Pb isotopic zoning of K-feldspar megacrysts determined by Laser Ablation Multi-Collector ICP-MS: Insights into granite petrogenesis

This study investigates Pb isotopic zoning in magmatic K-feldspar megacrysts from the Monte Capanne pluton (Elba, Italy) using Laser Ablation Multi-Collector-ICPMS. The studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open-system processes and better characterize the components involved in the genesis of complex magma systems. Earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal-derived magmas.The investigated K-feldspar megacrysts exhibit strong zoning in ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb, correlated with lead elemental variations. We interpret these variations as reflecting growth zoning, as opposed to secondary diffusive exchange. Despite a great variety of zoning patterns, we were able to correlate different events of megacryst growth, reflecting crystallization in a dynamic magma system. Our two-step model includes (1) growth of a granitic magma chamber by addition of low ²⁰⁸Pb/²⁰⁶Pb magma to a high ²⁰⁸Pb/²⁰⁶Pb magma contaminated with crustal material (i.e., the megacryst cores) and (2) recharge by mantle-derived magma (i.e., the megacryst rims). We interpret the thorogenic nature of the megacryst rims to reflect the mantle-derived component involved in the mixing process. Taking account of other data from the Tuscan Magmatic Province, the mantle source is inferred to have been metasomatized by continental material during subduction. TIMS Sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled.