Origin of olivine in kimberlite: Phenocryst or impostor?

Abstract

Kimberlite hosts two populations of olivine that are distinguished on the basis of grain size and morphology; the populations are commonly described genetically as xenocrysts and phenocrysts. Olivine xenocrysts or macrocrysts are thought to derive from disaggregation of mantle xenoliths whereas the smaller, euhedral olivine crystals are presumed entirely cognate to the kimberlite melt. Recent studies of zoning patterns of euhedral olivine in kimberlite have, however, cast doubt on the actual origins of the smaller olivine crystals. Here, we elucidate the nature and origins of the textural and chemical zonation that characterize both populations of olivine: macrocrysts (olivine-I) and euhedral crystals (olivine-II). Specifically, we show that both olivine-I and olivine-II feature chemically distinct overgrowths resulting from heterogeneous crystallization onto pre-existing olivine xenocrysts. Our analysis limits the total volume of olivine crystallized during transport to ≤ 5% in contrast to previous estimates of ~ 25%. The reduced extent of olivine crystallization allows for closer reconciliation of crystallized olivine compositions and estimates of Mg#s for primitive kimberlite melts. It also places constraints on processes involving orthopyroxene assimilation by kimberlite melt. If olivine crystallization and orthopyroxene assimilation are coupled, then orthopyroxene assimilation is limited to ~ 7%. Larger masses of orthopyroxene assimilation (i.e. 25%) are possible only if kimberlite magmas originate at super-liquidus (> 100 ºC) conditions and sub-equal amounts of olivine crystallization occurs.