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Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley.

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Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley. / Macdonald, R.; Navarro, J. M.; Upton, B. G. J. et al.
In: Journal of Volcanology and Geothermal Research, Vol. 60, No. 3-4, 05.1994, p. 301-325.

Research output: Contribution to Journal/MagazineJournal articlepeer-review

Harvard

Macdonald, R, Navarro, JM, Upton, BGJ & Davies, GR 1994, 'Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley.', Journal of Volcanology and Geothermal Research, vol. 60, no. 3-4, pp. 301-325. https://doi.org/10.1016/0377-0273(94)90057-4

APA

Macdonald, R., Navarro, J. M., Upton, B. G. J., & Davies, G. R. (1994). Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley. Journal of Volcanology and Geothermal Research, 60(3-4), 301-325. https://doi.org/10.1016/0377-0273(94)90057-4

Vancouver

Macdonald R, Navarro JM, Upton BGJ, Davies GR. Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley. Journal of Volcanology and Geothermal Research. 1994 May;60(3-4):301-325. doi: 10.1016/0377-0273(94)90057-4

Author

Macdonald, R. ; Navarro, J. M. ; Upton, B. G. J. et al. / Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley. In: Journal of Volcanology and Geothermal Research. 1994 ; Vol. 60, No. 3-4. pp. 301-325.

Bibtex

@article{984b0d5f4e4a4d32b24d2990baabc9f1,
title = "Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley.",
abstract = "The second caldera-forming ash-flow tuff erupted from the Menengai volcano is up to 80 m thick and represents ca. 30 km3 of peralkaline trachytic magma. Activity started with the emplacement of air-fall tuffs, possibly from a central vent, followed by welded ash-flow tuffs and then by unwelded flow material, the proximal facies of which is lag breccia. At least part of the ash-flow deposits was emplaced through the ring fracture. The tuff was erupted from a compositionally zoned magma chamber which showed strong roofward enrichment in Fe, Mn, Na, Nb, Rb, Th, Y, Zr and the REE, and depletion in Al, Mg, Ca, K, Ti, P, and Ba, corresponding to pantelleritic trachyte at the top, grading down through comenditic trachyte to more primitive (Ba-rich) trachyte. The maximum observed enrichment factor is ×4 for Zr. The chamber may also have been zoned isotopically, 87Sr/86Sr becoming less radiogenic (from 0.7062 to 0.7048) and Nd more radiogenic ( from 0 to −2), upwards. Alkali feldspar phenocrysts generally became more sodic, and olivine and clinopyroxene phenocrysts more Ferich, towards the roof. Variable matrix glass compositions and unusually sodic cores of feldspar phenocrysts in some rocks are thought to result from mixing of liquids from more than one compositional layer during magma withdrawal. The partitioning of Ba, Rb, Sr and the REE between alkali feldspar and liquid was apparently dependent on melt peralkalinity. As the eruption proceeded, a range of compositions was progressively tapped. Compositional zonation in the chamber is inferred to have been continuous. The zonation resulted from up to 75% fractional crystallization of an alkali feldspar-olivine-clinopyroxene-titanomagnetite-apatite assemblage from the least-evolved trachytes, accompanied by minor assimilation of wall rocks, especially in the deeper parts of the chamber.",
author = "R. Macdonald and Navarro, {J. M.} and Upton, {B. G. J.} and Davies, {G. R.}",
year = "1994",
month = may,
doi = "10.1016/0377-0273(94)90057-4",
language = "English",
volume = "60",
pages = "301--325",
journal = "Journal of Volcanology and Geothermal Research",
publisher = "Elsevier Science B.V.",
number = "3-4",

}

RIS

TY - JOUR

T1 - Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley.

AU - Macdonald, R.

AU - Navarro, J. M.

AU - Upton, B. G. J.

AU - Davies, G. R.

