Bivariate Diagrams Research Articles

Origin of glass in upper mantle xenoliths from the quaternary volcanics of Gees, West Eifel, Germany

Glasses have been analysed from six mantle-derived xenoliths (5 orthopyroxene and/or olivine-rich, 1 clinopyroxene-rich) from the Quaternary volcanics S.E. of Gees, West Eifel, Germany. The glasses in these xenoliths occur as pools surrounding and embaying spinels, as inclusions in spinels, as veins and stringers within phlogopiterich veins, and as jackets partially surrounding some of the xenoliths. Glasses analysed are felsic and characterised by low to intermediate SiO2 (40–62 wt.%), variable CaO (1–11 wt.%) and MgO (1–4 wt.%), high Al2O3 (14–21 wt.%), and up to 11 wt.% Na2O + K2O. The jacket glasses have the lowest SiO2, highest CaO and MgO. Variations in all of the glass compositions are similar and imply a unifying factor or process in their formation. Glass as pools and stringers within veins of phlogopite forms part of the same trends shown by all the glasses when plotted on bivariate (oxide vs SiO2) diagrams but can be distinguished from glass surrounding and enclosed by spinels. Glasses occurring as jackets are similar in composition to those in pools and veinlets associated with phlogopite but are of quite different composition to the glasses found within the xenoliths that they partially enclose. The occurrence and chemistry of the glasses do not support such glasses as representing original or differentiated magma trapped during formation of the xenolithic assemblages. The chemistry of the glasses also makes it unlikely that they were produced by dissociation of phlogopite during ascent of the xenoliths. The most likely origin for the glasses is that they represent volatile-rich melts which migrated through upper mantle material. These melts are likely to be responsible for the heterogeneous nature of the mantle underlying this part of the West Eifel region.

Generation, Diversification, and Emplacement of the Rockford Granite, Alabama Appalachians: Mineralogic, Petrologic, Isotopic (C & O), and P--T Constraints

The Rockford Granite is a plutonic suite of synmetamorphic, two-mica granitoids that intrude graphite-bearing, amphibolite-facies metasediments in the eastern Blue Ridge of the Alabama Appalachians. Geochemical classification indicates peraluminous granite and trondhjemite with subordinate granodiorite. The trondhjemites are products of pervasive, infiltrative Na-metasomatism; whereas the granites and granodiorites sufficiently retain primary traits for petrogenetic modelling. A significant proportion of the Rockford Granite magmatic phases may represent refractory restite, expressed as centimeter-size biotite clots, normal oscillatory zoned plagioclase with a subidiomorphic zone-boundary core, and rounded, zoned xenocrystic zircons. Major and trace element modelling indicate that the Rockford Granite could be a product of 40% equilibrium partial melting of a metagraywacke source producing felsic, granitic melt plus a restite assemblage of plagioclase49-quartz31-biotite14-apatite5 5-zircon0 5. The limited geochemical heterogeneity of the Rockford Granite is attributed to progressive restite-melt separation, reflected by systematic collinearity of calculated source and restite compositions in bivariate diagrams. Whole rock oxygen and reduced carbon isotope analyses support the metagraywacke source model for the Rockford Granite and also record regional isotopic exchange with metamorphic fluids from the volumetrically-dominant metapelites. Combined whole-rock oxygen and carbon isotope determinations both constrain granitoid source rock characteristics and measure the extent of C-O-H metamorphic fluid-rock interaction. Quantitative geothermobarometric estimates for the Rockford Granite and surrounding metasediments indicate kyanite-sillimanite facies series regional metamorphism, with a Barrovian geothermal gradient of 30°C/km. The prograde P-T path is abruptly terminated by nearly isothermal decompression, interpreted as due to diapiric intrusion and associated thermal doming. Rockford Granite and migmatite samples fall in the P-T regime where the maximum overlap between muscovite dehydration and minimum melt conditions exist suggesting that the former controlled anatexis. The tectonic setting and depositional environment of the host metasedimentary protoliths are interpreted as associated with opening of the lapetus Ocean. Structurally lowermost are interlayered amphibolites and quartzites, representing synrift volcanism and sedimentation, overlain by thick passive margin pelitic and graywacke deposits. The Rockford Granite contains a Grenville-age (l·0–1·2 Ga) inherited, xenocrystic zircon component interpreted to record crustal recycling via weathering of a Grenville gneiss source to produce metagraywacke protoliths, followed by regional metamorphism and anatexis to yield the granitic magmas. Barrovian regional metamorphism, associated crustal shortening, and anatexis in an Acadian continental-oceanic subduction regime best represents the tectonomagmatic setting of Rockford Granite genesis.

