(Phys.org)—A pair of researchers with
The Hebrew University of Jerusalem has found evidence that suggests
both gem quality and fibrous diamonds form from the same types of
fluids. In their paper published in Earth and Planetary Science Letters,
Brooke Matat Jablon and Oded Navon, describe their experiments with
beams of electrons and multiple diamonds, both gem quality and fibrous,
what they found and why they now believe all diamonds have a common
source.
For many years geologists and other scientists
have debated whether cloudy diamonds, known as fibrous, developed under
the same conditions as gem quality diamonds, with many arguing that the
process must have been different to account for such obvious differences
between the two. Scientists have known for many years, that the reason
fibrous diamonds are cloudy, is because they have carbonates in them,
bits of the same types of materials that is found in shells—known as
inclusions. Gem quality diamonds were thought to be very near perfect,
which meant they had no inclusions. Now, it appears such thinking was
wrong as the research pair found evidence of extremely small inclusions
in every diamond they studied, regardless of type or quality.
To find inclusions in gem quality diamonds, the researchers
used electron scanning techniques to search in ways that had never been
tried before, focusing most intently along borders. Among the inclusions
found were multiple instances of material that was identical to that
found in carbonate-bearing fluids that have been typically found in
fibrous diamonds. This, the team suggests, indicates very strongly that
the diamonds all came about from the same source, namely carbon material
deep under the crust that was subject to extreme pressure. They do
suggest that the timeframe involved likely played a role determining
which type of diamond was formed. Fibrous diamonds, they suggest formed
relatively quickly, on the order of just a few million years, while gem
quality diamonds appear to have taken from one to three and a half billion years.
The team notes that their findings also suggest that our
planet must have maintained diamond-forming conditions for billions of
years, which suggests plate tectonics would have been occurring that
long as well.
Explore further:
Diamonds grow like trees, but over millions of years
More information:
Brooke Matat Jablon, Most diamonds were created equal, Earth and Planetary Science Letters (2016). DOI: 10.1016/j.epsl.2016.03.013Abstract
Diamonds crystallize deep in the mantle (>150 km),
leaving their carbon sources and the mechanism of their crystallization
debatable. They can form from elemental carbon, by oxidation of reduced
species (e.g. methane) or reduction of oxidized ones (e.g.
carbonate-bearing minerals or melts), in response to decreasing carbon
solubility in melts or fluids or due to changes in pH. The mechanism of
formation is clear for fibrous diamonds that grew from the
carbonate-bearing fluids trapped in their microinclusions. However,
these diamonds look different and, based on their lower level of
nitrogen aggregation, are much younger than most monocrystalline (MC)
diamonds.
In the first systematic search for microinclusions in MC diamonds we examined twinned crystals (macles), assuming that during their growth, microinclusions were trapped along the twinning plane. Visible mineral inclusions (>10 μm) and nitrogen aggregation levels in these clear macles are similar to other MC diamonds.
We found 32 microinclusions along the twinning planes in eight out of 30 diamonds. Eight inclusions are orthopyroxene; four contain >50% K2O (probably as K2(Mg, Ca)(CO3)2); but the major element compositions of the remaining 20 are similar to those of carbonate-bearing high-density fluids (HDFs) found in fibrous diamonds. We conclude that the source of carbon for these macles and for most diamonds is carbonate-bearing HDFs similar to those found here and in fibrous diamonds. Combined with the old ages of MC diamonds (up to 3.5 Ga), our new findings suggest that carbonates have been introduced into the reduced lithospheric mantle since the Archaean and that the mechanism of diamond formation is the same for most diamonds.
Via ScienceNews
In the first systematic search for microinclusions in MC diamonds we examined twinned crystals (macles), assuming that during their growth, microinclusions were trapped along the twinning plane. Visible mineral inclusions (>10 μm) and nitrogen aggregation levels in these clear macles are similar to other MC diamonds.
We found 32 microinclusions along the twinning planes in eight out of 30 diamonds. Eight inclusions are orthopyroxene; four contain >50% K2O (probably as K2(Mg, Ca)(CO3)2); but the major element compositions of the remaining 20 are similar to those of carbonate-bearing high-density fluids (HDFs) found in fibrous diamonds. We conclude that the source of carbon for these macles and for most diamonds is carbonate-bearing HDFs similar to those found here and in fibrous diamonds. Combined with the old ages of MC diamonds (up to 3.5 Ga), our new findings suggest that carbonates have been introduced into the reduced lithospheric mantle since the Archaean and that the mechanism of diamond formation is the same for most diamonds.
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