Th and U isotopic systematics of basalts from the Juan de Fuca and Gorda Ridges by mass spectrometry

We have developed a mass spectrometric technique for the measurement of 230Th/232Th ratios in young volcanic rocks. We show that we can measure 230Th/232Th ratios on MORB samples of ∼- 1 gram with an accuracy and reproducibility of 0.5–1.0%. This represents an improvement of at least a factor of 5–10 in sample size and precision over conventional alpha spectrometry methods. Using this technique, we have measured distinct excesses of230Th activity relative to238U activity for axial samples from the Juan de Fuca (JDF) and Gorda Ridges. These enrichments are 13–15% but range up to 40% in one sample. Low boron concentrations and 234U/238U ratios corresponding to secular equilibrium verify the absence of detectable seawater contamination. From this we infer that primary magmatic processes are the source of the measured230Th activity excesses, and that Th is indeed more incompatible than U during partial melting of MORB sources. 230Th/232Th activity ratios for the axial samples from JDF appear to be relatively high for uncontaminated MORB, ranging from 1.35 to 1.41. From this we infer that the JDF basalts formed from a uniform source highly depleted in Th relative to U (Th/Uwt= 2.15–2.25). One Gorda axial sample has a somewhat lower 230Th/232Th activity ratio of 1.31, suggesting formation from a source only slightly less depleted (Th/U= 2.32 ± 0.02). Based on the small range in 30Th/232Th ratios for axial basalts from JDF, dating off-axis JDF basalts may be feasible.