Mantle plumes are now widely recognised as an integral part of the mantle convection system. The ultimate source of such plumes remains, however, a subject for debate. Most authors agree that they must originate from thermal boundary layers in the mantle, either the 670 km discontinuity or the core mantle boundary (D" layer). The HIMU mantle reservoir has been defined on the basis of the high 206Pb/204Pb (>20) ratios of alkali basalts and basanites derived from it by partial melting, typified by the magmatism of the Atlantic oceanic island of St.Helena (Chaffey et al., 1989). The distinctive Sr-Nd-Pb isotopic and trace element characteristics of the HIMU reservoir suggest that it may represent ancient (>1 Ga) recycled oceanic crust (Wilson, 1993).
Tertiary-Quaternary alkali basalts, basanites and melilitites with HIMU-like Sr-Nd-Pb isotope and trace element characteristics occur at widely scattered localities throughout western and central Europe, the Mediterranean and offshore in the Eastern Atlantic. The magmatism is frequently associated with domal basement uplifts with a wavelength of 200-500 km and occurs in both oceanic and continental sectors. A recent seismic tomographic experiment across the French Massif Central (Granet et al., 1996) indicates that here the magmatism is associated with the upwelling of a discrete, small scale, HIMU mantle "hot-blob" from a layer deeper than 300 km within the upper mantle. Similar "hot blobs" are inferred to occur beneath the other Cenozoic volcanic fields. These may represent remobilization of a much larger scale HIMU plume head (or heads) which had stalled and spread out within the upper mantle, perhaps at the 400 km seismic discontinuity. Unpublished fission track data (Hurford, pers.comm) suggests that the precursor plume-event may have been Early Cretaceous in age (145-135 Ma). Within the North Atlantic domain the Icelandic plume system, which initiated in the Late Cretaceous, also has HIMU characteristics, although the HIMU fingerprint is significantly diluted in the high degree tholeiitic magmas emplaced along the spreading axis by partial melts of a depleted mantle component which may be derived from the lower mantle (Thirlwall et al., 1994).
Upwelling of the initial head of the St. Helena mantle plume (>1000 km in diameter) appears to have played an important role in weakening the continental lithosphere during the break-up of the Equatorial Atlantic during the Early Cretaceous (Wilson, 1992). There is, however, no record of extensive magmatism associated with the major rifts (e.g. Benue Trough of Nigeria; Wilson and Giraud, 1992), suggesting that the plume head was neither hot nor particularly volatile-rich by the time it impacted on the base of the lithosphere. During Tertiary times alkaline magmatism along the Cameroon volcanic "hot-line" has been attributed to re-working of the "fossil" St. Helena plume head attached to the base of the lithosphere (Halliday et al., 1990). In the Middle East (Israel, Jordan, Lebanon) an Early Cretaceous HIMU plume caused extensive alkaline mafic magmatism preceded by a brief phase of tholeiitic dyke emplacement.
New Sr-Nd-Pb isotopic data for primitive mafic magmas with HIMU mantle source characteristics from Tertiary-Quaternary occurrences within Europe and Mesozoic occurrences within Africa and the Middle East are combined with published data for Icelandic basalts and St. Helena in an attempt to model the evolution of the HIMU mantle reservoir over the past 150 Ma.
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