From source to crust: Tracing magmatic evolution in a kimberlite and a melilitite using microsample geochemistry

We present an integrated microsampling trace element and isotopic study of primary minerals within the Jos kimberlite, Canada, in order to observe how different phases record progressive crustal interaction in the evolving kimberlite. Identification of the least contaminated phases provides the best information on kimberlite source geochemistry. We also carried out an analogue study on an olivine melilitite from Saltpetre Kop, South Africa, examining a similar mineral suite, with the addition of melilite. The two studies show that all phases except phenocryst olivine show some evidence of crustal modification. Perovskite has recently been used as a proxy for source isotope composition. Although perovskite may provide a better constraint on the isotopic composition of the kimberlite than the whole rock, it does not necessarily provide the best constraint on the source region. The lower initial 87Sr/ 86Sr ratio recorded by early crystallising phases in the kimberlite is similar to those of the low-Cr megacryst suite, strengthening the genetic relationship between kimberlite activity and megacryst formation.