Basalts uranium/lead isotopic

O Hara MJ (1968) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci Rev 4 69-133 O Nions RK, McKenzie D (1993) Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. Phil Trans Royal Soc 342 65-77 Oversby V, Gast PW (1968) Lead isotope compositions and uranium decay series disequilibrium in reeent volcanic rocks. Earth Planet Sci Lett 5 199-206... 

Altliough plagioclase, unlike calcium-rich pyroxene, ilmenite, or zircon, does not readily incorporate uranium or thorium, isotopic compositions of lead in plagioclase are of particular interest. Plagioclase is a dominant mineral phase in the lunar crust and a major constituent of mare basalts that can incorporate lead into cation sites normally occupied by potassium, sodium, or calcium and thereby play an important role in the U-Th-Pb isotopic systematics. 

Ocean Basins. The most fundamental rocks upon which to obtain isotopic data are currently the abyssal basalts from oceanic ridges and rises (Tatsumoto, 1966 b) and basalts from islands in open ocean structures such as Hawaii (references in Table 17). The data on abyssal basalts which are given in Table 18 are shown in enclosed areas along with all Hawaiian data in Fig. 18. The isotope ratios are clearly not uniform, although the average of all samples would approach a modern lead within the limits of error that such models may be calculated. The age of origin for the source of abyssal basalts may be quite old, on the order of 1,000 m.y., and there may be little differentiation of uranium relative to lead as Tatsumoto has demonstrated (also see Ulrych, 1967). Tatsumoto has also shown that the Th/Pb is apparently equal to or less than that in the source. 

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