Geochronology and Geochemistry of Existing Samples From the Walvis Ridge and Rio Grande Rise: Aims, Status and First Results of the SPP1375 Project WALVIS-I

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Hauff, F. , Deppe, J. , Hoernle, K. , Werner, R. , Garbe-Schönberg, D. , O'Connor, J. and Jokat, W. (2009): Geochronology and Geochemistry of Existing Samples From the Walvis Ridge and Rio Grande Rise: Aims, Status and First Results of the SPP1375 Project WALVIS-I , SAMPLE Colloquium , May 04-06 2009 at Benediktbeuern Abbey, Benediktbeuern, Germany.. .
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The crudely linear trend and apparent age progression of the Walvis Track (Walvis aseismic volcanic ridge and Guyot Province to the SW) is commonly believed to reflect a mantle-plume-related hotspot track connecting the Etendeka Flood Basalts with Tristan and Gough Islands. The validity of the few age data and thus age progression for the Tristan/Gough hotspot track, however, are severely questioned. The plume hypothesis itself is also increasingly challenged by The Great Plume Debate and requires critical field tests. In addition, the Walvis Ridge is considered a type locality for the enriched mantle (EM) I endmember of intraplate basalts. The project WALVIS I conducts an integrated Ar/Ar geochronological and geochemical study (major and trace element and Sr-Nd-Pb-Hf-O isotopes) on existing samples from the Walvis Track and Rio Grande Rise. Our sample collection includes >100 samples from 34 sites of various international cruises and from five DSDP sites. These samples are used to test for age progressive magmatism along the Walvis Track, to evaluate changes in chemical composition through time, particularly to see if there is a geochemical zonation along this volcanic lineament, and to constrain further the ultimate origin of the EM-I mantle endmember. Due to logistical reasons the appointment of Mrs. Joana Deppe as Ph D student has been delayed until summer 2009. Nevertheless initial screening of the entire sample suite, making of thin sections as well as preparation of 22 samples from the Guyot Province for XRF (major elements), ICPMS (trace elements) and TIMS (Sr-Nd-Pb isotopes) has been carried out since January 2009. This geochemical work serves as a critical test for a possible zonation of the Tristan-Gough mantle source region through time and first results will be presented at the SPP1375 workshop. Available data from both island groups possess distinct trace element and Pb isotopic compositions (e.g. Ba/Th and 207 Pb/204Pb), which could reflect a zoned plume. Still the longevity of these components has not been established yet, because the Gough component can presently only be tracked to McNish seamount (8 Ma), to the east of Gough, and RSA (19 Ma) seamount. Therefore, the dredge samples collected by R/V Polarstern in 2006 at the morphological termination of the Gough track play a crucial role. Full scale preparation of key samples for Ar-Ar geochronological work will take place this summer and a first batch of mineral separates will be irradiated this fall so that age dates will become available early 2010. Overall our study also serves as the basis for a more detailed survey along the Walvis Ridge with R/V Meteor; a proposal of our workgroup that has been positively reviewed recently by the Senatskommission für Ozeanographie.

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