Abstract

A detailed geochemical and lithological profile through unit 14 of the Rhum Eastern Layered Series is presented and interpreted to trace the evolution of this cyclic peridotite-troctolite unit. The lower 15.5 m of the unit are peridotite which has an abrupt contact with the upper 35.5 m of troctolite. Numerous thin subsidiary peridotites occur within the troctolite. Variation in the modes, whole rock 87Sr/86Sr ratios, and Ni and Fo in olivine provide most information about the evolution of the unit. Discontinuities in these parameters divide the unit into six subunits: (I) the lower peridotite, (II) the middle main peridotite, (III) the upper main peridotite and lowermost troctolite, (IV) the middle troctolite, (V) the upper troctolite and (VI) the disturbed troctolite. These subunits are interpreted as the result of magma replenishment by picritic and by basaltic magmas. Field evidence suggests that the subsidiary peridotites are later sills intruded into solid or semi-consolidated troctolite. The olivine Fo and Ni values are similar to those of surrounding troctolite suggesting either re-equilibration with, or assimilation of, the troctolite. Comparison with unit 10 shows that the unit 14 resident basaltic liquid was more mafic than that of unit 10. This factor, along with the basaltic replenishment, can explain many of the differences in geochemical and lithological variation between the two units.

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