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New insights concerning the influence of water during the formation of podiform chromitite

Edwards, Stephen John, Pearce, Julian A. and Freeman, Jonathan 2000. New insights concerning the influence of water during the formation of podiform chromitite. Special Papers- Geological Society of America 349 , pp. 139-147.

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Abstract

Podiform chromitites are pod-like bodies dominated by chromian spinel and enveloped by dunite. Most major bodies are Cr rich and occur in the harzburgitic mantle section and the mantle-crust transition zone of ophiolitesformed in supra-subduction zone environments. Current models for the origin of podiform chromitites require a combination of crystal fractionation and melt-peridotite reaction as basaltic melt migrates through the upper mantle. Although such models in general account for large accumulations of chromian spinel and the field and textural relationships in and around podiform bodies, they do not adequately deal with the problem of the low solubility of Cr in basaltic melts, which begs the questions: How does Cr enter melt and why is it transported to a restricted zone in a melt conduit where accumulation of chromian spinel takes place? The present study suggests that the answer may lie with water and its ability to depolymerize the silica network of the melt. The formation of most, if not all, melts capable of forming podiform chromitites requires the involvement of water in the zone of melt generation. Water not only promotes partial melting of refractory peridotites, it also dissolves in the resulting melt. This hydrous melt will have a greater abundance of octahedral sites than its anhydrous equivalent, and this modification promotes greater solubility of Cr in the hydrous melt owing to the high octahedral site preference energy of Cr3+-the major ion controlling Cr solubility. Cr is held in the melt until the melt and peridotite reaction raises the Si content and extent of polymerization of the melt (so that the concentration of octahedral sites is reduced), at which point the Cr becomes insoluble and chromian spinel precipitates. Growth of chromian spinel takes place in an open system in which there is a continuous supply of Cr-rich melt during a melting episode. An examination of the partial-melting history of peridotites preserved in the Troodos ophiolite, Cyprus, indicates that Cr behaves as an incompatible element during the hydrous melting of refractory peridotites. The highest Cr contents of melts that separate from these peridotites are rarely seen in the extrusive sequences of the Troodos ophiolite, suggesting that these melts fractionated substantial volumes of chromian spinel prior to eruption.

Item Type: Article
Date Type: Submission
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: chemical composition chromitite chromium crust crystal fractionation igneous rocks lower crust mantle melts metals ophiolite plutonic rocks solubility ultramafics upper mantle water
Publisher: Geological Society of America
ISSN: 0072-1077
Last Modified: 04 Jun 2017 02:05
URI: http://orca-mwe.cf.ac.uk/id/eprint/8600

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