Initial Pb?Sr(?Nd) isotopic heterogeneity in a single allanite?epidote crystal: implications of reaction history for the dating of minerals with low parent-to-daughter ratios

Abstract

We analyzed 17 fragments from a zoned allanite–epidote crystal (ca 2.2 mm × 4.0 mm), which had formed during different prograde and retrograde stages of ultra high pressure (UHP) and amphibolite facies metamorphism (240–230 Ma, Sulu Belt, E China), for the isotopic composition of Pb, Nd, and Sr and contents of Pb, U, and Th, Sr and Rb, and Nd and Sm. Since most fragments had 238U/204Pb and 232Th/204Pb values less than 1, corrections for in situ Pb growth are small and uncertainties in the recalculation of the Pb isotopic compositions to 240 Ma are insignificant. The recalculated Pb falls on a linear trend in the 206Pb/204Pb vs 207Pb/204Pb diagram with the allanite defining the low−206Pb/204Pb end (17.07) of this trend and the epidote defining its high−206Pb/204Pb end (17.56). The recalculated data scatter in the 206Pb/204Pb vs 208Pb/204Pb diagram, which implies that the initial Pb isotopic variation reflects the involvement of at least three different Pb sources. The low 87Rb/86Sr values account for a change in 87Sr/86Sr by in situ 87Sr growth of less than 0.0007, which implies that the isotopic heterogeneity of 87Sr/86Sr (0.70601–0.7200) is a primary feature. The Pb and Sr isotope data unequivocally demonstrate that contributions from different precursor minerals result in initial isotopic heterogeneity in the metamorphic reaction product. It is likely that such an initial isotopic heterogeneity also exists for Nd, but it could not be resolved in the present study. Initially heterogeneous Pb and Sr isotope compositions imply that age differences between core and rim of large crystals may result in the determination of highly arbitrary geological rates, especially for minerals with relatively low parent-to-daughter ratios.