Fe-Mn Geochemistry of OIB From the Azores
Abstract
The iron content of the mantle is one of the major compositional controls of its density and, therefore, of its buoyancy. Variations in iron in the mantle may originate from heterogeneities in the lower mantle, or from core-mantle interaction at the core-mantle boundary. Recent measurements of the Fe/Mn ratio in OIB have shown that Hawaii, Reunion, and Tahiti, have high Fe/Mn, relative to Iceland, interpreted to reflect differences in Fe source composition. An alternative interpretation maintains that Fe/Mn increases with increasing lithospheric thickness in OIB and reflects melting processes rather than source composition. The Azores islands are situated upon an oceanic plateau where four of the nine emerging islands are located along the ultraslow spreading Terceira Rift. We report precise, mass spectrometric determinations of the Fe/Mn ratio, and of first-row transition metals, in 20 young Azorean lavas previously analyzed for U-series and Os isotopes. The Fe/Mn ratio in Azorean lavas (51-73) spans the widest range yet observed for any terrestrial lava. At a given MgO, lavas from the islands of Pico, Faial, and Terceira, plot at Fe/Mn lower than Kilauea lavas (used as a reference here), while lavas from São Jorge and São Miguel have higher Fe/Mn than Kilauea lavas. There exists a rough correlation between Fe/Mn and chondrite-normalized (Tb/Yb) ratios, indicating possible depth control on the Fe/Mn ratio. A clear relationship to either a high-iron source for São Miguel, or to control of Fe/Mn by lithospheric thickness, is lacking with lavas from São Jorge (highest Fe/Mn) providing the exception. We have not yet obtained a clear fractionation sequence for any of the Azorean volcanoes. An important objective is to extend the dataset, including to lavas of Flores and Corvo.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V52A..03H
- Keywords:
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- 1025 Composition of the mantle;
- 1033 Intra-plate processes (3615;
- 8415);
- 1038 Mantle processes (3621);
- 1065 Major and trace element geochemistry;
- 8499 General or miscellaneous