Fluid-driven element mobility resets plagioclase rubidium strontium and barium clocks while potassium feldspar resists

Abstract Rubidium, cesium, strontium, barium and their isotopes are employed as tracers of magmatic differentiation and fluid-mediated metasomatism, but their mobility during mineral-scale alteration remains uncertain. Here we carry out an in situ elemental and isotope analysis of feldspars from altered pegmatites in the North China Craton to trace element mobility in feldspar during fluid–rock interaction. Our results show that plagioclase readily re-equilibrates with invading magmatic-hydrothermal fluids, erasing magmatic rubidium, cesium, strontium, and barium signature, whereas potassium feldspar only partly yields, selectively leaking cesium and rubidium while shielding strontium and barium. Lead isotopes act as an internal measure of reaction progress, quantifying this mineral-scale interplay: plagioclase tracks the fluid composition within millimetres, whereas potassium feldspar preserves most of its primary signature even in strongly altered zones. Comparing both feldspars offers a practical check on whether feldspar-based isotope signatures record primary magmatic values or later fluid overprinting.