Analysis of the zoned plagioclase of different magmatic environments: A preliminary ion-microprobe study |
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| Authors: | N Shimizu |
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| Institution: | Laboratoire de Géochimie et Cosmochimie (LA 196), Institut de Physique du Globe, Université de Paris 6, 75230 Paris Cedex 05 France |
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| Abstract: | A method for quantitative analysis of silicates with an ion microprobe has been developed which suppresses molecular ion interferences with an energy filtering technique based on the difference in the kinetic energy distributions between single-atom ion and molecular ion species.Using this technique, the distribution patterns (zoning) of the major and trace elements were compared in plagioclase crystals from different magmatic environments. The phenocryst of an ocean-ridge tholeiite (mid-Indian Ocean Ridge) is characterized by homogeneous distribution on all elements analyzed. The apparent partition coefficient for K is more than a factor of two higher than the value found in the literature. The plagioclase grain in a Lower Zone of the Skaergaard Intrusion consists of a cumulus part with uniform An content and an intercumulus part with normal major element zoning. The cumulus part shows an outward increase of Sr and a decrease of Mg and Fe. Potassium is uniform but high, giving an apparent partition coefficient close to unity. The phenocryst in an alkali basalt from Terceira, Azores, consists of an inner part of bytownite and an outer part of labradorite with a sharp transition in between.It is suggested that the distribution of K in the Skaergaard plagioclase could represent a steady-state crystallization and that uniformity of a phase with respect to an element may not be adequate as a criterion for equilibrium. |
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