学习的古诗Chapter2
Isotope Fractionation Process of Selected Elements
The foundations of stable isotope geochemistry were laid in1947by Urey’s classic paper on the thermodynamic properties of isotopic substances and by Nier’s de-velopment of the ratio mass spectrometer.Before discussing details of the naturally occurring variations in stable isotope ratios,it is uful to describe some generalities that are pertinent to thefield of non-radiogenic isotope geochemistry as a whole. 1.Isotope fractionation is pronounced when the mass differences between the iso-
topes of a specific element are large relative to the mass of the element.There-fore,isotope fractionations are especially large for the light elements(up to a mass number of about40).Recent developments in analytical techniques have opened the possibility to detect small variations in elements with much higher mass numbers.The heaviest element for which natural variations have been re-ported is thallium with isotopes of mass203and205(Rehk¨a mper and Halli-day1999).
2.All elements that form solid,liquid,and gaous compounds stable over a wide
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国际医生节temperature range are likely to have variations in isotopic composition.Gener-ally,the heavy isotope is concentrated in the solid pha in which it is more tightly bound.Heavier isotopes tend to concentrate in molecules in which they are prent in the highest oxidation state.
3.Mass balance effects can cau isotope fractionations becau modal proportions
of substances can change during a chemical reaction.They are especially impor-tant for elements in situations where the coexist in molecules of reduced and oxidized compounds.Conrvation of mass in an n component system can be described by
δ(system)=∑x iδi(2.1) where“x i”is the mole fraction of the element in question for each of n phas within the system.
4.Isotopic variations in most biological systems are mostly caud by kinetic ef-
fects.During biological photosynthesis,bacterial process)the lighter isotope is very often enriched in the reaction product relative to the sub-strate.Most of the fractionations in biological reactions generally take place J.Hoefs,Stable Isotope Geochemistry,35©Springer-Verlag Berlin Heidelberg2009
362Isotope Fractionation Process of Selected Elements during the so-called rate determining step,which is the slowest step.It commonly involves a large rervoir,where the material actually ud is small compared to the size of the rervoir.
2.1Hydrogen
Until1931it was assumed that hydrogen consisted of only one isotope.Urey et al.(1932)detected the prence of a cond stable isotope,which was called deuterium.(In addition to the two stable isotopes there is a third naturally oc-curing but radioactive isotope,3H,tritium,with a half-life of approximately12.5 years).Rosman and Taylor(1998)gave the following average abundances of the stable hydrogen isotopes:
1H:99.9885%
2D:0.0115%
The isotope geochemistry of hydrogen is particularly interesting,for two reasons: 1.Hydrogen is omniprent in terrestrial environments occurring in different oxi-
dation states in the forms of H2O,H3O+,OH−,H2and CH4,even at great depths within the Earth.Ther
efore,hydrogen is envisaged to play a major role,directly or indirectly,in a wide variety of naturally occurring geological process.数学电子小报
2.Hydrogen has by far the largest mass difference relative to the mass of the ele-
ment between its two stable isotopes.Conquently hydrogen exhibits the largest variations in stable isotope ratios of all elements.
The ranges of hydrogen isotope compositions of some geologically important rer-voirs are given in Fig.2.1.It is noteworthy that all rocks on Earth have somewhat similar hydrogen isotope compositions,which is a characteristic feature of hydro-gen,but not of the other elements.The reason for this overlap in isotope composi-tion for rocks is likely due to the enormous amounts of water that have been cycled through the outer shell of the Earth.
2.1.1Preparation Techniques and Mass Spectrometric
Measurements
Determination of the D/H ratio of water is performed on H2-gas.There are two different preparation techniques:(1)equilibration of milliliter-sized samples with gaous hydrogen gas,followed by mass-s
pectrometric measurement and back cal-culation of the D/H of the equilibrated H2(Horita1988).Due to the very large frac-tionation factor(0.2625at25◦C)the measured H2is very much depleted in D,which complicates the mass-spectrometric measurement.(2)water is converted to hydro-gen by passage over hot metals(uranium:Bigelein et al.1952;Friedman1953;
2.1Hydrogen37
Fig.2.1δD ranges of some geologically important rervoirs
Godfrey1962,zinc:Coleman et al.1982,chromium:Gehre et al.1996).This is still the classic method and commonly ud.
