ALBERT

Description: Oceanic flood basalts are poorly understood, short-term expressions of highly increased heat flux and mass flow within the convecting mantle. The uniqueness of the Caribbean Large Igneous Province (CLIP, 92–74 Ma) with respect to other Cretaceous oceanic plateaus is its extensive sub-aerial exposures, providing an excellent basis to investigate the temporal and compositional relationships within a starting plume head. We present major element, trace element and initial Sr–Nd–Pb isotope composition of 40 extrusive rocks from the Caribbean Plateau, including onland sections in Costa Rica, Colombia and Curaçao as well as DSDP Sites in the Central Caribbean. Even though the lavas were erupted over an area of ∼3×106 km2, the majority have strikingly uniform incompatible element patterns (La/Yb=0.96±0.16, n=64 out of 79 samples, 2σ) and initial Nd–Pb isotopic compositions (e.g. 143Nd/144Ndin=0.51291±3, ϵNdi=7.3±0.6, 206Pb/204Pbin=18.86±0.12, n=54 out of 66, 2σ). Lavas with endmember compositions have only been sampled at the DSDP Sites, Gorgona Island (Colombia) and the 65–60 Ma accreted Quepos and Osa igneous complexes (Costa Rica) of the subsequent hotspot track. Despite the relatively uniform composition of most lavas, linear correlations exist between isotope ratios and between isotope and highly incompatible trace element ratios. The Sr–Nd–Pb isotope and trace element signatures of the chemically enriched lavas are compatible with derivation from recycled oceanic crust, while the depleted lavas are derived from a highly residual source. This source could represent either oceanic lithospheric mantle left after ocean crust formation or gabbros with interlayered ultramafic cumulates of the lower oceanic crust. High 3He/4He in olivines of enriched picrites at Quepos are ∼12 times higher than the atmospheric ratio suggesting that the enriched component may have once resided in the lower mantle. Evaluation of the Sm–Nd and U–Pb isotope systematics on isochron diagrams suggests that the age of separation of enriched and depleted components from the depleted MORB source mantle could have been ≤500 Ma before CLIP formation and interpreted to reflect the recycling time of the CLIP source. Mantle plume heads may provide a mechanism for transporting large volumes of possibly young recycled oceanic lithosphere residing in the lower mantle back into the shallow MORB source mantle.

Description: Major element, trace element and Sr–Nd–Pb isotopic data of volcanic rocks from the Madeira Archipelago (eastern North Atlantic) and seamounts of the Madeira hotspot track (Ampère, Coral Patch and Ormonde) are presented in this study. Although the Sr and Nd isotopic ratios are similar to those in normal mid ocean ridge basalt, the incompatible element signatures and Pb isotopic compositions (206Pb/204Pb=18.7–19.8) show similarities to the high time integrated 238U/204Pb mantle component. On the 206Pb/204Pb versus 207Pb/204Pb isotope diagram, all samples plot below the Northern Hemisphere reference line (Δ 7/4=−1.0 to −7.2) and form a 430 Ma isochron. The Pb, as well as Sm–Nd, isotope data are consistent with the presence of recycled Paleozoic (≤500 Ma) oceanic crust in the Madeira source. Variations in major element and isotopic geochemistry (e.g. positive correlation between SiO2 and FeOT with Pb isotope ratios in primitive samples) point to a heterogeneous plume source containing upper (primarily hydrothermally altered basaltic) and lower (primarily unaltered gabbroic) ocean crust and lithospheric mantle. The more fertile basaltic crustal component is preferentially sampled during the shield stage of volcanism, whereas the more depleted lower crust and lithospheric mantle components are preferentially sampled during the post-erosional stage. We propose that plume material becomes progressively depleted through melt extraction as it spreads out along the base of the lithosphere in the direction of plate motion. A systematic decrease in 143Nd/144Nd and increase in 207Pb/204Pb isotopic ratios with increasing age along the hotspot track and proximity to the Iberian peninsula are attributed to increased contamination by continental lithosphere.