Showing papers on "Phenocryst published in 1972"
TL;DR: In this article, it was shown that in general the observed partition coefficients for trace element distribution between phenocrysts and host lava are not equilibrium coefficients, as far as trace element partitioning is concerned is due to an imbalance between the rate of crystal growth and trace element diffusion in the lava and in the crystal.
273 citations
Abstract: Submarine basalts from the mid-Atlantic ridge, Red Sea rift, and Joides site 105 in the western Atlantic have been studied in ultra thin, doubly polished thin sections. Most of the samples are pillow lava fragments containing a variety of skeletal crystal growth forms that can be related to three major textural zones in the pillows. Olivine appears as diffuse, lattice-like growths near quenched pillow margins, as elongated chainlike or hollow lanternlike forms within a partly crystallized variolitic zone, and as larger lantern shaped crystals within the holocrystalline pillow centers. Plagioclase forms acicular hollow prisms elongated parallel to a in the variolitic zone. In cross section, these crystals often show sector zoning developed on the 001 and 010 faces. Each sector is also progressively zoned outward from a more calcic to a more sodic composition. In holocrystalline pillow cores, plagioclase forms more nearly equidimensional solid crystals with no trace of sector zoning. Pyroxene is rarely well crystallized and is typically found as feathery crystals intimately intergrown with plagioclase in the groundmass. Spinel and magnetite are ubiquitous in the samples examined, although the glassy pillow rims usually contain spinel but not magnetite. Within the variolitic zone and holocrystalline pillow interiors, magnetite appears both as skeletal crystals and as overgrowths on spinel. Sulphides (mainly pyrite) are especially abundant in the Red Sea lavas. Phenocrysts of olivine and plagioclase and subhedral and euhedral groundmass olivine and plagioclase show no evidence of growth by filling in of hollow or lattice-like crystals similar to those of the outer pillow zones, It is suggested that development of these skeletal forms is restricted to supercooled, highly viscous magma in which relatively rapid growth is combined with low rates of diffusion. Slower growth or more rapid diffusion rates or both at crystallization temperatures maintained close to the liquidus appear to promote more even growth rates and compositional homogeneity on all crystal faces; this possibility accounts for the more nearly equidimensional form and lack of sector zoning in phenocrysts and microphenocrysts. Petrographic observations support the division of sea floor basalts into ‘Olivine tholeiites’ and ‘plagioclase tholeiites’ which has been suggested by other workers. The only large pyroxene crystals observed in the samples examined appear to be xenocrysts, and pyroxene is not a prominent quench (liquidus) phase in this or other samples studied.
162 citations
TL;DR: In this paper, the authors used semi-empirical equations for the dispersive pressure and found that a plug flow velocity profile yields a phenocryst distribution very similar to those observed in the field.
Abstract: When a magma containing phenocrysts in concentrations greater than about 8 percent intrudes rock to form a dike or sill, mechanical interactions between the phenocrysts create a grain dispersive pressure. To maintain a balance in the dispersive pressure, the phenocryst concentration must decrease toward the walls of the intrusion because of the increase in the velocity gradient and the fluid viscosity. This mechanism offers an explanation for the observed rapid, but gradational, increases in content from phenocryst-poor margins to a phenocryst-rich center, especially in picritic dikes and sills. Using semi-empirical equations for the dispersive pressure, I found that a plug flow velocity profile yields a phenocryst distribution very similar to those observed in the field.
If the phenocryst interactions are sufficiently intense, then the grain dispersive pressure would be also dependent upon the size of the phenocrysts. This dependence would cause the larger phenocrysts to migrate preferentially to the center of the intrusion and the smaller phenocrysts to the walls, giving a size variation as well as a concentration variation. It is uncertain whether the interactions are sufficiently intense to give size sorting by this mechanism.
