Abstract
We present the results of a detailed petrographic study of fresh coherent samples of the Menominee kimberlite sampled at site 73, located in Menominee County, MI, USA. Our objective is to account for its unusual and complex paragenetic sequence. Several generations of olivine, ilmenite, and spinel-group minerals are described. Early olivine and ilmenite are xenocrystic and were replaced or overgrown by primary minerals. Zoned microcrysts of olivine have a xenocrystic core mantled by a first rim in which rutile, geikielite, and spinel s.s. (spinel sensu stricto) cocrystallized. The in situ U–Pb dating of a microcryst of primary rutile yielded 168.9 ± 4.4 Ma, which was interpreted as the age of emplacement. The groundmass consists of olivine, spinel s.s., a magnesian ulvöspinel–ulvöspinel–magnetite (MUM) spinel, calcite, and dolomite. An extremely low activity of Si is suggested by the crystallization of spinel s.s. instead of phlogopite in the groundmass. The presence of djerfisherite microcrysts indicates high activities of Cl and S during the late stages of melt crystallization. The occurrence of two distinct spinel-group minerals (spinel s.s. and qandilite-rich MUM) in the groundmass is interpreted as clear evidence of the mingling of a magnesiocarbonatitic melt with a dominant kimberlitic melt.
Highlights
Kimberlites are mantle-derived hybrid rocks consisting of a mixture of crystals, some derived from the disaggregation of xenoliths and some that grew directly in the carrier magma [1]
A petrographic examination of rock samples from Menominee revealed a complex paragenetic sequence that strongly suggests the injection of a magnesiocarbonatitic liquid into a resident kimberlitic melt
Spinel s.s. likely crystallized from the kimberlitic melt, whereas qandilite-rich magnesian ulvöspinel–ulvöspinel–magnetite (MUM) crystallized from the carbonate melt
Summary
Kimberlites are mantle-derived hybrid rocks consisting of a mixture of crystals, some derived from the disaggregation of xenoliths and some that grew directly in the carrier magma [1] These rocks clearly present a petrographic challenge due to the hybrid nature and common strong overprint by late hydrothermal and supergene processes. The mixture of different pulses of magma can be diagnosed by disequilibrium mineral assemblages, bimodal mineral compositions, and normal and reverse zoned minerals [7] This is why our research team investigated the mineral associations found in kimberlites and related rocks using around 400 samples from worldwide occurrences that form part of the collection of the University of Barcelona. In this paper we aim to demonstrate, with examples from two selected samples from a kimberlite (site 73 in Menominee, MI, USA), that spinel groundmass associations can be a criterion to reveal the mingling processes in kimberlites
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