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Phenocryst

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


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TL;DR: In this article, the plagioclase phenocrysts are divided into three parts: core, mantle, and rim, and the inner rim is typically oscillatory zoned.
Abstract: Phyric basalts recovered from DSDP Legs 45 and 46 contain abundant plagioclase phenocrysts which occur as either discrete single grains (megacrysts) or aggregates (glomerocrysts) and which are too abundant and too anorthitic to have crystallized from a liquid with the observed bulk rock composition. Almost all the plagioclase crystals are complexly zoned. In most cases two abrupt and relatively large compositional changes associated with continuous internal morphologic boundaries divide the plagioclase crystals into three parts: core, mantle and rim. The cores exhibit two major types of morphology: tabular, with a euhedral to slightly rounded outline; or a skeletal inner core wrapped by a slightly rounded homogeneous outer core. The mantle region is characterized by a zoning pattern composed of one to several spikes/plateaus superimposed on a gently zoned base line, with one large plateau always at the outside of the mantle, and by, in most cases, a rounded internal morphology. The inner rim is typically oscillatory zoned. The width of the outer rim can be correlated with the position of the individual crystal in the basalt pillow. The presence of a skeletal inner core and the concentration of glass inclusions in low-An zones in the mantle region suggest that the liquid in which these parts of the crystals were growing was undercooled some amount. The resorption features at the outer margins of low-An zones indicate superheating of the liquid with respect to the crystal.

114 citations

Journal ArticleDOI
TL;DR: The phase relations of an uncontaminated (primitive), ferropicrite lava from the base of the Early Cretaceous Parana´- Etendeka continental flood basalt province have been determined between 1 atm and 7GPa.
Abstract: The anhydrous phase relations of an uncontaminated (primitive), ferropicrite lava from the base of the Early Cretaceous Parana´- Etendeka continental flood basalt province have been determined between 1 atm and 7GPa. The sample has high contents of MgO (� 14� 9 wt %), FeO* (14� 9 wt %) and Ni (660ppm). Olivine phenocrysts have maximum Fo contents of 85 and are in equilibrium with the bulk rock, assuming a K Olliquid DFeMg of 0� 32. A comparison of our results with previous experimental studies of high-Mg rocks shows that the high FeO content of the ferropicrite causes an expan- sion of the liquidus crystallization field of garnet and clinopyroxene relative to olivine; orthopyroxene was not observed in any of our experiments. The high FeO content also decreases solidus temper- atures. Phase relations indicate that the ferropicrite melt last equili- brated either at � 2� 2GPa with an olivine-clinopyroxene residue, or at � 5GPa with a garnet-clinopyroxene residue. The low bulk-rock Al2O3 content (9 wt %) and high (Gd/Yb)n ratio (3� 1) are consistent with the presence of residual garnet in the ferropicrite melt source and favour high-pressure melting of a garnet pyroxenite source. The garnet pyroxenite may represent subducted oceanic litho- sphereentrainedbytheupwellingTristanstartingmantleplumehead. During adiabatic decompression, intersection of the garnet pyroxenite solidus at � 5GPa would occur at a mantle potential temperature of � 1550 � C and yield a ferropicrite primary magma. Subsequent melting of the surrounding peridotite at � 4� 5GPa may be restricted by the thickness of the overlying sub-continental lithosphere, such that dilution of the garnet pyroxenite melt component would be signific- antly less than in intra-oceanic plate settings (where the lithosphere is thinner). This model may explain the limited occurrence of ferropicrites at the base of continental flood basalt sequences and their apparent absence in ocean-island basalt successions.

114 citations

Journal ArticleDOI
TL;DR: In this paper, infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at Pantelleria, Italy, show that the magmas contained moderate pre-eruptive H2O contents, ranging from 1.4 to 2.1 wt%.
Abstract: Infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at Pantelleria, Italy, show that the magmas contained moderate pre-eruptive H2O contents, ranging from 1.4 to 2.1 wt.%. Melt H2O concentrations increase linearly with incompatible elements, demonstrating that H2O contents were not buffered significantly during fractionation by any crystalline or vapor phase. The relatively low H2O contents of pantellerites are consistent with an origin by partial melting of alkali gabbros rather than fractional crystallization of basalt. Preeruptive H2O concentrations do not correlate with the volume or explosivity of pantellerite eruptions; decompression history is critical in determining the style of pantellerite (and other) eruptions.

