Plume-ridge interaction studied at the Galápagos spreading center: Evidence from ²²⁶Ra-²³⁰Th-²³⁸U and ²³¹Pa-²³⁵U isotopic disequilibria

New ²³⁸U-²³⁰Th-²²⁶Ra and ²³¹Pa-²³⁵U disequilibria data measured by TIMS are presented for ridge-centered MORB glasses dredged during the R/V Sonne 158 cruise at the Galápagos or Cocos-Nazca Spreading Center (GSC) between 86.0°W and 92.3°W. The application of U-series isotopes to the GSC region, situated a few hundred kilometres to the north of the Galápagos hotspot, allows assessment of fundamental questions related to the dynamics of plume-ridge interaction. These include (1) the relationship between long-lived source variations, U-series disequilibria and extent of differentiation, (2) partial melting during solid upwelling, and (3) the nature and rates of plume-ridge mass transfer. The along axis U-series disequilibria variation show gradational patterns that locally are correlated with geochemical and isotopic parameters such as La/Sm, Tb/Yb, ²⁰⁶Pb/²⁰⁴Pb and ¹⁴³Nd/¹⁴⁴Nd as well as major element compositions. The correlation of (²³⁰Th)/(²³⁸U) with radiogenic isotopes and Tb/Yb provides constraints on the plume source influence on the melting process, reflecting an increase in the amount of melting at depth in the presence of garnet or aluminous clinopyroxene. Moreover, the correlation between U-series signatures, radiogenic isotopes, incompatible element abundance and MgO content indicates a causative relationship between the melting of plume source materials and how these lavas differentiate at shallow depths. We speculate that this involves loss of alkalis from ascending melts to shallow peridotite and crustal gabbro, resulting in increased olivine fractionation from the magmas. The U-series data place stringent constraints on the timing of plume-ridge mass transfer and thus distinguish whether mass transfer occurs by movement of melts or solid mantle. Within the likely conditions proposed by the model of (Braun and Sohn [EPSL 213 (2003): 417-430] and with knowledge of (²³¹Pa)/(²³⁵U) and (²³⁰Th)/ (²³⁸U) observed in Galápagos Islands lavas [A. Saal, personal communication], we show that all ²²⁶Ra excess will be lost and the initial ²³¹Pa and ²³⁰Th excesses will be largely decayed. Therefore, we conclude that the plume influence on the GSC lavas results from a solid mantle flow process instead of through migration of plume-derived melts to the ridge.