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. ¿ American Geophysical Union 1993