Blue (type IIb) diamonds owe their color to boron, an element abundant in the Earth’s continental and oceanic crust. According to new research led by the Gemological Institute of America, these diamonds formed up to four times deeper in the Earth’s mantle than most other diamonds.
A blue, boron-bearing diamond with dark inclusions of a mineral called ferropericlase, which were examined as part of this study. This gem weighs 0.03 carats. Image credit: Evan Smith / Gemological Institute of America.
“Type IIb diamonds are tremendously valuable, making them hard to get access to for scientific research purposes, and it is very rare to find one that contains inclusions, which are tiny mineral crystals trapped inside the diamond,” said Dr. Evan Smith, lead author of the study.
The team’s analysis of the trapped mineral grains in 46 blue diamonds indicates that they crystallized in rocks that only exist under the extreme pressure and temperature conditions of the Earth’s lower mantle.
The researchers determined that blue diamonds form at least as deep as the transition zone between the upper and lower mantle — or between 255 and 410 miles (410-660 km) below the surface.
Several of the samples even showed clear evidence that they came from deeper than 410 miles, meaning they originated in the lower mantle. By contrast, most other gem diamonds come up from between 93 and 124 miles (150-200 km).
“So how did the boron get down there if it is an element known for residing predominately in the shallow crust? According to our hypothesis, it came from seafloor that was conveyed down into the Earth’s mantle when one tectonic plate slid beneath another — a process known as subduction,” the scientists said.
“Boron from the Earth’s surface was incorporated into water-rich minerals like serpentine, which crystallized during geochemical reactions between seawater and the rocks of the oceanic plate.”
“This reaction between rock and water is a process called serpentinization and can extend deep into the seafloor, even into the oceanic plate’s mantle portion.”
The team’s discovery reveals that the water-bearing minerals travel far deeper into the mantle than previously thought, which indicates the possibility of a super-deep hydrological cycle.
“Most previous studies of super-deep diamonds had been carried out on diamonds of low quality,” said co-author Dr. Steven Shirey, from the Carnegie Institution for Science.
“But between our recent finding that the world’s biggest and most-valuable colorless diamonds formed from metallic liquid deep inside Earth’s mantle and this new discovery that blue diamonds also have super-deep origins, we now know that the finest gem-quality diamonds come from the farthest down in our planet.”
The study appears in the journal Nature.
_____
Evan M. Smith et al. 2018. Blue boron-bearing diamonds from Earth’s lower mantle. Nature 560: 84-87; doi: 10.1038/s41586-018-0334-5
Source link: https://www.sci.news/geology/blue-boron-bearing-diamonds-earths-lower-mantle-06281.html
