Separation Of Plagioclase Research Articles

Petrogenesis of Lavas from Cinder Cone Fields behind the Volcanic Front of Central America

Small, monogenetic cinder cones and shield volcanoes occur in platform cone fields behind the volcanic front of Central America. Their lavas (BVF suite) are distinctly different from typical volcanic front lavas (VF suite). Most VF lavas are nearly aphyric basalts, containing only olivine and plagioclase phenocrysts. They have high , Ba, and Sr contents and moderate Ni, Cr, and Rb contents. In contrast to petrographic observations, major and trace element modeling and the decrease in the ratio with fractionation require substantial clinopyroxene fractionation and moderate degrees of fractionation for the BVF lavas. The contradiction between petrography and compositional trends can be resolved by the following model: (1) crystallization and separation of plagioclase, clinopyroxene, and olivine at approximately 5-10 kb; and (2) rapid magma ascent and surface eruption of nearly aphyric lavas. Near-surface crystallization involves only plagioclase and olivine due to the simultaneous expansion of the primary phase volumes of both with decreasing pressure. The moderate Cr contents of BVF lavas indicate a mantle origin for BVF primary magmas. Lavas with petrographic and compositional characteristics similar to Central American BVF lavas are often found associated with clusters of cinder cones at other convergent plate margins, like in the Cascades and Mexico.

Basalts as probes of planetary interiors: Constraints on the chemistry and mineralogy of their source regions

Basalt magmas, derived by the partial melting of planetary interiors, have compositions that reflect the pre-accretionary history of the material from which the planet formed, the planets, subsequent evolutionary history, the chemistry and mineralogy of the source regions, and the intensive thermodynamic parameters operating at the source and emplacement sites. Studies of basalt suites from the Earth, its Moon, and the eucrite parent body reveal compositional differences intrinsic to their source regions which are, in turn, a characteristic of the planet and its formational and evolutionary history. Major interplanetary differences are observed in iron, Mg (Mg + Fe 2+) , TiO 2, Al 2O 3, Na 2O, Cr, Ni, and in volatile element abundances. The most primitive mare basalts have Mg#s ∼ 0.6, on the Earth they are 0.70–0.72 for mid-ocean ridge basalts (MORBs) and up to 0.9 for Archean peridotitic komatiites. Eucrites have Mg#s approaching 0.5 (excepting Binda). These differences reflect inherent differences in Mg (Mg + Fe 2+) of their sources. Striking differences in the TiO 2 abundances of the planetary basalts reflect both inter- and intra-planetary variations in source chemistry. Primitive MORBs and primitive oceanic intraplate tholeiites have a factor of 2–3 difference in TiO 2 at comparable Mg# (0.7–1.2 vs 2–3 wt.% respectively). Three major titania groups are recognized in the mare suite; high TiO 2 (8–13 wt.%), low TiO 2 (2–5 wt.%) and very low TiO 2 (<1 wt.%). The eucrites have TiO 2 contents <1 wt.%. The mare basalts and eucrites have pronounced Na 2O depletion relative to all terrestrial basalts. This is a consequence of the preplanetary accretion loss of volatiles from the material that formed the Moon and the eucrite parent bodies. Mare basalts have consistently lower Al 2O 3 contents than the terrestrial basalts. This may be due either to an inherently lower content of Al 2O 3 in the mare sources or it may reflect Al 2O 3 retention in an aluminous phase. The transition metals are fractionated in all three basalt suites. For terrestrial basalts this may reflect core-separation; however, in the case of the Moon and eucrite parent bodies pre-accretionary separation of metal and silicates is a more reasonable explanation. A pronounced Cr anomaly is observed in terrestrial MORBs but not in the mare basalts. This appears to be related to f O 2 differences in the respective mantles. Overall rare earth element abundances are comparable between all three objects. Mare basalts have a pronounced negative Eu anomaly which is inherited from their source region and is record of plagioclase removal from crystallizing magma ocean early in lunar history (4.4–4.6 Ga). Early separation of plagioclase on the Earth appears to have been a relatively unimportant process.