At the phenocryst concentrations normally encountered, a review of experimental studies indicates that the Magnus-effect and similar single particle forces are small in magnitude in comparison to the forces of grain interaction. They would have only a modifying effect on the phenocryst distribution produced by the grain dispersive pressure.
Modifications of the phenocryst distribution produced by gravity during and after intrusion are examined. During intrusion, gravity settling of the phenocrysts would be balanced by a gradient of the grain dispersive pressure, causing the phenocryst distribution to be asymmetrical with respect to the center of the intrusion. Phenocryst interactions during gravity settling following intrusion, in which the faster settling larger grains capture smaller grains within their boundary layers, may be responsible for sharp discontinuities observed in the distributions of phenocrysts.
122 citations
Journal Article•
121 citations
TL;DR: A slightly metamorphosed igneous sequence is exposed beneath upper Eocene limestone on Eua, the southernmost of the eastern line of limestone-covered islands of the Tonga group as discussed by the authors.
Abstract: A slightly metamorphosed igneous sequence is exposed beneath upper Eocene limestone on Eua, the southernmost of the eastern line of limestone-covered islands of the Tonga group. This igneous sequence occurs as lava, agglomerate, conglomerate, and tuff; compositions include high-alumina tholeiite, basaltic andesite, quartz gabbro, olivine basalt, and dacite tuff. Well-defined dikes of acid andesite (also pre-limestone) cut this sequence. Phenocryst mineralogy is typically highly calcic plagioclase, augite, and titanomagnetite, plus alteration products (mainly uralite, chlorite, epidote, calcite, and pyrite). Characteristic chemical features of the Eua igneous suite include relatively high alumina, very low K 2 O, P 2 O 5 , Rb, Ba, and REE, and relatively low TiO 2 , Ni, and Cr. The Na/K, V/Ni, and K/Rb ratios are high, and Rb/Sr ratios are very low. Measured Sr 87 /Sr 86 ratios (= initial) of a tholeiitic basalt and an acid andesite give 0.7034 and 0.7038, respectively. The Eua igneous suite evidently represents a very early stage of island arc evolution. Geochemically, these Eua rocks are decidedly less fractionated (or more “primitive”) than the recent Tongan basaltic andesite-dacite association which constitutes the western line of active volcanic islands of Tonga. This latter suite can be classified as one of the circum-Pacific “low potash andesite suites” (or “island arc tholeiitic series”). In terms of the trace and minor element data, the Eua igneous suite is similar in many respects to ocean-floor basaltic lavas, although perhaps characterized by lower TiO 2 , Ni, and Cr than is typical of most ocean floor lavas. It is concluded that the over-all chemistry and mineralogy of the Eua igneous suite suggest close similarities with the volcanic and mafic plutonic components of described ophiolite complexes; this is partly supported by the reported occurrence of peridotite and dunite from the nearshore flank of the Tonga trench. Thus, it is postulated that on Eua we have the topmost part of an underlying ophiolite complex exposed, possibly developed at the location of and at the inception of the site of initial subduction of lithosphere that has subsequently been responsible for the development of the Tonga arc system.
104 citations
TL;DR: In this article, a relation for the grain dispersive pressure is solved together with the momentum equation of fluid flow, utilizing an empirical relation for apparent viscosity of the phenocryst suspension.
Abstract: Mechanical interactions between phenocrysts during magma flow give rise to a grain-dispersive pressure. During intrusion into a dike or sill, in the absence of forces other than those of grain interaction, the grain-dispersive pressure must be constant across the flow width. As a result, the concentration of phenocrysts must decrease toward the walls to offset the increase in the velocity gradient as the walls are approached. This mechanism has been offered as an explanation for the observed rapid but gradational increases in content from phenocryst-poor margins to a phenocryst-rich center, especially in picritic dikes and sills.
