 | Изотопы |
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Isotopes
A. Terminology
Element = chemical entity having a set number of protons in nucleus
Isotope = nuclide = atoms of the same element having different numbers of neutrons in nucleus (=> different atomic weights)
B. Types
Stable isotopes don''t change with time
Radioactive isotopes (unstable) spontaneously change over time = radioactive decay
II. Stable isotopes
A. Fractionation
Isotopes by definition are the same chemical element, so they don''t fractionate chemically.
Isotopes can be separated by mass fractionation = differentiation by mass ("weight").
Lighter isotope always fractionates into the phase with weaker bonding: liquid over solid, vapor over liquid.
B. Oxygen
3 isotopes: 18 and 16 used
Stable isotopes usually referred to a standard. For O, this is Standard Mean Ocean Water (SMOW).
delta notation delta (18O) > 0 means a sample has been enriched in 18O
C. Uses in igneous petrology
O isotopes - distinguish between rocks from mantle melts and those from crustal melts
H isotopes - decipher the origin of hydrothermal ores
C isotopes (in carbonate minerals, diamond, or graphite) - magmatic origin vs. low-T alteration
D. Uses in metamorphic petrology
Can narrow down protolith
Identify fluids involved in metamorphism
Determine temperature of reactions (must be applied carefully, as rocks can be affected by later events)
III. Radionuclides
A. Definitions
Radioactive isotope = parent = the one that decays. Radiogenic isotope = daughter = product of decay - may itself be radioactive. Decay can occur in several ways. Problems: was there original nonradiogenic D present? Has any radiogenic D* been lost?
B. K-Ar system
40K > 40Ar = inert gas - escapes from a hot system.
As magma cools, it starts with K, no Ar, so all Ar is radiogenic.
Two-step decay process
In general, measure Ar and K and get date
C. Rb-Sr system
87Rb > 87Sr
Useful for igneous ages and differentiating sources
Systematics: Rb compatible in micas, amphiboles, Kspar ; Sr compatible in plag.
Problem: 87Sr in a sample = original amount + radiogenic amount - no way to separate these.
Get around this by measuring 87Sr/86Sr (latter is non-radiogenic).
Rb/Sr systematics in upper mantle and crust
D. Sm-Nd system
147Sm > 143Nd
Dating equation with isochrons similar to Rb/Sr for mantle and crust evolution
Nd ratios are very small, so epsilon notation is used
Epsilon > 0 if sample derived from a depleted mantle source
Epsilon < 0 if sample is derived from an enriched mantle source
E. U-Th-Pb systems
3 systems :238U > 206Pb ; 235U > 207Pb ;232Th > 208Pb
All produce a series of intermediate decay products.
All end in stable Pb.
There are 3 isochron equations, which can be treated as before.
Or, use the 2 U/Pb systems together
If rock is undisturbed after formation, U/Pb ratios change along curve = concordia
If a thermal or fluid event occurs, Pb can be removed from system, in the 206/207 ratio at the time => disturbed U/Pb values will lie along a line to the origin = discordia
Subsequently, both systems will decay, and each sample''s U/Pb ratios will change along new curves. At a later time, samples will line up along a different discordia. This second discordia intersects the concordia at 2 points: right point = age of original rock, left point = age of thermal event