Geochemistry of North Central North Fiji Basin Sediments

Abstract

Pelagic sediment cores from eleven stations in the north central North Fiji Basin plus a coring station on the Vanuatu archipelagic apron were geochemically characterized as part of an intensive geological survey of backarc spreading centers in the region. In order to assess recent hydrothermal contributions to these sediments from active backarc spreading, surface samples were chemically and mineralogically analyzed Analyses included phase and size partitioning and Q-mode factor analysis of the chemical data. The cores were analyzed for sedimentation and element accumulation rates by the excess 230Th method, using alpha-track autoradiography.The sampled sediments are largely carbonate oozes whose noncarbonate material is dominated by volcanic detritus in the > 2-µm fraction and by Fe-montmorillonite, amorphous ferromanganese oxides and fine-grained volcanic ash in the < 2-µm fraction. The sediments are slightly enriched in Fe, Mn, Cu, Zn, Ni and Pb relative to other North Fiji Basin and southwest Pacific sediments. Iron accumulation rates are similar to those of metalliferous sediments from the East Pacific Rise. The mean Mn accumulation rate is only about twice the global authigenic rate, whereas Al, because of volcanic ash contributions from the Vanuatu island arc and intra-basin volcanism, is accumulating at rates that are about ten times those of eastern and equatorial Pacific pelagic sediments.Q-mode factor analysis produced three major factors—hydrothermal, authigenic and detrital — which together account for more than 90% of the sample variance in both size fractions of the noncarbonate material. The aerial distribution of the < 2-µm fraction factor components reveals a major hydrothermal source associated with the South Pandora Ridge spreading center, in general agreement with the Mn and As accumulation rate patterns. Two smaller hydrothermal sources were revealed by the factor analysis which are not associated with any known active tectonic features, suggesting that back-arc rifting and associated hydrothermal activity in this portion of the North Fiji Basin may follow both ordered and complex regional modes.KeywordsAccumulation RateSpreading CenterPiston CorePelagic SedimentSediment GeochemistryThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.