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Isotope Geology Claude J. (cambridge)

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fractionation is due tomass alone
and isnota¡ectedby the element’s chemical characteristics.
Ionic bombardment in secondary-ionmass spectrometry (SIMS)
The solid sample (rock, mineral) containing the chemical element for analysis is cut,
polished, andput into thevacuum chamber where it isbombardedbya‘‘primary’’ beamof
ions (argon, oxygen, or cesium). This bombardment creates a very-high-temperature
plasma at about 40 000K inwhich the element is atomized and ionized.The development
ofhigh resolution secondary-ionmass spectrometers (ionmicroprobes)means in-situ iso-
topemeasurements can bemade onvery small samples and, above all, on tinygrains.This
is essential for studying, say, thefewgrainsof interstellarmaterial contained inmeteorites.
Remark
All of the big fundamental advances in isotope geology have been the result of improved
sensitivity or precision in mass spectrometry or of improved chemical separation reducing con-
tamination (chromatographic separation using highly selective resins, use of high-puritymaterials
such as teflon). These techniques have recently become automated and automation will be more
systematic in the future.
1.3 Isotopy
Assaid,eachchemicalelementisde¢nedbythenumberofprotonsZ in itsatomicstructure.
It is the numberofprotonsZ thatde¢nes the element’s position in theperiodic table. But in
11 Isotopy
eachposition there are several isotopeswhich di¡erby the numberofneutronsN they con-
tain, that is, by their mass.These isotopes are created during nuclear processes which are
collectively referred to as nucleosynthesis and which have been taking place in the stars
throughoutthehistoryoftheUniverse (seeChapter 4).
The isotopic composition of a chemical element is expressed either as a percentage or
more convenientlyasaratio.A reference isotope is chosen relative towhichthequantitiesof
other isotopes are expressed. Isotope ratios are expressed in terms of numbers of atoms
andnotofmass.Forexample, to study variations inthe isotopic compositionofthe element
strontium brought about by the radioactive decay of the isotope 87Rb, we choose the
87Sr/86Sr isotope ratio. To study the isotopic variations of lead, we consider the
206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pbratios.
1.3.1 The chart of the nuclides
The isotopic composition of all the naturally occurring chemical elements has been deter-
mined, that is, the numberof isotopes and their proportionshavebeen identi¢ed.The¢nd-
ings havebeen plotted as a (Z,N) graph, that is, the numberofprotons against the number
ofneutrons.Figure1.3, detailsofwhicharegiven intheAppendix,promptsafewremarks.
Firstofall, thestableisotopesfall intoaclearlyde¢nedzone,knownasthevalleyofstability
because it corresponds to theminimum energy levels of nuclides. Initially this energy valley
follows the diagonalZ¼N.Then, afterN¼ 20, thevalley falls away fromthe diagonalonthe
side of a surplus of neutrons. It is as if, asZ increases, an even greater numberof neutrons is
neededtopreventthe electricallychargedprotons fromrepellingeachotherandbreaking the
nucleusapart. (Thingsareactuallymore complicatedthanthis simplistic imagesuggests!)
80
60
40
20
20 40 60 80
Neutron number (N)
Pr
o
to
n
 n
u
m
b
er
 (
Z)
100 120
Z =
 N
Na
tur
al s
tab
le i
sot
op
es
Figure 1.3 The distribution of natural stable isotopes in the neutron–proton diagram. The diagram is
stippled because natural or artificial radioactive isotopes lie between the stable isotopes. After N¼ 20,
the zone of stable nuclei moves away from the diagonal for which the number of neutrons equals the
number of protons. For N> 20, the number of neutrons then exceeds the number of protons. This zone
is called the valley of stability as it corresponds to a minimum energy level of the nuclides.
12 Isotopes and radioactivity
Asecond remark relates toparity. Elements for whichZ is an even numberhave farmore
isotopes than elements for which Z is an odd number. Fluorine (Z¼ 9), sodium (Z¼ 11),
phosphorus (Z¼ 15), andscandium(Z¼ 21)have justa single isotope.
