『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