『Abstract
40/39Ar dating of Mn-oxides from the Upper
Kandanga (UKA) Mn deposit, southeast Queensland, Australia shows
that the weathering profile overlying the deposit is younger than
1Ma. Seventy plateau ages obtained for 81 grains from 17 samples
fall into three groups: I. 1000-800 ka (n=2), II. 630-510 ka (n=2),
and III. 400-50 ka (n=66). The last age group can be further resolved,
using the mixture modelling solution by Sambridge and Compston
[Sambridge, M.S., Compston, W., 1994. Mixture modeling of multi-component
data sets with application to ion-probe zircon ages. Earth Planet.
Sci. Lett. 128, 373-390.], into two major age clusters (sub-groups,
IIIa and IIIb) with peak values at 313±4 ka (n=43, with an outlier)
and 213±7 ka (n=22), respectively.
The results suggest that a supergene blanket in a Mn deposit
may develop relatively fast if weathering and erosional conditions
are appropriate. The age peaks measured in this study correspond
to warm periods identified in SPECMAP (oxygen isotope records
of deep-sea sediments) and local paleoclimatic data. The correlation
of weathering age peaks with oceanic climatic indicators suggest
a climatic control on continental chemical weathering, where warm
and humid conditions are conclusive to increasing weathering rates.
The clustering of supergene Mn-oxide precipitation ages suggests
that weathering rates vary with time, supporting an episodic weathering
history for the profiles investigated.
Keywords: 40/39Ar dating; Mn-oxides; supergene
enrichment; Quaternary; Paleoclimatology』
1. Introduction
2. Geology, climate, and geomorphology
3. Sample processing and analytical procedures
4. Results
4.1. Petrographic examination and electron microprobe analysis
(EPMA)
4.2. 40Ar/39Ar results
4.2.1. Type A
4.2.2. Type B
4.2.3. Type C
4.2.4. Type D
4.2.5. Type E
5. Discussion
5.1. Age difference among grains from a same sample and its
influence on histograms
5.2. Resolving individual peaks in probability density plots
5.3. Rates of weathering and supergene enrichment in Mn-oxides
deposits
5.4. Climatic control on weathering and supergene enrichment
of Mn-oxides deposits
6. Conclusions
Acknowledgment
Appendix A. Supplementary data
References