A relation for the grain dispersive pressure is solved together with the momentum equation of fluid flow, utilizing an empirical relation for the apparent viscosity of the phenocryst suspension. Solutions for steady flow between parallel walls demonstrate pluglike velocity profiles as well as phenocryst-concentration increases toward the center away from the walls. The velocity is nearly the maximum value within the central half of the flow. Therefore very strong pseudoplastic non-Newtonian behavior of the magma need not be assumed to explain the observed phenocryst concentration variations.
97 citations
TL;DR: A suite of garnet-bearing andesites and dacites from the Ordovician of N. W. England is described and major and trace-element analyses of the garnet phenocrysts are presented as mentioned in this paper.
Abstract: A suite of garnet-bearing andesites and dacites from the Ordovician of N. W. England is described and major- and trace-element analyses of the garnet phenocrysts are presented. The garnets are of almandine-pyrope composition, with minor amounts of spessartine and grossular, and often show marked reversed zoning; the crystal becoming progressively enriched in pyrope towards the margin. Garnets from the dacites are consistently richer in almandine and spessartine than are those from the andesites.
74 citations
TL;DR: In this article, the authors investigated the relationship between compositional variation and the sequence of phenocryst phases available for fractionation in basaltic rocks from the New Georgia Group and found that the discrepancy appeared explicable in terms of two low-pressure crystal fractionation mechanisms.
Abstract: Study of the data provided by Stanton and Bell (1969) for certain basaltic rocks from the New Georgia Group reveals an apparent discrepancy between compositional variation and the sequence of phenocryst phases available for fractionation. The discrepancy none-the-less appears explicable in terms of two low-pressure crystal fractionation mechanisms. The first of these we term compensated crystal settling, a process which, it is postulated, allows a substantial amount of magma undergoing crystal settling to maintain its overall composition since crystals settling from it are continually replaced by compositionally similar crystals which settle into it from higher levels. The second process involves selective fractionation of phases sinking at different rates. Slow sinking of plagioclase relative to ferromagnesian minerals is believed to produce cumulus enrichment in plagioclase in the upper part of the chamber, the resultant magmas being erupted as highly porphyritic, high-alumina, basaltic andesites.
53 citations
TL;DR: In this paper, the distribution of Rb and Ga in basic volcanic rocks and their constituent groundmass and mineral phases is studied in terms of random adsorption and site occupancy of the cations.
53 citations
TL;DR: The late Cenozoic lavas of the Raton-Clayton region of New Mexico can be divided into five groups: (1) the raton-clayton lavas, widespread alkali olivine basalts; (2) the hornblende andesites and dacites of the Red Mountain lavas; (3) a separate group of pyroxene andesite from Sierra Grande volcano; (4) a feldspathoidal group of basanites and olivined nephelinites; and (5)
Abstract: The late Cenozoic lavas of the Raton-Clayton region of New Mexico can be divided into five groups: (1) the Raton-Clayton lavas, widespread alkali olivine basalts; (2) the hornblende andesites and dacites of the Red Mountain lavas; (3) a separate group of pyroxene andesite from Sierra Grande volcano; (4) a feldspathoidal group of basanites and olivine nephelinites; and (5) the Capulin-type basaltic lavas, which are characterized by abundant “cloudy” plagioclase phenocrysts. The olivines of the feldspathoidal rocks zone toward calcium enrichment rather than iron enrichment as in more silicic rocks. This trend can be related to the silica activity of the magma and to changes in pressure during crystallization. The pyroxenes in the lavas of this suite are rich in alumina, and those of the feldspathoidal rocks show strong sector zoning. Ilmenite from one basanite contains over 40 mole percent of the geikelite (MgTiO 3 ) component, and other oxides also have high contents of MgO. The distribution of Mg between oxide phases and olivine follows predictions from thermodynamic data, but does not agree with experimental results. The Fe-Ti distribution in coexisting oxide phases indicates oxygen fugacities near FMQ for the Raton-Clayton and Capulin basaltic rocks, and higher values for the feldspathoidal, andesitic, and dacitic rocks. Estimates of partial pressures of water based on the Kudo-Weill feldspar geothermometer range from 5 kb for a basanite, 2 to 3 kb for the andesites and dacites, to near 1 kb for the Capulin magmas. Fractional crystallization and crustal contamination do not appear to have been significant processes in the origin of these lavas. Partial melting in the lower crust (Red Mountain lavas) and upper mantle under different physical conditions seems to be a more likely explanation of the diversity of rock types displayed in this suite. High water pressures, in particular, may have played a significant role in the genesis of the feldspathoidal and Sierra Grande andesitic rocks.