Lastly, andnot least importantly, theheaviest elementwith stable isotopes is lead.8
1.3.2 Isotopic homogenization and isotopic exchange
As the isotopes ofanygiven chemical element all have the same electron suite, theyall have
prettymuch the same chemical properties. But in all chemical, physical, or biological pro-
cesses, isotopes ofanygiven elementbehave slightlydi¡erently from eachother, thusgiving
rise to isotopic fractionation. Such fractionation is very weak and is apparent above all in
lightelements. It is also exploited in isotopegeologyas shallbe seen inChapter 7.
Initially we shall ignore such fractionation, exceptwhere allowancehas tobemade for it
as with 14C or when making measurements with a mass spectrometer where, as has been
seen, correctionmustbemade formass discrimination.Thisvirtually identicalbehaviorof
chemical isotopes entails a fundamental consequence in the tendency for isotopic homogen-
ization to occur.Where two or more geochemical objects (minerals within the same rock,
rocks in solution, etc.) are in thermodynamic equilibrium, the isotope ratios of the chemical
elements present are generally equal. If theyare unequal initially, they exchange some atoms
until theyequalize them. It is important tounderstand that isotopic homogenization occurs
through isotopic exchange without chemical homogenization. Each chemical component
retains its chemical identity,ofcourse.Thispropertyof isotopichomogenization‘‘across’’che-
micaldiversity isoneofthe fundamentalsof isotopegeochemistry.Asimplewayofobserving
this phenomenon is to put calcium carbonate powder in the presence ofa solution of carbo-
nate in water in proportions corresponding to thermodynamic equilibrium.Therefore no
chemical reaction occurs. Repeat the experiment but with radioactive 14C in solution in the
formofcarbonate. Ifafter10daysorsothesolid calciumcarbonate is isolated, itwillbefound
tohavebecome radioactive. It will have exchanged some of its carbon-14 with the carbonate
ofmass12and13whichwere inthesolution.
Exercise
A liter of water saturated in CaCO3 whose Ca
2þ content is 1 � 10�2 moles per liter is put in the
presence of 1 g of CaCO3 in solid form. The isotopic ratio of the solid CaCO3 is
40Ca/42Ca¼ 151.
The isotopic ratio of the dissolved Ca2þ has been artificially enriched in 42Ca such that
40Ca/42Ca¼ 50. What is the common isotopic composition of the calcium when isotopic
equilibrium is achieved?
Answer
40Ca/42Ca¼ 121.2.
As said, when two or more geochemical objects with di¡erent isotopic ratios are in
each other’s presence, atom exchange (which occurs in all chemical reactions, including at
8 Until recently it was believed to be bismuth (Z¼ 83), whose only isotope is 209Bi. In 2003 it was
discovered to be radioactive with a half-life of 1.9 � 1019 years!
13 Isotopy
equilibrium) tends tohomogenize thewhole in termsof isotopes.This is known as isotopic
exchange. It isakineticphenomenon,dependingthereforeonthetemperatureandphysical
state of the phases present. Simplifying, isotope exchange is fast at high temperatures and
slow at low temperatures like all chemical reactions which are accelerated by temperature
increase. In liquids andgases, di¡usion is fast and so isotope exchange is fast too. In solids,
di¡usion is slowandsoisotopeexchangeis slowtoo. Inmagmas(high-temperatureliquids),
then, both trends are compounded and isotope homogenization occurs quickly.The same
istrueofsupercritical£uids, thatis,£uidsdeepwithintheEarth’scrust.Conversely, insolids
atordinary temperatures, exchange occurs very slowlyand isotopeheterogeneities persist.
Two important consequences follow from these two properties.The ¢rst is that a magma
has the same isotope compositionas the solid source fromwhich ithas issuedby fusion, but
notthe same chemical composition.The