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Melt mixing and crystallization under Theistareykir, northeast Iceland

Maclennan, J. and McKenzie, D. and Grönvold, K. and Shimizu, N. and Eiler, J. M. and Kitchen, N. (2003) Melt mixing and crystallization under Theistareykir, northeast Iceland. Geochemistry, Geophysics, Geosystems, 4 (11). 2003GC000558. ISSN 1525-2027. doi:10.1029/2003GC000558.

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Analysis of the compositions of crystals and melt inclusions from a suite of 40 gabbroic and wehrlitic nodules in a single eruptive body provides a record of concurrent mixing and crystallization of melts under NE Iceland. The crystals in the nodules have a similar range of compositions to those found as phenocrysts in the flow, and many of the nodules may have been generated by crystallization of a magma with a similar composition to that of the host flow. While plagioclase is only present in nodules where the average forsterite content of olivines is <Fo88.5, clinopyroxene is found in nodules with average olivine forsterite contents of up to Fo91.6. The order of crystallization ol → ol + cpx → ol + cpx + plg is consistent with generation of the nodules at pressures >0.8 GPa and is in agreement with estimates of crystallization pressures for the host basalt. The relationship between the compositional variability of melt inclusions and the forsterite content of the host olivine is revealed by REE analyses of over 120 melt inclusions. The degree of variability in REE concentrations and REE/Yb ratios decreases with falling forsterite content of the host olivine, as expected if melt mixing and fractional crystallization are operating together. The standard deviation of the REEs falls by a factor of ~4 between Fo90 and Fo87. This change in olivine composition can be produced by crystallization of 20% which occurs on cooling of ~50°C. The relative rates of mixing, cooling and crystallization may provide constraints upon the dynamics of magma bodies. The oxygen isotopic composition of olivines from the nodules and phenocrysts is highly variable (δ18O from 3.3–5.2 per mil) and shows little correlation with the forsterite content of the olivine. The full range of oxygen isotope variation is present in olivines with Fo89–90, and the low δ18O signal is associated with melts of high Mg# and La/Yb. Such geochemical relationships cannot be produced by assimilation of low Mg# crustal materials alone, and may reflect oxygen isotopic variation within the mantle source. The geochemistry of the melt inclusions and their host crystals can be accounted for by fractional melting of a mantle source with variable composition, followed by concurrent mixing and crystallization beneath the Moho.

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Additional Information:Copyright 2003 by the American Geophysical Union. Received: 31 March 2003; Revised: 9 September 2003; Accepted: 15 September 2003; Published: 15 November 2003. Theme: The Oman Ophiolite and Mid-Ocean Ridge Processes Guest Editors: Peter Kelemen, Chris MacLeod, and Susumu Umino Graham Layne and Stephen Reed are thanked for their help with the SIMS and EMP analyses respectively. Iris Van Der Zander found the nodule that set this study in motion. We would like to acknowledge the constructive and thorough reviews of Marc Spiegelman, Laurence Coogan and Susumu Umino. Erik Hauri provided useful communication on his work on Borgarhraun inclusions and Leonid Danyushevsky supplied his program PETROLOG. The Royal Society, NERC and NSF (Grant EAR990400) are thanked for their financial assistance. This is Earth Sciences contribution number ES.7549.
Subject Keywords:Iceland; basalt; mixing melt inclusion; oxygen isotopes; olivine; 3640 Mineralogy and Petrology: Igneous petrology; 3035 Marine Geology and Geophysics: Midocean ridge processes; 8439 Volcanology: Physics and chemistry of magma bodies
Issue or Number:11
Record Number:CaltechAUTHORS:MACggg03
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Official Citation:Maclennan, J., D. McKenzie, K. Grönvold, N. Shimizu, J. M. Eiler, and N. Kitchen, Melt mixing and crystallization under Theistareykir, northeast Iceland, Geochem. Geophys. Geosyst., 4(11), 8624, doi:10.1029/2003GC000558, 2003.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1123
Deposited By: Archive Administrator
Deposited On:21 Dec 2005
Last Modified:08 Nov 2021 19:07

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