The Petrology of Lavas and Associated Plutonic Inclusions of Ascension Island

Harris, Chris

doi:10.1093/petrology/24.4.424

Cited by 93 publications

(41 citation statements)

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“…Unlike reported syenitic ejecta from Terceira , Ascension (e.g. Harris 1983), and nearby Fogo volcano (Ridolfi et al 2003), the Furnas syenites do not contain any of the Na-K-Ca-Ti-Zr silicate minerals that characterise agpaitic phase assemblages (e.g. aenigmatite, astrophyllite, eudialyte, cf.…”

Section: Syenite Nodulescontrasting

confidence: 54%

Temporal evolution of a post-caldera, mildly peralkaline magmatic system: Furnas volcano, São Miguel, Azores

Jeffery

Gertisser

O’Driscoll

et al. 2016

Contrib Mineral Petrol

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This is considered to take place in a density-stratified reservoir, with alkali basalt magmas at the base and hydrous trachytes forming an upper cap or cupola. The presence of this reservoir at shallow crustal depths beneath the caldera likely inhibits the ascent and subsequent eruption of mafic magmas, generating a compositional Daly Gap. Rare syenitic ejecta represent in situ crystallisation of trachytic magmas in the thermal boundary zone at the top of the reservoir. Trachytic enclaves within these syenites, in addition to banded pumices and ubiquitous clinopyroxene antecrysts in the UFG pumice falls, provide evidence for mingling/mixing processes within the magmatic system. Despite relatively uniform major element compositions, systematic trace element variations within individual eruptions highlight the importance of fractional crystallisation during late-stage evolution of the trachytes. This is facilitated by the accumulation of water and the development of mild peralkalinity, which contribute to low pre-eruptive melt viscosities and efficient crystal settling. Compositional zoning patterns between individual eruptions cannot be accounted for by periodic tapping of a single magma batch undergoing fractional crystallisation. Instead, up to four individual cycles are recognised, in which a zoned cap of eruptible trachytic magma, formed at the top of the reservoir, was erupted in one or more eruptions and was re-established via intermittent replenishment and subsequent fractional crystallisation. Keywords Furnas volcano · Peralkaline trachyte · Fractional crystallisation · Zoned magma reservoir · Post-caldera volcanismAbstract Furnas is one of three active central volcanoes on São Miguel Island, Azores, and is considered to be one of the most hazardous in the archipelago. In this study, the pre-eruptive magma plumbing system of the 10 young (<5 ka), intra-caldera, sub-Plinian, trachytic eruptions of the Upper Furnas Group (UFG) is investigated via whole rock major and trace element geochemistry, mineral chemistry, thermobarometry, and petrogenetic modelling. The main aim of this work is to elucidate the petrogenesis of the Furnas trachytes, constrain the P-TfO 2 conditions under which they evolve, and investigate the temporal evolution of the magma plumbing system. Results indicate that the trachytes are derived predominantly from extended fractional crystallisation of alkali basalt parental magmas, at depths between ~3 and 4 km.Communicated by Jochen Hoefs. Electronic supplementary materialThe online version of this article

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Section: Syenite Nodulescontrasting

confidence: 54%

Temporal evolution of a post-caldera, mildly peralkaline magmatic system: Furnas volcano, São Miguel, Azores

Jeffery

Gertisser

O’Driscoll

et al. 2016

Contrib Mineral Petrol

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“…Natural lavas from three sodic silica-saturated alkalic suites from Trend 1 (of Fig. 3) (Nandewar volcano, Australia, gray circles, Stolz [1985]; Azores, open triangles, White et al [1979]; Ascension Island, open squares, Harris [1983]) are shown for comparison with experimental liquids. The composition of the Martian rock "Backstay" ) is indicated by the large black circle.…”

Section: Constraints On Liquid Compositionsmentioning

confidence: 99%

“…2). Natural feldspars from Ascension Island (a Trend 1 suite) lie along the dark gray curve (based on data from Harris [1983]). Those from Thingmuli lavas (a low-pressure intra-plate tholeiitic suite) lie along the black curve (from Carmichael 1964).…”

Section: Tholeiitic Versus Alkalic Parental Liquidsmentioning

confidence: 99%

Alkalic parental magmas for chassignites?

