loading page

Ongoing alteration of mantle peridotite in the weathering horizon: Initial results from the Oman Drilling Project Multi-Borehole Observatory
  • +15
  • Peter B Kelemen,
  • James A Leong,
  • Juan Carlos de Obeso,
  • Juerg Matter,
  • Eric T Ellison,
  • Alexis S Templeton,
  • Daniel B Nothaft,
  • Alireza Eslami,
  • Katy Evans,
  • Marguerite Godard,
  • Jude A Coggon,
  • Nehal H. Warsi,
  • Philippe Pézard,
  • Saebyul Choe,
  • Damon Teagle,
  • Katsuyoshi Michibayashi,
  • Eiichi Takazawa,
  • Zaher Al Sulaimani
Peter B Kelemen
Columbia University

Corresponding Author:[email protected]

Author Profile
James A Leong
LDEO, Columbia University
Author Profile
Juan Carlos de Obeso
University of Calgary
Author Profile
Juerg Matter
University of Southampton
Author Profile
Eric T Ellison
University of Colorado
Author Profile
Alexis S Templeton
University of Colorado Boulder
Author Profile
Daniel B Nothaft
University of Pennsylvania
Author Profile
Alireza Eslami
ISTerre, Université Grenoble Alpes
Author Profile
Katy Evans
Curtin University of Technology
Author Profile
Marguerite Godard
Universite de Montpellier
Author Profile
Jude A Coggon
School of Ocean & Earth Science, University of Southampton
Author Profile
Nehal H. Warsi
Alara Resources Ltd.
Author Profile
Philippe Pézard
CNRS Géosciences, Université de Montpellier
Author Profile
Saebyul Choe
Dept. of Earth & Planetary Sciences, American Museum of Natural History
Author Profile
Damon Teagle
National Oceanography Centre Southampton
Author Profile
Katsuyoshi Michibayashi
Nagoya University
Author Profile
Eiichi Takazawa
Niigata University
Author Profile
Zaher Al Sulaimani
Oman Water Society
Author Profile


The Oman Drilling Project “Multi-Borehole Observatory” (MBO) samples an area of active weathering of tectonically exposed peridotite. This paper reviews the geology of the MBO region, summarizes recent research, and provides new data constraining ongoing alteration. Host rocks are partially to completely serpentinized, residual mantle harzburgites and replacive. Dunites show evidence for “reactive fractionation”, in which cooling, crystallizing magmas reacted with older residues of melting. Harzburgites and dunites are 65-100% hydrated. Ferric to total iron ratios vary from 50 to 90%. In Hole BA1B, alteration extent decreases with depth. Gradients in water and core composition are correlated. Serpentine veins are intergrown with, and cut, carbonate veins with measurable 14C. Ongoing hydration is accompanied by SiO2 addition. Sulfur enrichment in Hole BA1B may result from oxidative leaching of sulfur from the upper 30 m, coupled with sulfate reduction and sulfide precipitation at 30-150 m. Oxygen fugacity deep in Holes BA3A, NSHQ14 and BA2A is fixed by the reaction 2H2O = 2H2 + O2 combined with oxidation of ferrous iron in serpentine, brucite and olivine. fO2 deep in Holes BA1A, BA1D and BA4A is 3-4 log units above the H2O-H2 limit, controlled by equilibria involving serpentine and brucite. Variations in alteration are correlated with texture, with reduced, low SiO2 assemblages in mesh cores recording very low water/rock ratios, juxtaposed with adjacent veins recording much higher ratios. The proportion of reduced mesh cores vs oxidized veins increases with depth, and the difference in fO2 recorded in cores and veins decreases with depth.
Dec 2021Published in Journal of Geophysical Research: Solid Earth volume 126 issue 12. 10.1029/2021JB022729