Rare Earth Element Geochemistry

511 Pages·1984·8.177 MB·English

by  P. Henderson (Eds.)

Developments
in
Geochemistry
R A RE E A R TH
E L E M E NT
G E O C H E M I S T RY
edited by
P. HENDERSON
Department of Mineralogy, British Museum (Natural History), London, U.K.
ELSEVIER
Amsterdam - Oxford - New York - Tokyo 1984
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PREFACE
Nearly two centuries ago, in 1787, Carl Alex Arrhenius collected an
unusual black mineral from a feldspar quarry at Ytterby, near Stockholm.
From this mineral, later to be named gadolinite, Gadolin in 1794 extracted
the earth "yttria" — a mixture of several rare earth oxides. Thus began
the discovery and separation of the rare earths, but it was not until 1907
that Urbain completed the separation of all the naturally-occurring rare
earths by extracting lutetium. The problem facing the early chemists was the
extreme chemical coherence of the rare earth element group, which makes
separation of the individual members so difficult. This coherence is shown in
their geochemical behaviour but sufficiently significant fractionations
do occur, which have proved to be of great importance in the interpretation
of petrogenetic processes.
This book reflects the remarkable developments that have come about
in our knowledge of the chemistry and geochemistry of the rare earth
elements since the beginning of the century. The geochemical advances
stem from the introduction of new analytical techniques, together with the
recognition that rare earth fractionation occurs naturally in various ways
and is capable of interpretation. These advances were aided by improvements
in data presentation, especially that of chondrite-normalization, which
allows a clear visual appreciation of the type and degree of fractionation.
Much of the geochemical work has been in the field of igneous rock pedo-
genesis; this book, therefore, also has that emphasis. More information on
metamorphic rocks and processes would have been welcomed but this
proved to be unforthcoming. For the future, it seems clear that the applica-
tion of rare earth element geochemistry to the interpretation of the processes
of metasomatism, ore formation, rock alteration, and mineral authigenesis
in marine and fresh waters is likely to yield beneficial results.
The following chapters have been written by recognized authorities in
their chosen field; they reflect the current state of knowledge in an area of
science undergoing rapid development. They help to show the value of the
rare earths in geochemistry but illustrate also how interpretations obtained
from rare earth element data often need to be supported by other geo-
chemical or geological evidence. The book is addressed to all those who
VI
study geochemistry and petrology, whether they be undergraduates,
lecturers, or researchers. Some of the chapters emphasizing general principles
(e.g., Chapter 4: "Petrogenetic modelling — use of rare earth elements") will
be of especial use to the student while others (e.g., Chapter 2: "Mineralogy
of the rare earth elements") will be of most value to those needing a source
of reference. In all, the book attempts to provide a much-needed synthesis
of a diverse geochemical field at a time of its continuing growth.
I wish to thank the many people who acted as reviewers for the following
chapters or who made many useful suggestions in discussion.
PAUL HENDERSON
October, 1982.
LIST OF CONTRIBUTORS
W.V. BOYNTON Department of Planetary Sciences and Lunar and Planetary
Laboratory, University of Arizona, Tucson, AZ 85721,
U.S.A.
A.M. CLARK Department of Mineralogy, British Museum (Natural History),
London SW7 5BD, UK
R.L. CULLERS Department of Geology, Kansas State University, Manhattan,
KS 66506, U.S.A.
A.J. FLEET Exploration and Production Division, BP Research Centre,
Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN,
UK
F.A. FREY Department of Earth and Planetary Sciences, Massachusetts
Institute of Technology, Cambridge, MA 02139, U.S.A.
J.L. GRAF Department of Geology, Kansas State University, Manhattan,
KS 66506, U.S.A.
L.A. HASKIN Department of Earth and Planetary Sciences and McDonnell
Center for the Space Sciences, Washington University, St.
Louis, MO 63130, U.S.A.
C.J. HAWKESWORTH Department of Earth Sciences, Open University, Walton Hall,
Milton Keynes, MK7 6 A A, UK
P. HENDERSON Department of Mineralogy, British Museum (Natural History),
London SW7 5BD, U.K.
D.E. HIGHLY Minerals Strategy and Economics Research Unit, Institute
of Geological Sciences, London SW7 2DE, U.K.
