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The origin of water in ocean island volcanism: correlating volatile and radiogenic isotope signatures in Atlantic OIB

Final Activity Report Summary - OIBH2O (The origin of water in ocean island volcanism: correlating volatile and radiogenic isotope signatures in Atlantic OIB)

Oceanic island basalts (OIB) display significant heterogeneity in their radiogenic isotope ratios. The extremely radiogenic Pb isotope signature of so-called HIMU OIB has long been attributed to the presence in recycled oceanic crust in the mantle sources of these lavas. Yet, this view has become increasingly questioned. Given the element loss occurring during subduction the presence of recycled crust in HIMU source should produce a negative correlation between 206Pb/204Pb and significant element ratios such as Ba/Nb. The opposite should instead be expected if HIMU signature is related to small degree melts recycled in ancient lithosphere. The same should be observed with water and volatiles much of which are lost during the dehydration of the subducting slab. Thus, a relationship between recycling of subducted material and 'enriched' components (EM1, EM2, HIMU) should produce correlated variation in H2O content and Pb within individual HIMU volcanoes. Moreover, if elevated U/Pb of HIMU component was caused by subduction zone dehydration such components should have low water contents; whereas the opposite would be true if HIMU were caused by small degree melts in ancient oceanic lithosphere.

Correlated variations between Pb isotopes, trace elements and water contents are easier to investigate in single localities rather than by comparing a global dataset of OIB. Melt inclusion are particularly appealing for this test since they are believed to report a much wider Pb isotope variation than the whole rocks of any single location. In this study we combined volatiles, incompatible trace element and Pb isotope compositions of melt inclusions from 2 OIB locations in the Atlantic Ocean: La Palma (Canary Islands), as representative of HIMU OIB, and Sao Miguel (Azores archipelago), which has been explained with recycling of under-plated basalt rather than upper oceanic crust.

The most striking feature of our results is the modest variations observed. The in situ analyses slightly extend the observed range of whole rock Pb isotopic compositions of each island, but this is barely significant given the greater errors of the SIMS measurements. This is surprising, given the previous work of Saal et al. on Mangaia and importantly highlights that extreme variability of Pb isotopes in individual ocean island is perhaps an exception and by no means the rule. Likewise the trace element analyses show little variability beyond that encountered in the whole rocks. We had hoped to examine co-variations of Ba/Nb with Pb isotopic ratios, but any possible correlation within such minor variation was difficult to discern. It is plausible that the variability observed in Mangaia samples reflects interaction with a carbonatite metasomatised lithosphere. The little Pb isotopic variability observed may then suggest that the studied localities have interacted little with the lithosphere through which they pass.

Despite careful sample selection we found highly variable water contents in the analysed melt inclusions from La Palma, ranging from 0.03 wt.% to 0.7 wt.%. These sometimes-low water contents suggest a possible role for H loss via diffusion. It is thus difficult to obtain detailed relationships between H2O and Pb isotopes as hoped but we can make a minimum estimate on the water content of the island in general. Our maximum values are lower than those reported for melt inclusions and submarine glasses from Hawaii. Therefore, whilst we cannot definitively rule out a component of diffusive loss, our values suggest a rather dry source for La Palma, in keeping with a recycled origin. Similar maximum water contents were measured by FTIR in phenocrysts in quenched glass.