Exploration of Continental Lithospheric Mantle Evolution Using Ta/Th Ratios in Volcanic Rocks

The ratio of tantalum to thorium (Ta/Th) in basaltic volcanic rocks has been recently proposed to be an indicator of the style of melting. Low values of Ta/Th (<0.2) are correlated with flux melting triggered by hydration, eg., subduction zones. High values (0.6 to 1.2) are correlated with decompression melting, e.g., mid-ocean ridges. The southwestern part of North America (SWNA) has undergone a transition from subduction to extension in Cenozoic time and we see variably complete transitions from low to high Ta/Th, with intermediate values (0.2 to 0.6) in-between. We analyze Ta/Th ratios vs age for basalts in spatial tiles across SWNA and characterize the observed transition in each tile by fitting a sigmoid function and noting the optimal parameters for midpoint and rate. We find a consistent transition from low to high Ta/Th across SWNA and, where extension is present, the midpoint of the transition agrees with the onset of extension. Focusing on regions that show intermediate Ta/Th, we test whether these intermediate values arise from the transport of magma through the CL (continental lithosphere). Using a 1D transport model, we show that with low transport velocity and a low diffusive constant for Th, intermediate Ta/Th can be produced even when the initial Ta/Th is not intermediate. This suggests that transport processes should be investigated in further work as they may provide clues to better understand the variability of magma-CL interactions.