C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/436%20lectures/L19-OIB.html http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html


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


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