[1] A new method for calculating fractionation pressures of mid-ocean ridge basalts (MORB) that are saturated in clinopyroxene and plagioclase is presented. This mineral assemblage is the major control on the CaO versus Mg # (molar Mg/(Mg + Fe-tot), all Fe as Fe2+) chemical variations of basaltic liquids. By combining new equilibrium and fractional crystallization experiments on primitive mid-ocean ridge basalts at high pressure with previously published experiments on natural basaltic systems, we derive an expression for fractionation pressures that depends only on the CaO content and the Mg # of the liquid. P (kbar) = [CaO (wt %) - 3.98(+/- 0.17) - 14.96(+/- 0.34) x Mg # (molar)]/[-0.260(+/- 0.008)], r(2) = 0.92. We compare our formulation of a liquid barometer with those of Grove et al. (1992), Yang et al. (1996), and Herzberg (2004). The equation can be used to predict crystallization pressures of dry tholeiitic liquids with Mg # < 0.6. Low H2O contents in the liquid (< 1 wt %) do not significantly affect the calculated pressures compared to anhydrous liquids. Pressure calculations have been performed on MORB glasses for three mid-ocean ridge systems with different spreading rates (East Pacific Rise, Mid-Atlantic Ridge, Southwest Indian Ridge). Average pressure estimates correlate negatively with spreading rate when the MORB data are filtered for hot spot-affected compositions. Major element variations indicate that MORB crystallizing at elevated pressures also require lower degrees of mantle partial melting. Locations and compositions of MORB glasses crystallized at elevated pressures can be explained with models where conductive cooling is enhanced, either at segmented ridges with slow spreading rates or along ridge segment terminations at fast spreading ridges.