Daylesford, Victoria, Australia

In this part of the world the youngest volcanoes are 0.5 to 1 million years old - so young that they still retain their volcanic shape.

The researchers find grains of feldspar, or some other suitable crystal which contains significant amounts or potassium.  In the case of the Daylesford sample, this was a 2cm long feldspar crystal embedded in basalt from the old quarry on Ridge Road at the corner of Stony Creek Road, 3km south-south-west of Daylesford (Google Maps).  From the above article [Ismail-2016]:


There are three naturally occurring isotopes of potassium ³⁹K, ⁴⁰K and ⁴¹K, with 20, 21 and 22 neutrons respectively in their nuclei.  All have 19 protons, which is what defines these nuclei as being of potassium atoms, since the 19 protons attract 19 electrons to form an electrically neutral atom.  The outermost electron of a neutral potassium atom is in its own "shell" (really a cloud of probability where the electron might be at any moment, if we think of it as a little ball, which is not quite the whole story . . .).  The shell below is full, with 8 electrons.  The lone outer electron is unstable and the total energy of the atom is significantly reduced if it can get rid of the electron.  So potassium in compounds does this, and becomes a positively charged ion.  In potassium chloride, the chlorine atoms have seven electrons in their outer "shell", which is also in a high-energy unstable state.  They accept the electrons from the potassium atoms and so too have full, low-energy, totally self-satisfied outer electron "shells".  The negatively charged chlorine ions are attracted to the positively charged potassium ions and form a neat cubic crystal.  Sodium chloride - table salt - is the same.

The number of neutrons in the nucleus of an atom does not affect how the atom forms compounds, so all three potassium isotopes are chemically indistinguishable.  All three had their nuclei formed by the extreme pressures and temperatures inside a supernova explosion of a star - one or more stars whose supernovae debris formed the cloud from which the Solar System formed about 4.6 billion years ago.  No-one knows how long ago these supernovae were, but these potassium nuclei have been unchanged since they were first formed by lighter nuclei being forced together in the supernova, so forcibly that their mutual repulsion was overcome and they stuck together as new, larger, nuclei.

³⁹K and ⁴¹K are perfectly stable.  However, ⁴⁰K is unstable AKA radioactive.  0.012% of potassium in and on the Earth is ⁴⁰K.  (Wikipedia link) This has a half life of 1.251 billion years, so after this amount of time, half the atoms of ⁴⁰K in a sample would be gone, since their nuclei will have broken down (decayed) into the nuclei of isotopes of the metal calcium and the noble gas argon.  10.72% of these decays involve an inner electron from the atom entering the nucleus and converting one of the protons into a neutron, with the release of electromagnetic energy as a gamma ray and a neutrino - an enigmatic particle which can usually pass right through the Earth without hitting anything!

So in any one year, there is a small, but precisely known, chance that a ⁴⁰K
nucleus will transform itself from having 21 neutrons and 19 protons into having 22 neutrons and 18 protons, which means it is no longer a potassium nucleus, but the nucleus of the noble gas argon - specifically the stable (non-radioactive) isotope ⁴⁰Ar. 

Since it now has one less positive charge, the nucleus only attracts 18 electrons to form an electrically neutral atom.  This is argon, and its outer electron shell is completely self-satisfied with 8 electrons - the whole atom is in a low energy state and there isn't any addition or subtraction of electrons which would cause it to be in a still lower energy state.

This means that argon does not bond covalently (electron sharing) or bionically (as with sodium and potassium, by giving up or gaining and electron) with any other atoms.  Argon, along with neon and krypton, are noble gasses which exist in the air we breath.  By mass 0.934% of air is argon, and 99.6% of this is ⁴⁰Ar produced, as just described, by the decay of ⁴⁰K in rocks, magma etc. within the whole Earth.  (There used to be a lot more ⁴⁰K when the Earth was first formed.  It used to be the most important source of radioactive heating of the Earth's core and mantle, but it is now the third-most important after isotopes of uranium and thorium.)

