Résumé
Oman Drilling Project Hole BT1B drilled 300 meters through the basal thrust of the Samail ophiolite and provides a unique record of interaction between peridotite in the leading edge of the mantle wedge and hydrous, CO (sub 2) -rich fluids derived from subducting lithologies. The upper 200 meters are dominated by listvenites (carbonated peridotites) with two serpentinite/ophicarbonate bands: One 20 m thick (depth 80-100 m) and one 3 m thick (183-186 m). Below the ultramafic sequence a 70 cm fault zone separates the mantle lithologies from the metamorphic sole of the ophiolite. The serpentinites and listvenites have (super 87) Sr/ (super 86) Sr significantly more radiogenic that average mantle and ambient peridotite in Oman with an average value of 0.7119+ or -0.0014. The two-serpentinite bands are characterized by less radiogenic (super 87) Sr/ (super 86) Sr than the listvenites, with values dropping to 0.7090 in the lower serpentinite. The values of listvenites and serpentinites are significantly more radiogenic than the underlying metamorphic sole (average 0.7060+ or -0.0008), modern seawater (0.709) or Cretaceous seawater (0.7075), but similar to the allochthonous Hawasina formation that underlies the ophiolite (0.7116+ or -0.0041; Falk & Kelemen 2015). Carbon contents above the basal thrust vary from less than 1 wt% with mantle-like d (super 13) C ( approximately -5 ppm) in the serpentinite bands to more than 11.5 wt% with d (super 13) C approximately -1 ppm in listvenites. Two component mixing models with mantle like and Hawasina (super 87) Sr/ (super 86) Sr and d (super 13) C end members suggests decarbonation reactions of the Hawasina units during subduction produced the radiogenic (super 87) Sr/ (super 86) Sr and carbon-rich fluids responsible for the converting peridotite to listvenite.