Showing papers in "Journal of Petrology in 1986"

Subsolidus and Partial Melting Reactions in the Quartz-excess CaO+MgO+Al2O3+SiO2+H2O System under Water-excess and Water-deficient Conditions to 10 kb: Some Implications for the Origin of Peraluminous Melts from Mafic Rocks

Abstract: Experimental results up to 10 kb pressure are presented on the stability of amphibole in the quartz-excess CaO + MgO + Al2O3 + SiO2 + H2O (CMASH) system under H2O-excess and H2Odeficient conditions. Amphibole is stable above the solidus under H2O-excess conditions whereas under H2O-deficient conditions dehydration melting of amphibole-bearing assemblages defines the solidus. The successive appearance of amphibole, talc, and zoisite with increasing pressure considerably modifies the plagioclase-pyroxene-garnet-kyanite reactions documented experimentally in the CaO + MgO + Al2O3 + SiO2 system for gabbro-granulite-eclogite transitions. Although both clinopyroxene and cordierite (with anorthite + orthopyroxene + quartz) may melt eutectically at one atmosphere to form diopside-normative and corundum-normative melts respectively, at higher pressures under H2O-excess conditions the peritectic melting of mafic rock compositions produces corundum-normative liquids together with either clinopyroxene or amphibole. Dehydration melting produces melts which are not corundum-normative. These data are used to discuss the origins and evolution of contrasting basalt-andesite-dacite-rhyolite volcanic suites and granitic plutons, many of whose silicic variants are corundum-normative in character, such as the Toba Tuff ignimbrites, Indonesia (Beddoe-Stephens et ai, 1983) and I-type granite minimum melts (White & Chappell, 1977). In contrast, it is proposed that for the Cascades basalt-andesite-dacite-rhyolite suite the orthopyroxene-plagioclase-quartz thermal divide was maintained up to rhyolite compositions, thereby prohibiting the derivation of corundum-normative rocks from diopside-normative parent magmas. The deduced reaction relations between pyroxenes, amphibole, plagioclase, quartz, and liquid are used to explain the absence or extreme scarcity of hydrous phases in some hydrous magmas. These phase relations can also explain the development of later plagioclase overgrowths on resorbed plagioclase cores in granitic intrusives, and the general absence of resorption and overgrowths in chemically equivalent extrusive rocks. A theoretical analysis of the partial melting of forsterite-bearing assemblages in the CaO + MgO + Al2O3 + SiO2-l-H2O system shows that under H2O-excess conditions partial melting may generate corundum-normative (but low SiO2) melts from a peridotite source at shallow depths.

On the Origin of High-Alumina Arc Basalt and the Mechanics of Melt Extraction

Abstract: Most models of high-alumina arc basalt petrogenesis rely heavily on the supposition that the abundances of certain trace elements, in particular the relatively unfractionated Rare Earth Element (REE) patterns and the unusually high concentrations of K, Rb, Sr, and Ba are incompatible with a garnet-bearing subducted oceanic crustal (quartz eclogite) source rock. We have carefully examined this apparently unequivocal evidence in light of recent progress on the physics of melt extraction and the heat transfer and mechanics of magma ascent. The weakest element of all trace element models involving a quartz eclogite source is the assumption that the element concentrations are fixed at the source and only later modified in the near-surface environment. We expand on such models by monitoring the concentrations of R EE and major and trace elements during initial melting, ascent, and extraction of magma. This is done by combining calculated cooling curves for ascending magmatic bodies with high pressure phase equilibria. The amount that each phase contributes to the melt is monitored along with the composition of the melt and residual solids. With quartz eclogite, initial melting initiates gravitational instability of the entire source material (melt plus solids) before melt extraction can occur. During ascent of this mush, melting increases until the solids can be repacked to free the melt. This extraction takes place some 15-20 km above the slab, after about 50 per cent melting, at which point the melt has a pattern of REE and other trace element concentrations almost identical to those observed in high-alumina arc basalts, assuming an initial composition equivalent to altered oceanic crust plus 5 per cent pelagic sediment. Sr abundances are the only ones which are not well-matched by this process. The major element concentrations of the extracted melt also closely match those of high-alumina arc basalt. A similar, but less detailed evaluation of both fertile and depleted peridotite source rocks yields good agreement for the REE and other trace element concentrations assuming a LREE-enriched source rock strongly enriched in K, Rb, Sr, and Ba. Ni, Cr, and Co abundances are satisfied only through substantial low pressure fractionation of mafic phases, in particular olivine. Though not rigorously tested, such a process may be compatible with the observed major element concentrations of high-alumina basalt. However, the experimentally verified fact that high-alumina basalts could never have been in equilibrium with either an olivine-beari ng magma or source rock eliminates this possibility altogether. Thus, the simultaneous consideration of the mechanics of ascent and melt extraction along with phase equilibria clearly shows that partial melting of quartz eclogite best satisfies the chemical constraints of major, trace, and REE characteristics of high-alumina arc basalts.

