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Origin of Argon-Lead Isotopic Correlation in Basalts

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Science  29 Oct 1999:
Vol. 286, Issue 5441, pp. 871
DOI: 10.1126/science.286.5441.871a

Sarda et al. (1) show that there is a correlation between 206Pb/204Pb and maximum40Ar/36Ar values in Atlantic basalts. The authors argue that this correlation is due to mixing between a degassed-depleted upper mantle (40Ar/36Ar > 40,000 and206Pb/204Pb ≈ 17.8) and a recycled component in the mantle (HIMU) that has unradiogenic Ar resulting from recycling of atmospheric Ar (40Ar/36Ar = 300). However, devolatilization of subducting material is well documented in arc environments and it seems extraordinary that Ar, a highly volatile element, can be recycled in significant amounts. I aim to show that the Ar content required of the recycled slab is implausibly high and that most of the variation in Ar isotopic composition could be the result of shallow-level atmospheric contamination.

From the curvature of the hyperbola in40Ar/36Ar–206Pb/204Pb space, Sarda et al. (1) suggest a40K/36Ar ratio of 3000 to 6000 in the recycled endmember. The K content of the HIMU endmember is difficult to constrain, but is unlikely to be less than 0.1 wt % K2O (2), implying a minimum 36Ar content of 20 × 10−9 cm3 STP g−1 in the HIMU endmember. This is similar to the highest values measured in basalts (≈ 2 − 20 × 10−9 cm3 STP g−1) (3), and implies that oceanic crust is subducted without36Ar loss. If average basalt was subducted (≈ 6 × 10−9 cm3 STP g−1) without volatile loss, the r coefficient in206Pb/204Pb–40Ar/36Ar space would be ≈ 1 (a straight line). It is reasonable to expect that gas loss during subduction would result in r values >> 1 (the opposite curvature to that observed). It is difficult to envisage a scenario where subducted Ar can result in an rcorrelation of 0.15 as reported by Sarda et al. (1); it is therefore necessary to examine an alternative mechanism for the observed correlations.

There is a broad anti-correlation between40Ar/36Ar and eruption depth (Fig. 1) in the data presented by Sardaet al. (1). This can reasonably be attributed to more extensive magmatic degassing at shallow levels, producing basalts more susceptible to atmospheric contamination. Basalts with elevated206Pb/204Pb ratios in the Atlantic are commonly associated with topographic highs. Yet the correlation between40Ar/36Ar and206Pb/204Pb, rather than the consequence of recycled Ar in the HIMU source, may equally be the result of preferential atmospheric contamination of the more shallow erupted HIMU basalts.

Figure 1

40Ar/36Ar versus depth of eruption for all data in Sarda et al.(1); squares, Azores; filled circles, north Atlantic; open circles, south Atlantic. Shaded area indicates probable mantle composition. In general, the deepest samples have the highest40Ar/36Ar ratios. The considerable scatter in the correlation is likely due to variable atmospheric contamination.

This argument does not eliminate the possibility of Ar recycling to the mantle; the correlation between40Ar/36Ar and depth may result from a 36Ar–rich HIMU component which then produces topographic highs on the seafloor. However, given the problems associated with recycling atmospheric Ar in the quantities required, and the viable alternative explanation to the correlation between Ar and Pb isotopes, subduction of atmospheric Ar is not yet proven.


Response: Burnard states that Ar cannot be recycled in significant amounts. Our data show that argon and lead do correlate at the scale of the whole Atlantic Ocean (and with a data dispersion that is of the same magnitude as for the Pb-Pb correlation); therefore, Ar may be recycled in concentrations high enough in the recycled material to be seen in MORB melts. This does not necessarily contradict the fact that important devolatilization occurs at subduction, and does not necessarily imply that argon is recycled with such high concentrations that the degassed-mantle 40Ar/36Ar isotopic anomalies should be erased (they are not).

That this correlation holds at the scale of the Atlantic Ocean is difficult to understand if the correlation is produced by contamination. Here, we suggest that contamination is an erratic process and produces scattered values. See, for example, the rare gas concentration patterns for samples with40Ar/36Ar < 10,000 shown in Staudacher et al. (1, p. 124, figure 4a).

