Diamond Research Gives Clues to the Formation of the Continents
April 25, 2019
This is a summary of the article "Sulfur Isotopes in Diamonds Reveal Differences in Continent Construction" by Karen V. Smit, Steven B. Shirey, Erik H. Hauri and Richard A. Stern published on April 26, 2019, in Science.
Diamonds, one of the most valuable gemstones, are also very valuable to geoscientists. Mineral inclusions in diamonds are the most direct samples we have from the inaccessible depths of Earth. Inclusions in diamonds have given geoscientists information about water in the earth’s interior, mineralogy of the deep earth and metallic phases in the deep earth. Also, because there is no direct way to determine a diamond’s age from the diamond itself, mineral inclusions trapped within diamonds provide the only way to date diamonds. So, although inclusions in diamonds are often considered to be undesirable in the gem trade, they are extremely valuable scientific samples.
In our study, we measured the sulphur and rhenium-osmium isotopes in sulphide inclusions in diamonds from the Zimmi region of Sierra Leone. The sulphides are tiny minerals, often between 100 and 300 microns across, trapped in the diamonds during growth.
We found that the sulphides recorded two episodes of subduction in the West African continent. Subduction is when the oceanic crust is thrust under another tectonic plate during collision into the deep earth. The first subduction event recorded by Zimmi sulphides occurred around three billion years ago and the second was around 650 million years ago.
3 billion-year-old subduction
The sulphides have isotopic compositions that indicate the sulphur was cycled through the ancient atmosphere prior to the rise of oxygen 2.5 – 2.3 billion years ago (Farquhar et al., 2001). This is indicated by mass-independently fractionated (MIF) sulphur isotopes. Any modern sulphur (or sulphur that did not cycle through the ancient atmosphere) will not have these MIF isotopes1. The presence of MIF sulphur in these sulphide inclusions indicates that they have a surficial origin in the earth’s ancient atmosphere. The sulphides were likely emplaced into the deep earth around 3 billion years ago – a time that has been well established for subduction and incorporation of oceanic crust material into the mantle2,3.
650 million-year-old subduction
We also measured the rhenium-osmium isotopes in these sulphide inclusions. Rhenium-osmium is the most widely used technique to date the time of diamond formation4. Zimmi diamonds were found to have 650-million-year-old ages5, an age that overlaps with subduction and collisional mountain building in the region, between 700 and 550 million years ago. Subduction of oceanic crust and subsequent dehydration of the oceanic crust would have introduced carbon-bearing fluids into the deep earth for diamond formation6.
Why do we find this interesting?
Earth’s oldest continents (called cratons) are stabilized by lithospheric mantle keels. The stability of Earth’s continents in the face of destructive tectonic activity is an essential geologic backdrop for the emergence of life on our planet. Since this is the only tectonically active, rocky planet that we know, understanding the geology of how our continents formed is a crucial part of discerning what makes Earth habitable.
The stability of cratons depends on the underlying mantle keels that are around 150 – 200 km thick. The processes for how these mantle keels form are still debated, and there are several different theories for their origin. Some of the models for craton formation involve subduction-style plate tectonics where plates are subducted into the deep earth and stabilize the cratonic keels. Other models do not invoke subduction, and instead, require deeper mantle processes like melting in mantle plumes or melting at oceanic plateaus.
Luckily these mantle keels have the ideal conditions for diamond formation. The majority of natural diamonds form in these cratonic mantle keels. Diamonds become important samples that can be used to investigate how the stabilizing keels below the oldest continents are formed.
Sulfur isotopes in diamonds, combined with their Re-Os ages, can be used to track multiple subduction events during craton growth, even those separated by billions of years. Subduction processes were essential to the growth and modification of the West African craton over a period of 2 billion years.
We compared our results to diamonds from southern Africa and northern Canada. We found that this combined isotopic approach also reveals differences in craton construction worldwide. Similar to the Zimmi diamonds, diamonds from the Jwaneng and Orapa mines in southern Africa do contain MIF sulfur1,7. This indicates that subduction was also an important process in the construction of the cratonic mantle in southern Africa.
But diamonds mined in northern Canada do not show the same sulphur chemistry. Diamonds from the Ekati mine have 3.5-billion-year-old rhenium-osmium ages and do not have MIF sulphur8,9. This means that the mantle keel in this region originated in some way that did not incorporate surface material. The sulphur in the Canadian diamonds does not tell us how the mantle keel formed, only how it did not.
Our work shows that sulphide inclusions in diamonds are a powerful tool to investigate craton construction processes.
How did we do this?
To characterize the diamonds and image the inclusions, we first laser cut and polished double-sided plates of the diamonds.
