=?Windows-1252?Q?RE:_Radioactive_potassium_may_be_major_heat_source_in_Ea?==?Windows-1252?Q?rth=92s_core?=

[John R Johnson <idias@interchange.ubc.ca>]

Susan



Very interesting. I,like I think most of us, was taught it was due to U-Nat

and Th-Nat.



Thanks for posting it.



John

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-----Original Message-----

From: owner-radsafe@list.vanderbilt.edu

[mailto:owner-radsafe@list.vanderbilt.edu]On Behalf Of Susan L Gawarecki

Sent: December 15, 2003 2:13 PM

To: RADSAFE

Subject: Radioactive potassium may be major heat source in Earth’s core





Radioactive potassium may be major heat source in Earth’s core

http://snipurl.com/3fhk



BERKELEY – Radioactive potassium, common enough on Earth to make

potassium-rich bananas one of the “hottest” foods around, appears also

to be a substantial source of heat in the Earth’s core, according to

recent experiments by University of California, Berkeley, geophysicists.



Radioactive potassium, uranium and thorium are thought to be the three

main sources of heat in the Earth’s interior, aside from that generated

by the formation of the planet. Together, the heat keeps the mantle

actively churning and the core generating a protective magnetic field.



But geophysicists have found much less potassium in the Earth’s crust

and mantle than would be expected based on the composition of rocky

meteors that supposedly formed the Earth. If, as some have proposed, the

missing potassium resides in the Earth’s iron core, how did an element

as light as potassium get there, especially since iron and potassium

don’t mix?



Kanani Lee, who recently earned her Ph.D. from UC Berkeley, and UC

Berkeley professor of earth and planetary science Raymond Jeanloz have

discovered a possible answer. They’ve shown that at the high pressures

and temperatures in the Earth’s interior, potassium can form an alloy

with iron never before observed. During the planet’s formation, this

potassium-iron alloy could have sunk to the core, depleting potassium in

the overlying mantle and crust and providing a radioactive potassium

heat source in addition to that supplied by uranium and thorium in the core.



Lee created the new alloy by squeezing iron and potassium between the

tips of two diamonds to temperatures and pressures characteristic of

600-700 kilometers below the surface - 2,500 degrees Celsius and nearly

4 million pounds per square inch, or a quarter of a million times

atmospheric pressure.



“Our new findings indicate that the core may contain as much as 1,200

parts per million potassium -just over one tenth of one percent,” Lee

said. “This amount may seem small, and is comparable to the

concentration of radioactive potassium naturally present in bananas.

Combined over the entire mass of the Earth’s core, however, it can be

enough to provide one-fifth of the heat given off by the Earth.”



Lee and Jeanloz will report their findings on Dec. 10, at the American

Geophysical Union meeting in San Francisco, and in an article accepted

for publication in Geophysical Research Letters.



“With one experiment, Lee and Jeanloz demonstrated that potassium may be

an important heat source for the geodynamo, provided a way out of some

troublesome aspects of the core’s thermal evolution, and further

demonstrated that modern computational mineral physics not only

complements experimental work, but that it can provide guidance to

fruitful experimental explorations,” said Mark Bukowinski, professor of

earth and planetary science at UC Berkeley, who predicted the unusual

alloy in the mid-1970s.



Geophysicist Bruce Buffett of the University of Chicago cautions that

more experiments need to be done to show that iron can actually pull

potassium away from the silicate rocks that dominate in the Earth’s mantle.



“They proved it would be possible to dissolve potassium into liquid

iron,” Buffet said. “Modelers need heat, so this is one source, because

the radiogenic isotope of potassium can produce heat and that can help

power convection in the core and drive the magnetic field. They proved

it could go in. What’s important is how much is pulled out of the

silicate. There’s still work to be done “



If a significant amount of potassium does reside in the Earth’s core,

this would clear up a lingering question - why the ratio of potassium to

uranium in stony meteorites (chondrites), which presumably coalesced to

form the Earth, is eight times greater than the observed ratio in the

Earth’s crust. Though some geologists have asserted that the missing

potassium resides in the core, there was no mechanism by which it could

have reached the core. Other elements like oxygen and carbon form

compounds or alloys with iron and presumably were dragged down by iron

as it sank to the core. But at normal temperature and pressure,

potassium does not associate with iron.



Others have argued that the missing potassium boiled away during the

early, molten stage of Earth’s evolution.



