Magnesium and iron isotopes in 2.7 Ga Alexo komatiites: Mantle signatures, no evidence for Soret diffusion, and identification of diffusive transport in zoned olivine

摘要

Komatiites from Alexo, Canada, are well preserved and represent high-degree partial mantle melts (～50%). They are thus well suited for investigating the Mg and Fe isotopic compositions of the Archean mantle and the conditions of magmatic differentiation in komatiitic lavas. High precision Mg and Fe isotopic analyses of 22 samples taken along a 15-m depth profile in a komatiite flow are reported. The δ~(25)Mg and δ~(26)Mg values of the bulk flow are -0.138±0.021‰ and -0.275±0.042‰, respectively. These values are indistinguishable from those measured in mantle peridotites and chondrites, and represent the best estimate of the composition of the silicate Earth from analysis of volcanic rocks. Excluding the samples affected by secondary Fe mobilization, the δ~(56)Fe and δ~(57)Fe values of the bulk flow are +0.044±0.030‰, and +0.059±0.044‰, respectively. These values are consistent with a near-chondritic Fe isotopic composition of the silicate Earth and minor fractionation during komatiite magma genesis. In order to explain the early crystallization of pigeonite relative to augite in slowly cooled spinifex lavas, it was suggested that magmas trapped in the crystal mush during spinifex growth differentiated by Soret effect, which should be associated with large and coupled variations in the isotopic compositions of Mg and Fe. The lack of variations in Mg and Fe isotopic ratios either rules out the Soret effect in the komatiite flow or the effect is effaced as the solidification front migrates downward through the flow crust. Olivine separated from a cumulate sample has light δ~(56)Fe and slightly heavy δ~(26)Mg values relative to the bulk flow, which modeling shows can be explained by kinetic isotope fractionation associated with Fe-Mg inter-diffusion in olivine. Such variations can be used to identify diffusive processes involved in the formation of zoned minerals.