'Rock stars' solve long-standing diamond conundrum
Queensland University of Technology

Date:
March 21, 2023
Source:
Queensland University of Technology
Summary:
Two researchers have used a standard laptop computer and a humble
piece of rock -- from the 'waste pile' of a diamond mine -- to
solve a long- held geological conundrum about how diamonds formed
in the deep roots of the earth's ancient continents.


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FULL STORY ==========================================================================
Two QUT researchers have used a standard laptop computer and a humble
piece of rock -- from the 'waste pile' of a diamond mine -- to solve
a long-held geological conundrum about how diamonds formed in the deep
roots of the earth's ancient continents.


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The paper "Deep, ultra-hot-melting residues as cradles of mantle diamond,"
has been published in the academic journal Nature by lead author QUT
PhD student Carl Walsh, along with QUT Professor Balz Kamber and Emma
Tomlinson from Trinity College, Ireland.

Mr Walsh said the study, for his MSc research, involved computer modelling
on a rock from the African continent and recovered from the bottom of
the lithosphere, the outer part of the Earth between about 30km and
250km below the surface.

Mr Walsh said the dominant part of a continent was the part that you
never see.

"If you think of an iceberg -- the visible part -- if you just had an
iceberg floating on the ocean surface it would tip over like a boat. This
is like the keel of an iceberg," Mr Walsh said.

"We basically had a known starting composition of a rock, which is representative of the earth's mantle at an early time in the history of
the earth before all the continents were formed," Mr Walsh said.

"We took that starting composition and modelled what would happen to it
if it was progressively melted, and what would be left over. And that
material is what forms the bulk of the roots of ancient continents that
are still around today." Professor Kamber, from QUT's Faculty of Science, School of Earth and Atmospheric Sciences, said the aim of this research
was to use a computer model to see how these deep roots might have formed.

"The model essentially predicts which minerals and melts will be present
as you change the temperature of the mantle. So, it's a predictive tool
you can compare with the composition of actual minerals and rocks,"
Prof Kamber said.

The piece of rock used for the advanced computer modelling was mined
sometime between 1871 and 1914 and ended up in the 'waste-pile' of
the legendary Kimberley diamond mine, best known as 'The Big Hole' --
a combination open-pit and underground mine -- in Kimberley, Northern
Cape in South Africa.

The piece of rock they have modelled, garnet harzburgite, was brought
to the surface in a kimberlite pipe. The rock was retrieved by Professor
Kamber -- who specialises in petrology, a branch of geology that studies
rocks and the conditions under which they form.

He carefully sledgehammered the rock down to a size that he could
successfully ship home.

"It contains a jumble of minerals that were entrained on the way up
as they ripped through the base of the whole continent in a supersonic
volcanic eruption -- the likes of which we have never seen," Professor
Kamber said.

"The minerals in this rock sample are so badly hurt, they are screaming
still today, they were absolutely smashed." "It is so exciting to see
this preserved, it is extremely old -- 3.3 billion years old. Probably
the oldest rock most people will ever hold in their hands," Professor
Kamber said.

Mr Walsh said the study solved the conundrum of diamonds and the
temperatures in which they formed, given a diamond will turn into graphite
if heated up too much.

"But yet, when we look at the rocks that contain diamonds, they must
have been heated to massive temperatures," Mr Walsh said.

"So why is it that it is exactly those rocks that experienced the highest temperatures that ended up having diamonds?" Their research challenges
the existing two-step shallow "melting and stacking" explanation.

"Previously, it was believed that most of the ancient deep roots of
continents would have been host to diamonds, and that these diamonds were destroyed over time, because the base of the continent is continually
invaded and eroded by volatile rich melts and fluids," Mr Walsh said.

"Our work suggests that actually this might not be the case, that diamonds
are rare today -- and were in fact always rare." "And that's because
we can for the first time know what is missing from the cradle of the
diamond and we can go hunt for it at the surface." Professor Kamber
said on the present-day earth the heat and temperature distribution in
the mantle is not uniform.

"We have areas of relatively uniform mantle temperature, and areas where
the mantle is a lot hotter. These are known as mantle plumes. And we
have expressions of these in Hawaii and Iceland," Professor Kamber said.

"What we're studying is the effect of ancient plumes -- when much hotter
plumes than we have now would have hit the base of a growing continent."
Since conducing the research, Mr Walsh has travelled to Canberra
to recreate similar rocks in the lab at the Research School of Earth
Sciences at the Australian National University.

* RELATED_TOPICS
o Matter_&_Energy
# Energy_and_Resources # Chemistry # Nature_of_Water #
Thermodynamics
o Earth_&_Climate
# Earth_Science # Geology # Climate # Atmosphere
* RELATED_TERMS
o Ice_age o Landslide o Feldspar_mineral o Glacier o Ocean o
Sedimentary_rock o Ocean_current o Metamorphic_rock

========================================================================== Story Source: Materials provided by
Queensland_University_of_Technology. Note: Content may be edited for
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========================================================================== Journal Reference:
1. Carl Walsh, Balz S. Kamber, Emma L. Tomlinson. Deep,
ultra-hot-melting
residues as cradles of mantle diamond. Nature, 2023; 615 (7952):
450 DOI: 10.1038/s41586-022-05665-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/03/230321112642.htm

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