Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
162
CHAPTER V
Geochemistry
163
5.1 INTRODUCTION
The chemical composition of a soil will be highly controlled by the composition of
the geological parent material, during the early pedogenesis, whereas the chemical
composition of mature soils strongly reflects the effects of weathering environment.
With time, soil composition diverges progressively from that of the parent material
under the influence of pedogenic processes determined by vegetation, topography
and, in particular, climate.
The divergence may be manifested initially by redistribution of elements within the
soil fabric, then between profile horizons and finally between soils within the
landscape (Jenkins and Jones, 1980).
The mobilization and redistribution of elements during weathering follow various
pathways as different elements are affected differently by the various pedogenic
processes including dissolution of primary minerals, formation of secondary minerals,
redox reactions, transport of material, and ion exchange (Middleburg et al., 1988).
There are several approaches to paleosols; it can be employed as litho-stratigraphic
markers or to define paleo-morphology. Paleosols provide an excellent reservoir of
information on weathering and climate conditions. For example compositions of
paleosols have been effectively used to estimate the PO2 of the atmosphere prevalent
at the time of their development (Holland and Zbinden 1988; Pinto and Holland 1988;
Zbinden et al., 1988; Holland et al., 1989; Holland and Beukes 1990; Holland 1994;
MacFarlane et al., 1994; Ohmoto 1996; Rye and Holland 1998).
The study of the Geochemistry of paleosols in the Precambrian sequences of India has
so far received very little attention. Some of the important literature on paleosols of
India is given here:
Dash et al (1987) demonstrated that the khondalites of Orissa probably represent
metamorphosed Precambrian aluminous lateritic soils. Similar high Al2O3 soils have
been reported as protoliths of the Precambrain sillimanite-corundum deposits of
Sonapahar of Meghalaya state, northeastern India (Golani 1989). The study by
Sharma, (1979) clarified some pyrophyllite-diaspore deposits of the Bundelkhand
Complex of Central India which have been interpreted as paleosols.
164
In an attempt to chemically distinguish metamorphosed paleosols from metapelites of
the Precambrian of Peninsular India, Sreenivas and Srinivasan (1994) proposed that
khondalites, especially of the northern parts of the Eastern Ghats mobile belt may
represent metamorphosed Precambrian paleosols, whereas some high alumina rocks
of the Holenarasipur schist belt of the Dharwar Craton are metamorphosed
hydrothermal alteration products.
Apart from these, large deposits of fine grained micaceous rock at the base of the
Proterozoic Aravalli sequence (being mined as “Pyrophyllite”) have been interpreted
by Roy and Paliwal (1981) as possible paleosols. A major element geochemical study
by Banerjee (1996) has also suggested that these rocks may be Precambrian paleosols.
To understand their origin, much more detailed examination including trace and rare
earth element geochemical studies are essential. In this chapter, the chemical
composition of selected bed rock as well as weathered zones and top soil of seven
profiles of study areas have studied and discussed. Seven soil profiles that developed
on different bed rocks along with different horizons in the profile in the study areas
were analyzed for major and trace element.
The analytical data obtained from these rock samples and respected horizons are
presented in tables- 5.1 to 5.7.
Based on literature survey, not much work has been done on development of
paleosols in selected bed rocks in the study area and also mobility of elements along
the profile horizons. This study is an attempt to understanding the process of
weathering in Paleosols which developed on varieties of bed rocks that have been
identified and described in chapter 3.
5.2 SAMPLING
Based on different depths and horizons of the soil profile, comprehensive sampling
technique for chemical analysis was adopted. The sample patterns covering all the
facies consisted of sample sections in the vertical lithosection as well as weathered
and soils so as to give vertical as well as lateral variation of elemental distribution
patterns. A total of 35 samples were analyzed, of which 5 samples from Amphibolite
and 7 samples from Gneiss in Chikkahali area, 5 samples from Quartzo -feldspathic
165
rock in Belagula area, 5 samples from calc-silicate rocks in Bettadabidu area, 3
samples from Amphibolite in Sargur area, Nugu dam, 4 samples from Hornblende
gneiss in Gundlupet area and 6 samples from Ultramafic rock in Doddakanya area.
5.3 ANALYTICAL PROCEDURE
The major and trace elements were analyzed by XRF (Philips, Holand) respectively at
CESS, Thiruvananthapuram.
5.3.1 X-Ray Fluorescence Spectrometry
Collapsible aluminum cups were filled with 9 gm of boric acid, which acts as binding
material. 1 gm of ~ 200 mesh homogenized sample powder was sprayed upon it by
covering the boric acid uniformly and about 15 tons of pressure was applied using
Herzog hydraulic press (H/100) to obtained pressed pellet of 40 mm diameter. The
pressed powder pellets allows trace element determinations, with limits of detection
up to 1 ppm for selected elements. The elements like K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu,
Zn, Rb, Sr, Zr, Co, Ba, Ce, Pb, La and P were determined with same method. Light
elements like Si, Al, Mg and Na are less precise by this method. All major and trace
elements are determined on sample pellets for which fused glass disk cannot be made
(for e.g. river and marine sediments, soils etc.).
Fused glass disk is prepared on a Claisse Fluxy instrument in the sample preparation
lab of CESS. One gram of finely powdered sample is mixed with 5 gram of flux
(LiT/LiM/LiBR 49.75/49.75/0.50, Pure) and fused in a platinum crucible. The fluxer
is microprocessor controlled and uses LPG and in controlled manner raises the
temperature up to 1000°C. It has built in programmes for different sample types.
Rotating and stirring of crucible for uniform mixing and pouring the fused liquid on to
a platinum mold to produce 30 mm glass disk is done automatically. It is also possible
to create need based programmes with different gas flow, crucible tilt, mixing speed
and length of each step.
Fused disks are excellent for analyzing major elements as it reduces matrix effects;
eliminates particle size effects; and provides a homogeneous specimen. Samples
which contain higher than normal concentrations of elements like lead, tin, arsenic
and/or antimony (as they can seriously damage the platinum ware) and samples which
166
contain organic matter are not suitable for this method of sample preparation.
Elements determined by this method are Na, Mg, Al, Si, P, K, Ca, Ti, Mn, and Fe.
The samples were analyzed for major elemental composition by Philips Pw 1400
microprocessor controlled wavelength dispersive sequential X-Ray Fluorescence
spectrometer. The system was interfaced to a Philips P851 online minicomputer for
preparing calibration curve relating the concentration and intensity level in a
standards as well as unknown samples after due matrix corrections.
Software available in the computer was able to take care of dead time; background
and line overlap correction after regression and converting the counts into correction
with the help of the calibration curves, finally giving the output directly as
concentration in oxide percentage or in ppm as required. A spinner was used to spin
the samples inside the spectrometer while measuring to have uniform counts. Certain
elements were analyzed using a Rhodium target X-ray tube, while a Chromium X- ray
target tube estimated Na, Al and Mg, since the concentration levels of these elements
were very low. All the elements were estimated under a high vacuum condition (10-6
Torr). The major and minor elemental data estimated by XRF are reproducible with a
precision range of ± 5%.
