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Department of Geology and Geophysics and Hawaii Center for Volcanology, University of Hawaii, 2525 Correa Road, Honolulu, HI, 96822, USA
Tectonic and magmatic segmentation of ridges occurs at several scales that overlap or form a continuum. A good working hypothesis is that segmentation is hierarchical and that magmatic and tectonic segmentation are linked and related by mantle flow and upwelling patterns. The largest scale of magmatic segmentation is isotopic, reflecting differences in mantle history and composition spanning a range of scales as large as individual ocean basins or larger. Superimposed, is the most conspicuous scale of segmentation, defined by large transforms and/or non-transform offsets. Magmatically, this scale of segmentation seems to reflect differences in the melting process together with the effects of smaller scale mantle heterogeneity. There appear to be differences between slow- and fast-spreading magmatic segments, although other factors such as magma supply, plume influence and ridge obliquity may also have important effects. Recurring patterns at individual segments, at least partly independent of spreading rate, include: constancy v. variability in inferred melting parameters, regular v. irregular spatial distribution of enriched basalts, MgO values, and other parameters, and correlation vs. no correlation of chemistry with axial depth.
The smallest scales of magmatic segmentation require the most closely spaced and detailed sampling. Since there have been relatively few studies of this type, there are many questions remaining. It is likely that this scale of segmentation is controlled by deep crustal and shallow upper mantle processes. However, the superimposed effects of mantle heterogeneity and time-dependent processes such as volcanic-magmatic cycles complicate the picture.
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