CUMULATES FROM STRONGLY DEPLETED MID-OCEAN-RIDGE BASALT

RECENT studies of abyssal peridotites1, mid-ocean-ridge basalts (MORBs )2 and their entrained melt inclusions3,4 have shown that fractional m elting of the upwelling sub-oceanic mantle produces magmas with a much wider range of compositions than erupted MORBs. In particular, it see ms that strongly depleted primary magmas are routinely produced by mel ting beneath ridges1. The absence of strongly depleted melts as erupte d lavas prompts the question of how long such magmas survive beneath r idges, before their distinctive compositions are concealed by mixing w ith more enriched magmas. Here we report mineral compositions from a u nique suite of oceanic cumulates recovered from DSDP Site 334 (ref. 5) , which indicate that the rocks crystallized from basaltic liquids tha t were strongly depleted in Na, Ti, Zr, Y, Sr and rare-earth elements relative to any erupted MORB. It thus appears that the magmatic plumbi ng system beneath the Mid-Atlantic Ridge permitted strongly depleted m agmas to accumulate in a magma chamber and remain sufficiently isolate d to produce cumulate rocks. Even so, spatial heterogeneity in the com positions of high-calcium pyroxenes suggests that in the later stages of solidification these rocks reacted with infiltrating enriched basal tic liquids.