Th–U fractionation and mantle structure

In the Rio Grande rift (RGR) asthenospheric mantle-derived (high ϵNd) alkali basalts are strongly enriched in 230Th over 238U (10–27%). In contrast, lithospheric mantle-derived alkali and tholeiitic lavas (high ϵNd) have secular equilibrium (230Th/238U) values, within analytical error. These lavas are young, 3–50 ka old. Asmerom et al. reported (231Pa/235U) for one of the tholeiites of 1.40 [Y. Asmeron, H. Cheng, R. Thomas, M. Hirschmann, R.L. Edwards, 231Pa–235U and 230Th–238U Isotope Systematics in Continental Basalts, Mineral. Mag. 62 (1998) 81–92]. Thus, the lack of 230Th enrichment is not due to magma transport or post-eruption related decay. The contrast in the (230Th/238U) 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 ϵNd value lavas with no 230Th enrichment on one hand and the high ϵNd lavas with large 230Th excesses, 10–40% on the other, irrespective of whether they are tholeiites or alkali basalts. This observation, in corroboration with (231Pa/235U) 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 230Th enrichment are postulated to have a spinel lherzolite source, while a garnet peridotite source for lavas with 230Th enrichment. It is shown that, degree of 230Th 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 230Th enrichment, similar to MORB.