Constrains of seepage fluids based on the characteristics of authigenic deposition from Conical serpentinite mud volcano in the Mariana forearc

Authigenic depositions induced by low-temperature alkaline seepage fluids occur on the top of the Mariana forearc serpentinite mud volcanoes, which are archives of the seepage fluids and are significant for studying the material circulation of the subduction zone. However, little is known about the features of the authigenic depositions composed of multiple minerals and their recording of seepage fluids. In this paper, we investigated the petrology, mineralogy and major and trace element compositions of authigenic depositions collected from Conical serpentinite mud volcano in Mariana forearc. The authigenic depositions from Conical serpentinite mud volcano are loose and extremely friable into lamellar and spherical fragments. The lamellar fragments are white, mainly composed of needle-like aragonite and prismatic calcite, with high CaO contents (49.3%~53.3%) and low MgO contents (2.3%~4.5%). The spherical fragments are yellow or white, made of amorphous magnesium silicate, with high MgO contents (25.5%~29.1%) and low CaO contents (0.5%~2.9%). ΣREE of the carbonate fragments range from 227.2 ng/g to 4136.6 ng/g, while the ΣREE of the amorphous magnesium silicate fragments are from 115.4 ng/g to 364.9 ng/g. All samples show flat distribution patterns with slight enrichment of heavy rare earth elements. The rare earth element distribution patterns of authigenic depositions indicate that the contribution of seepage fluids is higher than 90% except for two carbonate samples with relatively high rare earth element contents. This suggests that all samples should form in the intense seepage environments, but the carbonates and magnesium silicates may be induced by varied types of seepage fluid, namely, "low-silica type" and "high-silica type".