Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 2, pp. 139-143, 2005

ArticlesApplication of k0-based INAA Method in the Studies of Rare Earth and Other Elements in
Manganese Nodules from Indian Ocean
R. K. Dutta,a, b R. Acharya,b A. G. C. Nair,b S. N. Chintalapudi,c V. Chakravortty,aA. V. R. Reddy,*, b and S. B. ManoharbaDepartment of Chemistry, Utkal University, Vani Vihar, Bhubaneswar 751004, India
Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, Indiac
IUC for DAE facilities, Calcutta Centre, III/LB-8, Bidhan Nagar, Calcutta 700 091, Indiab

Received: May 23, 2005; In Final Form: October 24, 2005
Five manganese nodules obtained from different locations with varying water depths of the Indian Ocean wereanalysed by k0-based instrumental neutron activation analysis (k0-INAA) method. A total of 22 elements were
estimated including nine rare earth elements (REE). The accuracy of the method has been evaluated by analysing
USGS manganese nodule reference material NOD P1. The nodules have been classified into hydrogenous anddiagenetic on the basis of their Mn/Fe ratios. Data on elemental concentrations were used to explain the possible
differences in the trace element distribution. The minor elements were found to be enriched in the hydrogenous
nodules compared to the diagenetic one. An attempt was made to characterise the distribution of these minor
elements in the light of known geochemical evidences of Pacific Ocean nodules. The dissimilarity of the geochemistry of Ce and Mn in the Indian Ocean nodules has been discussed. The manganese nodules under investigation
exhibit a positive cerium anomaly, indicating an oxidising environment. The possible mechanism of incorporationof elements like REE into the iron oxyhydroxide phase has been discussed.

1. Introduction
Manganese nodules are small, dark, potato-shaped mixturescontaining sources of useful metals such as Mn, Fe, Cu, Ni, Co,
and Zn, making them a potential ore of these valuable elements.
These nodules are built of onion like layers of manganese andiron oxides. The mineral content varies from place to place
and the nodules in some areas contain about 30% manganese,
15% iron, 1.25% nickel, 1% copper, and 0.25% cobalt, muchhigher concentrations than are usually found in the land ores.
The chemical composition of manganese nodules are reported
to vary from ocean to ocean.1−3 The variation in the chemicalcomposition of these nodules might be due to mineralogicaleffects4, 5 as well as different metal sources.6 The nodules werediscovered by the Challenger expedition (1873 – 76) and since
1950s, mining of manganese nodules has been the subject ofstudy and experimentation. Although the nodules grow at an
estimated rate of 0.1 mm/1000 years, they are already present
in vast quantities. They are found in the areas of slow sedimentation, usually five millimetres per thousand years or less.
The North and South Pacific Oceans hold the greatest concentration of manganese nodules; in some places 90% of the surfaceof the Ocean floor is covered by nodules. Similar high coverageof ocean floor is also found in the southernmost South Atlantic.
The Indian Ocean floor is largely devoid of manganese nodules.
Because seawater is supersaturated in manganese, the direct
precipitation of the element onto an available surface is the most
likely mode of nodule formation.
The characterisation of manganese nodules may provide
important information on physical, chemical, and biologicalactivities taking place in the Ocean as a whole. The knowledge
on the elemental concentration levels is relevant in understanding the inter-elemental relationship and the distribution of
the elements in the manganese and/or iron oxyhydroxide
phases.7 This helps in differentiating the deposits into hydroge*Corresponding author. E-mail: avreddy@magnum.barc.ernet.in.
FAX: +91-22-2550 5151.

