Lead isotopes in feldspars trace fluid circulation in deforming granitoids

The U–Th–Pb system is a potential impactful tracer of fluid sources and pathways in the continental crust. Different rock types exhibit distinct U/Th/Pb ratios, and within a single rock, heterogeneities arise from different minerals having different U, Th and Pb contents. These differences result in distinct Pb isotope compositions over time, at the scale of mineral to whole rock. Fluids circulating in the continental crust thus inherit different Pb isotope ratios upon interaction with different rocks along their pathways. Feldspars dominate the granitic continental crust and typically contain tens of µg/g of Pb but negligible U or Th, and thus they retain their primary Pb isotope composition. However, feldspars are also readily altered by fluid–rock interaction processes, and they incorporate the Pb isotope composition of fluids with which they interact. Such modified feldspars can thus provide information on the nature of crustal fluids.

This concept is applied to feldspars in post-Variscan mantle-derived granitoids from the Aar Massif (central Swiss Alps). In this setting, fluids circulated during Permian and Mesozoic rifting, and during the (Miocene) Alpine orogeny. The combination of Pb–Sr–O–H isotope data in hydrothermal epidote revealed that Permian and Miocene fluids had external sources and exploited various pathways to infiltrate the granitoids. Triassic seawater infiltration was inferred from biotite Rb–Sr data.

Feldspar grains were separated from the granitoids, and subsequently leached to remove alteration minerals until they appeared transparent. Feldspar Pb isotope ratios were measured by solution MC-ICP-MS after acid digestion and ion exchange chromatography. The Pb isotope ratios in these leached feldspar fractions, reported here as ²⁰⁷Pb/²⁰⁶Pb ratios of 0.8249–0.8050 and ²⁰⁸Pb/²⁰⁶Pb ratios of 2.042–2.021 for direct comparison to LA-ICP-MS data of fluids, are more radiogenic than model values at the time of granitoid emplacement (ca. 300 Ma). This is attributed to post-magmatic processes resetting Pb isotope ratios of feldspars. This hypothesis is explored further by comparing the Pb isotope ratios of feldspars to that of Permian (²⁰⁷Pb/²⁰⁶Pb = 0.8326–0.8296; ²⁰⁸Pb/²⁰⁶Pb = 2.064–2.051) and Miocene fluids (²⁰⁷Pb/²⁰⁶Pb = 0.8118–0.7308; ²⁰⁸Pb/²⁰⁶Pb = 2.021–1.904), and to the Pb isotope evolution of the granitoids (whole rocks) from their emplacement until the present day.

The agreement of feldspar Pb isotope data with those of Permian and Miocene fluids suggests that the fluids altered the initial Pb isotope ratios of feldspars, imparting a more radiogenic composition. However, the overlap of feldspar Pb isotope data with the Pb isotope evolution of the granitoids suggests that the Pb isotope composition of feldspars includes components inherited by redistributing Pb mobilized from variably U-/Th-enriched phases within the granitoids themselves. This suggests that the local, fluid-induced heterogeneities in Pb isotope ratios within the studied feldspars result from fluid-induced redistribution of Pb originating from the granitoids themselves, rather than by fluid-mediated addition of externally derived Pb. This hypothesis has implications for our understanding of Pb sources and redistribution – with possible enrichment into ore deposits – in the granitic continental crust.