Enrichment and influencing factors of fluid-mobile elements in breccia serpentinite from serpentinite mud volcanoes in Mariana Forearc

The serpentinite mud volcano is considered as a direct window into subduction in Mariana Forearc. Serpentinites in serpentinite mud volcano are rich in water and fluid-mobile elements, and an archive of information about serpentinized fluids, fluid/rock interactions, and element cycles in subduction zones. We reviewed the FME (fluid-mobile element) behaviors of breccia serpentinite collected from five serpentinite mud volcanoes that erupted from the Mariana subduction zone in gradually elevated depths (Yinazao, Fantangisña, Asut Tesoru, South Chamorro, and Conical). We summarized and compared the FME characteristics of breccia serpentinite samples from different mud volcanoes at different depths of the same mud volcano and from microsamples in the same block, aiming at uncovering the features and sources of the serpentinized fluids, and the formation mechanisms of the Mariana forearc serpentine mud volcanoes. The FME characteristics of breccia serpentinite from all five serpentinite mud volcanoes show enrichments compared with those in depleted mantle (DM): strong enrichments of B, Cs, and As (often over 100×DM (depleted mantle)), moderate enrichments of Li, Rb, and Sb(>10×DM), and weak enrichments of Ba, Sr, and Pb(<10×DM). The contents of B, Sr, Ba, and Pb are the highest in breccia serpentinite samples from shallow mud volcano and show a decreasing trend from shallow mud volcano to deep mud volcano. On the contrary, the contents of Li, Rb, Cs, As, and Sb show the opposite trend, and these elements are lowest in breccia serpentinite samples from the shallow mud volcano and increase gradually from shallow mud volcano to deep mud volcano. The systematic variation of FMEs of breccia serpentinite in the five different mud volcanoes indicates that subducted plate-derived fluid is the main serpentinized fluid of the breccia serpentinite. The plate-derived fluid and main serpentinized fluid of Yinazao that is close to the trench, are extremely rich in B and slightly rich in Li, Ba, Sr, and Pb that are originated from opal dehydration processes at subduction interface below 80℃. The plate-derived fluids and the main serpentinized fluids of the farthest serpentinite mud volcanoes are rich in Ba, Sr, Li, Rb, and Cs that are mainly generated by carbonate mineral decomposition and certain altered oceanic crust dehydrated and clay-mineral transformed materials at subduction interface over 200 ℃. The Fantangisña in a moderate distance away from the trench shows plate-derived fluids and serpentinized fluids that featured with enrichment of B, Li, Rb, Cs, and Ba, which should be resulted from clay mineral dehydration and transformation at subduction interface in temperature 80-200 ℃. Samples in the upper 50 mbsf have the highest FME contents, indicating that seawater weathering has an effect on some FMEs, especially B and Sr. The contents of FME varied in different microstructures of the same breccia serpentine with the highest content in pure serpentine area, showing that serpentine products had a certain influence on FMEs. The contents of Li, Rb, and Cs in bastite are higher than those of the serpentine formed by olivine alteration, which proved that the original mineral types also had a certain influence on the enrichment of some FMEs.