Oxygen and hydrogen isotope studies of the serpentinization of ultramafic rocks in oceanic environments and continental ophiolite complexes

Abstract

Oxygen and hydrogen isotopic analyses have been obtained on 19 \noceanic chrysotiles and lizardites and l oceanic antigorite from dredge-hauls at three \nlocations along the Mid-Atlantic Ridge (43°N, l°N, and 22°S), the Puerto Rican \nTrench, and the Blanco Fracture Zone in the Pacific Ocean. Lizardite-chrysotile serpentines \nwere also analyzed from continental ophiolite bodies at Vourinos, Greece \n(10 samples); San Luis Obispo, Calif. (16); Del Puerto, Calif. (4); Wilbur Springs, \nCalif. (l); Stonyford, Calif. (3); Canyon Mtn., Oreg. (3); and New Caledonia (3); in \naddition to 3 antigorites from Del Puerto. There is typically a wide variation in \nδO18 in a single ophiolite complex; the lowest δO18 values tend to occur in the interior, \npartially serpentinized portions of the ultramalic body. The isotopic compositions of \nthe oceanic and continental lizardite-chrysotile fall in different ranges (Oceanic: δD \n= -35 to -68, δO18 = +0.8 to +6.7; Vourinos: δD = -88 to -114, δO18 = +2.l to \n+12.7; San Luis Obispo: δD = -82 to -91, δO18 = +3.8 to +9.3; other ophiolites: \nδD = -80 to -149, δO18 = +6.3 to +9.2). Therefore, different types of waters must \nbe involved in the serpentinization of ophiolitic and oceanic ultramafic rocks. Heated \nocean water, perhaps mixed with some magmatic water, is apparently responsible for \nsubmarine serpentinization. Most of the lizardite-chrysotile in continental ophiolites \nprobably formed from hot, exchanged meteoric ground waters, mixed meteoric-magmatic \nwaters, and/or mixed meteoric-connate waters; the small quantities of antigorite \nin these bodies apparently form in the presence of non-meteoric metamorphic waters. \nThe isotopic evidence strongly suggests that most of the serpentine in ophiolite complexes \nwas not originally formed from ocean water. Some of the O18-depleted serpentines \nin the Vourinos and San Luis Obispo complexes can conceivably be interpreted \nas having formed in this manner, but if so, all these serpentines must have later \nundergone essentially total D/H exchange with other types of waters. Thus, if \nophiolite complexes truly represent exposures of oceanic crust and mantle, they probably \nwere largely unserpentinized prior to their emplacement on land.