Extremely low δ56Fe in arc tholeiites linked to ferrocarbonate recycling: Implications for Fe enrichment in the Awulale Arc, Central Asia

Abstract

Recycling of Fe-rich materials through subduction may affect the element budgets and redox properties of the mantle, thus influencing the differentiation trends and mineralization types of mantle-derived magmas. However, the effects of different recycled Fe-rich materials on the mantle are dependent on their lithologies, which are still poorly constrained. Stable Fe isotopes can act as useful tracers for distinguishing among different recycled Fe-rich lithologies, and their imprints may be documented in mantle-derived magmas. This study focuses on the Fe isotopes of ferrobasalts and the associated dacites and magnetite ores in the Chagangnuoer Fe deposit of the Awulale Arc, Central Asia, to identify recycled Fe-rich materials and explore the Fe enrichment mechanism in continental arcs. Our results indicate that the ferrobasalts and dacites possess the lowest known δ56Fe, −0.40‰ ± 0.04‰ (2SE), among their counterparts worldwide. The low δ56Fe signatures are considered to originate from a hybridized mantle source, which may have been modified by recycled ferrocarbonates. The recycled ferrocarbonates may have melted during the decompressional heating stage of the slab subduction of the South Tianshan Ocean, coupled with asthenospheric upwelling under “wet” mantle conditions. The addition of ferrocarbonate melts to the mantle might have decreased the oxygen fugacity of the mantle wedge to below the fayalite−magnetite−quartz buffer, accounting for the Fe enrichment in arc tholeiites and large-scale Fe mineralization along the Awulale. Notably, our study reveals a novel carbonate recycling pathway in the cold subduction zones, where ferrocarbonates were subducted into the mantle and then recycled by the upwelling asthenosphere to mix with the fluid-metasomatized mantle through mantle convection.