Detailed Step-Leaching Experiments on Plagioclase Feldspar Grains: Understanding the Physical and Chemical Distribution of Pb Isotopes

Plagioclase feldspar Pb isotope systematics were investigated by MC-ICP-MS analyses of leachates and residues from six detailed step-leaching experiments, combined with SEM imaging of the residues. The grains represent the least magnetic fraction (N3) of a leucogabbro (SR246) from the 1.43 Ga Laramie anorthosite complex, Wyoming. They have an anorthite content of 47.7±2.8 (1sd, n=75), and contain minor oxide needles, polyphase solid inclusions (silicates, Fe-sulfides and -oxides, apatite, zircon and baddeleyite), and antiperthite. The first leachates yielded extremely radiogenic lead isotopic ratios (206Pb/204Pb=83.116-96.023, 207Pb/204Pb=21.524-22.685 and 208Pb/204Pb=97.42-109.80) and are associated with dissolution of grain fractures, soluble inclusion phases (apatite and sulfides), and antiperthitic areas. The least radiogenic leachate was measured after six leaching steps, having 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 16.679, 15.526, and 35.968, respectively. Dissolution features include etching of grain surfaces, dissolution of antiperthite exsolution, and reduction of overall grain size. The isotopic composition of intermediate and final residues shows a positive correlation with the number of leaching steps. Regression of leachate data results in a 207Pb/206Pb age of 1431.3±2 Ma (MSWD=0.11), i.e. slightly younger that the U-Pb baddeleyite/zircon age of the rock (1436.2±0.6 Ma). We identify the presence of different Pb reservoirs: (1) "labile" Pb and highly soluble inclusions easily removed by leaching; (2) a plagioclase isotopic signature associated with structurally bound Pb; and (3) possible contribution of refractory phases discernible after final dissolution to explain the increasing radiogenic nature of residues. A key result is the observed closed-system radiogenic decay of U and Th in the plagioclase feldspars, and preservation of the parental isotopic signature of the magma. These conclusions are supported by the Pb/Pb linear arrays and similar age of leached material and whole rock.