113 citations

Journal ArticleDOI
TL;DR: In this article, the earliest fluids to circulate are recorded in potassically altered (orthoclase- and biotite-stable) veins as vapor-rich H 2 O-CO 2 inclusions and hypersaline (30-70 equiv. wt % NaCl + or - KCl) inclusions which homogenize over a wide temperature range from 240 degrees to greater than 800 degrees C.
Abstract: Mineralogic and fluid inclusion investigations conducted in the laboratory combined with in situ observations of temporal relationships among geologic features demonstrate that systematic changes in hydrothermal fluid characteristics occurred with time during evolution of mineralization and alteration of the granodiorite stock at the Santa Rita porphyry copper deposit. The earliest fluids to circulate are recorded in potassically altered (orthoclase- and biotite-stable) veins as vapor-rich H 2 O-CO 2 inclusions and hypersaline (30-70 equiv. wt % NaCl + or - KCl) inclusions which homogenize over a wide temperature range from 240 degrees to greater than 800 degrees C. The high-salinity fluid inclusions are complex chloride brines with significant concentrations of sodium, potassium, copper, iron, and sulfur, but sulfide minerals are not associated with the early potassic assemblage produced by this fluid. Oxygen isotope ratios of quartz in the orthoclase- and biotite-stable veins are identical to phenocryst quartz in the intrusion (8.2ppm). Later solutions circulated through newly formed fractures and older reopened veins, and are recorded in low-salinity (less than 15 equiv. wt % NaCl) fluid inclusions which homogenize primarily in the temperature range of 220 degrees to 360 degrees C. These hydrothermal fluids formed two distinct alteration assemblages as they reacted with potassically altered rock: chlorite is the dominant silicate gangue mineral of the earlier chalcopyrite-bearing, orthoclase- and clay-stable assemblages and sericite characterizes the later phyllic alteration assemblage with quartz and pyrite. Oxygen isotope ratios of quartz of the later assemblages (8.6-10.4ppm) are heavier than quartz of the earlier potassic assemblages.It is concluded that the earliest hydrothermal fluids to circulate within the stock localizing the orebody at Santa Rita could have been derived by phase separations of fluids exsolved from crystallizing magma into condensed, hypersaline liquids and H 2 O-CO 2 vapors. As these fluids moved through fractures, potassic alteration of rock occurred without concomitant deposition of sulfides. Later solutions of dominantly meteoric origin progressively formed hypogene copper mineralization associated with chlorite-stable alteration and then phyllic alteration.

113 citations

Journal ArticleDOI
TL;DR: In this article, in situ Pb/U ion microprobe (SHRIMP-RG) and detailed cathodoluminescence imaging was used to analyze zircons from the Devils Kitchen rhyolite in the Pleistocene Coso Volcanic field, California.
Abstract: Zircons from the Devils Kitchen rhyolite in the Pleistocene Coso Volcanic field, California have been analyzed by in situ Pb/U ion microprobe (SHRIMP-RG) and by detailed cathodoluminescence imaging. The zircons yield common-Pb-corrected and disequilibrium-corrected Pb/U ages that predate a previously reported K–Ar sanidine age by up to 200 kyr, and the range of ages exhibited by the zircons is also approximately 200 kyr. Cathodoluminescence imaging indicates that zircons formed in contrasting environments. Most zircons are euhedral, and a majority of the zircons are weakly zoned, but many also have anhedral, embayed cores, with euhedral overgrowths and multiple internal surfaces that are truncated by later crystal zones. Concentrations of U and Th vary by two orders of magnitude within the zircon population, and by 10–20 times between zones within some zircon crystals, indicating that zircons were transferred between contrasting chemical environments. A zircon saturation temperature of 750 C overlaps within error a previously reported phenocryst equilibration temperature of 740 25 C. Textures in zircons indicative of repeated dissolution and subsequent regrowth are probably caused by punctuated heating by mafic magma input into rhyolite. The overall span of ages and large variation in U and Th concentrations, combined with calculated zircon saturation temperatures and resorption times, are most compatible with crystallization in magma bodies that were emplaced piecemeal in the crust at Coso over 200 kyr prior to eruption, and that were periodically rejuvenated or melted by subsequent basaltic injections.

113 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202399
2022142
2021105
2020100
2019103
2018109