HR: 14:55h
AN: V43F-06 INVITED    [Abstracts]
TI: Integrated Nd, Hf and Pb View of Mantle Evolution
AU: * Vervoort, J D
EM: vervoort@wsu.edu
AF: Washington State University, School of Earth and Environmental Sciences, Pullman, WA 99164-2812, United States
AB: Recent developments in the ^147,146Sm-^143,142Nd and ¹⁷⁶Lu-¹⁷⁷Hf systems during the past several years have drastically altered our view of the evolutionary history of the early Earth. These new developments, in particular from ¹⁴⁶Sm-¹⁴²Nd systematics and the changes in the initial ¹⁷⁶Hf/¹⁷⁷Hf data for early Archean samples using the lower ¹⁷⁶Lu decay constant, have produced a data set that, on face value, appears to be in conflict and require more complicated models for early planetary differentiation and subsequent evolution. For example, the record of Hf mantle evolution using previous decay constant of 1.94 e^-11 y^-1 was entirely congruent with the Nd isotopic record and broadly characterized by linear trend of radiogenic values today (ε_Hf, ε_Nd of +16 and +10, respectively) through slightly radiogenic values for the early Archean (ε_Hf, ε_Nd of ~ +4 and +2) and projecting to chondritic values within the first few 100 m.y. of Earth's history. These data seem to be explained by a rather straight-forward model with differentiation of the mantle and the development of enriched and depleted reservoirs. Using the most recent ¹⁷⁶Lu decay constant estimates, however, the Hf isotope record of the oldest terrestrial rocks and zircons is characterized by chondritic to negative initial ε_Hf values. In contrast, the Nd isotopic record of similar rocks are characterized by positive ε_Nd values. The ¹⁴³Nd record is now consistent with constraints from the short-lived isotope ¹⁴⁶Sm-¹⁴²Nd system which indicate very early and widespread differentiation of the mantle and possibly sequestration of the enriched component to the hidden mantle. The Pb isotopic record of galenas from the Late Archean Abitibi greenstone belt of the Superior Province also records a pervasive mantle differentiation event in the first hundred m.y. of Earth's history and therefore are broadly consistent with the ¹⁴²Nd and ¹⁴³Nd data. The apparent decoupling of the Hf systems with the Nd and Pb systems (if the ¹⁷⁶Lu decay constant values are correct) must reflect processes operating during early differentiation of the planet. This differentiation is responsible for widespread fractionation of Sm/Nd and production of enriched, low Sm/Nd and depleted, high Sm/Nd reservoirs, but does not fractionate Lu/Hf effectively between these reservoirs. The ¹⁴⁶Sm- ¹⁴²Nd systematics require sequestration of the early enriched reservoir to an unseen part of the mantle. This is consistent with the positive ε_Nd values that are characteristic of most Archean terranes. Post-4.0 Ga, the Nd and Hf isotopic systems behave congruently and record the progressive evolution of enriched and depleted reservoirs due to Lu/Hf and Sm/Nd fractionations such as we see during mantle melting processes operating on the Earth today.
DE: 1020 Composition of the continental crust
DE: 1021 Composition of the oceanic crust
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting