Magmatic processes at slow spreading ridges: implications of the RAMESSES experiment at 57 degrees 45 ' N on the Mid-Atlantic Ridge

This paper is the first in a series of three (this issue) which present the results of the RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics). RAMESSES was an integrated geophysical study which was carefully targeted on a magmatically active, ax ial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57 degr ees 45'N. It consisted of three major components: wide-angle seismic profil es along and across the AVR, using ocean-bottom seismometers, together with coincident seismic reflection profiles; controlled-source electromagnetic sounding (CSEM); and magnetotelluric sounding (MT). Supplementary data sets included swath bathymetry, gravity and magnetics. Analyses of the major components of the experiment show clearly that the su b-axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids. The AVR is underlain by a significant crus tal magma body, at a depth of 2.5 km below the sea surface. The magma body is characterized by low seismic velocities constrained by the wide-angle se ismic data; a seismic reflection from its upper surface; and a region of an omalously low electrical resistivity constrained by the CSEM data. It inclu des a thin, ribbon-like melt lens at the top of the body and a much larger region containing at least 20 per cent melt in a largely crystalline mush z one, which flanks and underlies the melt lens. RAMESSES is the first experi ment to provide convincing evidence of a significant magma body beneath a s low spreading ridge. The result provides strong support for a model of crus tal accretion at slow spreading rates in which magma chambers similar to th ose at intermediate and fast spreading ridges play a key role in crustal ac cretion, but are short-lived rather than steady-state features. The magma b ody can exist or only a small proportion of a tectono-magmatic cycle, which controls crustal accretion, and has a period of at least 20 000 years. The se findings have major implications for the temporal patterns of generation and migration of basaltic melt in the mantle, and of its delivery into the crust, beneath slow-spreading mid-ocean ridges.