5.4 Patterns of Element Concentration ( Depth–wise variation of elements)
The major, minor and the trace element abundances in all the seven representative
Paleosol profiles are analyzed and data has been presented in Tables- 5.1 to 5.7.
Major, minor element concentrations are given in weight percent as oxides and trace
elements in parts per million.
Variations of elements within the profile as a function of depth and a comparison of
mobile and immobile elemental concentrations (major and trace element) are made
for the parent and pedogenic horizons of the Paleosols profiles of the study areas
(Figures- 5.1 to 5.4).
167
5.4.1 Major elements
a) Alumina (Al2O3)
The Al2O3 content of the profile developed on amphibolite ranges from 10.33% in the
parent rock to 15.34% in paleosols zone. Its content in profile developed on gneiss
ranges from 14.7 in bed rock to 17.13% in top soil horizon. In paleosol developed on
quartzo-feldespatic rock the Al2O3 content was 18.95% whereas in the lower
unaltered horizon was 10.76%. calc-silicate rock profile, the range of Aluminum
oxide content in less altered zone was 4.9% and 10.77% in soil zone. In the soil
profile developed on amphibolite in Sargur area, Al2O3 content are ranges from 5.24%
in upper portion of profile to 11.18 in bed rock. In soil profile developed on
hornblende gneiss, Al2O3 content gradually increased toward the soil horizon due to
slow alteration and its content correlates well with the proportion of the various
aluminum bearing minerals. Paleosol developed on rock of Doddakanya area also
shows very clear result with the gradual increase in alumina in the different separated
zones from the bed rock zone to soil horizon in top of the profile tables- 5.1 to 5.7.
Al2O3 is commonly assumed to be retained in the soil during weathering because Al
compounds are relatively insoluble under normal pH conditions and much of the Al is
incorporated in clay minerals (Birkeland, 1974). The study conducted by
Balasubramaniam (1978), demonstrated that, only under very low pH aluminum is
more soluble, but between pH 4 and 3 it is virtually insoluble and when the acidic
solution is naturalized alumina remains in solution.
Therefore the concentration of Al generally increases in the profile relative to the
parent rock during weathering since it is retained in the soil, while other components
are removed by leaching. This is generally the scenario in all paleosol profiles in the
current studies in and around Mysore (Figureures- 5.1 to 5.4). Mineral gibbsite is a
main carrier of alumina followed by kaolinite, allophone, diaspore and hydrous mica
in that order of sequence (Govinda M. S et al., 1997).
168
b) Silica (SiO2)
Silica is one of the distinctive major elements in chemical features. The silica content
of the amphibolite profile in Chikkahali area varies from 50.3% in the parent material
to 53.9% in soil horizon, similar result also observed in profile developed on
amphibolite in Sargur area whereas the SiO2 content was varied from 55.16% in bed
rock to 63.69% in top soil (tables- 5.1- 5.7). In the profile developed on gneiss, the
silica content decreased from bed rock toward the paleosol horizon, which indicates
that it is slowly leaching out as result of chemical weathering on the other hand clay
formation is reflected in the decreased SiO2 and increased Al2O3 toward the surface.
Generally the bulk silica content in the zones above the altered basement rock is
represented by kaolinite and quartz. Compared to all the study profiles the soil profile
developed on ultramafic rock in Doddakanya area shows the less concentration of
silica in the parent materials as well as developed soil. The variations of silica in
entire profile in the study area are shown in Figures- 5.1 to 5.4).
c) Iron (Fe2O3)
The iron content discussed here is expressed as Fe2O3. Basically the soils developed
in humid condition in tropical and subtropical environments are more thoroughly
leached and possess Fe-rich in B horizon. There is a wide variation in the iron content
of the profile of studies area (tables- 5.1 to 5.7). The minimum, content of Fe2O3
(0.121 %) is recorded in unaltered hornblende gneiss of Gundlupet area and
maximum (27.58 %) is in the completely weathered amphibolite in Sargur area
(Figures- 5.1 to 5.4). This data shows the enrichment of iron oxide in the top most and
completely weathered zone of the paleosols profiles in the study region. There is a
gradual increase in the content of Fe2O3 from fresh rocks to slightly weathered and
completely weathered zone of the profiles.
d) Titanium (TiO2)
The low content of titanium is yet another important chemical feature of the paleosols
of the study areas. The total TiO2 content of the fresh parent amphibolite in
Chikkahali area is 0.43% and gradually increase in this amount is observed in top soil
around 0.8%. The gradual increase of TiO2 content in the upper zone of the profile
tabulated in tables- 5.1 to 5.7. The minimum content (0.26%) of TiO2 have been
169
recorded in the fresh calc-silicate rock and maximum (1.14%) have been reported
from completely weathered zone of the profile developed on ultramafic rock in
Doddakanya region tables- 5.1-5.7 and Figures- 5.1 to 5.4.
The enrichment of titanium in the top soil is probably due to the presence of
crystallized or dispersed submicroscopic anatase which formed during the process of
weathering. Titanium precipitated normally at ˃ pH 2.5 and to be retained in a soil
profile because Ti compounds are also relatively immobile during weathering
(Sposito, 1989). Between the pH 4 and 5 elements like Ti, Al and Fe may become
relatively enriched by precipitation from solution (Loughman, 1969).
e) Loss On Ignition (LOI)
Results of loss on ignition (LOI) tests on samples from different depths are shown in
Tables- 5.1 to 5.7. The LOI test generally measures the molecular water content in
soil minerals (Nishida, 1999). Since most water is contained in the crystal structure of
clay minerals rather than in the rock minerals. The LOI value increased with
increasing degree of weathering probably due to the presence of hydroxide minerals.
5.4.2 Minor Elements
The details of each of the minor elements present in the profile are discussed below
and data expressed as percentage and showed in tables- 5.1 to 5.7.
a) Sodium (Na2O)
Sodium forms a soluble salt during weathering and is removed by circulating rain
water/meteoritic water. The content of soda in the entire profile range from 0.17% in
unaltered amphibolite and 0.25% in completely weathered zone respectively. In the
profile developed on gneiss the range was 3.73% in unaltered to 4.36% in highly
weathered respectively, but depleted in top soil due to influenced of the rain water. In
the quartzofeldspathic rock the sodium content was only 0.062% and strongly leached
toward the upper zones (1.04%) (Tables- 5.1 to 5.7 and Figures- 5.1 to 5.4).
