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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 authors examine whether, for an initially crystal-free sill solidifying inwardly from top and bottom, crystals nucleated and grown along the roof can escape incorporation, survive resorption in settling through the hotter central region, and reach the basal solidification front to form a cumulate.
Abstract: Basal cumulate zones are common in sills formed of magma-carrying phenocrysts upon emplacement, and the process leading to these cumulates and the corresponding roofward depleted zones are well understood. Many large sills which seem to have been emplaced with phenocryst-free magma, are nearly uniform in composition and show little tendency to differentiate by crystal settling. We examine whether, for an initially crystal-free sill solidifying inwardly from top and bottom, crystals nucleated and grown along the roof can escape incorporation, survive resorption in settling through the hotter central region, and reach the basal solidification front to form a cumulate. We call this process solidification front fractionation and build a relatively simple model to examine the rheologic, dynamic, and crystal growth rate conditions leading to the downward escape of newly grown, single crystals from the roof of a sheet-like body of mafic magma cooling by conduction. Our results suggest that, whereas a thin micro-...

61 citations

Journal ArticleDOI
TL;DR: The petrology and geochemistry of East Island have been investigated for the first time in this article, where X-ray fluorescence analyses were made of 43 lavas for the major elements plus Cr, Ni, Rb, Sr, Ba, Pb, and Th and the minerals were analysed by electron microprobe.
Abstract: The petrology and geochemistry of East Island have been investigated for the first time. The island is a deeply dissected remnant of a Pleistocene shield volcano, one of several emerging from an oceanic rise forming part of the southwest branch of the Indian Ocean ridge system. The lavas form a flat-lying sequence of oceanites, ankaramites, olivine basalts and feldsparphyric basalts, the ankaramites containing 1 cm phenocrysts of diopsidic clinopyroxene. X-Ray fluorescence analyses were made of 43 lavas for the major elements plus Cr, Ni, Rb, Sr, Ba, Pb, and Th and the minerals were analysed by electron microprobe. The elements Mg, Cr, and Ni are strongly concentrated in spinel, olivine and clinopyroxene phases and in the ankaramites and oceanite lavas with maximum concentrations of 18% MgO, 1,000 ppm Cr, 380 ppm Ni, while Al, Ti, K, Rb, Ba, Th, Na, P, Sr concentrate in the groundmass and in the feldspathic and aphyric basalts. The elements Si, Ca, Fe and Mn remain virtually constant throughout the series.

61 citations

Journal ArticleDOI
TL;DR: In this article, the authors used the balance of dispersive pressure to deduce the velocity profile of the magma flowing through a picritic dike from the observed distribution of olivine phenocrysts.
Abstract: Mechanical interactions between phenocrysts during magma flow give rise to a grain dispersive pressure. Considerations of the balance of the dispersive pressure are utilized to deduce the velocity profile of the magma flowing through a picritic dike from the observed distribution of olivine phenocrysts. The dike, from the Isle of Skye, Scotland, has a gradual increase in phenocryst concentration from the walls to the center. The analysis of this dike yields a pluglike velocity profile in which the velocity is within 1 percent of the maximum velocity throughout the central half of the dike, with large velocity gradients near the walls. Factors such as a viscosity increase toward the walls due to cooling and variations in the proportionality coefficient in the grain dispersive relationship are considered in the analysis. Other possible corrections required to improve the results are indicated. The velocity profile obtained in the analysis is first compared to profiles for pseudoplastic and Bingham-plastic non-Newtonian fluids. Poor agreement is found, and it is concluded that non-Newtonian fluid models are unsatisfactory. The data are then compared to a model in which the pluglike velocity profile results from the simultaneous requirement of a constant grain dispersive pressure and a balance between the pressure gradient producing the magma intrusion and the viscous dissipation of its momentum. This comparison was successful, and I concluded that such balances cause the pluglike velocity profile as well as the increase in phenocryst concentration toward the dike center.

