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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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Journal ArticleDOI
01 Sep 2010-Lithos
TL;DR: The high-Mg rocks vary in composition from LREE-depleted, low-Ti komatiites to LREEenriched, high-Ti picrites in the Emeishan Large Igneous Province (ELIP) as mentioned in this paper.

87 citations

Journal ArticleDOI
TL;DR: The composition of primary magmas and their mantle sources can be successfully inferred from the study of melt inclusions trapped in spinel phenocrysts as discussed by the authors, particularly in the case of severely altered rocks, in which spinel and spinel-hosted melt-inclusions usually retain primary magmatic information.

87 citations

Journal ArticleDOI
17 Jan 2013-Nature
TL;DR: It is proposed that a less-degassed nickel-rich source formed by core–mantle interaction during the crystallization of a melt-rich layer or basal magma ocean, and that this source continues to be sampled by mantle plumes.
Abstract: High (3)He/(4)He ratios in some basalts have generally been interpreted as originating in an incompletely degassed lower-mantle source. This helium source may have been isolated at the core-mantle boundary region since Earth's accretion. Alternatively, it may have taken part in whole-mantle convection and crust production over the age of the Earth; if so, it is now either a primitive refugium at the core-mantle boundary or is distributed throughout the lower mantle. Here we constrain the problem using lavas from Baffin Island, West Greenland, the Ontong Java Plateau, Isla Gorgona and Fernandina (Galapagos). Olivine phenocryst compositions show that these lavas originated from a peridotite source that was about 20 per cent higher in nickel content than in the modern mid-ocean-ridge basalt source. Where data are available, these lavas also have high (3)He/(4)He. We propose that a less-degassed nickel-rich source formed by core-mantle interaction during the crystallization of a melt-rich layer or basal magma ocean, and that this source continues to be sampled by mantle plumes. The spatial distribution of this source may be constrained by nickel partitioning experiments at the pressures of the core-mantle boundary.

87 citations

Book ChapterDOI
01 Jan 1985
TL;DR: The origin of monocrystalline quartz, the dominant detrital component of most sandstones, can still not be determined quantitatively with the petrographic microscope and published classifications as discussed by the authors.
Abstract: The origin of monocrystalline quartz, the dominant detrital component of most sandstones, can still not be determined quantitatively with the petrographic microscope and published classifications. With the aid of a technically improved cathodoluminescence microscope that enables the study of low luminescent minerals, six classes of monocrystalline former high-quartz are distinguished and used as a guide to provenance. Quartz crystals from plutonic rocks and phenocrysts from volcanites show a wide variation of luminescence colours from blue through mauve to violet. Volcanic quartz phenocrysts usually show a zonation or irregular distribution of luminescence colours which generally enables distinction from plutonic quartz. Red luminescing quartz is also of volcanic origin and crystallizes at lower temperatures than phenocrysts. Quartz crystals which have been plastically deformed, as evidenced by strong undulatory extinction, luminesce bluish-black whereas brown quartz is derived from regional metamorphic rocks. Metamorphic quartz that recrystallised at high temperatures (hornfels, granulites) reverts to a blue luminescence colour comparable to plutonic quartz. Luminescence petrography also allows discrimination between various types of feldspars and rock fragments and to estimate quantitatively their abundance. Furthermore, cathodoluminescence enables detrital grains to be distinguished from syntaxial overgrowths in well cemented sandstones so that the original grain size and roundness parameters can be determined.

87 citations

Journal ArticleDOI
TL;DR: In this paper, the authors divided the Northern Canadian Cordilleran Miogeocline volcanic rocks into five main groups, namely, potassic and ultrapotassic basanites, based on geochemistry and geographic location.
Abstract: The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and NbIY and low ZrINb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group 11 rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher ZrINb, and lower NbIY. Group I11 rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and I1 rocks, and contain elevated Ba contents. Groups 1-111 are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and I1 basalts. Groups I-111 are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults.

87 citations


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