Phenocryst

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

Hydrothermal alteration of oxygen isotope ratios in quartz phenocrysts, Kidd Creek mine, Ontario: Magmatic values are preserved in zircon

Abstract: Zircons from rhyolites hosting the Kidd Creek volcanogenic massive sulfide deposit preserve magmatic oxygen isotope ratios (δ 18 O = 5.4‰ ± 0.3‰, n = 11) that are indistinguishable from average values of other plutonic and volcanic rocks of the Superior Province. Oxygen isotope ratios of petrographically pristine, primary quartz phenocrysts, however, are greatly elevated in δ 18 O and heterogeneous (δ 18 O = 14.2‰ to 16.1‰). Such phenocrysts are not a good monitor of magmatic processes. The presence of healed microcracks in quartz phenocrysts, as seen in cathodoluminescence images, and the variability of quartz oxygen isotope ratios suggest that recrystallization is the process of δ 18 O elevation. In contrast to the variability of quartz, the δ 18 O zircon values do not vary regardless of the degree of magnetic susceptibility. The δ 18 O values of zircon are interpreted to reflect the primitive, magmatic oxygen, consistent with the rhyolite geochemistry, for magmas derived by partial melting of a tholeiitic source. Our data show that the δ 18 O values of quartz and whole rock at Kidd Creek are a good monitor of hydrothermal exchange and are 5‰ higher than the magmatic value that is preserved in zircon due to its refractory nature.

High-temperature alpine-type peridotite from venezuela

Abstract: The Tinaquillo peridotite is the least serpentinized of the ultramafic masses included in the N. 75 E.-trending belt of Cretaceous metamorphic rocks in Northern Venezuela. It is a south-dipping tabular mass of intensely crushed and foliated dunite bounded on the north by a south-dipping regional thrust fault and on the south by regionally metamorphosed hornblende-quartz-plagioclase gneisses. Near the intrusion, these gneisses have been contact-metamorphosed by the dunite to garnet-augite-hornblende-andesine gneisses. Large tabular masses of gabbroic composition within the peridotite are interpreted as inclusions of contact-metamorphosed country rock. The peridotite contains thin hornblende and pyroxene layers parallel to the foliation and, in addition, remarkable lath-shaped orthopyroxene phenocrysts flattened in the planes of foliation and oriented dimensionally and crystallographically parallel to one another. These laths were formed by the stretching-out of primary equant-shaped phenocrysts, probably during final-stage solid intrusion of the peridotite. The structure and composition of the pyroxene and garnet minerals in the contact rocks indicate that the temperature of intrusion was between 800°C. and 1000°C. During cooling, some of the thin pyroxene layers were altered hydrothermally to hornblende. Later and at still lower temperature, a belt of dunite along the Manrique thrust fault was altered to serpentinite and asbestos. The other peridotites in the coastal range of northern Venezuela, which are moderately to intensely serpentinized, have no contact metamorphic aureoles, and some show conflicting age relationships toward the country rocks. Repeated upward movement during successive orogenies may explain these phenomena associated with serpentinites both in Venezuela and elsewhere.

Interaction of Meteoric Water with Magma: An Oxygen-Isotope Study of Ash-Flow Sheets from Southern Nevada

Abstract: Glassy rocks from compositionally zoned ash-flow sheets and cogenetic lava flows, erupted from the late Miocene Timber Mountain–Oasis Valley caldera complex, show systematic oxygen-isotope variations (90 analyses) among all major phenocryst phases: quartz, sanidine, plagioclase, biotite, augite, and magnetite. Oxygen-isotope compositions of each phenocryst phase become lighter in O18 with decreasing age and are interpreted as indicating major interaction between meteoric ground water and batholithic-sized bodies of silicic magma prior to eruption. Each of the ash-flow sheets is compositionally zoned from silicic and phenocryst-poor at the base of more mafic and phenocryst-rich at the top; this pattern represents in inverse order the original compositional zonation in the magma chamber. The changing isotopic fractionations between phenocrysts also reflect crystallization temperatures that varied vertically over several kilometers in the differentiated magma. Posteruption oxidation and cooling effects have not significantly modified oxygen-isotope compositions of phenocrysts from glassy rocks.

Magma Storage Conditions of Large Plinian Eruptions of Santorini Volcano (Greece)

Abstract: The intensive variables of dacitic-rhyodacitic magmas prior to four large Plinian eruptions of Santorini Volcano over the last 200 kyr (Minoan, Cape Riva, Lower Pumice 2 and Lower Pumice 1) were determined by combining crystallization experiments with study of the natural products, including the volatile contents of melt inclusions trapped in phenocrysts. Phase equilibria of the silicic magmas were determined at pressures of 1, 2 and 4 kbar, temperatures of 850-900°C, fluid (H2O + CO2)-saturation, XH2O [= molar H2O/(H2O + CO2)] between 0*6 and 1 (melt H2O contents of 2-10 wt %), and redox conditions of FMQ (fayalite-magnetite-quartz buffer) or NNO + 1 (where NNO is Ni-NiO buffer). Experiments were generally successful in reproducing the phenocryst assemblage of the natural products. The phase relationships vary significantly among the investigated compositions, revealing a sensitivity to small variations in whole-rock compositions. Our results show that the pre-eruptive storage conditions of the four silicic magmas were all very similar. The magmas were stored at T = 850-900°C and P ≥ 2 kbar, under moderately reduced conditions (ΔNNO = −0*9 to −0*1), and were poor in fluorine (∼500-800 ppm) and sulphur (≤100 ppm), but rich in water and chlorine (5-6 wt % and 2500-3500 ppm, respectively). In all cases, the melts were slightly undersaturated with respect to H2O, but most probably saturated with respect to H2O + Cl ± CO2 and a brine. The Santorini magma plumbing system appears to be dominated by a large, long-lived (≥200 kyr) predominantly silicic magma storage region situated at ≥8 km depth, from which crystal-poor melt batches were extracted during the largest caldera-forming eruptions of the volcanic system.