A difficulty in measuring D/H isotope ratios is that,along with the H2+and HD+formation in the ion source,H3+is produced as a by-product of ion-molecule collisions.Therefore,a H3+correction has to be made.The amount of H3+formed is directly proportional to the number of H2molecules and H+ions.Generally the H3+current measured for hydrogen from ocean water is on the order of16%of the total mass3.The relevant procedures for correction have been evaluated by Brand (2002).联欢晚会现场直播
Analytical uncertainty for hydrogen isotope measurements is usually in the range ±0.5to±5‰depending on different sample materials,preparation techniques and laboratories.
Burgoyne and Hayes(1998)and Sessions et al.(1999)introduced the continu-ouslyflow technique for the D/H measurement of individual organic compounds. The preci measurement of D/H ratios in a He carrier pos a number of analyt-ical problems,related to the tailing from the abundant4He+onto the minor HD+ peak as well as on reactions occurring in the ion source that produce H3+.However, the problems have been overcome and preci hydrogen isotope measurements of individual organic compounds are possible.
2.1.2Standards
There is a range of standards for hydrogen isotopes.The primary reference standard, the zero point
of theδ-scale,is V-SMOW,which is virtually identical in isotopic composition with the earlier defined SMOW,being a hypothetical water sample orginally defined by Craig(1961b).
382Isotope Fractionation Process of Selected Elements Table2.1Hydrogen isotope standards
Standards Descriptionδ-value
V-SMOW Vienna Standard Mean0
Ocean Water
GISP Greenland Ice Sheet
Precipitation−189.9
V-SLAP Vienna Standard Light
停车管理员Antarctic Precipitation−428
NBS-30Biotite−65
V-SMOW has a D/H ratio that is higher than most natural samples on Earth, thusδD-values in the literature are generally negative.The other standards,listed in Table2.1,are generally ud to verify the accuracy of sample preparation and mass spectrometry.
2.1.3Fractionation Process
治男人喝酒最狠的方法The most effective process in the generation of hydrogen isotope variations in the terrestrial environment are pha transitions of water between vapor,liquid,and ice through evaporation/precipitation and/or boiling/condensation in the atmosphere,at the Earth’s surface,and in the upper part of the crust.Differences in H-isotopic com-position ari due to vapor pressure differences of water and,to a smaller degree,to differences in freezing points.Becau the vapor pressure of HDO is slightly lower than that of H2O,the concentration of D is lower in the vapor than in the liquid pha.In a simple,but elegant experiment Ingraham and Criss(1998)have mon-itored the effect of vapor pressure on the rate of isotope exchange between water and vapor,which is shown in Fig.2.2.Two beakers with isotopically differing wa-ters were juxtapod in a aled box to monitor the exchange process at different temperatures(in this ca21and52◦C).As shown in Fig.2.12in the52◦C ex-periment the isotopic composition of the water changes rapidly and nearly reaches equilibrium in only27days.
Horita and Wesolowski(1994)have summarized experimental results for the hy-drogen isotope fractionation between liquid water and water vapor in the tempera-ture range0–350◦C(e Fig.2.3).Hydrogen isotope fractionations decrea rapidly with increasing temperatures and become zero at220–230◦C.Above the crossover temperature,water vapor is more enriched in deuterium than liquid water.Fraction-ations again approach zero at the critical temperature of water(Fig.2.3).
From experiments,Lehmann and Siegenthaler(1991)determined the equilib-rium H-isotope fractionation between ice and water to be+21.2‰.Under natural conditions,however,ice will not necessarily be formed in isotopic equilibrium with the bulk water,depending mainly on the freezing rate.
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2.1Hydrogen39
Fig.2.2δD values versus time for two beakers that have equal surface areas and equal volumes undergoing isotopic exchange in aled systems.In both experiments at21and52◦C isotope ratios progress toward an average value of−56‰via exchange with ambient vapour.Solid curves are calculated,points are experimental data(after Criss,1999)
Fig.2.3Experimentally determined fractionation factors between liquid water and water vapour from1to350◦C(after Horita and Wesolowski,1994)
