Lead- and strontium-isotope geochemistry of the Karmutsen Formation, Vancouver Island, British Columbia

Abstract

Lead-isotope whole-rock ratios of the Triassic flood basalts of the Karmutsen Formation are heterogeneous, with 206Pb/2MPb = 18.72–21.51, 207Pb/204Pb = 15.56–15.77, and 208Pb/204Pb = 38.16–40.15. Whole-rock lead initial ratios are coincident with galena analyses having 206Pb/204Pb = 18.1–19.0, 207Pb/204Pb = 15.53–15.60, and 208Pb/204Pb = 38.2–38.6. Lead, uranium, and thorium concentrations are greater than for mid-ocean-ridge basalts, less than for island arcs, but similar to those in ocean-island and continental flood basalts. Isotopic similarities exist between the Columbia River – Chilcotin continental flood basalts and the Karmutsen Formation, suggesting similar origins for these basalts. The heterogeneity in whole-rock ratios can be explained in terms of isotopic mixing between a lead-rich average crustal lead type and an ocean-island basalt-type mantle source. Thus the Karmutsen Formation displays evidence for sediment contamination of the mantle.