『Abstract
To study the rate and mechanism of granitic rock weathering,
field weathering experiments using granodiorite tablets (diameter
3.5 cm, height 1.1 cm, weight 30 g) were conducted at a catchment
over 10 yr. The tablets were exposed at three positions having
different weathering conditions: ground surface, above aquifer,
and in aquifer. The weight of the tablets decreased linearly with
time: 0.022 wt% yr-1 for the ground surface, 0.013
wt% yr-1 above the aquifer, and 0.42 wt% yr-1
in the aquifer. Only minor changes of the tablet surface were
observed after 10 yr of weathering at the ground surface and above
the aquifer. In contrast, the tablets in the aquifer were remarkably
weathered, and weathering of biotite was particularly noticeable,
whereas that of plagioclase, K-feldspar, quartz, and hornblende
was minor or moderate. The weathering of granodiorite is inferred
to proceed by initial dissolution of mineral grain boundary (chemical
process) and subsequent detachment of the mineral grain (physical
process). To evaluate the amount of weathering caused only by
the chemical process, a laboratory dissolution experiment was
conducted on the granodiorite tablet using a flow-through reactor
at 20℃ and pH 6-7. The obtained rates, 0.0078-0.010 wt% yr-1,
were 〜50 times slower than the field rate in the aquifer. This
result shows that the contribution of the physical process to
granodiorite weathering in the aquifer is very large compared
to that of the chemical process, and the relative intensities
of the two processes during the 10 yr weathering are estimated
to be chemical : physical = 1 : 〜50.
Keywords: chemical weathering; physical weathering; weathering
rate; granodiorite; grain boundary; dissolution』
Introduction
Field weathering experiment
Laboratory weathering experiment
Grain boundary dissolution and grain detachment
Conclusions
Acknowledgments
References cited