S.E. HUMPHRIS Sea Education Association and Woods Hole Oceanographic
Institution, Woods Hole, MA 02543, U.S.A.
C.R. NEARY Minerals Strategy and Economics Research Unit, Institute
of Geological Sciences, London SW7 2DE, U.K.
R.J. PANKHURST British Antarctic Survey, Natural Environment Research
Council, c/o Institute of Geological Sciences, 64-78 Gray's
Inn Road, London WC1X8NG, U.K.
A.D. SAUNDERS Department of Geology, Bedford College, London NW1 4NS,
U.K.
P.W.C. van CALSTEREN Department of Earth Sciences, Open University, Walton Hall,
Milton Keynes, MK7 6AA, U.K.
SYMBOLS
Most of the symbols, abbreviations and acronyms used in this book are
standard ones, and many are defined within the body of the text. The more
commonly-used, and less usual, symbols are listed below.
D Nernst distribution coefficient
distribution coefficient (sol id/liquid) summed for all minerals in a rock
oxygen fugacity
equilibrium constant
R gas constant
half-life
a year
t tonne
cn chondrite normalized
1 liquid
s solid
HREE heavy rare earth elements, Gd—Lu
LREE light rare earth elements, La—Eu
M a chemical element
RE rare earth(s)
REE rare earth element(s)
REO rare earth oxide (no specific stoichiometry is implied)
EPR East Pacific Rise
10R Indian Ocean Ridge
MAR Mid-Atlantic Ridge
MORB mid-oceanic ridge basalt(s)
The rare earth elements, in alphabetical order, are:
Ce cerium Nd neodymium
Dy dysprosium Pm promethium
Er erbium Pr praseodymium
Eu europium Sm samarium
Gd gadolinium Tb terbium
Ho holmium Tm thulium
La lanthanum Yb ytterbium
Lu lutetium
Chapter 1
GENERAL GEOCHEMICAL PROPERTIES AND ABUNDANCES OF THE
RARE EARTH ELEMENTS
PAUL HENDERSON
1.1. Introduction
The rare earth elements, lanthanum to lutetium (atomic numbers 57—71),
are members of Group IIIA in the periodic table (Fig. 1.1) and all have
very similar chemical and physical properties. This uniformity arises from
the nature of their electronic configurations, leading to a particularly stable
3+ oxidation state and a small but steady decrease in ionic radius with
increase in atomic number for a given co-ordination number. Despite the
similarity in their chemical behaviour, these elements can be partially frac-
tionated, one from the other, by several petrological and mineralogical
processes. The wide variety of types and sizes of the cation co-ordination
polyhedra in rock-forming minerals provides the means for this chemical
fractionation: it is this phenomenon which has important consequences in
geochemistry.
The significant growth of interest in the geochemistry of the rare earth
elements (REE) has come about because of the realization that the observed
(measured) degree of REE fractionation in a rock or mineral can be a pointer
Gr1oup
VIIIB
2
H HA MB IVB VB VIB VIIB He
3 5 6 7 8 9 10
1Li Be B c N o F Ne
Na 12M g GGrroouupp 13A l 14S i 15 P 16 s 17C I 18A r
IIIIIIAA IIVVAA VVAA VVIIAA VVIIIIAA >> VVIIIIII ,, IIRR IIIIBB
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
K Ca Sc Ti 4V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
37 38 39 40 42 43 44 45 46 47 48 49 50 51 52 53 54
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
5C5 s 56B a 57L a 72H f 73T a 74w 75R e 76O s 77 lr 78P t 79A u 80H g 81T I 82P b 83B i 84P o 85A t 86R n
8877 8888 8899
FFrr RRaa AAcc
5588 5599 6600 6611 6622 6633 6644 6655 6666 6677 6688 6699 7700
CCee PPrr NNdd PPmm SSmm EEuu GGdd TTbb DDyy HHoo EErr TTmm YYbb LLuu
90 91 92 93 94 95 96 97 98 99 100 101 102 03
Th Pa u Np Pu Am Cm Bk Cf Es Fm Md No Lw
Fig. 1.1. The periodic table. The rare earth elements are in bolder type.
