radiogenic isotopes8 - 240 Using radiogenic isotope data...

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Unformatted text preview: 240 Using radiogenic isotope data 87Sr/“Sr (a) 206Pb/204Pb 0.5130 143Nd/144Nd 0.5126 0.5122 Figure 6.12 (a) 87sr/ 8681‘ vs 206Ph/2 and EMII, enriched mantle; HIMU, mantle with high U/Pb ratio; PREMA, frequently observed PREvalent MAntle composition. The stippled field is mid—ocean ridge basalts (MORE). The 206Pb/204Pb value of the bulk Earth differs from the value in Table 6.5, which is taken from Allegre et a]. (1988). 207Pb/204Ph = 0.1084 (2°6Pb/204Ph) at 13.491 [6.6]. 2°8Pb/204Pb = 1.209 (2°6Pb/204Pb) + 15.627 [6.7] The isotopic anomaly was expressed by Hart (1984) in terms of A 7/4 and A 8/4, defined as the vertical deviation from the reference line so that for a given dat —set (DS): A 7/4 = [(207131) / 2041319135 _ (207Pb/204Pb)NHRL] X 100 [6.8] A 8/4 = [(208Pb/204Pb)es — (2°8Pb/2°4Pb)mt1 X 100 [6-9] A similar notation was used for 87Sr/86Sr: A st = “fist/“sons — 0.7] x 10 000 [6.10] ...
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