Temperatures of serpentinization of ultramafic rocks based on O 18 /O 16 fractionation between coexisting serpentine and magnetite

摘要: 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 magnetite (δO18 magnetite=−7.6 −4.6 relative SMOW). Nine antigorites (mainly Vermont S. E. Pennsylvania) distinctly smaller 8.7 4.8 magnetite=−2.6 +1.7 mil). Two lizardite chrysotile dredged Mid-Atlantic Ridge exhibit 10.0 12.4 magnetite=−6.8 −7.9 mil, respectively), whereas an oceanic antigorite shows a value 8.2 magnetite=−6.2). These data all clearly indicate that formed at higher temperatures than chrysotilelizardites. Electron microprobe analyses magnetites above samples they are chemically homogeneous essentially pure Fe3O2. However, some certain other wide variation Cr content also give very erratic isotopic results, suggesting non-equilibrium. An approximate serpentine-magnetite geothermometer curve was constructed by (1) extrapolation observed O18 chlorites Fe-Ti oxides in low-grade pelitic schists whose known quartz-muscovite geothermometer, (2) estimates fractionation factor chlorite (assumed be equal unity). This suggests equilibrium as follows: continental lizardite-chrysotile, 85° 115° C; chrysotile, 130° C 185° C, respectively; antigorite, 235° antigorites, 220° 460° C.