Contribution a l'etude du volcanisme de l'arc des Nouvelles-Hebrides; caracterisation de deux series magmatiques de l'ile d'Erromango

Abstract The volcanic series of Mt Rantop and Robertson's Thumb, Erromango Island, New Hebrides, formed by fractional crystallization of orogenic basaltic magma of near-island-arc tholeiitic type. Differentiation was controlled mainly by separation of plagioclase, olivine and clinopyroxene. The Mt Rantop series is predominantly tholeiitic (plagioclase at the liquidus, late appearance of magnetite, pigeonite in microphenocrysts, and Fe and Ti remaining constant or increasing in the early stages of differentiation); those of Robertson's Thumb are mostly calc-alkaline (magnetite at the liquidus, late appearance of plagioclase, olivine quickly becoming unstable, orthopyroxene in phenocrysts and early decrease of Fe and Ti). The compositional differences reflect higher fO &amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and PH &amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; O in Robertson's Thumb during fractional crystallization.

Vector analysis of chemical variation in the lavas of Par�cutin volcano, Mexico

Compositional variations in the lavas of Paricutin volcano, Mexico, have been examined by an extended method of Q-mode factor analysis. Each sample composition is treated as a vector projected from an original eight-dimensional space into a vector system of three dimensions. The compositions represented by the vectors after projection are closely similar to the original compositions except for Na2Oand Fe2O3.The vectors in the three-dimensional system cluster about three different planes that represent three stages of compositional change in the Paricutin lavas. Because chemical data on the compositions of the minerals in the lavas are presently lacking, interpretations of the mineral phases that may have been involved in fractional crystallization are based on CIPW norm calculations. Changes during the first stage are attributed largely to the fractional crystallization of plagioclase and olivine. Changes during the second stage can be explained by the separation of plagioclase and pyroxene. Changes during the final stage may have resulted mostly from the assimilation of a granitic material, as previously proposed by R. E. Wilcox.

Elemental and isotopic geochemistry of nonhydrated quartz latite glasses from the Eureka Valley Tuff, east-central California

The Eureka Valley Tuff, erupted from the late Miocene Little Walker volcanic center, east-central California, includes two major ash-flow sheets: the more voluminous, widespread, and more silicic Tollhouse Flat Member and the overlying By-Day Member. Nonhydrated quartz latitic glasses from the Tollhouse Flat and By-Day Members have, respectively, 65.5 and 63.3 wt percent SiO 2 and are extremely rich in large-ion lithophile elements. The K 2 O contents and K/Rb ratios are 5.3 wt percent and 195 for the Tollhouse Flat glass and 5.5 wt percent and 190 for glasses from the By-Day Member. FeO*/MgO (FeO* = total Fe as FeO) ratios are transitional between values characteristic of basaltic and calc-alkalic suites. Initial 87 Sr/ 86 Sr ratios of 0.7054 to 0.7055 are consistent with the pattern of northwestward decrease in 87 Sr/ 86 Sr observed for Mesozoic plutonic and Cenozoic volcanic rocks in the western Great Basin and Sierra Nevada provinces. The glasses are higher in Rb than are Mesozoic granitic rocks of the Sierra Nevada batholith that have comparable Sr contents and initial 87 Sr/ 86 Sr ratios. Pb isotopic compositions fall at the upper end of arrays found for upper Cenozoic calc-alkalic rocks from the Cascade Mountains and nearly on trends found for rocks of the Sierra Nevada batholith. The moderately high FeO*/MgO ratios, very low Co, Cr, and Ni contents, high Ba/Sr and K/Sr ratios, and approximately 25 percent negative Eu anomalies demonstrate that the glasses are the products of appreciable crystal fractionation. This in turn strongly suggests that the Eureka Valley Tuff was derived from a hypersthene-normative or only slightly quartz-normative parent magma. A model is outlined in which a potassic mafic parent magma was produced by a moderate degree of partial melting of diapirically upwelling, undepleted ultramafic mantle material followed by zone refining of upwelling mantle and other material within the asthenosphere and lithosphere. Extensive fractional crystallization and crystal separation subsequently enriched the melt further in large-ion lithophile elements. The evidence for appreciable plagioclase separation demonstrates that much fractionation took place well within the Earth9s crust. The Tollhouse Flat and By-Day Members were erupted from discrete — although closely related — bodies of magma. Lower water and halogen contents of the By-Day magma may have prevented the intratelluric crystallization of biotite and thereby allowed the magmatic liquid to become richer in K and Rb than that of the more highly fractionated Tollhouse Flat Member.

