Caesium as a geochemical tool to define crustal-mantle interaction processes within North Atlantic LIP

In the ongoing 'Plume Debate' a major unsolved problem, is the contribution from the continental lithosphere to the production of melt and to the contamination of mantle melts during Large Igneous Province (LIP) magmatic episodes. Divergent opinions originate mainly from the difficulties in distinguishing trace element and isotopic characteristics of "fresh" continental lithospheric signatures from those acquired by involvement of recycled lithosphere within an ascending plume. This is an inherent limitation of our geochemical tools. As a result, a better characterization of LIP magmas and alternative petrogenetical approaches are needed to ultimately clarify which of the observed variations is due to a deep, respectively shallow mantle source. To this end, this study has used Caesium, along with traditional geochemical tools (trace elements and radiogenic isotopes) to investigate the potential influence of continental crustal contamination / assimilation processes on LIP magmas. Cs with its [Xe] 6s1 electron configuration is the most "lithophile" of all elements. This trace element shows the greatest contrast in abundance between mantle melts, lower crust and especially upper crust. In geodynamic earth models Cs is always strongly enriched in the continental crust and depleted in all possible geochemical reservoirs within the earth's interior. During subduction Cs should be liberated very early on from the subducting slab during dehydration processes. As a result, Cs concentrations in magmas are extremely sensitive to continental crust / mantle interactions. A systematic investigation of Cs from mid-Norwegian Volcanic Rifted Margin rocks (ODP Leg 104) defined a decreasing influence of continental crust during the excess magma formation. The Cs signature of the continental rift related ('Lower Series') rocks (6.13 to 0.12 ppm) confirms the crustal anatectic contribution of these rocks, and the data of the rift to drift transition ('Upper Series') tholeiitic MORBs (0.14 to 0.01 ppm) indicate that a minor amount of crustal material might have been involved during the SDRS formation. Due to positive correlations of Cs with both, Sr and Nd isotopic systems it is unlikely that the enhanced Cs content reflects redistribution of Cs, e.g. by secondary seawater alteration processes. The correlation of enriched classical isotopic signatures with the Cs data, points to a major continental source for this enrichment. Project supported by EUROMARGINS and RM founded by BFR 05/133