Accumulation rates, chemical partitioning, and Q-mode factor analysis of metalliferous sediments from the North Fiji Basin

Abstract

In order to assess recent hydrothermal contributions to pelagic sediments from active back-arc spreading, surface samples collected from the north central North Fiji Basin and the vanuatu Archipelagic Apron were chemically and mineralogically analyzed, including phase and size partitioning, and Q-mode factor analysis of the chemical data. Sediment cores were analyzed for accumulation rates by the excess 230Th method, using alpha-track autoradiography. The north central North Fiji Basin sediments are largely carbonate oozes in which the non-carbonate 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. Phase and size partitioning indicate that metal concentrations in the <2 μm fraction approach those for metalliferous sediments from the Eastern Pacific, and that the metalliferous material in the majority of these sediments is concentrated in the oxide portion of the <2 μm fraction, suggesting potential widespread colloidal dispersion from the source areas. Iron accumulation rates are similar to those of metalliferous sediments from the East Pacific Rise. The mean Mn accumulation rate is about twice the global authigenic rate, whereas Al is accumulating about ten times faster than in Eastern and Equatorial Pacific pelagic sediments because of volcanic ash contributions from the vanuatu island arc and possible intra-basin volcanism. Q-mode factor analysis of the compositional data produced three major factors—hydrothermal, authigenic and detrital—which together account for 90% of the sample variance in both size fractions of the non-carbonate 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.