Characteristics of Sr, Nd and Pb isotopic compositions of hydrothermal Si-Fe-Mn-oxyhydroxides at the PACMANUS hydrothermal field, Eastern Manus Basin

Abstract

Si-Fe-Mn-oxyhydroxides dredged at the PACMANUS (Papua New Guinea-Australia-Canada-Manus) hydrothermal field, Eastern Manus Basin, have Sr-87/Sr-86=0.708 079-0.708 581; epsilon Nd=5.149 833-6.534 826; Pb-208/Pb-204=38.245-38.440; Pb-207/Pb-204=15.503-15.560; Pb-206/Pb-204=18.682-18.783. Sr-87/Sr-86 isotope ratios are relatively homogeneous and close to the value of the surrounding seawater (0.709 16). The content of Sr in the samples contributed by seawater was estimated to be 76.7%-83.1% of total amount. The mixing temperature of hydrothermal fluids and seawater were ranging from 53.2A degrees C to 72.2A degrees C and the hydrothermal activities were unstable when the samples precipitated. The eNd values of all the samples are positive, which differ from the values of ferromanganese nodules (crusts) with hydrogenic origin. Nd was mainly derived from substrate rocks leached by hydrothermal circulation and preserved the hydrothermal signature. Pb isotopic compositions of most samples show minor variability except Sample #9-2 that has relatively high values of Pb isotopes. The Pb may be derived from the Eastern Manus Basin rocks leached by the hydrothermal fluid. The slightly lower Pb-208/Pb-204 and Pb-207/Pb-204 values of the samples indicated that the hydrothermal circulation in PACMANUS was not entire and sufficient, or that hydrothermal circulation had transient changes in the past. Si-Fe-Mn-oxyhydroxides in the samples preserved the heterogeneities of local rocks.