Geochemistry of Suspended Particulate Matter (SPM) in the Murray ...

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previous investigation of the origin of SPM in the Murray-Darling River system (MDRS) ... All samples in this study were collected from the Murray-Darling River.
Aquatic Geochemistry 5: 167–194, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.

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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 G. B. DOUGLAS1,?, B. T. HART1, R. BECKETT1, C. M. GRAY2 and R. L. OLIVER3 1 Water Studies Centre and Department of Chemistry, Monash University, Clayton, Victoria,

Australia 2 School of Earth Sciences, La Trobe University, Bundoora, Victoria, Australia 3 Murray-Darling Freshwater Research Centre, Albury, New South Wales, Australia

(Received May 1998) Abstract. A conceptual isotopic/geochemical model is presented to explain the variation of major, trace and rare earth element (REE) geochemistry and Sr isotope systematics in suspended particulate matter (SPM) as a function of particle/colloid size. This conceptual model is an extension of a previous investigation 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 rise to the often coherent trends in major, trace and REE geochemistry and Sr isotopic signature as a function of particulate (>1 µm) and colloidal (1 µm) fractions (high 87 Sr/86 Sr ratio), which have a geochemical and Sr isotopic signature that closely resembles precursor mineralogies, (ii) the differential weathering of Na, Ca-feldspars (plagioclase) which decompose to form clay minerals in the colloidal (1 µm) and colloidal (1 µm) and colloidal (1 µm) and colloidal (1 µm) and colloidal ( 1 µm) fractions. Noteworthy, however, is the adjacent clustering of Cu, Zn and P with LOI, suggesting that in particulate fractions total Cu, Zn and P concentrations may still be subtly influenced even by relatively low organic matter contents. Group 4. (Sr, CaO, MgO, TiO2 , Sc, aliphatic carbon, carboxylic carbon, SiO2 ). This cluster consists of elements (Sr, CaO, MgO, TiO2 , Sc) that in many cases are probably associated with clay minerals as discrete/amorphous mineral inclusions (e.g. Ti in rutile/leucoxene) or as interlayer cations within the particulate fractions. As a result of their similar geochemistry, Sr and Ca are particularly closely associated. These elements are, however, considerably less strongly associated with organic matter functional groups (principally carboxylic carbon) than the colloidal fractions, which is consistent with the conceptual isotopic/geochemical model. The poor association of SiO2 with any variables is probably due to the predominance of quartz in the coarser particulate size fractions (cf. high SiO2 contents, Tables II, III). Quartz can vary independently in abundance due to factors such as variation in local supply within a catchment or particle size/streamflow (and hence rates of resuspension) and contains little or no other major, trace or REE’s. The poor association between aliphatic and carboxylic carbon in this cluster and aromatic, carbohydrate and ketone/aldehyde carbon in cluster 3 probably reflects its low

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abundance in the particulate (>1 µm) fractions relative to that of colloidal (1 µm) geochemistry. The small influence of organic matter on overall particle geochemistry (due to its low abundance) is also consistent with the conceptual isotopic/geochemical model (Table I).

4. Conclusions A conceptual isotopic/geochemical model for the variation of major, trace and rare earth element (REE) geochemistry and Sr isotope systematics in suspended particulate matter (SPM) as a function of particle size has been developed. This model was based on a previous model that explained the origin of SPM in the MDRS using Sr isotopic systematics. The original model is augmented here by statistical analysis (hierarchical clustering) of the major, trace and REE geochemistry of 23 SPM samples (each fractionated into two particulate and three colloidal fractions) from the MDRS to explain geochemical associations of elements in terms of particle/colloid structure, differential weathering processes and precursor mineralogy. The new augmented model explains trends in major, trace and REE geochemistry and Sr isotopic variation in colloidal material previously recognised in quasi-isochron and Sr mixing diagrams by Douglas et al., (1995) and highlights major geochemical differences between particulate (>1 µm) and colloidal (1 µm) fractions (high 87 Sr/86 Sr ratio), which have a geochemical and Sr isotopic signature that closely resembles precursor mineralogies, (ii) the differential weathering of Na-, Ca-feldspars (plagioclase) which decompose to form clay minerals in the colloidal (1 µm) and colloidal (