Multi-isotopic (Fe-Cu-Zn) constraints on the magmato-hydrothermal history during mantle exhumation at slow-spreading centers

Abstract

At slow to ultraslow-spreading ridges, tectonic mantle exhumation and magmatic processes accounts for heterogeneity in the lithosphere and drives deep hydrothermal circulation and fluids venting at the seafloor. However, the spatio-temporal evolution and the interplay between magmatic and hydrothermal processes during mantle exhumation, as well as their consequences for chemical exchange at mid-ocean ridges are poorly constrained.

We carried out a Fe, Cu and Zn isotope study of mantle rocks drilled at the Mid-Atlantic Ridge Kane (MARK) area (23′30°N) to decipher the consequences of magmatic versus hydrothermal chemical exchange on lithospheric mantle composition. At MARK, mantle rocks undergo complex melt-rock interaction during melt percolation overprinted by high temperature (HT, > 350 °C) hydrothermal circulation that leads to the formation of secondary mineral assemblages (e.g., amphibole, chlorite, ilvaite, hydro-andradite, clinopyroxene, talc, serpentine). Serpentinized peridotites cut by hydrothermally overprinted magmatic veins have increased isotopic heterogeneity to both lighter and heavier isotope compositions (δ⁵⁶Fe from −0.44 to 0.07 ± 0.03 ‰; δ⁶⁶Zn from −0.24 to 0.32 ± 0.04 ‰), expending the predictive unaltered composition of the primitive mantle (δ⁵⁶Fe = 0.025 ± 0.025 ‰ and δ⁶⁶Zn = 0.16 ± 0.06 ‰). Such variability is ascribed to diffusion-related kinetic isotope fractionation during the percolation of Fe- and Zn-rich melt in mantle rocks. Low isotopic values are due to preferential diffusion of lighter isotope in mantle rocks, while high values may involve mixing of serpentinized peridotites with isotopically heavy magmatic veins. The lower Cu content (0.5 to 23.9 ppm) and either lower or higher δ⁶⁵Cu (−0.11 to 0.32 ± 0.04 ‰) of abyssal peridotites, compared to the primitive mantle (30 ppm Cu, δ⁶⁵Cu = 0.07 ± 0.1 ‰), can be explained through Cu leaching during hydrothermal alteration of sulfide, and possibly oxide, at high temperature (∼ 450–600 °C). Hydrothermal veins in serpentinites formed at decreasing temperature (∼ 300 °C) from a metal- and sulfur-rich fluid interacting with serpentinized peridotites. Iron, Cu and Zn isotopes record the inventory of magmato-hydrothermal processes during mantle exhumation at (ultra-)slow spreading centers, from HT melt-rock interaction to late low-temperature (LT) fluid-rock interaction.