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 |
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| Authors: | GB Douglas BT Hart R Beckett CM Gray R L Oliver |
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| Institution: | (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 |
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| 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) size in the MDRS have been identified using an enhanced SPM size fractionation
technique as a basis to not only obtain a broad range of particle/colloid size ranges, but also to provide sufficient material
for subsequent geochemical and isotopic analysis. The conceptual isotopic/geochemical model proposed here contains three major
components: (i) the differential weathering of micas and alkali (K-) feldspars to form the majority of the particulate (<1
μm) fractions (high 87Sr/86Sr 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) fractions (low 87Sr/86Sr ratio), with a range of geochemical signatures related to the relative proportions
of inorganic and organic constituents, and (iii) the presence of natural organic matter as coatings on the particulate (<1
μm) and colloidal (>1 μm) matter and possibly as organo-colloids which exert an increasing influence in particular on bulk
colloid geochemistry with decreasing colloid size. This conceptual isotopic/geochemical model also accounts for the distinct
variation in major, trace and REE geochemistry and Sr isotopic systematics between the particulate (<1 μm) and colloidal (>1
μm) fractions, the variation being primarily a function of the distinctly different precursor mineralogies of the SPM fractions
and geochemical fractionation during the weathering and transport. Additionally, this model explains a systematic fractionation
of REE apparent within colloidal (>1 μm) fractions. 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 particulate and colloidal size fractions and to provide additional supporting evidence for the conceptual isotopic/geochemical
model.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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| Keywords: | suspended particulate matter particulate colloid Sr isotope fractionation Murray-Darling River system |
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