Titanium isotopes indicate a magmatic transition at the Hadean-Archean - Dr Sarah Aarons

Titanium isotopes indicate a magmatic transition at the Hadean-Archean boundary in the Acasta Gneiss Complex Sarah Aarons Scripps Institution of Oceanography Earth’s oldest presently known rocks are found in the Acasta Gneiss Complex (AGC; Slave craton, Canada). These rocks have experienced multiphase metamorphism, deformation, and igneous intrusion, which has resulted in the partial obliteration of primary textures and possibly bulk compositions. Stable titanium (Ti) isotopes have recently been applied to the rock record to understand crustal differentiation because they are tracers of fractional crystallization. Recently, it was also demonstrated that Ti isotopes are sensitive tracers of magmas produced in different tectonic settings. Specifically, rocks formed in settings with anhydrous (dry) magmas fractionate Ti isotopes more significantly at a given SiO2 content compared to rocks from hydrous (wet) magmas. Here, I apply the Ti isotope proxy to orthogneisses from the Acasta Gneiss Complex spanning the Hadean to Eoarchean transition. Hadean tonalitic gneisses have Ti isotopic compositions comparable to modern evolved tholeiitic magmas, formed by differentiation of dry parental magmas in plume settings. Younger Eoarchean gneisses have Ti isotopic compositions comparable to modern calc-alkaline magmas produced in convergent arcs. This data documents a unique shift from tholeiitic to calc-alkaline style magmatism between 4.02 and 3.75 billion years ago in the Slave craton.

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