glass from the lower half of the section. The SiO2-phase associated with devitrification contains alkalis and Al and is believed to be cristobalite. Needle-like apatite crystals in the groundmass glass are Siand Fe-bearing fluorapatites containing appreciable rare earths (predominantly Ce) and variable Cl. The grain-size and maximum An content of the cores of plagioclase grains were controlled by cooling rate and are at a maximum at the center of the section. The most homogeneous pyroxene (and olivine, Moore and Evans, 1967), most equilibrium pyroxene trends, most abundant alkali feldspar, and most equilibrium feldspar trends are found at 160 to 190 feet, which is appreciably below that part of the lake which was slowest to crystallize. Volatile pressure, increasing with depth, possibly controlled the degree of attainment of equilibrium more than cooling rate. Since they are dependent on cooling history, some of the modal criteria commonly used for recognizing basalt types, such as the absence of Ca-poor pyroxene, presence of groundmass olivine, and the presence of alkali feldspar, should be applied with caution. Petrographic comparison of basalts from one flow, volcano, or province, with another, should recognize the possible variations due to cooling history alone.

The Mid-Atlantic Ridge near 45° N. II. Basalts from the area of Confederation Peak

Abstract: Assisted by detailed bathymetric and bottom photographic coverage, a series of closely spaced samples of rock were taken by dredging along a traverse from the center of the Median Valley to the adjacent crest mountains on the Mid-Atlantic Ridge. The specimens show a gradation from tholeiitic to alkali basalts. Chemical variations, and the alkali content in particular, can be correlated with the depth of extrusion and with the topographic relationship of the volcanoes to the axis of the Median Valley.Although the basalts show considerable evidence of gravity-controlled crystal fractionation, the trends so established are evidently not responsible for alkali enrichment, but appear to be superimposed onto the more fundamental, continuous trend from tholeiitic to alkali basalts.

Lunar and martian surfaces: petrologic significance of absorption bands in the near-infrared.

Abstract: A reflection minimum at 1 micron, reported for Moon and for Mars, indicates olivine or iron- and calcium-bearing clinopyroxene, or both-major constituents of many basaltic rocks. If the 1-micron absorption features are real, both chondritic and acidic rocks are ruled out as primary constituents of the surfaces of the bodies. The reflectance spectrum of Mars matches closely that of an oxidized basalt.

Tholeiitic and alkalic basalts of the Eocene Siletz River Volcanics, Oregon Coast Range

Abstract: These rocks consist of a thick lower unit of tholeiitic basalt and a thin upper unit of alkalic basalt and associated porphyries. Petrochemical studies indicate that older basaltic pillow lavas of the lower unit have the most primitive composition. The abundance of porphyries in the upper unit suggests that crystal fractionation was important in production of the aphyric alkalic rocks, although some volatile transfer may have occurred. Plots of analyzed basalts show similarities to those of the Hawaiian tholeiitic and alkalic suites. The marine volcanic accumulation is typical of flows in the Tertiary eugeosyncline of the Oregon-Washington coast in early to middle Eocene, though the rocks of the upper unit are more localized. The lowermost basalts appear to be typical of basalt from the adjacent Pacific Basin whereas the upper part of the lower unit and the upper unit are similar to Hawaiian basalts.

Isotopic Dating and Palaeomagnetic Polarity in the Faeroe Islands

Abstract: THE Faeroe Islands consist of almost 3,000 m of plateau basaltic lavas in which three main series have been recognized1. The Lower Series consists of forty to fifty moderately massive flows of average thickness 20 m, separated from the thin flows of the Middle Series by about 10 m of sediment containing coal horizons2 of probable Eocene age. The Middle Series occupies about 1,300 m. The Upper Series consists of well marked flows intercalated with tuffs; the flows (which average 10 m in thickness) are somewhat less massive than those of the Lower Series, and appear to have been erupted without noticeable break from those of the Middle Series. Intrusive rocks are uncommon, although some sills are very well developed within the Middle Series.

