Th-U fractionation and mantle structure

In the Rio Grande rift (RGR) asthenospheric mantle-derived (high epsilon(Nd)) alkali basalts are strongly enriched in Th-230 over U-238 (10-27%). In contrast, lithospheric mantle-derived alkali and tholeiitic lavas (high epsilon(Nd)) have secular equilibrium (Th-230/U-238) values, within analytical error. These lavas are young, 3-50 ka old. Asmerom et al. reported (Pa-231/U-235) for one of the tholeiites of 1.40 [Y. Asmeron, H. Cheng, R. Thomas, M. Hirschmann, R.L. Edwards, Pa-231-U-235 and Th-230- U-238 Isotope Systematics in Continental Basalts, Mineral. Mag. 62 (1998) 81-92]. Thus, the lack of Th-230 enrichment is not due to magma transport or post-eruption related decay. The contrast in the (Th-230/U-238) between asthenospheric and lithospheric mantle lavas is exactly similar to the contrast preciously described by Asmerom and Edwards [Y. Asmerom, R.L. Edwards, U-series isotope evidence for the origin of continental basalts, Earth Planet. Sci. Lett. 134 (1995) 1-7], based on a study of lavas from the Colorado Plateau and the Pinacate volcanic field (PVF), Mexico. Collectively there is a distinct separation between low End value lavas with no Th-230 enrichment on one hand and the high epsilon(Nd) lavas with large Th-230 excesses, 10-40% on the other, irrespective of whether they are tholeiites or alkali basalts. This observation, in corroboration with (Pa-231/U-235) data, show that source mineralogy of the point of initiation of melting is the deciding factor in Th-U fractionation during mantle melting. The lavas with no Th-230 enrichment are postulated to have a spinel lherzolite source, while a garnet peridotite source for lavas with Th-230 enrichment. It is shown that, degree of Th-230 enrichment during dynamic melting reflects the length and depth of the melting column. The factors that modulate the depth and length of the melting column include, variations in lithospheric thickness and heat flow. During the transient thermal pulse associated with igneous activity in continental rifts, the solidus should mirror the topology of the lithosphere. This is due to the thermal inertial of the lithosphere. The second related factor is the physical thickness of the lithosphere, which determines the height to which the melting column can rise to melt. Thirdly, the mantle solidus deepens in areas of high heat flow. The combined effects modulate the structure of the mantle solidus, resulting in differences in the length of the melting column and depth of initiation of melting. Thus, during dynamic melting, the areas with the thinnest Lithosphere and highest heat flow, i.e. the longest and deepest melting column, have the highest degree of Th-230 enrichment, similar to MORE. (C) 1999 Elsevier Science B.V. All rights reserved.