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Deep-seated fractionation during the rise of a small-volume basalt magma batch: Crater Hill, Auckland, New Zealand

Smith, I. E. M.; Blake, S.; Wilson, C. J. N. and Houghton, B. F. (2008). Deep-seated fractionation during the rise of a small-volume basalt magma batch: Crater Hill, Auckland, New Zealand. Contributions to Mineralogy and Petrology, 155(4) pp. 511–527.

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Crater Hill is a small volume alkali olivine basalt volcano in the Auckland volcanic field. Crater Hill consists of a sequence of pyroclastic and effusive eruptive units of which the earliest have low silica, ferromagnesian elements and Mg/Fe ratios, high incompatible elements and are more silica undersaturated while the last material to be erupted has higher silica, ferromagnesian elements and Mg/Fe ratios but relatively low incompatible elements. Through the sequence, Mg-number changes from 59 to 67 and LaN/LuN decreases by a factor of 3. This systematic compositional variation is interpreted to be the result of clinopyroxene ± spinel fractionation at pressures of at least 1.4–1.9 GPa, from a primary magma generated by small-degree partial melting in the garnet peridotite stability field (>2.5 GPa). Fractionation occurred where early crystals grew and accumulated along the conduit walls. The rising magma evolved along a polybaric liquid line of descent until it encountered lithosphere cold enough to chill the dike margin. Above this depth, further cooling resulted only in the growth of suspended phenocrysts in a magma separated from the country rock by a chilled margin. This process is observed in the Auckland volcanic field because the rate of magma production is very small allowing compositional features to be preserved that would be overwhelmed in a larger scale magmatic system.
Electronic supplementary material The online version of this article (doi:10.1007/s00410-007-0255-z) contains supplementary material, which is available to authorized users.

Item Type: Journal Article
ISSN: 0010-7999
Keywords: Alkali basalt; Magma zoning; Crystal fractionation; Auckland volcanic field; Mantle processes; Pyroxene paradox;
Academic Unit/Department: Science > Environment, Earth and Ecosystems
Interdisciplinary Research Centre: Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR)
Item ID: 10603
Depositing User: Users 2315 not found.
Date Deposited: 16 Apr 2008
Last Modified: 14 Jan 2016 16:55
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