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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: Local saturation is an attractive mechanism for enhancing fractionation during crystallization differentiation as discussed by the authors, but it is not a process that applies to all cases and some inconsistencies remain in local saturation origin for accessory phases that cannot be evaluated without additional information.

183 citations

Journal ArticleDOI
TL;DR: In this paper, it was shown that andesites are generated through the interaction of mantle-derived magmas with lower crustal melts and restites and differences between Ruapehu and Taranaki andesite reflect contrasting physical conditions during melting as well as subtle variations in the mantle and crustal source compositions.

181 citations

Journal ArticleDOI
TL;DR: The Taylor Creek Rhyolite of southwest New Mexico comprises at least 20 lava domes and flows, each of the lavas was erupted from its own vent, and the vents are distributed throughout a 20 km by 50 km area.
Abstract: The Tertiary Taylor Creek Rhyolite of southwest New Mexico comprises at least 20 lava domes and flows. Each of the lavas was erupted from its own vent, and the vents are distributed throughout a 20 km by 50 km area. The volume of the rhyolite and genetically associated pyroclastic deposits is at least 100 km3 (denserock equivalent). The rhyolite contains 15%–35% quartz, sanidine, plagioclase, ±biotite, ±hornblende phenocrysts. Quartz and sanidine account for about 98% of the phenocrysts and are present in roughly equal amounts. With rare exceptions, the groundmass consists of intergrowths of fine-grained silica and alkali feldspar. Whole-rock major-element composition varies little, and the rhyolite is metaluminous to weakly peraluminous; mean SiO2 content is about 77.5±0.3%. Similarly, major-element compositions of the two feldsparphenocryst species also are nearly constant. However, whole-rock concentrations of some trace-elements vary as much as several hundred percent. Initial radiometric age determinations, all K−Ar and fission track, suggest that the rhyolite lava field grew during a period of at least 2 m.y. Subsequent 40Ar/39Ar ages indicate that the period of growth was no more than 100 000 years. The time-space-composition relations thus suggest that the Taylor Creek Rhyolite was erupted from a single magma reservoir whose average width was at least 30 km, comparable in size to several penecontemporaneous nearby calderas. However, this rhyolite apparently is not related to a caldera structure. Possibly, the Taylor Creek Phyolite magma body never became sufficiently volatile rich to produce a large-volume pyroclastic eruption and associated caldera collapse, but instead leaked repeatedly to feed many relatively small domes and flows.

179 citations

Journal ArticleDOI
TL;DR: In this article, the authors show that the diffusivities calculated using the standard approach actually represent the product of the true volume diffusivity (D) and the helium solubility, as represented by the distribution coefficient KDv (defined by CcrystalCfluid).

179 citations

Journal ArticleDOI
TL;DR: Ewart et al. as discussed by the authors inferred that the mafic lavas are derived from the Goboboseb-Messum Centre and the Messum Carter Basalts (MCB) and showed evidence for crystal fractionation, have 'arc-like' trace element signatures, correlated e-SiO, e-Ti/γ and -Ti/Zr.
Abstract: The Goboboseb Mountains and Messum Complex represent a major Cretaceous (132 Ma) bimodal eruptive centre in the southern Etendeka continental flood basalt (CFB) province. The eruptives compris the Awahab Formation and are represented by a lower sequence of mafic lavas, followed by the Goboboseb quartz latite members, the Messum Mountain Basalts, and finally the Springbok quartz latite. The sequence is cut by numerous dolerite dykes, sills and plugs, rare rhyolite, and carbonatite. The mafic lavas comprise two distinct series, which although corresponding broadly to the Etendeka regional low Ti and Zr basalts (LTZ type), are distinguished by Ti/Zr ratios into the LTZ.H (higher Ti/Zr) and LTZ.L (lower Ti/Zr) series. The LTZ.H basalts have no previously described extrusive equivalent in the Etendeka (or Parana) CFB, and consist of magnesian, mildly alkaline to tholeiitic lavas, dominated by oliv + cpx phenocryst assemblages which fractionate (near the surface) to phono-tephrite. They are identified as predominantly mantle plume melts (Tristan-Walvis plume). The LTZ.L lavas are less magnesian, extending to icelandites, are tholeiitic, with cpx ± oliv + pl + Fe-Ti oxide phenocryst assemblages and groundmass pigeonite and subcalcic augite. Stratigraphically, the LTZ.H lavas are interbedded with LTZ.L types in the lower part of the sequence and also occur as dykes. Within the Messum Complex, a remnant early sequence of basalts (Messum Carter Basalts) are in part transitional between the LTZ.L and LTZ.H series. The LTZ.H, and at least some of the LTZ.L lavas are inferred to have been erupted from the Goboboseb-Messum Centre. Chemically, the LTZ.H melts are broadly intermediate between E-MORB and OIB magmas, with higher Ti/Zr Sm/ γb and Ti/γ ratios than the LTZ.L types, which suggest segregation depths between the garnet and spinel peridotite stability fields. The Pb-Nd-Sr isotopic compositions of the LTZ.H eruptives are similar to, but not identical with the modern Tristan plume composition, and the observed variability is attributable to limited lower-crust assimilation and/or Atlantic MORB source mixing. The LTZ.L lavas show evidence for crystal fractionation, have 'arc-like' trace element signatures, correlated e-SiO, e-Ti/γ and -Ti/Zr, e-1/Sr and 1/Nd-e variations, and relatively radiogenic Pb, evolved Sr(e, 58-174) and low Nd(e -6·1 to -9·5) isotopic compositions. Their geochemistry is inferred to be AFC (assimilation-fractional crystallization) controlled, and is modelled by three-component mixing involving mantle plume derived melt, mafic lower crust and silicic mid-upper crust. The voluminous quartz latites (Part II, Ewart et al., 1978) extend these geochemical trends.

178 citations


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