Showing papers on "Basalt published in 1975"

Structure and constitution of the lower oceanic crust

Abstract: Vp from 6.g to 6.5 km/s within gO m.y.; both the mantle and layer 3 are statistically anisotropic. Dredge lithologies consist predominantly of serpentinized ultramafics and mafic igneous rocks ranging from basalt to gabbro, the gabbro often showing evidence of fractionation. Metamorphism of mafic rocks from zeolite to amphibolite facies grade is common. Velocities in oceanic serpentinites and basalts are generally lower than layer 3 refraction velocities. Unaltered gabbros have compressional wave velocities of approximately 7.0 km/s, which is high for layer 3, together with shear wave velocities V, of 3.g km/s and values of Poisson's ratio Uof 0.30. Metabasites containing hornblende and plagioclase have values of Vp = 6.g km/s, V, = 3.g km/s, and u = 0.2g, in good agreement with those of layer 3. On the basis of petrology and velocity it is suggested that layer 3 is composed of hornblende metagabbro underlain by normal gabbro. In a model consistent with geophysical observations of heat flow, seismicity, gravity, and seismic structure at the ridge it is proposed that layer 2 and the upper levels of layer 3 form near the median valley but that deeper levels of layer 3 thicken for 40 m.y. by intermittent offridge intrusion fed from the underlying anomalous mantle. Ophiolites in such a model represent segments of thin immature ridge crest obducted onto continental margins during subduction of a spreading ridge.

Origin of Ross Island basanitoids and limitations upon the heterogeneity of mantle sources for alkali basalts and nephelinites

Abstract: Primary basanitoids from Ross Island, Antarctica have REE patterns and Pb isotope ratios similar to those for primary alkali basalts and nephelinites on ocean islands. The lead data from all volcanics on Ross Island have a spread of 4% in the 206/204 ratio and give a two-stage model lead age of 1500 m.y. The age is interpreted to be the time since the development of the chemical heterogeneity of the mantle source, presumably during an earlier melting process.

K-Ar dating, Quaternary and Neogene volcanic rocks of the Snake River Plain, Idaho

Abstract: Ninety-eight K-Ar dates for lavas of the Snake River Plain (SRP) and vicinity provide calibration for the stratigraphic framework discussed by Malde and Powers (1962). The Idavada Volcanics, predominantly silicic volcanic rocks, are 9 to 13 m.y. old in the western SRP. Rocks of similar type and comparable stratigraphic position are 8 to 10 m.y. old in the central SRP and as young as 4 to 5 m.y. old in the eastern SRP. This time-transgressive silicic volcanic assemblage merges in the east with the silicic volcanic units erupted during the last 2 m.y. from Island Park, Yellowstone, and related volcanic centers. The Idaho Group, consisting of intercalated basalt flows and sedimentary deposits in the western half of the SRP, ranges in age from about 0.6 to 11.5 m.y. (71 refs.)

Constitution of the Lower Continental Crust Based on Experimental Studies of Seismic Velocities in Granulite

Abstract: Rocks of the granulite facies have been proposed as major constituents of the lower continental crust. To evaluate this possibility, compressional and shear wave velocities have been determined to pressures of 10 kb for 10 granulite samples, thus enabling comparisons of seismic data for the lower crust with the velocities and elastic properties of granulite rocks. The samples selected for this study range in composition from granitic to basaltic, with bulk densities of 2.68 to 3.09 g/cm 3 . At 6 kb, compressional ( V p ) and shear ( V s ) wave velocities range from 6.39 to 7.49 km/sec and from 3.36 to 4.25 km/sec, respectively. Velocities in granulite rocks are shown to vary systematically with variations in mineralogical constitution. Both V p and V s , increase with increasing pyroxene, amphibole, and garnet. Velocities increase with an increasing ratio of pyroxene to amphibole in hornblende-granulite subfacies rocks of approximately equivalent chemical compositions. Decreasing quartz content in granulite rocks produces an increase in V p and an accompanying decrease in V s , thereby significantly changing Poisson9s ratio. The range of velocities measured for the granulite samples is similar to the range of seismic velocities reported for the lower continental crust; thus, the hypothesis that granulite rocks are major lower crustal constituents is further strengthened. Furthermore, it is shown that lower crustal composition is extremely variable, and therefore valid discussions of composition must be limited to specific regions where seismic velocities are well known. The use of seismic velocities in estimating lower crustal composition is illustrated for the Canadian Shield in Ontario and Manitoba.

