Geochemical Characteristics Of Silt And Clay-grade Sediments From The Seafloor At South Chamorro Serpentinite Mud Volcano In The Mariana Forearc

Serpentinization of ultramafic rocks mainly refers to the process that the magnesium-rich silicate minerals such as olivine,orthopyroxenite and clinopyroxenite in dunite,lherzolite and harzburgite interact with fluid and altered to serpentine at medium and low temperatures.Serpentinization,as an exothermic reaction,will lead to the formation of low-temperature hydrothermal fluid,which is characterized by low temperature,rich in methane and hydrogen,and highly alkaline.At the same time,it can nourish chemoautotrophy communities at the seafloor and provide material and energy for the formation and evolution of life through inorganic processes.In addition,serpentinization leads to a significant increase in the p H value of the environmental fluid,which is considered as a potential carbon storage site.Therefore,serpentinization is of great significance in consideration of energy resources,origin of life and environmental governance,which is attracting more and more scientists'attentions to this territory.The forearc of Mariana subduction zone is the only geological province where active serpentinite mud volcanoes have been found,where about 19 active mud volcanoes with diameter of 10 to 30km and height of 0.5 to 2km locate.Serpentinite mud volcanoe are composed of unconsolidated silty and clayey serpentinite muds,generally rich in debris of serpentinized ultramafic rocks,metamorphic rocks,bioclastic debris and pyroclastic debris.The rocks,fluids and other materials that constitute serpentinite mud volcanoes originate from the deep mantle wedge and subduction oceanic crust which is far beyond the depth that can be reached by current technology directly.Therefore,the mud volcanoes are helpful to explore the chemical characteristics of the deep Earth and are considered as direct windows to explore a series of geological processes in subduction zones.At the same time,the study of the fluid activity of the serpentinite mud volcanoes will help to understand the serious imbalance between the volatile matter input from the subduction oceanic crust and the output from the island arc volcanoes and the back arc,and will also be the basis for the interpretation of the ancient similar geological structures.Therefore,it is of great scientific significance to focus on this territory.In this paper,silt and clay serpentinite muds and pore water from two shallow deposits collected by gravity column in December 2018 from SMV4 station at the summit of South Chamorro serpentinite mud volcano were researched.SMV4-1 is composed of serpentine(an average of 96.2%),a small amount of talc,calcite and aragonite,while SMV4-2 is composed of serpentine(an average of 62.9%),chlorite(an average of 21.9%),talc(an average 10.3%),amphibole(an average of 3.1%),a small amount of mica and aragonite.Carbonates in silt and clay serpentinite muds vary from 1.5%to 9%,and the?¹³C(VPDB)varies from-1‰to 1‰and?¹⁸O(VPDB)varies from-5‰to 5‰.Carbonates are derived from bioclastic or authigenic deposits with oxygen isotope as low as-5‰,indicating the possible input of carbonates from the subducted western Pacific Ocean crust.The content of TOC ranges from 0.1%to 0.25%,and the?¹³C(VPDB)ranges from-29.2‰to-27.2‰.The organic matters are most likely derived from the abiogenic organic matters generated during the serpentinization of mantle wedge-peridotites,more or less mixed with the organic matters from the subducted western Pacific oceanic crust rocks and/or Mariana forearc marine sediments.The SMV4 serpentinite muds is mainly composed of serpentinite with a few metamorphic minerals.The average total content of silicate,magnesium and iron(SiO₂+Mg O+Fe₂O_3T)is more than 95%wt(after normalization).Mineral compositions and major elements suggest that chlorite,talc,amphibole and mica of SMV4-2 may have been largely originated from the metamorphic process of oceanic crust.The?REE of serpentinite rocks in the background of Mariana Forearc serpentinite mud volcanoes is between 0.01 and 1 ppm,while the metamorphic rocks are beyond 1 ppm,even up to hundreds ppm.The average contents of?REE of SMV4-1 and SMV4-2 are 4.41 ppm and 8.38 ppm,respectively.Under the influence of the distribution patterns of serpentinite and metamorphic rocks(after Chondrite standardization),the serpentinite muds is characterized by enrichment of LREE and depletion of HREE.?Ce and?Eu are negative abnormality.In the trace element spider diagram,Sc and Ga also indicate the input of oceanic metamorphic products to SMV4.South Chamorro contains the metamorphic products of ocean island basalts(OIB)and mid-ocean ridge basalt(N-MORB)from subducted Pacific Ocean crust,as well as Mariana Forearc basalts(FAB)and Mariana Forearc boninite.However,the relative low ratio of Zr/Hf and Nb/Ta indicates the input of products from N-MORB and/or FAB to SMV4-2.Fluids defluidized from the subducted Pacific plate and serpentinized the overlying mantle wedge,as a result,the fluid mobile elements(such as As,B,U,Sr,Ba,Li,Cs,Rb,Ba,K,Pb,etc.)were enriched in the serpentinite rocks.In addition to FME,the serpentinite muds also contain high content of high field strength elements(HFSE,such as Th,Nb,Ta).However,a small elevated mixture of metamorphic rocks in the serpentinite mud bulk would results in very large increase in some trace elements owing to the high contents of certain trace elements in metamorphic rocks.It is difficult to distinguish the element mobile activity of serpentinite and metamorphic rocks in the process of sediment formation.Relative to the ocean bottom water,South Chamorro pore water represents the deep source seepage properties which are typically characterized by higher p H,alkalinity,NH₃,Ba,B,Na,K,Fe and Si than ocean bottom water,but lower Li,Cl,Mg,Al,Ca,Sr,REE than ocean bottom water.SO₄,Na,K/Cl ratio in SMV4 pore water are higher than in ocean bottom water,and the Li content may be affected by the deep fluid.However,conventional indicators such as high p H,and high alkalinity,B,Ba,Sr,Rb,Cs contents are not shown in our pore fluids,and on the contrary,our pore waters are even more depleted in these index relative to the ocean bottom water.DIC decreases with depth,and its?¹³C(VPDB)gradually decreases from-2.6‰to-9‰,which may be influenced by the interaction between CH₄ and SO₄ in the seepage fluid.REE in SMV4 is enriched relative to the bottom water,while the deep source seepage fluid is depleted.The positive anomaly of?Ce and?Eu indicate that the environment of fluid is reductive.