Stable isotope geochemistry - IntroductionStudying abundance, ratio and distribution of isotopes in geophases –

object and mission of geochemistry. Development in modern physics, nuclear physics, discovery of deuterium (H.

Urey, 1931), and development in analytical chemistry.

Isotopes: same chemical properties, but different mass (Δm >5% (same atomic number - Z, different atomic mass - A).D. Mendeleev (1869) observed chemical anomaly based on atomic mass at couples of Ar-K, Co-Ni and Te-I. Discovery of isotopes (Soddy, 1911 -radioactive; Thompson, 1913 - stable - 22Ne, but he did not believe it; Aston, 1919 – proved it by mass spectrograph, and other 212 stabil isotopes were discovered) solved Mendeleev’s problem.

Ratio of stable isotopes of elements > Sc:1/ constant (or mass difference cannot be measured yet), the increase the atomic number, the decrease the mass differences (detection of decreasing Δm/m causes more complications) + generally, ionic bondand one oxidation state;2/ large differences in mass (e.g., Pb) radioactive decay: 232Th 208Pb, 238U 206Pb, 235U 207Pb (radiogenic stable isotopes).

Ratio of stable isotopes of elements < Sc (light elements):abundances measurable (by MS at least 0.0000X); constant values stable isotopes.

Stable isotopes: there is no radioactive decay (or T1/2 > 15 billion years);if even number of proton and neutron nucleus is more stable (so higher abundances).

Generally, 3 stable isotopes in light elements; but Sn: 10, radioactive elements (21): 0)Increasing role of stable isotopes (of light elements) in geochemical

research (mantle, hydrogeochemistry, environmental geochemistry, cosmo(geo)chemistry).

Traditional stable isotopes: H (D/H [2H/1H], C (13C/12C), N (15N/14N), O (18O/16O), S (34S/32S):

- low atomic mass; relatively high mass differences (Δm/m) (advantage in analysis:look mass of D and H)

- abundances of heavier stable isotopes relatively high, similar ratios in comparison to lighter stable isotopes, lighter isotopes have generally higher abundances than heavier ones (relative difference in mass: 0,1-5 %, except for H: D (~100-200 ppm inaccuracy during analysis),

- covalent bond (mass-depending), diverse oxidations states (C, N, S), in several minerals(compounds) (O), natural solid and fluid phases and materials.

Today: Li - (7Li/6Li), B - (11B/10B), Ne - (22Ne/20Ne), Mg - (26Mg/24Mg), Si - (30Si/28Si), Cl - (37Cl/35Cl), Ar - (40Ar/36Ar), Ca - (44Ca/40Ca), Fe - (56Fe/54Fe), Se - (Se76/Se82),

The abundances of the isotopes present in solar system matter are plotted as a function of mass number A (the solar system abundances for the heavy elements are those compiled by Palme and Jones (see Chapter 1.03 in Treatise on Geochemistry, 2003). Truran Heger, 2003

1930-1940: A. Nier: developed a dual convertor gas ion source mass spectrometer for measure stable isotopes1947: H. Urey: established theoretical bases and geological application of the stable isotope geochemistry (The Thermodynamics Properties of Isotopic Substances); study of natural materials, experiments, T-dependent isotope fractionation

Change of percentage of stable isotopes (namely, their relative amounts) called as isotope fractionation: common, natural physical and chemical (diffusion, precipitation etc.) (and biological), but notnuclear process;

Isotope fractionation is detectable in light elements (<Sc): traditional stable isotopes and others (e.g. 6Li-7Li, 10B-11B, 28Si-30Si, 24Mg-26Mg, 40Ca-48Ca), and some heave elements (>Sc) (e.g. 54Fe-56Fe, 76Se-82Se); process of isotope fractionation is the same, but analytical difficulties (bond types, low variability in coordination, oxidation state, low relative mass difference slight fractionation

N (neutron)

Z (proton)

Stable isotopesCosmogen isotopesNatural radioactiveisotopes

Plot of Z vs. N for nuclides up to tin (Z=50) showing the "stable" valley of the nuclides. The Z : N ratio is 1 for the light nuclides and increases towards 1.5 for the heavier nuclides. Increases or decreases in N for given element produces increasingly unstable isotopes (decreasing T½).

CaAr

TiCr

Fe

As Se

Faure, 1998

Nuklid táblázat könnyű elemekre

Könnyű elemekstabil izotópjai

Faure, 1998

Henderson, 1982

Abundances of stable isotopes in additional light elements

35Cl 75.7737Cl 24.53

Szelektált izotóp mennyiségek

Brownlow, 1989

Isotope fractionation:

Distribution of stable isotopes of an element among/between phases, minerals, natural materials, etc. driven:

1) exchange reaction – redistribution of the stable isotopes,2) kinetic processes, reaction rates control isotopic rations in

reagents and products (one-way and not completed reactions), 3) physicochemical processes (vaporization/condensation,

melting/crystallization, solution/precipitation, adsorption/desorption, diffusion, etc.).

