Showing papers on "Basalt published in 1977"

Clinopyroxene composition in mafic lavas from different tectonic settings

Abstract: Many metamorphosed and weathered basalts contain fresh clinopyroxene crystals set in an altered groundmass. Microprobe analysis of these relict grains can be used to identify the magma type of the host lava. Statistical discrimination of clinopyroxenes from known magma types provides a test of the effectiveness of this method, showing that any attempt to classify an unknown clinopyroxene as either an ocean-floor basalt, a volcanic arc basalt, a within plate tholeiite or a within plate alkali basalt magma type should have a 70% chance of success. Identification of within plate alkali basalts is most likely to be successful because their pyroxenes characteristically have high Na and Ti and low Si contents. Within plate tholeiites can usually be distinguished from volcanic arc basalts because their pyroxenes contain more Ti, Fe and Mn. However, neither of these last two magma types can be easily distinguished from ocean floor basalts on the basis of pyroxene analyses. Diagrams of pyroxene composition based on discriminant functions and on Na2O vs MnO vs TiO2, SiO2 vs TiO2 and SiO2 vs Al2O3 provide the basis for visual discrimination. The discrimination achieved is mainly due to differences in the bulk chemistry of the host magmas and in the partitioning of cations into the pyroxene lattice; differences in temperature and crystallization histroy of the magmas are of lesser, but nevertheless finite, importance. Application of this technique to pyroxenes in metabasalts from Othris, Greece gave results consistent with, but more ambiguous than, results obtained from immobile trace element studies.

Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near lat 36°49′N

Abstract: Fifty acoustically positioned samples of fresh basalt were collected by the submersible Alvin from the median valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (FAMOUS) in the summer of 1974. The samples show regular compositional variations from the center of the rift valley (central lava flows) out to the rift valley walls (flank lava flows). The central lava samples show higher ratios of olivine relative to clinopyroxene and plagioclase phenocrysts and contain chrome spinel. Glasses of the flank lava samples are enriched in SiO 2 , TiO 2 , K 2 O, H 2 O, and FeO/MgO relative to central lava samples. Studies of the thickness of palagonite and manganese crusts indicate that the flank lava flows are considerably younger than the inferred spreading age of the crust on which they occur. Flank lavas are generally older than central lavas, but notable exceptions occur. The composition of the flank lava glass can be derived by the removal of approximately 29 wt percent of analyzed phenocrysts (in the ratio 5.7 plagioclase, 2.5 olivine, 1.8 clinopyroxene) from the central lava glass. In addition, other processes (possibly involving volatile transfer) must enrich the flank lavas in K 2 O, TiO 2 , and H 2 O. A model is proposed whereby this crystal fractionation occurs in a shallow, narrow (6-km-wide) magma chamber underlying the median valley. The chamber is compositionally zoned, and central lavas are fed from dikes tapping its hotter axial zone, whereas flank lavas are fed from the cooler, differentiated melt on the margins. The nature of the chemical variations in the lavas permits an estimate of the composition and thickness of the cumulates forming at the base of the chamber.

The sources of island arcs as indicated by Nd and Sr isotopic studies

Abstract: Island arc lavas from New Britain and the Marianas have ^(143)Nd/^(144)Nd similar to other oceanic basalts and distinctly different from continental flood basalts and thus appear to be derived from a high Sm/Nd, light-REE-depleted reservoir. Consideration of both Nd and Sr isotopes suggests seawater involvement in the generation of some island arc lavas and thus indicates that they may be derived from altered subducted oceanic crust. Other island arc lavas show no evidence of seawater involvement and may be derived from mantle reservoirs with affinities to the sources of ocean island basalts. Andesite and rhyolite from an Andean volcano reflect assimilation of old continental crust. Nd and Sr in basaltic and ultrapotassic continental rocks indicate that some mafic magmas in continental regions may be derived from old low-Sm/Nd reservoirs or are heavily contaminated with old continental crustal material. Fish debris from the ocean floor provides an estimate of ^(143)Nd/^(144)Nd in seawater and indicates that light-REE in the marine environment are derived mainly from continents. Basalts erupted above sea level in oceanic and continental areas are isotopically distinct from those erupted on the ocean floor, suggesting a relationship between parental reservoirs and hydrostatic head.