nous or diagenetic types. The diagenetic nodules occur at the
sediment-sea water interface, at a depth of about 5000 m andare formed by the upward supply of the elements from theunderlying sediment column. These types of nodules aremostly formed on siliceous zones and their Mn/Fe ratios arehigh (> 2.5). On the contrary, the hydrogenous types are theones where the metal constituents are obtained by direct
precipitation from the seawater in favourable oceanic condition. The Mn/Fe ratios of these deposits are low (1 – 1.5).
Such deposits are mostly formed on red clays where δ-MnO2 isthe major phase.
The rare earth elements (REE), La to Lu, are widely utilized
as tracers for a range of geological processes because of theirstrikingly similar electronic configurations that give rise tosubtle, predictable chemical behaviour along the series. Studies
have revealed that REE patterns in sedimentary systems areboth influenced by depositional environment and diagenetic
processes.8 The shale normalised REE pattern are useful forunderstanding the geochemical aspects of the formation ofmanganese nodules and the prevailing local oceanic physicochemical environment. Investigations on the REE behaviour
and particularly cerium anomaly give an idea about the redox
condition at the depositional environment.9 Extensive studies
on the REE content of ferromanganese nodules of Pacific Ocean
have been carried out for understanding their accretionary
processes. 9−12 Similar studies are available on the Atlantic
Ocean nodules.13, 14 A few reports are available on the characterization of the elements namely, Sc, Sb, W, Hf, and Th in the
Pacific Ocean nodules.9, 15 Studies on REE of Indian Ocean
manganese nodules are rare and are mostly concentrated on the
nodules of the Central Indian Basin.16−19 To the best of ourknowledge, such detail studies including characterization ofelements like that in Pacific Ocean nodules have not been carriedout on the Indian Ocean nodules.
The success of such studies and interpretation of the datasolely depend on accurate and precise analysis of concernedsamples by a good, matured, and reference analytical technique.
One such technique is the instrumental neutron activation© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 11/07/2005

Dutta140 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005

Sample Description. The samples, which were obtained
from National Institute of Oceanography, Goa, India, werecollected from different locations in the Indian Ocean duringcruises of the Scandi Surveyor and the 35th Sagar Kanya. The
geographical locations and water depths of these manganese
nodules are given in Table 1.
TABLE 1: Sample Locations and Descriptions
SampleCode

Latitude(S)

Longitude(E)

Water Depth
(m)

N1
N2
N3
N4
N5

9.88˚
11.05˚14.5˚
14.56˚
NA

64.25˚
63.63˚
75.73˚
75.5˚
NA

4120
3600
5175
5200
>5000

NA – not available
Note: Geographical location is not available for the nodule N5,
which is originated from Central Indian Basin.

