Geochemistry of Rare Erath Elements and Trace Metals along the Western Continental Shelf of India

241 Pages·2011·4.46 MB·English

GEOCHEMISTRY OF RARE EARTH ELEMENTS
AND TRACE METALS ALONG THE WESTERN
CONTINENTAL SHELF OF INDIA
Thesis submitted to the
COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY
in Partial Fulfilment of the
C!rllemical (@ceanngrap1nJ
Under the faculty ofM arine Sciences
SIBY VARGHESE
NATIONAL INSTITUTE OF OCEANOGRAPHY
REGIONAL CENTRE, KOCHI
KOCHI· 682014
MAY 2004
May, 2004
CERTIFICATE
This is to certify that the thesis titled "Geochemistry of Rare Earth
Elements and Trace Metals along the Western Continental Shelf of
India" is an authentic record of the research work carried out by
Siby Varghese under our supervision and guidance in the National
Institute of Oceanography, Regional Centre Cochin, in partial fulfilment
of the requirements for PhD degree of Cochin University of Science and
Technology and no part of this has been presented before for any degree
in any university.
~
Dr. N. Chandramohanakumar Dr.K.K. C.Nair
(Supervising guide) (Co-guide)
Professor, Scientist-in-Charge,
Dept. of Chemical Oceanography National Institute of Oceanography
CUSA T, Cochin-16 Regional Centre, Cochin-14
Rare earth elements (REEs) are in high demand on a global
perspective, and the main reservoir of these elements is believed to be the
ocean floor. A lot of studies on REEs are going on throughout the world for
its quantification and exploitation. Even though. some preliminary attempts
have been made in selected areas for the quantitative study of REEs in the
EEZ of western continental shelf of India, no comprehensive work has been
reported so far. In this context, this work has been initiated to study the
distribution of REEs and other major and trace elements, and its
geochemical behaviour in the EEZ of the marine environment.
REEs in the sedimentary phases are increasingly being used as
indicators of several geological and oceanographic processes; hence the
study of REEs is relevant. These elements fonn a very coherent group,
although two elements cerium and europium may develop anomalies due to
changes in their oxidation states. Most of the REEs exist in +3 oxidation
state, while Ce and Eu changes their oxidation state in certain geochemical
conditions. The distribution of uranium and thorium ant its geochemical
properties along the west coast of India also have been addressed in this
study.
In the present study, surface sediment samples were collected from the
western continental shelf of India to estimate the distribution pattern and
geochemical behaviour of REEs, major and trace elements. Bottom water
Preface
samples were also taken at each station, and analysed for temperature,
salinity and dissolved oxygen. Surface sediment samples were collected
from the Kerala coast during pre monsoon and post monsoon to examine the
seasonal variations of REEs and trace elements. Sediment cores were taken
from the Andaman Backarc basin to study the downcore variation of REEs
and other trace elements and to see the signatures of hydrothennal
influences through REE fractionation indices.
The thesis is divided into six chapters. First chapter covers a brief
introduction about rare earth elements and its chemical properties, the
occurrence and abundance of REEs, its behaviour in the oceanic system
under two different phases; such as seawater and sediments. The survey of
previous works and the aim and scope of present study is also mentioned in
this chapter.
Second chapter deals with the description of study area, sampling
locations and procedures and various analytical methods adopted for the
work. Variations of hydrographic parameters such as, temperature,
dissolved oxygen and salinity of bottom waters and textural characteristics
of sediments are also given.
Third chapter deals with the distribution and geochemical behaviour
of REEs, uranium and thorium in the sediments of western continental shelf
of India. The anomalous properties of cerium and europium, fractionation
indices and element excess studies are also described in this chapter. The
behaviour of Ce and U in the oxygen minimum zone is also addressed in
this chapter.
Preface
Fourth chapter gives the distribution of major elements such as, AI, Fe
etc. and trace elements such as Ni, eu, Co, Sc, V, Ba, Zr, Hf etc. The
relationship between organic carbon and bottom water dissolved oxygen,
enrichment factors of the elements and upper continental crust normalisation
of the elements are also given in this chapter. The statistical techniques such
as correlation matrix and factor analysis used in this study are also given in
this chapter.
Fifth chapter explains the seasonal variation of rare earth elements and
trace metals along the Kerala coast during pre monsoon and post monsoon
season. Comparison of coastal sediments samples with the offshore samples
of Arabian Sea is also given in this chapter.
Sixth chapter deals with the concentration and down core variation of
REEs and trace elements in the sediment cores taken from the Andaman
Backarc basin. The hydrothermal and detrital sources of the sediments are
explained with the use of inter-elemental ratios and discrimination plots.
Mass accumulation studies of the sediment samples are also given in this
chapter.
The summary given at the end of the thesis explains the distribution
and geochemical behaviour of rare earth elements and trace elements in the
western continental shelf of India. The seasonal behaviour of REEs in the
coastal area and the hydrothermal-detrital sources of sediments in the
Andaman basin are also given. This study contributes significantly to the
national database of REEs and trace elements on the western continental
shelf of India and Andaman Backarc basin.
