Conclusions The RAMESSES Experiment III: The mid-crustal low resistivity

Discussion

Despite the wide variation in mid-ocean ridge morphology, the basic structure of mature oceanic crust is remarkably similar regardless of the spreading rate at which is formed (Raitt, 1963). At fast and intermediate spreading ridges, crustal magma chambers have been imaged seismically, and provide an explanation of the method of formation of new oceanic crust (Detrick etal , 1987; Vera etal , 1990; Collier & Sinha, 1992). Until now, there has been no evidence of similar melt accumulations in the crust beneath a slow-spreading mid-ocean ridge. Thermal arguments suggest that no steady state magma body can exist at spreading rates less than 20-40 mm/ yr (Sleep, 1975; Kusznir & Bott, 1976; Phipps-Morgan & Chen, 1993). Two alternative hypotheses to explain the formation of crust at slow spreading ridges have been proposed.

In the first scenario, melt rises in small discrete packets, which feed individual seamounts and flows on the seafloor through a series of dikes (Nisbet & Fowler 1978; Smith & Cann, 1992; Magde & Smith, 1995). Each small melt body is likely to solidify before the next one is emplaced, with the result that a magma chamber as such never develops.

In the alternative hypothesis, melt injection into the crust is periodic. Periods of magmatic quiescence are punctuated by influxes of melt from the mantle. This view is supported by the observations of Murton & Parson (1993), who used deep towed sidescan sonar data to infer an AVR lifecycle of volcanic activity and quiescence at the Reykjanes Ridge. In this case, substantial melt accumulations may exist beneath the axes of slow spreading mid-ocean ridges, but would be highly transient features, accompanying fresh melt inputs from the mantle.

A melt fraction of 20% suggests a total volume of 3 kmof melt per kilometre beneath the AVR. A crustal thickness of 7.5 km is formed at a rate of 20 mm/ yr at this point on the Reykjanes Ridge, so the mid-crustal low resistivity zone contains enough melt to feed crustal accretion at this AVR segment for 20,000 years. Thermal arguments make it likely that the melt body is a transient feature. In addition, if such large magma chambers were typical features of slow spreading ridges, it is unlikely that they would have escaped detection for so long. The discovery of a melt accumulation of this size lends support to the model of cyclic crustal accretion accompanying periodic influxes of melt from the mantle at slow spreading rates.

It is probable that volcanic activity at the seafloor accompanies fresh inputs of melt from the mantle. Since a melt input of the size detected is only required once in every 20,000 years to produce the 7.5 km thickness of crust observed seismically, this implies that the period between episodes of constructive volcanism is also of the order 20,000 years. This is of the same order as the periodicities of active volcanism on slow spreading ridges estimated to be about 10,000 years, based on the spatial density of volcanos in the crust and the spreading rate (MacDonald, 1982). Between these times the AVRs are faulted and broken up by tectonic extension (Murton & Parson, 1993).

A useful quantity to estimate is the time taken for a partially molten body of the type detected beneath the Reykjanes Ridge to solidify. It is thought that hydrothermal circulation is the dominant mechanism of heat loss and therefore it may be assumed that the latent and specific heat of injected melt is convected upward through the hydrothermal system (Sleep & Wolery, 1978;Henstock etal, 1993). If it is assumed that at fast spreading rates, crustal accretion is a steady state process (Phipps-Morgan & Chen, 1993) then the amount of heat dissipated by hydrothermal circulation is equal to the heat input by the melt. For a ridge with a spreading rate, v, where a thickness of crust, t is formed then the heat dissipated per unit length of the ridge in a year is

where is the density, L is the latent heat of crystallisation, is the heat capacity and is the drop in temperature as the melt cools. For a melt body of width, w, the heat loss per unit area, A, across the top of the body is given by

At the East Pacific Rise, a crustal thickness, t, of 6.5km is formed at a rate,v, of 120 mm/yr. The width, w, of the melt body imaged seismically is on average 1km (Kent etal , 1990). The latent heat and heat capacity of basaltic rocks are 3.410 J/kg and 800 J/kgK respectively (Henstock etal , 1993). The heat contained in the low resistivity anomaly detected at the Reykjanes Ridge can be evaluated similarly :

where is the volume of the low resistivity anomaly and a 20% melt fraction is assumed. Since it is likely that the melt body at the Reykjanes Ridge represents a single influx of melt from the mantle, it may be assumed that there is no further heat input after the initial injection of melt. If it is further assumed that the dominant mechanism of heat loss is through hydrothermal circulation, and that the the rate of heat loss per unit area across the top of the melt body is the same at the Reykjanes Ridge as at the fast spreading East Pacific Rise, then the time taken for the melt body to lose enough heat to solidify is given approximately by

where is the thickness of the low resistivity anomaly and it has been assumed that the density, , is the same at both fast and both spreading rates, since in both instances normal oceanic crust is formed. For a temperature drop of 500°C, sufficient to cool basaltic melt from above the liquidus to below the solidus, the time taken for the melt in the mid-crustal low resistivity anomaly to solidify is on the order of 1500 years. No account is taken of conductive heat loss, which may be significant at slow spreading rates, and would reduce the time taken for the melt body to solidify (Sleep, 1975).

The calculation of the solidification time of the melt body is very approximate, however it illustrates that the time for which the melt body is present beneath the ridge is significantly less than the time between melt influxes. For over 90% of an accretionary cycle there would be little or no observable evidence for the presence of a melt body, either in the form of a low velocity or a low resistivity region. Given that the period for which a new input of melt is observable, compared to the overall time between successive influxes of melt, the majority of slow-spreading ridge segments will not be underlain by an observable magma chamber.

Conclusions The RAMESSES Experiment III: The mid-crustal low resistivity

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