Conflicts of Interest The authors declare no conflicts of interest. Supporting Information Filename Description rcm10015-sup-0001-Suppl_materials.docxWord 2007 document , 20.1 KB Table S1. Potassium isotopic compositions of the pure K solutions. Table S2. Long-term reproducibility of BHVO-2 over a 6-month period. References 1 and , “ 4.1-Composition of the Continental Crust,” in Treatise on Geochemistry, eds. H. D. Holland and K. K. Turekian (Elsevier, 2003): 1–64.