170
b) Potash (K2O)
Less amount of K2O are detected in all studies profile and like soda, k2O also strongly
depleted in paleosols horizons (tables- 5.1 to 5.7). The K-feldspar is the major mineral
containing substantial potash in the basement rock (granitic gneiss and also partly
from mica). During the hydrolysis, potassium is released from the mineral and forms
soluble salt and subsequently leached out.
c) Magnesia (MgO)
Magnesia almost depleted in all the zones of the profile toward the upper horizons,
MgO shows a gradual decrease in content from the parent rock to paleosols zone
(tables- 5.1 to 5.7), and shows less mobility during the weathering process. In the
profile developed on ultramafic rocks in magnesite mine in Doddakanya region,
unaltered parent material contains almost 32% magnesium whereas the top soil
exhibit only 10.9% and depletion being 3 times in the horizon. The variation of
percentage of MgO in paleosols profiles of the current study are shown in Figures- 5.1
to 5.4.
d) Limo (CaO)
The profile developed on calc-silicate rock shows high lime content (19.1%) in parent
material due to existence of calcite minerals and depleted through the profile horizons
(11% in top soil). The CaO also shows strong depletion in all the zones of the studies
profiles (tables- 5.1 to 5.7 and Figures- 5.1 to 5.4). In the early stage of weathering
calcium easily goes into solution than any other minor elements. In acidic condition
the calcite mineral converted to calcium bicarbonate and leached out and can enter in
to the underground water.
5.4.3 Trace Element
During the pedogenesis several geochemical process like selective mobilization,
leaching and partial precipitation of elements are involved in the weathering profile.
The behavior of trace elements during the weathering has been discussed by several
researchers (Gardon and Murata, 1952; Adams and Richardson, 1960; Chowdry et al.,
171
1965; Balasubramanian and Srikant, 1984; Khandali and Devaraju, 1986; Krishna
Rao et al., 1987).
The role of trace elements is usually linked with major elements, but in the case of
residual deposit such a linkage is untenable (Balasubramanian and Srikant, 1984).
Therefore, the geochemical behavior of individual trace element during the process of
weathering is considered.
a) Vanadium (V)
The paleosols are a potential source of vanadium as a by-product of weathering. There
is a gradual increase in the concentration of V from the parent rock toward the upper
horizons in the almost studies profile (Tables- 5.1 to 5.7 and Figures- 5.1 to 5.4). The
maximum content of vanadium is recorded from top soil developed on ultramafic rock
in Doddakanya area (204 ppm), followed by paleosols developed on amphibolite
profile in Chikkahali region (154 ppm). Vanadium probably occurs in magnetite,
biotite, hornblends and Ti- minerals of fresh bed rock.
Vanadium is closer related to ferric iron than aluminum (Goldschmidt, 1954). During
the process of weathering the precipitation of soluble vanadate is aided by hydroxides
of ferric iron, Therefore the concentration of V is always more in the rich iron soils.
b) Chromium (Cr)
In the process of weathering, chromium presented in pyroxene and probably
amphibolite and biotite might have released in solution. Whereas, Cr which is
presented in the structure of magnetite and limonite gets concentrated mechanically as
these minerals are resistances to chemical weathering (Rankama and Shama, 1950).
The high concentration of Cr can also occur in Kaolinite (Mclaughlin, 1959), which is
very similar to those of Al+3
and Fe+2
. Therefore, it can have an isomorphous
substitution in aluminous and ferric iron minerals.
The highest content of Cr is recorded in the parent rock horizon of the profiles
developed on ultramafic rock (5467 ppm), followed by the in unaltered amphibolite in
Chikkahali area (2698 ppm) (Tables- 5.1 to 5.7). The variations of Cr in the studied
profiles are shown in Figures- 5.1 to 5.4.
172
c) Nickel (Ni)
Most of the nickel in the parent rock is represented by the ferromagnesian minerals as
well as ferric hydroxide minerals. High concentration of Ni is recorded from the
highly weathered zones of profile developed on ultramafic rock in magnesite mine
(2751 ppm), probably related to rather high iron oxide minerals present in the area
(Tables- 5.1 to 5.7). The concentration of Ni in the paleosols profiles depend on the
degree of alteration, on the other hand in highly or completely weathered horizon the
concentration of the Ni is high (Figures- 5.1 to 5.4).
d) Cobalt (Co)
Cobalt is incorporated in the ferromagnesian minerals of parent rocks (Carr and
Tunekian, 1961). Such minerals of parent rock (gneiss, hornblende gneiss,
quartzofeldspathic …) under the condition of chemical weathering are converted to
goethite and limonite, which are likely to contain Co in different lithounits of the
profile.
The maximum Co content is recorded in the hornblende gneiss profile (32 ppm) and
the minimum in the calc-silicate rock (9 ppm) see tables- 5.1 to 5.7. There is not much
variation in the concentration of Co in the profile horizons (Figures- 5.1 to 5.4).
e) Copper (Cu)
The copper contents of parent rocks in different paleosols profiles in the study areas
were varied from 12 ppm to 62 ppm. The maximum content of cupper was recorded
in amphibolite followed by ultramafic rock and minimum was observed in quartzo-
feldspathic rock in the profile examined from Belagula region (Tables- 5.1 to 5.7).
Moderate variation was observed through the profiles especially in weathered
horizons (Figures- 5.1 to 5.4).
Based on the ionic potential of cu, it is expected to pass into ionic solution and
thereby get depleted during the process of weathering (Rao and Murthy, 1981). The
study conducted by Rankama and Shama (1950), demonstrated that the higher
concentration of Cu in highly weathered zones may be due to the highest iron content
as cu replace ferrous iron due to their similarity in ionic radii.
173
f) Zinc (Zn)
Zn is usually present in the ferromagnesian minerals. The geochemistry Zn is similar
to magnesium iron group of metals due to their large ionic radii, especially between
zinc and iron (Goldschmidt, 1954). Less variation of Zn content have observed in the
profiles might be due to the slow rate of weathering. High Zn contents have recorded
in highly weathered horizons can be related to the presence of higher contents of iron
minerals (Rao and Murty, 1981). High concentration of Zn has been recorded from
weathered amphibolite in Sargur region (Tables- 5.1 to 5.7). The depth wise variation
diagram of Zn in the studied profiles is also shown in the variation diagram Figures-
5.1 to 5.4.
g) Rubidium (Rb)
The variations of Rb in the studied profiles were low to moderate, high concentration
of Rb clearly recorded from weathered zones followed by top soils. Among the all
studied profile, weathered ultramafic rock in Doddakanya area shows high Rb content
(Tables- 5.1 to 5.7 and Figures- 5.1 to 5.4). The chemical behavior of alkali Rb is very
similar to that of potassium. During the chemical weathering, Rb and other alkali
metals are also brought into solution and subsequently removed and precipitated or
absorbed by the clay due to their small ionic potential (Rankama and Sahama, 1950).
h) Strontium (Sr)
The high content of Sr in bed rock of the study areas is belong to Amphibolite (388
ppm), followed by gneiss (313 ppm) then calc-silicate (54 ppm) and finally
hornblende gneiss (27 ppm). Slow to moderate depletion through the upper horizons
of the mentioned profiles is reported (Tables- 5.1 to 5.7 and Figures- 5.1 to 5.4).