CONTENTS
Chapter 1 Introduction ............................................................................ 1
Chapter 2 Materials and Methods ........................................................ 21
Chapter 3 Rare Earth Elements, Uranium and
Thorium in the Sediments of Western
Continental Shelf of India ................................................... S3
Chapter 4 Major Elements and Trace in the Sediments
~etals
of Western Continental Shelf of India .............................. 109
Chapter S Seasonal Variability of Sediment Geochemistry
Along the Kerala Coast During Pre Monsoon
and Post Monsoon ............................................................... 153
Chapter 6 Rare Earth Element Geochemistry of
Andaman Backarc Basin .................................................... 185
Summary ............................................................................. 217
List of Abbreviations .......................................................... 223
Appendix .............................................................................. 225
INTRODUCTION
1.1 Rare Earth Elements
1.2 Chemical properties
1.3 Occurrence and Abundance
1.3.1. Rare earth minerals
1.4 Lanthanides in nature
1.5 Oceanic system
1.5.1 REE supply to the Oceans
1.5.2 REE in sea water
1.5.3 REE contents of sediments and sedimentary rocks
1.6 Previous works
1.7 Aim and Scope of present study
Chapter 1
• 1.1 Rare Earth Elements
The Rare Earth Elements (REEs) from lanthanum to lutetium
(atomic numbers 57-71) are members of Group IlIA in the periodic table
and all have very similar chemical and physical properties.
Atomic Elements & Atomic Electronic Configuration
No Symbol weight
57 Lanthanum, La 138.90 [Xe] 5dT6?
58 Cerium, Ce 140.12 [Xe] 4fl5dl6s2
59 Praseodymium, Pr 140.90 .[Xe] 4f36s2
60 Neodymium, Nd 144.24 [Xe] 4f46s2
61 Prometheum, Pm* 145 [Xe] 4f56s2
62 Samarium, Srn 150.4 [Xe] 4f66s2
63 Europium, Eu 151.96 [Xe] 4f76s2
64 Gadolinium, Gd 157.25 [Xe] 4f7 5dl6s2
65 Terbium, Tb 158.92 [Xe] 4f96s2
66 Dysprosium, Dy 162.5 [Xe] 4flO6s2
67 Holmium, Ho 164.93 [Xe] 4f 116s2
68 Erbium, Er 167.26 [Xe] 4f 126s2
69 Thulium, Tm 168.93 [Xe] 4f 136s2
70 Ytterbium, Yb 173.04 [Xe] 4f 146s2
71 Lutetium, Lu 174.97 [Xe] 4f 14 5d 16s2
[Xe] = configuration of xenon: Is2 2s 2 2p 632s 3p6 3d, ]0 4s_ 2 4p6 4d,] 0 5s_ 2 5p_ 6
Table 1.1 Atomic weight and ground state electronic configurations of rare
earth elements. (* Does not exist in nature).
2
Introduction
They fonn a very coherent group, although two elements Ce and Eu
may develop anomalies due to changes in their oxidation states. Pm, a
lanthanide between Nd and Srn which can be produced by nuclear reactions,
does not exist in nature in significant concentrations.
It has been found convenient to divide the REEs into two sub
groups: those from La to Srn (ie, lower atomic numbers and masses) being
referred to as the Light Rare Earth Elements (LREEs) and those from Gd to
Lu (higher atomic numbers and masses) being referred to as the Heavy Rare
Earth Elements (HREEs). Very occasionally, the tenn Middle Rare Earth
Elements (MREEs) is loosely applied to the elements from about Pm to
about Ho.
Yttrium (Y, z =39) is also a member of group IlIA and shows similar
chemistry to that of the REE, and is sometimes included with them in
descriptive accounts. The tenn 'lanthanons' (abbreviated Ln) is applied to
the sixteen elements in the group La to Lu plus Y. The significant growth of
interest in the geochemistry of REE has come about because of the
realization that the observed degree of REE fractionation in a rock or
mineral can indicate its genesis, and accurate elemental analysis is now
possible eventhough these elements occur at very low concentration
(Henderson, 1984).
1.2 Chemical properties
REEs are difficult to separate because of their chemical similarity.
The chemical similarity is with regard to the electronic configuration of the
atoms and ions of the individual elements (Maller, 1968)
3
Chapter 1
Lanthanum has an outer electronic configuration in the ground state of 5d
I
6s2. but the next element, Ce has an electron in the 4f sub-shell (Table 1.1).
The following elements have the electrons entering the 4f sub-shell, until at
ytterbium the 4f sub-shell is filled. The 4f electrons are well shielded by the
eight electrons in the 5s2 and 5p6 sub-shells, so that they are not significantly
involved in chemical reactions. Hence, any difference in the number of
electrons in the 4f sub-shell does not lead to much difference in chemical
behaviour, nor to significant ligand field affects. The REEs, therefore, tend
to occur in any natural forms. as a group rather than singly or as a
combination of a few of their number (Henderson, 1984).
The REEs occupy a wide variety of co-ordination polyhedra in
minerals, from six fold to twelve fold or even higher co-ordinations. The
smaller REE ions can occupy six fold (CN = 6) co-ordination sites but do so
only rarely in minerals. A correlation between co-ordination and ionic
radius is observed, ie, the larger ions will tend to occupy larger sites and
vice versa. Most of the REEs show a constant valency of three in their
chemistry and geochemistry, while Ce and Eu exist in different oxidation
states.
Lanthanide Contraction: The REEs exhibit a gradual and steady decrease in
their atomic volumes with increase in atomic number as a result of imperfect
shielding of one electron by another in the same sub-shell, so that the
effective nuclear charge acting on each 4f electron increases with increasing
atomic number, thereby leading to a reduction in the size of the 4fsub-shell.
This reduction is referred to as the lanthanide contraction and is reflected by
a steady decrease in ionic radius of the REE with increase in atomic number.
4