D/H and O¹⁸/O¹⁶ studies of serpentinization of ultramaflc rocks

Abstract

Oxygen and hydrogen isotope analyses have been obtained on serpentine and related metasomatic talc and 'blackwall' minerals from the following ultramaflc-rock types: 1. (1) alpine associations in unmetamorphosed or low-grade metamorphic terranes; 2. (2) alpine associations from medium- to high-grade regionally metamorphosed areas; 3. (3) magmatic ultramafic intrusions such as the layered stratiform complexes, the concentrically-zoned Alaskan-type bodies, the high-temperature Mt. Albert alpine intrusion, and the kimberlite body at Moses Rock, Utah; 4. (4) dikes and sills of Precambrian shield areas; and 5. (5) deweylites, which are serpentine-like mineraloids of probable near-surface weathering origin. The δD values of the lizardite-chrysotile serpentines from all of the ultramafic rock types exhibit a total range from −59 to −205 per mil and, except for the Precambrian samples, most show an exceptionally good correlation with geographic position and latitude of the sample locality. The lizardite-chrysotile samples show a progressive change in δD that parallels the present-day variation in δD of meteoritic waters across the North American continent: Caribbean (−59 to −71); Central America (−78 to -97); California (−85 to −108); Oregon (−107 to −149); Washington (−127 to −142); S.E. Alaska ( − 127 to −140); southern British Columbia ( − 150 to −163); northern British Columbia ( −162 to −205). The δO18 values of lizardite-chrysotile range from +8.7 to −5-0 per mil and are more correlative with the δO18 of the country rocks than with latitude. These relationships, combined with model estimates of the isotopic compositions of waters involved in such serpentinization, suggest that much of this lizardite-chrysotile serpentinization probably formed by waters of meteoric-hydrothermal origin at relatively shallow levels in the Earth's crust. Inasmuch as sedimentary formation waters typically contain a major meteoric-water component, they are also possible agents of this type of serpentinization. In contrast to lizardite-chrysotile, the pure antigorites have very restricted ranges of δD (−39 to δ-66) and δO18 ( +4.7 to +8.7) similar to values in metamorphic chlorites. This indicates that the antigorites generally have formed during regional metamorphism in the presence of non-meteoric waters; similar relationships are also observed for all metasomatic talc and 'blackwall' minerals. The deweylites are consistently higher in δO18 than all true serpentines, compatible with a very low temperature of formation (~15–30°C) and serpentine-H2O fractionation factors of 1.0185 () and 0.946, (D/H). The observed isotopic values of all serpentines, combined with our experiments on isotopic exchange rates in the serpentine-H2O system, suggest that most samples examined in this work probably have largely preserved the isotopic compositions they acquired at the time of their formation.

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