Figure 1

Pressure–temperature (P–T) pseudosection for a mid-ocean ridge basalt (MORB), displaying the metamorphic facies for this protolith and the PT boundaries of key minerals (inset). Different shading intensity of the fields represents changes in variance of the phase assemblage, the red shading represents fields denser than the ambient mantle. Warm and cool slab-parallel geotherms are displayed for reference (dashed red lines). Isopycnals for the metamorphosed MORB are shown in dashed black lines, with assemblage fields above the density threshold delineated in red (3.33 gcm⁻³). Labelled boxes represent the P–T conditions inferred for natural eclogite terranes, which correlate well with the predicted mineral assemblages (Table S2).