Strontium, Rubidium, Potassium, and Calcium Variations in Quaternary Lavas, Crater Lake, Oregon, and Their Residual Glasses

Strontium and rubidium contents of 22 whole-rock specimens and 15 residual glasses from Crater Lake, Oregon, range from 650 to 725 ppm and 23 to 33 ppm, respectively, for the less differentiated hypersthene andesites and basaltic andesites; to 150 to 400 ppm and 50 to 80 ppm for the more salic whole-rock and residual glass specimens. The behavior of these elements largely reflects the separation of large amounts of phenocrystic plagioclase. Sr content closely follows that of calcium. More sodic plagioclase exhibits a greater absolute affinity and a slightly greater relative affinity for Sr than more calcic compositions. K/Rb decreases progressively from about 440 at a K 2 O content of 1.2 weight percent to 330 at 3.4 percent K 2 O as a result of separation of plagioclase with a K/Rb ratio several times that of the associated magmatic liquid. Two factors support the hypothesis that the late dacites and rhyodacites of Mount Mazama were derived by low-pressure crystal fractionation of intermediate magma(s) that are very similar in composition to the much more voluminous andesites of the volcano. They are (1) the coincidence of the whole-rock–residual liquid tie lines, with general variation trends defined by the combined whole-rock and residual liquid data; and (2) the similarity of the coherent interelement variations, with trends inferred from data on elemental distribution between coexisting phenocryst minerals and magmatic liquid. Removal of 45 to 65 percent crystalline material from andesitic magma with 25 to 30 ppm Rb is required to produce the more salic rocks containing 50 to 65 ppm Rb.

A point of phase equilibria interpretation in connection with lavas from the Apollo 12 site

The pattern of phase equilibria, observed as a function of temperature and normative olivine content, in a group of lava samples which are (I) randomly chosen primary magmas; liquids related by fractionation of olivine, with or without the additional separation of plagioclase and pyroxene (II–IV); or (V) liquids related by accumulation of olivine plus pyroxene into a cotectic liquid, is explored using a three-component system analogy. The patterns actually observed in Apollo 12 lava samples belong to type (V), and result principally from local accumulation of a small proportion of olivine, pyroxene and spinel phenocrysts present in liquids which were also close to saturation with plagioclase at low pressure.

Rubidium-Strontium Age and Elemental and Isotopic Abundances of Some Trace Elements in Lunar Samples

Data on six lunar crystalline rocks give an apparent Rb-Sr isochron age of 4.42 +/- 0.24 x 10(9) years (95 percent confidence limits) and initial (87)Sr/(86)Sr ratio similar to that in a basaltic achondrite. Relationships between K, Rb, Sr, and Ba and depletion of Eu in these samples point to plagioclase separation from the melts that produced these rocks. The abundance of (157)Gd in the three lunar samples is similar to terrestrial abundance within < 0.2 percent, thus setting a limit of < 6 x 10(15) neutrons per square centimeter for the integrated thermal neutron flux difference between lunar and terrestrial materials.