52 citations
TL;DR: In this article, a 5.5 meter thick sill of fourchite near Montreal, Quebec, has several cumulate layers of titanaugite phenocrysts in its lower half and a corresponding number of zones of ocelli, small rounded bodies of feldspathic rock, in its upper half.
TL;DR: A Pleistocene pyroclast flow of dacitic andesite composition from the island of Dominica, Lesser Antilles is described in this article, where several properties of the flow are discussed, including specific gravity, flattening ratio and grain size.
Abstract: A Pleistocene pyroclast flow of dacitic andesite composition is described from the island of Dominica, Lesser Antilles. Lateral zonation, from welded tuff to a tuffaceous zone of no welding and pumiceous distal facies is observed with distance from the vent. Comparable but less distinct zonation occurs in vertical sections. Variations in physical properties of the pyroclast flow, such as specific gravity, flattening ratio and grain size are described, and related to the degree of welding. Source of the pyroclast flow is located in the former crater of Micotrin volcano, now plugged by a dome. Eleven chemical analyses reveal small, but significant variations in composition, which, together with systematic variations in phenocryst content, are related to crystal sinking in the magma column prior to eruption.
TL;DR: The Spanish Peaks intrusions of southcentral Colorado range in age from 22 to 25 my, as shown by four K-Ar dates as mentioned in this paper, as well as other intrusions in the Sangre de Cristo Range and southern high plains.
Abstract: The intrusive rocks of the Spanish Peaks, south-central Colorado, range in age from 22 to 25 my, as shown by four K-Ar dates These rocks as well as other intrusions in the Sangre de Cristo Range and southern high plains appear to be correlated in time with the early Miocene maximum of igneous activity in the Basin and Range province The volcanic fields of northeastern New Mexico are products of a distinct 8-my-to-Holocene phase of activity Widespread olivine basalt 2 to 5 my in age was erupted in these fields during approximately the same period that similar basalt was erupted west of the Sangre de Cristo Range in the Rio Grande depression Feldspathoidal basanite and nephelinite were erupted about 19 my ago in the Raton-Clayton field The Capulin basalt flows are of Holocene age, and are characterized by the presence of resorbed plagioclase phenocrysts These rocks resemble the Hinsdale basalt of a much earlier period that are located west of the Rio Grande depression The relation of basalt magma type and position relative to the depression is complex, or asymmetric in time and space
TL;DR: The Madeiran suite of minor intrusives and lavas consists of parental, unusually soda rich, alkali olivine basalts with hawaiite, mugearite and essexite derivatives as mentioned in this paper.
Abstract: Petrological and geochemical investigations have been conducted on the little studied Neogene basaltic rocks of the Madeiran Islands. The Madeiran suite of minor intrusives and lavas consists of parental, unusually soda rich, alkali olivine basalts with hawaiite, mugearite and essexite derivatives. Olivine and clinopyroxene are dominant phenocryst and cumulus nodule phases. Low pressure fractionation of the parental magma by precipitation of these minerals gave rise to the hawaiitic trend. That olivine settling precedes clinopyroxene in the fractionation process can be deduced from Ca and Ni variations in the analysed rocks and phenocryst separates. Late stage feldspar flotation in a hawaiitic derivative liquid led to extrusive mugearites and an intrusive essexite.
