HR: 1330h
AN: V42D-0384    [PDF]
TI: Lead Paradoxes as a Result of the Secular Evolution of Crustal Recycling Processes
AU: * Mata, J
EM: jmata@fc.ul.pt
AF: Departamento e Centro de Geologia, Universidade de Lisboa, Faculdade de Ciencias, Campo Grande C2 - 5 Piso, Lisboa, 1749 - 016 Portugal
AU: Kerrich, R
EM: robert.kerrich@usask.ca
AF: Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, S7N 5E2 Canada
AB: The positioning of both the upper mantle and upper crust to the right of the Geochron, and the inconsistency between the Th/U ratio of depleted mantle and their time-integrated values calculated from Pb-isotope compositions, constitute the two terrestrial Pb-isotope paradoxes. Based on estimates of present-day continental uranium added to oceanic crust, several authors have attempted to explain the paradoxes by oxygenation at $\sim$ 2.2 Ga and consequent uranium fluxing to the mantle, via subduction of U enriched oceanic crust. However, reasoning founded in trace element ratios involving U in several types of oceanic basalts is consistent with significant loss of U from the slab to arc magmas, such that little U is recycled to the mantle. This constraint on U recycling requires an alternative explanation for the long-term mantle evolution characterized by U/Pb increase and Th/U decrease. We propose that this evolution, inferred from the lead isotopic signatures, can be accounted for secular variations in subduction zone trace element fractionation, where the upper mantle composition is constrained by the mixing of recycled oceanic crust and harzburgite. Archean continental crust formed at convergent margins dominantly from high Al, high La/Yb$_{n}$ tonalitic liquids generated by slab melting under higher geothermal gradients, whereas post-Archean arc magmatism is characteristically generated by slab dehydration-peridotite wedge melting under relatively lower geothermal gradients. We consider Th/U decrease in the mantle to have started through magmatic processes associated with Archean partial melting of subducting slabs, governed by garnet, and recycling of a complementary low Th/U residue. This explains evolution of the Th/U from the primordial value of $\sim$ 4.2 to $\sim$ 3.8 at 2.6 Ga. This decrease continued subsequently as a result of the Th/U fractionation associated with the generation of MORB and consequent recycling of the complementary depleted harzburgitic residue. In contrast, the U/Pb increase, which explains the positioning of oceanic basalts to the right of the Geochron, is thought to be produced by non-magmatic processes through the recycling of dehydrated, lead depleted, oceanic crust in post-Archean times. However, the ante-2.6 Ga $\mu$ decrease from the primitive mantle value of $\sim$ 9.2 to $<$ 8, inferred from the two stages model for recent oceanic basalts, is considered the result of D$_{U}$ $<$ D$_{Pb}$ during slab melting induced by the high Archean geathermal gradients. This model also explains the time-integrated Nd/Sm and Rb/Sr mantle ratios inferred from isotopic signatures of modern basalts. The depleted $^{143}$Nd/$^{144}$Nd signature is considered a consequence of mantle depletion tendency initiated during Archean crustal-forming events, whereas the low $^{87}$Sr/$^{86}$Sr reflects essentially the recycling of a low Rb/Sr residue stemming from the high Rb mobility at Phanerozoic subduction zones.
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting