Geochemistry Study Of Peridotites From The Southern Mariana Forearc

Detailed petrology, mineral and geochemistry study was carried out on serpentinized peridotites dredged from the southern Mariana forearc. To study the histories of partial melting and melt-mantle interaction occurred in the Mariana forearc mantle, and to reveal the serpentinization process and its influence on peridotites. Melting model result suggests that all samples have undergone high degrees of melting (15-25%). The relatively flat PGE patterns indicate these peridotites may suffer interaction with melt.The constituent minerals of these southern Mariana forearc peridotites are olivine, amphibole and spinel, as well as serpentine, chlorite and talc. The absence of magnetite in the serpentinized peridotites is due to the incomplete serpentinization, the adding of later SiO₂-rich fluid leding to the lack of brucite and the occurrence of talc. Scarcity of H₂O, low porosity and permeability, as well as the actual situation of initial serpentinization and the low temperature in our study area, results in the absolute prevalence of lizardite over chrysotile and antigorite.Talcs mainly occur as meshwork veinlets and large veins, the sufficiently low frictional strength and inherently stable sliding behavior may have great significance on downdip limit of subduction thrust earthquakes. Chromites typically exhibit distinct chemical zones, the most reasonable mechanism for alteration of chromites should be the postserpentinization alteration, during which ferrian chromite leaves as chromite diffuse out A1 and Mg. The variability of chromtie zoning charater between sites D7 and D8 may be due to the difference in fluid-rock ratios.Serpentinization of the study peridotites resulted in the lost of MgO and SiO₂, and the influence degree on this two elements are similar. Besides, serpentinization could also lead to the lost of CaO in the peridotites. The influence of serpentinization on trace elements in the study peridotite mainly embodies the addition of fluid-mobile elements Pb, Sr and Li, while the concentration increase of U in the study samples is considered due to the interaction of peridotites with seawater. In addition, the Mariana forearc peridotites have distinct negative Ce anomaly, which may indicate the significant contribution of the subducted material to the melting source region of the Mariana wedge mantle.