Generally, the mineral plagioclase contains as appreciable amount of Sr (Rankama
and Sahama, 1950) and the geochemical behavior of Sr in weathering process is
similar to that of calcium. It rapidly enters into an aluminous solution and removed by
circulating water.
174
Table- 5.1: Major (in oxide weight percent) and Trace (in ppm) elements abundances
in the profile developed on amphibolite in Chikkahali area.
T: Trace
ND: Not detected / below detection limit
C: bed rock, B1: moderately weathered, B2: highly weathered, B3: completely weathered
Sample C B1 B2 B3 Soil
SiO2
50.351
56.125
52.081
51.785
53.949
Al2O3 10.333 8.412 9.403 9.799 15.347
TiO2 0.437 0.396 0.411 0.441 0.789
MnO 0.135 0.177 0.106 0.124 0.158
Fe2O3 11.162 10.145 9.729 9.935 11.678
CaO 5.922 3.339 3.675 2.823 2.435
MgO 16.965 17.305 14.984 14.394 5.494
Na2O 0.174 0.202 0.133 0.118 0.258
K2O 0.008 0.055 0.026 0.083 0.561
P2O5 0.049 0.033 0.035 0.032 0.055
LOI 4.100 3.400 9.100 10.100 9.000
Total 99.636 99.589 99.683 99.634 99.724
Trace elements (in ppm)
V 136 109 100 104 152
Cr 2506 2698 2107 2432 1269
Co 24 24 22 23 23
Ni 666 932 657 728 388
Cu 65 30 8 47 64
Zn 82 75 72 75 78
Rb 35 35 74 64 67
Sr 22 14 20 18 59
Zr ND ND ND ND 266
Ba 20 114 28 74 256
La 17 22 16 21 28
Ce 35 35 34 38 72
Pb 19 19 26 24 21
175
Table- 5.2: Major (in oxide weight percent) and Trace (in ppm) elements abundances
in the profile developed on gneiss in Chikkahali area.
Sample C B1 B2 B3 B4 B5 Soil
SiO2 72.002 68.906 69.865 69.934 69.932 72.842 65.692
Al2O3 14.726 14.077 15.757 16.595 15.228 16.184 17.133
TiO2 0.427 0.587 0.411 0.355 0.474 0.395 0.791
MnO 0.023 0.066 0.012 0.014 0.026 0.013 0.057
Fe2O3 0.548 4.492 0.670 0.659 1.453 0.624 4.521
CaO 0.644 2.889 1.721 1.145 0.545 1.044 1.911
MgO 0.770 2.330 1.165 1.013 0.884 0.690 1.677
Na2O 3.734 3.053 4.436 4.366 3.446 3.311 2.263
K2O 3.386 2.304 2.252 2.696 2.785 3.213 2.716
P2O5 0.078 0.113 0.084 0.077 0.079 0.032 0.057
LOI 3.500 1.050 3.500 3.000 5.000 1.500 3.000
Total 99.838 99.867 99.873 99.854 99.852 99.848 99.818
Trace elements (in ppm)
V 59 94 55 54 69 56 90
Cr 31 71 29 27 34 20 61
Co 10 14 11 10 11 10 14
Ni 36 47 44 35 33 24 52
Cu 12 30 21 4 13 21 31
Zn 56 79 55 54 53 48 85
Rb 135 151 128 109 143 118 146
Sr 313 275 357 379 163 254 196
Zr 269 139 195 306 148 258 395
Ba 501 283 234 348 618 580 478
La 46 36 23 26 34 32 96
Ce 119 87 76 71 110 63 132
Pb 23 26 29 23 28 27 29
T: Trace
ND: Not detected / below detection limit
C: bed rock, B1: moderately weathered, B2, B3: highly weathered, B4, B5: completely weathered
176
Table- 5.3: Major (in oxide weight percent) and Trace (in ppm) elements abundances
in the paleosols developed on quartzo-feldspathic rock in Belagula area.
Sample C B1 B2 B3 Soil
SiO2 57.174 48.193 51.650 53.587 67.546
Al2O3 18.955 9.365 9.271 8.823 10.764
TiO2 0.925 0.945 0.867 0.752 0.584
MnO 0.086 0.163 0.135 0.094 0.065
Fe2O3 7.151 14.698 13.942 13.493 7.206
CaO 1.691 7.609 7.193 4.616 2.673
MgO 1.700 14.963 12.922 12.527 5.367
Na2O 0.062 1.266 0.829 0.557 1.043
K2O 0.365 0.402 0.279 0.264 1.852
P2O5 0.116 0.160 0.120 0.053 0.049
LOI 11.600 1.850 2.400 4.850 2.600
Total 99.825 99.614 99.608 99.616 99.749
Trace elements (in ppm)
V 159 196 186 150 104
Cr 533 2003 2177 2238 1226
Co 18 29 27 26 18
Ni 159 1069 1069 1008 497
Cu 43 89 54 73 43
Zn 82 87 83 78 66
Rb 85 34 12 43 75
Sr 16 117 118 61 106
Zr 277 18 30 ND 63
Ba 195 115 78 73 214
La 41 21 20 23 21
Ce 89 41 44 44 34
Pb 28 18 15 20 20
T: Trace
ND: Not detected / below detection limit
C: bed rock, B1: moderately weathered, B2: highly weathered, B3: completely weathered
177
Table- 5.4: Major (in oxide weight percent) and Trace (in ppm) elements abundances
in the profile developed on calc-silicate rock in Bettadabidu area.
Sample C B1 B2 B3 Soil
SiO2 58.705 92.502 13.946 54.649 44.645
Al2O3 4.907 5.125 6.102 6.345 9.777
TiO2 0.262 0.267 0.322 0.318 0.595
MnO 0.384 0.082 0.590 0.665 1.768
Fe2O3 1.414 T 2.445 4.630 12.592
CaO 19.104 0.353 47.954 19.014 11.063
MgO 8.883 0.465 12.105 10.342 8.400
Na2O 0.031 T 0.044 0.057 0.063
K2O T T 0.503 0.132 0.483
P2O5 0.031 0.025 0.053 0.060 0.087
LOI 6.250 1.150 15.900 3.750 10.500
Total 99.971 99.969 99.964 99.962 99.973
Trace elements (in ppm)
V 37 41 49 55 101
Cr ND 16 19 30 132
Co 10 9 11 13 19
Ni 11 22 25 39 59
Cu 9 3 5 3 27
Zn 57 42 49 71 66
Rb 19 64 46 25 34
Sr 54 2 41 19 19
Zr ND ND ND ND 26
Ba 14 18 36 23 190
La 13 19 15 18 27
Ce 34 34 38 41 48
Pb 16 25 19 17 17
T: Trace
ND: Not detected / below detection limit
C: bed rock, B1: moderately weathered, B2: highly weathered, B3: completely weathered
178
Table- 5.5: Major (in oxide weight percent) and Trace (in ppm) elements abundances
in the profile developed on amphibolite in Sargur area.
