Uranium-Series Geochemistry

677 Pages·2003·20.092 MB·English

by  Bernard Bourdon, Gideon M Henderson, Craig C Lundstrom, Simon P Turner, Eds.

REVIEWS in MINERALOGY
and GEOCHEMISTRY
Volume 52 2003

URANIUM-SERIES GEOCHEMISTRY

EDITORS:
Bernard Bourdon Institut de Physique du Globe de Paris
Paris, France
Gideon M. Henderson University of Oxford
Oxford, United Kingdom
Craig C. Lundstrom University of Illinois, Urbana-Champaign
Urbana, Illinois, USA
Simon P. Turner University of Bristol
Bristol, United Kingdom

FRONT COVER: The background of the front cover image is an exert
from a paper published exactly 100 years before this volume. That paper
was the first to calculate the half life for 234Th, or U-X as this newly
discovered radioactive substance was then called. Further details of the
early history of U-series science can be found in the second preface to
this volume. Superimposed on this background is the full series of
radioactive nuclides produced by the initial decay of 238U, and finally
resulting in the formation of 206Pb. Nuclides with half lives longer than a
year are shown in red, while those with half lives between one day and
one year are shown in blue.

Series Editors: Jodi J. Rosso & Paul H. Ribbe

GEOCHEMICAL SOCIETY
MINERALOGICAL SOCIETY of AMERICA
COPYRIGHT 2003

MINERALOGICAL SOCIETY OF AMERICA

The appearance of the code at the bottom of the first page of each chapter in this volume
indicates the copyright owner’s consent that copies of the article can be made for
personal use or internal use or for the personal use or internal use of specific clients,
provided the original publication is cited. The consent is given on the condition, however,
that the copier pay the stated per-copy fee through the Copyright Clearance Center, Inc.
for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law.
This consent does not extend to other types of copying for general distribution, for
advertising or promotional purposes, for creating new collective works, or for resale. For
permission to reprint entire articles in these cases and the like, consult the Administrator
of the Mineralogical Society of America as to the royalty due to the Society.

R M
EVIEWS IN INERALOGY
G
AND EOCHEMISTRY
( Formerly: REVIEWS IN MINERALOGY )
ISSN 1529-6466

Volume 52

U-series Geochemistry

ISBN 0-939950-54-5

Additional copies of this volume as well as others in
this series may be obtained at moderate cost from:

