Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.v43f..06v&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #V43F-06
Physics
1020 Composition Of The Continental Crust, 1021 Composition Of The Oceanic Crust, 1025 Composition Of The Mantle
Scientific paper
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 (ɛHf, ɛNd of +16 and +10, respectively) through slightly radiogenic values for the early Archean (ɛHf, ɛNd 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 ɛHf values. In contrast, the Nd isotopic record of similar rocks are characterized by positive ɛNd 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 ɛNd 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.
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