UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Primitive kimberlite magmas from Jericho, N.W.T., Canada: constraints on primary magma chemistry Price, Susannah Eloise

Abstract

In this thesis I present petrographic and geochemical data for six aphanitic kimberlite samples from the Jericho kimberlite pipe, N.W.T., Canada. Three samples derive from chilled margins where kimberlite dykes cooled against wall rock; three samples represent flow differentiated margins of a kimberlite diatreme. The chilled margin samples (JD51, JD69 and JD82) are texturally representative of a melt phase with relatively few microphenocrysts (<30 vol%) set in a groundmass of predominantly calcite and serpentine. The other samples (LGS07, 4S and 4SA) are similar in texture to typical macrocrystal kimberlite (Phase 1 kimberlite at Jericho); they are a mixture of melt and small crystals, although with a higher proportion of melt than macrocrystal kimberlite. Crustal contamination was identified in samples 4S, 4SA, and weakly in JD51, with samples having elevated U and Th attributed to contamination by the U- and Th-bearing host granite (Contwoyto batholith). Based on petrographic and geochemical evidence, samples JD69 and JD82 represent the most primitive melts identified within the Jericho kimberlite to date, with several characteristics indicative of primary melts. They have high Mg#s (86-88), high Cr (1300-1900 ppm) and Ni (800-1400 ppm) contents indicative of primary melts. They also have high CO2 contents (10-17 wt%) suggesting they have suffered little devolatilization, and these values are a minimum for the primary magma. Furthermore, incompatible element contents (Nb, Zr, Y) of aphanitic samples are high relative to other phases of the Jericho kimberlite, suggesting these samples are the closest to a primary magma. The microphenocrysts in these samples are, however, completely serpentinized (and partly altered to other Mg-silicate hydroxides and chlorite), which affects the Mg/Si ratio of the rock. Geochemical comparisons with experimentally generated partial melts from a carbonated garnet lherzolite source indicate that the primary kimberlite magma at Jericho may have been generated from a similar source at a pressure of 6 GPa (approximately 200 km). The Jericho geotherm suggests that the kimberlite melt may have been generated at slightly higher pressures (6.5 GPa; 220 km) and at temperatures greater than 1300°C. [Scientific formulae used in this abstract could not be reproduced.]

Item Media

Item Citations and Data

Rights

For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.