Sample C B1 Soil
SiO2 55.167 63.501 63.698
Al2O3 11.188 5.104 5.246
TiO2 0.687 0.272 0.278
MnO 0.127 0.023 0.034
Fe2O3 21.419 22.893 27.588
CaO 4.593 5.278 1.151
MgO 2.577 1.167 0.808
Na2O 0.556 T T
K2O 0.263 T T
P2O5 0.099 0.087 0.119
LOI 3.250 1.650 1.050
Total 99.926 99.975 99.972
Trace elements (in ppm)
V 167 47 53
Cr 134 31 18
Co 29 28 32
Ni 71 18 13
Cu 62 ND ND
Zn 69 51 53
Rb 12 ND ND
Sr 27 4 2
Zr ND ND ND
Ba 74 11 19
La 23 17 18
Ce 40 34 34
Pb 14
11
11
T: Trace
ND: Not detected / below detection limit
C: bed rock, B1: completely weathered
179
Table- 5.6: Major (in oxide weight percent) and Trace (in ppm) elements
abundances in the profile developed on hornblende gneiss in Gundlupet area.
Sample C B1 B2 Soil
SiO2 75.181 46.619 51.876 69.358
Al2O3 13.824 17.769 13.391 17.544
TiO2 0.328 1.329 1.175 0.534
MnO 0.019 0.218 0.154 0.046
Fe2O3 0.121 17.306 14.140 3.158
CaO 1.189 7.301 8.708 1.375
MgO 0.321 2.484 2.619 1.077
Na2O 4.026 0.732 1.652 1.927
K2O 3.025 0.519 0.489 1.797
P2O5 0.038 0.110 0.207 0.059
LOI 1.800 5.500 5.500 3.000
Total 99.872 99.887 99.911 99.875
Trace elements (in ppm)
V 42 285 234 72
Cr 7 100 62 67
Co 10 28 24 13
Ni 16 79 43 45
Cu 22 101 93 25
Zn 57 132 105 66
Rb 122 19 24 122
Sr 388 62 97 258
Zr 112 22 43 186
Ba 398 193 88 260
La 20 25 20 22
Ce 43 56 38 55
Pb 23 15 16 29
T: Trace
ND: Not detected / below detection limit
C: bed rock, B1: moderately weathered, B2: completely weathered
180
Table- 5.7: Major (in oxide weight percent) and Trace (in ppm) elements abundances
in the profile developed on ultramafic rock in Doddakanya area.
Sample C B1 B2 B3 B4 Soil
SiO2 36.422 33.445 45.313 27.686 40.178 47.248
Al2O3 5.273 5.565 5.025 6.689 5.190 9.482
TiO2 0.292 0.283 0.268 0.321 0.274 1.148
MnO 0.120 0.108 0.120 0.037 0.103 0.162
Fe2O3 9.206 8.339 9.139 1.174 7.753 14.738
CaO 1.615 7.961 1.429 9.737 2.743 9.974
MgO 32.083 24.634 22.285 29.742 22.411 10.948
Na2O 0.128 0.114 0.072 0.092 0.085 1.378
K2O T T T T T 0.265
P2O5 0.034 0.033 0.036 0.026 0.032 0.130
LOI 14.000 18.800 15.600 24.200 20.600 4.250
Total 99.173 99.282 99.287 99.704 99.369 99.723
Trace elements (in ppm)
V 53 59 48 49 52 204
Cr 5467 4697 3543 1417 4435 1518
Co 27 25 27 13 24 27
Ni 2492 2159 2751 1129 2052 528
Cu 10 10 ND 7 8 94
Zn 62 59 54 48 57 89
Rb 54 24 62 94 37 25
Sr 6 36 3 91 10 122
Zr ND ND ND ND ND 6
Ba 17 32 52 14 60 69
La 13 14 17 16 16 23
Ce 35 39 34 38 34 48
Pb 23 17 24 31 20 17
T: Trace
ND: Not detected / below detection limit
C: bed rock, B1: moderately weathered, B2: highly weathered, B3, B4: completely weathered
181
Figure- 5.1: Abundances and distribution of major, minor (in oxide weight %) and
trace element (in ppm) of Paleosols profiles; (A) developed on amphibolite and (B)
developed on gneiss in the Chikkahali area.
A
B
182
Figure- 5.2: Abundances and distribution of major, minor (in oxide weight %) and
trace element (in ppm) of Paleosols profiles; (A) developed on quartzo-feldspathic
rock in Belagula area and (B) developed on Calc-silicate rock in the Bettadabidu area.
B
A
183
Figure- 5.3: Abundances and distribution of major, minor (in oxide weight %) and
trace element (in ppm) of Paleosols profiles; (A) developed on amphibolite in the
Sargur region and (B) developed on hornblende gneiss in Gundlupet area.
B
A
184
Figure- 5.4: Abundances and distribution of major, minor (in oxide weight %) and
trace element (in ppm) developed on ultramafic rock in Doddakanya region
(magnesite mine).
185
5.5 CHEMICAL VARIATIONS
Geochemical parameters can be used as paleo-climate indicators and for inferring the
weathering conditions. However, most of the elements can be variably mobile during
post-depositional and metamorphic events. The geochemical trends in mature paleosol
profiles reflect the weathering of minerals, gravitational processes of leaching and
translocation, and the capillary processes that produce calcification or salinization
(Gill and Yemane, 1996).
Leaching, transfer and dissolution of the major and trace elements during pedogenesis
is greatly controlled by chemical constituents of the country rock and the surface,
besides the prevailing climatic conditions (Nesbitt and Young, 1982, 1984, 1989;
Maynard, 1992; Mc Lennan et al. 1993; Soil Survey Staff, 1999). Therefore, relative
geochemical variations across a profile inclusive of parent, gives an insight into the
degree of pedogenesis and environmental conditions persisted during pedogenesis
(Retallack, 1995; Sheldon et al. 2002). On the other hand depletion of mobile and
enrichment of immobile components in the Parent material and their corresponding
paleosols are distinct features of any paleosol profiles (Nagendera G et al., 2009).
5.5.1 Concentration Ratio of Major, Minor and Trace Elements Related to
Al2O3 and TiO2
The Al and Ti are highly conserved in the almost soil profile, the nature of chemical
weathering can be determined by making concentration ratio (CR) diagrams based on
the assumption that Al and Ti are retained within the soil profile. Naturally, some Al
and Ti are moved during weathering, although their solubility and mobility is
relatively low (Gay and Grandstaff, 1980). To understand the enrichment and
depletion of major and trace elements in the studied paleosols developed on different
parent material the method described by Gay and Grandstaff (1980), have been used.
