U-series disequilibria in MORB from the Garrett Transform and implications for mantle melting

Here, we report 238U–230Th–231Pa–226Ra disequilibria and 87Sr/86Sr measurements in 11 mid-ocean ridge basalt (MORB) glasses from the Garrett Transform (∼13°30′S latitude on the East Pacific Rise [EPR]) whose compositions range from primitive, depleted high-MgO basalts to evolved basalts. U and Th concentrations of samples range between 3 and 75 ppb and between 6 and 220 ppb, respectively, with a corresponding large variation in Th/U (1.5–2.9). (230Th)/(232Th) varies from 1.2 to 1.6 such that (230Th)/(238U) range from ∼15% excess 230Th in a high-Th/U evolved sample to ∼25% excess 238U in a high-MgO sample with low Th/U. Out of 11 samples, 7 have 238U excess, an unusual feature for MORB. All samples have 226Ra excesses, with (226Ra)/(230Th) varying between 1.3 and 3.8 constraining ages since eruption to <8000 years and the measured (230Th)/(232Th) to be within a few percent of its value at eruption. 87Sr/86Sr ratios range between 0.7022 and 0.7024 and poorly correlate with (230Th)/(238U). ing the Garrett Transform to the Siqueiros Transform shows a remarkable correspondence between sample setting, composition and disequilibria systematics. Both settings produce linear trends of (230Th)/(238U) as a function of Th/U, consistent with mixing between two melts derived from different depths in the melting column. The mixing relationships are identical in both locations: The most incompatible rich samples with the highest Th/U and 230Th excess come from the ridge–transform intersection (RTI), whereas the most incompatible element poor basalts with the lowest (230Th)/(238U) and Th/U are erupted along leaky transform faults. Samples with intermediate Th/U and (230Th)/(238U) all come from within intra-transform spreading centers, consistent with the spreading centers acting to homogenize these diverse magmas. The cause of variation in Th/U could reflect either melting processes or different long-lived sources. No clear indication exists within these data.