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

Abstract

The whole rock 143Nd 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 147Sm–143Nd isotope compositions of rocks from AGC, however, fall on an array with an errorchron date of ∼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 ∼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 ∼3.3 Ga whole rock errorchron further supporting Sm–Nd modification at ∼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 143Nd isotope signatures.