The transition from mafic to felsic upper continental crust (UCC) is crucial to habitability of Earth, and may be related to the onset of plate tectonics. Thus, defining when this crustal transition occurred has great significance for the evolution of Earth and its inhabitants. We demonstrate that V isotope ratios (reported as δ⁵¹V) provide insights into this transition because they correlate positively with SiO₂ and negatively with MgO contents during igneous differentiation in both subduction zones and intraplate settings. Because δ⁵¹V is not affected by chemical weathering and fluid-rock interactions, δ⁵¹V of the fine-grained matrix of Archean to Paleozoic (3 to 0.3 Ga) glacial diamictite composites, which sample the UCC at the time of glaciation, reflect the chemical composition of the UCC through time. The δ⁵¹V values of glacial diamictites systematically increase with time, indicating a dominantly mafic UCC at ~3 Ga; the UCC was dominated by felsic rocks only after 3 Ga, coinciding with widespread continental emergence and many independent estimates for the onset of plate tectonics. On Earth, the timing of the transition from mafic crust to felsic continental crust may have influenced continental emergence, the rise of atmospheric oxygen, the start of plate tectonics, and habitability. However, this timing is currently debated. New V isotope data for the fine-grained matrix of lithified glacial tills, deposited between 3000 to 350 Mya, indicate that the exposed crust was dominantly mafic early on and became more felsic in the post-Archean. These observations are free of the ambiguity seen in previous isotopic proxy studies and provide a reliable assessment of the compositional evolution of the upper continental crust. Keywords: continental crust, glacial diamictite, vanadium isotopes, crustal evolution Abstract The transition from mafic to felsic upper continental crust (UCC) is crucial to habitability of Earth, and may be related to the onset of plate tectonics. Thus, defining when this crustal transition occurred has great significance for the evolution of Earth and its inhabitants. We demonstrate that V isotope ratios (reported as δ 51 V) provide insights into this transition because they correlate positively with SiO 2 and negatively with MgO contents during igneous differentiation in both subduction zones and intraplate settings. Because δ 51 V is not affected by chemical weathering and fluid–rock interactions, δ 51 V of the fine-grained matrix of Archean to Paleozoic (3 to 0.3 Ga) glacial diamictite composites, which sample the UCC at the time of glaciation, reflect the chemical composition of the UCC through time. The δ 51 V values of glacial diamictites systematically increase with time, indicating a dominantly mafic UCC at ~3 Ga; the UCC was dominated by felsic rocks only after 3 Ga, coinciding with widespread continental emergence and many independent estimates for the onset of plate tectonics. status released display-pdf yes is-olf no is-manuscript no is-preprint no is-journal-matter Within our solar system, this crustal duality is unique to Earth and is a direct manifestation of the operation of plate tectonics, which is widely considered to generate felsic continental crust ( 1 – 3 ). The dominance of felsic continental crust is crucial to planetary habitability, as the transition from mafic to felsic crust may have impacted atmospheric composition, climate stabilization, and nutrient supplies to the oceans ( 4 – 6 ). Thus, determining when the emerged crust transitioned from mafic to felsic is critical in understanding the evolution of our planet ( 2 , 3 ). Secular evolution of the composition of the UCC has implications for the feedback between the onset of plate tectonics, accumulation of O 2 in the atmosphere and ocean, and nutrient supply to the oceans. Estimates for the timing of initiation of plate tectonics range from early Hadean to ~0.85 Ga based on many different lines of evidence ( 54 , 55 ). Difficulties in determining this timing mainly arise from the fragmentary rock record for the early Earth and debate on the tectonic setting (subduction or non-subduction) of Archean tonalite–trondhjemite–granodiorite (TTG) suites ( 56 – 59 ). Regardless of the TTG formation mechanism, the globally synchronous occurrence of intermediate-felsic crust must be catalyzed by subduction of water into the mantle, which is greatly facilitated by subduction ( 60 – 62 ). Therefore, the compositional change in UCC from a dominantly mafic to a more intermediate or felsic composition may mark the onset of global plate tectonics, indicating global plate tectonics initiated either since the early Archean, or, at the end of Archean (~2.9 to 2.2 Ga) ( 2 , 3 , 7 , 12 ). Our findings support the view that global plate tectonics probably had not started earlier than 2.9 Ga. Furthermore, recycling of surface materials into the mantle is a direct consequence of subduction. Inert noble gas systematics serve as a tracer of atmospheric input to the mantle. Recent Xe isotopic studies indicate that effective recycling of Xe and likely other volatile components could not have occurred before 2.8 Ga, which supports a late Archean start of global plate tectonics ( 63 , 64 ). Collectively, our V isotopic data and model for reconstruction of the secular evolution of Earth’s upper continental crust composition, and by inference, timing of onset of global plate tectonics, provide important insights into the changing habitability of Earth. Methods summary Sample powders containing 6 to 10-µg V were dissolved in a 3:1 (v/v) mixture of concentrated hydrofluoric acid and nitric acid. The samples were purified using the ion-exchange chromatography method ( 71 ). The total procedural V blank was 1 to 2 ng, which is negligible compared with the total amount of V processed. The recovery rates of the purification procedures were higher than 99%.