• 'Rock stars' solve long-standing diamond

    From ScienceDaily@1:317/3 to All on Tue Mar 21 22:30:26 2023
    '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.


    ==========================================================================
    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.

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    ========================================================================== Story Source: Materials provided by
    Queensland_University_of_Technology. Note: Content may be edited for
    style and length.


    ========================================================================== 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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