The impact of primary processes and secondary alteration on the stable isotope composition of ocean island basalts

Abstract

Molybdenum and uranium are redox sensitive elements that experience isotope fractionation in the Earth-surface environment. Recent studies have explored the transfer of these fractionated signals in subduction zones to arc magmas, but less is known about the full variation of these isotopic tracers in the mantle, particularly domains of the mantle that may feed hot spots. Here, we present Mo and U isotope data for lava samples from Pitcairn, the Samoan Islands, and St. Helena, representing classic localities for the EM-1 (enriched mantle 1), EM-2 (enriched mantle 2) and HIMU (high μ, where μ is 238U/204Pb) endmembers.Mo and U isotope compositions are highly variable for each location, and the range of variation largely overlaps between the three localities. However, some of this variation may be attributable to chemical weathering and low-temperature alteration effects, as revealed by 234U/238U disequilibrium, also reported here; many samples, especially from Pitcairn, have a 234U deficit, while a few others have a 234U excess. Samples with the greatest 234U deficit tend to have the highest Ce/Mo for a given locality and samples with the lowest δ98Mo typically show considerable 234U/238U disequilibrium, suggesting that Mo was leached during weathering with preferential loss of heavy isotopes.Of the samples with 234U and 238U activity in equilibrium from the three localities, most have δ98Mo values lower than the published average for MORB, but they have δ238U values within uncertainty of MORB. In contrast, two Samoan samples have δ98Mo values similar to MORB but have isotopically light δ238U. While the low δ98Mo seen in many samples could be attributable to the incorporation of subducted sediments into the plume sources, such sediment would have needed to be deposited under oxic oceanographic conditions that were not prevalent until the Neoproterozoic; in contrast, radiogenic isotope compositions tend to point towards a much greater age of recycled components in the plume sources. Instead, we consider the most likely cause of the light Mo isotope compositions to be incorporation into the source region of ancient subducted ocean crust that experienced Mo isotope fractionation during devolatilization, which favored loss of the heavy isotopes, as has been proposed in several recent studies. This is in line with models for the origin of the EM-1 and HIMU source, along with a portion of the Samoan source. The two Samoan samples with isotopically light U, but MORB-like δ98Mo, may reflect influence of a distinct pool of mid-Proterozoic recycled ocean crust.