Investigating Pb Isotope Systematics Using 3D Geodynamic Models.

The Pb isotope signature of oceanic basalts is characterised by positive linear correlation between ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb. The gradient of the regression line in ²⁰⁶Pb/²⁰⁴Pb vs ²⁰⁷Pb/²⁰⁴Pb space (pseudo-isochron) can be used to calculate a 'bulk' melting age (τ_Pb), which is around 1.9 Gyr for a global set of fresh oceanic basalts. Reconciling this age in geodynamic models could help us to understand relative importance of different processes on the U-Th-Pb system. To do this we test a range of parameters to unpick the effect that each has on τ_Pb and the scatter. We build upon previous work by modelling in 3D spherical geometry and introducing methods to redistribute continental material into the mantle and mantle material to a continental reservoir. We also touch on the differences in Pb isotope ratios between different oceanic basins and assess whether current models can usefully investigate potential causes of this.

We find that increasing the difference between the partition coefficients of Pb and its parent U and Th isotopes has a negligible effect on τ_Pb but sees scatter (weighted average orthogonal distance from regression line) increase over 4x. Increasing the difference between the partition coefficients of Pb and its parent isotopes at 2.4Ga to simulate influence of GOE, reduces τ_Pb to around 1.8 Gyr. However, this also decreases the scatter by an order of magnitude, so the full range of Pb ratios that are measured in oceanic rocks cannot be generated. Allowing Pb to be eroded from the continental reservoir into the mantle after 2.4 Ga also decreases τ_Pb from around 3.0 Gyr to 1.8 Gyr but increases the scatter by an order of magnitude, bringing the model predictions closer to that of the data. The degree of processing (how much of the mantle has melted) also plays a role in decreasing τ_Pb and generating more scatter.