In:
Earth and planetary science letters, Amsterdam [u.a.] : Elsevier, 1966, 265(2008), 1/2, Seite 167-182, 1385-013X
In:
volume:265
In:
year:2008
In:
number:1/2
In:
pages:167-182
Description / Table of Contents:
Late Cretaceous (66.2±0.5 Ma amphibole and 66.7±0.2 Ma phlogopite 40Ar/39Ar ages) nephelinitic volcanic rocks from Godzilla Seamount in the eastern North Atlantic (34°N latitude) have trace element and SrNdPbHf-isotope compositions similar to the Enriched Mantle I (EM-I) endmember, except for their low 207Pb/204Pb relative to 206Pb/204Pb ratios (206Pb/204Pbin=17.7, 207Pb/204Pbin=15.34) plotting below the Northern Hemisphere Reference Line on the uranogenic Pb isotope diagram. O isotope data on amphibole separates are mantle-like (γ18O=5.65.8 promille). Age and location of the isolated Godzilla Seamount, however, preclude it from being derived from the Madeira or Canary hotspots, making a lower-mantle origin unlikely. Therefore we propose derivation from a shallow (lithospheric/asthenospheric) melting anomaly. As observed in mid-ocean-ridge and ocean-island basalts, there is a systematic decrease of 207Pb/204Pb ratios (and Delta 7/4) in the individual EM-I endmember type localities towards northern latitudes with Godzilla lying on the extension of this trend. This trend is mirrored in ultra-potassic volcanic rocks such as lamproites and kimberlites, which reflect the composition of enriched subcontinental lithospheric mantle. Therefore, a global pattern in 207Pb/204Pb ratios and Delta 7/4 is suggested. The geochemical composition of EM-I endmember type localities, including Godzilla lavas, and the enriched (DUPAL) anomaly in the southern hemisphere could reflect derivation from ancient, metasomatized subcontinental lithospheric mantle. We propose a two-stagemodel to explain the trace element and isotopic composition of the EM-Imantle endmember localities worldwide: 1) during the early history of the Earth, subcontinental lithosphere was metasomatized by melts from subducted slabs along convergent margins generating high My (238U/204Pb) sources, and 2) as the Earth cooled, hydrous fluids replaced hydrous melts as the main slab component metasomatizing the subcontinental lithospheric mantle (generating EM-I sources with lower My). In accordance with this model, the global variations in 207Pb/204Pb ratios and Delta 7/4 could reflect geographic differences in My and/or the age at which the transition from stages 1 to 2 took place in the Archaean lithosphere. The model would require a re-definition of the EM-I endmember to low 206Pb/204Pb, high 208Pb/204Pb (positive Delta 8/4) but variable 207Pb/204Pb (positive and negative Delta 7/4).
Type of Medium:
Online Resource
Pages:
graph. Darst., Kt
ISSN:
1385-013X
DOI:
10.1016/j.epsl.2007.10.001
Language:
English
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