Figure 12.
A. Abundance ratios of (Th/Nb)pm vs. (La/Nb)pm (subscript pm indicates normalized to primitive mantle ratios of Sun and McDonough, 1989). Most oceanic island basalts, including ~100 basalts from the Kerguelen Archipelago, have (Th/Nb)pm < 1 and (La/Nb)pm = ~1. In contrast, most contin ental crust, especially upper crust, is relatively depleted in Nb (and Ta) (e.g., Thompson et . al., 1984) with (Th/Nb)pm = 5.46 and (La/Nb)pm = 2.17 in average bulk crus t (Rudnick and Fountain, 1995, except with Nb and Ta values of upper crust from Plank and L angmuir, 1998). As indicated, there are considerable differences between lower (LC) and upper (U C) crust composition with the lower crust value of Weaver and Tarney (1984) (W&T), which has much higher (La/Nb)pm. Dredge 8 basalts from eastern Broken Ridge and Kerg uelen Plateau basalts from Site 738 and dredge basalts from the 77° graben lie outside th e oceanic basalt field, thereby showing that they contain a continental crustal component, an inference that is consistent with isotopic data (Figs. 7, 8). Also, Kerguelen Plateau samples from Site 747 are of fset to high (La/Nb)pm, but they have normal (Th/Nb)pm; these basalts m ay have a smaller proportion of a different continental crustal component. Although all Ke rguelen Archipelago flood basalts lie within the field for normal oceanic basalts, lavas of the Hear d Island-Big Ben basaltic series trend to high (Th/Nb)pm. This trend is accompanied by increasi ng 87Sr/86Sr (Fig. 9), and it is also a trend reflecting an i ncreasing role for a continental crust component (Barling et al., 1994). Other data sources are Davie s et al., 1989; Storey et al., 1992; Mahoney et al., 1995; Yang et al., 1998; Frey et al., 1999, in press.
B. Same plot as in A, but with an enlarged scale to include data for continental basalts that have bee n attributed to result from the Kerguelen plume, that is, the Bunbury Basalt (southwest Australia) and Rajmahal Basalt (northeast India). These basalts show a trend of variable contamination by a con tinental crust component (Frey et al. 1996; Kent et al., 1997). Estimates of average, upper and lower continental crust are from Rudnick and Fountain (1998) and Weaver and Tarney (1984) and are labeled (R&F) and (W&T) in the figure. Also shown are North Atlantic MORBs recovered during Le g 152 in a transect away from Greenland. The lowermost lavas in Hole 917A, the drill site c losest to Greenland, define a trend that is consistent with variable contamination by lower crustal g ranulites. In fact, Fitton et al., (1998a, 1998b) concluded that two different crustal components are prese nt in lavas from Hole 917A; the oldest lavas contain a component derived from granulite-facies Archean crust, whereas some of the younger lavas contain a component derived from amphibolite-facies Ar chean crust. Relative to Kerguelen Plateau basalts, the much stronger continental signature in these North Atlantic MORBs probably reflects the lower abundances of incompatible elements in MORBs r elative to plume magmas.