Derivation of Internally-Consistent Thermodynamic Data by the Technique of Mathematical Programming: a Review with Application the System MgO-SiO2-H2O

Abstract: The problem of deriving an optimal set of thermodynamic properties of minerals from a diverse experimental data base is reviewed and a preferred methodology proposed. Mathematical programming(MAP) methods extend the linear programming (LIP) approach first presented by Gordon (1973), and make it possible to account for the type of information conveyed, and the uncertainties attending both phase equilibrium data and direct measurements of phase properties. For phase equilibrium data which are (in most cases) characterized by non-normal error distributions across experimental brackets, the midpoint of a bracket is no more probable than other points, and the data are best treated by considering the inequality in the change in Gibbs free energy of reaction at each half-bracket. Direct measurements of phase properties can be assumed to have approximately normal error distributions, and the MAP technique optimizes agreement with these values by using the principles of least squares in the definition of an objective function. The structure of this problem, treatment of uncertainties in various types of experimental data, and method of optimizing final solutions are discussed in some detail. The method is applied to experimental data in the MgO-SiO2-H2O system, where inconsistencies among the data are resolved and an optimal set of thermodynamic properties is presented. The derived standard state entropies and volumes agree with all direct measurements (within their uncertainties), as do enthalpies of formation from the elements except for those of talc (+16 kJ mol" •), anthophyllite (+ 14 kJ mol~'), and brucite (— 1 kJ mol~'). Stable phase relations in the system have the topology predicted by Greenwood (1963, 1971), with quartzand forsterite-absent invariant points at 683 "C-6-4 kb and 797 °C-12 kb respectively, repeating at 552 °C-120 b and 550 °C-55 b. The thermodynamic analysis indicates little remaining flexibility in the phase relations, which, when combined with suitable activity models for solid solution, should allow for accurate determination of the conditions of metamorphism of ultramafic rocks.

Komatiites II: Experimental and Theoretical Investigations of Post-emplacement Cooling and Crystallization