Np /LT**
1 f + Q0(α) ε
SDCWCi (µg·g ) =
·
·
·
,
*
k0 f + Q0(α) εNp /LTSDCw−1

13

2. Experimental

Irradiations. The samples were powdered separately in an
agate mortar and were heated in an air oven at 105 ˚C for about
24 h and cooled for 8 h before sample preparation. Samples
weighing in the mass range of 75 – 100 mg were sealed in a
polypropylene tubes (2 mm i.d.). The sealed tubes containing
the samples together with the accurately weighed gold standard (5 µg), prepared from a standard gold solution, weredoubly sealed in polythene. The manganese nodule control
sample (NOD P1) was also sealed in the similar way to that ofsamples. Samples and standards were irradiated at the E8
position of APSARA reactor at BARC, Trombay, Mumbai.
The neutron flux at E8 position of APSARA reactor is ~ 1012
cm−2s−1. The duration of irradiation was varied between 5 min
to 7 h depending on the half-life of the radionuclide of interest.
Radioactive assay. After an appropriate cooling period, thesamples were mounted on standard Perspex plates and assayedfor gamma activity using a 16% HPGe detector coupled to a
PC based 4K channel analyser in an efficiency calibrated position with reproducible sample-to-detector geometry. The resolution of the detector was found to be 2.0 keV at 1332 keV. A
multi gamma ray standard of 152Eu source was used for efficiency calibration of the detector. A 5 min irradiation time anda longer cooling period of about 8–10 h were selected for thedetermination of Mn using 56Mn (half-life 2.58 h). The irradiation time in most of the cases was 7 h. The elements e.g. Na,
K, As, La, Lu, Sb, Sm, W, Yb, and Ho were determined withan experimental condition of 7 h irradiation time and 3 – 7 daysof cooling, while Ce, Nd, Co, Eu, Fe, Hf, Sc, and Tb were
determined after 12 days of cooling. Half-life of the activation
product of Mn, 56Mn is 2.58 h and the measurements were madeafter allowing it to decay. On the other hand the activation
product of Fe, 59Fe has a half-life of 44.5 days and therefore did
not contribute to dead time for the radioactive measurement andalso its γ-rays did not interfere in the spectrum. Therefore, it was
not difficult for measurement of other nuclides/elements in thepresence of high amounts of Mn and Fe. The sample to detector
distance was maintained between 8 and 15 cm depending upon
the level of radioactivity in the sample to avoid any coincidence
effects. Peak areas under the full energy peaks were evaluated
via peak fit software PHAST, developed at our institute.26 The
software has features for energy calibration and determinationof peak shape parameters. A second order polynomial inenergy was used to calibrate the full width at half maximum(FWHM) of the individual peaks and is used for deconvolutionof multiple peaks in the gamma ray spectrum.
Calculations. The concentration of an element, Ci, wascalculated using the following formula of k0-INAA method.21, 24

68

analysis (INAA). It has been extensively used for non-destructive multielement determination in various fields like geology,
environmental science, and biology. The INAA method iscapable to determine major, minor, and trace elements simultaneously. This is one of the best methods for analyzing REE as
well as other elements like Sc, Sb, Cs, W, Hf, and Th due totheir favourable nuclear properties.20 The method of INAA
does not require special sample preparation and chemical pretreatment, and experiences negligible matrix interferences due
to high penetration power of neutron, activating source, and γrays, the measured radiation. The advent of high-resolution
gamma ray spectrometer with multichannel analyzer (MCA)
using high efficiency HPGe detector has increased the potential of this technique. The use of the single comparator methodof INAA, generally known as k0-INAA method21 is versatile formultielement analysis and is easy to adopt. The most important
advantage of this method over relative method is that it uses asingle comparator instead of multielement standards. The k0
technique has been evolved over the years by De Corte et al.21, 22
to achieve better precision and accuracy of the results. It provides
an opportunity to determine all elements that are amenable to
INAA. The k0-INAA method was adapted at Radiochemistry
Division, Bhabha Atomic Research Centre, Trombay, Mumbai,
India and in use since 1995 for the analysis of varieties of samples
including manganese encrustations.23 As a part of standardization of this method in our laboratory, the k0-factors for prominent gamma lines of 38 nuclides with respect to gold single
comparator were experimentally determined using elemental
standards. A good agreement between the determined and theliterature k0,Au-factors was observed.24 The developed methodof k0-INAA was validated using several certified referencematerials.24 Kunzendorf and Gwozdz estimated W, Th, and U
contents in the US Geological Survey (USGS) nodule standards
NOD A1 and NOD P1 for the first time by INAA method.25
In this paper, we report the results of elemental concentrationsof five manganese nodule samples of Indian Ocean by k0-INAA
method. Gold was used as the single comparator. Data on
elemental concentrations were used to understand the possible
differences in the trace element distribution. An attempt has
been made to characterize the Indian Ocean nodules by correlating the distributions of Sc, Sb, W, Hf, Th, and REE, especially
the Ce anomaly in the light of known geochemical evidences of
those of the Pacific Ocean nodules. The USGS manganese
nodule reference material NOD P1 was used as a control forevaluating the accuracy of the method.