The demonstration by Lee and Jeanloz that potassium can dissolve in iron

to form an alloy provides an explanation for the missing potassium.



“Early in Earth’s history, the interior temperature and pressure would

not have been high enough to make this alloy,” Lee said. “But as more

and more meteorites piled on, the pressure and temperature would have

increased to the point where this alloy could form.”



The existence of this high-pressure alloy was predicted by Bukowinski in

the mid-1970s. Using quantum mechanical arguments, he suggested that

high pressure would squeeze potassium’s lone outer electron into a lower

shell, making the atom resemble iron and thus more likely to alloy with

iron.



More recent quantum mechanical calculations using improved techniques,

conducted with Gerd Steinle-Neumann at the Universität Bayreuth’s

Bayerisches Geoinstitüt, confirmed the new experimental measurements.



“This really replicates and verifies the earlier calculations 26 years

ago and provides a physical explanation for our experimental results,”

Jeanloz said.



The Earth is thought to have formed from the collision of many rocky

asteroids, perhaps hundreds of kilometers in diameter, in the early

solar system. As the proto-Earth gradually bulked up, continuing

asteroid collisions and gravitational collapse kept the planet molten.

Heavier elements – in particular iron - would have sunk to the core in

10 to 100 million years’ time, carrying with it other elements that bind

to iron.



Gradually, however, the Earth would have cooled off and become a dead

rocky globe with a cold iron ball at the core if not for the continued

release of heat by the decay of radioactive elements like potassium-40,

uranium-238 and thorium-232, which have half-lives of 1.25 billion, 4

billion and 14 billion years, respectively. About one in every thousand

potassium atoms is radioactive.



The heat generated in the core turns the iron into a convecting dynamo

that maintains a magnetic field strong enough to shield the planet from

the solar wind. This heat leaks out into the mantle, causing convection

in the rock that moves crustal plates and fuels volcanoes.



Balancing the heat generated in the core with the known concentrations

of radiogenic isotopes has been difficult, however, and the missing

potassium has been a big part of the problem. One researcher proposed

earlier this year that sulfur could help potassium associate with iron

and provide a means by which potassium could reach the core.



The experiment by Lee and Jeanloz shows that sulfur is not necessary.

Lee combined pure iron and pure potassium in a diamond anvil cell and

squeezed the small sample to 26 gigapascals of pressure while heating

the sample with a laser above 2,500 Kelvin (4,000 degrees Fahrenheit),

which is above the melting points of both potassium and iron. She

conducted this experiment six times in the high-intensity X-ray beams of

two different accelerators - Lawrence Berkeley National Laboratory’s

Advanced Light Source and the Stanford Synchrotron Radiation Laboratory

- to obtain X-ray diffraction images of the samples’ internal structure.

The images confirmed that potassium and iron had mixed evenly to form an

alloy, much as iron and carbon mix to form steel alloy.



In the theoretical magma ocean of a proto-Earth, the pressure at a depth

of 400-1,000 kilometers (270-670 miles) would be between 15 and 35

gigapascals and the temperature would be 2,200-3,000 Kelvin, Jeanloz said.



“At these temperatures and pressures, the underlying physics changes and

the electron density shifts, making potassium look more like iron,”

Jeanloz said. “At high pressure, the periodic table looks totally

different.”



“The work by Lee and Jeanloz provides the first proof that potassium is

indeed miscible in iron at high pressures and, perhaps as significantly,

it further vindicates the computational physics that underlies the

original prediction,” Bukowinski said. “If it can be further

demonstrated that potassium would enter iron in significant amounts in

the presence of silicate minerals, conditions representative of likely

core formation processes, then potassium could provide the extra heat

needed to explain why the Earth’s inner core hasn’t frozen to as large a

size as the thermal history of the core suggests it should.”



Jeanloz is excited by the fact that theoretical calculations are now not

only explaining experimental findings at high pressure, but also

predicting structures.



“We need theorists to identify interesting problems, not only check our

results after the experiment,” he said. “That’s happening now. In the

past half a dozen years, theorists have been making predictions that

experimentalists are willing to spend a few years to demonstrate.”



The work was funded by the National Science Foundation and the

Department of Energy.

--

.....................................................

Susan L. Gawarecki, Ph.D., Executive Director

Oak Ridge Reservation Local Oversight Committee

102 Robertsville Road, Suite B, Oak Ridge, TN 37830

Toll free 888-770-3073 ~ www.local-oversight.org

.....................................................





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