The geochemistry and evolution of early precambrian mantle

Abstract: Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and 87Sr/86Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times. Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution:

Cooling and crystallization of tholeiitic basalt, 1965 Makaopuhi Lava Lake, Hawaii

Abstract: ___________________________________

The geology of the Viking Lander 1 site

Abstract: Stereo pictures show that Viking Lander 1 landed on volcanic terrain of undulating topography in the plains of Chryse. The bedrock is exposed along several ridge crests, and blocks are more numerous than can be attributed to impact ejecta. The presence of a variety of rock types suggests in situ weathering of extrusive and near-surface basaltic igneous rocks along a linear volcanic vent. Fine-grained sediment is present in drift complexes and isolated drifts. A small patch of fine-grained sediment slumped down one of the drift faces during the course of the Viking mission. Otherwise, no other morphological changes unrelated to spacecraft activity have been observed.

Andesites and high-alumina basalts from the central-south Chile high Andes: Geochemical evidence bearing on their petrogenesis

Abstract: High-alumina basalts from seven High-Andean stratovolcanoes (37 °30′S to 41 °S) have major and trace element (including rare earth elements, REE) that are consistent with derivation by partial melting (typically 10–15%) of garnet-free peridotite followed by fractional crystallization of olivine and pyroxene. High-alumina basalts from two stratovolconoes require significantly lower degrees of melting (<5%) or melting of an incompatible, element-enriched source. However, a poorly understood feature of all of these basalts-and calc-alkaline rocks in general-is the mechanism for causing their low TiO2 and heavy REE content relative to oceanic basalts. Further north in Chile (33 °–34 °S and 21 °–22 °S) amphibole-bearing andesites have REE abundances consistent with derivation from a garnet-bearing source such as incompatible, element-rich eclogite (e.g., Franciscan eclogites) or garnet peridotite. The marked petrological and geochemical changes along strike of the Andes are probably related to the varying nature of the subduction process; e.g., dips of the downgoing slab varying from 10 to 30 °.

Nucleation and growth of plagioclase, Makaopuhi and Alae lava lakes, Kilauea Volcano, Hawaii

Abstract: The Hawaiian lava lakes offer an unparalleled opportunity to study the processes that occur during the crystallization of basaltic magma. This paper presents estimates of the rates of nucleation and growth of plagioclase in the Kilauea lava lakes, Makaopuhi and Alae, and a discussion of the processes that control the nucleation and growth. The observed growth rates perpendicular to (010) vary from 1,7 to 11.0 × 10 −10 cm sec −1 . The nucleation rates vary from 6.8 × 10 −3 to 2.0 cm −3 sec −1 . In general the rates increase with increasing crystallization at any point, decrease with increasing distance from the surface, and are higher in the shallower lake, Alae. For the most part, nucleation appears to occur heterogeneously on previously existing crystals. The growth appears to be controlled by the interface attachment kinetics and not by diffusion in the melt. The observed results are in qualitative agreement with theoretical predictions.