The equations used to obtain concentration ratio (CR) is:
( )
( )
And
186
( )
( )
Where, M weathered is the concentration of major and trace elements in the
weathered zone and M parent is the concentration major and trace elements in the
parent rock.
The method of Muhs et al., (1987) also has been used to estimate the depletion and
enrichment of SiO2 relative to immobile constituents R2O3= (Al2O3+Fe2O3+TiO2)
compared to the presumed parent rock, the following equation is used:
5.5.1.1 Profile 1 (Chikkahali area)
The profile developed on amphibolite as a parent material in Chikkahali area have
been subjected for the calculation of concentration ratio (CR) of major, minor and
trace elements through the profile. The relative changes in abundances of the major,
minor and trace elements in the above profiles relative to Al2O3 and TiO2 are shown
in Figure- 5.5 A and B.
SiO2 shows slight to moderate depletion at the upper horizon toward the top of the
profile relative to Al2O3 (Figure- 5.1 A). Relative to TiO2, SiO2 is very slightly
depleted evenly throughout the profile, with the exception of the lower portion of the
B horizon (Figure- 5.1B). Weight percent ratios of SiO2 to R2O3 (Al2O3+Fe2O3+TiO2)
in this profile is showing depletion of SiO2 relative to (Al2O3+Fe2O3+TiO2) in the
profile Figure- 5.12 A.
Sposito (1989) demonstrated that SiO2 is conserved in the soil profile under low to
moderate leaching conditions. SiO2 is usually retained in clay minerals under such
conditions (Gay and Grandstaff, 1980). However, under intense leaching conditions,
SiO2 is removed and depleted in top horizon (Sposito, 1989).
Fe2O3 shows less depletion in the middle of the profile, but Fe2O3 is moderately
depleted from the surface of the profile (Figure. 5.5 A and B). During weathering and
diagenesis the oxidation state of iron changes (Gay and Grandstaff, 1980). Diagenesis
187
usually changes the iron from ferric (Fe+3
) to ferrous (Fe+2
) state, with little changes in
the total Fe content (Veizer, 1973).
Changes in the iron content can however be used to show whether oxidation or
reduction has occurred during the process of weathering. If conditions are oxidizing
during soil formation, iron mobilized by dissolution of primary minerals is oxidized to
insoluble Fe+3
and precipitates in the soil profile as ferric iron oxide or hydroxide
minerals but under reducing conditions iron is reduced to the more soluble Fe+2
,
which may be removed by groundwater (Soil Survey Staff, 1975).
Figure- 5.5 illustrate that iron was lost from the A horizon. Therefore surface loss and
subsurface gain of iron in the profile suggests that during weathering, soil waters were
under oxidizing conditions. The colors mottling occurring in the profile suggests that
reduction (gleying) may have occurred in this profile due to iron oxidation state
variations. Diagenesis may have had an impact in causing color changes in the
paleosol profile.
The concentration ratio of TiO2 related to Al2O3 in the B horizon was slightly depleted
whereas in top soil very less enriched but in terms of Al2O3 related to TiO2 the result
was reverse.
The concentrations ratio of MgO, CaO, K2O, Na2O, P2O5, MnO, Ni, Zn, and Cr
related to Al2O3 and TiO2 decrease from the parent rock to the paleosol (Figure- 5.5 A
and B). The depletion of Na2O was high in B horizon but the degree of depletion
decreased in A horizon. Cupper ratio decreased in weathered horizon and in top soil
slightly enriched. Sr and Rb gradually enriched toward the paleosols and Ba element
concentration strongly depleted in weathered horizon then gradually enriched toward
the top soil.
5.5.1.2 Profile 2 (Chikkahali area)
The paleosol developed on gneiss in Chikkahali area have been tested for chemical
variation through the profile. The relative changes in abundances and variation of the
major, minor and trace elements in this profile, relative to Al2O3 and TiO2 are shown
in Figure- 5.6 A and B.
188
The concentration ratio of SiO2 related to Al2O3 didn’t change significantly but
weight-percent ratios of SiO2 related to R2O3 shows slightly depletion in surface of the
profile (Figure- 5.12 B). The CR value of Al2O3 related to TiO2 shows slight
enrichment in upper horizon but in top soil it had similar concentration with parent
material. Iron oxide concentration in the profile strongly depleted in less weathered
and highly weathered horizons but in the paleosols, degree of depletion significantly
decreased might be due to oxidation stage of iron.
Same as amphibolite profile in the same region the concentration ratio of MgO, CaO,
K2O, Na2O, P2O5, MnO, Ni, Zn, Rb, V, Cu, Sr and Cr related to Al2O3 and TiO2
decrease from the parent rock to the paleosol (Figure- 5.6 A and B). In modern soils,
most of these constituents are depleted from the profile (Sposito, 1989). This is the
case for the above said profile in Chikkahali region. Ba ratio related to Al2O3 shows
graduate enrichment towards the profiles but in relation with TiO2 the degree of
enrichment is strong.
5.5.1.3 Profile 3 (Belagula area)
The profile developed on quartzo-feldspathic rock in Belagula region was
geochemically analyzed and concentration ratio of major and trace elements related to
Al2O3 and TiO2 calculate. The Figure- 5.7 A and B shows the relative changes in
abundances and variation of the major, minor and trace elements in this profile.
The SiO2 concentration ratio was slightly enriched from parent material to top soil;
same result also came out when weight-percent ratios of SiO2 related to R2O3
calculated (Figure- 5.12 C). Iron oxide shows the graduate depletion toward the upper
horizons, in the case of Al2O3 related to TiO2 slightly enrichment has observed. The
concentration ratio of MgO, CaO, P2O5, MnO, Ni, Zn, Rb, V, Cu, Sr and Cr related to
Al2O3 and TiO2 decrease from the parent rock to the paleosol (Figure- 5.7 A and B).
CR value of Na2O was depleted in weathered zone the enriched in the paleosols. Ba
and K2O related to TiO2 shows gradual increasing in the Cr value through the profile.
5.5.1.4 Profile 4 (Bettadabidu area)
The chemical variations of paleosol developed on calc-silicate rock in betadabeddue
area have been examined. Soil profiles of arid and semi-arid climates often contain
189
pedogenic calcretes which can be used as an indicator for the maturity of paleosols.
The relative changes in abundances of the major and trace elements in the profile
relative to Al2O3 and TiO2 are shown in Figure- 5.8 A and B.
