Origin and length scale of compositional anomalies in the convecting Earth: constraints from the Azores
Zusammenfassung der Projektergebnisse
Basalts erupted in oceanic intraplate settings provide a unique window into the upper mantle because it is here that the lithospheric lid through which the ascending melts have to path is relatively thin. The incompatible element and radiogenic Sr-Nd-PbHf-Os isotope signatures of ocean island basalts (OIB) are uniquely enriched when being compared to the more homogenous geochemical signatures along the midocean ridges. The origin of these geochemical signatures has been a matter of debate and two of the competing models include a melt metasomatism in the lithosphere or the presence of (ancient), recycled component in the upper mantle. This project aimed at increasing our understanding of the sources underneath oceanic intraplate volcanoes from the Azores that are well-known for their incompatible element and radiogenic isotope heterogeneity. We have used the stable isotopes of oxygen to better constrain the nature of the Azores mantle. We find, that in contrast to the radiogenic isotopes, the stable isotopes of oxygen (and lithium and boron) are relatively homogenous in the pristine Azores mantle. This is in stark contrast to the erupted lavas that display a stunning correlation of mineral major element geochemistry (i.e. forsterite content) and oxygen isotope signatures which we interpret as the result of assimilation of crustal material into the ascending melt. The implications from these observations are that the assimilation of oceanic crustal material may occur more widely than previously believed and that the radiogenic and stable isotope signatures are largely decoupled in the Azores. However, our results also show that the combined use of mineral major element and stable oxygen isotope data on the same sample aliquot may sufficiently provide information on the petrogenetic processes influencing the melt composition. These results may not only be applicable to the Azores and we aim at further our understanding of these processes in other OIB settings.
Projektbezogene Publikationen (Auswahl)
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(2012) Assessing the heterogeneous source of the Azores mantle plume. PhD Cotutelle Thesis, Friedrich-Alexander Universität Erlangen, Nürnberg and Macquarie University, Sydney, 208pp
Genske, F.S.
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(2013). Oxygen isotopes in the Azores islands: Crustal assimilation recorded in olivine. Geology 41, 491
Genske, F. S., Beier, C., Haase, K. M., Turner, S. P., Krumm, S. & Brandl, P. A.
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(2014). Lithium and boron isotope systematics in lavas from the Azores islands reveal crustal assimilation. Chemical Geology 373, 27-36
Genske, F. S., Turner, S. P., Beier, C., Chu, M.-F., Tonarini, S., Pearson, N. J. & Haase, K. M.
