The influence of melt flux and crustal processing on Re-Os isotope systematics of ocean island basalts : constraints from Galápagos.

• Published in: Earth and Planetary Science Letters
• Main Authors: Gibson, S.A., Dale, C.W., Geist, D.J., Day, J.A., Brügmann, G., Harpp, K.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2016
• Subjects: Bouvet; Pacific; St. Helena; Tristan; Iceland; Ocean Island
• Online Access: http://dro.dur.ac.uk/18764/; http://dro.dur.ac.uk/18764/3/18764P.pdf; http://dro.dur.ac.uk/18764/1/18764.pdf; http://dro.dur.ac.uk/18764/2/18764.pdf; https://doi.org/10.1016/j.epsl.2016.05.021

Galápagos basalts with the most radiogenic 187Os/188Os (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 187Os/188Os (0.32), than previously proposed for oceanic gabbros. We suggest the inferred, exceptionally-high radiogenic 187Os of the Pacific crust may be localised and due to sulfides precipitated from hydrothermal systems established at the Galápagos Spreading Centre. High 187Os/188Os 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 187Os/188Os 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 187Os/188Os. The Os concentration threshold below which the 187Os/188Os 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 187Os/188Os ratios in other ocean island settings, especially those where large variations in 187Os/188Os have been linked to heterogeneity in mantle lithology or sulfide populations: the effect of crustal contamination on 187Os/188Os 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.