Magnesium Isotopes of Talc and Ultramafic Rock from Seafloor Hydrothermal Vents in the Central Indian Ridge

Abstract

In a typical submarine hydrothermal system, magnesium is depleted in vent fluid because it is removed from the seawater by the fluid-rock reaction within the oceanic crust. Thus, hydrous magnesium silicate, talc, found from slow-spreading mid-ocean ridges often considered a product of alteration of ultramafic host rock exposed on the seafloor. However, at Onnuri Vent Field (OVF) in the Central Indian Ridge, talc occurs not only in the mounds and sediments vicinity of the vent field but also within the chimneys, suggesting that the talc might be precipitated directly from the Mg-rich vent fluid as well as originated from the serpentinized ultramafic host rock. Based on the appearance of the precipitated talc, the OVF is presumed to be composed of hybrid type hydrothermal vents formed by a mixture of hydrothermal fluid and unmodified seawater, but the lack of evidence for the origin of Mg in the vent fluid prevents to reveal of hydrothermal circulation scale and structure. Since Mg isotopes tend to be fractionated by direct precipitation (carbonation) rather than hydrothermal alteration (serpentinization), we present the Mg isotope compositions of the talc and host rock of the OVF and attempt to identify the formation mechanism of talc in the slow-spreading mid-ocean ridge hydrothermal system.