Pb, Sr, and Nd isotopic systematics of rocks from the Galapagos Microplate

Abstract

Pb, Sr, and Nd isotopic analyses were performed on a suite of compositionally variable rock samples representing an approximately 2500 m thickness of oceanic crustal lithologies from the deep rift valleys and spreading centers along the periphery of the Galapagos Microplate at the junction of the Pacific, Cocos, and Nazca plates. Samples were obtained from lower peridotite and gabbro layers, Fe–Ti basalts, very recent neovolcanic flows, and the thin sedimentary cover. The samples were dredged, cored, or video-grabbed during the research cruise Sonne 60. For unaltered igneous rocks, 206Pb/204Pb ratios range from 17.70 to 18.90, 87Sr/86Sr ratios from 0.7024 to 0.7035, clustering around 0.7027, while 143Nd/144Nd ratios range from 0.51307 to 0.51324. Based on these data, little systematic isotopic variation can be associated with any specific area or lithology of the microplate. One exception is a series of very thin volcanic flows from the Summit Graben area, where Pb and Nd isotope ratios change gradually, with each successive flow. The extent of the observed Pb isotopic variation covers almost the entire range of ratios for the East Pacific Rise mid-oceanic ridge basalts published to date, suggesting that the part of the mantle responsible for formation of the Galapagos Microplate rocks is no more isotopically homogeneous than the East Pacific Rise mantle as a whole, taken over a length of several thousands of kilometres.