where Np is the net counts, S is the saturation factor (1-e−λti), D isthe correction factor for decay during cooling (e−λtd), C is thecorrection factor for decay during counting ((1-e−λ LT)/λ LT), ti
is the duration of irradiation, td is the cooling time, LT is the
live-time of counting, W is the mass of the sample, w is the massof the comparator, k0 is the literature recommended k0-factor,22
and Qo(α) is the ratio of cross sections and is equal to Io(α) / σ0,
where Io(α) is the infinitely dilute resonance integral corrected
for the non-ideal epithermal neutron flux distribution. The
symbol “*” refers to the corresponding parameters of thecomparator. The determined values of f and α for the E8 position of APSARA reactor are 52.2 ± 2.7 and –0.016 ± 0.004
respectively. 24 The relevant nuclear data were taken from
Reference 22.

J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005 141

Application of k0-based INAA Method

3. Results and Discussion
The determined concentrations of various elements in the
USGS NOD P1 and the nodule samples are presented in Tables
2 and 3, respectively. The measured elemental concentrationsin the tables are the mean values from four independent meaTABLE 2: Determined Elemental Concentrations (in mg
kg–1 unless % is indicated) of the USGS Manganese Nodule
Reference Material, NOD P1 by k0-INAA
Element

This work

Na %
K%
Mn %
Fe %
Sc
Co
As
Sb
Cs
Hf
W
Th
La
Ce
Nd
Sm
Eu
Tb
Ho
Yb
Lu

1.7 ± 0.1
(9.8 ± 0.4) × 10−1
29.3 ± 0.1
5.8 ± 0.1
(49.6 ± 0.3) × 10−1
(2.26 ± 0.05) × 103
37.9 ± 1.8
38.3 ± 1.6(2.14 ± 0.06) × 103
1.4 ± 0.1
31.7 ± 2.1
27.1 ± 1.7(1.03 ± 0.04) × 102
279 ± 19
(1.26 ± 0.06) × 102
27.9 ± 1.2
7.2 ± 0.4
1.4 ± 0.1
(12.3 ± 0.8) × 10−1
12.3 ± 0.4
2.0 ± 0.1

Certified, (info)
1.63
1.0
29.1
5.82
NA
2240
NA
NA
NA
NA
NA
NA
(104)
(290)
(120)
(30)
(7.5)
NA
NA
(13)
(1.8)

surements of samples with their standard deviations (±1σ). The
determined values of the elemental concentration in the reference material (Table 2) are in good agreement with those of thecertified values,27 which reflects the accuracy of the method foranalysing manganese nodules. In addition to the elements certified in the USGS Reference material NOD P1 other minor elements e.g. Sc, As, Sb, Cs, Tb, Ho, Hf, W, and Th, which are notavailable in the certificate, have been estimated.
The Mn/Fe ratio, frequently used as an indicator for hydrogenous or diagenetic nodules,7, 28 were found to be in the range of
0.9–1.5 for nodules N1–N4 and may thus be considered to behydrogenous nodules. The higher Mn/Fe ratio for the Central
Indian Basin nodule N5 (3.1) may be due to early diagenetic
influence mixed with hydrogenous contribution. From Table
3, it is seen that the classification of the nodules on the basis of
their Mn/Fe ratios is well reflected from their minor elemental
composition. Most of the minor elements, namely, Sc, Co, As,
Sb, Hf, W, Th, and REE are observed to be enriched in thehydrogenous nodules (N1–N4) compared to the diagenetic one(N5). On the other hand, the Na and K contents are mostly
unaffected by the variation in the Mn/Fe ratio whereas the Cs
content is found to be higher for the nodule whose Mn/Fe ratio
is high (3.1). These observations are in good agreement with thecorresponding elemental profiles of the Pacific Ocean nodules.9
In general the mechanism of incorporation of elements like
REE into iron hydroxide/oxyhydroxide precipitate is governedby co-precipitation. The two principal ways in which the coprecipitation takes place are adsorption and occlusion. Foreignions are brought down to the surface of the iron hydroxide
precipitate by adsorption and also by occlusion within the crystal
structure of the precipitate. All precipitates even colloidal,
gelatinous and curdy ones show crystalline structure and thesurface area plays a major role on the extent of adsorption ofions. Thus elements like REE, Th, Hf, As, Sb, and W that form
either hydroxide or oxyanions are easily brought down by iron