The concentration ratio of SiO2 related to Al2O3 and TiO2 shows moderate to slight
depletion in weathered and top soil respectively, weight-percent ratios of SiO2 related
to R2O3 also shows slight enrichment in A horizon (Figure-5.12 D). Fe2O3 is strongly
enriched from parent material to paleosols might be due to extreme environment
condition of earth in past. Precambrian paleosols on Earth clearly formed in a world
of running water and atmosphere, and may provide evidence of variation in their fluid
environment with time (Holland, 1984). Atmospheric oxygenis the main oxidizing
agent in the present atmosphere and its effect on weathering is profound. Iron
liberated by dissolution of minerals is oxidized to ferric oxides (Fe2O3 or Hematite),
hydroxides or oxyhydrates, in which iron is in the trivalent state (Fe3+
).
Under reducing conditions, iron released from minerals remains in its reduced state
(Fe2+
), within drab coloured silicates (such as chlorite), sulphides (pyrite) or
carbonates (siderite). Since bivalent iron is much more soluble than trivalent, iron
tends to be lost from soils formed under reducing conditions.the iron content and
colour of paleosols are potentially useful guides to the changing oxygen content of
past atmospheres.
The concentration ratio of MgO, CaO, Na2O, P2O5, MnO, Ni, Zn, Ba, V, Cu, Sr and
Cr related to Al2O3 and TiO2 increased from the parent rock to the paleosol (Figure-
5.8 A and B).
5.5.1.5 Profile 5 (Gundlupet area)
In the paleosols developed on hornblende gneiss, CR value of SiO2 slightly decreased
from parent rock to top soil (Figure- 5.12E). Same as calc-silicate profile the CR of
Al2O3 increased but the trend of increasing was moderate might be due the clay
formation which is reflected in the decreased SiO2 and increased Al2O3 toward the
surface (Figure- 5.9 A).
The concentration ratio of MgO, CaO, Ni and Cr related to Al2O3 and TiO2 decreased
from the parent rock to the paleosol whereas The concentration ratio of Na2O, P2O5,
190
MnO, Zn, Ba, V, Cu and Sr related to Al2O3 and TiO2 increased from the parent rock
to the paleosol (Figure- 5.9 A and B).
5.5.1.6 Profile 6 (Sargur area)
A paleosol developed on amphibolite in Sargur area was analyzed for variation of
major and trace elements in different horizons of the profile. The concentration ratio
of major and trace elements related to Al2O3 and TiO2 are shown in Figure- 5.10 A
and B.
Weight-percent ratios of SiO2 related to R2O3 are strongly to moderately decrease in
weathered zone to top soil respectively (Figure- 5.12 F). The concentration ratio of
major and trace elements such as of TiO2, MgO, CaO, P2O5, MnO, V, Cu, and Sr
related to Al2O3 decreased from the parent rock to the paleosol where as the CR value
of Na2O, K2O, Zr, Rb and Ba slightly increased in upper zones (Figure- 5.10 A and
B). Ni, Zn and Cr shows opposite result when calculated with TiO2. Ni related to TiO2
shows strong enrichment in surface compare with parent material. However, the
enrichment of this element is generally greatest in the lower half of the profile along
with clay and iron oxides (Figure- 5.10 B).
5.5.1.7 Profile 7 (Doddakanya area)
One profile of the paleosols developed on ultramafic rock in Doddakanya area has
been geochemically analyzed to check the variation of elements in the profile. The
concentration ratio of major and trace elements related to Al2O3 and TiO2 are shown
in the Figure- 5.11 A and B.
In this profile the weight-percent ratios of SiO2 related to R2O3 slightly depleted
(Figure- 5.12 G). TiO2 shows less enrichment towards the soil surface. Iron oxide
depleted in weathered zone and slight enrichment are observed in A horizon (Figure-
5.11 A and B)
The concentration ratio of major and trace elements such as of MgO, CaO, MnO, Rb,
Ni, and Zn related to Al2O3 decreased from the parent rock to the paleosol whereas
the CR value of Na2O, P2O5, Sr, Cu, V and Ba slightly increased in upper zones
(Figure- 5.11 A).
191
Figure- 5.5: Concentration ratio diagram for profile 1 (Paleosols developed on
amphibolite) showing enrichment and depletion of constituents of major and trace
elements; A) relative to Al2O3 B) relative to TiO2.
A
B
192
Figure- 5.6: Concentration ratio diagram for profile 2 (Paleosols developed on
gneiss) showing enrichment and depletion of constituents of major and trace
elements; A) relative to Al2O3 B) relative to TiO2.
A
B
193
Figure- 5.7: Concentration ratio diagram for profile 3 (Paleosols developed on
quartzofeldspathic rock) showing enrichment and depletion of constituents of major
and trace elements; A) relative to Al2O3 B) relative to TiO2.
A
B
194
Figure- 5.8: Concentration ratio diagram for profile 4 (Paleosols developed on calc-
silicat rock) showing enrichment and depletion of constituents of major and trace
elements; A) relative to Al2O3 B) relative to TiO2.
A
B
195
Figure- 5.9: Concentration ratio diagram for profile 5 (Paleosols developed on
amphibolite) showing enrichment and depletion of constituents of major and trace
elements; A) relative to Al2O3 B) relative to TiO2.
A
B
196
Figure- 5.10: Concentration ratio diagram for profile 6 (Paleosols developed on
hornblende gneiss) showing enrichment and depletion of constituents of major and
trace elements; A) relative to Al2O3 B) relative to TiO2.
A
B
197
Figure- 5.11: Concentration ratio diagram for profile 7 (Paleosols developed on
ultramafic rock) showing enrichment and depletion of constituents of major and trace
elements; A) relative to Al2O3 B) relative to TiO2.
B
A
198
Figure- 5.12: Plots of weight-percent ratios of SiO2 to R2O3 in the all studied profiles
of the studied areas showing depletion and enrichment of SiO2 relative to immobile
constituents (Al2O3+Fe2O3+TiO2) compared to the presumed parent rock.
199
5.6 Intensity of Weathering Conditions (Molecular Weathering Ratios)
Molecular weathering ratios reveal changing chemical proportions and estimation of
various ratios of the mobile and immobile oxides as a result of processes or properties
like salinization (Na2O/ K2O), calcification ((CaO + MgO)/ Al2O3), clayeyness
(Al2O3/SiO2), Hydrolysis (Al2O3/(CaO + MgO + Na2O + K2O)), leaching (Ba/ Sr),
chemical index of alteration (CIA), chemical index of alteration (CIW) and
Weathering (Fe2O3/SiO2) based on description demonstrated by Nesbitt and Young,
(1982, 1984) and Sawyer, (1986).
These approaches have been widely used (Retallak 1983, 1991) because it is
relatively free from assumption except that the material in question is indeed a
paleosol and that changing molar proportions of elements are related to common
weathering reactions found in soils today.
