Isotopes

See http://plate-tectonic.narod.ru/petrographyigneouslinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html
http://ijolite.geology.uiuc.edu/08SprgClass/geo436/436%20lectures/L12-Isotope.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

B. 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 (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

C. 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): straight line = isochron

-See Rb/Sr systematics in upper mantle and crust

D. Sm-Nd system

-147Sm > 143Nd

-Dating equation becomes 9-19, with isochrons similar to Rb/Sr

-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)