NA – not available; (info) – Information value

TABLE 3: Elemental Concentrations (in mg kg–1 unless % is indicated) of Manganese Nodules of Indian Ocean
Elements
Na %
K%
Mn %
Fe %
Sc
Co
As
Sb
Cs
Hf
W
Th
La
Ce
Nd
Sm
Eu
Tb
Ho
Yb
Lu
Ce*
Mn/Fe
(La/Lu)*
ΣREE
Ce/La

N1

N2

N3

N4

N5

2.44 ± 0.12
0.57 ± 0.03
15.3 ± 0.5
16.5 ± 0.7
15.7 ± 0.33220 ± 163
76 ± 818.9 ± 0.7
321 ± 7
6.52 ± 0.37
67.1 ± 4.3
42.5 ± 2.4
142.2 ± 6.4
951 ± 34
137.4 ± 7.2
28.2 ± 1.1
6.74 ± 0.56
3.24 ± 0.33
4.78 ± 0.64
19.32 ± 0.59
3.48 ± 0.13
+ 0.480.93 ± 0.05
0.611296 ± 35
6.7 ± 0.3

2.56 ± 0.13
0.58 ± 0.04
19.1 ± 0.7
21.2 ± 0.8
22.9 ± 0.85362 ± 214
110 ± 921.6 ± 0.8
379 ± 16
6.61 ± 0.35
77.8 ± 6.8
50.5 ± 3.3
257.5 ± 10.2
1862 ± 83
196.6 ± 9.8
42.8 ± 2.1
11.14 ± 0.66
4.82 ± 0.34
6.89 ± 0.91
32.43 ± 1.08
6.22 ± 0.24
+ 0.540.90 ± 0.05
0.622420 ± 84
7.2 ± 0.5

1.75 ± 0.12
0.40 ± 0.03
18.6 ± 0.6
13.9 ± 0.7
13.7 ± 0.4
2180 ± 96
77 ± 619.5 ± 0.8
216 ± 12
8.54 ± 0.46
49.2 ± 3.9
48.5 ± 2.9
204.8 ± 7.91246 ± 48
167.7 ± 7.6
42.1 ± 2.1
8.70 ± 0.83
3.31 ± 0.37
6.53 ± 0.87
24.22 ± 1.03
4.62 ± 0.13
+ 0.461.3 ± 0.1
0.661708 ± 49
6.1 ± 0.3

2.08 ± 0.14
0.50 ± 0.03
16.9 ± 0.5
11.3 ± 0.4
15.3 ± 0.82174 ± 106
61 ± 517.9 ± 0.6
267 ± 97.14 ± 0.49
46.6 ± 4.5
47.2 ± 2.9
184.4 ± 8.6
948 ± 39138.4 ± 9.7
34.4 ± 1.1
7.55 ± 0.72
3.09 ± 0.29
5.78 ± 0.64
21.41 ± 0.98
3.82 ± 0.13
+ 0.391.5 ± 0.1
0.721347 ± 41
5.1 ± 0.3

2.32 ± 0.11
0.48 ± 0.03
17.2 ± 0.4
5.5 ± 0.3
10.9 ± 0.41128 ± 49
39 ± 412.2 ± 0.6
718 ± 24
3.40 ± 0.22
42.5 ± 4.4
24.6 ± 1.4
98.6 ± 8.2
502 ± 2388.3 ± 7.2
20.3 ± 0.9
4.76 ± 0.38
2.62 ± 0.33
2.97 ± 0.32
9.85 ± 0.63
1.58 ± 0.11
+ 0.373.1 ± 0.2
0.94
731 ± 25
5.1 ± 0.3

