LEAD INCORPORATION DURING CRYSTAL GROWTH AND THE MISINTERPRETATION OF GEOCHRONOLOGICAL DATA FROM LOW-238U/204PB METAMORPHIC MINERALS

Abstract

The isotopic composition of lead available for incorporation during metamorphic reactions is heterogeneous, depends on the reaction history of the metamorphic rock, and is commonly not accessible for measurement as the precursor minerals have been consumed during the growth of the metamorphic phases. The initial lead composition has a significant effect on the age of low-238U/204Pb metamorphic phases (e.g. garnet, rutile, titanite, staurolite, vesuvianite and ilmenite). Using a distinct value (e.g. leached K-feldspar Pb, model Pb) rather than a geologically reasonably constrained range may result in apparently precise, yet inaccurate ages. Since age data from metamorphic minerals are widely used to unravel the P-T-t-d evolution of orogens, inaccurate ages result in: (1) incorrect timing (duration) of P-T loops and associated with it the heat budget and mass transfer in orogens; (2) arbitrary rates (based on the age difference between core and rim) for mineral growth, P-T evolution and deformation; and (3) apparent sequences of isotopic closure for the U-Pb system of contrasting minerals.