Geochronology and petrogenesis of MORB from the Juan de Fuca and Gorda ridges by 238U230Th disequilibrium
A highly precise mass spectrometric method of analysis was used to determine 238 U 234 U 230 Th 232 Th in axial and off-axis basalt glasses from Juan de Fuca (JDF) and Gorda ridges. Initial 230 Th activity excesses in the axial samples range from 3 to 38%, but generally lie within a narrow range of 12 to 15%. Secondary alteration effects were evaluated using δ 234 U and appear to be negligible; hence the 230 Th excesses are magmatic in origin. Direct dating of MORB was accomplished by measuring the decrease in excess 230 Th in off-axis samples. 238 U 230 Th ages progressively increase with distance from axis. Uncertainties in age range from 10 to 25 ka for U Th ages of 50 to 200 ka. The full spreading rate based on U Th ages for Endeavour segment of JDF is 5.9 ± 1.2 cm/yr, with asymmetry in spreading between the Pacific ( 4.0 ± 0.6 cm/yr) and JDF ( 1.9 ± 0.6 cm/yr) plates. For northern Gorda ridge, the half spreading rate for the JDF plate is found to be 3.0 ± 0.4 cm/yr. These rates are in agreement with paleomagnetic spreading rates and topographic constraints. This suggests that assumptions used to determine ages, including constancy of initial 230 Th/ 232 Th ratio over time, are generally valid for the areas studied. Samples located near the axis of spreading are typically younger than predicted by these spreading rates, which most likely reflects recent volcanism within a 1–3 km wide zone of crustal accretion. Initial 230 /Th/ 232 Th ratios and 230 Th activity excesses were also used to examine the recent Th/U evolution and extent of melting of mantle sources beneath these ridges. A negative anomaly in 230 Th/ 232 Th for Axial seamount lavas provides the first geochemical evidence of a mantle plume source for Axial seamount and the Cobb-Eickelberg seamount chain and indicates recent depletion of other JDF segment sources. Large 230 Th activity excesses for lavas from northern Gorda ridge and Endeavour segment indicate formation from a lower degree of partial melting than other segments. An inverse correlation between 230 Th excess and 230 Th/ 232 Th for each ridge indicates that these lower degree melts formed from slightly less depleted sources than higher degree melts. Uniformity in 230 Th excess for other segments suggests similarity in processes of melt formation and mixing beneath most of the JDF-Gorda ridge area. The average initial 230 Th/ 232 Th activity ratio of 1.31 for the JDF-Gorda ridge area is in agreement with the predicted value of 1.32 from the Th Sr isotope mantle array.