Spreading rate exerts a significant control on the nature of the magmatic
and tectonic processes that create crust at ocean ridges. Purdy
et al. [1992] showed that a systematic relation exists between spreading
rate and the depths to zones of elevated temperatures and/or partial melt.
The dependence of ridge-axis morphology and magma supply on spreading rate
was investigated by Phipps Morgan and Chen [1993a]. Niu
and Batiza [1993] investigated the spreading rate dependence of
the occurrence of the `local trend' of Klein and Langmuir [1989]
and showed that the local trend (defined by a positive correlation
between Na
O and FeO abundance exhibited by MORB collected from
a single ridge segment and corrected to 8.0 wt. % MgO) is more likely
to occur at slower spreading rates than at faster spreading rates.
The results of a thermal and mechanical model for the genesis of
oceanic crust presented by Phipps Morgan and Chen [1993b] suggest
that a delicate balance between magmatic heat injection during crustal
accretion and hydrothermal heat removal leads to different crustal
thermal structure at fast and slow spreading ridge axes. Crustal
construction at the slow-spreading mid-Atlantic ridge was suggested
to be complex and highly variable [ Smith and Cann, 1993], and
to involve a large component of seamount volcanism [ Smith and
Cann, 1992]. Bryan et al. [1994] presented detailed
comparative volcanology of small, discontinuous axial eruptive
centers from the slow spreading MARK area on the MAR. In contrast,
crustal construction at the ultrafast spreading EPR south of the
Garrett fracture zone was suggested to be uniform
[ Kent et al., 1994].