Other
Scientific paper
Aug 1974
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1974gecoa..38.1255w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 38, Issue 8, pp.1255-1286
Other
9
Scientific paper
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 D 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 D that parallels the present-day variation in D 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 O 18 values of lizardite-chrysotile range from +8.7 to -5-0 per mil and are more correlative with the O 18 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 D (-39 to -66) and O 18 ( +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 O 18 than all true serpentines, compatible with a very low temperature of formation (~15-30°C) and serpentine-H 2 O fractionation factors of 1.0185 ( O 18 / O 16 ) and 0.946, (D/H). The observed isotopic values of all serpentines, combined with our experiments on isotopic exchange rates in the serpentine-H 2 O system, suggest that most samples examined in this work probably have largely preserved the isotopic compositions they acquired at the time of their formation.
Taylor Hugh P. Jr.
Wenner David. B.
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