Geo-Note 2003-37

Geo-Note 2003-37

GEO 2003-37

Radiogenic Isotope Geochemistry of Kimberlitic Rocks in Northern Alberta: Constraints for Source of Magmatism and Emplacement Age

Author(s) Eccles, D.R. Heaman, L.M. Creaser, R.A. Date 2003-11-01

Kimberlitic rocks from the K4 and K6, and Legend, Phoenix and Kendu pipes from the Buffalo Head Hills and Birch Mountains fields, respectively, in the northern Alberta kimberlite province (NAKP) were selected for radiogenic isotopic analysis, including Rb-Sr, Sm-Nd and Pb-Pb on whole rock, U-Pb on perovskite and rutile, Sr on perovskite and Rb-Sr on mica (phlogopite).

Good correlations between whole-rock εNd, 87Sr/86Sr and Pb-isotopes, in conjunction with enriched incompatible elements and low contamination index (SiO2 + Al2O3 + Na2O/MgO + K2O), suggest the isotopic content of the NAKP kimberlitic rocks can be used to make relevant observations on the mantle source characteristics in northern Alberta. Thus, the difference in isotopic character between the northern Alberta pipes must reflect chemical differences in their mantle source rocks, which may, for example, be derived from differing sources in a heterogeneous mantle or by the mixing of two or more components of vastly differing isotopic characteristics.

The K6 pipe (Buffalo Head Hills) is representative of melts derived from a depleted mantle and plot in the field for South African Group I kimberlites. The Phoenix and Legend pipes (Birch Mountains) have similar εNd to K6, but more radiogenic 87Sr/86Sr, possibly related to their evolved nature. The K4 (Buffalo Head Hills) and Kendu (Birch Mountains) pipes contain lower εNd, higher 87Sr/86Sr, and less radiogenic Pb isotopic ratios, and therefore contain: 1) at least some contribution from an old source region enriched in Rb/Sr and light rare-earth element (LREE; low U/Pb and Sm/Nd) material, 2) crustal contamination, or 3) both.

A compilation of known biostratigraphic and isotopic dates show the emplacement age of the NAKP is Late Cretaceous (Coniacian to Maastrichtian). The isotopic results for mineral separates analyzed in this study provided a robust emplacement age for the Phoenix pipe, 77.6±1.1 Ma by U-Pb perovskite. This date is older than previously reported ages of 70.3±1.6 and 70.9±0.4 Ma by U-Pb perovskite and Rb-Sr phlogopite, respectively (Aravanis, 1999). The variance in ages is possibly the result of separate eruption events, which have subsequently mixed in the crater environment.

Eccles, D.R., Heaman, L.M. and Creaser, R.A. (2003): Radiogenic isotope geochemistry of kimberlitic rocks in northern Alberta: constraints for source of magmatism and emplacement age; Alberta Energy and Utilities Board, EUB/AGS Geo-Note 2003-37, 27 p.