Topic
Phenocryst
About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.
Papers published on a yearly basis
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Papers
TL;DR: In this article, the authors studied the effect of pre-emptive and preemptive gradients in T and O 2 in a variety of compositionally zoned ash flow tuffs.
Abstract: Every large eruption of nonbasaltic magma taps a magma reservoir that is thermally and compositionally zoned. Most small eruptions also tap parts of heterogeneous and evolving magmatic systems. Several kinds of compositionally zoned ash flow tuffs provide examples of preemptive gradients in T and ƒO2, in chemical and isotopic composition, and in the variety, abundance, and composition of phenocrysts. Such gradients help to constrain the mechanisms of magmatic differentiation operating in each system. Roofward decreases in both T and phenocryst content suggest water concentration gradients in magma chambers. Wide compositional gaps are common features of large eruptions, proving the existence of such gaps in a variety of magmatic systems. Nearly all magmatic systems are ‘fundamentally basaltic’ in the sense that mantle-derived magmas supply heat and mass to crustal systems that evolve a variety of compositional ranges. Feedback between crustal melting and interception of basaltic intrusions focuses and amplifies magmatic anomalies, suppresses basaltic volcanism, produces and sustains crustal magma chambers, and sometimes culminates in large-scale diapirism. Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components, some of which may influence chemical transport in the overlying, more silicic zones. Basaltic magmas become andesitic by concurrent fractionation and assimilation of partial melts over a large depth range during protracted upward percolation in a plexus of crustal conduits. Zonation in the andesitic-dacitic compositional range develops subsequently within magma chambers, primarily by crystal fractionation. Some dacitic and rhyolitic liquids may separate from less-silicic parents by means of ascending boundary layers along the walls of convecting magma chambers. Many rhyolites, however, are direct partial melts of crustal rocks, and still others fractionate from crystal-rich intermediate parents. The zoning of rhyolitic magma is accomplished predominantly by liquid state thermodiffusion and volatile complexing; liquid structural gradients may be important, and thermal gradients across magma chamber boundary layers are critical. Intracontinental silicic batholiths form where extensional tectonism favors coalescence of crustal partial melts instead of hybridization with the intrusive basaltic magma. Cordilleran batholiths, however, result from prolonged diffuse injection of the crust by basalt that hybridizes, fractionates, and preheats the crust with pervasive mafic to intermediate forerunners, culminating in large-scale diapiric mobilization of partially molten zones from which granodioritic magmas separate. Much of the variability among magmatic systems probably reflects the depth variation of relative rates of transport of magma, heat, and volatile components, as controlled in turn by the orientation and relative magnitudes of principal stresses in the lithosphere, the thickness and composition of the affected crust, and variations in the rate and longevity of basaltic magma supply. Extension of the lithosphere may reduce the susceptibility of basaltic magmas to hybridization in the crust, but it can also enhance the role of mantle-derived volatiles in chemical transport.
1,559 citations
TL;DR: In this article, amphibole is used as a filter for water dissolved in mantle-derived arc magmas, and amphibole cumulates may act as a fertile source of intracrustal melts and fluids.
Abstract: Pressure-temperature-time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes, and hence the compositions of liquid fractionates. Here we use La/Yb and Dy/Yb versus SiO2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma. Production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid-lower crust, where amphibole is stable. Because amphibole is rarely a phenocryst phase, we term this “cryptic amphibole fractionation.” If this process is as widespread as our investigation suggests, then (1) amphibole cumulates may act as an effective filter for water dissolved in mantle-derived magmas; (2) amphibole cumulates may act as a fertile source of intracrustal melts and fluids; and (3) recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle.
997 citations
TL;DR: In this article, a histogram of the total phenocryst content measures the probability of the magma to be erupted as lava, and the eruption probability is defined as the product of the probability for finding the magmas at any state of crystallinity (thermal probability) and the rheological probability (Rpheological probability).
