Primitive magma from the Jericho pipe, N.W.T., Canada: Constraints on primary kimberlite melt chemistry

Abstract

We report the first estimates of primary kimberlite melt composition from the Slave craton, based on samples of aphanitic kimberlite from the Jericho kimberlite pipe, N.W.T., Canada. Three samples derive from the margins of dykes where kimberlite chilled against wall rock (JD51, JD69 and JD82) and are shown to be texturally consistent with crystallization from a melt. Samples JD69 and JD82 have geochemical characteristics of primitive melts: they have high MgO (20-25 wt %), high mg-numbers (86-88), and high Cr (1300-1900 ppm) and Ni (800-1400 ppm) contents. They also have high contents of CO2 (10-17 wt %). Relative to bulk macrocrystal kimberlite, they have lower mg-numbers and lower MgO but are enriched in incompatible elements (e.g. Zr, Nb and Y), because the bulk kimberlite compositions are strongly controlled by accumulation of mantle olivine and other macrocrysts. The compositions of aphanitic kimberlite from Jericho are similar to melts produced experimentally by partial melting of a carbonate-bearing garnet lherzolite. On the basis of these experimental data, we show that the primary magmas from the Jericho kimberlite could represent 0.7-0.9% melting of a carbonated lherzolitic mantle source at pressures and temperatures found in the uppermost asthenosphere to the Slave craton. The measured CO2 contents for samples JD69 and JD82 are only slightly lower than the CO2 contents of the corresponding experimental melts; this suggests that the earliest hypabyssal phase of the Jericho kimberlite retained most of its original volatile content. As such these samples provide a minimum CO2 content for the primary kimberlite magmas from the Slave craton.