The determination of along-axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle, eruption history along the ridge crest, and the process of crustal accretion. Seismic reflection images(1-4) have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths (250-4,500 m), but variations in its seismic properties and thickness have remained elusive, despite several attempts to constrain these properties(5-7). Here we report that the melt sill along the southern East Pacific Rise, which is about 50 m thick, undergoes abrupt changes in its internal properties, ranging from pure melt to mush. The 60-km-long ridge-crest segment near 14 degrees 00＇ S is characterized by three 2-4-km sections containing pure melt embedded within a magma chamber rich in mush. These small pure melt pockets may represent a fresh supply of magma from the mantle, capable of erupting and forming the upper crust. Conversely, the 80-90% of the magma chamber which is mushy is unlikely to erupt and may influence the lower-crustal accretion.