March 1965 eruption of Kilauea volcano and the formation of Makaopuhi Lava Lake

Abstract: From March 5 to 15, 1965, Kilauea volcano erupted over 15 million cubic meters of tholeiitic basalt along the upper east rift zone between Makaopuhi and Kalalua craters. A pond of basalt 83 meters deep was left within the west pit of Makaopuhi crater. The eruption was preceded by inflation of the Kilauea summit and was accompanied by collapse of the summit. Observed horizontal, vertical, and tilt changes accompanying inflation of Kilauea before and after the eruption cannot fit any simple model for inflation of a single magma chamber. Data for the collapse are even farther from agreement with the draining of a single chamber. Seismic activity was exceptionally low in the months preceding the eruption and during the eruption. Amplitude of harmonic tremor correlates with the rate of fountaining during the eruption. Temperatures of the erupted lavas have been estimated at 1190°–1200°C by extrapolation of thermocouple data obtained in Makaopuhi lava lake. A comparison with temperatures of other Kilauea eruptions measured by optical pyrometer indicates that the best optical pyrometry yields temperatures 35°–75° lower than the true temperature of eruption. The erupted lavas were olivine-poor tholeiitic basalts. Lavas erupted at successively later times down rift are shown to be differentiated from the lavas erupted at Makaopuhi crater by removal of olivine, clinopyroxene, and minor plagioclase during transport in the rift zone.

Age of Precambrian events in the Northeastern Front Range, Colorado

Abstract: Precambrian metasedimentary rocks of the northeastern Front Range have undergone a long and complex geologic history involving multiple periods of metamorphism, deformation, and intrusive activity. The first event following sedimentation involved deformation which produced large east-west isoclinal folds and metamorphism in the greenschist facies and possibly higher. It has been impossible to date this first event by radiometric techniques. The second major event was a period of dominantly medium- to high-grade regional metamorphism accompanied by folding and syntectonic intrusions, such as the Boulder Creek granite. Rb-Sr dating of high-grade gneisses, pegmatites, and Boulder Creek granite places this event at 1700–1800 m.y. ago. Precambrian metasedimentary rocks younger than 1700 m.y. have not been recognized in the northeastern Front Range. A major period of plutonism is dated at 1390–1450 m.y. ago during which time the Sherman and Silver Plume granites, pegmatites, and basalt and andesite dikes were emplaced. The regional heating of the country rock at this time was effective in lowering most of the Rb-Sr and K-Ar mineral ages determined on prebatholithic rocks and probably caused the retrogressive metamorphism which locally affected the gneisses and schists. Post-batholith faulting and cataclasis, possibly at about 1300 m.y., may have been effective in further reducing some mineral ages.

Variation of Palaeomagnetic Stability and Other Parameters in a Vertical Traverse of a Single Icelandic Lava

Abstract: An intensive study has been made of 30 samples from within, and two bakes samples from below a single Tertiary basalt in Eastern Iceland The samples are spaced along a measured vertical traverse from bottom to top The lava is 16·8 m thick, and is reversely magnetized The quantities measured for each core were: 1 Oxidation state as revealed by microscope studies in polished sections of the iron-titanium oxide 2 Oxidation state of the whole rock as determined by FeO and Fe2O3 analyses 3 Titanomagnetite and separate ilmenite grain sizes 4 Magnetic susceptibility 5 Saturation magnetization 6 Curie points and total heating curve in a strong magnetic field 7 Natural magnetization after demagnetization in several alternating fields 8 Koenigsberger's Q factor, slightly modified 9 A new measure of the resistance of the specimen to alternating field demagnetization, the ‘stability factor’ 10 The position of the core above the base of the lava The above quantities vary greatly with position in the lava The results of these measurements show the relationships of these variables to each other; in particular how the palaeomagnetic stability depends on oxidation state of the iron-titanium oxides Stability is high in the lower two-thirds of this lava, and not just at the chilled margins Two anomalous results are (1) that the Curie point is virtually constant regardless of oxidation state (unlike some other lavas), and (2) that the whole rock oxidation state measured by Fe2O3/(2FeO + Fe2O3) does not correlate simply with the oxidation state of the iron-titanium oxides as seen in the microscope

Quaternary lavas from the southern Cascades, western U.S.A.

