Depleted mantle sources through time: Evidence from Lu-Hf and Sm-Nd isotope systematics of Archean komatiites
Résumé
In this study, we present Lu-Hf isotope systematics and Lu and Hf abundances for komatiites from the lowermost part of the 2.8 Ga Kostomuksha greenstone belt in the Baltic Shield and compare these, as well as available Sm-Nd isotope data, with those for the best characterized Archean komatiite systems. The Lu-Hf isotope compositions of four spatially associated differentiated lava flows from the Kostomuksha greenstone belt yield an isochron (MSWD = 1.6) with an age of 2931 +/- 300 Ma. which represents the first Lu-Hf isochron obtained for a suite of co-magmatic komatiite lavas. The calculated mean initial (176)Hf/(177)Hf for the Kostomuksha komatiite samples is 0.281107 +/- 3 (2 sigma(mean)), which corresponds to an initial epsilon(176)Hf of + 4.9 +/- 0.1 (2 sigma(mean)). Assuming that mantle differentiation occurred 10 Ma after Earth's accretion at 4.558 Ga, this precise initial ratio requires a time-integrated (176)Lu/(177)Hf = 0.03759 +/- 8, which is identical to the average time-integrated (176)Lu/(177)Hf = 0.0375 +/- 6 calculated for the best characterized late Archean komatiite systems. Together with the calculated average time-integrated (147)Sm/(144)Nd = 0.2091 +/- 4 for the same late Archean komatiite systems, these parameters represent our best estimate of the Lu/Hf and Sm/Nd properties in the late Archean mantle and indicate derivation of komatiite magmas from around the globe from long-term melt-depleted sources that were remarkably homogenous in terms of lithophile trace element systematics. These time-integrated ratios are identical to the respective values of 0.0375 and 0.209 calculated by Boyet and Carlson (2006) for the so-called Early Depleted Reservoir (EDR), and may indicate that the late Archean mantle was similar in composition to the putative EDR, whereas early Archean systems had higher, and Proterozoic systems lower time-integrated Lu/Hf and Sm/Nd ratios. The observed decrease in time-integrated Lu/Hf and Sm/Nd in komatiite sources over time is interpreted as strong evidence for the existence of a hidden enriched reservoir complementary to the EDR that has been gradually mixed back into the mantle over time. The overall depletion of the early mantle likely occurred very early in Earth's history as a result of either global magma ocean differentiation or extraction and subsequent long-term isolation of primordial terrestrial crust. (C)