GEOS 459/559 - Thermochronology. Today: Why take this class? Intro to nuclear stability and decay . Who needs this?. Tectonicists and petrologists working with multi-Ma rocks Some 85% of the papers published in the journal Tectonics contain some quantitative geochron / thermochron data;
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GEOS 459/559 - Thermochronology
Today: Why take this class?
Intro to nuclear stability and decay
Class syllabus - subject to minor changes in the future
*-non biostratigraphic ages, i.e. no fossils
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
K (radioactive parent) - Ar (daughter)
At T’s above a certain # (say, Tc), all or some Ar atoms are lost from the system considered the “chronometer”.
K (radioactive parent) - Ar (daughter)
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)
Stability of nuclei as a function of proton (Z) vs. neutron (N) numbers
A (mass #)= Z+N
Isotopes (equal z’s)
Isobars- same mass #, A (=N+Z)
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 (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!!