Crustal interaction during construction of ocean islands: PbSrNdO isotope geochemistry of the shield basalts of Gran Canaria, Canary Islands

Abstract

The isotopic compositions of ocean island basalts are usually taken to reflect the compositions of their mantle sources. We report PbSrNdO isotopic data for 52 samples, and chemical data for 70 samples from the major early Miocene subaerial shield construction phase of Gran Canaria, an ocean island located on the passive margin of northwest Africa. Small systematic differences in isotopic composition exist between the five vertical sequences of basalts sampled, with tight collinear PbSrNdO isotopic correlations in the lower parts of three sections, and very restricted isotopic compositions elsewhere. Negative 206 Pb 204 Pb − 207 Pb 204 Pb correlations are observed, with increases in Δ7 4 Pb and 87 Sr 86 Sr , and decrease in ϵ Nd, accompanied by increasing δ 18O cpx values. These require that isotopic compositions of the basalts were primarily controlled by mixing between high- 206 Pb 204 Pb mantle and a crustal component. This crustal component must have been introduced into the magmas within the ocean island crust, since relatively large variations in δ 18O cpx values (+5.2 to +6.8‰) are observed over a narrow range in 87 Sr 86 Sr (0.70320 to 0.70390). The isotopic data of the lower parts of the sections can be precisely modelled by up to 8% bulk assimilation of NW African passive margin sediments by the least contaminated lavas: this is sufficient for 50% of the Pb in the most contaminated basalt to be sediment-derived, and to change substantially Ce Pb , Ba Nb and La Nb ratios. No correlations exist between isotopic composition and fractionation indices, even in lavas that are closely spatially related, suggesting that contamination processes were not simple AFC-type processes. It follows that the absence of isotope composition-fractionation index relationships cannot be used as evidence that crustal contamination did not take place. It is very difficult to identify unequivocally an uncontaminated magma composition, since the least sediment-contaminated lavas, and most samples with 206 Pb 204 Pb > 19.55 , have δ 18O cpx values significantly below MORB and lunar values, and below mantle δ 18O cpx values determined by laser fluorination. Uncontaminated magmas could be derived from a mantle source with high 206 Pb 204 Pb (> 19.8), high Ce Pb and negative Δ7 4 Pb that resembles resembles the compositions of the late Miocene undersaturated volcanics of Gran Canaria. If so, this source must have low δ 18O values (≤ +5‰), requiring an origin by recycling of hydrothermally-altered oceanic crust. Alternatively, the uncontaminated magmas could be derived from a mantle source on the Northern Hemisphere Reference Line with 206 Pb 204 Pb ≈ 19.6 , which would imply that magmas with higher 206 Pb 204 Pb and low δ 18O values, which are mostly quite evolved, acquired their characteristics by assimilation of hydrothermally-altered igneous crust with HIMU characteristics.