Calculation of the composition of fractionated solid as deduced from chemical profiles in tholeiitic lava |
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Authors: | Wallace H MacLean |
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Institution: | (1) Department of Earth and Planetary Sciences, McGill University, 3450 University Avenue, Montreal, Quebec H3A 2A7, Canada Tel.: +1-514-3984888; Fax: +1-514-3984680; e-mail: whm@eps.mcgill.ca, CA |
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Abstract: | The composition of solid precipitated from cooling magma is calculated from chemical differentiation profiles and an incompatible
element monitor of fractionation. In a period of cooling, magma evolves to a derivative liquid and a solid; liquid compositions
are obtained from the chemical profiles, and the fractionation interval via the incompatible element and the Rayleigh distillation
equation. The composition of the solid, the only unknown, can be calculated for both equilibrium and continuous fractionation
models. A sequence of low-K tholeiitic laves (basalt to rhyolite) from an Archean greenstone belt in northwestern Quebec are
used to describe the procedure. Chemical trends in the lava exhibit strong iron enrichment followed by depletion and evolution
to rhyolite. Trends in the calculated solid are amplified relative to lava, and bulk distribution coefficients, normative
minerals, mg-number, and other parameters also show significant divergence. Oxidation state and amount of “trapped liquid”
have important effects on solid composition. Solids have notably more basic chemical compositions than coexisting liquids,
and this produces a “lag” in differentiation indices between the two. The high values of bulk distribution coefficients calculated
for Fe, Ti and P (D ∼ 3 to 4+) in late stages of fractionation make it possible to produce the extremely Fe–Ti–P-rich solid fractions in layered
igneous complexes from quartz-normative liquids.
Received: 28 June 1999 / Accepted: 13 December 1999 |
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