Mantle Plume Mixing Along the Reykjanes Ridge Axis: Lead Isotopic Evidence

Abstract: Gradients of lead isotopic ratios from basalts erupted along the Reykjanes Ridge and Median Neovolcanic Zone of Iceland confirm mantle plume mixing with the depleted asthenosphere along the ridge axis

Granitic to ultramafic rock complexes of the Indian Ocean ridge system, western Indian Ocean

Abstract: During five expeditions of the Scripps Institution of Oceanography to the western Indian Ocean, more than 4,500,000 sq km of the Central Indian Ridge and its branching Southeast Indian Ridge and Southwest Indian Ridge were explored by bathymetric, magnetic, and seismic-reflection profiling. In some 2,800,000 sq km of this region, igneous rocks of the crust, lower crust, and possible upper mantle are exposed by faulting or volcanism. Fifty-six dredge hauls of these igneous rocks were obtained, largely from the major cross-fractures (transform faults) or clefts trending athwart the volcanically active ridges. From north to south, the cross-fractures most intensively sampled were the Vema Fracture Zone, which crosses the crestal area near 9°S, Argo Fracture Zone near 13°30′S, Marie Celeste Fracture Zone near 17°30′S, and the newly delineated “Melville Fracture Zone” trending north-south for more than 600 km near 60°30′E on the Southwest Indian Ridge. Our field and laboratory studies indicate that under a capping of young flow basalt, there is a regional complex of igneous rocks produced by magma generated under the ridges, trapped and differentiated into sill-like, podiform, and larger, crudely stratified to well-stratified sheets. Rocks from the stratiform masses include abundant Iherzolite and minor harzburgite, orthopyroxenite, olivine- and two-pyroxene gabbros, Ti-ferrogabbros, norite, and anorthosite. Some associated diabase intrusions are granophyric and are cut by late-stage dikelets of quartz monzonite and Na-rich trondhjemite. Both calc-alkaline and alkalic lines of differentiation are indicated. The granitic dikelets contain clear, doubly-terminated crystals of zircon, unusual in a terrane of large-cation–depleted rocks. The overlying basalt flows are pillowed with chemical and mineralogical characteristics typical of olivine-bearing tholeiite from the ridge-rise systems of the world oceans. The ubiquitous nature of the crustal complex found throughout the western Indian Ocean, together with data from the Atlantic and Pacific Oceans, suggest that similar rock complexes, dominated in their lower parts by stratiform bodies, are characteristic of most of the igneous crust throughout the world oceans.

Rare‐Earth variations across ‘normal segments’ of the Reykjanes Ridge, 60°–53°N, Mid‐Atlantic Ridge, 29°S, and East Pacific Rise, 2°–19°S, and evidence on the composition of the underlying low‐velocity layer

Abstract: Supplement is available with entire article on microfiche. Order from American Geonhvsical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J75-002; $1.00. Payment must accompany order. Rare-earth (RE) abundance patterns in 26 basalts occurring across normal segments of the Reykjanes ridge, 60°–53°N; the mid-Atlantic ridge, 29°S (Deep-Sea Drilling Project leg 3); and the East Pacific rise, 2°–19°S, including the Nazca plate, are reported. Without exception, all the RE patterns were found to be characteristically light RE depleted, reflecting to a first-order approximation, a mantle source with uniquely defined RE content. Computer multiphase and eutectic, equilibrium-partial melting models show that the reconstituted RE pattern of the mantle underlying these regions must be more depleted in light RE than the basalts generated. This is so whether a lherzolite (spinel free or bearing), garnet peridotite, or plagioclase peridotite is considered for the primary mantle source. The source is thought to be the low velocity layer apparently depleted in light RE and other large ionic lithophile elements. A lherzolite phase assemblage is found to be preferable for the depleted low-velocity layer source under normal midocean ridge segments at a depth greater than 30–38 km. Above 30–38 km, equilibration of normal ridge basalts with a plagioclase peridotite cannot be ruled out. Rare-earth results across these three normal ridge segments further emphasize (1) the spatial and temporal RE uniformity of the depleted low-velocity layer source, (2) the widespread geographic coverage of this mantle source, (3) its RE composition contrast with mantle source regions, such as those beneath Iceland, the Azores, and the Afar, which have been assumed to represent rising plumes (or blobs), and finally (4) the passive nature of volcanism along normal ridge segments occurring in response to the spreading as a simple ‘healing’ process.

