Trace element mobility during early diagenesis of volcanic ash: Applications to stratigraphic correlation

Abstract

We have analyzed the relative mobility of trace elements in five rhyolitic and two basaltic tephra layers preserved in both altered and unaltered states, as a means of developing a rapid, whole-rock method for correlating altered tephra layers. The tephra layers were altered to zeolites or clay minerals in either saline-alkaline pore fluids, characteristic of small ephemeral lakes, or slightly more dilute pore fluids associated with perennial lakes and brackish coastal groundwaters. Comparison of whole rock trace elements analyses from the unaltered and altered portions of each tephra layer indicates that immobile elements, such as Ti, Zr, Hf, and Ta, retain their initial concentrations. Relatively more reactive alkaline and alkaline-earth elements, such as Rb, Sr, and Cs, are generally mobile. The lanthanide elements display variable mobility. The Light Rare Earth Elements (LREE) generally retain their initial concentrations, whereas the Heavy Rare Earth Elements (HREE) are generally depleted in alkaline solutions but immobile in more dilute pore fluids. We suggest that differences between LREE and HREE mobility result from the combined effects of LREE partitioning into stable accessory minerals and HREE transport as solution complexes, although our data set was not designed for quantitative analysis of these proposed mechanisms. Where both the diagenetic environment, and the accessory mineral compositions are well-constrained, the trace element signatures of the unaltered and altered tephra layers can be used to correlate an altered tephra layer to its unaltered equivalent, and to distinguish between different altered tephra layers. Four of the seven altered tephra layers can̈be distinguished on the basis of ratios of immobile elements, e.g. Zr Ti vs. Hf Ti . The remaining three, all W-type rhyolitic ashes, can be largely differentiated using cluster analysis on a combination of immobile elements and elements with similar mobilities.