Oxygen and hydrogen isotopic compositions of ultramafic rocks at the antigorite stage of serpentinization: A case of voikarite from the Polar Urals

Wide variation of the hydrogen isotopic composition (deltaD from -155 to -25parts per thousand) was established for antigorite from a peculiar olivine-antigorite ultramafic rock in the Polar Urals. It suggests the mixing of meteoric and metamorphic water in the fluid responsible for antigoritization. The temperature of the process was estimated as 460-610degreesC from the oxygen isotope equilibrium between antigorite and magnetite, which is close to the upper temperature limit of antigotitization. The oxygen isotopic composition of olivine remnants after antigoritization could change, as well as that of the newly formed antigorite (delta(18)O = 5.7-7.9parts per thousand), depending on the width of the voikarite zone, i.e., on the intensity of the process. As a result, an inhomogeneity in oxygen isotopic composition arose in ultramafic rocks and retained at subsequent deserpentinization.