Focused mantle upwelling beneath mid-ocean ridges: evidence from seamount formation and isostatic compensation of topography

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Scientific paper

Observational evidence for the geometry of mantle flow beneath mid-ocean ridges is often considered limited. I offer two constraints supporting interpretations of strong focusing of upwelling through the depths where MORB magmas are generated. The first is based on a modification of the model of Davis and Karsten [1] for the formation of near-axis seamounts by the melting of fertile mantle heterogeneities. Several types of observations indicate that most seamounts in the eastern Pacific form in a remarkably narrow range of distance from the axis, about 6-15 km. The outer limit of this range is interpreted to represent the width of the upwelling zone in the upper part of the garnet stability field (depth ~ 80 km) where the most fertile mantle melts, while the inner limit indicates the width of most magma generation (perhaps depths of 30-50 km). The diverse and depleted chemistry of young seamount lavas is inferred to reflect originally enriched magmas generated at depth that bypass the principal zone of melting and interact with strongly depleted material at intermediate depths. A comparable and entirely independent constraint on the width of most magma generation is available from the assumption that anomalously shallow topography at ridge axes is isostatically compensated by melt at depth. The width of the topographic anomaly limits the melt zone to within 10 km from the axis, while the amplitude of the gravity anomaly, at least in some places, requires most of the melt to be deeper than 15 km. If the fraction of melt is assumed to be small ( ~ 2%), the partial melt must extend to depths exceeding 40 km.

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