1、iiiURANIUMSERIES GEOCHEMISTRYURANIUMSERIES GEOCHEMISTRY52Reviews in Mineralogy and Geochemistry52FOREWORDAnniversaries 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
2、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-speci
3、alists 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 meetin
4、g 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 proce
5、ss.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 ACKNOWLEDGMENTSWe would like to thank all the individuals and organizations who have made possible the publication of this volume including
6、 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 comprehen
7、sive 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 lEnergie Atomique(Atomic Energy Commission,Fran
8、ce),The French Agence National des Dchets 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 t
9、hank 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 U-series Geochemistry Preface ivPREFACE URANIUM DECAY SERIESURANIUM DECAY SERIESKarl K.Turekian The disco
10、very 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 Ion
11、ium)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 23
12、8U 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 Piccio
13、tto and Gilvain in 1954 using photographic emulsions,it was not until alpha spectrometry was developed in the late 1950s 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 va
14、riety 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
15、 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
16、 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 m
17、ass 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 mater
18、ials 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-l
19、ikeness 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
20、 U and Th isotopes in geological materials as well as cosmic materials with the same refinement and accommodation for small sample size.U-series Geochemistry Preface v Soon after this discovery the harnessing of the technique to the measurement of all the U isotopes and all the Th isotopes with grea
21、t 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 r
22、adiocarbon 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 bott
23、om 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
24、 for determining the uranium and thorium decay chain nuclides in Earth materials are discussed.The range of problems solvable with this approach is remarkablea 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 SCIE
25、NCEONE HUNDRED YEARS AGO:THE BIRTH OF URANIUM-SERIES SCIENCEGideon 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 Curies doctoral thesis;t
26、he 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 discove
27、ry 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
28、 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 discoverydiscovery that underlies all the geochemical applications detailed in the chapters that follow.As the end of the 19th century approac
29、hed,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 experime
30、nt,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 pheno
31、menon was U-series Geochemistry Preface viinitially 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
32、 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 compou
33、nds.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 18
34、98).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 elementspolonium and radiumwhich the Curies were successfully able to isolate from pitchblende(Curie and Curie 1898;Curie et al.1898).For radium,the presence of a new
35、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“r
36、adiation 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
37、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).Rut
38、herford named the non-penetrating form-rays,and the penetrating form-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 disco
39、vered 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
40、-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 theref
41、ore the first to report an exponential decrease in radioactivity with time,and his 1900 paper on the subject introduced the familiar equation dN/dt=?N,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
42、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 ho
43、urs(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
44、 1900)and turned out U-series Geochemistry Preface viito 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
45、 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 radioactivi
46、ty 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 descript
47、ion 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
48、).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 therefo
49、re 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):Urani
50、um 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
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