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OCEAN RIDGES 133

not yielded any convincing evidence for a magma ing an outer transition zone made up of a hot, mostly

chamber or melt lens (Whitmarsh, 1975; Fowler, 1976; solidified crust with small amounts of interstitial melts
Purdy & Detrick, 1986; Detrick et al., 1990). However, and an inner zone of crystal mush with suffi cient melt
Calvert (1995), in reanalyzing the data of Detrick et al. for it to behave as a very viscous fluid. A melt lens only

(1990) acquired at 23°17′N, isolated reflections from a develops in fast-spreading ridges where there is a suffi -

presumed magma chamber at a depth of 1.2 km and ciently high rate of magma supply for it to persist at the
with a width of 4 km. top of the mush zone (Fig. 6.11a). This lens may extend
It seems unlikely therefore that steady state magma for tens of kilometers along the ridge crest, but is only
chambers exist beneath the axes of slowly spreading 1–2 km wide and tens or hundreds of meters in thick-
ridges. Transient magma chambers, however, related to ness. Slow-spreading ridges are assumed to have an
influxes of magma from the mantle, may exist for short insufficient rate of magma supply for a melt lens to

periods. In order to test this hypothesis a very detailed develop (Fig. 6.11b) and that eruptions only occur when

combined seismic and electromagnetic experiment was there are periodic influxes of magma from the mantle.
carried out across the Reykjanes Ridge south of Iceland Such a model is consistent with the seismic data from
(Sinha et al., 1998). This study was deliberately centered ocean ridges and petrologic observations which require

on a magmatically active axial volcanic ridge (AVR) on magma to have been modified by fractionation within
the Reykjanes Ridge at 57°45′N, and did reveal a melt the crust, which could not occur in a large, well-mixed
lens and crystal mush zone analogous to those imaged chamber. It also explains why less fractionation occurs
on the East Pacific Rise. In this instance the melt lens in the volcanic rocks of slow-spreading ridges. A

occurs at a depth 2.5 km beneath the sea fl oor. The problem with this model, however, is that it is not
results of this study provide strong support for the apparent how the layered gabbros of layer 3 might
hypothesis that the process of crustal accretion on slow- develop.
spreading ridges is analogous to that at fast-spreading Subsequent work by Singh et al. (1998), involving
ridges but that the magma chambers involved are short- further processing of the seismic refl ection data
lived rather than steady state. Despite its proximity to obtained by Detrick et al. (1993a) near to 14°S on the
the Iceland hot spot, the ridge crest south of 58°N on East Pacific Rise, was specifically targeted at identifying

the Reykjanes Ridge has the characteristics of a typical any along-axis variations in the seismic properties and
slow-spreading ridge: a median valley, and normal thickness of the melt lens. Their results suggest that
crustal thickness and depth. only short, 2–4 km lengths of the melt lens contain pure
The logistically complicated seismic experiments melt capable of erupting to form the upper crust. The
required to test for the presence or absence of a melt intervening sections of the melt lens, 15–20 km in
lens have yet to be carried out on the very slow- and length, are rich in crystal mush and are assumed to
ultraslow-spreading Gakkel Ridge. It seems extremely contribute to the formation of the lower crust. It seems
unlikely that melt lenses exist beneath the amagmatic probable that the pockets of pure melt are related to
segments of this ridge, in that these consist of mantle the most recent injections of magma from the mantle.
peridotite with only a thin carapace of basalts, but pos-
sible that transient melt lenses occur beneath the mag-
matic segments and volcanic centers (Section 6.9).
However, in 1999 seismological and ship-borne sonar 6.7 ALONG-AXIS
observations recorded a long-lived magmatic-spreading
event on the Gakkel Ridge that had characteristics more
consistent with the magma being derived directly from SEGMENTATION OF
mantle depths than from a crustal magma chamber
(Tolstoy et al., 2001). OCEANIC RIDGES
Sinton & Detrick (1992), taking account of the
seismic data available at that time and incorporating
new ideas on magma chamber processes, proposed a Many early investigations of ocean ridges were essen-
model in which the magma chambers comprise narrow, tially two-dimensional in that they were based on quite
hot, crystal-melt mush zones. In this model magma widely spaced profiles oriented perpendicular to their

chambers are viewed as composite structures compris- strike. More recently “swath”-mapping systems have