MELT TO MUSH VARIATIONS IN CRUSTAL MAGMA PROPERTIES ALONG THE RIDGE CREST AT THE SOUTHERN EAST PACIFIC RISE

The determination of along-axis variations in melt properties within t he crustal axial magma chamber beneath fast spreading axes is importan t for understanding melt delivery from the mantle, eruption history al ong the ridge crest, and the process of crustal accretion. Seismic ref lection 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 r emained elusive, despite several attempts to constrain these propertie s(5-7). Here we report that the melt sill along the southern East Paci fic Rise, which is about 50 m thick, undergoes abrupt changes in its i nternal properties, ranging from pure melt to mush. The 60-km-long rid ge-crest segment near 14 degrees 00' S is characterized by three 2-4-k m sections containing pure melt embedded within a magma chamber rich i n 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 unlike ly to erupt and may influence the lower-crustal accretion.