The influence of melt flux and crustal processing on Re–Os isotope systematics of ocean island basalts: Constraints from Galápagos

New rhenium–osmium data for high-MgO (>9 wt.%) basalts from the Galápagos Archipelago reveal a large variation in $^{187}$Os/$^{188}$Os (0.1304 to 0.173), comparable with the range shown by primitive global ocean island basalts (OIBs). Basalts with the least radiogenic $^{187}$Os/$^{188}$Os occur closest to the Galápagos plume stem: those in western Galápagos have low $^{187}$Os/$^{188}$Os, moderate $^{87}$Sr/$^{86}$Sr, $^{143}$Nd/$^{144}$Nd, $^{206}$Pb/$^{204}$Pb and high $^{3}$He/$^{4}$He whereas basalts in the south also have low $^{187}$Os/$^{188}$Os but more radiogenic $^{87}$Sr/$^{86}$Sr, $^{143}$Nd/$^{144}$Nd, $^{206}$Pb/$^{204}$Pb and $^{3}$He/$^{4}$He. Our new Os isotope data are consistent with the previously established spatial zonation of the common global isotopic mantle reservoir “C” and ancient recycled oceanic crust in the mantle plume beneath western and southern parts of Galápagos, respectively.

Galápagos basalts with the most radiogenic $^{187}$Os/$^{188}$Os (up to 0.1875) typically have moderate MgO (7–9 wt.%) and low Os (<50 pg g−1) but have contrastingly unenriched Sr, Nd and Pb isotope signatures. We interpret this decoupling of chalcophile and lithophile isotopic systems as due to assimilation of young Pacific lower crust during crystal fractionation. Mixing models show the assimilated crust must have higher contents of Re and Os, and more radiogenic $^{187}$Os/$^{188}$Os (0.32), than previously proposed for oceanic gabbros. We suggest the inferred, exceptionally-high radiogenic $^{187}$Os of the Pacific crust may be localised and due to sulfides precipitated from hydrothermal systems established at the Galápagos Spreading Centre.

High $^{187}$Os/$^{188}$Os Galápagos basalts are found where plume material is being dispersed laterally away from the plume stem to the adjacent spreading centre (i.e. in central and NE parts of the archipelago). The extent to which crustal processing influences $^{187}$Os/$^{188}$Os appears to be primarily controlled by melt flux: as distance from the stem of the Galápagos plume increases, the melt flux decreases and crustal assimilation becomes proportionally greater, accounting for co-variations in Os and $^{187}$Os/$^{188}$Os. The Os concentration threshold below which the $^{187}$Os/$^{188}$Os of Galápagos basalts are contaminated (100 pg g−1) is higher than the canonical value (<50 pg g−1) assumed for many other global OIBs (e.g. for Iceland, Grande Comore and Hawaii). This most likely reflects the low overall melt flux to the crust from the Galápagos plume, which has only a moderate excess temperature and buoyancy flux. Our findings have implications for the interpretation of $^{187}$Os/$^{188}$Os ratios in other ocean island settings, especially those where large variations in $^{187}$Os/$^{188}$Os have been linked to heterogeneity in mantle lithology or sulfide populations: the effect of crustal contamination on $^{187}$Os/$^{188}$Os may be greater than previously recognised, particularly for basalts associated with weak, low melt flux mantle plumes, such as Tristan, Bouvet, Crozet and St Helena.Natural Environment Research Council (Grant ID: RG57434)This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.epsl.2016.05.02