5.6.1 Salinization
Based on the equation introduced by Retallack (2001), Salinization of the paleosols
profiles (i.e., accumulation of soluble salts during weathering) can be calculated using
the following equation:
The values less than 0.2 are below the salinization, whereas soils with values greater
than 2 have been affected by salinization (Retallack, 2001). There is no increase in the
ratio near the top of the paleosol profiles, which is also consistent with minimal to
moderate salinization in all studied profile. Almost in the entire profiles, weathered
zone more affected by salinization and top paleosols shows minimal salinization. The
variations of salinization ratio in all the studied profile are presented in Figure- 5.13,
Whereas A) is represented the profile developed on amphibolite profile, B) gneiss, C)
quartzo-feldspathic, D) calc-silicate and E) hornblende gneiss as parent rocks.
5.6.2 Clayeyness
The second molecular weathering ratio that was applied is "clayeyness". The relative
clayeyness suggests significant clay accumulation especially in the bottom portion of
200
the B horizon, where the ratio should increase due to clay production during
pedogenesis (e.g., Sheldon, 2006; Sheldon and Tabor, 2009).
The following equation widely used to estimate the rate of clay formation during the
pedogenesis:
The data obtained from this ratio shows clayeyness, parameters increases with higher
pedogenic levels in the almost all the profiles and only in the profiles developed on
calc-silicate rock and quartzo-feldspathic rock minimal decreasing trend are observed.
The clayeyness data calculated from the studied profiles of all studied profiles and the
variation in the profile are shown in Figure- 5.14. The profiles developed on
amphibolite (A), gneiss (B), quartzo-feldspathic (C), hornblende gneiss (F) and
ultramafic rocks shows slow clayeyness process whereas the profile developed on
calc-silicate rock shows moderate trend toward the upper horizon.
5.6.3 Calcification
The calcification parameter varies in complement with the abundance of calcretes
accumulation in the paleosol horizons. The calcification is an evident in the paleosol
profile. The calcification ratio can obtain by the following formula:
( )
The paleosols developed on calc-silicate (D), amphibolite (profile A, E) and
ultramafic rocks (G) decreasing with higher pedogenic levels while the profiles
developed on gneiss (B), quartzofeldspathic (C) and hornblende gneiss (F) rocks
shows low increases trend. The Figure- 5.15 shows the variation of calcification ratio
in the all seven studied profiles.
5.6.4 Hydrolysis (base loss)
Application of the hydrolysis ratio is based on the idea that as minerals are weathered,
base cations are lost relative to Al during clay formation. These values indicate the
removal of mobile cations by extreme leaching (Gill and Yemane, 1996).
201
Hydrolysis is calculated with the following equation (Retallack, 2001):
( )
The profiles developed on amphibolite (A and E) and ultramafic rocks (G) shows high
hydrolysis toward the upper horizon whereas the profiles developed on gneiss (B),
calc-silicate (D) and hornblende gneiss (F) shows moderate hydrolysis in paleosols
zone (Figure- 5.16). In the paleosols profile developed on quartzo-feldspathic rock
(C), the hydrolysis was decreased in top horizons. The degree of hydrolysis in the soil
profiles work as an indicator for high leaching activity during the pedogenesis.
Positive and negative peaks of the base loss, respectively indicates the relative
abundance of the bases and the refractive element (Al2O3). In the paleosols developed
in the study area, relative base loss is very strong (Figure- 5.16).
5.6.5 Leaching
One method of measuring the degree of chemical weathering (or leaching) in the soil
profile is obtained by observing molecular weathering ratios as given by Gill and
Yemane (1996). The degree of leaching may be assessed by considering the following
equation (Retallack, 2001):
All the seven profiles in the study area subjected for calculation of degree of leaching
based on different parent material. The solubility differences between the two
elements (Ba < Sr) gives rise to higher values in more leached samples. The profiles
developed on amphibolite (A and E), gneiss (B) and calc-silicate (D) rocks shows
strong leaching whereas the paleosols profiles developed on quartzo-feldspathic (C),
hornblende gneiss (F) and ultramafic rocks (G) shows low leaching (Figure- 5.17).
Strong loss of mobile cations and clay enrichment is typical of soils that form in warm
tropical and subtropical environments (Brady, 2002). The variations of leaching ratio
in the all studied profiles are shown in Figure- 5.17.
202
5.6.6 Chemical Index of Alteration (CIA)
Soil profiles characterized by high degree of weathering are enriched in hydrous clay
mineral phases. Another method can be used to measure the degree of chemical
weathering in paleosol profiles is the chemical index of alteration (CIA) of the profile
in terms given by Nesbitt and Young (1982) using molecular proportions:
( )
Based on description of Nesbitt and Young, (1982) Changes in CIA are reflected
mainly by changes in the proportion of feldspars and clay minerals or caused by grain
size sorting during original deposition of the parent material for the profile. Different
parent materials have different initial CIA values. The parental materials of the study
area are shown wide range of CIA for example, amphibolite (A) in Chikkahali shows
CIA value of 62.8 whereas calc-silicate (D) rock in Bettadabidu shows CIA value of
20.4 (Figure-5.18).
Except profiles developed on quartzo-feldspathic rock (G), and ultramafic rock in all
studied profiles significant increasing in CIA value of top horizon are observed this
result is an agreement with hydrolysis value obtain in same profiles. The changes in
these CIA values are illustrated in Figure-5.18. CIA values for the profile are more
than 80 suggesting the presumed parent material was weathered under intense
leaching conditions (Nesbitt and Young, 1982).
5.6.7 Chemical Index of Weathering (CIW)
Chemical Index of Weathering for the first time proposed by Harnois (1988), he
considers that the alteration index calculated on K2O-free basis will represent better
the intensity of weathering. Later on Condie, (1992) considers CIW as a best measure
of intensity of chemical weathering.
The chemical index of weathering measure the degree of weathering experienced by a
material relative to its source rock and increased with depilation of Na and Ca in soil
203
profile relative to Al (Harnois,
1988). The following equation is used in the
calculation of CIW:
( )
The CIW, just as CIA values are found to be maximum for paleosols developed on
amphibolite (A) in Chikkahali area (85) followed by hornblende gneiss (F) in
Gundlupet area. However, the gneiss (B) and quartzo-feldspathic (C) profiles which
showed low CIA values, are characterized by high CIW, suggesting the low CIA may
be due to post depositional enrichment of K2O in these profiles. Figure- 5.19 shows
variation of CIW index in all seven studied profile in and around Mysore district.
204
Figure- 5.13: Variation of salinization values in the selected Paleosol profiles in the
study areas.
205
Figure- 5.14: Variation of clayeyness values in the Paleosol profiles in the study
areas.
206
Figure- 5.15: The calcification parameter variation in the Paleosols developed on
different parent rocks.
207
Figure- 5.16: The hydrolysis parameter variation in the Paleosols developed on
different parent rocks.
208
Figure- 5.17: The leaching parameter variation in the Paleosols developed on
different parent rocks.
209
Figure- 5.18: The variation of CIA in the Paleosols developed on different parent
rocks.
210
Figure- 5.19: The variation of CIW in the Paleosols developed on different parent
rocks.