A 3.8 billion-year-old bedrock
The Shandong diamond deposits are located in the North China Craton (also called the Sino-Korean Craton), one of the oldest, complex blocks of continental crust on Earth. The craton's basement rocks are Precambrian, with some minerals dating back 4.1 billion years and the oldest rocks clocking in at 3.8 billion years old. This immense geological structure covers about 1.5 million square kilometers and finalized its formation around 1.85 billion years ago when several smaller micro-continental blocks collided and fused.
Diamonds form under immense pressure and heat, conditions found deep within the stable, thick lithospheric mantle beneath ancient cratons. The North China Craton once had a thick mantle keel, extending more than 200 km deep, which provided the perfect environment for diamond crystallization. These precious minerals were then transported rapidly to the surface in explosive volcanic eruptions through conduits called kimberlite pipes. The primary diamond deposits in Shandong are found within the Mengyin cluster of kimberlite pipes and dikes, which erupted during the Paleozoic era, around 493 to 383 million years ago.
Trapped messages from the mantle
The diamonds from the Mengyin region are scientific samples. Using Raman spectroscopy on mineral inclusions trapped inside the diamonds, scientists have calculated their formation depths to be between 147 and 176 kilometers. This corresponds to pressures between 4.45 and 5.35 gigapascals (GPa).
These inclusions are tiny specks of other minerals that were encapsulated as the diamond crystal grew. They provide direct samples of the Earth's mantle at these extreme depths. Geologists classify these inclusions into two main types: peridotitic and eclogitic. Peridotitic inclusions, like olivine, pyrope garnet, and diopside, are typical of the upper mantle. Eclogitic inclusions, such as coesite (a high-pressure form of quartz), suggest a different origin story. The presence of coesite provides strong evidence for the subduction of ancient oceanic crust deep into the mantle. This process recycled surface materials, introducing unique chemical ingredients that influenced diamond formation. The mix of both peridotite-type and eclogite-type inclusions shows that the mantle beneath the North China Craton was a complex environment, altered by these deep geological processes.
A peculiar feature of the North China Craton is that its deep, stable root was largely destroyed during the Mesozoic era, thinning from around 200 km to less than 80 km. This "decratonization" was likely caused by the subduction of the Pacific oceanic plate beneath the continent, which hydrated and weakened the lithosphere. The Shandong diamonds, having formed before this event, show the composition and structure of the craton's original, deep mantle keel.