Silica activity and P
total in igneous rocks

Nicholls, J.; Carmichael, I. S. E.; Stormer, John C.

doi:10.1007/bf00373791

Cited by 112 publications

(26 citation statements)

References 26 publications

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“…The Ca content is similar to that in olivines from the Igaliko complex (Stephenson, 1974). The content of Ca in olivine (the larnite component) is dependent on the silica activity of the magma, low silica activity and low pressure favouring highCa olivines (Nicholls et al, 1971).…”

Section: Olivinementioning

confidence: 94%

Petrogenesis of a silicic magma system: Geochemical evidence from Bamenda Mountains, NW Cameroon, Cameroon Volcanic Line

Kamgang

Njonfang

Nono

et al. 2010

Journal of African Earth Sciences

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Section: Olivinementioning

confidence: 94%

Petrogenesis of a silicic magma system: Geochemical evidence from Bamenda Mountains, NW Cameroon, Cameroon Volcanic Line

Kamgang

Njonfang

Nono

et al. 2010

Journal of African Earth Sciences

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“…assumption is correct. Quartz commonly becomes unstable in silica-rich magmas below 6 kbar (Nicholls et al 1971). This iv+ermediate temperature may represent the conditions during magma mixing.…”

Section: Petrology Of Mount Pinatubo 15 June 1991 Eruptionmentioning

confidence: 98%

Buffering of melt oxygen fugacity by sulphur redox reactions in calc-alkaline magmas

Matthews

Jones

Beard

1994

JGS

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Calculated oxygen fugacities of magmas from Lascar Volcano, Northern Chile, and the 1991 eruption of Mount Pinatubo, Phillipines, are seen to increase with decreasing temperature, relative to the fayalite-magnetite-quartz buffer. Isopleths of constant H,S/SO, at moderate pressures (2-4 kbar) lie parallel to the data. We conclude that magma mixing prior to eruption liberated large quantities of SO, gas by oxidation of dissolved sulphide in the mafic end-member. Oxygen fugacity of the magmas was then buffered by reduction of this gas to H$, leading to increasingly oxidized magmas. Such a process might be used to explain the highly oxidized nature of other sulphur-rich evolved calc-alkaline magmas, notably the El Chichon trachyandesite.Oxygen fugacity (f,,) is an important variable in magmatic processes since it can have a strong effect on phase relations and mineral chemistry due to the positive correlation between f , , and melt Fe3+/Fe2+ ratios. An increase in f , , would therefore tend to stabilize magnetite and increase the Mg No. (Mg/Mg + FeZ*) of ferromagnesian minerals.Preferential crystallization of magnetite relative to silicates is likely to increase the silica activity of the melt, further affecting the mineral assemblage. An example of this would be increased stability of orthopyroxene relative to olivine. In primitive mantle-derived magmas the f,, is likely to be inherited from the source region, providing valuable information about mantle mineralogy and heterogeneity and helping to constrain models of melting mechanisms in different tectonic environments.Oxygen fugacity of silicate liquids is commonly buffered during magmatic differentiation by iron redox reactions.Examples are fayalite = magnetite + quartz (FMQ) and magnetite = hematite (MH). These reaction curves are parallel in temperature-f,, space with f , , decreasing with decreasing temperature. In many magmas, complex reactions involving ferrous and ferric iron take place as the temperature drops, buffering f,, along trends parallel to the fayalite-magnetite-quartz buffer. Changes in f , , relative to the fayalite-magnetite-quartz buffer during magmatic differentiation may reflect processes such as assimilation of crustal material and degassing of the magma.

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“…Similar basic xenoliths are widespread in andesites and dacites; they have been observed in numerous other islands of the Lesser Antilles arc (Guadeloupe, St. Vincent, St. Lucia, Dominica, Les Saintes, Nevis), in the Cascades volcanoes (Crater Lake, Mt. Hood, Glacier Peak, Chaos Crags; EICHEI~ BERGER, 1978a;HEIKEN and EICHELBER-GER, 1980), in Japan (EICHELBERGER, 1978b;SAKUYAMA, personal communication), in the Aleutian arc (CONRAD et al, 1983) and in some Agean lavas (NICHOLLS, 1971). The purpose of this paper is a systematic petrological investigation of basic xenolith-acid host lavas relations in Martinique in order to draw general conclusions regarding magma mixing mechanism which occur in xenoliths-bearing calc-alkaline lavas.…”

Section: Introductionmentioning

confidence: 94%

Petrology of basaltic xenoliths in andesitic to dacitic host lavas from Martinique (lesser antilles): Evidence for magma mixing

et al. 1984

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Some recent calc-alkaline andesites and dacites from southern and central Martinique contain basic xenoliths belonging to two main petrographic types: -The most frequent one has a hyalodoleritic texture (~>) with hornblende + plagioclase + Fe-Ti oxides, set in an abundant glassy and vacuolar groundmass.-The other one exhibits a typical porphyritic basaltic texture (~( B type >>) and mineralogy (oli~_ne + plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxides and scarce, or absent hornblende).Gradual textural and mineralogical transitions occur between these two types (<< I type >>) with the progressive development of hornblende at the expense of olivine and pyroxenes. Mineralogical and chemical studies show no primary compositional correlations between the basaltic xenoliths and their host lavas, thus demonstrating that the former are not cognate inclusions; they are remnants of basaltic liquids intruded into andesitic to dacitic magma chambers. This interpretation is strengthened by the typical calc-alkaline basaltic composition of the xenoliths, whatever their petrographic type (<>, <> or <>). The intrusion of partly liquid, hot basaltic magma into colder water-saturated andesitic to dacitic bodies leads to drastic changes in physical conditions, The two components; the basaltic xenoliths are quenched and homogeneized with their host lavas with respect to T°; f02 and pH20 condi-

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Silica activity and P total in igneous rocks

Cited by 112 publications

References 26 publications

Petrogenesis of a silicic magma system: Geochemical evidence from Bamenda Mountains, NW Cameroon, Cameroon Volcanic Line

Petrogenesis of a silicic magma system: Geochemical evidence from Bamenda Mountains, NW Cameroon, Cameroon Volcanic Line

Buffering of melt oxygen fugacity by sulphur redox reactions in calc-alkaline magmas

Petrology of basaltic xenoliths in andesitic to dacitic host lavas from Martinique (lesser antilles): Evidence for magma mixing

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