Ce* = Magnitude of cerium anomaly, calculated using the relation log [3Ce/(2La + Nd)], as given by Elderfield and greaves.9(La/Lu)* = (Lasample / LaNASC) / (Lusample / LuNASC)

Dutta142 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005
2500

TOTAL REE (ppm)

2000

Diagenetic one

1500

1000

500
3

8

13

18

23

Fe (%)
Figure 1. Scatter plot showing variation of total REE content with Fecontent in hydrogenous nodules of Indian Ocean.
1.5log [(REE)sample / (REE)NASC ]hydroxide precipitate. Adsorption is dependent on the pH of
the zero point charge (pHzpc) of the sorbing medium. The pHzpc
of ferrihydrite is 8.0 and that for manganese oxides is 2.2. 29
Since the pH of the sea water is ~ 8.0, the anionic species inthe sea water environment are thus expected to be adsorbed on
positive surface charge of the ferric oxyhydroxide while thecationic species would adsorb on the negative surface chargeof the δ-MnO2 phase.
It may be observed from Table 3 that higher Co content wasobserved for the hydrogenous nodules originating at relatively
shallower depth, i.e. on the elevated areas (nodules N1 and N2)
compared to the hydrogenous nodules originating at greater
water depths (N3 and N4). Such a negative correlation
between Co content in nodules and water depth is consistent
with the earlier report by Cronan and Tooms.30 The Na and K
contents do not vary appreciably with the variation in theircorresponding Mn/Fe ratio and their contribution could be due
to sea salt component and terrigenous input as described by
Kunzendorf et al.31 It may be assumed that these mono-valent
species are deposited in the nodules as a separate phase and are
not taken up by either Fe or Mn oxide phases. However, it
could be observed from Table 3 that the Cs content is significantly larger in the Central Indian Basin nodule (N5) comparedto the hydrogenous nodules. Caesium, which exists as Cs+ inthe seawater environment may have been adsorbed on thenegatively charged surface of δ-MnO 2 . The W content isfound to be higher in the nodules originated at elevated areas,
i.e. nodules N1 (67.1 mg kg−1) and N2 (77.8 mg kg−1) than in
the nodules occurring at greater depth (43 – 49 mg kg−1). The
information on the distribution of W in the Indian Ocean
manganese nodules are rare compared to the Pacific Ocean
nodules.9, 31 Tungsten exists as an oxyanionic species (WO42−)
in seawater 32 and are thus adsorbed on the surface of thepositvely charged ferric oxyhydroxide phase. Kunzendorf and
Glasby suggested that tungsten is not deposited by direct
precipitation from seawater rather the oxyanionic species areadsorbed on the charged surface of the sorbing colloids.31
The distribution pattern observed for As and Sb, too, are inclose resemblance to that of the distribution of W in the Indian
Ocean nodules. This is well expected as these elements also
form oxyanionic species and are thus sorbed on the ferric
oxyhydroxide phase. The Sb concentration in Indian Ocean
nodules are found to be less compared to those of equatorial
and SW Pacific Ocean nodules.9 It should be mentioned herethat the correlation of Sb with Mn as previously reported by
Moore et al. in Pacific Ocean nodules33 could not be observed.
It is interesting to note here that the Th concentrations for thehydrogenous nodules (N1 – N4) are of nearly similar values inspite of the variation in their Fe and Mn contents. However, theTh concentration in the iron depleted nodule of the Central
Indian Basin (N5) is significantly low. This indicates theaffinity of Th towards iron oxyhydroxide phase. Thoriumhydroxide may have been co-precipitated with ferric oxyhydroxide phase. Our finding gets support from the reports of
Shaw and Francois,34 who explained the co-precipitation of Th
with Fe and Al hydroxide.
Rare Earth Elements (REE). It is evident from Figure 1
that the Fe enriched nodules of Indian Ocean are associatedwith higher sum of the rare earth elements (ΣREE) (Table 3),
which indicate that the REE are preferentially distributed in theiron oxyhydroxide phase. Significantly large ΣREE contents in
the hydrogenous nodules are attributed to slow growth rate ofthese deposits and thereby favorable for co-precipitation/adsorption from seawater and are preferably carried by the ferricphosphate phase.35 The depletion of ΣREE in the nodule N5
is thought to reflect the diagenetic influence. 28 The North
American Shales Composite (NASC) data, taken from thereference by Henderson,36 have been used for normalizing the
REE contents of nodules, i.e. [REEsample /REENASC]. It is evident

