Braun,J.-J., Descloitres,M., Tiotte,J., Fleury,S., Barbiero(eの頭に´),L., Boeglin,J.-L., Violette,A., Lacarce,E., Ruiz,L., Sekhar,M., Hohan Kumar,M.S., Subramanian,S. and Depre(eの頭に´),B.(2009): Regolith mass balance inferred from combined mineralogical, geochemical and geophysical studies: Mule Hole gneissic watershed, South India. Geochimica et Cosmochimica Acta, 73, 935-961.

『鉱物学と地球化学と地球物理学を結びつけた研究から推定されたレゴリスのマスバランス:南インドのミュール・ホール片麻岩流域』


Abstract
 The aim of this study is to propose a method to assess the long-term chemical weathering mass balance for a regolith developed on a heterogeneous silicate substratum at the small experimental watershed scale by adopting a combined approach of geophysics, geochemistry and mineralogy. We initiated in 2003 a study of the steep climatic gradient and associated geomorphologic features of the edge of the rifted continental passive margin of the Karnataka Plateau, Peninsular India. In the transition sub-humid zone of this climatic gradient we have studied the pristine forested small watershed of Mule Hole (4.3 km2) mainly developed on gneissic substratum. Mineralogical, geochemical and geophysical investigations were carried out (i) in characteristic red soil profiles and (ii) in boreholes up to 60 m deep in order to take into account the effect of the weathering mantle roots. In addition, 12 Electrical Resistivity Tomography profiles (ERT), with an investigation depth of 30 m, were generated at the watershed scale to spatially characterize the information gathered in boreholes and soil profiles. The location of the ERT profiles is based on a previous electromagnetic survey, with an investigation depth of a about 6 m. The soil cover thickness was inferred from the electromagnetic survey combined with a geological/pedological survey.
 Taking into account the parent rock heterogeneity, the degree of weathering of each of the regolith samples has been defined using both the mineralogical composition and the geochemical indices (Loss on Ignition, Weathering Index of Parker, Chemical Index of Alteration). Comparing these indices with electrical resistivity logs, it has been found that a value of 400 Ohm in delineates clearly the parent rocks and the weathered materials. Then the 12 inverted ERT profiles were constrained with this value after verifying the uncertainty due to the inversion procedure. Synthetic models based on the field data were used for this purpose. The estimated average regolith thickness at the watershed scale is 17.2 m, including 15.2 m of saprolite and 2 m of soil cover.
 Finally, using these estimations of the thicknesses, the long-term mass balance is calculated for the average gneiss-derived saprolite and red soil. In the saprolite, the open-system mass-transport function τ indicates that all the major elements except Ca are depleted. The chlorite and biotite crystals, the chief sources for Mg (95%), Fe (84%), Mn (86%) and K (57%, biotite only), are the first to undergo weathering and the oligoclase crystals are relatively intact within the saprolite with a loss of only 18%. The Ca accumulation can be attributed to the precipitation of CaCO3 from the percolating solution due to the current and/or the paleoclimatic conditions. Overall, the most important losses occur for Si, Mg and Na with -286×106 mol/ha (62% of the total mass loss), -67×106 mol/ha (15% of the total mass loss) and -39×106 mol/ha (9% of the total mass loss), respectively. Al, Fe and K account for 7%, 4% and 3% of the total mass loss, respectively. In the red soil profiles, the open-system mass-transport functions point out that all major elements except Mn are depleted. Most of the oligoclase crystals have broken down with a loss of 90%. The most important losses occur for Si, Na and Mg with -55×106 mol/ha (47%of the total mass loss), -22×106 mol/ha (19% of the total mass loss) and -16×106 mol/ha (14% of the total mass loss), respectively. Ca, Al, K and Fe account for 8%, 6%, 4% and 2% of the total mass loss, respectively.
 Overall these findings confirm the immaturity of the saprolite at the watershed scale. The soil profiles are more evolved than saprolite but still contain primary minerals that can further undergo weathering and hence consume atmospheric CO2.』

1. Introduction
2. Field settings
3. Materials and methodology
 3.1. Previous studies and sampling
 3.2. Protolith/regolith geochemistry and mineralogy
 3.3. Geophysical investigations
4. Results
 4.1. Boreholes and soil profiles
  4.1.1. Fresh gneiss and weathering products
  4.1.2. Fresh and weathered amphibolite (BH6)
 4.2. ERT profiles
5. Discussion
 5.1. Determination of fresh and weathered materials
  5.1.1. Determination of fresh gneiss, gneiss-derived saprolite and red soil
  5.1.2. Determination of the fresh amphibolite and amphibolite-derived saprolite
 5.2. Assessment of regolith thickness with ERT
 5.3. Mass balance calculation
  5.3.1. Selection of the inert element
  5.3.2. Strain and elemental gain or loss in the average gneiss-derived saprolite
  5.3.3. Strain and elemental gain or loss in the average gneiss-derived red soil
 5.4. Long-term chemical weathering rate and minimum age of the saprolite
 5.5. Consequence of chemical weathering on the alkalinity production potential on the Karnataka Plateau
6. Conclusion
Acknowledgments
References


ホーム