THE MINERALOGICAL SOCIETY OF AMERICA
1015 EIGHTEENTH STREET, NW, SUITE 601
WASHINGTON, DC 20036 U.S.A.
ii
DEDICATION
Dr. William C. Luth has had a long and distinguished career in
research, education and in the government. He was a leader in
experimental petrology and in training graduate students at
Stanford University. His efforts at Sandia National Laboratory and
at the Department of Energy's headquarters resulted in the
initiation and long-term support of many of the cutting edge
research projects whose results form the foundations of these short
courses. Bill's broad interest in understanding fundamental
geochemical processes and their applications to national problems
is a continuous thread through both his university and government
career. He retired in 1996, but his efforts to foster excellent basic
research, and to promote the development of advanced analytical
capabilities gave a unique focus to the basic research portfolio in
Geosciences at the Department of Energy. He has been, and
continues to be, a friend and mentor to many of us. It is appropriate
to celebrate his career in education and government service with
this series of courses in cutting-edge geochemistry that have
particular focus on Department of Energy-related science, at a time
when he can still enjoy the recognition of his contributions.
URANIUM–SERIES GEOCHEMISTRY
52 52
Reviews in Mineralogy and Geochemistry
FOREWORD
Anniversaries always cause us to reflect upon where we have been and where we are
going. Exactly 100 years before the publication of this volume, the first paper which
calculated the half-life for the newly discovered radioactive substance U-X (now called
234Th), was published. Now, in this volume, the editors Bernard Bourdon, Gideon
Henderson, Craig Lundstrom and Simon Turner have integrated a group of contributors
who update our knowledge of U-series geochemistry, offer an opportunity for non-
specialists to understand its basic principles, and give us a view of the future of this
active field of research. It was prepared in advance of a two-day short course (April 3-4,
2003) on U-series geochemistry, jointly sponsored by GS and MSA and presented in
Paris, France prior to the joint EGS/AGU/EUG meeting in Nice.
As Series Editor, I thank Bernard, Gideon, Craig and Simon for the considerable
time and effort that they put into the preparation and organization of this volume. I also
thank the many authors who contributed to this volume for their timely thoroughness
during the preparation and review process. And, as always, I thank my infinitely patient
and supportive family, Kevin, Ethan and Natalie.
Jodi J. Rosso, Series Editor
West Richland, Washington
January 31, 2003
ACKNOWLEDGMENTS
We would like to thank all the individuals and organizations who have made
possible the publication of this volume including the Board of directors of the
Geochemical Society and the Mineralogical Society of America. Special thanks are due
to Scott Wood and Jodi Rosso for handling the production of this volume. Karl K.
Turekian is thanked for writing the preface and all the authors are thanked for their
prompt and comprehensive contributions. We would like to thank Seth Davis (GS) and
Alex Speer (MS) who have helped organize and muster support for the short course
associated with this volume (Paris, April 2003). We would like to thank the US
Department of Energy, the Commissariat à l’Energie Atomique (Atomic Energy
Commission, France), The French Agence National des Déchets Radioactifs (Radioactive
Waste National Agency) and the Thermo-Finnigan Company who have all provided
financial support for this short-course. We would also like to thank Claude Jaupart,
director of IPGP for provision of the venue. Finally, we would also like to thank all the
reviewers who significantly helped to improve the quality and inclusiveness of the
chapters in this volume.
Bernard Bourdon
Gideon Henderson
Craig Lundstrom
Simon Turner
January 2003
iii
U-series Geochemistry – Preface
PREFACE
URANIUM DECAY SERIES
Karl K. Turekian
The discovery of the 238U decay chain, of course, started with the seminal work of
Marie Curie in identifying and separating 226Ra. Through the work of the Curies and
others, all the members of the 238U decay chain were identified. An important milestone
for geochronometrists was the discovery of 230Th (called Ionium) by Bertram Boltwood,
the Yale scientist who also made the first age determinations on minerals using the U-Pb
dating method (Boltwood in 1906 established the antiquity of rocks and even identified a
mineral from Sri Lanka-then Ceylon as having an age of 2.1 billion years!)
The application of the 238U decay chain to the dating of deep sea sediments was by
Piggott and Urry in 1942 using the “Ionium” method of dating. Actually they measured
226Ra (itself through 222Rn) assuming secular equilibrium had been established between
230Th and 226Ra.
Although 230Th was measured in deep sea sediments by Picciotto and Gilvain in
1954 using photographic emulsions, it was not until alpha spectrometry was developed in
the late 1950’s that 230Th was routinely measured in marine deposits. Alpha spectrometry
and gamma spectrometry became the work horses for the study of the uranium and
thorium decay chains in a variety of Earth materials. These ranged from 222Rn and its
daughters in the atmosphere, to the uranium decay chain nuclides in the oceanic water
column, and volcanic rocks and many other systems in which either chronometry or
element partitioning, were explored.
Much of what we learned about the 238U, 235U and 232Th decay chain nuclides as
chronometers and process indicators we owe to these seminal studies based on the
