Page 154 - Global Tectonics

140 CHAPTER 6

order effects are caused by the geochemical differences highs, which correspond to the centers of segments
in magma supply. (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
6.8 PETROLOGY OF partial melting and degree of fractionation. They con-
clude that these patterns imply the presence of a deep
OCEAN RIDGES 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
Under normal conditions the peridotite of the upper lithology and chemistry of basalts generated at mid-
mantle does not melt. However, the high heat fl ow at ocean ridges show a simple correlation with spread-
ocean ridges implies that the geothermal gradient ing rate. The differences are not related to processes
crosses the peridotite solidus at a depth of about 50 km in the upper mantle, as the primary melts appear to

(Wyllie, 1981, 1988), giving rise to the parental magma be identical. They are believed to reflect the fraction-
of the oceanic crust (Fig. 2.36). A similarly high geo- ation environment after partial melting. Slow-spread-
thermal gradient is believed to exist beneath oceanic ing systems are characterized by a complex magma
islands as oceanic lithosphere traverses a mantle plume chamber in which there is widespread accumulation
or hotspot (Section 5.5), so generating basaltic rocks by of calcic plagioclase, the presence of phenocryst-
a similar mechanism. liquid reaction morphologies, and pyroxene-domi-
Mid-ocean ridge basalts (MORB) have the compo- nated fractionation extracts. These phenomena are
sition of olivine tholeiite (Kay et al., 1970), and exhibit consistent with fractionation at many different pres-
only minor variation in major element composition sures in a chamber that appears to be transient. This
caused by variable alumina and iron contents. They conclusion is in accord with the pattern of rare earth
may contain phenocrysts of olivine or plagioclase or, elements in basalts sampled from the Mid-Atlantic
rarely, clinopyroxene (Nisbet & Fowler, 1978). The Ridge (Langmuir et al., 1986). Although a homoge-
simplest interpretation of the chemistry of oceanic neous mantle source is suggested, the variations in
basalts, suggested from experimental petrology, is that rare earth chemistry apparent in samples from adja-
separation of the partial melt occurs at a depth of cent areas indicate a complex subsequent history of
15–25 km. However, a wide range of alternative inter- differentiation. Fast-spreading ridges, however, suggest
pretations exist. The analysis of trace elements reveals low-pressure basalt fractionation trends to iron-rich
that much of the compositional variation in the compositions with little plagioclase accumulation or
basalts is explicable in terms of high-level fraction- crystal–liquid interaction. This is consistent with the
ation. To explain the most extreme variations, magma chamber being a stable and steady state
however, it is necessary to invoke the mixing of feature.
batches of magma. The frequent presence of xeno- Basalts from very slow- and ultraslow-spreading
crysts of deep-level origin indicates that the rocks ridges have lower sodium and higher iron contents
only spend a very short time in a high-level magma than typical MORB, reflecting a smaller degree of

chamber. mantle melting and melting at greater depths. The
On a smaller scale, a detailed sampling of the East geochemistry of the peridotites dredged from such
Pacific Rise by Langmuir et al. (1986) revealed a series ridges also indicates that the extent of mantle melting

of basalts that are diverse in their major and trace beneath the ridge is low. The great variation in the rate
element chemistry. This compositional variation has at which magma is supplied along the length of the
been interpreted in terms of a series of magmatic injec- Gakkel Ridge, and its lack of correlation with spread-
tion centers along the crest of the ridge which correlate ing rate, suggests that additional factors must be
with bathymetric highs spaced about 50–150 km apart. involved. Different thermal regimes or varying mantle
Magma moves outwards from the injection points composition along the length of the ridge, or lateral
along the ridge so that the temperature of eruption migration of melts in the upper mantle are some of
decreases regularly from maxima at the bathymetric the possibilities. Indeed, because of the smaller vertical