Isotopes Atom of an element that has the same number of protons but different number of neutrons, thus they have different atomic masses. Isotope means “at the same place” (Greek isos topos). and it was first used by Margaret Todd Two kinds: –Stable (e.g., O 2 = 16 O, 17 O, 18 O) –~79% of nuclides on Earth are stable (out of 339 natural) –Unstable (e.g., radiogenic such as 26 Al)

Isotopes - Stable Three isotopes: 16 O, 17 O, 18 O of which 16 O is the most abundant at %. They are typically expressed in , which are deviations in part per thousand (permil, ‰) in the 17 O/ 16 O and 18 O/ 16 O ratios of a measured composition referenced from a standard (typically SMOW, Standard Mean Ocean Water) with 17 O/ 16 O = and 18 O/ 16 O =

Isotopes - Stable  17 O SMOW = [( 17 O/ 16 O ) sample / ( 17 O/ 16 O) SMOW -1] x 1000  18 O SMOW = [( 18 O/ 16 O ) sample / ( 18 O/ 16 O) SMOW -1] x 1000 Or:  17 O = ([( 17 O/ 16 O ) sample - ( 17 O/ 16 O) SMOW ]/ ( 18 O/ 16 O) SMOW ) x 1000  18 O = ([( 18 O/ 16 O ) sample - ( 18 O/ 16 O) SMOW ]/ ( 18 O/ 16 O) SMOW ) x 1000 Thus, positive  values indicate enrichments of a sample in that isotope relative or compared to SMOW, whereas negative values imply depletion of those isotopes in a sample relative to SMOW.

Isotopes - Stable Isotopes fractionate between sources: 1.Exchange reactions that redistribute isotopes of an element among different molecules containing that element. 2.Unidirectional reactions where rates of reactions depend on isotopic compositions of the reactants and products. 3.Physical processes that produce concentrations or temperature gradients in which mass differences com into play (e.g., evaporation, crystallization, diffusion, etc.).

Isotopes - Stable They are commonly expressed graphically on an oxygen three-isotope (  18 O vs  17 O) diagram. Material related to SMOW by mass fractionation fall on a nearly linear curve on this graph described by the equation:  17 O SMOW ~ 0.52  18 O SMOW Typically the equation is treated as an equality with mass fractionation shown as a straight line with a slope –Terrestrial Fractionation Line (TFL or TF). –Isotopes fractionate based on their vapor pressure and kinetic effects that typically follow Rayleigh distillation: R/R o = f (  -1) where R is the a ratio ( 18 O/ 16 O ) of the remaining vapor, R o is the ratio ( 18 O/ 16 O) of a vapor before condensation begins, f is the fraction of the vapor remaining, and  is the isotope factionation factor (which is an equilibrium approach).

Isotopes - Stable Non-mass-dependant variations, which do not fall on the TF are described with the parameter  17 O, which is essentially deviations from the TF but are parallel to the TF. These are defined by:  17 O =  17 O SMOW  x 18 O SMOW Methods of analysis: Use of mass spectrometer. –Fluorination of bulk material –Laser fluorination (in situ) –Secondary ion mass spectrometry (in situ)

Isotopes - Stable When a nuclei of an unstable atom undergoes spontaneous transformation that involves the emission of particles and radiation energy. The Z and N of the original or parent atom or isotope is changed leading to its transformation to another isotope. –Alpha (  ), beta (  ), and/or gamma (  ).

Radiogenic Nuclei Here we refer to decay and radioactive atoms. –Simply stated, a nuclei of an atom spontaneously transforms because it is not energetically stable through a process of the emission of particles and radiant energy. –A radioactive atom decays, which results in changes in Z (# of protons) and N (# of neutrons) of that parent atom to produce a daughter atom/element that is new. Three different types of rays or decay mechanisms, alpha (particle with 2 protons and 2 neutrons), beta (electron emission), and gamma (electromagnetic energy-same element).

Radiogenic Nuclei N = N 0 e - t (# of parent atom at time, t) N = radioactive parent atoms, N 0 = original number of atoms present at t = 0. D* = N 0 (1- e - t ) Number of stable radiogenic daughter atoms (none lost or gained, none present at t = 0). The term = decay constant. T 1/2 = (ln 2)/ = 0.693/l (half-life and decay constant).

Radiogenic Nuclei Discussion of decay typically refers to the time required for one-half of a given number of radionuclide to dcay, which is termed half-life (T 1/2 ): –T 1/2 = (ln 2)/ = 0.693/ (half-life and decay constant). Another means of discussing and quantifying decay is through the mean life of a radioactive species: –  = 1/ The mean life is longer than the half-life by the factor 1/0.693.

Radiogenic Nuclei For the method to produce ‘ages’ the following criteria must be understood: 1.The system must not have gained or lost either parent or daughter atoms, it must be a closed system. 2.We must determine a realistic value of Do. 3.The value of the decay constant must be known accurately. 4.The determination of D and N must be accurate and be representative of the sample to be dated. 1.In the case of Solar System materials the isotope in question is assumed to be heterogeneous within the rock-forming region of the disk.

Radiogenic Nuclei

Cosmogenic radionuclei: –These are produced from atoms that have been hit by a high-energy particle or a cosmic ray produced by the Sun or by galactic or extragalactic objects. ~ 90% are protons ~ 9% are helium nuclei (alpha particles) Rest are electrons Reactions occur in atmospheres, asteroids, etc. 10 Be, 14 C, 26 Al, 32 Si, 36 Cl, 39 Al, 53 Mn, 59 Ni, 81 Kr.