We have determined noble gas abundances and isotopic compositions of 10 MORB glasses from the East Pacific Rise (EPR) between 14° and 23°S. In that region, distinct latitudinal variations in trace element compositions, radiogenic isotopes, and 3He/4He ratios have been observed (Sinton et al., 1991; Bach et al., 1994; Mahoney et al., 1994). However, the patterns observed do not agree: Whereas ratios of highly to moderately incompatible elements, such as La/Sm, K/Ti, K/P, Rb/Nd, or Nb/Zr form a smooth trend of increasing ratios from south to north, the isotope ratios 87Sr/86Sr, 143Nd/144Nd, and 206,207,208Pb/204Pb define a prominent peak with a maximum (or a minimum in the case of 143Nd/144Nd) around 17°S. 3He/4He ratios (unpublished work of Kurz et al. discussed in Mahoney et al., 1994) also show a peak, but it is narrower than that for Sr, Nd, and Pb ratios. Maximum 3He/4He ratios up to 11RA (where RA=1.39·10-6 is the atmospheric ratio) observed near 17°S indicate the presence of a plume-like mantle component in that area. Our work was aimed at investigating whether such a component can be seen in other noble gases as well, and how the latitudinal patterns relate to those for Sr, Nd, and Pb.
Our He results (Fig. 1a) are in almost perfect agreement with those of Kurz et al. (plotted in Fig. 2 of Mahoney et al., 1994). The well-known excesses of 20Ne and 21Ne in mantle gases are also present. Just as expected for a three-component mixture of atmospheric, MORB-like, and plume-like Ne, the data points in a Ne three-isotope plot (not shown) lie in an array bounded by the MORB and Loihi-Kilauea correlation lines, as given by Sarda et al., (1988) and Honda et al. (1991), respectively. 21Ne excesses are smaller relative to 20Ne excesses in samples from the ridge segment between 16° and 20°S, implying larger plume-type contributions there. Plotting the ratio of relative excesses, d20Ne/d21Ne, versus latitude yields a pattern similar, but not equal to that for He (Fig. 1b). North of 16°S, where Ne data are consistent with MORB (d20Ne/d21Ne~0.22), 3He/4He ratios persist on a level above the typical MORB value of 11.2·10-6 (8.2RA). In contrast, samples from 18°-19°S have 3He/4He ratios lower than those north of 16°S and quite similar to MORB, but their d20Ne/d21Ne are distinctly higher, indicating that at 18°-19°S the relative contribution of the plume component to Ne is larger than to He.
40Ar/36Ar ratios do not show a smooth latitudinal trend, probably because of varying amounts of atmospheric Ar, which cannot be corrected for in a straightforward way as in the case of Ne. Nevertheless, in combination with Ne data three latitudinal groups can be discerned, suggesting that south of 20°S Ar is a mixture of atmospheric and MORB gases only, whereas between 17° and 18°S there is a strong contribution of plume-type Ar. North of 17°S, all three components seem to be present. Kr and Xe isotopic compositions are generally atmospheric within uncertainties and, therefore, do not allow to recognize any mantle signatures.
In summary, the noble gas data indicate a plume-like mantle component first showing up in He, then in Ne, and finally in Ar as one progresses from ~15° to 20°S. Our results are compatible with a model proposed by Mahoney et al., (1994) of a discrete mantle heterogeneity which enters the subaxial melt zone at ~15°S and moves southward parallel to the ridge as far as the 20.7°S overlapping spreading center. On its way from north to south the most incompatible element He would be extracted earlier than Ne and Ar. The patterns observed suggest that incompatibilities of Ne and Ar are closer to those of Sr, Nd, and Pb than to that of He.
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Honda, M., McDougall, I., Patterson, D.B., Doulgeris, A. & Clague, D.A., Nature 349, 149-151 (1991).
Mahoney, J.J., Sinton, J.M., Kurz, M.D., Macdougall, J.D., Spencer, K.J. & Lugmair, G.W., Earth Planet. Sci. Lett. 121, 173-193 (1994).
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Sinton, J.M., Smaglik, S.M., Mahoney, J.J. & Macdonald, K.C., J. Geophys. Res. 96, 6133-6155 (1991).
Fig. 1: Latitudinal patterns of 3He/4He (a) and *20Ne/*21Ne (b) for MORB glasses from the East Pacific Rise, 14°-23°S. The dashed horizontal lines represent the isotopic compositions of MORB-type gases; data points above those lines indicate contributions from a plume-like mantle component.