TL;DR: The olivine phenocrysts of four basalts (12004, 12008, 12009 and 12022) are concentrically zoned and have core compositions about as magnesian as experimentally produced liquidas olivines, features which suggest fractional crystallization and absence of Fe-Mg reequilibration as mentioned in this paper.
TL;DR: In this article, the results of a whole-rock analysis of the Fantale volcano were presented. But this does not correspond to the age of the volcano, since it is assumed that the feldspar phenocrysts crystallized at a high level in the magma chamber and then fell into a less radiogenic, less fractionated environment.
Abstract: The Quaternary Ethiopian volcano Fantale is a central volcano with a summit caldera. The most recent flows are pantellerites characterized by high Rb/Sr ratios. These cover earlier weakly peralkaline rhyolites and trachytes. Results of Sr 87 /Sr 86 analyses on selected samples from the volcano are presented. The whole-rock results on a basalt, two trachytes, and nine more silicic rocks show a correlation of the Sr 87 /Sr 86 ratio with Rb/Sr. Eleven of the rocks fall on an isochron giving an apparent age of 2.6 m.y. However, this does not correspond to the age of the volcano. Data from separated feldspar phenocrysts and host residual glassy silicic matrix show that the feldspars are enriched in radiogenic strontium relative to the matrix. Chemical analyses of the lavas suggest that the composition of the upper part of the sub-volcanic magma chamber at Fantale changed progressively from that of a trachyte to that of a weakly peralkaline rhyolite, finally becoming pantelleritic. It is suggested that the isotopic composition also changed with increasing enrichment in Rb/Sr and Sr 87 /Sr 86 , thus accounting in a general way for the isochron relation noted above. Data from the Fantale tuff, a 2 km 3 welded ash flow, indicates that the Fantale magma chamber was chemically zoned. The contrasting isotopic compositions of the feldspar phenocrysts and matrix support the concept of feldspar fractionation in this zoned magma chamber. It is suggested that the phenocrysts crystallized at a high level in the magma chamber and then fell into a less radiogenic, less fractionated environment.
TL;DR: An anorthoclase phenocryst separate from a pantellerite welded tuff and a slightly peralkaline nonhydrated sodatrachyte glass both have 87 Sr/ 86 Sr ratios of 0.7030 ± 0.0002.
Journal Article•
TL;DR: In this article, Bhattacharji et al. observed a gradual decrease in the grain size and content of olivine and pyroxene from the centre to the margin of the dykes.
Abstract: Picritic dykes of oceanite and ankaramite composition occur around Dedan in the Kathiawar peninsula of Gujarat State. They have been studied across their width, with reference to the distribution and crystallisation of olivine and pyroxene. Olivine and pyroxene occur both as large phenocrysts and as small crystals in the groundmass. Considerable variation in the concentration of olivine and pyroxene is observed from the central portion of the dykes to the margins. A gradual decrease in the grain size and content of these minerals is seen from the centre to the margin of the dykes It is also observed that the grain size of these two minerals shows a direct relationship with the percentage of these minerals. This is explained by flow differentiation proposed by Bhattacharji and Smith for the Muskox feeder dyke.
01 Jan 1972
TL;DR: The interpretation of immature crystallization and some lunar textures characterized by it are the principal objectives of this investigation as discussed by the authors, and particular reference is made to the form and textural relations of olivine in 12009 and of pyroxenes and plagioclase in 12021, and to terrestrial analogs.