Nekvasil

Filiberto

McCubbin

et al. 2007

Meteorit & Planetary Scien

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Abstract-Detailed analysis of cumulate and melt inclusion assemblages in the chassignites provide important constraints on the nature of the melt trapped as inclusions in cumulus olivine (and, by extension, parental magma compositions), the pressures of crystallization, and magmatic volatile contents. These mineral assemblages show strong similarities to the experimental fractionation assemblages that produce the sodic silica-saturated alkalic lavas on Earth (e.g., Ascension Island, Azores, the Nandewar volcano of Australia). The experimental assemblages were produced from silica-saturated hawaiite at pressures above 4.3 kbar with dissolved water contents above 0.5 wt%. Such pressures are consistent with Ti:Al ratios of the melt-inclusion pyroxenes in the Chassigny meteorite. Pyroxene compositions suggest early high crystallization temperatures and thus relatively low initial water and F contents. Feldspars indicate that melt evolution proceeded to rhyolite compositions both within the interstices of the cumulate olivine and within the melt inclusions, even though rhyolitic glass is only found within olivine-hosted polyphase melt inclusions. The observed rhyolite glass is compositionally similar to the alkali-rich rhyolite of Ascension Island which is produced experimentally by crystallization of hawaiite. It is proposed that the melt trapped in cumulus olivine of the Chassigny dunite was similar to a terrestrial silica-saturated hawaiite, while that trapped in olivine of the Northwest Africa (NWA) 2727 dunite was less evolved, perhaps mildly alkalic basalt. Melts similar to terrestrial intra-plate tholeiite could be parental to the cumulus minerals and evolve upon crystallization at pressures above 4.3 kbar and water contents abovẽ 0.4 wt% to mildly alkalic basalt, silica-saturated hawaiite, and alkali-rich rhyolite. The melt inclusion assemblages are inconsistent with either crystallization of a low-Al, high-Fe basalt, or lowpressure crystallization of a terrestrial-like tholeiite.

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“…13 and 14 illustrate ocean island suites for which crystal fractionation of basaltic magmas has been invoked as the dominant process generating intermediate and felsic rocks (Harris, 1983;LeRoex, 1985;Macdonald et al, 1990). Also illustrated in could reflect removal/accumulation of only small amounts of accessory minerals, whereas the ocean island suites may not have been strongly influenced by accessory mineral fractionation.…”

Section: Crystal Fractionation Models and The Sodic Seriesmentioning

confidence: 99%

Petrology and geochemistry of late-stage intrusions of the A-type, mid-Proterozoic Pikes Peak batholith (Central Colorado, USA): implications for petrogenetic models

Smith

Noblett

Wobus

et al. 1999

Precambrian Research

164

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. (1999). Petrology and geochemistry of late-stage intrusions of the a-type, mid-Proterozoic pikes peak batholith (central Colorado, USA): Implications for petrogenetic models. Precambrian Research,, 271-305. doi:10.1016/ S0301-9268(99) Gabbros and mafic dikes associated with the sodic granitoids have ߳ Nd (1.08 Ga) of −3.0 to +3.5, which are lower than depleted mantle at 1.08 Ga, and their trace element characteristics suggest derivation from mantle sources that were previously affected by subduction-related processes. However, it is difficult to characterize the mantle component in these magmas, because assimilation of crust during magma ascent could also result in their observed geochemical features.The Pikes Peak batholith is composed of at least two petrogenetically different granite types, both of which exhibit geochemical characteristics typical of A-type granites. Models proposed for the petrogenesis of the granitoids imply the existence of mafic rocks at depth and addition of juvenile material to the crust in central Colorado at ~1.1 Ga.3

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The Petrology of Lavas and Associated Plutonic Inclusions of Ascension Island

Cited by 93 publications

References 0 publications

Temporal evolution of a post-caldera, mildly peralkaline magmatic system: Furnas volcano, São Miguel, Azores

Temporal evolution of a post-caldera, mildly peralkaline magmatic system: Furnas volcano, São Miguel, Azores

Alkalic parental magmas for chassignites?

Petrology and geochemistry of late-stage intrusions of the A-type, mid-Proterozoic Pikes Peak batholith (Central Colorado, USA): implications for petrogenetic models

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