PY - 1994/5

Y1 - 1994/5

N2 - The second caldera-forming ash-flow tuff erupted from the Menengai volcano is up to 80 m thick and represents ca. 30 km3 of peralkaline trachytic magma. Activity started with the emplacement of air-fall tuffs, possibly from a central vent, followed by welded ash-flow tuffs and then by unwelded flow material, the proximal facies of which is lag breccia. At least part of the ash-flow deposits was emplaced through the ring fracture. The tuff was erupted from a compositionally zoned magma chamber which showed strong roofward enrichment in Fe, Mn, Na, Nb, Rb, Th, Y, Zr and the REE, and depletion in Al, Mg, Ca, K, Ti, P, and Ba, corresponding to pantelleritic trachyte at the top, grading down through comenditic trachyte to more primitive (Ba-rich) trachyte. The maximum observed enrichment factor is ×4 for Zr. The chamber may also have been zoned isotopically, 87Sr/86Sr becoming less radiogenic (from 0.7062 to 0.7048) and Nd more radiogenic ( from 0 to −2), upwards. Alkali feldspar phenocrysts generally became more sodic, and olivine and clinopyroxene phenocrysts more Ferich, towards the roof. Variable matrix glass compositions and unusually sodic cores of feldspar phenocrysts in some rocks are thought to result from mixing of liquids from more than one compositional layer during magma withdrawal. The partitioning of Ba, Rb, Sr and the REE between alkali feldspar and liquid was apparently dependent on melt peralkalinity. As the eruption proceeded, a range of compositions was progressively tapped. Compositional zonation in the chamber is inferred to have been continuous. The zonation resulted from up to 75% fractional crystallization of an alkali feldspar-olivine-clinopyroxene-titanomagnetite-apatite assemblage from the least-evolved trachytes, accompanied by minor assimilation of wall rocks, especially in the deeper parts of the chamber.

AB - The second caldera-forming ash-flow tuff erupted from the Menengai volcano is up to 80 m thick and represents ca. 30 km3 of peralkaline trachytic magma. Activity started with the emplacement of air-fall tuffs, possibly from a central vent, followed by welded ash-flow tuffs and then by unwelded flow material, the proximal facies of which is lag breccia. At least part of the ash-flow deposits was emplaced through the ring fracture. The tuff was erupted from a compositionally zoned magma chamber which showed strong roofward enrichment in Fe, Mn, Na, Nb, Rb, Th, Y, Zr and the REE, and depletion in Al, Mg, Ca, K, Ti, P, and Ba, corresponding to pantelleritic trachyte at the top, grading down through comenditic trachyte to more primitive (Ba-rich) trachyte. The maximum observed enrichment factor is ×4 for Zr. The chamber may also have been zoned isotopically, 87Sr/86Sr becoming less radiogenic (from 0.7062 to 0.7048) and Nd more radiogenic ( from 0 to −2), upwards. Alkali feldspar phenocrysts generally became more sodic, and olivine and clinopyroxene phenocrysts more Ferich, towards the roof. Variable matrix glass compositions and unusually sodic cores of feldspar phenocrysts in some rocks are thought to result from mixing of liquids from more than one compositional layer during magma withdrawal. The partitioning of Ba, Rb, Sr and the REE between alkali feldspar and liquid was apparently dependent on melt peralkalinity. As the eruption proceeded, a range of compositions was progressively tapped. Compositional zonation in the chamber is inferred to have been continuous. The zonation resulted from up to 75% fractional crystallization of an alkali feldspar-olivine-clinopyroxene-titanomagnetite-apatite assemblage from the least-evolved trachytes, accompanied by minor assimilation of wall rocks, especially in the deeper parts of the chamber.

U2 - 10.1016/0377-0273(94)90057-4

DO - 10.1016/0377-0273(94)90057-4

M3 - Journal article

VL - 60

SP - 301

EP - 325

JO - Journal of Volcanology and Geothermal Research

JF - Journal of Volcanology and Geothermal Research

IS - 3-4

ER -