『Abstract
A clast of low porosity basaltic andesite collected from the
B horizon of a soil developed on a late Quaternary volcaniclastic
debris flow in the Bras David watershed on Basse-Terre Island,
Guadeloupe, exhibits weathering like that observed in many weathered
clasts of similar composition in other tropical locations. Specifically,
elemental profiles measured across the core-rind interface document
that primary minerals and glass weather to Fe oxyhydroxides, gibbsite
and minor kaolinite in the rind. The earliest reaction identified
in the core is oxidation of Fe in pyroxene but the earliest reaction
that creates significant porosity is plagioclase dissolution.
Elemental loss varies in the order Ca≒Na>K≒Mg>Si>Al>Fe≒P>>Ti,
consistent with the surrounds a core of unaltered material that
is more spherical than the original clast. The distance from the
core-rind boundary to a visually prominent rind layer, L, was
measured as a proxy for the rind thickness at 36 locations on
a slab cut vertically through the nominal center of the clast.
This distance averaged 24.4±3.1 mm. Maximum and minimum values
for L, 35.8 and 20.6 mm, were observed where curvature of the
core-rind boundary is greatest (0.12 mm-1) and smallest
(0.018 mm-1)) respectively. Extrapolating from other
rinds in other locations, the rate of rind formation is estimated
to vary by a factor of about 2 (from 〜4 to 7×10-14
m s-1) from low to high curvature. The observation
of a higher rate of rind formation for a higher curvature interface
is consistent with a diffusion-limited model for weathering rind
formation. The diffusion-limited model predicts that, like rind
thickness, values of the thickness of the reaction front (h) for
a given reaction, defined as the zone over which a parent mineral
such as plagioclase completely weathers to rind material, should
also increase with curvature. Values of h were quantified as a
function of interface curvature using bulk chemical analysis (500<h<2000μm).
Values of h were also quantified by measuring loss of matrix glass
and increase in porosity as a function of curvature. In contrast
to rind thickness, h shows no consistent increase with curvature.
This contradiction is attributed to the mm-scale roughness of
the interface which is related to phenocryst grain size. Therefore,
the overall rind formation rate is strongly affected by curvature
measured at the scale of the clast, while mineral reaction rates
documented by reaction front thickness are strongly affected by
curvature at the scale of phenocrysts. Similarly, the weathering
advance rate (m s-1) for the entire Bras David watershed
can be extrapolated from the clast weathering rate if roughness
at the watershed scale equals values of approximately 400-800.
Keywords: Chemical weathering; Weathering rinds; Basalt weathering;
Guadeloupe』
1. Introduction
2. Geologic setting
3. Method
3.1. Field sampling
3.2. Clast sectioning and characterization
3.3. Textural and mineralogic analyses
3.4. Chemical analyses
4. Results
4.1. Clast characterization
4.2. Textural and mineralogic analyses
4.3. Chemical analyses
5. Discussion
5.1. 2-D diffusion model of weathering rind development
5.2. Weathering reactions
5.3. Mass balance calculations
5.4. Reaction fronts
5.5. Comparison to river chemistry and watershed scale denudation
rates
6. Conclusions
Acknowledgements
Appendix A. Supplementary material
References