Abstract
Five lizardite-chrysotile type serpentinites from California, Guatemala and the Dominican Republic show oxygen isotope fractionations of 15.1 to 12.9 per mil between coexisting serpentine and magnetite (δO18 magnetite=−7.6 to −4.6 per mil relative to SMOW). Nine antigorites (mainly from Vermont and S. E. Pennsylvania) show distinctly smaller fractionations of 8.7 to 4.8 per mil (δO18 magnetite=−2.6 to +1.7 per mil). Two lizardite and chrysotile serpentinites dredged from the Mid-Atlantic Ridge exhibit fractionations of 10.0 and 12.4 per mil (δO18 magnetite=−6.8 and −7.9 per mil, respectively), whereas an oceanic antigorite shows a value of 8.2 per mil (δO18 magnetite=−6.2). These data all clearly indicate that the antigorites formed at higher temperatures than the chrysotilelizardites. Electron microprobe analyses of magnetites from the above samples show that they are chemically homogeneous and essentially pure Fe3O2. However, some magnetites from certain other samples that show a wide variation of Cr content also give very erratic oxygen isotopic results, suggesting non-equilibrium. An approximate serpentine-magnetite geothermometer curve was constructed by (1) extrapolation of observed O18 fractionations between coexisting chlorites and Fe-Ti oxides in low-grade pelitic schists whose isotopic temperatures are known from the quartz-muscovite O18 geothermometer, and (2) estimates of the O18 fractionation factor between chlorite and serpentine (assumed to be equal to unity). This serpentine-magnetite geothermometer suggests approximate equilibrium temperatures as follows: continental lizardite-chrysotile, 85° to 115° C; oceanic lizardite and chrysotile, 130° C and 185° C, respectively; oceanic antigorite, 235° C; and continental antigorites, 220° to 460° C.
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Contribution No. 2029 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91109.
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Wenner, D.B., Taylor, H.P. Temperatures of serpentinization of ultramafic rocks based on O18/O16 fractionation between coexisting serpentine and magnetite. Contr. Mineral. and Petrol. 32, 165–185 (1971). https://doi.org/10.1007/BF00643332
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DOI: https://doi.org/10.1007/BF00643332