Abstract: Given a set of comagmatic lavas of similar composition but varying crystallinity, a diagram can be constructed using only the modes of the phenocrysts that quantitively shows the sequence of crystallization This is done by plotting the amount of each phenocryst against the total crystallinity or percentage of melt of the lava itself A histogram of the total phenocryst content measures the probability of the magma to be erupted as lava This eruption probability (P
E
) is the product of the probability of finding the magma at any state of crystallinity (thermal probability, P
T
) and the rheological probability (P
R
) of the magma being physically able to erupt (ie P
E
=P
T
P
R
) It is shown that P
E
is given by dX/dT, where X is the crystallinity of the magma as a function of temperature (T) Because crystal production is generally nonlinear—in most rocks it is step-like—P
E
is a bellshaped curve stradling the temperature at which the magma is one half crystallized Near the liquidus it is most favorable rheologically for the magma to erupt But the probability is small of sampling a magma near its liquidus, because it cools quickly there It is maximum when there are high rates of crystal production, because it then cools slowly As the crystallinity increases, it reaches a critical point of maximum packing (ie lowest porosity) around 50–60% crystals where it becomes rheologically impossible to erupt The magma looses its potential to become a lava and it becomes a pluton From a histogram of crystallinity and P
T
,P
R
can be found This technique, as well as the construction of the mode-crystallization (M-C) diagram, is illustrated using a set of Aleutian lavas These lavas also show that the point of critical crystallinity decreases with increasing silica content of the lava Because this critical crystallinity is much lower for granitic magmas, they are much more probable than basaltic magmas to become plutons Beyond this point, granitic magmas can only erupt as ash flows This correlation of critical crystallinity and silica content is used to show a method by which the viscosity of the magma can be estimated as a function of crystallinity This variation is found to compare favorably with Roscoe's equation of the dependence of viscosity on the concentration of suspended solids These results show that differentiation probably can not normally take place beyond this critical crystallinity The extraction of melt beyond this critical point by filter pressing is unlikely because the assemblage dilates upon stressing Only if the phenocrysts deform viscously can additional melt be extracted, and this can probably only occur with large (−30km) bodies
852 citations
TL;DR: In this article, the coexisting microphenocrysts of magnetite and ilmenite together with the ferromagnesian silicates in salic volcanic rocks have been analyzed with the electron microprobe.
Abstract: The co-existing microphenocrysts of magnetite and ilmenite together with the ferromagnesian silicates in salic volcanic rocks have been analysed with the electron microprobe. The temperatures and oxygen fugacities of the oxide equilibration have been estimated from the curves of Buddington and Lindsley (1965). The co-existing ferromagnesian silicate phenocrysts are either iron-rich olivine, or orthopyroxene or biotite and amphibole; for each of these groups of phenocrysts, the oxide equilibration data are specific and fall on three distinct curves, parallel to experimental oxygen buffer curves. Many of the investigated rhyolites were quenched at temperatures near 900°C, which may represent liquidus temperatures for those with sparse phenocrysts, and also the intrusion temperature of water-undersaturated granites. The composition of the biotite phenocrysts, which are Al-poor and Ti-rich, taken in conjunction with the oxide data, suggest that two Lassen dacites precipitated biotite at a water fugacity of approximately 400 bars. The composition of the later crystallizing ferromagnesian silicates, particularly the pyroxenes which show a wide range in Fe/Mg ratio, is strongly influenced by the prior crystallization of the oxide phases. If the biotite phenocrysts are typical of acid liquids, then they are incapable of generating by fractionation a peraluminous residual liquid; rather they would tend to make a liquid peralkaline.
782 citations
TL;DR: In this paper, the partitioning of 25 trace elements between high-silica rhyolitic glass and unzoned phenocrysts of potassic and sodic sanidine, biotite, augite, ferrohedenbergite, hypersthene, fayalite, titanomagnetite, ilmenite, zircon, and allanite has been determined by INAA on suites of samples from the mildly peralkaline lavas and tuff of the Sierra La Primavera, Mexico, and the metaluminous, compo
780 citations
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Performance Metrics
| Year | Papers |
|---|---|
| 2025 | 30 |
| 2024 | 25 |
| 2023 | 123 |
| 2022 | 145 |
| 2021 | 107 |
| 2020 | 102 |