Lead Isotope Relations in Oceanic Ridge Basalts from the Juan de Fuca-Gorda Ridge Area, N.E. Pacific Ocean

Abstract: Lead isotopic analyses of a suite of basaltic rocks from the Juan de Fuca-Gorda Ridge and nearby seamounts confirm an isotopically heterogeneous mantle known since 1966. The process of mixing during partial melting of a heterogeneous mantle necessarily produces linear data arrays that can be interpreted as secondary isochrons. Moreover, the position of the entire lead isotope array, with respect to the geochron, requires that U/Pb and Th/Pb values are progressively increased over the age of the earth. Partial melting theory also dictates analogous behavior for the other incompatible trace elements. This process explains not only the LIL element character of MOR basalts, but also duplicates the spread of radiogenic lead data collected from alkali-rich oceanic basalts. This dynamic, open-system model of lead isotopic and chemical evolution of the mantle is believed to be the direct result of tectonic flow and convective overturn within the mantle and is compatible with geophysical models of a dynamic earth.

Some basaltic and andesitic gases

Abstract: Igneous gases escape from igneous rocks and magmas. They may originate by effervescence or they may bubble through the melt from an external source. In this paper I review data (mostly post-1963) on H2O, B, CO2, N2, O2, Ne, sulfur gases, Cl, Ar36, Ar40, Br, Kr, I, Xe, Hg, and Rn. The compositions of volcanic gases are compatible with solubility and melt composition for H2O, Cl, and possibly sulfur gases. For other gases there are either inadequate data or conflicting data (CO2). Volcanic gas emitted at surface pressure is generally very rich in H2O because H2O is the principal gas dissolved in most melts. CO2-rich volcanic gases can be explained by separation of gas from melt at pressures of a few tens of atmospheres and by effervescence of magmas with high ratios of dissolved CO2/H2O (mostly alkaline basalts). There is no single magmatic gas phase. Glassy basalts from the sea floor have substantial concentrations of both active and noble gases. The concentrations of H2O and Cl correlate with the alkali concentration (especially K2O). The recorded concentrations of H2O and CO2 in sea floor basalts are far below saturation at 500 atm. Yet the basalts contain tiny vesicles. It appears likely that magmatic gas of basalts at sea floor pressure is 60% SO2, 20% H2O, 10% H2S, and 10% CO2 at 1200°C. The concentrations of Cl and H2O in basaltic and andesitic glasses trapped in large crystals in pumices from continental margins are about 10 times those of deep sea basalts. Detailed relationships suggest that at least some andesitic magmas develop in vapor-saturated environments. The restored or potential concentrations of H2O and Cl in andesitic magmas I estimate by allowing for vapor loss during production of andesite from a basaltic precursor. I use the restored concentrations as a basis for estimating the rate of igneous outgassing in island arcs and continental margins. The inferred rates of outgassing are such that the present quantities of gaseous elements in seawater and sediment would be exhaled in 109 to 1010 years for H2O, Cl, Br, S, I, and Hg. The rates of outgassing of CO2 from Kilauea volcano and of SO2 from Pacaya exceed the rate which could be maintained by outgassing of erupted lava. These relations imply the existence of a larger source of these gases than that of the volcanic rocks. Apparently, large subsurface reservoirs of melt discharge gas out of the volcanic vents. It seems probable that volcanic rocks are generally volumetrically small portions of subsurface reservoirs which may be vapor-saturated. Such a condition has major implications for crustal evolution, the origins of volcanic rocks, eruption mechanisms, and hydrothermal ore deposits.

