Abstract

Mid-ocean ridge basalt (MORB) is the most abundant magma on Earth. It is generated beneath mid-ocean ridges by decompression melting of upwelling mantle, and, following processing in lower crustal magma chambers, erupted onto the seafloor. For more then four decades igneous petrologists and geochemists have relied upon MORB as their major window into the mantle, deriving its composition, melting processes and melt migration mechanisms from the erupted lavas. However, this approach assumes that modification of melts in crustal magma chambers occurs exclusively by fractional crystallisation, and can thus be easily corrected for. Data from an extensive suite of lower crustal rocks from Hess Deep (equatorial Pacific Ocean) demonstrate that melts do not simply evolve by fractional crystallisation. The gabbros crystallised from melts that underwent extensive reactive porous flow, which modified both their major- and trace element composition. The degree to which this reactive signature is present increases up section throughout the lower crust, suggesting that it occurs on a crustal scale. Thus, magma in the lower oceanic crust evolves by a combination of fractional crystallisation and melt-rock reaction. If a reactive signature is present in MORB , this requires a reassessment of its use as a messenger from the mantle.