measurement of radioactivity.
The discovery that mass spectrometry would soon usurp many of the tasks
performed by radioactive counting was in itself serendipitous. It came about because a
fundamental issue in cosmochemistry was at stake. Although variation in 235U/238U had
been reported for meteorites the results were easily discredited as due to analytical
difficulties. One set of results, however, was published by a credible laboratory long
involved in quality measurements of high mass isotopes such as the lead isotopes. The
purported discovery of 235U/238U variations in meteorites, if true, would have
consequences in defining the early history of the formation of the elements and the
development of inhomogeneity of uranium isotopes in the accumulation of the
protoplanetary materials of the Solar System.
Clearly the result was too important to escape the scrutiny of falsification implicit in
the way we do science. The Lunatic Asylum at Caltech under the leadership of Jerry
Wasserburg took on that task. Jerry Wasserburg and Jim Chen clearly established the
constancy and Earth-likeness of 235U/238U in the samplable universe. In the hands of
another member of the Lunatic Asylum, Larry Edwards, the methodology was
transformed into a tool for the study of the 238U decay chain in marine systems. Thus the
mass spectrometric techniques developed provided an approach to measuring the U and
Th isotopes in geological materials as well as cosmic materials with the same refinement
and accommodation for small sample size.
iv
U-series Geochemistry – Preface
Soon after this discovery the harnessing of the technique to the measurement of all
the U isotopes and all the Th isotopes with great precision immediately opened up the
entire field of uranium and thorium decay chain studies. This area of study was formerly
the poaching ground for radioactive measurements alone but now became part of the
wonderful world of mass spectrometric measurements. (The same transformation took
place for radiocarbon from the various radioactive counting schemes to accelerator mass
spectrometry.)
No Earth material was protected from this assault. The refinement of dating corals,
analyzing volcanic rocks for partitioning and chronometer studies and extensions far and
wide into ground waters and ocean bottom dwelling organisms has been the consequence
of this innovation.
Although Ra isotopes, 210Pb and 210Po remain an active pursuit of those doing
radioactive measurements, many of these nuclides have also become subject to the mass
spectrometric approach.
In this volume, for the first time, all the methods for determining the uranium and
thorium decay chain nuclides in Earth materials are discussed. The range of problems
solvable with this approach is remarkable—a fitting, tribute to the Curies and the early
workers who discovered them for us to use.
ONE HUNDRED YEARS AGO:
THE BIRTH OF URANIUM-SERIES SCIENCE
Gideon M. Henderson
One hundred years ago: the date is 1903, and it is an auspicious year in the history
of radiochemistry. 1903 witnessed the first published version of a radioactive decay
chain; the submission of Marie Curie’s doctoral thesis; the award of the Nobel prize for
physics to Becquerel and the Curies; and the recognition that radioactivity released both
heat and He, with important implications for the age of the Earth and for absolute dating.
These events were part of the rapid development of a new science that followed the
discovery of radioactivity in 1896. For those geochemists familiar with U-series
geochemistry, the early history of the field can make fascinating reading. In these early
years, armed only with simple chemistry (the mass spectrometer, for instance, was not to
be invented until 1918), the pioneers of the field were able to piece together an almost
complete picture of the three naturally occurring decay series. This preface provides a
brief introduction to this period of discovery—discovery that underlies all the
geochemical applications detailed in the chapters that follow.
As the end of the 19th century approached, several workers were investigating the
recently discovered X-rays. One of these, Henri Becquerel, discovered that
phosphorescent uranium salts released penetrating rays, distinct from X-rays, which were
capable of exposing photographic plates (Becquerel 1896b). In a key, but somewhat
fortuitous experiment, Becquerel demonstrated that the rays from the uranium salts did
not require light in order to be emitted and were therefore independent of the
phosphorescence (Becquerel 1896a). Becquerel had discovered radioactivity, although it
was two years before this name was coined (by Marie Curie) and the phenomenon was
v
U-series Geochemistry – Preface
initially termed “Becquerel” radiation, or “uranic” radiation. This discovery was pursued
by Marie Curie who checked the radioactivity of many compounds and minerals. She
demonstrated that radioactivity came particularly from uranium and thorium (Curie
1898). And she provided the first indication of its atomic, rather than molecular nature
because natural compounds emitted radioactivity proportionally to their U or Th content,
regardless of their chemical form.
One curious observation, however, was that pure U actually had a lower
radioactivity than natural U compounds. To investigate this, Curie synthesized one of
these compounds from pure reagents and found that the synthetic compound had a lower
radioactivity than the identical natural example. This led her to believe that there was an
impurity in the natural compound which was more radioactive than U (Curie 1898). Since
