Zoning in igneous plagioclase; normal and oscillatory zoning

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ID: 293568
1962
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Abstract
A sequence of oscillatory cores and thin, normally zoned, more sodic rims is characteristic of many igneous plagioclases, especially those of quartz diorites and granodiorites. The boundary between the core and rim may be euhedral or somewhat irregular and embayed; the core is commonly corroded, showing a patchy zoning formed by irregular inclusions of rim plagioclase composition. The oscillatory zoning is believed to have formed in response to recurrent supersaturation of the melt in anorthite adjacent to the individual crystals. The abrupt change to normally zoned rims in the zoning sequence is thought to reflect late-stage saturation of the residual melt in volatiles. After saturation, agitation by the escaping volatiles maintains a uniform melt composition equalizing the rates of diffusion and crystallization. This prevents further supersaturation, permitting uninterrupted crystallization of the rims. The occurrence of resorbed oscillatory cores in many plagioclases showing this zonal sequence supports the view that falling pressure accompanying rise of magma was a factor in bringing about saturation. As a result of the diminishing solubility of volatiles, further rise of these magmas led to rapid crystallization of the sodic rims and filling-in of the corroded cores. Formation of the oscillatory zoning through a mechanism of pressure changes due to recurrent release of volatiles cannot be reconciled with either the change in zoning or the timing of this change. This zonal sequence is believed to show that loss of volatiles, even in plutonic rocks, occurs principally as a result of late-stage saturation rather than of leakage or diffusion without saturation. Late saturation in volatiles as recorded in the zonal sequence bears out the generally accepted low water content of granitic magmas.
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Authors Joseph A. Vance
Journal american journal of science
Year 1962
DOI
10.2475/ajs.260.10.746
URL
https://ajs.scholasticahq.com/article/58900.pdf
Keywords Keywords not found

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