Geochemical evolution of the suboceanic mantle

Abstract: The suboceanic mantle has on the basis of incompatible element and isotope ratios two chemically distinct units; the mantle source for ocean island basalts and the mantle source for ocean ridge basalts. Ocean island alkali basalts have an enriched light rare earth element (REE) source with K/Rb = 400, K/Ba = 28, Zr/Nb = 6.6. Ocean ridge tholeiites have a depleted light REE source, with K/Rb = 1060, K/Ba = no, and Zr/Nb = 37. 87 Sr/ 86 Sr and 206 Pb/ 204 Pb ratios are generally lower in the ocean ridge basalts than in the basalts from ocean islands. It is suggested that these differences are not a result of simple partial melting, fractional crystallization, zone refining, residual minerals or isotopic disequilibrium between minerals and melt during melting. Instead, the differences between the sources are real, and have existed for some 1500 Ma or more. Nephelinite, alkali basalt, and tholeiites from ocean islands have similar isotopic and incompatible element ratios. The nephelinites and alkali basalts are, however, progressively much more enriched in the light REE than are the tholeiites, yet the light REE in nephelinites and alkali basalts appear to be incompatible. This may be explained by the occurrence of small (average 4 cm thick), closely spaced pyroxenite veins (within tens of cm) in the mantle source for ocean island basalts. The occurrence of a network of veins which melt preferentially may lead to melts which have apparent sources with a much greater enrichment in incompatible elements than the total mantle source and may also give the appearance of a source whose ratio for slightly compatible elements changes as a function of extent of melting.

Geochemistry and petrogenesis of a basalt-benmoreite-trachyte suite from the southern part of the Gregory Rift, Kenya

Abstract: In the southern Gregory Rift valley a series of transitional basalt, ferrobasalt, and benmoreite flows (1.65–1.4 Myr) is overlain by flood trachyte lavas (1.3–0.9 Myr). Mass balance calculations for major element compositions of rocks of this suite and their phenocrysts and microphenocrysts suggest that the ferrobasalts and benmoreites formed from magma resembling the most primitive basalt by closed system fractionation of plagioclase, clinopyroxene, olivine, titanomagnetite, and apatite. The trachytes formed from evolved magmas largely by alkali feldspar fractionation. Estimates of phenocryst and liquid densities and Rayleigh-law modelling of trace element contents support these conclusions. From Rayleigh-law modelling, we derived a set of effective distribution coefficients. Partial melting of crustal rocks or volatile transfer processes had no significant effect on the petrogenesis of this suite. The duration of the eruptive cycle, cooling time calculations, and mass balance calculations suggest that fractionation occurred in a magma reservoir with volume of at least 3 × 104 km3 during an interval of about 0.8 Myr. Temperatures during fractionation probably ranged from about 1200 °C to 900 °C, and pressures may have been roughly 5 to 8 Kb. We suggest that rift development was accompanied by large-scale injection of basaltic magma and dilation of the crust, extensive fractionation, preferential eruption of low-density and fluid trachytic flood lavas, and by several episodes of normal faulting.

Basaltic Magmatism and the Bulk Composition of the Moon. II: Siderophile and Volatile Elements in Moon, Earth and Chondrites: Implications for Lunar Origin

Abstract: Abundance patterns of siderophile and volatile elements imply that the Moon was derived from the Earth's mantle after the core had segregated. The relative abundances of siderophile and volatile elements in the Moon and in the Earth's mantle are obtained from a comparison of their abundances in terrestrial ocean-floor basalts and lunar low-Ti mare basalts. The abundances of a group of siderophile elements Ni, Co, W, Ir, Os, P, S and Se are found to be very similar in ocean-floor tholeiites and low-Ti mare basalts and this similarity is believed to extend to their respective source regions in the Earth's mantle and lunar interior. The abundances of the above siderophile elements in the Earth's mantle have been determined by the interaction of several complex processesunique to the Earth, which relate to core formation and non-equilibrium distribution of elements between metallic and silicate phases. Since these factors could not possibly have operated separately within the Moon, the similarity in siderophile element abundances therefore implies thatthe Moon was derived from the Earth's mantle after the Earth's core was formed. Abundance patterns of volatile elements in the Moon differ dramatically from those in ordinary chondrites and from those to be expected from condensation of a nebula of solar composition. These differences imply that the Moon was not formed from components which themselves had condensed directly from the solar nebula. For these reasons, current versions of the capture and binary planet hypotheses of lunar origin, which maintain that the Moon formed independently by accretion in the solar nebula, can be rejected. The drastic depletion of many volatile elements in the Moon compared to the Earth implies that the separation of material from the Earth's mantle to form the Moon occurred at very high temperatures. Depletions of this magnitude are best explained by the hypothesis that material from the Earth's mantle was totally evaporated, and then selectively recondensed, whilst the more volatile components were lost. The Moon then formed in Earth orbit by accretion from this volatile-depleted, mantle-derived condensate.

