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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, high and low-temperature modifications of albite are shown to have X-ray and optical properties different than those of the very pure natural albites from pegmatites, but these natural albits can, by heating, be converted into a form identical with the synthetic product.
Abstract: Synthetic albite is found to have X-ray and optical properties different than those of the very pure natural albites from pegmatites, but these natural albites can, by heating, be converted into a form identical with the synthetic product. High- and low-temperature modifications of albite are thus indicated. The inversion temperature is believed to be in the neighborhood of 700° C. Optical and X-ray studies show that the inversion extends into the plagiocla.se series approximately to $$An_{35}$$. These high-temperature plagioclases are common as phenocrysts in extrusive rocks but have not been found in plutonic rocks.

88 citations

Journal ArticleDOI
TL;DR: In this article, trace element data for olivines from the Karoo CFB Tuli and Mwenezi picrites and the Etendeka CFB Horingbaai/LTZ-L type picrites, all of which erupted in regions of thickened (>90 km) lithosphere in southern Africa.

88 citations

Journal ArticleDOI
TL;DR: In this paper, microprobe analyses of pyroxene phenocrysts occurring in two tephritic leucitites, two leucite phonolites and one trachyte from Vulsini are reported.

88 citations

Journal ArticleDOI
TL;DR: The Galapagos spreading center has produced a suite of volcanic rocks which include (1) typical ocean floor tholeiites, (2) iron-and titanium-enriched basalt and low-alumina andesite, and (3) rhyodacites.
Abstract: The Galapagos spreading center has produced a suite of volcanic rocks which include (1) typical ocean floor tholeiites, (2) iron- and titanium-enriched tholeiites, (3) andesites, and (4) rhyodacites. Interpretation of major element compositions determined for these rocks, associated glassy selvedges, and mineral phases suggests that the entire suite can be produced by fractional crystallization of the observed low-pressure phenocrysts. Early fractionation to extreme iron and titanium enrichment requires 50–75% crystallization of five parts plagioclase, three parts augite, and one part olivine. In the highly fractionated basalts, pigeonite replaces olivine as a phenocryst phase and is a minor phase in the fractionation. At extreme iron and titanium enrichment, titanomagnetite becomes a phenocryst phase, and fractionation produces a silica enrichment trend to andesite (79% total crystallization) and rhyodacite (87% total crystallization). Apatite occurs as a microphenocryst phase in the silicic rocks. This suite of iron- and titanium-enriched basalt and low-alumina andesite differs from the calc-alkaline suite of island arcs and continental margins. It is similar to the suite of fractionated lavas of the Galapagos Islands and Iceland, though these rocks are generally more alkaline than the Galapagos spreading center suite, which is similar to other ocean floor basalt suites though more fractionated. Magma mixing is suggested by the presence of basaltic xenoliths in the silicic rocks; the presence of high-silica glass inclusions in the glassy selvedges of the basalts, andesite, and rhyodacite; the common occurrence of reversely zoned phenocrysts in the silicic rocks; and the occurrence of some highly fractionated yet aphyric rocks. The unusual extent of fractionation for this region may be related to the size, morphology, and eruption and resupply rates of the magma chambers associated with the Galapagos spreading center. This and the regional distribution of these highly fractionated rocks suggest an important, though indirect, influence of the Galapagos hot spot.

88 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the mineralogy and chemistry of picrites from Mauna Loa and Kilauea and evaluated, for Hawaiian tholeiitic picrites, the contrasting genetic models which have been proposed for these Mg-rich volcanics, namely products of direct crystallization of high-Mg melts (20-25% MgO) or the result of accumulation of olivine phenocrysts into less Mgrich melts.
Abstract: The mineralogy and chemistry of picrites from Mauna Loa and Kilauea have been investigated to evaluate, for Hawaiian tholeiitic picrites, the contrasting genetic models which have been proposed for these Mg-rich volcanics, namely products of direct crystallization of high-Mg melts (20–25% MgO) or the result of accumulation of olivine phenocrysts into less Mg-rich melts. Genetic interpretations rely heavily on Mg-Fe partitioning relations between olivines and their picrite hosts. Although the 100 Mg/(Mg + Fe2+) ratios (M) of picrites are wide-ranging (M=73.6–82.9 for Fe2O3/FeO=0.15), with MgO as high as 27.8%, the average 100 Mg/(Mg+Fe) ratios (mg) of the cores of olivine phenocrysts (megacrysts) show only restricted compositional variation (mg=87.2–89.0). Successive olivine generations are progressively more Fe-rich. Olivine/liquid Mg-Fe partitioning data and the Mn and Ni abundances in olivine phenocrysts collectively indicate that they were precipitated by Mg-rich basaltic melts with 12–14% MgO. Spinel compositions range from liquidus magnesiochromites, occurring mainly as inclusions in olivine phenocrysts, to groundmass titanomagnetites which crystallized at nearsolidus temperatures. The Cr2O3 contents and M values of liquidus magnesiochromites suggest that their parent melts were neither Mg-rich picritic (MgO>20%) nor relatively Mg-poor basaltic types. On MgO variation diagrams (extending from approximately 7% to more than 25% MgO), Mauna Loa and Kilauea picrites and their respective microcrystalline/glassy groundmasses (the major component of quickly-cooled picrites) plot on linear regression lines (‘olivine control lines’). At a given MgO content, Kilauean picrites and tholeiites (M<75) generally contain more TiO2 FeO t , CaO, K2O and P2O5, and less SiO2 and Na2O than Mauna Loan types. The compositions of the groundmasses in picrites and Mg-rich ol-tholeiites equate closely with those of the Mg-poor tholeiites (7–9% MgO) which dominate the petrology of each shield. Low-pressure closed system differentiation of Hawaiian tholeiitic magmas (10–15% MgO) can yield picritic derivatives which differ, however, from the extrusive picrites by virtue of distinctly higher FeO t contents and correspondingly more Fe-rich olivines and Cr-spinels. The calculated Mg-Fe olivine megacryst-‘liquid’ partition coefficient K D for individual picrites indicate that lowpressure equilibria (K D =0.30–0.34) are defined only by melts with approximately 12–14% MgO (M∼ 71–74). Assessed in conjunction with Ni-MgO modeling, these data indicate that the more Mg-rich picrites (MgO> 14–15%) are indeed olivine-enriched and do not represent melt compositions. Olivine enrichment resulted from post-eruptive mechanical (flow) differentiation of extruded ‘mushes’ of intratelluric cognate olivine phenocrysts (mg∼88) and tholeiitic melts (M∼60), which are ‘residua’ of the parental magmas (12–14% MgO), following the crystallization of the olivine phenocrysts. The ‘parental’ magmas of both picrite suites were generated by 35–40% melting of relatively Fe-rich spinel lherzolites (mg∼84) containing kaersutitic amphibole as a major primary constituent.

88 citations


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