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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 paper, glass inclusions in olivine phenocrysts from the 1959 Kilauea Iki eruption were analyzed and found to have formed at pressures 2 kbar.
Abstract: Of 50 analyzed glass inclusions in olivine phenocrysts from the 1959 Kilauea Iki eruption, 41 formed at pressures 2 kbar. The surprisingly low formation pressures suggest that most 1959 olivines, including most of those with preeruptive equilibration temperatures above 1200 degrees C, crystallized in an upper part of Kilauea) s summit magma storage reservoir. The implication that the parental magma was buoyant relative to stored magma is consistent with an expected preeruptive bulk CO2 content near 0.2 wt% and published evidence for mixing between hot, newly arrived parental and preexisting magma. That the 1959 magma was rich not only in crystals but also in gas, as evidenced by its high lava fountains, suggests that the storage time in a shallow reservoir was too short for either crystals or gas to be lost. Therefore, the 1959 Kilauean magma probably is a near-parental magma that rose and formed a gas- and crystal-rich cap near the top of a shallow body of stored magma beneath Kilauea) s summit region. Whether newly arriving parental magma is buoyant relative to stored magma depends mainly on pressure and magma gas content. Consequently, it seems likely that the eruptive and degassing behavior of Kilauea is regulated in part by an interplay between the CO, content of parental magma and the pressure at which new magma intrudes stored, degassed magma.

148 citations

Journal ArticleDOI
TL;DR: In this article, anon-microprobe analyses of the Li concentration and Li isotopic composition of zoned clinopyroxene and olivine phenocrysts from within primitive arc lavas from the New Georgia Group in the Solomon Islands reveal that both Li and δ7Li vary widely from rim to core.

148 citations

Journal ArticleDOI
TL;DR: The Coso volcanic field in southeastern California has been studied in this article, showing that inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing.
Abstract: Basaltic lava flows and high-silica rhyolite domes form the Pleistocene part of the Coso volcanic field in southeastern California. The distribution of vents maps the areal zonation inferred for the upper parts of the Coso magmatic system. Subalkalic basalts (<50% SiO2) were erupted well away from the rhyolite field at any given time. Compositional variation among these basalts can be ascribed to crystal fractionation. Erupted volumes of these basalts decrease with increasing differentiation. Mafic lavas containing up to 58% SiO2, erupted adjacent to the rhyolite field, formed by mixing of basaltic and silicic magma. Basaltic magma interacted with crustal rocks to form other SiO2-rich mafic lavas erupted near the Sierra Nevada fault zone. Several rhyolite domes in the Coso volcanic field contain sparse andesitic inclusions (55–61% SiO2). Pillow-like forms, intricate commingling and local diffusive mixing of andesite and rhyolite at contacts, concentric vesicle distribution, and crystal morphologies indicative of undercooling show that inclusions were incorporated in their rhyolitic hosts as blobs of magma. Inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing. Inclusion magma was formed by mixing (hybridization) at the interface between basaltic and rhyolitic magmas that coexisted in vertically zoned igneous systems. Relict phenocrysts and the bulk compositions of inclusions suggest that silicic endmembers were less differentiated than erupted high-silica rhyolite. Changes in inferred endmembers of magma mixtures with time suggest that the steepness of chemical gradients near the silicic/mafic interface in the zoned reservoir may have decreased as the system matured, although a high-silica rhyolitic cap persisted. The Coso example is an extreme case of large thermal and compositional contrast between inclusion and host magmas; lesser differences between intermediate composition magmas and inclusions lead to undercooling phenomena that suggest smaller ΔT. Vertical compositional zonation in magma chambers has been documented through study of products of voluminous pyroclastic eruptions. Magmatic inclusions in volcanic rocks provide evidence for compositional zonation and mixing processes in igneous systems when only lava is erupted.

147 citations

Journal ArticleDOI
TL;DR: In this article, the authors reproduce melt inclusions in plagioclase in a series of partial dissolution experiments, and the compositional differences from the surrounding matrix melt were evaluated, showing that the molar ratio of Ca/(Ca + Na) and the MgO concentration in the synthesized melt inclusion are up to 17 and 61% lower than those in the matrix melt, respectively.

147 citations

Journal ArticleDOI
TL;DR: In this paper, micro-sampling of cm-scale feldspar crystals within an S-type granite from the Lachlan Fold Belt of southeastern Australia has revealed complex internal Sr and Nd isotopic variations.
Abstract: Microsampling of cm-scale feldspar crystals within an S-type granite from the Lachlan Fold Belt of southeastern Australia has revealed complex internal Sr and Nd isotopic variations. The observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system, the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle-like magmas, the latter stages of which are now represented in modified form by microgranular enclaves. Similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation. Feldspar rims have higher 87Sr/86Sri and lower ɛNd(i) than adjacent whole rock analyses, but match those of mineral separates from the surrounding enclave matrix. This suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high 87Sr/86Sr, low ɛNd(i) component such as metasedimentary wall rocks. Isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix, suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma. Our results reveal major isotopic heterogeneities on a single crystal and also inter-mineral scale in a pluton which shows well constrained evidence for magma mingling. These results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems, even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale.

146 citations


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