Hydration due to high-T brittle failure within in situ oceanic crust, 30°N Mid-Atlantic Ridge

Katsuyoshi Michibayashi, Takehiro Hirose, Toshio Nozaka, Yumiko Harigane, Javier Escartin, Heike Delius, Margaret Linek, Yasuhiko Ohara

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Abstract

Analysis of an in situ fault zone within the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge) provides clues to the relevant deformation mechanisms and their temporal evolution within oceanic crust. IODP EXP304/305 drilled a succession of gabbroic lithologies to a final depth of 1415 m below the sea floor (mbsf), with very high recovery rates of up to 100% (generally ∼ 80%). We identified an intra-crustal fault zone between 720 and 780 mbsf in a section of massive gabbro, olivine gabbro, oxide gabbro units, and minor diabase intrusions. Of particular interest is the section between 744 and 750 mbsf, which unfortunately was marked by low recovery rates (17%). Electrical borehole-wall images show a 1-m-thick zone of east-dipping fractures within this interval, which is otherwise dominated by N-S dipping structures. Despite the high fracture density in this section, the hole walls are smooth, with rare breakouts, suggesting that the low recovery rate was due to a change in lithology rather than well conditions. The recovered rocks include ultracataclasite and possibly incohesive fault gouge that formed in the upper amphibolite regime, with mostly amphibole infill. Logging data suggest that the gabbroic rocks in this interval are rich in hydrous phases, consistent with increased amounts of amphibole found in the core. Equilibration temperature conditions of about 640 °C were obtained for plagioclase clasts and aluminous actinolite, assuming a pressure of 200 MPa. The permeability of the fault zone is in the range of 10- 19 to 10- 17 m2. Although the permeability appears to be high within the fault zone relative to other parts of the section, it is no higher than that in typical lower crustal material. As a consequence, because brittle failure occurred at high temperatures, the fault zone was subsequently completely sealed by hydrous minerals, thereby preventing further fluid circulation and preserving water in the crust.

Original languageEnglish
Pages (from-to)348-354
Number of pages7
JournalEarth and Planetary Science Letters
Volume275
Issue number3-4
DOIs
Publication statusPublished - Nov 15 2008

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Keywords

  • core-log
  • fault
  • gabbro
  • IODP U1309D
  • Mid-Atlantic Ridge
  • permeability

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

Cite this

Michibayashi, K., Hirose, T., Nozaka, T., Harigane, Y., Escartin, J., Delius, H., Linek, M., & Ohara, Y. (2008). Hydration due to high-T brittle failure within in situ oceanic crust, 30°N Mid-Atlantic Ridge. Earth and Planetary Science Letters, 275(3-4), 348-354. https://doi.org/10.1016/j.epsl.2008.08.033