she had already tested all the other elements, this impurity seemed to be a new element.
In fact, it turned out to be two new elements—polonium and radium—which the Curies
were successfully able to isolate from pitchblende (Curie and Curie 1898; Curie et al.
1898). For radium, the presence of a new element was confirmed by the observation of
new spectral lines not attributable to any other element. This caused a considerable stir
and the curious new elements, together with their discoverers, achieved rapid public
fame. The Curies were duly awarded the 1903 Nobel prize in Physics for studies into
“radiation phenomena,” along with Becquerel for his discovery of “spontaneous
radioactivity.” Marie Curie would, in 1911, also be awarded the Nobel prize in chemistry
for her part in the discovery of Ra and Po.
Shortly after the discovery of these new radioactive materials it was recognized that
there were two different forms of radiation. All radiation caused ionization of air so that it
would conduct electricity, but only some radiation was capable of passing through
material, such as the shielding paper which was placed on photographic plates to prevent
them being exposed by light (Rutherford 1899). Rutherford named the non-penetrating
form α-rays, and the penetrating form β-rays.
The discovery of two new elements started a frenetic race to find more. Actinium
was soon unearthed (Debierne 1900) and many other substances were isolated from U
and Th which also seemed to be new elements. One of these was discovered somewhat
fortuitously. Several workers had noticed that the radioactivity of Th salts seemed to
vary randomly with time and they noticed that the variation correlated with drafts in the
lab, appearing to reflect a radioactive emanation which could be blown away from the
surface of the Th. This “Th-emanation” was not attracted by charge and appeared to be
a gas, 220Rn, as it turns out, although Rutherford at first speculated that it was Th vapor.
Rutherford swept some of the Th-emanation into a jar and repeatedly measured its
ability to ionize air in order to assess its radioactivity. He was therefore the first to
report an exponential decrease in radioactivity with time, and his 1900 paper on the
subject introduced the familiar equation dN/dt = (cid:237)λN, as well as the concept of half-
lives (Rutherford 1900a). His measured half-life for the Th emanation of 60 seconds
was remarkably close to our present assessment of 55.6 seconds for 220Rn.
Rutherford also noticed that the walls of the vessels in which Th emanation was
investigated became radioactive during the experiment. This “excited activity” lasted
longer than the activity of the Th emanation, but itself decayed away with a half-life of
about 11 hours (Rutherford 1900b). Unwittingly, he was working down the Th decay
chain and was measuring the decay of 212Pb, the grand-daughter of 220Rn, formed when it
decayed.
At about the same time, solid substances with strong radioactivity were separated
chemically from U and Th. That from U was named U-X (Crookes 1900) and turned out
vi
U-series Geochemistry – Preface
to be 234Th, while that from Th was named Th-X (Rutherford and Soddy 1902) and was
224Ra. Becquerel, returning to radioactivity research, noted that U, once stripped of its U-
X, had a dramatically lower radioactivity and that this radioactivity seemed to return to
the uranium if it was left for a sufficiently long time (Becquerel 1901). Rutherford and
Soddy pursued this idea using the more rapidly decaying Th-X. They separated Th-X
from Th and made a series of measurements that demonstrated an exact correspondence
between the return of the radioactivity to the Th, and the decay of radioactivity in the Th-
X. On this basis they deduced that much of the, “radioactivity of thorium is not due to
thorium itself but to the presence of a non-thorium substance in minute amount which is
being continuously produced.” And they went on to give the first description of secular
equilibrium:
“The normal or constant radioactivity possessed by thorium is an equilibrium value, where the
rate of increase of radioactivity due to the production of fresh active material is balanced by the
rate of decay of radioactivity of that already formed” (Rutherford and Soddy 1902).
In the same paper, Rutherford and Soddy also suggested that elements were undergoing
“spontaneous transformation.” The use of the word transformation smacked of alchemy
and Rutherford was loath to use it, but by then it seemed clear that elements were really
changing and that “radioactivity may therefore be considered as a manifestation of
subatomic change” (Rutherford and Soddy 1902).
The recognition of element transformation allowed the idea of a series of elements
forming sequentially from the decay of a parent element and led to the first published set
of “U-series” in 1903 (Rutherford 1903):
Uranium Thorium Radium
| | |
Uranium-X Thorium X Radium emanation
| | |
? Thorium-emanation Radium-excited
| activity I
Thorium-excited |
activity I ditto II
| |
ditto II ditto III
| |
? ?
By painstaking chemical separations, and careful study of the style and longevity of
radioactivity from the resulting separates, these series were rapidly added to and only a
year later more than 15 discrete radioactive substances were known, each with measured
half-lives, and all arranged into four decay series from U, Th, Ac, and Ra (Rutherford
1904).
Were all of these newly discovered substances also new elements? This question
would not be answered for some years but there was a flurry of other major discoveries
to keep the protagonists occupied. Pierre Curie discovered that radioactivity released
