Samples of plagioclase olivine websterite included in variously serpentinized harzburgite and lherzolite tectonites were collected by submersible on the northwest flank of the Gettysburg Seamount near 36°N in the northeastern Atlantic. The websterite exhibits a coarse porphyroclastic texture (70% porphyroclasts), and the composition of the pyroxene shows a progressive re-equilibration with decreasing temperature (and probably pressure) under anhydrous conditions from a near-solidus temperature of ~1250 °C (porphyroclasts) down to ~700 °C (granoblasts). In situ observations, as well as textural, mineralogical, and chemical evidence, suggest that the websterite can best be interpreted as a heteradcumulate rock that crystallized on the walls of narrow conduits from a MORB type magma flowing through the residual tectonites.The Mg #'s (= 100 × Mg/(Mg + Fe2+)) of coexisting clinopyroxene, orthopyroxene, and olivine (91.5–90.5, 90.0–89.7, and 89.5–88.5, respectively), the plagioclase composition (An88.7–87.9), and the mineral minor-element distribution are not compatible with the crystallization at low pressure of the websterite from N-MORB type parental magma. Instead, the calculated composition of the liquid in equilibrium with the websterite's most magnesian minerals and the bulk-rock composition are more akin to those of a poorly differentiated T-MORB than of N-MORB parental magma. Phase relations and mineral compositions produced by anhydrous melting experiments for synthetic and natural systems indicate that the pressure of crystallization of the plagioclase olivine websterite is constrained within the narrow range of 7–10 kbar. It is thought that fractionation of websterite is probably a minor but significant petrologic factor of MORB type magma differentiation. It could explain some of the compositional variations observed in magnesian MORB.