The origin of fluids associated with serpentinization processes: Evidence from stable-isotope compositions

Values of delta D for serpentine minerals from the Mesozoic Cassiar serpent inite in north-central British Columbia (delta D from 196 to -103 parts per thousand), and from several serpentinites from the Archean Abitibi greenst one belt in Ontario (delta D from -147 to -54 parts per thousand) are varia ble, low, and many are consistent with those expected from modern meteoric water, regardless of the probable age of serpentinization. In contrast, del ta(18)O values are relatively constant, although different for each area. S uch a pattern of variable hydrogen but constant oxygen isotopic composition s would be expected from hydrogen-isotope exchange, rather than rock-fluid interaction with varying water:rock ratios. Exchange is greater near faults , where enhanced flow of fluid would be expected. Values of delta(18)O corr elate with mineralogy and texture (whereas delta D does not), and the oxyge n isotope thermometer gives temperatures consistent with those inferred fro m mineral assemblages (e.g., 250 to 300 degrees C). Antigorite has higher d elta D values than lizardite and chrysotile because antigorite forms imperv ious rocks, attains a greater grain-size, and has a greater degree of cryst allinity relative to lizardite or chrysotile. Isotopic data from other stud ies of serpentinites show similar patterns and can be attributed to the sam e process of later hydrogen-isotope exchange. On the basis of data for anti gorite from serpentinized komatiitic flows from the Abitibi belt, Archean s eawater had delta D values similar to those of modem seawater.