Major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading (150 mm/yr) East Pacific Rise (EPR), 13°–23°S, and near‐ridge seamounts comprise 212 chemical groups. We interpret these groups to represent the average composition of individual lava flows or groups of closely related flows. Groups slightly enriched in K2O (T‐MORB) are distributed variably along the axis, in contrast to the Galapagos Spreading Center where T‐MORB are extremely rare. This result is consistent with the interpretation that T‐MORB magmas arise from low‐melting temperature, K‐rich heterogeneities in the subaxial EPR mantle. The Galapagos Spreading Center, which is migrating to the west in an absolute reference frame, is underlain by mantle previously processed and depleted in the T‐MORB component during melting events giving rise to earlier EPR magmas. Excluding T‐MORB, there are nearly monotonic, twofold increases in K/Ti and K/P of axial lavas from 23°S to 13°S. From 22°S to 17°S these gradients correlate with isotopic ratios, but north of 17°S there is a reversal of isotopic gradients, indicating (recent?) decoupling of the isotopic and minor element ratios in the subaxial mantle. A strong, southward increase in degree of differentiation for approximately 200 km north of the large offset at 20.7°S correlates with a gradient in bathymetry, consistent with previous interpretations that this offset is propagating to the south. Samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive, evolved fractionation signatures of rift propagation, suggesting that propagating rift tips have been abandoned preferentially to failing rift tips. Glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset, and possibly also south of 22°S; the latter region may be affected by deformation accompanying northward growth of the Easter Microplate. Near‐ridge seamounts on the Pacific Plate between 18°–19°S comprise two distinct populations: those aligned approximately parallel to the spreading direction are extremely variable in major element composition, but consistently enriched in Sr relative to nearby axial lavas; smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and Sr relative to axial lavas. The degree of differentiation of axial lavas between 18°–19°S can be related to the structural development of the rift axis and/or vigor of hydrothermal activity of individual segments. Glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area. Primary magmatic segmentation mainly reflects mantle source variations, the boundaries of which correlate with the largest physical offsets in the rise axis between the Easter Microplate and Garrett Transform Zone. A secondary magmatic segmentation, defined by the along‐axis continu...

An analysis is presented for the variation in chemistry of 162 normal mid‐ocean ridge basalt glass groups, collected with an average spacing of about 10 km along a −1100 km section of the East Pacific Rise between 13° and 23°S. Long‐wavelength periodicities have been determined for oxides Al2O3, CaO, Cr2O3, FeO*, K2O, MgO, MnO, Na2O, NiO, P2O5, SiO2, TiO2; for ratios FeO*/MgO, K/P, and K/Ti; for FeO*, Na2O, and SiO2 corrected for fractionation to 8.0 wt % MgO; and for bathymetry. The data were modeled as the sum of a long‐wavelength periodic component and a residual random component. The power spectrum of the periodic component was determined using the maximum entropy method. The spectral phase of significant spectral peaks was determined by cross correlation with a cosine wave with the wavelength of the respective peaks. In most cases the characteristics of the random component were found to be consistent with a normal distribution. Spatial aliasing considerations and the variance of the random component place confidence limits upon the spectral features. The results of the spectral analyses indicate substantial correlation in the power spectra of the oxides. The oxide data have spectral peaks with wavelengths in the vicinity of 575 and 225 km and a local minimum in spectral power at wavelengths between 300 and 500 km. In general, oxide components compatible in the crystallizing minerals olivine and plagioclase are approximately 180° out of phase with the incompatible oxides, indicating that fractionation is an important process in controlling compositional variations along this portion of the East Pacific Rise. Unlike the fractionation‐sensitive oxide data, periodic variations are not pronounced for K/Ti, indicating that the length scales of mantle composition being melted are uncorrelated with those of magmatic temperature variation. The analysis of fractionation‐corrected values of Na8.0 and Fe8.0 indicates that degree and depth of partial melting show a strong spectral peak near a wavelength about 430 km. There is also significant power in the spectrum of Na8.0 near 260 km and of Fe8.0 near 200 km, bounding the average spectral peak for the oxides at 225 km. There appears to be strong coupling between the degree and depth of melting, and magmatic temperature at two length scales, around 225 km and 400–600 km, corresponding to wavelengths of geoid undulations observed in the vicinity of the East Pacific Rise, and suggesting a relationship with convection.

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