2
to its genesis, and also because accurate quantitative analysis for the REE,
both as a group and individually, is now possible on a routine basis even when
the elements occur at very low concentrations (Chapter 13). The applica-
tion of REE abundances to petrogenetic problems has centred on the
evolution of igneous rocks where such processes as partial melting of crustal
or mantle materials, fractional crystallization, and/or mixing of magmas are
involved (Chapters 5 to 8). In these studies the matching of observed REE
abundance with those provided by the theoretical modelling of petrogenetic
processes (Chapter 4) has helped considerably to restrict the number of
possible hypotheses on the genesis of a rock or mineral suite.
Yttrium (Y, atomic number 39) is also a member of Group IIIA and
shows a similar chemistry to that of the REE, and is sometimes included
with them in descriptive accounts. The term "lanthanons" (abbreviated Ln)
is applied to the sixteen elements in the group La to Lu plus Y. The lightest
element in Group IIIA, scandium, shows a sufficiently distinct chemistry,
owing to the relatively small radius of its 3+ ion, to warrant separate de-
scription. The term lanthanides is sometimes used as a name for the fourteen
elements following lanthanum in the periodic table (i.e. Ce to Lu). This book
is concerned with the fifteen elements La to Lu where, to conform with past
and current geochemical usage, they are referred to as rare earth elements^.
Chapters 2 and 12 also include Y in their discussion.
It has been found convenient to divide the REE into two sub-froups:
those from La to Sm (i.e. lower atomic numbers and masses) being referred
to as the light rare earth elements (abbreviated LREE) and those from Gd
to Lu (higher atomic numbers and masses) being referred to as the heavy rare
earth elements (abbreviated HREE). Very occasionally the term middle
rare earth elements (abbreviated MREE) is loosely applied to the elements
from about Pm to about Ho. Atomic weights are given in Table 1.1.
The electronic configurations of the REE are given in Table 1.1 for the
ground state and for three different oxidation states. Llan2thanum has an
outer electronic configuration in the ground state of bd&s, but the next
element, cerium, has an electron in the 4f sub-shell (see Table 1.1). The
following elements have the electrons entering the 4f sub-shell, until at
ytterbium the 4f sub-she2ll is fille6d. The 4f electrons are well shielded by the
eight electrons in the 5s and 5p sub-shells, so that they are not significantly
involved in chemical interactions. Hence, any difference in the number of
electrons in the Af sub-shell does not lead to much difference in chemical
behaviour, nor to significant ligand field affects. The REE, therefore, tend to
occur in any natural occurrence as a group rather than singly or as a combina-
tion of a few of their number. They are lithophile, in that they concentrate
^In 1968 the International Union for Pure and Applied Chemistry recommended that
"rare earth elements" should refer to the elements scandium, yttrium, lanthanum and the
lanthanides, but this usage has not been generally adopted in the geochemical literature
and is not followed here.
3
TABLE 1.1
Atomic weights (1973) and ground state electronic configurations
Atomic Atomic Symbol Configuration
No. weight
0 + 1" + 2 + 3
2 2 1
? 1, 2 l l 1
57 138.9055 La [Xe^d'es [Xe]5d [Xe]5d [Xe]4/°
2 r 31 3
58 140.12 Ce [Xe]4/ 5d 6s [Xe)4f5d6s [Xe]4r [Xe]4r
2 1
59 140.9077 Pr [Xe]4r6s [Xe]4/ 6s [Xe]4/ [Xe]4r
5 2 ? 51 5 4
60 144.24 Nd [Xe]4f 6s [Xe]4f 6s [Xe]4/« [Xe]4r
f 62 6 1 5
61 (145) Pm [Xe]4f 6s [Xe]4/ 6s [Xe]4/ [Xe]4/
l
62 150.4 Sm [Xe]4/ 6s2 [Xe]4f6s [Xe]4f« [Xe]4f
r 71 2 P 71 1 7 !
