Carbonate Transfer during the Onset of Slab Devolatilization: New Insights from Fe and Zn Stable Isotopes

• Published in: Journal of petrology Vol. 59; no. 6; pp. 1145 - 1166
• Main Authors: Debret, B, Bouilhol, P, Pons, M L, Williams, H
• Format: Journal Article
• Language: English
• Published: Oxford University Press (OUP) 01.06.2018
• Subjects: Sciences of the Universe
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egy057

Long-term carbon cycling is a subject of recent controversy as new mass balance calculations suggestthat most carbon is transferred from the slab to the mantle wedge by fluids during subduction,limiting the efficiency of carbon recycling to the deep mantle. Here, we examine the large scale mobilityof carbon during subduction using new isotopic tracers sensitive to H–C–O–S–Cl fluids, namelyiron and zinc stable isotopes, in samples interpreted to represent residual slab (Queyras,Western Alps) and sub-arc mantle (Kohistan, Himalaya). We show that during subduction there areseveral stages of carbonate precipitation and dissolution at metasomatic interfaces between metasedimentaryand ultramafic rocks in the slab. During the early stages of subduction, before the slabreaches the 300–400C isotherms, the infiltration of sediment-derived fluids into ultramafic lithologiesenhances carbonate precipitation in antigorite-bearing serpentinites. Carbonate storage in serpentinites,therefore, acts as a temporary reservoir of carbon in subduction zones. This episode isaccompanied by a decrease in serpentinite iron isotope composition (d56Fe), due to interactionwith low-d56Fe sediment-derived fluids, and an increase in the concentrations of fluid-mobile elements(e.g. B, Li, As). At higher temperatures (>400C), carbonate is leached from the serpentinitesby fluids. This is accompanied by a decrease in serpentinite zinc isotope composition (d66Zn) whichwe interpret as the release of a carbonate-bearing fluid with an isotopically heavy d66Zn signature.Thermodynamic modelling shows that the sudden change in fluid carbon mobility is due to a decreasein the aCO2 of the fluids released during slab prograde metamorphism, which shifts fromsediment- to serpentinite-dominated dehydration. This demonstrates that slab fluids bearing oxidizedcarbon (e.g. CO2), associated with isotopically light Fe, heavy Zn and fluid-mobile elements,can be released before the slab reaches eclogite facies P-T conditions. These observations providestrong evidence for the mobility of carbon in fluids during the early stages of subduction.Moreover, the fluids released will act as a potential metasomatic agent for the fore-arc mantle (orslab/mantle interface). The observation of carbonate-bearing metamorphic veins in the Himalayansub-arc mantle with complementary light d56Fe and heavy d66Zn signatures provides further supportfor the large scale transfer of both sulphate- and carbonate-bearing fluids during the earlystages of subduction. This suggests that the fore-arc may have an important role in deliveringwater, sulfur and carbon to the source of arc-magmas.