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Geochronology and petrogenesis of MORB from the Juan de Fuca and Gorda ridges by 238U230Th disequilibrium

January 1, 1991

A highly precise mass spectrometric method of analysis was used to determine238U234U230Th232Th in axial and off-axis basalt glasses from Juan de Fuca (JDF) and Gorda ridges. Initial 230Th 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 usingδ234U and appear to be negligible; hence the 230Th excesses are magmatic in origin.

Direct dating of MORB was accomplished by measuring the decrease in excess 230Th in off-axis samples. 238U230Th ages progressively increase with distance from axis. Uncertainties in age range from 10 to 25 ka for UTh ages of 50 to 200 ka. The full spreading rate based on UTh ages for Endeavour segment of JDF is5.9 ± 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 be3.0 ± 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 230Th/232Th 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.

Initial230/Th/232Th ratios and230Th activity excesses were also used to examine the recentTh/U evolution and extent of melting of mantle sources beneath these ridges. A negative anomaly in230Th/232Th 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. Large230Th 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 between230Th excess and230Th/232Th for each ridge indicates that these lower degree melts formed from slightly less depleted sources than higher degree melts. Uniformity in230Th excess for other segments suggests similarity in processes of melt formation and mixing beneath most of the JDF-Gorda ridge area. The average initial230Th/232Th activity ratio of 1.31 for the JDF-Gorda ridge area is in agreement with the predicted value of 1.32 from the ThSr isotope mantle array.

Publication Year 1991
Title Geochronology and petrogenesis of MORB from the Juan de Fuca and Gorda ridges by 238U230Th disequilibrium
Authors S.J. Goldstein, M.T. Murrell, D.R. Janecky, J.R. Delaney, D. A. Clague
Publication Type Article
Publication Subtype Journal Article
Series Title Earth and Planetary Science Letters
Index ID 70016486
Record Source USGS Publications Warehouse