Phenocryst

About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.

The andesite aqueduct: perspectives on the evolution of intermediate magmatism in west-central (105–99°W) Mexico

Abstract: Intermediate calc-alkaline magma (52–65% SiO2) in western-central Mexico is the focus of this paper, and the typically porphyritic andesites (57–65% SiO2) form large central volcanoes, whereas basaltic andesites (52–57% SiO2) are less porphyritic, and they are found as cones and flows but are absent from central volcanoes. Several studies of experimental phase equilibria on these lavas relate water concentration to the phenocryst assemblages and to the degree of crystallinity, so that the abundance, composition and variety of phenocrysts can be used to constrain the amount of water dissolved in the magmas. Thus, the plagioclase-rich andesites of Volcan Colima, Mexico, become so as a result of decompressional crystallisation at ~950 °C (the pyroxene phenocryst temperature), and lose their dissolved water (2.5 to 4.5 wt% H2O) which is inversely proportional to the modal abundance of plagioclase. The feeding magma to V. Colima, North America's most productive central volcano, is represented by hornblende lamprophyre, a lava type without plagioclase phenocrysts which requires at least 6 wt% water to reproduce the phenocryst assemblage. Thus, degassing of the V. Colima magmas, and of those of the other central volcanoes in the western-central Mexican volcanic belt, contributes essentially all their dissolved water to the conduit or to the atmosphere. The source of this magmatic water is related to the source of the intermediate magmas. For some this must lie in the mantle, as the incorporation of hornblende-lherzolite nodules in a hydrous andesite with hornblende phenocrysts could only have occurred while ascending through the mantle. Consistent with a mantle source is the composition of the olivine phenocrysts in Mexican lavas with 10 to 5% MgO, which is in the mantle range of Fo88–92. Accordingly, basaltic andesites and andesites with >5% MgO are candidates for a mantle source. The equilibration of intermediate magmas with the mantle, as illustrated by the experiments of various workers, requires that the magmas be hydrous at pressure. An additional constraint is that the activity of silica in the mantle must be equal to that in the hydrous magma at equilibrium. Using published and new experiments to define RTlnγSiO2 in hydrous liquids, this quantity is shown to vary as a function of liquid composition (H2O, MgO, Na2O+K2O), and it approaches zero for quartz-saturated hydrous liquids. Using appropriate values of RTlnγSiO2 for three intermediate lavas, the amount of water required to equilibrate with an olivine-orthopyroxene mantle source is calculated, and within error indicates that only the most silica-rich magma is at water saturation in the mantle, in agreement with published experimental work. Hydrous intermediate magmas, ascending from their hornblende-lherzolite source regions (~1 to 1.5 GPa) along the hydrous adiabat, may not encounter any phase boundaries until 0.2–0.4 GPa because of the increase in the thermal stability of hornblende in water-undersaturated magmas. Therefore, the phenocryst assemblages of hornblende-free andesites equilibrate at low pressures. The virtual absence of basalt in west-central Mexico ( 50% crystallinity, evidently also an eruptible limit for V. Colima andesitic lavas. If the lower limit of water dissolved in Mexican intermediate magmas is accepted as that required for phenocryst equilibration (~6 wt% water), and the upper limit as saturation in the mantle source at 1 GPa (~16 wt%) then, with an estimate of the volcanic and plutonic magma delivery rate (km3/106 year) per km of volcanic arc, the flux of water returned from the mantle along the 35,000-km, global subduction-related arc system can be estimated. Measurements of the volcanic flux are woefully few, and estimates from Mexico, the Lesser Antilles and central America show a range from 4 to 20 km3/106 year×km which, if subtracted from the isotopically constrained continental growth rate, gives the plutonic flux rate. This suggests that, of the magma flux ascending to the continental crust, only about a fifth reaches the surface. If the dissolved magmatic water limits are coupled with the volcanic and plutonic emplacement rates, then the amount of water returned by magmatism to the crust is crudely in balance with that subducted.

Origin and differentiation of picritic arc magmas, Ambae (Aoba), Vanuatu

Abstract: Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg ∼80 to 93.4) and clinopyroxene (mg ∼80 to 92), and include accessory Cr-rich spinel (cr ∼50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to ∼20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with ∼10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo2 ∼FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has ∼15 wt% MgO and an mg Fe2 value of ∼81. It requires deep (∼3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (T p ∼1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.

Crystal capture, sorting, and retention in convecting magma

Abstract: The mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet-like chamber. A central conclusion is that inward-progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer. Convection is apparently confined to an essentially isoviscous, isothermal region that hugs the downward-growing roof zone. Strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity (N) called "rigid crust" (N ≥ 0.5), "mush" (0.5 ≥ N ≥ 0.25), and "suspension" (N ≤ 0.25). The strong increase in viscosity near the mush-suspension interface acts as a capture front that overtakes and traps slowly settling crystals. Initial phenocrysts mostly escape capture, but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m. Escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself. Crystal-laden plumes traverse the central, hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone. Convection is never vigorous but is part of an overall intimate balance between roofward heat loss, rigid-crust growth, crystallization kinetics, and transport and sorting of sinking escaped crystals. There is a strong similarity between these processes and those producing both varves and saline pan deposits. It is clear that lavas, lava lakes, and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over-all process. These aspects commonly also characterize the large mafic magmatic bodies. Because strong compositional changes in the residual melt occur largely outward (that is, at lower temperatures and higher crystallinities) of the capture front, which is immobile and mostly within rigid crust, the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited. This is in broad agreement with the compositional range observed in basaltic lava lakes, sills, and plutons like Skaergaard. The tuning of convection, crystallization kinetics, and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions.