Formation of early terrestrial crust in the Acasta Gneiss Complex.

The Acasta Gneiss Complex (AGC) is a unique exposure of 4.0 to 2.9 billion-year-old (Ga) meta-igneous rocks in the Northwest Territories, Canada. The AGC represents the oldest known silicic and zircon-bearing terrestrial crust and thus can provide critical insight on the tectonic processes operating on the early Earth. The primary objective of my work is to track crust-forming processes in the AGC for over a billion years of its history using radiogenic isotopes (U-Pb, 176Lu-176Hf, 147Sm-143Nd). I will share three major conclusions from my Hf isotope investigation: 1) precursor Hadean crust (>4.0 Ga) is an important source to the 4.0-3.6 Ga magmas; 2) there was rejuvenation of the magmatic source at ~3.6 Ga, for which I will offer possible tectonic models; and 3) there is an absence of highly depleted mantle in the generation of these rocks. I will discuss these results in light of previous work proposing evidence for contributions from extensively depleted mantle to the AGC and further address this disparity through a careful reevaluation of its highly-contested Nd isotopic record.

Speaker: Annie Bauer, Yale