large quantities of heat (Curie and Laborde 1903) which appeared mysterious—as if the
heat was coming from nowhere. This discovery provided an extra heat source for the
Earth and reconciled the estimates of a very old Earth, based on geological estimates,
with the young age calculated by Lord Kelvin from cooling rates. The year 1903 also
witnessed the first demonstration that α-decay released He (Ramsay and Soddy 1903).
The build up of He was soon put to use to date geological materials, initially by
Rutherford in 1905 who calculated the first ever radiometric age of (cid:167)500 Myr for a
pitchblende sample, and then by Strutt who examined a wide variety of minerals (Strutt
vii
U-series Geochemistry – Preface
1905). Shortly thereafter, Boltwood recognized that the Pb content of minerals
increases with age and it became clear that Pb was the final product of radioactivity.
Boltwood was also responsible for adding another substance to the decay series through
his discovery of ionium (230Th), and therefore for linking the U and Ra decay chains
(Boltwood 1907). The discovery of 234U, initially known as UrII, followed in 1912.
Increasingly, new attempts to use basic chemistry to separate substances from
radioactive material were meeting with failure. In many cases, two substances which
were known to have different radioactive properties and molecular masses simply could
not be separated from one another and appeared chemically identical. By 1910, this
problem led Soddy to speculate that there were different forms of the same element
(Soddy 1910). By 1913 he was confident of this interpretation and coined the term
“isotope” to describe the various types of each element, recognizing that each isotope had
a distinct mass and half-life (Soddy 1913b). In the same year he wrote that “radiothorium,
ionium, thorium, U-X, and radioactinium are a group of isotopic elements, the calculated
atomic masses of which vary from 228-234” (a completely accurate statement- we now
call these isotopes 228Th, 230Th, 232Th, 234Th, 227Th respectively). Soddy received the
Nobel prize for chemistry in 1921 for his work on isotopes.
The various U series were by now all but complete. Branched decays were
understood, and a daughter of U-X was discovered—234Pa as it is now known, but
initially named “brevium” to reflect it’s short half-life (Fajans and Gohring 1913). By
1913, a published 238U decay series (Fajans 1913) was remarkably close to that in use
today, differing only in the absence of some of the branched decays after Ra-A (218Po),
and in the precise values of some of the half-lives:
UrI →α UrX →β UrX →β UrIIα→ I α→ Ra →α RaEm→α RaAα→ RaB →β
o
2
5×109yrs 24.6 days 106yrs 106yrs 2000 yrs 3.86 days 3 min 26.7 min
RaC
2
1.4 min
RaC
1
19.5 min
RaC′α→RaD→β RaE→β RaF →α Pb
10-6sec 16 yrs 5 days 136 days
Working independently of one another, Fajans and Soddy also deduced the
displacement rule (Soddy 1913a) (Fajans 1913). Based on the chemical behavior of the
isotopes in the decay chains, and on their molecular masses, they realized that each time
an element changed by emitting an α-ray, the resulting element belonged to a group in
the periodic table shifted two to the left of the initial isotope. Similarly, each time an
element changed by emitting a β-ray, the resulting element was shifted one group to the
right. This enabled the decay series to be plotted on a figure of mass against atomic
number, as shown in Figure 1, and to look even more familiar to the modern U-series
geochemist.
The fundamental work to establish the sequence of isotopes in the U and Th decay
chains was therefore almost complete by 1913, only 17 years after the first discovery of
radioactivity. It would be another 40 years before techniques for the routine measurement
of some of these isotopes were developed (as detailed in Edwards et al. 2003) and the U-
series isotopes started to see their widespread application to questions in the earth
sciences.
viii
U-series Geochemistry – Preface
Figure 1. The three decay series from uranium, thorium, and actinium as published by Soddy in 1913
(Soddy 1913b).
REFERENCES
Becquerel AH (1896a) On the invisible rays emitted by phosphorescent bodies. Comptes Rendus de
Seances de l'academie de Sciences 122:501-503
Becquerel AH (1896b) On the rays emitted by phosphoresence. Comptes Rendus de Seances de l'academie
de Sciences 122:420-421
Becquerel H (1901) Sur la radioactivitie de l'uranium. Comptes Rendus de Seances de l'academie de
Sciences 83:977-978
Boltwood BB (1907) Note on a new radioactive element. Amer J Sci 24:370-372
Crookes W (1900) Radio-activity of Uranium. Proc R Soc London 66:409-422
Curie M (1898) Rays emitted by compounds of uranium and thorium. Comptes Rendus de Seances de
l'academie de Sciences 126:1101-1103
Curie P, Curie M (1898) Sur une nouvelle substance radioactive, contenue dans la pechblende. Comptes
Rendus de Seances de l'academie de Sciences 127:175-178
Curie P, Curie M, Bemont G (1898) Sur une nouvelle substance fortement radioactive, contenue dans la
pechblende. Comptes Rendus de Seances de l'academie de Sciences 127:1215-1217
Curie P, Laborde A (1903) On the heat spontaneously released by the salts of radium. Comptes Rendus de
Seances de l'academie de Sciences 86:673
Debierne A (1900) Sur un nouvel element radio-actif: l'actinium. Comptes Rendus de Seances de
l'academie de Sciences 130:906-908
Edwards RL, Gallup CD, Cheng H (2003) Uranium-series dating of marine and lacustrine carbonates. Rev
Mineral Geochem 52:363-405
Fajans K (1913) Radioactive transformations and the periodic system of the elements. Berichte der
Dautschen Chemischen Gesellschaft 46:422-439
ix