We broke the diamond plates using a small hammer and a steel cracker to release the sulphide inclusions. The inclusions are tiny and weighed between 3 and 162 micrograms. After breaking each inclusion out of the diamond, we imaged it and analyzed its major element composition using a scanning electron microscope.
In a clean laboratory, we then dissolved the sulphide in some acid along with a known amount of tracer solution known as an isotopic spike. Chemical procedures are done to separate rhenium and osmium into different acidic solutions. The solutions that contain rhenium and osmium are each dried down on a hotplate.
The dried-down salt of osmium is placed on a metallic filament, which is then placed in a thermal ionization mass spectrometer (TIMS). The filament is heated to produce ions that are then accelerated through the instrument and the different osmium isotopes are measured on a very sensitive detector.
The dried-down salt of rhenium was taken up into a dilute acid solution. That solution was introduced into a multi-collector inductively coupled mass spectrometer (MC-ICP-MS). The solution was ionized in a plasma and those ions were accelerated through the instrument, where the two different rhenium isotopes were measured simultaneously on very sensitive detectors.
The amount of rhenium and osmium that we measured in the sulphides is in the femtogram to picogram range - where 1 femtogram is 10-15 g or 1 part per quadrillion (ppq) and 1 picogram is 10-12 g or 1 part per trillion (ppt). Because we are measuring such tiny amounts, any small amount of contamination can ruin the sample. This is why it is essential to do this work in a clean laboratory with dedicated supplies for diamond inclusion work.
Rhenium-osmium analyses were done using chemistry labs and a Thermo-Fisher Triton instrument at the Department of Terrestrial Magnetism at the Carnegie Institution for Science.
Sulphide inclusions were not broken out of the diamonds for sulphur isotope measurements. Instead, we polished the diamond plates down further with a diamond scaife to expose the sulphide inclusions. It took a few hours to a day to expose each inclusion.
Diamond plates with the exposed sulphide inclusions were placed in a secondary ion mass spectrometer (SIMS). The samples were bombarded with a cesium ion beam to create small pits in the sulphides. The material released from the sulphides was then accelerated through the instrument and the four different sulphur isotopes were measured simultaneously on very sensitive detectors.
Sulphur isotope measurements were done using a Cameca IMS 1280 instrument at the University of Alberta. Preliminary measurements were done using a Cameca NanoSIMS at the Department of Terrestrial Magnetism at the Carnegie Institution for Science.
About the diamonds themselves
Diamonds for this study are from the Zimmi alluvial deposit in Sierra Leone. The Zimmi mining area is located south of the town Zimmi, near the Libera-Sierra Leone border. The Zimmi locality is known for producing yellow diamonds with abundant sulphide inclusions.
The Zimmi diamonds have nitrogen impurities in the rare isolated form that classifies them as Type Ib diamonds. Type Ib diamonds are exceptionally rare among natural diamonds and account for less than 0.1% of worldwide natural diamonds.
Karen V. Smit has been a research scientist at the Gemological Institute of America (GIA) since 2014. She is interested in using diamonds and their mantle host rocks to understand diamond formation processes in the deep earth. Her research focuses on the origin of the earth’s continents, how plate tectonic processes influence diamond stability, and most recently in using the spectroscopic features of natural diamonds to distinguish them from lab-grown and treated diamonds.
Steven B. Shirey has been a staff member at the Department of Terrestrial Magnetism of the Carnegie Institution for Science for 34 years. Shirey's research focuses on the evolution of Earth's continents, especially as a function of mantle evolution. His interest in diamonds began more than 20 years ago as a way to examine continent formation and plate tectonics from the deepest possible perspective in the mantle.
Erik H. Hauri (now deceased) was a staff member at the Department of Terrestrial Magnetism of the Carnegie Institution for Science for 24 years. He passed away in 2018. Hauri was interested in the geochemical cycles of volatiles on the earth and moon and their relationship to planetary dynamics. Hauri's remarkable expertise in secondary ion mass spectrometry led to breakthroughs in understanding the water content of the moon, and the analysis of stable isotopes in diamonds and their mineral inclusions.
Richard A. Stern is a research scientist and ion probe facility manager at the University of Alberta. His research over the last 25 years has focused on developments and applications of secondary ion mass spectrometry to a broad range of topics in geochemistry and geochronology, most recently emphasizing light stable isotopes.
1. Farquhar et al., 2002
2. Barth et al., 2002
3. Aulbach et al., 2019
4. Pearson et al., 1998
5. Smit et al., 2016
6. Smit et al., 2019
7. Thomassot et al., 2009
8. Westerlund et al., 2006
9. Cartigny et al., 2009
Karen V. Smit is a research scientist at GIA.