FELDSPATHIC INCLUSIONS IN THE GARDAR IGNEOUS ROCKS OF SOUTH GREENLAND AND THEIR RELEVANCE TO THE FORMATION OF MAJOR ANORTHOSITES IN THE CANADIAN SHIELD

Anorthosite xenoliths and plagioclase megacrysts occur in Gardar igneous rocks over an area of approximately 50 000 sq km in South Greenland. The anorthosite inclusions represent a top accumulate of labradorite, which crystallized at a relatively early stage from the developing Gardar magmas at depth, while the megacrysts are regarded as feldspars that remained suspended in their parent magmas for a considerable period at a later stage when the Gardar magmas had developed their dominantly alkaline character. The formation of anorthosite was a continuous process, which probably started about 1300 m.y. ago (the age of the earliest Gardar intrusions) and continued to about 1200 m.y.The Gardar anorthosites formed in the period immediately preceding the development of the Grenville metamorphic belt of the Canadian Shield. They are particularly relevant to the problem of the anorthosites in the Canadian Shield, since they provide evidence for the formation of anorthosites in pre-Grenville, post-Elsonian non-orogenic conditions. It is suggested that the main factor controlling the formation of the two main types of primary anorthosites—those of layered intrusions and those of the almost monomineralic Adirondack type—is the relative specific gravity of the plagioclase and its parent magma. In most layered intrusions, formed from uncontaminated basaltic magmas, the first-formed plagioclases are bytownitic and would sink in their parent magma. In contrast, in alkali provinces such as the Gardar and in orogenic environments where original basaltic magmas may have been contaminated, the first-formed plagioclase is commonly either labradorite or andesine. This will rise in either basaltic or dioritic magma. The separation of plagioclase by flotation from complementary mafic minerals provides a mechanism by which anorthositic rocks can form from basic magmas apparently unaccompanied by mafic layers.Once formed, anorthositic masses of this type would be extremely resistant and could survive several later plutonic episodes as large monomineralic masses in deep-seated mesozonal rocks or the upper katazonal rocks. In contrast the complementary layered mafic rocks would form layered masses in more deep-seated katazonal gneisses.

Petrographic province of Central Montana

Research Article| June 01, 1940 Petrographic province of Central Montana ESPER S. LARSEN ESPER S. LARSEN Search for other works by this author on: GSW Google Scholar Author and Article Information ESPER S. LARSEN Publisher: Geological Society of America Received: 24 Feb 1940 First Online: 02 Mar 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 © 1940 Geological Society of America GSA Bulletin (1940) 51 (6): 887–948. https://doi.org/10.1130/GSAB-51-887 Article history Received: 24 Feb 1940 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ESPER S. LARSEN; Petrographic province of Central Montana. GSA Bulletin 1940;; 51 (6): 887–948. doi: https://doi.org/10.1130/GSAB-51-887 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The petrographic province of Central Montana is arbitrarily defined to include the Tertiary igneous rocks near the eastern flanks of the Rocky Mountains from the Yellowstone National Park on the south to the Canadian boundary on the north (Fig. 1).Each mountain range is made up of one or more groups or subprovinces of igneous rock. The Yellowstone Park area contains at least three subprovinces, the Highwood Mountains four, and the Bearpaw Mountains about seven. The rocks of a subprovince may vary from mafic to felsic, but they are all related in mineral composition, and their chemical analyses fall near smooth variation curves. The mafic rocks of any subprovince differ from those of other subprovinces more than do the felsic rocks. The latter tend toward a granite of about the same composition for all the subprovinces. The mafic rocks of the various subprovinces range from ordinary basalts or gabbros to orthoclase gabbro, to plagioclase shonkinite, to shonkinite without plagioclase. In general, if the mafic rocks of a subprovince are rich in plagioclase all the rocks of that subprovince are rich in plagioclase whereas if the mafic rocks are poor or lacking in plagioclase all the rocks of that subprovince are poor or lacking in plagioclase.The parent magmas of the various subprovinces are believed to be represented by the mafic rocks. The gradational character of these from gabbro to shonkinite leads to the conclusion that they have a common origin. They are believed to have been formed by the slow differentiation at great depth of a basaltic magma by the separation of plagioclase and hypersthene with some olivine.From time to time parts of this changing magma were erupted toward the surface where they further differentiated to form the rocks we see. This latter differentiation was relatively rapid and was by crystal fractionation, modified in all the magmas except the shonkinitic magma by assimilation of granitic material. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.