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Crystallization of accessory phases in magmas by local saturation adjacent to phenocrysts

Geochimica et Cosmochimica Acta

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Abstract

Accessory minerals commonly occur attached to or included in the major crystalline phases of felsic and some intermediate igneous rocks. Apatite is particularly common as inclusions, but Fe-Ti oxides, pyrrhotite, zircon, monazite, chevkinite and xenotime are also known from silicic rocks. Accessories may nucleate near the host crystal/ liquid interface as a result of local saturation owing to formation of a differentiated chemical boundary layer in which accessory mineral solubility would be lower than in the surrounding liquid. Differentiation of this boundary layer would be greatest adjacent to ferromagnesian phenocrysts, especially Fe-Ti oxides; it is with oxides that accessories are most commonly associated in rocks. A boundary layer may develop if the crystal grows more rapidly than diffusion can transport incorporated and rejected elements to and from the phenocryst. Diffusion must dominate over convection as a mode of mass transfer near the advancing crystal/liquid interface in order for a boundary layer to exist. Accumulation of essential structural constituent elements of accessory minerals owing to their slow diffusion in evolved silicate melt also may force local saturation, but this is not a process that applies to all cases. Local saturation is an attractive mechanism for enhancing fractionation during crystallization differentiation. If accessory minerals attached to or included in phenocrysts formed because of local saturation, their host phenocrysts must have grown rapidly when accessories nucleated in comparison to lifetimes of magma reservoirs. Some inconsistencies remain in a local saturation origin for accessory phases that cannot be evaluated without additional information. ?? 1989.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Crystallization of accessory phases in magmas by local saturation adjacent to phenocrysts
Series title:
Geochimica et Cosmochimica Acta
Volume
53
Issue:
5
Year Published:
1989
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
1055
Last page:
1066
Number of Pages:
12