Oxygen isotope signatures in olivines from Sمo Miguel (Azores) basalts: implications for crustal and mantle processes

Oxygen isotope ratios were measured in olivines from eight São Miguel basalt lavas. With one exception (4.57‰), the olivines are indistinguishable from one another with an average δ18O of 4.92±0.03‰ (1σ). This value is slightly lower than that characteristic of upper mantle peridotite and MORB olivines (5.2±0.2‰). Assimilation of ≥10–20% of high-temperature altered lower oceanic crust or 4–9% hydrothermally altered volcanic edifice rocks could produce the low δ18O signatures in the São Miguel olivines; both of these assimilation models are permitted by the trace element and radiogenic isotope variations in the São Miguel basalts. However, the limited variation in δ18O despite eruption of the basalts through compositionally and tectonically variable lithosphere, and the lack of correlation of δ18O with olivine forsterite content, are more easily explained if the olivine δ18O signatures are inherited from their mantle source. If the δ18O signatures reflect mantle source compositions, then the relatively low and uniform δ18O signatures allow constraints to be placed on the origin of the mantle sources beneath São Miguel. Extreme variations in radiogenic isotope signatures have previously been attributed to two component source mixing between a predominant Azores plume source with mild HIMU-like characteristics, and an EMII-type mantle with very radiogenic Sr. The low δ18O signatures in the São Miguel basalt olivines suggest that the predominant Azores plume source contains >10% hydrothermally altered recycled oceanic crust. The limited variation in δ18O is consistent with a component of recycled sediment in the São Miguel EMII-type source, although, unlike the case for other EMII OIB (e.g. Samoa and Society), the relatively low δ18O signatures in São Miguel restrict any involvement of recycled sediment to <2% of a relatively low δ18O and very radiogenic Sr or high Rb/Sr sediment. Involvement of several percent metasomatized subcontinental lithospheric mantle could alternatively produce the EMII-type Sr–Nd–Pb isotope signatures without significantly affecting the plume-related low δ18O signatures. The São Miguel δ18O data are thus consistent with mixing between a low δ18O Azores plume source with a component of subducted, hydrothermally altered lower oceanic crust, and either minor recycled sediment or localized EMII-rich delaminated subcontinental lithospheric mantle. The latter could have been introduced into the lithosphere or shallow asthenosphere during opening of the Atlantic ocean basin.