A mean Th/U ratio, ~4, seems to characterize most
terrestrial, lunar, and meteoritic igneous materials and
major patterns of lead isotopic evolution develop principally in systems with Th/U about 3.7-4.0. Some
important crustal igneous subsystems show systematic
deviations from these values. Such fractionation generally
is attributed to the geochemical behavior of the large
lithophile actinide ions in various enriched minor phases.
This probably is true in highly differentiated series with
higher Th and U levels (>8 and 2 ppm). A different
fractionation mechanism may be more important in some
lower concentration systems. Isotope dilution mass spectrometric studies of U and Th in diverse igneous feldspar separates reveal significant U and Th partitioning into them with drastic fractionation of Th/U (values 0.2- 3.0). Alkali feldspars in granitic rocks display partition coefficients (feldspar/total rock) of about 0.005-0.02 for U
and 0.001-0.004 for Th; Th/U~0.5-3.0. In plagioclase
in gabbros, tonalites, and granodiorites with lower Th and
U, the coefficients appear larger (U~0.1-0.4, Th~0.04-0.2) but Th/U appears lower (Th/U~0.2-2.0) than in K-feldspar. Limited data suggests some pyraxenes and other major minerals may also fractionate Th/U to lower ratios. Differences in ionic radius and uranium oxidation states may contribute to the undefined fractionation mechanism.
Processes of primary differentiation (fractional
crystallization, partial melting) involving gabbroic systems
seem to reflect this fractionation. Oceanic tholeiites, end
massive gabbroic complexes display generally low Th/U
values. Their lead isotopes reflect reservoirs with more
normal Th/U ratios. This suggests limits on the number of
fractionation cycles, mantle mixing, and/or reservoir dimensions for primary basaltic systems. Th and U and their
associated lead isotope systems can be used with lanthanide
R.E.E. to assist in developmet of petrogenetic models