N1
N2
N3
N4
N5

1.0

0.5

0.0
La Ce

Nd

Sm Eu

Tb

Ho

Yb Lu

Figure 2. North American Shales Composite (NASC) normalised
REE patterns of manganese nodules of Indian Ocean.

from Table 3 that the ΣREE of the nodules vary inversely with
the Mn/Fe ratio. Such a phenomenon could be attributed tothe adsorption process and is in good agreement with reports
by Glasby and Thijssen.35 The nodules under investigations
exhibit a positive cerium anomaly (Figure 2), which is calculated by using the formula, log [3Ce/(2La+Nd)], where, Ce,
La, and Nd are NASC normalised values.10 The positive Ceanomaly indicates the oxidising bottom environment at thedepositional sites. In the oxidising condition, Ce3+ is oxidised
to Ce4+ and are precipitated as Ce(OH)4, preferentially on theferric hydroxide colloidal particles.9, 18 Further, a negative
correlation between Ce/La and Mn/Fe was observed in ourstudies as evident from Table 3. The highest Ce/La ratio (7.2)
is observed for the lowest Mn/Fe ratio (0.90) and our observation is consistent with the earlier observations from Pacific
Ocean nodules.9, 12, 37 This indicates the dissimilarity of thegeochemistry of Ce and Mn. The fractionation of light REE(LREE) and heavy REE (HREE) has been obtained from the(La/Lu)* ratio, i.e. [(La sample /La shale )/(Lu sample /Lu shale )]. The
magnitude of the (La/Lu)* ratio was found to be less than one(Table 3), which suggests marginal enrichment of HREE over
LREE. The enrichment of HREE might be due to small ion
size, whereby they can be incorporated more easily into iron
oxyhydroxide matrix than the LREE.

Application of k0-based INAA Method

4. Conclusions
INAA using k0-method has been successfully used to determine many elements in manganese nodules in presence of highamounts of Fe and Mn. The Fe-enriched hydrogenous nodules
were associated with higher concentration of minor elements.
The Co content was found to vary inversely with the depth ofthe occurrences of hydrogenous nodules. The nodules underinvestigations showed a positive Ce anomaly, which indicatedan oxidising bottom environment at the depositional sites. The
present work describes characterization of elements in Indian
Ocean manganese nodules. It has been observed that there aremany similarities in the elemental concentration profiles of oursamples with the nodules of Pacific Ocean. The similar observations are (i) the REE and minor elements namely Sc, Co, As,
Sb, Hf, W, and Th are enriched in hydrogenous nodules, (ii)the REE abundances increase with the increase in the Fecontent, and (iii) there is a dissimilarity of the geochemistry of
Ce and Mn which is conjectured with a negative correlation
between Ce/La and Mn/Fe.
Acknowledgments. The authors are thankful to the authorities of National Institute of Oceanography, Goa, India forproviding the nodule samples. The authors also thank theoperating staff of APSARA reactor, BARC, Mumbai, India for
irradiating the samples. The authors are grateful to Prof. G.P.
Glasby for the constructive suggestion for the interpretation ofdata. One of the authors (R.K.D) is grateful to IUC DAEF for
providing financial support.
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