Abstract
Among the nodular xenoliths erupted with kimberlites and alkali basalts some plagioclase-bearing and other granulite-fades nodules with metamorphic textures may represent rock samples plucked directly form the Earth's lower crust and Moho region. In studying these xenoliths it is tempting, as in the case of deeper mantle xenoliths1, to use temperature–pressure estimates from mineral compositions to reconstruct petrological–depth relationships and geotherms for the time of eruption.2–4 In such reconstructions it has been stressed5–9 that possible modification of the xenolith mineral equilibria during the processes of eruption must be considered; but we emphasize here that the reconstructions may also be invalid because mineral equilibria had become inert or frozen long before eruption. We pay particular attention to nodules from kimberlites in Lesotho, and present some new age data. We argue further that medium-and coarse-grained rocks lacking a fluid phase in stable tectonic regions are unlikely to maintain local equilibrium at ambient geotherm conditions until depths well within the upper mantle are reached.
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Harte, B., Jackson, P. & Macintyre, R. Age of mineral equilibria in granulite facies nodules from kimberlites. Nature 291, 147–148 (1981). https://doi.org/10.1038/291147a0
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DOI: https://doi.org/10.1038/291147a0
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