The Forming Mechnism Of Si-fe-mn Oxides And Implications For Hydrothermal Activity At The Pacmanus Hydrothermal Field, Eastern Manus Basin

The samples of Si-Fe-Mn oxides were dredged from PACMANUS(Papua New Guinea?Australia?Canada?Manus) hydrothermal field in Eastern Manus Basin. Major, trace and rare earth elements, Sr, Nd and Pb isotopic compositions were analyzed for studying the genesises and geochemical characteristics, and analyzing the ore-forming material sources. Utilizing Si, Fe and Mn concentrations within the end-member PACMANUS hydrothermal fluid, Si-Fe-Mn-H2 O thermodynamic systems-Pourbaix diagrams were constructed at 300°C and 25°C, respectively. The diagrams illustrated the main Si, Fe and Mn oxides species, precipitating sequences and stability field of each species precipitating from the hydrothermal fluid. Based on microscopic textures, scanning electron microscopy(SEM) and electron probe microanalysis(EPMA), typical biogenic filamentous structures were identified. The relationship between biotic and abiotic kinetic mechanism in the formation of Fe oxides and the precipitation mechanism of amorphous silicon were researched.The genesises of the Si-Fe-Mn oxides were complicated. The geochemical characteristics reflected a obvious hydrothermal origin, and diagenetic influence in the later period. However, seawater and volcanic materials did not make a big contribution. Sr and Nd in the hydrothermal Si-Fe-Mn oxides had two end-member sources(seawater and hydrothermal fluid). The seawater contributed a large amount of Sr about 76.7 %-83.1 % of total amount. Nd was mainly derived from hydrothermal fluid and preserved the hydrothermal signature. Pb isotopic compositions showed minor variability and was mainly derived from local basement rocks.The Pourbaix diagrams showed that the main Si, Fe and Mn oxides species precipitating from the hydrothermal fluid were SiO2, Fe(OH)3, Fe3(OH)8, Mn3O4, and Mn2O3 at 25°C. During mixing of hydrothermal fluid with seawater, SiO2 precipitated earlier than Fe-Mn oxides because of the lower stability boundary. Then Fe(OH)2 precipitated first, followed by Fe3(OH)8 and Fe(OH)3, and last, small amounts of Mn3O4 and Mn2O3 precipitated.Based on the physico-chemical conditions of PACMANUS hydrothermal fluid, a strict abiotic oxidation rate of Fe2+ to Fe3+ was calculated, which was approximately 0.012 g/min. 3.2 kg per year Fe oxides would be deposited in this vent. A mesh-like microenvironment constructed by biogenic filaments ensured adequate Fe2+ concentration and low oxygen content for the growth of bacteria. Moreover, this structure promoted the deposition of abiotic Si-Fe oxides. The precipitation of amorphous silicon was divided into two stages. In the first stage, amorphous silicon would precipitate on the Fe filaments. When the Fe-Si filaments were crusted, the second stage of precipitation of amorphous silicon began. In this stage, the size of Fe-Si filaments increased and a large amounts of globular rich in Si and Fe precipitating around these filaments.