Another point is that just as there is a vague correlation between argon and depth, so is there a rough correlation between lead and depth. No one attributes this to any kind of surface-related process, but rather to plume-ridge interaction. Moreover, as shown in our report, the osmium isotopic data have recently been shown to also correlate with lead, so that there is necessarily a vague correlation between osmium and depth as well. The same kind of reasoning that Burnard is making should thus also apply to osmium. Yet I have not found this suggested in the literature.

Finally, let us turn to simple quantification. Following Burnard, we attempted an order-of-magnitude calculation of the consequences for K of our Ar-Pb correlation. We started from the model used by Chauvel et al. (2) for the recycling of oceanic crust that explains the HIMU component. These authors assumed a melting rate of 0.5% for both the depleted upper mantle and the recycled oceanic crust and used a simple batch melting equation with the appropriate mineralogical compositions and partition coefficients.

With a beginning 36Ar concentration of 2 x 10−10 cm3/g in the degassed mantle from Allègre et al. (1986) (3), the resulting concentration in undegassed MORB melt is 4 × 10−8 cm3/g, a value similar to concentrations found in popping rocks. We assume that we have two melts that mix together, one from the degassed-depleted mantle, the other from the HIMU recycled matter. For our Ar-Pb correlation to be visible, the Ar concentration in the HIMU melt should be on the same order of magnitude. Let us first assume it is the same, that is, 4 × 10−8 cm3 36Ar/g melt. With the above melting rate, this is 2 × 10−10cm3 of 36Ar per gram of HIMU rock (unmelted). With a 40K/36Ar ratio of 3000 for the HIMU melt [see our report (4)], we have 0.2 wt % of K2O in this melt, and, assuming the same partition coefficients for K and U, 6.5 × 10−3 wt % of K2O in the HIMU rock. If one uses the same geochemical parameters as Chauvel et al. (2)—a mixture of 75% depleted mantle derived melt and 25% recycled matter derived melt—the yield is a total K2O concentration of 0.87 wt %. This is consistent with the K2O concentrations of 0.5–1.5 wt % measured in HIMU-type melts from Tubuaii, as reported in Chauvel et al. (2). Thus, this calculation supports the idea that atmospheric argon is recycled with oceanic crust at a 36Ar concentration of about 2 × 10−10 cm3/g. Compared to the data of Staudacher and Allègre (5) for altered oceanic crust, this represents a loss of argon by a factor of 16, in agreement with the idea of devolatilization at subduction. Of interest is that if we compare the K concentration obtained above for the HIMU rock to the value given by Staudigel et al. (6) for altered oceanic crust of 0.1 wt % (used by Burnard), the loss factor for K is 18, the same as that for Ar.

If we want to reproduce a “subduction barrier factor” of 59, as in Staudacher and Allègre (5), we need to have a 3.7 lower 36Ar concentration in the HIMU melt than in the degassed mantle derived melt. The K loss factor then becomes 66, and the K2O changes to 0.06 wt % in the HIMU melt and 0.84 wt % in the final mixed melt. Using a value of 6000 for the40K/36Ar ratio of the HIMU melt, the loss factor for K would be 1.7 times lower than that for Ar.

The calculations above are only first order, but they show that the Ar-Pb correlation that we found are not inconsistent with the model used by Chauvel et al. (2) for the HIMU source. Our value of 0.2 wt % K2O is compatible with the one used by Burnard, but applies to HIMU melt in the calculation above instead of to the altered oceanic crust in the work of Staudigel et al. (6) as cited by Burnard. This implies that K is also lost to some extent at subduction, as seen in studies of island arc volcanic rocks [See (7)].

For this model to work, Ar should not be too mobile to remain stored in the subducted matter until it melts. This is not impossible. Hart (8) showed that helium moves relatively slowly in mantle silicates. Argon must be even slower.

We thus conclude that, given present knowledge and the uncertainties about the different parameters involved, argon can possibly be recycled at a rather low concentration, that is, of the same order of magnitude as the degassed part of the mantle, due to devolatilization at subduction. This is nevertheless consistent with measured concentrations of both Ar and K in HIMU related lavas, and with the Ar-Pb isotopic correlation evidenced for the mid-Atlantic Ridge glass samples.

As a final note, Pb isotope data by L. Dosso et al., cited as “unpublished data” (35) in our report, has now been published (9).


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