A team led by DCO member and experimental petrologist Yingwei Fei (Carnegie Institution of Washington, USA) and field geologist Cheng Xu (Peking University, China) found that a rare sample of the garnet mineral majorite, discovered in North China, formed at least 370km (235 miles) below Earth’s surface [1]. Fellow DCO member and Peking University colleague Renbiao Tao also contributed to the study. They published their findings in Science Advances.
The majorite sample Fei’s team found in Northern China had several unusual characteristics, which prompted them to investigate its origins. It was encased inside a regular garnet, like mineralogical nesting dolls. The majorite also contained ferric iron, an oxidized form of iron, which is highly unusual for the mineral. The North China Craton, one of the oldest cratonic blocks in the world, had carried it to the surface.
The researchers used several different kinds of analytical techniques to determine the chemistry and structural characteristics of the majorite. To determine its exact depth of origin, Tao conducted high-pressure experiments mimicking the mineral’s formation conditions. The team pinpointed its origin to a depth of nearly 400 kilometers (250 miles), at the bottom of the soft part of the upper mantle, called the asthenosphere, which drives plate tectonics.
High-pressure majorite rarely survives transportation from such a depth. Strangely, the garnet that encased it formed at a much shallower depth, at about 200 kilometers deep (125 miles). The nesting doll sample formed from two separate geological events, which created a time capsule the researchers could use to better understand Earth’s deep history.
“This two-stage formation process offers us important clues about the mantle’s evolutionary stage at the time when the majorite first formed,” Fei explained.
The sample’s location and depth of origin indicate that it is a relic from the end of an era of supercontinent assembly that took place about 1.8 billion years ago. Called Columbia, the supercontinent’s formation built mountain ranges that persist today.
“More research is needed to understand how the majorite became so oxidized, or rich in ferric iron, and what this information can tell us about mantle chemistry. We are going back to the site this summer to dig deeper trenches and hope to find fresh rocks that contain more clues to the deep mantle,” Fei added.
Article adapted from materials provided by the Carnegie Institution for Science.
Image: The fragment of the metamorphic rock found in Northern China that contained the garnet with the majorite encased within. Credit: Image courtesy of Yingwei Fei and Science Advances.
