EVOLUTION OF THE ACASTA GNEISS COMPLEX THROUGH SR ISOTOPE ANALYSIS OF APATITE INCLUSIONS IN ZIRCON

Understanding the evolution of Earth’s earliest continents relies on the analysis of Eoarchean and Hadean rocks. However, such rocks are extremely scarce in the geological record. The Acasta Gneiss Complex (AGC), Northwest Territories, Canada, is one of the few occurrences where rocks of this age are preserved. Several key studies have been done on these rocks, revealing crucial aspects of the formation of the AGC [e.g. 1,2]. Nevertheless, much of the history of these rocks is still unknown and the nature of the Hadean source in particular is enigmatic. Rb/Sr and Sr isotope analysis may prove very useful in shedding new light on these aspects. Such analysis is used to elucidate the composition and depth of magmatic sources [3]. This is based on the premise that Rb is enriched in evolved, felsic reservoirs and consequently yields higher ⁸⁷Sr/⁸⁶Sr through the decay of ⁸⁷Rb to ⁸⁷Sr. In spite of its potential, the method is thus far underutilized in early Earth research. This is primarily because, on a bulk-rock scale, the Rb-Sr system has the propensity to show open-system behavior during deformation and sub-solidus alteration. To circumvent this issue, we focus on apatite inclusions in robust zircon host crystals. Apatite is Sr-rich and Rb-poor, and may preserve primary Sr signatures if protected in a primary zircon. Analysis of such small samples at the precision required for this study posed a major analytical challenge. Through a major effort in analytical development in laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS), we developed a new method that allows Sr isotope analysis of small apatite crystals using 30-50 μm spots. Analysis of inclusions in zircon combined with zircon U-Pb dating from the AGC, allow initial ⁸⁷Sr/⁸⁶Sr for rocks in this ancient complex to be constrained. We use this result to estimate the Rb/Sr of the enigmatic Hadean source, and so characterize its nature and composition.

References: [1] Mojzsis et al. (2014) Geochim. Cosmochim. Acta. 133, 68-96. [2] Reimink et al. (2016) Prec. Res. 281, 453-472. [3] Dhuime et al. (2015) Nat. Geosci. 8, 552-555.