Lecture 18: isotopes and dating

Concordia

A plot of 206Pb /238U vs. 207Pb /235U, a curved graphical line in U-Pb dating

discordia

If a sample was disturbed only once since it formed, multiple samples may fall on a straight line between the point where the sample formed and the point where it was disturbed so that both ages can be determined. We call such a line a "discordia" line, compared to a Concordia line

How can we use the Rb and Sr in rocks to tell us when a rock formed? (what kind of rocks do we use, what measurements do we make, how do we plot up the data, what age range does it cover?)

Rocks used: Igneous rocks When an igneous rock crystallizes all the minerals within the rock have the same initial 87Sr/86Sr,. 87Sr/86Sr vs. Rb/Sr is plotted, for minerals from the same sample will fall on a straight line, called an isochron, if the system has been undisturbed since the sample was formed. The slope of the line, which becomes steeper and steeper over time, can be used to determine the time since the sample was formed The method is applicable to very old rocks, meteorites

How old is the Earth? What samples do we use to determine the age? Why do these samples tell us the age of Earth formation?

The age of the Earth is between 4.56 and 4.53 Ga. Tungsten (W) and Hafnium (Hf) is used During the formation of Earth's core and mantle, most of the W in the initial Earth went into the core, most of the Hf in the initial Earth went into the mantle. The amount of excess 182W in the mantle suggests that the Earth must have separated into a mantle and core within 30 Ma of the supernova explosion that triggered the formation of the solar system. If the Earth separated into a core and mantle at a later date, the excess 182W would have gone to the core with the rest of the W.

Isochron

a straight graphical line comparing 87Sr/86Sr vs. Rb/Sr in Rb-Sr dating

radioactive element

an unstable element that breaks down into a different element, a daughter element, decay can be used as a dating technique

radiogenic element

elements produced during radioactive decay are called radiogenic isotopes. Some radiogenic isotopes are themselves radioactive and decay into other radiogenic isotopes, others are stable, and do not undergo any further decay.

stable isotope

isotopes that do not undergo further decay

How does carbon-14 dating work? (what kind of rocks do we use, what measurements do we make, how do we plot up the data, what age range does it cover?)

scientists compare 14C of a sample in relation to the atmosphere The half-life of 14C is only 5730 years the technique is usually not used for dating materials older than ~57,000 years. The carbon 14 method is used extensively for archeological research, only works for the bodies of originally living organisms, and it only works for the first ~50,000 years after death.

Chondrite

the first rock-sized bodies that formed in the solar nebula from dust grains

initial isotope ratio

the isotope ratio when a mineral was first formed

What is a half-life? How does it affect radioactive dating techniques?

the rate at which a radioactive isotope decays Most age determinations are based on measuring the amount of the radioactive parent isotope that was originally present in a sample when it formed, compared to the amount of radiogenic daughter isotope in the sample after some period of time.