Acknowledgements The RAMESSES Experiment III: Discussion

Conclusions

(1) The shallow resistivity structure on axis is well constrained to a depth of 1-1.5 km by the short range data recorded by Quail during the first source tow, at least in the vicinity of the instrument. Resistivities rise steeply from 1 at the seafloor, reaching approximately 10 at 1 km depth. The low resistivities suggest an interconnected porosity in excess of 40% in the shallow crust on axis falling to below 1% at 1 km depth.

(2) In order to explain the data recorded at longer range by the ELF instruments, the resistivity must increase with distance from the axis. However, the details of the resistivity structure outside the axial region are poorly constrained. This is primarily because of a lack of short range data collected with both the source and receiver off axis. The data are sensitive to the average off axis resistivity, rather than the details of its distribution. Values of resistivity used in the 2-dimensional modelling are broadly consistent with the results of previous electromagnetic experiments elsewhere on oceanic crust.

(3) The most striking feature of the model is the presence of a large zone of anomalously low resistivity at mid-crustal levels beneath the AVR axis. This is required to explain the large difference in the amplitude of predominantly radial and predominantly azimuthal electric fields detected by the ELF instruments at source-receiver offsets of 5-15 km.

(4) The resistivity of the sub-axial anomaly must be less than 2.5  in order to produce an adequate fit to the data. Although a melt lens similar to that imaged seismically (Navin etal , this issue) is consistent with the data, it cannot be constrained independently of the surrounding 2.5 region. The minimum thickness of the low resistivity zone is constrained to be greater than approximately 1.3 km, corresponding to a skin depth at the lowest transmitted frequency. No maximum value can be placed on the thickness using this CSEM dataset. The across axis extent of the anomaly is in the range 7-9 km. Although the shape of the low resistivity anomaly is not constrained by the CSEM data, a body coincident with the region in which the seismic P-wave velocity anomaly is greater than -0.4 km/s is compatible with the data.

(5) The mid-crustal low resistivity anomaly can be explained by the presence of basaltic melt in the crust. In order to produce a resistivity of 2.5 , the connected melt fraction in the low resistivity zone must be greater than 20%. A 7.5 km thickness of oceanic crust is formed at a rate of 20 mm/yr at this point on the Reykjanes Ridge. There is therefore enough melt in the low resistivity anomaly to feed this crustal accretion for approximately 20,000 years. However, it is probable that the melt body will solidify in less than a tenth of this time, supporting the hypothesis that at slow spreading rates crustal accretion is a cyclic process, accompanying periodic influxes of melt from the mantle.

Acknowledgements The RAMESSES Experiment III: Discussion

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