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

Abstract

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.

Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observations

“…Calvert (1995) suggested the presence of a small magma body beneath the segment immediately south of the Kane TF, but the seismic images available were of poor quality. A combined seismic and electromagnetic study along the Reykjanes Ridge (57845 ′ N) did identify an AML located about 2.5 km beneath a large AVR (Sinha et al 1998). In addition, a large swarm of hydroacoustically and teleseismically recorded earthquakes in 2001 in the Lucky Strike segment near 37820 ′ N was interpreted by Dziak et al (2002) as a magma/dyke emplacement event into the crust beneath the Lucky Strike volcano, a c. 1 km high edifice in the segment centre; however, there was no along-axis migration of seismicity to indicate lateral propagation of a dyke and subsequent surveys showed no indication of eruption on the seafloor.…”

Geophysical characterisation of the ocean–continent transition at magma-poor rifted margins

“…The velocity of normal oceanic lower crust is well defined around 6.9-7.0 km/s, but it is actually not easy to explain this observation from first principles [Korenaga et [57]" ---Note that the interpretation of ref. [57] has recently been challenged by Korenaga [2007, JGR]. In this regard, it's important to emphasize the virtue of the combined use of seismic and electromagnetic methods.…”

Crustal accretion of thick mafic crust in Iceland: implications for volcanic rifted margins

“…2 for locations (25-29 are not in the map area). (a) Iceland: 1-6 with increasing distance from the hotspot (Darbyshire et al 2000a); (b) platform and crust from Kolbeinsey Ridge (Hooft et al 2006); (c) Reykjanes Ridge (Bunch and Kennett 1980;Rizert and Jacoby 1985;Sinha et al 1998); (d) East Greenland Margin: 15-transition zone, 16-thick oceanic crust (Korenaga et al 2000); (e) Faeroe-Iceland Ridge (Bohnhoff and Makris 2004;Smallwood et al 1999;White et al 2008); (f) Hatton Bank (White et al 2008); (g) Vøring Margin (Mutter and Mutter 1993); (h) average oceanic crust (Bown and White 1994). [Colour online.…”