Disturbances in the Sm-Nd isotope system of the Acasta Gneiss Complex-Implications for the Nd isotope record of the early Earth

The whole rock Nd-143 isotope record of the Acasta Gneiss Complex (AGC) has been used to argue for the development of both depleted mantle and enriched crustal reservoirs during the Hadean. The Sm-147-Nd-143 isotope compositions of rocks from AGC, however, fall on an array with an errorchron date of similar to 3.3 Ga, despite the majority of samples yielding substantially older zircon U-Pb ages. This has led to the suggestion of a major Sm-Nd resetting event at this time in the AGC. To better understand the Sm-Nd bulk-rock systematics of the AGC, we investigate the U-Pb age and Sm-Nd isotope compositions of apatite and titanite from the AGC, using the laser ablation split stream (LASS) method. Apatite from all samples yield broadly similar U-Pb dates of similar to 1.89 to 1.83 Ga, in agreement with resetting during the 1.9 Ga Wopmay orogen. The Sm-Nd isotope compositions of the apatite, however, lie off the WR errorchron and show a high degree of initial Nd isotope heterogeneity. Conversely, titanite Sm-Nd systematics are consistent with the similar to 3.3 Ga whole rock errorchron further supporting Sm-Nd modification at similar to 3.3 Ga. Taken together, these show that the oldest rocks in the AGC underwent at least two post-crystallization events (at 3.3 and 1.9 Ga) that modified the Sm-Nd isotope system. These results also demonstrate the relative mobility of the Sm-Nd isotope system during orogenic events, which may compromise the ability to determine robust bulk-rock Nd-143 isotope signatures. (C) 2019 Elsevier B.V. All rights reserved.