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GEOS 459/559 - Thermochronology

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GEOS 459/559 - Thermochronology

Today: Why take this class?

Intro to nuclear stability and decay

- Tectonicists and petrologists working with multi-Ma rocks
- Some 85% of the papers published in the journal Tectonics contain some quantitative geochron / thermochron data;
- This field is rapidly expanding so new techniques and applications become available.

Class syllabus - subject to minor changes in the future

- 1. Radiogenic isotope chemistry
- 2. Diffusion theory
- 3. Individual chronometers (a) the standard story, and (b) new developments
- 4. Unraveling the thermal history of crustal rocks;
- Applications to tectonic/magmatic problems.

- To teach you about determining ages* of rocks;
- So then why not “Geochronology”?
- Most isotopic clocks start ticking below a certain temperature. The modern science of rock chronology combines isotopes with diffusion studies. The resulting discipline is “THERMOCHRONOLOGY”.

*-non biostratigraphic ages, i.e. no fossils

- Millions to tens of millions of years;
- Absolute ages ranging from a few Ma to Archean rocks;
- We do not deal with Quaternary geochronology.

Various isotopic systems start ticking the clock at different temperatures. Above these temperatures, parent and/or daughter isotopes move freely in and out of the system

Example

K (radioactive parent) - Ar (daughter)

K

Ar

At T’s above a certain # (say, Tc), all or some Ar atoms are lost from the system considered the “chronometer”.

Example

K (radioactive parent) - Ar (daughter)

K

Ar

Whn T is < than Tc, all Ar atoms remain within the system considered the “chronometer”, e.g. a K-spar grain.

To a first approximation, there is one temperature below which diffusion is so slow that radiogenic parent or daughter atoms become static.

The corollary is that every age we measure with an isotopic system records the time elapsed since the temperature cooled below that value.

Chronometer records age A(-0), whereas rock formed at an earlier time B (-0)

temp

Tc

A

0

B

time

- What is an isotope?
- What isotopes are stable?
- When are they “radioactive”?
- How do we quantify decay?
- Which systems are useful to geologists?

- Made of protons, neutrons, electrons
- Sum of protons and neutrons = mass number
- Only certain combinations of proton/neutron numbers are stable in nature;

Stability of nuclei as a function of proton (Z) vs. neutron (N) numbers

A (mass #)= Z+N

Isotones

Isotopes (equal z’s)

Isobars- same mass #, A (=N+Z)

isotopes

How many isotopes per element?

The “stability” line is a thick one with some isotopes that are energetically stable and others that tend to “decay” into a different nuclear state.

How many isotopes per element

Not all of these isotopes are stable as they depart from the idealized stability line.

The isotopes that are not stable will tend to decay into more stable configurations.

Let us look at the element Rb and its various isotopes.

Essentially there are only two isotopes that don’t decay away within short time scales, 87Rb and 85Rb. All others are not present in nature. Of these, one is stable (85Rb), and one is radiogenic (87Rb)

If isotopes decay away within laboratory time scales, that’s a no brainer - they are not stable.

Slower decaying species - need to know their:

Decay constant or

Half life

Half life (t1/2) = the time required for half of the parent atoms to decay, alternatively use:

The decay constant () = ln2/t1/2

Systems that have half lifes comparable to or longer than the age of the planet. Fast decaying systems are evidently no good.

E.g. 87Rb’s half life is ten times the age of the earth.

Some super slow decaying systems have yet to be figured out. In the meantime, they count as “stable” isotopes.

Decay systems of interest for geologists

We will examine all of them.

So far, nothing has been mentioned about laws of decay, physics of decay, mechanisms etc. We will quantify decay on Sept 1 in class. In the meantime, here are the basic mechanisms of decay:

Alpha decay () - emission of a He (alpha) particle. Resulting isotope has a mass A1=A0-4; e.g. 147Sm decays into 144Nd;

Beta decay (aka -) - transforming a neutron into a proton + an electron. Resulting isotope has a mass A1=A0, e.g. 87Rb decays into 87Sr.

Electron capture (aka +) is essentially the reverse of 2. E.g. 40K decays to 40Ar;

Gamma decay is the process of emitting a high energy photon - no examples in this class.

We made the point the geochronology is thermochronology because of the closure temperature. Give two general examples of rocks that, when dated with radiometric techniques truly record the age of formation and not just cooling. Due in class, next lecture.

Next lecture, Thursday September 1!!