These are:1) equilibrium, 2) kinetic, and 3) non mass-depending processes

Priciple of fractionation: modes of movements of molecules and atoms depending on their energy level/state mass-dependent process

Izotope fractionation

1/ Equilibrium fractionation: in equilibrium reaction molecules (in gas, liquid, solid phases) and atoms (in solid phases) move: - molecules, atoms translational, rotational andvibrational move in (E=1/2hν, h – Plank constant, ν – vibrational frequency);

- motion energy is mass-depending a system intends to reach the lowest energy configuration this is an excellent base for isotope fractionation:

molecules and atoms, consisting of heavier isotopes, have lower energy states than thosebuilt up by lighter isotopes heavier isotopesprefer more stable states and bonds

- solid>liquid>gas phases, - covalent bond>ionic bond, coordination

phases/compounds containing heavierisotopes more stable

The three modes of motion shown for a diatomic molecule. Rotations can occur about both the y and x axes; only the rotation about the y axis is illustrated. Since radial symmetry exists about the z axis, rotations about that axis are not possible. Three modes of translational motion are possible: in the x, y, and z directions.

White, 2003

1/ Equilibrium fractionation:

Gas-liquid:16O and 18O distrubution in liquid (water) and in gas (vapor)

Reaction:(H2

18O)l + (H216O)g < == > (H2

16O)l + (H218O)g

According to law of mass action:

(H218O/H2

16O)g/ (H218O/H2

16O)l = (18O/16O)g/18O/16O)l

= αog/l (T, P) (fracionation coefficient,

approaches to 1 on T increase (independent on P)

= K (distribution coefficient, if the materials are of different)

Can be determined for several isotope pairs, e.g.:D/H, 13C/12C, 15N/14N, 34S/32S, 37Cl/35Cl (gas, solution, melt, solid, etc.)

Liquid-solid:16O and 18O distribution in liquid (water) and in solid (quartz)(H2

18O, H217O, H2

16O, D218O, etc. and also Si18O2, Si17O2, etc.)

Reaction: (Si18O2)s + (2H216O)l < == > (Si16O2)s + (2H2

18O)l

which is

(H218O)2 (Si16O2)

-----------*--------- = K1 equilibrium constant (at constant T)(H2

16O)2 (Si18O2)

if T increases, how does K change?

Heavier isotopes prefer stronger chemical bonds. 1 > K1 < 1?

(K2 can be determined, for instants, for distribution of 17O and 18O)

Solid-solid:16O and 18O distribution in magnetite and in quartz

2Si16O2 + Fe318O4 = 2Si18O2 + Fe3

16O4qtz mt qtz mt

covalent <-> ionic bonds, T-dependent reaction, geothermometer!

How about plagioclace (albite-anorthite) and alkali feldspar (albite-K-feldspar)?Can they serve as useful geothermometer?

Crystal-lattice and replacement of atoms and ions have a secondary role!

Equilibrium fractionation is faster in gas and liquid than solid phases (in the latter, diffusion has a significant role).

Structure (e.g., O on the surface or not), T and t affect the process

25 oC

All phases in equilibrium at 25 oC with seawater

α = K1/n

What fractionation can be expected?

O isotope fractionation between CaCO3 és H2O

Brownlow, 1996

Urey (1947): formation T of calcite can be calculated based on O isotope ratios of calcite and seawater!calcite – measurableseawater - ?

α = K1/n

Principle: equilibrium between/among minerals during and following their formation, but there is no new equilibrium when the system cooled down on/closed to the surface.

2/ Kinetic fractionation:

fast, not completed, one-way reactions: e.g., vaporization, precipitation, diffusion, dissociation, etc. and several biological processes driven reactions and mechanisms (photosynthesis, sulfate consumer/redusing anaerob bacteria)

motion rates of gas molecules are of different:- kinetic energy of ideal gas molecules is the same for each molecule at

a given T,- difference in mass (between/among light and heavy isotopes) is

compensated by molecule speed

Ek = ½*mv2

In case of CO2 gas molecules of 12C16O2, 12C17O2, 12C18O2, 13C16O2, etc. are present, however selecting:

A: 12C16O2 (mass = 12 + 2*16 = 44 amu) andB: 13C16O2 (mass = 13 + 2*16 = 45 amu)

if Ek is the same for both ½*mAvA2 = ½*mBvB

2 (in case of ideal gas at given T)

speed of rate: vA/vB = (mB/mA)1/2 = (45/44)1/2 = 1.0113

therefore 12C16O2 takes longer distance via diffusion during agiven time

T-dependent, molecules have higher stability when they contain haevier isotopes lighter isotopes prefer (should take) weaker bonds they form reaction products, can leave the system via inequilibrium reactions and biological processes (e.g., photosynthesis /low 13C/, bacterial reduction /low 34S/)

3/ Not mass-dependent fractionation /in photochemical reactions of meteorites and atmosphere, where molecular symmetry plays role/

Meteorites (CAI) of different classes plot in distinct fields on an oxygen isotope diagram, (nucleosynthetic, and then photochemical effect).

