Origin and nature of peridotites from the East-Azores Fracture Zone
Final Report Abstract
Abyssal samples recovered from the East Azores Fracture Zone (EAFZ) during Meteor cruise M128 Azores Plateau have been studied during this project. Despite the severe seafloor alteration that modified these samples, we successfully evaluated their chemical and isotopic compositions. Further work needs to be done in terms of modelling the two major processes that imprinted chemically and isotopically, but also mineralogically onto the primary composition of these rocks. We have shown that the samples from the EAFZ represent a section of exposed upper mantle, that represents the residue after melting at the nearby Mid-Atlantic ridge. Both trace element compositions and radiogenic isotope signatures reveal unpredicted complexities that characterise these rocks. Whereas seafloor alteration below 250 °C is assumed responsible for the lack of pristine olivine, the high modal abundance of clinopyroxene over orthopyroxene begs the question whether these rocks may indeed represent layered pyroxenite intrusions or veins. However, depleted trace element signatures and strongly (ultra-) depleted isotopic compositions argue for a depleted mantle origin, which is exposed at this key locality when studying the (early) formation of the Azores plateau. Prior to or during melt depletion at the ridge, this portion of the mantle may have undergone melt rock interaction involving depleted melts and subsequent recrystallisation of overabundant clinopyroxene. A metasomatic event is also evident in the vertical Hf-Nd isotope relationship, but deciphering whether this event is ancient or related to the arrival of the Azores plume is highly ambiguous and needs further work.
Publications
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(2019). Abyssal peridotites from the East Azores Fracture Zone – insights into the composition of Earth’s mantle. German-Israeli Foundation Young Scientists Meeting, MARUM, Bremen
Genske, F., Rosemann, M., Beier, C., Bach, W., Stracke, A.
