Abstract
Spinels are commonly observed in alkali olivine basalts and olivine basalts that form the “Plateau Magma Series” of the British Tertiary Province. The spinels are either partly or wholly enclosed within olivine or may have adhered to olivine surfaces, and have undergone cation exchange and reaction with the cooling basaltic melt. Detailed microprobe traverses indicate complex exchanges involving Fe-Mg, Cr-Al, Fe3+-R3+ and Fe2+ Ti-R3+ substitutions. Some of these changes are due to a reaction with liquid that produced plagioclase and resulted in Al depletion in the spinel. A complex series of solid solutions between hercynite-magnesioferrite-chromite and Al-Cr-titomomagnetite, is indicated in a combination that precludes the disappearance of spinel by a simple peritetic reaction with the melt. The initial spinels are compositionally distinct from the chromites found in the Rhum layered series and underline the great compositional variability of liquidus spinels that can crystallise from basaltic liquid. Some of this variability may relate to the changing solubility of Cr, which behaves as a trace element, in basaltic liquids in response to slight changes in the structure of the melt.
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Ridley, W.I. The crystallisation trends of spinels in tertiary basalts from Rhum and Muck and their petrogenetic significance. Contr. Mineral. and Petrol. 64, 243–255 (1977). https://doi.org/10.1007/BF00371756
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DOI: https://doi.org/10.1007/BF00371756