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