Geochemistry of hypabyssal kimberlites from Lac de Gras, Canada: Comparisons to a global database and applications to the parent magma problem

Abstract

We present 104 whole-rock geochemical analyses of hypabyssal kimberlite from the Lac de Gras field. Screens using Yb versus Al 2O 3 and ln Si/Al versus ln Mg/Yb effectively discriminate crustally contaminated samples. The remaining “non-contaminated” kimberlites samples have variable (5 to 50%) entrainment of cratonic peridotite. It is problematic to effectively screen for small amounts (< 5%) of digested crust in samples with higher (> 20%) contents of peridotite contamination. We utilize the Lac de Gras data suite to calculate, by two different methods, parent magma compositions and identify two (and potentially three) geochemically distinct parent magma types. The Lac de Gras parent magma compositions are compared to those calculated from other localities in Canada, Greenland, South Africa and Russia. Together, these calculated parent magmas define a range, albeit limited, of viable, yet distinct, kimberlite parent magma compositions. Geochemically, kimberlite parent magmas have high volatile contents (H 2O and CO 2), high MgO, and low SiO 2, Al 2O 3 and alkalis, with K > Na and Na + K/Al < 1. It is difficult to reconcile differences between various calculated kimberlite parent magma compositions from different cratonic areas as merely due to the effects of craton specific lithospheric mantle contamination, indicating the intra- and inter-cratonic variation of parent magma compositions reflect differing source region characteristics and/or partial melting regimes.