Under normal conditions the peridotite of the upper mantle does not melt. However, the high heat flow at ocean ridges implies that the geothermal gradient crosses the peridotite solidus at a depth of about 50 km (Wyllie, 1981, 1988), giving rise to the parental magma of the oceanic crust (Fig. 2.36). A similarly high geo-thermal gradient is believed to exist beneath oceanic islands as oceanic lithosphere traverses a mantle plume or hotspot (Section 5.5), so generating basaltic rocks by a similar mechanism.
Mid-ocean ridge basalts (MORB) have the composition of olivine tholeiite (Kay et al., 1970), and exhibit only minor variation in major element composition caused by variable alumina and iron contents. They may contain phenocrysts of olivine or plagioclase or, rarely, clinopyroxene (Nisbet & Fowler, 1978). The simplest interpretation of the chemistry of oceanic basalts, suggested from experimental petrology, is that separation of the partial melt occurs at a depth of 15-25 km. However, a wide range of alternative interpretations exist. The analysis of trace elements reveals that much of the compositional variation in the basalts is explicable in terms of high-level fraction-ation. To explain the most extreme variations, however, it is necessary to invoke the mixing of batches of magma. The frequent presence of xeno-crysts of deep-level origin indicates that the rocks only spend a very short time in a high-level magma chamber.
On a smaller scale, a detailed sampling of the East Pacific Rise by Langmuir et al. (1986) revealed a series of basalts that are diverse in their major and trace element chemistry. This compositional variation has been interpreted in terms of a series of magmatic injection centers along the crest of the ridge which correlate with bathymetric highs spaced about 50-150 km apart. Magma moves outwards from the injection points along the ridge so that the temperature of eruption decreases regularly from maxima at the bathymetric highs, which correspond to the centers of segments (Section 6.7). Batiza et al. (1988) sampled along the axis of the southern Mid-Atlantic Ridge, and showed that there are regular patterns of chemical variation along it caused by differences in the depth and extent of partial melting and degree of fractionation. They conclude that these patterns imply the presence of a deep central magma supply, with limited melt migration along the axis and no large, well-mixed magma chamber in the crust.
Flower (1981) has shown that differences in the lithology and chemistry of basalts generated at mid-ocean ridges show a simple correlation with spreading rate. The differences are not related to processes in the upper mantle, as the primary melts appear to be identical. They are believed to reflect the fraction-ation environment after partial melting. Slow-spreading systems are characterized by a complex magma chamber in which there is widespread accumulation of calcic plagioclase, the presence of phenocryst-liquid reaction morphologies, and pyroxene-dominated fractionation extracts. These phenomena are consistent with fractionation at many different pressures in a chamber that appears to be transient. This conclusion is in accord with the pattern of rare earth elements in basalts sampled from the Mid-Atlantic Ridge (Langmuir et al., 1986). Although a homogeneous mantle source is suggested, the variations in rare earth chemistry apparent in samples from adjacent areas indicate a complex subsequent history of differentiation. Fast-spreading ridges, however, suggest low-pressure basalt fractionation trends to iron-rich compositions with little plagioclase accumulation or crystal-liquid interaction. This is consistent with the magma chamber being a stable and steady state feature.
Basalts from very slow- and ultraslow-spreading ridges have lower sodium and higher iron contents than typical MORB, reflecting a smaller degree of mantle melting and melting at greater depths. The geochemistry of the peridotites dredged from such ridges also indicates that the extent of mantle melting beneath the ridge is low. The great variation in the rate at which magma is supplied along the length of the Gakkel Ridge, and its lack of correlation with spreading rate, suggests that additional factors must be involved. Different thermal regimes or varying mantle composition along the length of the ridge, or lateral migration of melts in the upper mantle are some of the possibilities. Indeed, because of the smaller vertical extent of melting beneath such ridges (Section 6.9), small variations in mantle temperature and/or composition would lead to greater proportional changes in the volume of magma produced (Michael et al., 2003).
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