The RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics) targeted an apparently magmatically active axial volcanic ridge (AVR), centred on 57°45′N at the Reykjanes Ridge, with the aim of investigating the processes of crustal accretion at a slow spreading mid‐ocean ridge. As part of this multicomponent experiment, airgun and explosive wide‐angle seismic data were recorded by 10 digital ocean‐bottom seismometers (OBSs) along profiles oriented both across‐ and along‐axis. Coincident normal‐incidence seismic, bathymetry and underway gravity and magnetic data were also collected.  Forward modelling of the seismic and gravity data has revealed layer thicknesses, velocities and densities similar to those observed elsewhere within the oceanic crust near mid‐ocean ridges. At 57°45′N, the Reykjanes Ridge has a crustal thickness of approximately 7.5 km on‐axis. However, the crust is modelled to decrease in thickness slightly off‐axis (i.e. with age), which implies that full crustal thickness is achieved on‐axis and that it is subsequently thinned, most likely, by off‐axis extension. Modelling also indicates that the AVR is underlain by a thin (∼100 m), narrow (∼4 km) melt lens some 2.5 km beneath the seafloor, which overlies a broader zone of partial melt approximately 8 km in width. Thus the results of this study provide the first clear evidence for a crustal magma chamber beneath any slow spreading ridge. The size and depth of this magma chamber (the melt lens and underlying zone of partial melt) are similar to those observed beneath fast and intermediate spreading ridges, which implies that the processes of crustal accretion are similar at all spreading rates. Hence the lack of previous observations of magma chambers beneath slow spreading ridges is probably temporally related to the periods of magmatic activity being considerably shorter and more widely spaced in time than at fast and intermediate spreading ridges.

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, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57°45′N. It consisted of three major components: wide‐angle seismic profiles 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 sub‐axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids. The AVR is underlain by a significant crustal 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 seismic data; a seismic reflection from its upper surface; and a region of anomalously low electrical resistivity constrained by the CSEM data. It includes 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 zone, which flanks and underlies the melt lens. RAMESSES is the first experiment to provide convincing evidence of a significant magma body beneath a slow spreading ridge. The result provides strong support for a model of crustal accretion at slow spreading rates in which magma chambers similar to those at intermediate and fast spreading ridges play a key role in crustal accretion, but are short‐lived rather than steady‐state features. The magma body can exist for only a small proportion of a tectono‐magmatic cycle, which controls crustal accretion, and has a period of at least 20 000 years. These 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.

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Summary Presented in this paper are the results of a two‐stage analysis of gravity data acquired during a multidisciplinary geophysical survey of a magmatically active axial volcanic ridge (AVR) segment located at 57°45′N on the Reykjanes Ridge, part of the slow spreading Mid‐Atlantic Ridge south of Iceland. Modelling of the free‐air anomaly in 2‐D shows that, across‐axis, the observed anomaly results largely from density and layer thickness variation in the mid‐lower crust. Although seismic control on crustal thickness along‐axis is limited, modelling also suggests that both crustal density and thickness also vary towards AVR tips. Using the 2‐D modelling results as crustal reference, the residual mantle Bouguer anomaly (RMBA) is calculated to assess whether magma‐related density anomalies are present in the mantle and to investigate the structure of, and relationship between, adjacent AVRs along‐axis. RMBA lows are associated with both an along‐ridge trend encompassing a number of adjacent AVRs and with individual, more topographically robust AVRs. Modelling of the RMBA low associated with the 57°45′N AVR further suggests that along‐axis density variation is confined to the central region of this AVR and that the anomaly can largely be accounted for by density variation within Layer 3 and a degree of crustal thinning towards AVR tips. The nature of the along‐axis variation in crustal density further suggests that it may result from repeated phases of magma supply to the crustal system from the mantle. Within the resolution of the RMBA, modelling does not confirm or preclude the presence of a subcrustal density anomaly associated with retention of a small percentage of melt in the mantle. However, a melt‐free model for at least the top 40 km of the mantle is preferred as this is consistent with the results of modelling a coincident magnetotelluric data set. The ridge‐trend characteristics of the RMBA also suggest that magma delivery may take place along this trend, and in an episodic fashion that controls initiation of an AVR tectonomagmatic cycle. AVRs may, therefore, be created along a spreading‐orthogonal direction by tapping of a recent magma injection into the ridge‐trending crustal system, with flow of this magma laterally along AVR‐parallel faults and fissures accommodating spreading and crustal accretion.