Abstract: When komatiite lavas are emplaced on the sea floor most of the heat transfer occurs through the upper lava-seawater boundary. We have investigated the cooling and crystallization of komatiites using a series of analogue laboratory experiments with aqueous solutions and by theoretical analysis. In komatiites the viscosity is sufficiently low that convection occurs in the interior of the flow and these motions, due both to thermal and compositional variations, have an important influence on the characteristic features of komatiites such as the strong compositional and textural layering. The experiments have been conducted with crystallizing aqueous solutions which display the same overall dynamical processes as solidifying komatiites. The solutions used are simple eutectic systems having the property that crystallization from a solution which is substantially more concentrated than the eutectic composition leaves behind residual fluid which is less dense than the original fluid. This models the decrease in density of komatiite melts on cooling, due to the crystallization of olivine. Such solutions have been cooled strongly through the metal roof of an otherwise insulated container, using a typical fluid depth of 80 mm. Dendritic crystals grew down vertically from the roof and released light fluid, depleted in solute, which rose to form a zone of stagnant fluid at the top of the container, while the tips of the crystals extended just below the bottom of this light layer. A layer of solid eutectic, with a horizontal front, grew more slowly and filled in the space between the vertically oriented crystals. The growth of the crystals and the eutectic layer were monitored visually, and in some experiments the temperatures at the top and in the fluid were recorded, until solidification throughout the layer was complete. The solid block was sampled, and the melted products analysed to give vertical concentration profiles. Both the texture and composition are strongly influenced by the fluid conditions during crystal growth. The top concentration is that of the original solution, rapidly quenched against the roof, and the mean concentration through the region influenced by the stable fluid layer is also close to the original. At the bottom the concentration is high, reflecting the in situ growth of close-packed crystals, and there is a sharp decrease in concentration at an intermediate level, between the upper and lower crystal layers. The experiments and associated theory shed new light on the consolidation of komatiites and the development of their characteristic textures and compositions. Since the lava is convecting within the interior, the early stages of cooling are characterized by a rapid decrease in temperature. Initial cooling rates of 1 to 100 °C h\" 1 are calculated. At this stage the crust remains thin, but as the spinifex zone develops, convection progressively decreases in vigour and the cooling rate decreases. Spinifex texture is considered to form by constitutional supercooling which is controlled by compositional convection. As the spinifex texture develops, the olivine dendrites form a layer of depleted fluid. The tips of the crystals extend beyond this differentiated layer into a convecting lower region and grow preferentially to produce the characteristic vertically oriented spinifex texture. The composition of spinifex zones is shown to be close, but not identical, to the initial liquid composition. The compositional profiles of the solid products of the experiments are similar to those found in komatiites, with the most evolved [Journal of Petrology, Vol. 27, Part 2, pp. 397-437, 1986] © Oxford University Press 1986 at C am bidge U nirsity on July 9, 2010 http://petrooxfordjournals.org D ow nladed fom 398 J. S. TURNER ET AL. rock compositions being found just above the cumulate zone. The experiments also suggest an alternative explanation to crystal settling for the cumulate zone, in which growth of the spinifex zone by compositional convection concentrates crystals suspended within the turbulently convecting lower layer.

Petrogenesis of Dunite Xenoliths from Koolau Volcano, Oahu, Hawaii: Implications for Hawaiian Volcanism

Abstract: Ultramafic xenoliths from Koolau Volcano on the island of Oahu, Hawaii, are divided into spinel lherzolite, pyroxenite, and dunite suites. On the basis of a study of the petrography and mineral compositions of 43 spinel lherzolites, 12 pyroxenites, and 20 dunites, the following characteristics of the dunites in relation to the other nodule types and to Hawaiian lavas emerge. (1) The forsterite content of olivines in the Koolau dunites (Fo82.6-Fo89 7) overlap those of Hawaiian tholeiitic and alkalic lavas and are generally lower than those in abyssal lherzolites and dunites and in Koolau spinel lherzolites. (2) Most of the dunites contain no orthopyroxene, all except two contain chrome spinel, and a few contain interstitial plagioclase and clinopyroxene. (3) Chrome spinels from the Koolau dunites are distinctly higher in Cr/(Cr +Al), lower in Mg/(Mg + Fe2+), and higher in TiO2 than those from abyssal basalts and peridotites. Chrome spinels in the dunites correspond closely in composition to chrome spinels in Hawaiian tholeiitic and alkalic lavas. (4) The abundance of dunite relative to other nodule types decreases outward from the central part of the volcano. The dunites are interpreted not as residues of partial fusion of the mantle but as crystal accumulations stored at shallow depths beneath the central part of Koolau Volcano and derived from picritic magmas parental to the shield-building tholeiitic lavas.