Oxygen isotopic composition in the stratosphere and troposphere show the effects of mass independent fractionation. A few other atmospheric trace gases show similar effects. Essentially all other material from the Earth and Moon plot on the terrestrial fractionation line.(ozone is poor in symmetric (18O3, 16O3) molecules) Thiemens and Heidenreich, 1983; Theimens, 1999 (review)

White, 2003

The δ notation:variations in stable isotope ratios are typically in the parts per thousand to parts perhundred range and are most conveniently and commonly reported as permil deviations, δ, from a standard:

δ={[(isotope ratiosample-isotope ratiostandard)/isotope ratiostandard]}*103

δ18O = {[(18O / 16O)sample /(18O / 16O)SMOW]-1}*1000SMOW – standard for oxygen

negative and positive δ values (meaning of these?)

The fractiomation factor:

Analytical techniques:1/ Gas-source double-collection mass spectrometer invented by A. Nier has been used for measuring slight isotopic differences for elements which can be measured in gaseous state and ionized by electron bombardment. The ratio of the two electrical currents is directly related to the isotope ratios of samples and their standards. This mass spectrometer perfect for hydrogen (in form of H2 or H2O), carbon and oxygen as CO2, sulfur (in form of SO2 or SF6), nitrogen (N2), chlorine (Cl2), etc. 1a/ Laser-fluorination techniques using CIF3 reagent for O-isotopes in silicates and oxides. 2/ Solid-source mass spectrometry (thermal ionization mass spectrometry (TIMS) or inductively coupled plasma mass spectrometry (ICPMS)|, even the multi-collection ICPMS (MC-ICPMS).3/ In-situ probes: secondary ion mass spectrometry (SIMS) (ion microprobe) (highly precise and sensitive, but very expensive) and laser ablation ICPMS (LA-ICPMS).

NBS - National Bureau of Standards (USA) (to 1988)NIST- National Institute of Standards and Technology (USA) (from 1988)IAEA - International Atomic Energy AgencyV – Vienna (VPDB, VSMOW or SMOW-1)USGS Reston

Standards (NBS, NIST, V, IAEA): notation standard absolute ratio

H δD average (hypothetical) oceanic water (SMOW) D/H = 1,557*10-4

O δ18O SMOW, PDB (Pee Dee Belemnite) or VPDB 18O/16O = 2,0052*10-3

O δ17O SMOW 17O/16O = 3,76*10-4

S δ34S troilite from Canyon Diablo meteorite (CDT) 34S/32S = 4,43*10-2

C δ13C PDB or VPDB 13C/12C = 1,122*10-2

N δ15N air (NBS-14 or ATM or AIR) 15N/14N = 3,613*10-3

Li δ7Li NBS L-SVEC vagy NIST 8545 7Li/6Li = 12,086053(Li δ6Li NBS L-SVEC or NIST 8545 6Li/7Li = 8,274*10-2)B δ11B NBS 951 or NIST 951 11B/10B = 4,044Cl δ37Cl SMOC 37Cl/35Cl = 0,324Br δ81Br SMOB 81Br/79Br = 49.313He Ra (present atmospheric He-ratio) 3He/4He = 1,3841*10-6

87Sr (natr. adund. 87Sr = 7.0 86Sr = 9.86) 87Sr/86Sr = measured value

Izotope Ratio Adundances % Referencia (absolute ratio) Phases/substances used

2H 2H/1H 0.015 V-SMOW (1.5575 · 10–4) H2O, CH2O, CH4, H2, OH–minerals

3He 3He/4He 0.000138 Atmoszferikus He (1.3 · 10–6) He in water or gas, crustal fluids, basalts

7Li 7Li/6Li 92.5 L-SVEC (12,086053) Salty waters, rocks

11B 11B/10B 80.1 NBS 951 (4.04362) Salty waters, clays, borates, rocks

13C 13C/12C 1.11 V-PDB (1.1237 · 10–2) CO2, carbonates, dissolved inorganic carbon, CH4, organic matters

15N 15N/14N 0.366 AIR N2 (3.677·10–3) N2, NH4+, NO3

–, N-bearing organic matters

18O 18O/16O 0.204 V-SMOW (2.0052 · 10–3) V-PDB (2.0672 · 10–3)

H2O, CH2O, CO2, sulfates, NO3–, carbonates,

silicates, OH–minerals34S 34S/32S 4.21 CDT (4.5005 · 10–2) Sulfates, sulfides, H2S, S-bearing organic

matters37Cl 37Cl/35Cl 24.23 SMOC (0.324) Salty waters, rosks, evaporites, dissolving agents

81Br 81Br/79Br 49.31 SMOB Salty wasters

87Sr 87Sr/86Sr 87Sr = 7.0 86Sr = 9.86

Measured value Waters, carbonates, sulfates, feldspars

Most frequently analyzed substances/phases