Stable isotope studies of nephrite deposits from Fengtien, Taiwan

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Metasomatic nephrite deposits ( i.e. , rodingites) occur between serpentinites and muscovite-quartz schist in the Fengtien area, eastern Taiwan. The 18 O values of nephrite range from +4.5 to +5.3%. and the D values, from -33 to -68%. The narrow range in the 18 O values and wide variation in the D values indicate low water/rock ratios and multiple origins of the ambient H 2 O during nephrite formation. The ultimate sources of the ambient H 2 O are largely sea water for the early nephrite and meteoric water for the late nephrite. The temperature of nephrite formation is 320-420°C: the estimated O-isotopic fractionation between nephrite (tremolite) and serpentine (antigorite) is 1.0 ± 0.2%. in this temperature range. Diopsidefels and epidotite are two other kinds of rodingite in the deposits. The 18 O values of diopside from diopsidefels range from +3.7 to +4.5%., the D values of clinozoisite from epidotite range from -31 to -34%. and the 18 O value of one clinozoisite analyzed is +3.9%. These data suggest that most diopsidefels formed in isotopic equilibrium with nephrite and that clinozoisite was not in isotopic equilibrium with either nephrite or diopsidefels. O-isotope compositions of both nephrite and diopsidefels cannot give conclusive evidence of their parental rocks. Circumstantial evidence, however, implies that the nephrite is derived from serpentinite whereas diopsidefels formed from unaltered ultramafic rocks.

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