If the feldspar crystal is hot enough, the argon atom behaves as a gas and can work its way through the crystal structure and so get out of the whole crystal.  However, this does not occur if the crystal is cool enough.

Long ago - I guess  millions  to a few billion years ago - a crystal of feldspar formed, incorporating potassium atoms whose nuclei were unchanged since being forged in one or more supernovae over 4.5 billion years ago.  This crystal somehow got mixed up in a flow of magma under the Australian continent, where this magma, later called lava as it erupts from what is now Leonards Hill, is hot enough to be molten, but not hot enough to melt the feldspar crystal.  This high temperature is, however, enough to allow all the ⁴⁰Ar atoms which result from the radioactive decay of ⁴⁰K to escape the crystal, go into the magma/lava, and, as the lava flows from the volcano, escape into the atmosphere.

The freshly erupted lava, with this feldspar crystal in its mist, flowed down the edge of the volcano, and eventually comes to rest 3.3km away, where it cools and solidifies, forming the basalt at the quarry at the corner of Ridge  and Stony Creek Roads.  The lava would have cooled within days or months to the point where the continually produced ⁴⁰Ar atoms remain trapped in the crystal.

Because enquiring minds want to know, Rafika Ismail and colleagues went looking for such a crystal, found it, and took it back to the lab for analysis.  By measuring how many ⁴⁰Ar atoms are trapped in the crystal, and by determining (indirectly) how many ⁴⁰K atoms are in the crystal now (or were in the past few million years, since their numbers are declining due to radioactive decay) it is possible to calculate how long ago the crystal and its surrounding lava cooled - so giving the age of the presumably single phase of eruptions which produced Leonards Hill.

This was done by sending the crystal to a Canadian nuclear reactor facility, where it was bombarded by neutrons, which converted some proportion of the stable ³⁹K nuclei into radioactive ³⁹Ar  nuclei.  (The high speed neutron boots out a proton and takes its place.)

The exact proportion of this conversion is measured by analysing another sample which is subject to the same neutron bombardment.

Then the crystal is crushed into small grains and placed in a mass spectrometer specifically designed for five of the isotopes of argon: the Argus IV mass spectrometer at the University of Melbourne, School of Earth Sciences (link).

An infra-red laser heats the grains so that various gases, including all the argon atoms of various isotopes, escape the crystal.  (The potassium atoms stay behind, ionically bound, since the temperature is not enough to free them from their ionic bonds with neighboring atoms.) The argon atoms are then ionised (an electron is added or removed).  These individual ions have the same electric charge, but different masses, according to the total number of protons and neutrons in their nuclei.  (The electrons are about 1/1,837 the mass of protons and neutrons, so they hardly contribute little to the total mass.)

The argon ions are accelerated electrically in a vacuum to form a narrow beam.  The beam is processed by a velocity selector (link) so that all the ions which emerge have the same velocity.

The beam is bent about 90 degrees by a large electromagnet.  The bending force is the same for all the ions, since they have the same electrical charge.  However, they have different momenta in the original direction, since they are traveling at the same speed but have different masses.  The heavy ions are bent less than the light ion, so five separate beams emerge from the magnetic field, one for each of the five argon isotopes.  The number of ions in each of the five beams is measured simultaneously by the ions coming to a halt, with their electrical charges adding up to an electrical current, on five metal plates connected to five very sensitive amplifiers.

The ³⁹Ar  nuclei can only have come from the conversion of neutron beam conversion of  ³⁹K  nuclei.  Any such ³⁹Ar which might have been present in the crystal from millions of years ago would by now have disappeared, since it has a half-life of 269 years.  The count of ³⁹Ar can be used to calculate the crystal's ³⁹K content, from which its much smaller present day and past (since it is slowly diminishing) ⁴⁰K content can then be calculated.

