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A comparative study of lherzolite nodules in basaltic rocks from British Columbia Littlejohn, Alastair Lewis

Abstract

Lherzolite nodules in basaltic rocks from three localities in British Columbia include rocks of mantle origin and crystal cumulates. Partial chemical analyses show that the compositional ranges of the minerals are narrow for both major and minor elements and fall within the ranges reported for lherzolite nodules elsewhere. Each suite is characterised by a definite range of concentrations of some elements. Olivine in nodules from Castle Rock and Jacques Lake show fabrics resulting from deformation in the solid state prior to their incorporation into their host rocks but those from Nicola Lake are undeformed. The distribution of iron and magnesium between coexisting phases is examined using an ideal ionic solution model. Differences in the distribution coefficients between the suites are probably due to different temperature and pressure conditions at the source of the nodules. The distribution of iron and magnesium between coexisting spinel and olivine gives nominal temperatures of formation of 838°C for Nicola Lake nodules, 1085°C for Jacques Lake nodules and >l600°C for Castle Rock nodules. Differences among the suites in the distribution, of Ni, Co, Mn and Zn between coexisting silicates are independent of variations in composition and are apparently due to different conditions of formation. The Castle Rock and Jacques Lake lherzolites are residual fragments of the upper mantle left after extraction of an under-saturated basaltic liquid from parental mantle rock. The source of the Castle Rock nodules probably lies at greater depth than that of the Jacques Lake nodules. The Nicola Lake nodules are crystal cumulates and formed at an early stage of basalt genesis within the upper mantle or lower crust.

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