Geochemical Characteristics of the Nigerian anorogenic province

The anorogenic granitoids of Nigeria are classic examples of ring complexes associated with migrating mid-plate magmatism. Most complexes represent eroded calderas of once active volcanoes, and range from 1 to 30 km in diameter. Petrology varies from peraluminous biotite granite, through hypersthene and fayalite granites to peralkaline aegirine, riebeckite-arfvedsonite granites rich in perthitic alkali feldspar. The complexes have intruded crystalline basement of Pan-African age, and dating indicates a progressive younging to the south. The majority of the Nigerian anorogenic ring-complexes represent the roots of volcanoes with dominantly syenitic to granitic compositions. They are A-type granites of long-time duration, petrologically similar to, but chronologically older than, their counterparts to the north in Niger. In a few northern centres the volcanic rocks are preserved indicating an alkaline trend through hawaiite, mugearite, trachyte to rhyolite. Some mixing of contrasting magma compositions has occurred to produce trachyandesitic suites with deisequilibrium mineral reactions. Mixing can be identified from the major and trace chemical variations and petrological instability reactions. Extensive mineralization by columbite, cassiterite, sphalerite and wolframite suggest fluid interaction on a major scale which has affected much of the original mineral assemblages. In cases of extreme fluid activity, granites have been converted into microclinites. albitites and greisens. Chemical data for major and trace elements have been obtained for a comprehensive suite of Mesozoic volcanic and plutonic rocks from Nigeria. Analyses of the crystalline basement rocks are presented for comparative purposes. Petrogenetic trends have been highlighted using selected bivariate diagrams in an attempt to reach a consensus view of the chemical evolution of the province, with due regard given to the effects of post-intrusion subsolidus fluid interactions. The series Rb, Th, Nb, La, Ce, Zr, Hf, and Y have been selected to monitor crystal-fluid partitioning, with the more compatible elements showing greater partitioning to the fluid phase.

A Practical Guide to the Construction of Bivariate Scatter Diagrams

ABSTRACT A standard rule is given for eliminating the visual bias in bivariate scatter diagrams. Bias is eliminated if the reduced major axis is plotted at an angle of 45° or else the scales chosen for the variables are proportional to the errors of observation. The rule is readily adapted to computing algorithms for generating scatter diagrams.

Weather and Health

AN interesting report, prepared by Dr. Ellsworth Huntington with the advice of a strong committee, based on the daily meteorological and mortality records of the city of New York from April 15, 1882, to Mar. 24, 1888, appears as Bulletin No. 75 of the National Research Council (National Academy of Sciences, Washington). Gross mortalities at ages under five years and over five years are considered separately, also mortalities in these age periods from causes other than pneumonia and influenza. Mortality from pneumonia and influenza (all ages) forms a separate group. The data are expressed as percentages of the daily average of the particular year, and, when necessary, corrections for seasonal trend are introduced. For sufficient reasons, graphical methods are chiefly used, in particular climographs: that is, a third variable, daily deaths, is shown by contour lines on a bivariate diagram, for example, of temperature and relative humidity. It is contended that the method leads to clearer results than the use of correlation coefficients and massed averages.