Kamber et al.(2005)による〔『A new estimate for the composition of weathered young upper continental crust from alluvial sediments, Queensland, Australia』(1041p)から〕

『オーストラリアのQueenslandの沖積堆積物からの風化した若い上部大陸地殻の組成についての新しい見積り』


Abstract
 We preset new major element, trace element ad Nd-isotope data for 30 alluvial sediments collected from 25 rivers in Queensland, E Australia. Samples were chosen to represent drainage from the region's most important lithologies, including Tertiary intraplate volcanic rocks, a Cretaceous igneous province (and sedimentary rocks derived thereof) as well as Proterozoic blocks. In most chemical and isotopic aspects, the alluvial sediments represent binary or ternary mixing relationships, with absolute abundances implied to reflect the proportion of lithologies in the catchments.
 When averaged, the studied sediments differ from other proxies of upper continental crust (UCC) mainly in their relative middle rare earth element enrichment (including an elevated Sm/Nd ratio), higher relative Eu abundance and higher Nb/Ta ratio. These features are inherited from eroded Tertiary intraplate basalts, which commonly from topographic highs in the studied region.
 Despite the high degree of weathering strong to excellent coherence between similarly incompatible elements is found for all samples. From this coherence, we suggest revisions of the following upper crustal element ratios: Y/Ho = 26.2, Yb/Tm = 6.37, Th/W = 7.14, Th/Tl = 24 and Zr/Hf = 36.9. Lithium, Rb, Cs and Be contents do not seem depleted relative to UCC, which may reflect paucity of K-feldspar in the eroded catchments. Nickel, Cr, Pb, Cu and Zn concentrations are elevated in polluted rivers surrounding the state capital. River sediments in the Proterozoic Georgetown Inlier are elevated in Pb, Cu and Zn but this could be a natural phenomenon reflecting abundant sulphide mineralisation in the area.
 Except for relative Sr concentrations, which broadly anticorrelate with mean annual rainfall in catchments, there is no obvious relationship between the extent of weathering and climate types, which range from arid to tropical. The most likely explanation for this observation is that the weathering profiles in many catchments are several Myr old, established during the much wetter Miocene period.
 The studied sediment compositions (excluding those from the Proterozoic catchments) are used to propose a new trace element normalisation termed MUQ (MUd from Queensland), which serves as an alternative to UCC proxies derived from sedimentary rocks.』

1. Introduction
2. Regional geology and samples
3. Analytical details
4. Results
 4.1. Major elements
 4.2. Trace elements
  4.2.1. Conservative trace elements (REE, Y, Th, Tl, Sc and Co)
  4.2.2. Elements affected by mineral sorting (Zr, Hf, Ti, Nb, Ta, Cr and Sn)
  4.2.3. Elements affected by pollution (Pb, Cu, Zn)
  4.2.4. Elements affected by loss during weathering (Li, Rb, Cs, Be, Sr, Ba, V, W, U)
  4.2.5. Other elements (Ni and Ga)
 4.3. Nd-isotopes
5. Discussion
 5.1. Coherent geochemical UCC features of the studied sediments
 5.2. Suggested revisions to existing crustal estimates
 5.3. The MUQ normalisation
 5.4. The uniformity of weathering indices and implications for provenance studies
Acknowledgments
References


戻る