In situ characterization of forearc serpentinized peridotite from the Sulu ultrahigh-pressure terrane: Behavior of fluid-mobile elements in continental subduction zone

Serpentinized peridotites in the Yangkou (YK), Suoluoshu (SLS) and Hujialin (HJL) areas in the Sulu ultrahigh-pressure terrane represent the relic of ancient subcontinental lithospheric mantle below the North China Craton. Their protoliths, harzburgite and dunite, were variably hydrated by aqueous fluids released from subducting Yangtze continent. The rocks are enriched in fluid-mobile elements (FME) including Sb (42-333 times the depleted mantle value) and Pb (30-476 times). The degrees of the FME enrichment are comparable to that of the Himalayan forearc serpentinites, and greater than forearc mantle serpentinites from Marianas, suggesting that the degrees of FME enrichment in the forearc serpentinites are greater in continental subduction zones than those in the oceanic subduction zones. Lizardite after olivine in the SLS serpentinite shows higher degrees of enrichment in Sb and As than those for antigorite after both olivine and orthopyroxene in the YK area. The antigorite has highly enriched in Pb, U, Cs, and LREE, but not for the lizardite. The abundance of FME in two different species of serpentine reflects the different temperature of hydration. At temperature lower than 300℃, formed lizardite at shallow depths of the mantle wedge incorporates elements that are fluid mobile at low temperatures, such as Sb and As. When the temperature greater than 300℃, formed antigorite at a relatively deep mantle wedge incorporate more FME from the subducting continental slab (or fragments), including Pb, U, Cs, LREE as well as Sb and As. The eventual breakdown of antigorite (600-700℃) in prograde metamorphism would discharge water as well as FME into the subducting channel and/or the overlying mantle.