Hydrothermal processes along mid-ocean ridges: An experimental investigation

Abstract: An experimental investigation of high-temperature seawater/basalt interactions has been conducted in order to better evaluate the geochemical and economic implications of hydrothermal circulation of seawater in the oceanic crust along active mid-ocean ridges. The results indicate that, as seawater reacts with basalt between 200 ° C and 500 ° C at 500–800 bars, the fluid tends to change from an oxygenated, slightly alkaline, Na+, Mg++, SO4 =, Cl− solution to a reducing, acidic, Na+, Ca++, Cl−, solution with Fe, Mn and Cu concentrations up to 1500, 190 and 0.3 ppm respectively. Silica concentrations in the fluid reach concentrations of 200–600 ppm; however, Al abundances remain very low (∼0.5 ppm). Gray and green smectites, anhydrite, albite, tremolite-actinolite, chalcopyrite, pyrrhotite and hematite were the dominant alteration products formed. These data imply that large-scale circulation of seawater in the oceanic crust could account for the Al-deficient metalliferous sediments associated with mid-ocean ridges and could be important in the genesis of certain Fe-Cu sulfide ore deposists. The process could also affect the geochemical budgets of certain elements and exert substantial control of the steady-state composition of seawater by removing excess Na and Mg and adding Ca, Si, and H to the oceans.

Lead and strontium isotopes in post-glacial basalts from Iceland

Abstract: We report here our Pb and Sr isotopic study of some post-glacial basalts (less than 12,000 yr old) from the southern part of Iceland, and use our new data to evaluate various proposed models for the genesis of volcanic rocks on Iceland. The interpretation for Icelandic rocks is also extended to the genesis of volcanic rocks on other oceanic islands of the world.

A late-Quaternary monogenetic volcano field in central Mexico

Abstract: The western end of the “Grupo Chichinautzin“, a segment of the east-west Mexican Volcanic Belt, contains 41 volcanic cones and associated lava flows, ranging in age from 8,390±100 to 40,000 years BP. There are two main cone types: cinder cones and cones made up mainly of lava blocks. The cone density is 0.1/km2 and the height range is 15 m–260 m with crater rim diameters of from 175 m to 500 m (mean 280 m). The relative ages of the volcanoes and flows were established using geomorphological parameters. With increasing age, height/width ratios decrease from 0.21 to 0.10 whilst the ratio of crater width to basal width increases from 0.40 to 0.83. Three of the cinder cones produced a total of > 0.75 km3 of ash covering > 200 km2. The other cone type appears to have erupted only small amounts of ash. The lava flows are of aa or block lava with an average thickness from 10 m to 70 m and a maximum length of 1 km to 13.5 km. The aspect ratio varies from 20 to 120. The younger, Holocene, flows, made up mainly of two-pyroxene andesites, are longer, thinner, and originally less viscous than the shorter, thicker flows from the older volcanoes which are relatively richer in olivine. The Holocene flows cover ∼ 115 km2 with a volume of ∼ 4.5 km3. The rocks contain 52% to 64% SiO2 and are essentially andesitic but some extremes could be classified as alkalic basalts and dacites. Sufficient parameters are available to characterize the “violent strombolian” eruptions of this volcano field, which show features of both hawaiian and normal strombolian activity.

Metagabbros from the Mid-Atlantic Ridge at 06°N: Contact-Hydrothermal-Dynamic Metamorphism beneath the Axial Valley

Abstract: Metagabbros with a transitional greenschist-amphibolite-facies mineralogy are exposed on the lower slopes of the eastern wall of the axial valley of the Mid-Atlantic Ridge at 06°N. These metagabbros are associated with metabasalts in the greenschist facies and with only mildly or nonmetamorphic basalts. Among the metagabbros some are finely banded, consisting of alternating amphibole-rich and plagioclase-rich zones, each about 1 mm thick. Specimens showing gabbro-basalt contacts are interpreted as representing the margins of basaltic dikes originally crossing through the gabbroic lower oceanic crust. The chemical composition of the metagabbros appears to be nearly unchanged from that of their parent rocks, which probably were gabbros with intermediate Fe/Mg ratio, and with olivine tholeiite normative composition and low K content characteristic of abyssal tholeiites. One of the metagabbros was dated at between 2 and 3 m.y. by the fission-track method on zircon crystals. The metamorphic processes which gav...

