Rare earth element mobility during granite alteration: Evidence from southwest England

Geochemical analyses of granitic rocks from southwest England reveal that the rare earth elements (REE) were potentially mobile during hydrothermal and supergene alteration. In particular, trivalent REE were removed from the system during K-silicate alteration, Eu was lost during sericitic alteration; all REE were lost during tourmalinization, and light REE were lost during chloritization and argillic alteration. The fluids themselves had low concentrations of REE; in only one case (chloritization) were heavy REE introduced during alteration. Analysis of separated minerals indicated that the behaviour of the REE could be partly explained in terms of their different affinities for the primary and secondary assemblages. Thermodynamic calculations indicated that REE mobility is enhanced by the presence of fluorine in the alteration fluids partly because REE form more stable complexes with F than with Cl and partly because elements such as Ti, Zr and P that form REE-bearing minor phases are themselves potentially mobile.     

Marine sand and gravel production in areas off the northeast coast of the United States

Agencies responsible for issuing permits authorizing excavation of marine sands and gravels have provided information on the amounts of aggregates removed from coastal waters off the New England and Middle Atlantic states. This information is reported herein. Major increases in marine mining are projected. The significance to fisheries of mining marine aggregates is discussed.     

The magnetic properties and morphology of metallic iron produced by subsolidus reduction of synthetic Apollo 11 composition glasses

Metallic iron has been precipitated from a synthetic high-iron, high-titanium Apollo 11 composition glass powder in a furnace with the oxygen fugacity controlled by gas mixing techniques. Measurable quantities of iron, as determined with a vibrating sample magnetometer capable of detecting 0.01 wt% iron in the absence of ferromagnetic minerals, were produced in experiments at temperatures between 700°C and 1045°C, with run times between 3 hr and 95 hr, and oxygen fugacities between 1 and 2 orders of magnitude below the iron-wustite (IW) buffer curve. Such conditions of ?_O2 and T are probably not greatly different from those occurring in a large lunar ejecta blanket. The oxygen fugacity determines the amount of iron produced for a given time and temperature, with about 1% produced if log ?_O2 is 1.4 units below the IW buffer curve and about 3.5% produced if log ?_O2 is 1.7 units below at 990°C. Above 950°C essentially all the iron is multidomain (>300Å) while below 950°C as much as 15% is single domain (150Å–300Å) and an appreciable quantity remains even smaller. Compaction of the sample slows the rate of reduction but does not influence the grain size of metal. The quantities and size distribution of the reduced iron in a number of the experimental runs are strongly analogous to certain lunar soils and breccias and indicate that reduction in an ejected blanket could partly account for the excess iron of lunar soils and breccias relative to the igneous rocks.     

Hydro-geomechanical modelling of seal behaviour in overpressured basins using discontinuous deformation analysis

A coupled hydro-geomechanical modelling environment, developed to evaluate the coupled responses of fluid flow in deforming discontinuous media, is described. A staggered computational framework is presented, where the two simulations tools, HYDRO and DDA, communicate via the mapping of an equivalent porosity (and related permeabilities) from the rock system to the fluid phase and an inverse mapping of the pressure field. Several algorithmic and modelling issues are discussed, in particular the computational procedure to map the current geometry of the discontinuous rock blocks assembly into an equivalent porosity (and permeability) field. A generic, geometrically simple, overpressured reservoir/seal system is analysed for illustration. Further examples investigate discontinuous, fractured configurations in flexure causing a degree of spatial variability in the induced stresses. Model predictions show that the combination of hydraulic and mechanical loads causes a dilational opening of some pre-existing fractures and closure of others, with strong localisation of the modified flow pattern along wider fracture openings.     

Volcano-stratigraphy and geochemistry of collision-related volcanism on the Erzurum–Kars Plateau, northeastern Turkey

The Eastern Anatolia Region exhibits one of the world's best exposed and most complete transects across a volcanic province related to a continental collision zone. Within this region, the Erzurum–Kars Plateau is of special importance since it contains the full record of collision-related volcanism from Middle Miocene to Pliocene. This paper presents a detailed study of the volcanic stratigraphy of the plateau, together with new K–Ar ages and several hundred new major- and trace-element analyses in order to evaluate the magmatic evolution of the plateau and its links to collision-related tectonic processes. The data show that the volcanic units of the Erzurum–Kars Plateau cover a broad compositional range from basalts to rhyolites. Correlations between six logged, volcano-stratigraphic sections suggest that the volcanic activity may be divided into three consecutive Stages, and that activity begins slightly earlier in the west of the plateau than in the east. The Early Stage (mostly from 11 to 6 Ma) is characterised by bimodal volcanism, made up of mafic-intermediate lavas and acid pyroclastic rocks. Their petrography and high-Y fractionation trend suggest that they result from crystallization of anhydrous assemblages at relatively shallow crustal levels. Their stratigraphy and geochemistry suggest that the basic rocks erupted from small transient chambers while the acid rocks erupted from large, zoned magma chambers. The Middle Stage (mostly from 6–5 Ma) is characterised by unimodal volcanism made up predominantly of andesitic–dacitic lavas. Their petrography and low-Y fractionation trend indicate that they resulted from crystallization of hydrous (amphibole-bearing) assemblages in deeper magma chambers. The Late Stage (mostly 5–2.7 Ma) is again characterised by bimodal volcanism, made up mainly of plateau basalts and basaltic andesite lavas and felsic domes. Their petrography and high-Y fractionation trend indicate that they resulted from crystallization of anhydrous assemblages at relatively shallow crustal levels. AFC modelling shows that crustal assimilation was most important in the deeper magma chambers of the Middle Stage. The geochemical data indicate that the parental magma changed little throughout the evolution of the plateau. This parental magma exhibits a distinctive subduction signature represented by selective enrichment in LILE and LREE thought to have been inherited from a lithosphere modified by pre-collision subduction events. The relationships between magmatism and tectonics support models in which delamination of thickened subcontinental lithosphere cause uplift accompanied by melting of this enriched lithosphere. Magma ascent, and possibly magma generation, is then strongly controlled by strike-slip faulting and associated pull-apart extensional tectonics.     

The analysis of zoning in magmatic crystals with emphasis on olivine

Many of the observed features of zoning in magmatic phenocrysts may be due to the orientation of the section rather than inherent properties of the crystals. An ideal section for the studying of zoning in magmatic crystals has two characteristics: it goes through the center of the crystal, and is perpendicular to one or more crystal faces. Using a model zoned olivine crystal, it is possible to construct accurate zoning profiles for different types of section (centered, symmetrical and skewed). The probability of obtaining a random section which passes within x% of the center of a crystal is shown to be P=0.0x, while the probability that a random section will be within A degrees of perpendicular to a given plane is P=sin(A). A systematic approach to the study of zoned crystals is outlined. In particular, it is suggested that composition be plotted against distance cubed, in order to correct for the volume versus size problem. A method of determining if a given section goes through (or near) the center of a zoned crystal is also presented. The reasoning in this work applies to other types of magmatic crystals such as pyroxenes and plagioclase.