Platinum group element mobilization in the mantle enhanced by recycled sedimentary carbonate

Sulfides are the main host of platinum-group elements (PGE) in the mantle and the stability of sulfide controls PGE mobilization and enrichment. The enrichment of PGE in mantle melts is generally interpreted as the result of complete dissolution of sulfide during the high-degree melting. However, the mechanism by which PGE are transferred from sulfides to the melt is not well known. Here we show that in areas where sedimentary carbonate is recycled, PGE mobility in the mantle could be greatly increased by the transformation of sulfide into sulfate. Carbonatitic xenoliths from the Neogene Dalihu basalt in Inner Mongolia represent sedimentary carbonates recycled into the mantle, documented by limestone-like trace element patterns and high Sr-87/Sr-86 ratios. They also contain micro-diamonds and silicate minerals with chemical features of mantle peridotite, showing that they must have resided in the mantle. However, the carbonatitic xenoliths contain 1000 times more PGE than sedimentary carbonate, indicating the incorporation of mantle sulfide. Moreover, delta S-34(sulfate) values (9.6-10.2 parts per thousand) are much lower than in the original limestone source (25-35 parts per thousand), and higher Ni, Cu contents in the carbonate groundmass than in sedimentary carbonates suggest that sulfide had been transformed into sulfate. With the destruction of sulfide, PGE contained in sulfide was released into the carbonate melt, thus promoting their mobility in the mantle. (C) 2020 Elsevier B.V. All rights reserved.