63 151.96 Eu [Xe]4r6s [Xe]4f6s [Xe]4r [Xe]4r
2 f 91 8
64 157.25 Gd [Xe]4/ 5d 6s [Xe]4/ 5d 6s [Xe]4/- 5d [Xe]4r
r l 20 f , 01 10
65 158.9254 Tb [Xe]4f6s [Xe]4/ 6s [Xe]4r [Xe]4/
? 12, r l 1l l 10
66 162.50 Dy [Xe]4/ 6s [Xe]4/ 6s [Xe]4/ [Xe]4f
1 22 ? 112 2 1
67 164.9304 Ho [Xe]4/ 6s [Xe]4/ 6s [Xe]4r [Xe]4/
3 2 P 113 3 2
68 167.26 Er [Xe]4/ 6s [Xe]4/ 6s [Xe]4r [Xe]4r
? 124 P , 14 4 13
69 168.9342 Tm [Xe]4r 6s [Xe]4/ 6s [Xe]4r [Xe]4r
I 41 2 1 42 f l 41 14
70 173.04 Yb [Xe]4/ 6s [Xe]4/ 6s [Xe]4r [Xe]4/
71 174.97 Lu [Xe]4/ 5d 6s [Xe]4/* 6s [Xe]4/ 6s [Xe]4f
2 2 6 2 6 1 02 6 , 02 6
[Xe] = configuration of xenon: l5 2s 2p 3s 3p 3d 4s 4p 4d 5s 5p .
predominantly in the silicate rather than the metal or sulphide phases when
these coexist. They also tend to be "dispersed" elements since they are
present in trace quantities in many minerals, only rarely occurring at high
concentrations (see Chapter 2).
This chapter reviews some of the fundamental aspects of REE geochemistry
and gives data on abundances in the solar system, the bulk Earth and the
Earth's crust. It describes the state of knowledge on the partitioning of the
REE, especially in igneous rock systems, and cites reference works concerned
with the REE.
1.2, Abundances in the solar system, Sun and Earth
Estimates of the composition of the solar system based on the concen-
trations of elements in carbonaceous chondrites and in young stars have
provided data on the relative abundances of the REE. Part of a recent
compilation by Cameron (1973) is given in T6able 1.2, together with an
estimate of the relative abundances (to Si = 10 atoms) in the solar atmo-
sphere determined from spectral analysis (Ross and Aller, 1976). The solar-
system abundances are plotted in Fig. 1.2 where the rhythmic alternation
in abundance between elements of even and odd atomic number can be
seen. This alternation arises from variations in the binding energy, and hence
4
s
m
o
at
60
1
=
Si
m/
e
st
y
S
ar
ol
S
6
Fig. 1.2. Rare earth element abundances (log scale), in the solar system, relative to 10
atoms of silicon, plotted against atomic number. Data from Cameron (1973).
stability, of a nucleus being dependent on whether the neutron number (N)
and the proton number (Z) are odd or even. Those nuclei with both N and
Z even are the stablest while those with both N and Z odd are the least stable.
Table 1.2 shows that the low relative abundance values of the REE follow
the general trend of decreasing abundance with increasing atomic number.
Perhaps not surprisingly it is more difficult to determine element concen-
trations in the bulk Earth than in the Sun's atmosphere. In the lack of direct
evidence, it is necessary to make assumptions as to the nature of proportions
of the different components that aggregated to form the bulk Earth, or to
make assumptions about the available materials which could be representative
of the interior parts of our planet. Ganapathy and Anders (1974) in their
attempt to estimate the composition of the bulk Earth, used the former
approach with the assumption that the inner planets were produced by the
same process that gave chondritic meteorites, and also used theoretical
condensation sequences of nebular gases. They concluded that the early
condensate material from the solar nebula was the sole contributor of REE
to the bulk Earth. Smith (1977), using the latter approach, summed feasible
contributions from the hydrosphere, atmosphere, crust, mantle and core to
obtain estimates for most elements including the REE (Table 1.3). For the
REE, both estimates are subject to large error, because of problems relating
to element condensation sequences, and the existence of varied REE concen-
trations in chondritic meteorites as well as in material that is likely to be
representative of early condensates (see discussion by Boynton, Chapter 3).
There is also uncertainty of the REE concentrations in the major shells that
constitute the Earth. Hence, the values in Table 1.3 should be treated
with caution.
There have been several attempts to establish the composition of the
Earth's crust but most of these have not included the REE. One important
exception is in the work of Taylor (1964) who used the very different abun-
dances of the REE in granitic and basaltic rocks as a basis for estimating
the composition of the continental crust. He showed that a mix of 1:1 mafic