With care, the number of ⁴⁰Ar ions detected by the mass spectrometer reflects exactly what was in the crystal.  The crystal has been imbedded in solid, cool, basalt (through which argon atoms cannot move) since the lava flow cooled.  Ideally, the sample and the spectrometer will not contain any stray  ⁴⁰Ar atoms from the atmosphere.

With this simultaneous measurement of the crystal's ⁴⁰Ar and ³⁹Ar content, the amount of  ⁴⁰K (the original radioactive potassium isotope, created in a supernova) can be calculated for the exact same grains and laser heated gas release temperatures as are used for the  measurement of the amount of ⁴⁰Ar in the crystal  (the decay product of this, collected in the years since the crystal and its surrounding lava cooled).  This overcomes various errors which would result from trying to measure the potassium and ⁴⁰Ar content separately.  This reduction of error makes it possible to calculate dates when even when only a very small fraction of the ⁴⁰K has decayed since the crystal cooled enough to trap all the ⁴⁰Ar.  "Small fraction" in this case means a small fraction of a billion years or so.  So the technique is good for measuring the last melting time of rocks in the million to billion year range.  However, as far as I know, it is not the best method of measuring much shorter times, such as of a few thousand years since the Mt Gambier or Mt Napier eruptions.

Plugging these two measurements, together with a measurement from the reference sample from the neutron, provides the time since the cooling.  For the Daylesford sample, this was 2.01 million years, with +/- 110,000 years error margin.

I mention this because it indicates that it is in fact quite difficult to accurately determine the age of rocks!  (This explanation is for general interest: geology students should read the article regarding corrections for entrapped atmospheric argon, and the hypothesis that the feldspar crystals are xeoncrysts, rather than being formed as the lava cooled.)

http://vrosearch.agriculture.vic.gov.au/results.php?q=newer%20volcanic

https://www.researchgate.net/profile/Julie_Boyce

https://www.google.com/maps/d/viewer?mid=znL0MWNL51Mw.keepDtG_Zq1k

http://www.australiangeographic.com.au/topics/science-environment/2017/05/volcanic-victoria

http://www.piecesofvictoria.com/2016/10/six-must-visit-volcanoes-in-victoria/

http://www.victorianvolcanoes.com

https://womsat.org.au/womsat/map.aspx

This was published by the Daylesford Historical Society - now the Daylesford & District Historical Society http://www.daylesfordhistory.com.au/ . 

The second edition of the book is now available from The Society at the Daylesford Museum (this is not its proper title, but I guess people will search for this term) and was  launched there at 6:30PM on 9th December 2016.  The Society's collection (museum) is located at 100 Vincent St, just south of the post office. 

I guess that this book was written and published in the early 1980s.   The first sentence of the author's biography is: "H. T. Maddicks, well known in Daylesford as a 1920's Radio and later TV Engineer, a business he ran in Daylesford for 40 years.

The last page mentions "Abco Print, Daylesford".  This indicates that it was printed and perhaps typeset (Linotype) here.  Linotype machines were still used in those days before computers enabled phototypesetting.  A few historical books printed (or perhaps published) by this company can be found with this Google search.  This library page refers to a 1981 book manufactured by "Daylesford Abco Printing Service".

http://earthresources.efirst.com.au/product.asp?pID=282&cID=9
Castlemaine Deep Leads 1:100,000 map, Geological Survey of Victoria, 1989.

The area referred to in evidence to the Gold Rewards Board was in Wombat Creek behind the present day Wentworth Hotel - Old Bakery area, now covered by the head waters of Lake Daylesford.

http://www.daylesfordcfa.org.au/weather/

The Goldfields Track Walking and Cycling Guide.  This booklet has many map pages and has full details of the track which runs from Mt Bunninyong, south-east of Ballarat, through Creswick, Daylesford, Hepburn Springs, Vaughn,  Fryerstown and Castlemaine to Bendigo.  The book is available from The Book Barn on Lake Daylesford and from  http://www.goldfieldstrack.com.au .

Two maps we bought from http://www.melbmap.com.au:

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