Oxygen and hydrogen isotopic compositions of ultramafic rocks at the antigorite stage of serpentinization: a case of voikarite from the polar urals

摘要

Wide variation of the hydrogen isotopic composition (deltaD from -155 to -25 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 460-610 deg C from the oxygen isotope equilibrium between antigorite and magnetite, which is close to the upper temperature limit of antigoritization. The oxygen isotopic composition of olivine remnants after antigoritization could change, as well as that of the newly formed antigorite (delta~(18)O = 5.7-7.9 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.