How can coupled Nd-Sr isotopic systematics in ancient terrestrial and extraterrestrial rocks be used to reconstruct the timing and extent of early planetary differentiation, and what does this reveal about the hidden reservoirs in Earth's mantle? I ask this question because this question challenges the understanding of geochemical tracing, radiogenic isotope evolution, and deep Earth processes, drawing on complex interactions between mantle-crust differentiation, isotopic reservoirs, and planetary formation history. Feel free to discuss ur valuable ideas here.