Abstract: The interpretation of immature crystallization and some lunar textures characterized by it are the principal objectives of this investigation A comparative and selective approach is adopted, and particular reference is made to the form and textural relations of olivine in 12009 and of pyroxenes and plagioclase in 12021, and to terrestrial analogs The optic orientation of the olivines in 12009 is determined, and their skeletal crystallization is illustrated and evaluated Microprobe and optical data are associated in a textural analysis of an analog from the upper contact of a minor intrusion in Skye The optic orientation of pyroxene enclosed in plagioclase cores is determined, and the results are plotted stereographically The need for greater precision in the use of textural terms is stressed, and a new term - intrafasciculate - is introduced for textures in which pyroxene has crystallized within hollow, skeletal plagioclase Apollo 12 pyroxene-phyric basalts are texturally reviewed, and the crystallization of the phenocrysts is discussed, emphasis being placed on size-independence of skeletal growth
TL;DR: In this article, a genetic model for granitic rocks dredged from the Aves Rise is proposed, which involves a sodium-enriched hydrothermal fluid phase, shallow emplacement, and significant derivation of components from the upper mantle.
TL;DR: In this article, the authors explored phase equilibria in a group of lava samples which are (I) randomly chosen primary magmas, (II) liquids related by fractionation of olivine, with or without the additional separation of plagioclase and pyroxene (III, IV), or (V) liquid related by accumulation of Olivine plus pyroxenes into a cotectic liquid, using a three-component system analogy.
05 Nov 1972-The Journal of the Japanese Association of Mineralogists,Petrologists and Economic Geologists
TL;DR: Composite lava flows of ankaramite and alkalic olivine basalt on the western flank of Haleakala contain abundant phenocrysts of olivines and augite in their upper parts, but are nearly devoid of phenocryst in their lower parts as discussed by the authors.
Abstract: Composite lava flows of ankaramite and alkalic olivine basalt on the western flank of Haleakala contain abundant phenocrysts of olivine and augite in their upper parts, but are nearly devoid of phenocrysts in their lower parts. They resulted from crystal settling in the magma column below the volcano, followed by eruption first of the phenocryst-poor upper part of the magma, and later the phenocryst-enriched lower part. The latter overrode the earlier lava of the same flow while the flow was still hot and active.
TL;DR: In this article, high-flux instrumental neutron-activation analysis was used to evaluate trace-element variation in individual plagioclase and individual hornblende crystals, and correlation coefficients between phenocryst masses and trace element contents of the phenocrysts, and between elements, were calculated.
TL;DR: In Apollo 12 sample 12052 as discussed by the authors, a porphyritic basalt, a weak, planar preferred orientation and a lineation was determined by crystal-elongation measurements and universal-stage measurements in orthogonal thin sections.
Abstract: Pyroxene phenocrysts in Apollo 12 sample 12052, a porphyritic basalt, have a weak, planar preferred orientation and a lineation as determined by crystal-elongation measurements and universal-stage measurements in orthogonal thin sections. The structures are probably the result of laminar flow. Numerous vugs and the flow foliation in sample 12052 suggest crystallization as a surface flow or a near-surface sill. Flowage appears to have ceased by the time of crystallization of the variolitic groundmass of the sample. The fabric data suggest a two-stage crystallization of sample 12052.
01 Jan 1972
TL;DR: In this paper, anon-thinned samples of rock 12052 have been examined in the electron microscope at 100kV, and the cores of the pyroxene phenocrysts consist of a calcium-poor pyroxenes, pigeonite, with an epitaxially grown rim of a calcite-rich pyrite, augite.
Abstract: Ion-thinned samples of rock 12052 have been examined in the electron microscope at 100kV. This rock contains numerous phenocrysts of pyroxene, Ca1−p(Mg, Fe)1 + PSi2O6, and a few rounded olivine crystals (Mg, Fe)2 SiO4 set in a finergrained groundmass of pyroxene, plagioclase (Na, Ca) (Al, Si)4O8, ilmenite, FeTiO3 and other minor minerals (Champness et al., 1971). The cores of the pyroxene phenocrysts consist of a calcium-poor pyroxene, pigeonite, with an epitaxially grown rim of a calcium-rich pyroxene, augite. Both the pigeonite and augite are zoned, and indication of non-equilibrium crystallisation (Bence et al., 1970).