Picrite basalts and related lavas from the Deccan Traps of Western India

Abstract: Detailed chemical and mineralogical data are given for three sequences of basalts and picrite basalts from bore-holes in Western India. The picrite basalts show bulk compositional variation generated by the fractionation of olivine and chromite. Evolved picrite basalt magma appears to have given rise to basalt by the fractionation of olivine+clinopyroxene, despite the presence of abundant plagioclase phenocrysts. It is suggested that a slow settling rate for plagioclase relative to clinopyroxene and olivine is sufficient to account for this feature. The high degree of equilibrium crystallisation which many of the lavas have apparently undergone is interpreted in terms of the mechanism of compensated crystal settling (Cox and Bell, 1972). Experimentally determined atmospheric pressure phase relations are used to model dyke-like magma chambers in some detail. Finally volumetric and age relationships are used to argue that the picrite basalts, despite their porphyritic nature, crystallised from ultramafic liquids containing in some cases at least 16% MgO.

Ocean floor metamorphism in the Betts Cove ophiolite, Newfoundland

Abstract: The mineralogical and geochemical features of the lower Ordovician Betts Cove ophiolite of northeastern Newfoundland indicate that hydrothermal circulation of seawater near a mid-ocean ridge has been involved in the metamorphism of the complex. The degree of greenschist facies metamorphism increases with stratigraphie depth in the ophioli te. Calcite, hematite and epidote distributions show that the metamorphosing fluid penetrated downward and was reduced with depth. The mobilities of major and trace elements support the hypothesis of the interaction of seawater and basalt: Fe2O3, MgO, Na2O and H2O increase whereas CaO and Cu decrease in the rock after alteration; SiO2, total iron, K2O, Ba and Rb can either be depleted or enhanced in the altered material; TiO2, P2O5, Zr, Y, Cr and Ni remain stable during the metamorphic episode. Finally, the occurrence of massive sulphides and incipient rodingitic gabbro is explicable in a circulatory seawater system.

Hydrothermal manganese in the Galapagos Rift

Abstract: HYDROTHERMAL emanations originating at mid-ocean ridges have been thought1–5 to provide a substantial source of manganese to the ocean but the evidence supporting this hypothesis has been indirect. Anomalous manganese concentrations have been measured in naturally occurring systems where seawater is in direct contact with lava flows6–8. Laboratory studies have shown that seawater tends to leach manganese from basalts at elevated temperatures and pressures9–11. Anomalously high manganese accumulation rates have also been determined for sediments adjacent to active ridge systems, most notably the East Pacific Rise12–14. No measurements of manganese concentrations in seawater near mid-ocean ridges, or in hydrothermal fluids emanating from these ridges, have yet been made, however. We report here the results of the first such direct measurements, which show that manganese is being injected into the deep sea by hydrothermal circulation of seawater through newly-formed oceanic crust.

Sr-isotope, K, Rb, Cs, Sr, Ba, and rare-earth geochemistry of basalts from the FAMOUS area

