Microstructural and experimental constraints on the rheology of partially molten gabbro beneath oceanic spreading centers

Aaron S. Yoshinobu, Greg Hirth

Research output: Contribution to journalArticlepeer-review

18 Scopus citations

Abstract

Flow laws for high-temperature creep of olivine, plagioclase, and diabase are used to place constraints on the rheology of partially molten lower oceanic crust. This analysis is motivated by the observation of olivine lattice preferred orientations and subgrain microstructures in oceanic gabbros that lack evidence for dislocation creep in coexisting plagioclase and pyroxene. Extrapolation of experimental flow laws indicates that at temperatures above 1100°C and stresses less than 10 MPa, olivine may be the weakest phase in rocks with gabbroic composition. By accounting for variations in the melt fraction (0-10%) and grain size of partially molten plagioclase aggregates we can constrain the rheological conditions where olivine deforms by dislocation creep while plagioclase deforms by diffusion creep. Calculated effective viscosities range from 1015 to 1019 Pa s; based on observations of the geometry of the partially molten zone beneath the East Pacific Rise and the microstructural and experimental constraints we favor a value of ~1018 Pa s. This value approaches estimates for the viscosity of the upper mantle beneath ridge axes, but is significantly higher than previously suggested for the partially molten lower crust. Such high viscosities are inconsistent with ridge evolution models that require large amounts of lower crustal flow to accommodate melt redistribution. However, the results are compatible with recent models that favor local magma replenishment from the mantle at closely spaced intervals along the spreading center axis in a 2D, 'sheet-like' fashion.

Original languageEnglish
Pages (from-to)1101-1107
Number of pages7
JournalJournal of Structural Geology
Volume24
Issue number6-7
DOIs
StatePublished - 2002

Keywords

  • Flow laws
  • Hypersolidus deformation
  • Microstructures
  • Mid-ocean ridges
  • Ophiolites
  • Partially molten gabbro
  • Rheology

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