Diamond’s 2 billion year growth charts tectonic shift in early Earth’s carbon cycle


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A study of tiny mineral ‘inclusions’ within diamonds from Botswana has shown that diamond crystals can take billions of years to grow. One diamond was found to contain silicate material that formed 2.3 billion years ago in its interior and a 250 million-year-old garnet crystal towards its outer rim, the largest age range ever detected in a single specimen. Analysis of the inclusions also suggests that the way that carbon is exchanged and deposited between the atmosphere, biosphere, oceans and geosphere may have changed significantly over the past 2.5 billion years.

Diamond's 2-billion-year growth charts tectonic shift in early Earth's carbon cycle
Gem quality diamond from Letlhakane, containing multiple orange garnets 
[Credit: M. Gress, VU Amsterdam]

‘Although a jeweller would consider diamonds with lots of inclusions to be flawed, for a geologist these are the most valuable and exciting specimens,’ said Prof Gareth Davies, of Vrije Universiteit (VU) Amsterdam, who co-authored the study. ‘We can use the inclusions to date different parts of an individual diamond, and that allows us to potentially look at how the processes that formed diamonds may have changed over time and how this may be related to the changing carbon cycle on Earth.’

Diamond's 2-billion-year growth charts tectonic shift in early Earth's carbon cycle
A composite of 9 catholuminescence images recording the growth
history in an individual gem quality diamond 3 mm in 

diameter. The
general tree ring structure defined by the different blue colours record
variations in the nitrogen content 

of the diamond. Black equates to
less than 10 parts per million and the brightest colours to ~500 ppm.
The diamond 

has a complex history with multiple periods of growth. The
irregular centre is surrounded by regular but rounded 

growth zones due
to the diamond suffering resorption. This occurs when a diamond is eaten
away by fluids deep 

in the Earth’s interior (> 150 km). Dating of
inclusions from different growth zones allows the time

 taken for diamond
growth to be determined [Credit: M. Gress, VU Amsterdam]

Sixteen diamonds from two mines in north eastern Botswana were analysed in the study: seven specimens from the Orapa mine and nine from the Letlhakane mine. A team at VU Amsterdam measured the radioisotope, nitrogen and trace element contents of inclusions within the diamonds. Although the mines are located just 40 kilometres apart, the diamonds from the two sources had significant differences in the age range and chemical composition of inclusions.

Diamond's 2-billion-year growth charts tectonic shift in early Earth's carbon cycle
A selection of unprocessed inclusion-bearing gem quality diamonds from Letlhakane. The dark areas surrounding the 
shiny metal like inclusions (sulphide) are graphite in cracks that from due to the differential expansion of the 
sulphide and diamond when brought to the surface from a depth of over 150 km. Bottom left diamond 
contains an orange garnet and a green clinopyroxene [Credit: M. Gress, VU Amsterdam]

The Orapa diamonds contained material dating from between around 400 million and more than 1.4 billion years ago. The Letlhakane diamond inclusions ranged from less than 700 million and up to 2-2.5 billion years old. In every case, the team were able to link the age and composition of material in the inclusions to distinct tectonic events occurring locally in the Earth’s crust, such as a collision between plates, continental rifting or magmatism. This suggests that diamond formation is triggered by heat fluctuations and magma fluid movement associated with these events.

Diamond's 2-billion-year growth charts tectonic shift in early Earth's carbon cycle
A plate cut through the centre of a gem quality diamond from Letlhakane, containing multiple orange garnets 
and green clinopyroxenes. Fractures in the diamond caused by the laser cutting and subsequent polishing 
[Credit: M. Gress, VU Amsterdam]

The Letlhakane diamonds also provided a rare opportunity to look back in time to the early Earth. The oldest inclusions date back to before the Great Oxidation Event (GOE) around 2.3 billion years ago, when oxygen produced by multicellular cyanobacteria started to fill the atmosphere, radically changing the weathering and sediment formation processes and thus altering the chemistry of rocks.

Diamond's 2-billion-year growth charts tectonic shift in early Earth's carbon cycle
An orange garnet exposed at a broken diamond surface. Note the well-developed crystal face at the top left 
that implies that the diamond imposed its crystal form on the garnet during growth of the garnet 
[Credit: M. Gress, VU Amsterdam]

‘The oldest inclusions in the diamonds contain a higher proportion of the lighter carbon isotope. As photosynthesis favours the lighter isotope, carbon 12, over the heavier carbon 13, this ‘light’ ratio finding suggests that organic material from biological sources may have been more abundant in diamond-forming zones early in the Earth’s history than we find today,’ explained Suzette Timmerman, lead author on the study. ‘Higher temperatures in the Earth’s interior before the GOE may have affected the way that carbon was released into the diamond forming regions beneath the Earth’s continental plates and may be evidence of a fundamental change in tectonic processes. However, we are currently working with a very small dataset and need further studies to establish if this is a global phenomenon.’

Source: Europlanet Media Centre [February 23, 2017]



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