Geochemistry of suspended particulate matter (SPM) in the Murray-Darling River system: A conceptual isotopic/geochemical model for the fractionation of major, trace and rare earth elements

A conceptual isotopic/geochemical model is presented to explain the variati on of major, trace and rare earth element (REE) geochemistry and Sr isotope systematics in suspended particulate matter (SPM) as a function of particl e/colloid size. This conceptual model is an extension of a previous investi gation of the origin of SPM in the Murray-Darling River system (MDRS) that utilised Sr isotope systematics to examine aspects of SPM( particle/colloid ) origin, structure and mineralogy. The geochemical processes that give ris e to the often coherent trends in major, trace and REE geochemistry and Sr isotopic signature as a function of particulate (>1 mu m) and colloidal (<1 mu m) size in the MDRS have been identified using an enhanced SPM size fra ctionation technique as a basis to not only obtain a broad range of particl e/colloid size ranges, but also to provide sufficient material for subseque nt geochemical and isotopic analysis. The conceptual isotopic/geochemical m odel proposed here contains three major components: (i) the differential we athering of micas and alkali (K-) feldspars to form the majority of the par ticulate (>1 mu m) fractions (high Sr-87/Sr-86 ratio), which have a geochem ical and Sr isotopic signature that closely resembles precursor mineralogie s, (ii) the differential weathering of Na, Ca-feldspars (plagioclase) which decompose to form clay minerals in the colloidal (<1 mu m) fractions (low Sr-87/Sr-86 ratio), with a range of geochemical signatures related to the r elative proportions of inorganic and organic constituents, and (iii) the pr esence of natural organic matter as coatings on the particulate (>1 mu m) a nd colloidal (<1 mu m) matter and possibly as organo-colloids which exert a n increasing influence in particular on bulk colloid geochemistry with decr easing colloid size. This conceptual isotopic/geochemical model also accoun ts for the distinct variation in major, trace and REE geochemistry and Sr i sotopic systematics between the particulate (>1 mu m) and colloidal (<1 mu m) fractions, the variation being primarily a function of the distinctly di fferent precursor mineralogies of the SPM fractions and geochemical fractio nation during the weathering and transport. Additionally, this model explai ns a systematic fractionation of REE apparent within colloidal (<1 mu m) fr actions. Statisitcal (hierachical cluster) analysis of two particulate and three colloidal fractions from 23 samples from the MDRS is used as a basis to investigate geochemical and mineralogical associations within the partic ulate and colloidal size fractions and to provide additional supporting evi dence for the conceptual isotopic/geochemical model.