We examined an expanded global data set of mid-ocean ridge basalt (MORB) major element analyses. In agreement with previous results, we show that slow spreading ridges tend to have more primitive (high Mg/Fe) lavas than fast spreading ridges. Fractionation-corrected values of Na(8) and Ca(8)/Al(8) (indices of the extent of melting) and Si(8)/Fe(8) (an index of the pressure of melting) do not vary systematically with spreading rate. Assuming a mantle that is generally homogeneous in major elements, we conclude that average mantle temperature in the region of melting below mid-ocean ridges is independent of spreading rate. Using data for 32 best sampled ridge segments of variable length, we show that the so-called global and local trends of chemical variation (Klein and Langmuir, 1989) are systematically distributed with spreading rate. The global trend (positive correlation between extent of melting and melting pressure) occurs at fast spreading ridges (>60 mm/yr), while the local trend (negative correlation between extent of melting and melting pressure) occurs at slow spreading ridges (<50 mm/yr). This distribution is independent of geographic length scale. Among the 32 ridge systems we examined, the slopes of the two trends on chemical diagrams show some variability, but no regular pattern, such as fanning. The global trend is well-explained by differences in average mantle temperature occurring at several length scales within mantle rising passively in response to plate separation. We propose that the local trend arises from processes occurring in buoyant diapirs undergoing melting and melt-solid reequilibration. Several lines of geophysical and geological evidence point to the importance of buoyant, three-dimensional mantle upwelling beneath slow spreading ridges. Petrologic modeling presented here is consistent with this hypothesis, as is the existence of the local trend at seamounts on the flanks of the East Pacific Rise. ¿ American Geophysical Union 1993