INSIGHT FROM CHLORINE ON ELEMENT ACCUMULATION IN FEMN CRUSTS

For decades, geochemists have been interested in the high concentrations of many elements in ferromanganese (FeMn) crusts compared to that in seawater; however, the mechanism for acquisition and accumulation of most remains an enigma. Rare elements are the focus of many genetic studies for FeMn crusts because their economic value is of interest for mining. However, the concentration of many rare elements can vary with water depth and geographic location, which may be reflected in the crust layers as they slowly grow making it difficult to determine the cause of high concentrations in particular deposits or crust layers. Therefore, this study addresses the distribution and incorporation of Cl into FeMn crusts, which is one of the most abundant elements in seawater and has a near-constant concentration globally that is conserved with depth.

An initial dataset of 40 FeMn crusts from the global ocean has been analyzed for Cl concentration using ISE. The Cl content in crusts has striking variability (354 - 12,371 ppm; average 3,256) but no obvious geographic relationship to ocean basin and no correlation with water depth, latitude, or longitude. Correlations of Cl with Fe, Mn, Si, and Al calculated using bulk concentrations as well as spatial analyses by EDS reveal no significant relationship of Cl to any of the main mineral phases of FeMn crusts. The passive incorporation of Cl as a salt or ligand bound to metals as a neutral compound was also considered; however, Cl does not associate with Na or K spatially in EDS analyses and does not correlate in concentration with Hg, Cu, Au, Ag, or Cd, whose speciation is dominated by neutral chloride complexes in seawater. These preliminary results show that the incorporation of Cl into FeMn crusts does not follow a first-order electro-chemical model for crust growth, and investigation into the chemical speciation of Cl and its precise spatial distribution in crusts is the important next step in understanding its role in their formation.

The data presented here will thus show results from analyses of Cl in FeMn crusts by microprobe, spectroscopy, and synchrotron radiation that help address these critical questions. Knowledge gained about Cl may then be applied to interpret the mechanism by which additional, rarer, and more valuable elements accumulate in FeMn crusts.