Abstract: Ten basalt samples recovered from the FAMOUS area were selected so as to obtain representatives of a wide geographical and compositional range. The samples were analyzed for 87Sr/86Sr, K, Rb, Cs, Sr, Ba, and rare-earths. Sr-isotope ratios fall in the narrow range of 0.70288 to 0.70307, which implies that these samples were derived from an isotopically homogeneous source. The FAMOUS area lies in a geochemical transition zone between the Azores Plateau and “normal” ridge areas south of lat 33°N. The LIL (large-ion-lithophile) and Sr-isotope geochemistry of FAMOUS basalts is thus influenced by the Azores mantle plume; this results in higher Sr-isotope and LIL concentrations in these basalts than is typical of Mid-Atlantic Ridge basalts. Trace-element distributions in FAMOUS area basalts cannot be entirely accounted for by fractional crystallization models that are based on major-element chemistry. The LIL distribution in FAMOUS basalts could be due to variable extents of partial melting. Zonation within the magma chamber may result from incomplete mixing of successive batches of magma entering the chamber and could be further enhanced by fractional crystallization. The variation in partial melting would require significant increases in mantle temperature over a relatively short period of time. According to this model, the Mount Pluto magma represents the highest degree of partial melting and may mark the initiation of a new cycle of eruptive activity in the median valley.

Serenitatis and Imbrium impact melts - Implications for large-scale layering in the lunar crust

Abstract: The early intense bombardment of the moon has not necessarily gardened most of the crust to more than a few kilometers depth Deep crustal material sampled by the largest impacts is most likely to be preserved as melt-rock deposits on or near the rims of mare basins The Apollo 17 melt-rock boulders and the matrices of the Apollo 15 'black-and-white' rocks (15445, 15455) are considered the most likely of all lunar samples to represent deep crustal material These samples have the composition of low-K Fra Mauro (LKFM) basalt A two- or three-layer crustal model is proposed, in which a layer of anorthositic gabbro, which forms most of the exposed surface of the lunar highlands, is underlain by a layer of LKFM basalt If the Apollo 15 and 17 heat-flow measurements are representative of the mean lunar heat flow, they constrain the LKFM layer to be no more than 20 km thick

Evolution of oceanic crust: 3. Petrology and chemistry of basalts from the East Pacific Rise and the Siqueiros Transform Fault

Abstract: Basalt samples obtained from the Siqueiros transform fault/fracture zone and the adjacent East Pacific Rise are mostly very fresh oceanic tholeiite and fractionated oceanic tholeiite with Fe+3/ Fe+2 ∼ 0.25; however, alkali basalts occur in the area as well. The rocks of the tholeiitic suite are ol + pl phyric and ol + pl + cpx phyric basalts, while the alkali basalts are ol and ol + pl phyric. Microprobe analyses of the tholeiitic suite phenocrysts indicate that they are Fo68–Fo86, An58–An75, and augite (Ca34Mg50Fe16). The range of olivine and plagioclase compositions represents the chemical variation of the phenocryst compositions with fractionation. The phenocyrsts in the alkali basalts are Fo81 and An69. The suite of tholeiites comprises a fractionation series characterized by relative enrichment of Fe, Ti, Mn, V, Na, K, and P and depletion of Ca, Al, Mg, Ni, and Cr. The fractionated tholeiites occur on the median ridge (which is a sliver of normal oceanic crust) of the double Siqueiros transform fault, on the western Siqueiros fracture zone, and on the adjoining East Pacific Rise, while the two transform fault troughs contain mostly unfractionated or only slightly fractionated tholeiite. We suggest that the fractionated tholeiites are produced by fractional crystallization of more ‘primitive’ tholeiitic liquid in a crustal magma chamber below the crest of the East Pacific Rise. This magma chamber may be disrupted by the transform fault troughs, thus explaining the paucity of fractionated tholeiites in the troughs. The alkali basalts are found only on the flanks of a topographic high near the intersection of the northern transform trough with the East Pacific Rise.

