A Zinc Isotope Perspective on Fluid/Mantle Interactions and Recycling During Subduction

Abstract:

Zinc isotope abundances are fairly constant in the majority of igneous rocks and shales (δ⁶⁶Zn ~0.3 ‰), and are typically unmodified by mid-ocean ridge hydrothermal processes, such that ocean ridge serpentinites formed by hydration and oxidation of mantle peridotites have igneous-like δ⁶⁶Zn values (0.29 ± 0.1 ‰). In contrast, serpentinites from Mariana forearc mud volcanoes are markedly depleted in ⁶⁶Zn, with a mean δ⁶⁶Zn value of -0.02 ‰¹. These isotopically light δ⁶⁶Zn values may reflect percolation of the forearc shallow mantle wedge by slab-derived CO₂-rich fluids. In this study, we further investigate the fate of Zn during prograde metamorphism of serpentinite in subduction zones, and the its subsequent recycling into the mantle.

The sample set includes (1) oceanic serpentinites (Gakkel ridge), (2) a sequence of Western Alps serpentinites recording a prograde subduction path, (3) Mariana forearc mud volcanoes samples, (4) fluid-derived olivines from the Kohistan arc mantle and (5) Himalayan mantle wedge serpentinites.

Subduction prograde metamorphism induces gradual transition from lizardite (Liz) to antigorite (Atg) in slab serpentinites – Fe³⁺-rich phases are reduced and fluid mobile elements (e.g. S, B) are released in an oxidized fluid. The Alpine ophiolites samples record a progressive loss of heavy Zn isotopes during the prograde path, with average d⁶⁶Zn values decreasing from 0.33 ‰ in oceanic Liz-serpentinites to 0.16 ‰ in eclogitic Atg-serpentinites. Our interpretation of these data is that Zn is released from the serpentinites during the Liz-Atg transition via oxidized, S-rich fluids, where it is bound by sulfate, a ligand that favours uptake of heavy Zn. In the Marianas, serpentinite ⁶⁶Zn depletion is even more pronounced which may reflect the release of CO₂-rich fluids enriched in ⁶⁶Zn.

Both shallow (mud volcano) and deeper (Liz/Atg transition) processes produce high δ⁶⁶Zn fluids that react with the mantle wedge. In agreement with these, samples representative of the fluid-enriched mantle wedge and arc mantle (Himalayan mantle wedge serpentinites, Kohistan mantle olivine²) have extremely heavy δ⁶⁶Zn values (> 0.74‰). Future work will focus on arc lavas in order to assess the recycling of Zn in subduction zones.

¹ Pons et al, 2011 PNAS v 108  ² Bouilhol et al, 2009 CanMin v 50