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Radiometric Age Dating


Requirements for a geologic clock:

• Begin at the time a rock was formed

• Operate at a constant rate

• Not be influenced by other processes

(not rain, cold, dark of night)


Conditions met by:

RADIOACTIVITY


Conditions met by:

RADIOACTIVITY


What is it?

Review of chemistry fundamentals:

Rock: composed of one or more minerals

Mineral: composted of one or more elements

Molecule: multiple atoms

Atom: smallest particle of an element.


Example:

Table Salt is made of two elements

Sodium & Chlorine

(Na Cl) solid


Example:

Table Salt is made of two elements

Sodium & Chlorine

dissolved in water

(Na+ and Cl-)


Example:


Why is Gold ashiny yellow metal?

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Elemental properties are dictated

by the number of subatomic particles:

Elemental properties are dictated

by the number of subatomic particles:


Specifically the numberof protons in thenucleus.

Protons:

• positive charge

• in nucleus,

• number defines the

type of element.

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7 protons= Nitrogen


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Nitrogen

Atomic Number:number of protons.

7 protons= Nitrogen

Electrons:

• negative charge

• orbit nucleus,

• number commonly the same as protons

(but may vary).


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Neutrons:

• neutral charge

• in the nucleus,

• approximately the same number as protons

(but varies).


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Atomic Mass:

Number of Protons+

Number of Neutrons


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7 Protons + 7 Neutrons = Atomic Mass of 14

Nitrogen



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Atomic Mass:

Number of Protons+

Number of Neutrons

Oxygen



Atomic Mass:

Number of Protons+

Number of Neutrons

Carbon

+ ++

++

+

NN

NN

N

N

Carbon 12


Isotope:

Atom with unequal

number of Protons and Neutrons

Carbon

+ ++

++

+

NN

NN

N

N

N


Isotope:

Many isotopes are "stable".

(they do not change with time).

Carbon

+ ++

++

+

NN

NN

N

N

N



Atomic Mass:

Number of Protons+

Number of Neutrons

Carbon

+ ++

++

+

NN

NN

N

N

N

Carbon 13

Stable Isotope


Isotope:

Some isotopes are ”unstable".

(they spontaneously change to a more stable state through radioactive decay).

Carbon

+ ++

++

+

NN

NN

N

N

N N



Atomic Mass:

Number of Protons+

Number of Neutrons

Carbon

+ ++

++

+

NN

NN

N

N

N

Carbon 14

N

Unstable Isotope


A number of ways in which decay can occur:

sub-atomic shifts that give off small particles with energy

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Radioactive Decay:

• some kinds with minor energy• some with lethal energy

• some one step to stable• some multiple


Uranium 238

Lead 206

Many steps,each with energy


Radioactive Decay:

• rate is constant (for an isotope)

• decay is spontaneous & random


Half Life:

• the time it takes for 1/2 of

the unstable parent atoms

to decay to the stable daughter

form.

0.5half-life


Obtaining Radiometric Dates:

• as minerals crystallize they

incorporate atoms of some

radioactive isotopes.


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zircon

Granite

trace amounts

Uranium 238



• unstable isotopes decay at a constant rate.



• as % of parent decrease,

% of daughter increases

Total remains the same (100%)



• Measure the ratio of

Parent : Daughter

(not absolute count of atoms)


Mass Spectrometer:instrument to measure isotopic ratios


Calculating age:

1. Convert % of parent remaining

to the number of 1/2 lives

this represents. 100% 0 - half life

50% 1 - half life

25% 2 - half life

12.5% 3 - half life

Parent


Calculating age:

2. Multiple the number of 1/2 lives by the time it takes for one half of the isotope to decay


Calculating age:

Half Life

Carbon 14 = 5,730 yrs

Potassium 40 = 1,300,000 yrs

Uranium 238 = 4,500,000,000 yrs


Calculating age: example

12.5% parent

= 3 half lives



(one half life for Uranium = 4.5 billion yrs)

Age = 3.0 half-lives x 4.5 billion yrs

Age = 13.5 billion years

billion yrs



This dates the time that the

mineral grain formed.

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Calculating age:

If rock is made out of pieces

of other rocks (sediment) . . .

date is the parent rock.


Calculating age:

Therefore igneous rocks are most

often used for radiometric dating.

rhyolite - andesite - basalt

granite - diorite - gabbro


Calculating age:

Love the bentonite = volcanic ash bed.

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Calculating age:

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Love the bentonite = volcanic ash bed.


Sedimentary rocks are often dated by:

(1) bounding with igneous rock datesand

(2) correlation with fossils


Assumptions:

• range of half-life must be appropriate

- short enough for measurable decay

- long enough for parent to still be present


Assumptions:


Uranium - Lead

igneous rocks greater than

100 million years


Assumptions:


Potassium - Argon

igneous rocks greater than

100,000 years


Assumptions:


Carbon 14

organic material less than

80,000 years


Assumptions:

• must be a closed system

no parents added or removed

no daughters added or removed


Assumptions:

• must be a closed system

metamorphism: "opens" the system and resets the clock

modern weathering: "opens" the system


How Accurate?:

• Very first date:Cambrian - Precambrian

~ 600,000,000 yrs


How Accurate?

• Acceptable error:

current standard (+ or – 1.0%)


How Accurate?

• Acceptable error:

500,000,000 x 0.01

+ or – 5 million yrs


How Accurate?

• Knowledge of your own weight?

1. ________ actually ________

difference ________

as % ________


Fission Tracks:


Lake varves:

• seasonal deposits of mud in lakes

• very accurate record up to 100,000 yrs.


Tree-rings:

• annual rings climate affects the width

• patterns can be correlated regionally

• 8000 yr. record


Ice cores:

• annual accumulation of snow on glaciers

• ~100,000 year record

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Documents

radiometric age datingwhy

gif7 protons

nitrogen atomic number

unequalnumber of protons

numberof protons

age datingelemental

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gifatomic mass