Reconnaissance geology of coastal Sonora between Puerto Lobos and Bahia Kino

Abstract: Coastal Sonora between Puerto Lobos and Bahia Kino can be subdivided into four structural-petrographic subprovinces: an inland subprovince in which unmetamorphosed upper Precambrian and Cambrian strata rest on older Precambrian gneiss and three other subprovinces in which Cenozoic volcanic strata rest on metamorphosed strata of post-Precambrian age intruded by granitic rocks of Mesozoic age. One of these latter subprovinces displays basin-and-range fault blocks; a second has northwest-trending strike-slip(?) faults; and the third, Isla Tiburon, shows structure related to the Neogene dilation of the Gulf of California depression. The pregranitic rocks include upper Precambrian and Cambrian carbonate rocks, a chert-graywacke-volcaniclastic sequence of probable Carboniferous age, and volcanic-volcaniclastic rocks of Jurassic age. The granitic rocks range from gabbro to granite and have K-Ar cooling ages of from 91 to 30 m.y. Dikes of basaltic to dacitic composition and quartz porphyritic bodies of late Mesozoic or early Cenozoic age cut the granitic rocks. The lowermost Cenozoic volcanic strata (pre–22 m.y. B.P.) are predominantly composed of rhyolite and basalt. These are followed by a sequence of predominant andesite (∼20 to 18 m.y. old), a sequence of partially marine conglomerate and pyroclastic deposits, and a widespread, predominantly rhyolitic sequence (∼14 to 10 m.y.). All strata 10 m.y. old or older are involved in basin-and-range–type tilting. Volcanic strata less than 8 m.y. old are nearly flat-lying.

Shock metamorphism of lunar and terrestrial basalts

Abstract: Lonar Crater (India) basalt and lunar basalt 75035 were shock loaded under controlled laboratory conditions up to 1000 kbar, generally in a CO/CO2 (1:1) environment evacuated to 10 to the minus seventh power torr. The Kieffer et al. (1976) classification scheme of progressive shock metamorphism is found to apply to lunar basalts. The major shock features of the five classes that span the range 0 to 1000 kbar are described. Only three out of 152 basalt specimens show shock effects in their natural state as severe as Class 2 features. The scarcity of shocked basalt hand samples in contrast to the abundance of shock-produced agglutinates and homogeneous glass spheres in the lunar regolith indicates the dominant role of micrometeorite impact in the evolution of the lunar regolith. The overall glass content in asteroidal and Mercurian regoliths is considered.

Chemistry of Apollo 12 mare basalts - Magma types and fractionation processes

Abstract: Major and trace element data for a large suite of petrographically diverse Apollo 12 mare basalts are presented, and magma types sampled at the Apollo 12 site are characterized. The data confirm earlier classifications of the basalts into olivine, pigeonite, ilmenite, and feldspathic basalts. The olivine and pigeonite basalts are shown to be comagnetic and related by olivine fractionation. The other types, which differ in trace element and isotopic characteristics, are derived from different sources within the lunar interior. The spatial relations between the main basalt types are discussed in terms of local cratering events, and it is suggested that the younger ilmenite basalts overlie the olivine-pigeonite basalts. The role of olivine-dominated near-surface crystal fractionation in causing chemical variation is examined, and a relation between inferred cooling rate and the position of a sample in the fractionation sequence is determined.

Mantle helium in the Red Sea brines

Abstract: HELIUM isotope studies on terrestrial samples have revealed the existence of two helium components which are clearly distinct from atmospheric helium. The first of these, which we term ‘crustal helium’, was identified in 1946 in natural gas wells1. This crustal component is produced by radioactive decay of U and Th to 4He, with 3He production by (n, α) reactions on Li; the resulting helium is characterised by 3He/4He ≃ 10−7, one-tenth of the atmospheric ratio2. The second component, ‘mantle helium’, was discovered as ‘excess 3He’ in deep ocean water, attributed to a flux of primordial helium from the mantle3. Studies of the 3He/4He ratio in deep water on the East Pacific Rise4 and in helium trapped in submarine basalt glasses5,6 have shown that this mantle component is characterised by 3He/4He ≃ 10−5, about 10 times the atmospheric ratio and 100 times the ratio in crustal helium. Basalt glasses from the Western Pacific Lau Basin, the East Pacific Rise, and the Mid-Atlantic Ridge contain trapped helium with similar 3He/4He ratios, indicating that mantle helium in at least three areas in which new lithosphere is being formed has a unique and uniform isotopic signature.