Petrology of rodingites from the equatorial Mid-Atlantic fracture zones and their geotectonic significance

Abstract: Rodingites were dredged from fracture zones of the equatorial Mid-Atlantic Ridge along with serpentinized ultramafics, and fresh and metamorphosed basalts and gabbroids. These rodingites were generated by a metasomatic process at low temperature involving an enrichment in lime and water, and a loss of silica and alkalis. The parent rocks were gabbronorites which intruded ultramafic material as it ascended from the upper mantle to its present location in the upper oceanic crust. The gabbronorites were probably altered to rodingites while they were still in the lower oceanic crust. Since the rodingitization process appears to be concomitant, complementary and simultaneous with the serpentinization of the host ultramafic rocks, we infer that the serpentinization process also took place in the deeper part of the oceanic crust. These two simultaneous metasomatic processes thus predate the major phase of tectonic events which uplifted these blocks as cold, solid diapiric emplacements of ultramafic material and accompanying rodingites to their present positions along lines of weakness expressed as fracture zones.

Petrography and K-Ar Ages of Dredged Volcanic Rocks from the Western Hawaiian Ridge and the Southern Emperor Seamount Chain

Abstract: Alkalic basalt dredged from Yuryaku Seamount in the southernmost Emperor Seamount chain and from the western Hawaiian Ridge at Pearl and Hermes Reef and at two unnamed seamounts 160 and 380 km west of Midway is similar to the alkalic basalt that caps the volcanoes in the Hawaiian Islands Conventional and 40 Ar/ 39 Ar K-Ar analyses give best weighted mean ages of 423 ± 16 my for Yuryaku Seamount, 273 ± 04 my and 267 ± 05 my for the two unnamed seamounts, and 201 ± 05 my for the volcano that forms Pearl and Hermes Reef The data show that the age of the Hawaiian-Emperor bend is about 41 to 43 my Although the volcanoes in the Hawaiian-Emperor chain generally increase in age to the north and west of the island of Hawaii, the measured age-distance relations along the chain are not linear in detail A phonolite, possibly a differentiated member of a posterosional nephelinic suite and the first found on the Hawaiian Ridge, was recovered from Pearl and Hermes Reef Samples of analcime tephrite recovered from the unnamed seamount 380 km west of Midway may also be derived from a posterosional nephelinic suite

Diffusion coronas around quartz xenocrysts in andesite and basalt from Tertiary volcanic region in northeastern Shikoku, Japan

Abstract: Coronas around quartz xenocrysts in andesite and basalt from Tertiary volcanics in northeastern Shikoku, Japan, have been described. The coronas are composed mainly of Ca-rich clinopyroxene and glass. Compositional profiles across the corona glass show monotonous variation of major elements except for alkalis. Preliminary experiment on the reaction between basaltic melt and quartz has shown that alkalis diffused against their concentration gradients. This particular feature of alkali enrichment in corona glass is explained by a diffusion model, in which non-ideality of alkalis in silicate melt is assumed. Preferred crystallization of Ca-rich clinopyroxene in coronas of orthopyroxene andesite is discussed using a chemical potential diagram in the system SiO2-CaO-RO (RO=MgO+FeO), and it is suggested that higher (Na+K)/Al ratio of the corona glass, which increases the effective CaO concentration and thus increases the μ CaO/μ RO, is responsible for the preferred crystallization of Ca-rich clinopyroxene.

Rare earth element concentrations in a suite of basanitoids and alkali olivine basalts from Grenada, Lesser Antilles

Abstract: A suite of basanitoids and alkali olivine basalts from Grenada, Lesser Antilles were analyzed for rare earth elements. The REE concentrations of these rocks are characterized by a small variation in the heavy REE (7 to 9 times chondrite) and a large variation in the light REE (17 to 93 times chondrite). Among the possible mechanisms to account for the REE variations, fractional crystallization processes at low and high pressures, and partial melting processes (both batch melting and fractional melting) were examined, using the partition relationships of REE among silicate minerals and melts. It is suggested that the observed REE variations are best explained by variable degrees of batch partial melting, in which garnet is present as one of the solid phases through 2 to 17% melting of a garnet lherzolite parent rock.

Is there an Icelandic mantle plume

Abstract: SCHILLING1 has interpreted the chemistry of basalt lavas from the Reykjanes Ridge and Iceland, identifying two distinct upper mantle sources for these lavas, one of which rises in a primordial hot mantle plume beneath Iceland. I questioned2 this interpretation on the grounds that insufficient attention had been given to the possibility that the magmas were derived from a homogeneous source; the differences could arise from by fractional crystallisation during the ascent of the lava.