Yoshida,H., Metcalfe,R., Nishimoto,S., Yamamoto,H. and Katsuta,N.(2011): Weathering rind formation in buried terrace cobbles during periods of up to 300ka. Applied Geochemistry, 26, 1706-1721.

『30万年前までの埋没段丘大礫での風化皮殻の形成』


Abstract
 Weathering rinds formed in Mesozoic sandstone and basalt cobbles buried in terrace deposits for up to 300 ka have been investigated. The aim was to determine the formation process and elemental mass balances during rind development. The ages of terraces distributed in the western part of Fukui prefecture, central Japan have been determined as 50 ka, 120 ka and 300 ka based on a tephro-stratigraphic method. Detailed investigations across the weathering rinds, consisting of microscopic observations, porosity measurements, and mineralogical and geochemical analyses using X-ray diffractometry (XRD), X-ray fluorescence (XRF), secondary X-ray analytical microscopy (SXAM), scanning electron microanalyser (SEM) and electron probe microanalysis (EPMA) have been carried out. The results revealed that the Fe concentrations in the weathering rind of a basaltic cobble slightly decreased from the cobble's surface (rim) towards the unweathered core. In contrast, in a sandstone cobble formed under the same environmental conditions over the same period of time there is an Fe-rich layer at some distance below the cobble's surface. Elemental mass balances across the rinds were determined by using open system mass balance (τi,j) calculations and show that the Fe was precipitated as Fe-oxyhydroxides in the basalt cobbles, although Fe was slightly removed from the rims. In sandstone cobbles, on the other hand, Fe migrated along a Fe concentration gradient by diffusion and precipitated as Fe-oxyhydroxide minerals to form the weathering rinds. Presumably, precipitation was due to relatively higher pH conditions caused by mineral dissolution within the pores, principally involving calcite, but probably also silicates including feldspar. The detailed characterization of the weathering rinds revealed the influence of lithology on the accumulation and dissolution of Fe-oxyhydroxides, causing weathering rinds with different characteristics to develop in different kinds of buried cobbles under the same conditions. Relatively large climatic changes in the study area did not cause discernible variations in the mean formation rates of the studied rinds, which were in the order of 10-8 m/a for both basalt and sandstone cobbles. These rates are 1-2 orders of magnitude slower than those reported for tropical areas elsewhere, most probably due to the lower rainfall in the studied area.』

1. Introduction
2. Geological setting
3. Materials and methods
 3.1. Thickness of the weathering rinds
 3.2. Detailed observation and physical characterization
 3.3. Geochemical characterization
4. Results
 4.1. Features and thickness of weathering rind
 4.2. Physical properties
 4.3. Geochemical characteristics
5. Discussion
 5.1. Differences between weathering rinds in basalt and sandstone
 5.2. Water conditions and Fe transportation
 5.3. Element fluxes during weathering rind formation
6. Conclusions
Acknowledgments
References


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