The effects of partial melting and metasomatism on peridotite water contents: insights from Shuangliao Volcano Group, Northeast China

Abstract

Many minerals within the subcontinental lithospheric mantle (SCLM) contain structurally bonded hydrogen (commonly referred to as “water”), which significantly impacts their physical properties and associated geodynamic processes. Observations from different localities worldwide make understanding the behavior of hydrogen during partial melting and mantle metasomatism a contentious issue, as different localities reveal either melting or metasomatism as the controlling factor. To provide new insights, major elements, trace elements and water contents of peridotite xenoliths from three volcanoes of the Shuangliao Volcano Group in Northeast China were analyzed. Minerals display variations in major and trace elements, particularly in clinopyroxene. Most olivine contain no observable water, while orthopyroxene and clinopyroxene retained 14-157 μg/g and 46-351 μg/g of water, respectively. Samples were divided into 3 types according to trace element patterns, namely, type 1: Light Rare Earth Elements (LREE)-depleted samples; type 2: “spoon-shaped” samples featuring slight enrichment of the most incompatible elements (La and Ce) and relative depletion of Mid-REE; and type 3: LREE-enriched samples. Combined with major element trends, the Shuangliao SCLM experienced varying degrees of partial melting and cryptic metasomatism. Different water contents in the Shuangliao SCLM are a combined result of melting and metasomatism: less metasomatized samples (LREE-depleted and “spoon-shaped”) preserved the control of water contents by partial melting, while strongly metasomatized samples (LREE-enriched), equilibrated at higher oxygen fugacity and temperature, display considerable post-melting modifications of water contents, possibly associated with Fe redox. These characteristics suggest that hydrous and oxidized melts/fluids likely released by the stagnant Pacific slab in the Big Mantle Wedge (BMW) have metasomatized the shallow SCLM beneath Shuangliao, which indicates the circulation of materials released by the stagnant slab throughout the upper mantle.