Abstract: A series of basaltic and andesitic lavas from three centers in the Cascades (Lassen, Medicine Lake, Mt. Shasta) have been investigated. The lavas are weakly porphyritic, containing phenocrysts of plagioclase, augite, and olivine or orthopyroxene; these phases are also found in the groundmass. Titanomagnetite is a groundmass phase in most lavas but it appears to be absent in some. A sub-calcic augite is found in the groundmass in some of the basic lavas. Orthopyroxenes are present only in the salic lavas and show an increase in calcium with increasing iron. The range in composition shown by both phenocryst and groundmass plagioclase is very similar except that the phenocrysts extend to slightly more calcic compositions. The residual glasses in many of the lavas have a rhyolitic composition. However, only those from the Shasta andesites have normative salic constituents that plot near the ternary minimum in the Ab-Or-Qtz system at 500 bars. Both chemical and mineralogical data allow the lavas of the different centers to be distinguished from one another. The most likely origin for the orogenic lavas of the Cascades is by partial melting of the upper mantle.

Geochemistry of the pantellerites of Mayor Island, New Zealand

Abstract: Recent pantelleritic lavas comprise the whole of the isolated and outlying volcano of Mayor Island. Mineralogically, they are characterised by phenocrystic anorthoclase-sodic27 sanidine, quartz, sodic ferrohedenbergite, and cossyrite. Nine new chemical analyses of the lavas are presented (including one residual glass), confirming their strongly sodic and peralkaline nature. One analysis is also given of trachybasalt, which occurs as common inclusions in the mantling pumice deposits. These inclusions are characterised by abundant feldspar phenocrysts. Detailed trace element data is presented for five of the lava samples, representing the mam volcanic phases and the trachybasalt inclusions. The following conclusions are presented: The data is interpreted to indicate that the pantellerites were derived by crystal differentiation from a postulated mildly alkali olivine basalt parent — feldspar fractionation is considered to have been extremely important in this process. It is shown that the element enrichment occurring in the younger lavas may not be wholely explained by crystallisation differentiation alone — it is possible that some additional process is required. It is also shown that the observed enrichment of sodium in the youngest lavas can only occur during crystal fractionation if quartz, as well as anorthoclase, separate from the magma. This is due to the higher alkali abundances of the anorthoclase phenocrysts, relative to the pantellerite compositions. There is limited evidence that post-eruptive devitrification of some of the lavas has resulted in some modification of the lava chemistry, notably sodium loss.

Partition Coefficients of Rare-Earth Elements and Barium between Igneous Matrix Material and Rock-Forming-Mineral Phenocrysts—I.

Abstract: Partition coefficients between natural melts, as represented by igneous matrix material, and amphibole, augite, olivine and plagioclase phenocrysts have been determined for eight rare-earth elements and barium by a mass-spectrometric stable-isotope-dilution technique. Peridotite nodules, crystallization of plagioclase and relationships between basalts are briefly discussed in terms of the partition coefficient data.

Ultramafic Rocks from the Mid-Atlantic Ridge

Abstract: ACCORDING to the ideas of Hess1 the backbone of active mid-oceanic ridges should consist of peridotites which originated in the Earth's mantle and were subsequently upthrusted into the crust along the axis of ridges, above the up welling limbs of mantle's thermal convection cells. The ultramafics should lie below a layer of basalt about 1 km thick, generated by partial melting of the upwelling mantle material.

The oxygen fugacity of alkaline basalt and related magmas, Tristan da Cunha

Abstract: Intergrown phenocrystic titaniferous magnetite-ferrian ilmenite, reduction of ferrian ilmenite by liquid rather than by oxidation of titaniferous magnetite, oxygen fugacity-temperature relationships as criteria of mode of origin

Puu Hou littoral cones, Hawaii

Abstract: The Puu Hou littoral cones, on the south shoreline of Hawaii, were built in 5 days by steam explosions from two narrow lava streams of the 1868 Mauna Loa lava flows as they entered the sea. Explosions occurred in localized areas of both streams, from foci that migrated seaward as the flow built jetties into the sea. Debris, radially ejected from the migrating explosion centers, fell partly on land and partly at sea, and thus formed crescent shaped rims (half-cones) on land which overlap one another. The half-cones are breached through their centers by debris-free basalt corridors, because fragments that fell onto the flowing lava were carried seaward beyond the accumulating rims.