EMORB – sea-floor basalt in the wrong place at the wrong time

Eruption of mid-ocean ridge basalt (MORB) constitutes approximately 80% of the earth’s volcanism (by volume), and a very high proportion of that activity takes place within about 1 km of the 56,000 linear km of the ocean-floor spreading centres (Elliot, 2002).  Recent work by some US and British geologists has focussed on understanding the nature and origin of some of the off-axis volcanism on the East Pacific Rise (Zou et al., 2002)

Before we can discuss the off-axis volcanism it is important to have a clear understanding of the processes of on-axis volcanism at a typical ridge.  The picture of the mid ocean ridges that has emerged over the past 20 years is summarized on the diagram below.

The slow flow (cm/y) of the solid mantle is shown as the heavy black lines.  As convection moves mantle material towards surface beneath the ridge its temperature drops, but its melting point also decreases because of the drop in pressure.  The pressure-related melting point decreases at a faster rate than the drop in temperature, and mantle material crosses the solid-liquid boundary at a depth of about 60 km.  At this point only a very small fraction of the mantle rock starts to melt, but as the material continues to ascend the degree of melting increases, until the amount of melt reaches a maximum of between 25 and 40% just below the ridge axis (Turcotte and Phipps Morgan, 1992, Langmuir et al., 1992).  The average degree of partial melting is close to 10%, which explains why partial melting within a 60 km thick zone of mantle produces oceanic crust that is consistently around 6 km thick.

The zone of partial melting defines an upside-down triangle in cross-section, with a depth of 60 km, and a base between 60 and 100 km wide.  This shape is related to the strong heat flow in ridge areas, and the fact that temperatures drop as material is moved laterally away from the ridge.  Material within this triangular zone is still largely solid, but it has varying amounts of melt. The melt, being lighter than the surrounding solid mantle material, has a tendency to rise, and the magma migration rate is estimated to be 2 to 3 orders of magnitude greater than the solid flow rate (Turcotte and Phipps Morgan, 1992).  The upward flow is not vertical, but is strongly focussed towards the actual ridge crest.  There are several possible reasons why this is the case, including:

  • fluid flow is towards the area of least pressure, which is at the centre of passive spreading,
  • fluid flow is constrained by the low permeability of the re-frozen mantle material outside of the triangular zone, and
  • because of the change in direction of the solid mantle flow there is preferential development of porosity and permeability in the direction towards the ridge crest.

MORB has MgO levels in the 9% range, which is less than is expected from partial melting of mantle material at 0 to 60 km depth.  Based on this observation it is interpreted that most magma in this setting undergoes fractionation due to separation of olivine crystals, and that the magma which makes its way to the sea floor is primarily the residual melt produced by this fractionation process (Hess, 1992).

While most spreading-ridge volcanism takes place within about 1 km of the ridge axis, some eruptions can be as much as 3 to 4 km away, and there is evidence of eruptions taking place several tens of kilometres away.  Zou et al. (2002) have collected basalt samples from the sea floor up to 35 km away from the ridge on either side of the East Pacific Rise at 9.5 degrees north, off of the coast of Central America.  They have carried out elemental and isotopic analysis of these samples, firstly to determine their age, and secondly to provide some information about the source of the magma.

Zou et al. have found that while most sea-floor basalt samples in ridge areas are derived from the normal magma forming processes (normal or NMORB), and appear to have ages appropriate to their distance from the ridge, some basalts of apparently recent origin are situated up to 20 km from the ridge crest.  Of these, some are geochemically similar to NMORB, while others are clearly different.  These samples – which are enriched in the so-called incompatible elements, such as Rb, Ba, Th, U, Nb and the light rare-earth elements – are referred to as EMORB.

References

Elliot, T., Caught offside, Science, V. 295, p.55-56.

Hess, P., 1992, Phase equilibria constraints on the origin of ocean floor basalts, in Phipps Morgan, J., Blackman D. and Sinton, J. (eds) Mantle flow and melt generation at mid-ocean ridges, Geophysical Monograph 71, American Geophysical Union, p. 67.

Langmuir, C., Klein, E. and Plank, T., 1992, Petrological systematics of mid-ocean ridge basalts: constraints on melt generation beneath ocean ridges, in Phipps Morgan, J., Blackman D. and Sinton, J. (eds) Mantle flow and melt generation at mid-ocean ridges, Geophysical Monograph 71, American Geophysical Union, p. 183.

Turcotte, D. and Phipps Morgan, J., 1992, The physics of magma migration and mantle flow beneath a mid-ocean ridge, in Phipps Morgan, J., Blackman D. and Sinton, J. (eds) Mantle flow and melt generation at mid-ocean ridges, Geophysical Monograph 71, American Geophysical Union, p. 155-182.

Zou, H., Zindler, A. and Niu, Y., 2002, Constraints on melt movement beneath the East Pacific Rise from 230Th-238U disequilibrium, Science, V. 295, p. 107-110 (January 2002)

Steven Earle, 2002. Return to Earth Science News