The importance of broad, integrated data sets for mantle plume studies - South Atlantic combined with South African evidence
Abstract
Geochemistry alone cannot distinguish between shallow enriched sources and plume sources including deep recycled material, because both types are formed by differentiation processes near the surface of the Earth [1]. The observation of elevated 3He/4He isotope ratios above the values typical in MORB has been arguably the least equivocal geochemical tracer in support of mantle plumes from the lower mantle [2,3]. In the oceans, elevated 3He/4He isotope ratios are almost exclusively found in volcanic rocks related to an age-progressive track of seamounts, aseismic ridges or islands. Similarly, geochemically anomalously enriched MORB related to such an age-progressive track has been taken as evidence for plume-ridge interaction. The presence of continental material from shallow recycling has been suggested, in most cases, based on continental samples not spatially related to the respective oceanic basalts, but based on their extreme continental-type composition and/or continental material from other places. However, each of these arguments by themselves are equivocal. Here it is argued that putting oceanic basalt samples in a broader context allows better constraints on the origin of mantle plume geochemical signatures. For example, sampling continental material spatially related to oceanic basalts through rifting processes allow testing the role of shallow sources from recent continental breakup. In addition, sampling seamounts adjacent to bathymetric anomalies in the oceans can place constraints on the low temperature melting components in the upper mantle specific to this area, and thus allows distinction between plume and ambient mantle sources. This approach is applied to the source composition of the four South Atlantic plumes that have been related to the edge of the African LLSVP [4]. The Shona plume has been shown to be active for 80 Ma and its track extends to the African continental shelf [4]. Extending the age progression of this plume onto the continent, we find that only type II kimberlites have the correct ages to be related to the Shona plume, which is consistent with previous studies [5]. Western Cape melitites (WCM) and type I kimberlites are too young. The composition of the WCM, however, constrains the composition of metasomatized lithospheric mantle of the tectonic unit of the continent that might have been extended or detached into the ocean basin during Gondwana breakup. This endmember of the WCM coincides in Nd-Pb-Hf isotope space with a composition of a Cape Rise seamount basalt, that otherwise lies outside the range of global oceanic basalts. Another unusual Cape Rise seamount composition, despite its location close to the African continent and its slight negative Nb anomaly, has a different composition and an origin from continental breakup is not indicated. Similar compositions in other South Atlantic plume tracks relate tectonically to continental crust and mantle of variable ages and compositions. Given the previously suggested shared plume source of the South Atlantic plumes [6], this suggests that the continental-type compositions have been recycled into the deep plume source. 1. Hofmann, Nature 1997 2. Kurz, Jenkins & Hart, Nature 1982 3. Courtillot et al., EPSL 2003 4. O'Connor et al., Nature Geoscience 2012 5. le Roex, Nature 1986 6. Class et al., AGU 2013
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMDI21A2255C
- Keywords:
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- 1025 GEOCHEMISTRY Composition of the mantle;
- 1033 GEOCHEMISTRY Intra-plate processes;
- 1040 GEOCHEMISTRY Radiogenic isotope geochemistry;
- 1038 GEOCHEMISTRY Mantle processes