『Abstract
Chemical weathering indices are useful tools in characterizing
weathering profiles and determining the extent of weathering.
However, the predictive performance of the conventional indices
is critically dependent on the composition of the unweathered
parent rock. To overcome this limitation, the present paper introduces
an alternative statistical empirical index of chemical weathering
that is extracted by the principal component analysis (PCA) of
a large dataset derived from unweathered igneous rocks and their
weathering profiles. The PCA analysis yields two principal components
(PC1 and PC2), which capture 39.23% and 35.17% of total variability,
respectively. The extent of weathering is reflected by variation
along PC1, primarily due to the loss of Na2O
and CaO during weathering. In contrast, PC2 is the direction along
which the projections of unweathered felsic, intermediate and
mafic igneous rocks appear to be best discriminated; therefore,
PC1 and PC2 represent independent latent variables that correspond
to the extent of weathering and the chemistry of the unweathered
parent rock. Subsequently, PC1 and PC2 were then mapped onto a
ternary diagram (MFW diagram). The M and F vertices characterize
mafic and felsic rock source, respectively, while the W vertex
identifies the degree of weathering of these sources, independent
of the chemistry of the unweathered parent rock.
The W index has a number of significant properties that are not
found in conventional weathering indices. First, the W index is
sensitive to chemical changes that occur during weathering because
it is based on eight major oxides, whereas most conventional indices
are defined by between two and four oxides. second, the W index
provides robust results even for highly weathered sesquioxide-rich
samples. Third, the W index is applicable to a wide range of felsic,
intermediate and mafic igneous rock types. Finally, the MFW diagram
is expected to facilitate provenance analysis of sedimentary rocks
by identifying their weathering trends and thereby enabling a
backward estimate of the composition of the unweathered source
rock.
Keywords: Chemical weathering;p Weathering index; Logratio analysis;
Principal component analysis』
1. Introduction
2. Materials and methods
2.1. Database for input
2.2. Statistical framework
3. Results
3.1. PCA results
3.2. MFW diagram
4. Discussion
4.1. Evidence of the weathering of rock-forming minerals
on the MFW diagram
4.2. Evaluation of weathering intensity using the W index
4.3. Application of the MFW diagram in different rock types
4.4. Implications for provenance studies
4.5. Limitations of the MFW scheme
5. Conclusion
Acknowledgements
Appendix A. Concepts of compositional data analysis
A.1. Simplex; a sample space of compositional data
A.2. Logratio analysis
References