61 citations

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate that the color variations correspond to both textural and compositional variations between the clast types, and they suggest that grey pumice provides evidence not only for cryptic silicic recharge in a large caldera system but also a probable trigger for the eruption.
Abstract: Triggering mechanisms of large silicic eruptions remain a critical unsolved problem. We address this question for the ~2.08-Ma caldera-forming eruption of Cerro Galan volcano, Argentina, which produced distinct pumice populations of two colors: grey (5%) and white (95%) that we believe may hold clues to the onset of eruptive activity. We demonstrate that the color variations correspond to both textural and compositional variations between the clast types. Both pumice types have bulk compositions of high-K, high-silica dacite to low-silica rhyolite, but there are sufficient compositional differences (e.g., ~150 ppm lower Ba at equivalent SiO2 content and 0.03 wt.% higher TiO2 in white pumice than grey) to suggest that the two pumice populations are not related by simple fractionation. Trace element concentrations in crystals mimic bulk variations between clast types, with grey pumice containing elevated Ba, Cu, Pb, and Zn concentrations in both bulk samples (average Cu, Pb, and Zn concentrations are 27, 35, and 82 in grey pumice vs. 11, 19, and 60 in white pumice) and biotite phenocrysts and white pumice showing elevated Li concentrations in biotite and plagioclase phenocrysts. White and grey clasts are also texturally distinct: White pumice clasts contain abundant phenocrysts (44–57%), lack microlites, and have highly evolved groundmass glass compositions (76.4–79.6 wt.% SiO2), whereas grey pumice clasts contain a lower percentage of phenocrysts/microphenocrysts (35–49%), have abundant microlites, and have less evolved groundmass glass compositions (69.4–73.8 wt.% SiO2). There is also evidence for crystal transfer between magma producing white and grey pumice. Thin highly evolved melt rims surround some fragmental crystals in grey pumice clasts and appear to have come from magma that produced white pumice. Furthermore, based on crystal compositions, white bands within banded pumice contain crystals originating in grey magma. Finally, only grey pumice clasts form breadcrusted surface textures. We interpret these compositional and textural variations to indicate distinct magma batches, where grey pumice originated from an originally deeper, more volatile-rich dacite recharge magma that ascended through and mingled with the volumetrically dominant, more highly crystalline chamber that produced white pumice. Shortly before eruption, the grey pumice magma stalled within shallow fractures, forming a vanguard magma phase whose ascent may have provided a trigger for eruption of the highly crystalline rhyodacite magma. We suggest that in the case of the Cerro Galan eruption, grey pumice provides evidence not only for cryptic silicic recharge in a large caldera system but also a probable trigger for the eruption.

61 citations

Journal ArticleDOI
TL;DR: In this article, the authors estimated time scales of magma-mixing processes just prior to the 2011 sub-Plinian eruptions of Shinmoedake volcano to investigate the mechanisms of the triggering processes of these eruptions.
Abstract: We estimated time scales of magma-mixing processes just prior to the 2011 sub-Plinian eruptions of Shinmoedake volcano to investigate the mechanisms of the triggering processes of these eruptions. The sequence of these eruptions serves as an ideal example to investigate eruption mechanisms because the available geophysical and petrological observations can be combined for interpretation of magmatic processes. The eruptive products were mainly phenocryst-rich (28 vol%) andesitic pumice (SiO2 57 wt%) with a small amount of more silicic pumice (SiO2 62–63 wt%) and banded pumice. These pumices were formed by mixing of low-temperature mushy silicic magma (dacite) and high-temperature mafic magma (basalt or basaltic andesite). We calculated the time scales on the basis of zoning analysis of magnetite phenocrysts and diffusion calculations, and we compared the derived time scales with those of volcanic inflation/deflation observations. The magnetite data revealed that a significant mixing process (mixing I) occurred 0.4 to 3 days before the eruptions (pre-eruptive mixing) and likely triggered the eruptions. This mixing process was not accompanied by significant crustal deformation, indicating that the process was not accompanied by a significant change in volume of the magma chamber. We propose magmatic overturn or melt accumulation within the magma chamber as a possible process. A subordinate mixing process (mixing II) also occurred only several hours before the eruptions, likely during magma ascent (syn-eruptive mixing). However, we interpret mafic injection to have begun more than several tens of days prior to mixing I, likely occurring with the beginning of the inflation (December 2009). The injection did not instantaneously cause an eruption but could have resulted in stable stratified magma layers to form a hybrid andesitic magma (mobile layer). This hybrid andesite then formed the main eruptive component of the 2011 eruptions of Shinmoedake.

60 citations


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