'Nesting doll' minerals offer clues to Earth's mantle dynamics

Science Daily ^ | 4/7/2017 | Carnegie Institution for Science

[JimSEA]

Recovered minerals that originated in the deep mantle can give scientists a rare glimpse into the dynamic processes occurring deep inside of Earth and into the history of the planet's mantle layer. A team led by Yingwei Fei, a Carnegie experimental petrologist, and Cheng Xu, a field geologist from Peking University, has discovered that a rare sample of the mineral majorite originated at least 235 miles below Earth's surface. Their findings are published by Science Advances.

Majorite is a type of garnet formed only at depths greater than 100 miles. Fascinatingly, the majorite sample Fei's team found in Northern China was encased inside a regular garnet -- like mineralogical nesting dolls. It was brought to surface as an eclogite xenolith in the North China Craton, one of the oldest cratonic blocks in the world. What's more, the majorite was rich in ferric iron, an oxidized form of iron, which is highly unusual for the mineral.

All of these uncommon factors prompted the team to investigate the majorite's origins.

They used several different kinds of analytical techniques to determine the chemistry and structural characteristics of this majorite formed deep inside Earth. In order to determine the exact depth of its origin, Carnegie's postdoc Renbiao Tao conducted high-pressure experiments that mimicked the formation conditions of natural majorite. The team pinpointed its origin to a depth of nearly 250 miles (400 kilometers), at the bottom of the soft part of the upper mantle, called the asthenosphere, which drives plate tectonics.

[JimSEA]

Eclogite Xenolith formed 235 miles plus beneath the crust in the mantle. The red spots are a type of garnet.

It is extremely unusual that a high-pressure majorite could survive transportation from such a depth. Adding to the strange circumstances is the fact that it was later encased by a garnet that formed at a much shallower depth of about 125 miles (200 kilometers). The nesting-doll sample's existence required two separate geological events to explain, and these events created a time capsule that 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 was 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.

[JimSEA]

Xenoliths are bits of older rock that are caught up in magma that erupts to the surface.

A xenolith of schist caught up in pink granite.

[Rebelbase]

Strange rock.

[aquila48]

Bookmark

[Axenolith]

Yes?

[JimSEA]

White Mountain /Death Valley/ Numerous mines / Inyo Mine / Grand View Mine (https://youtu.be/OmLw8KPnMFA)

photo of the Empire near Grass Valley well north of Death Valley but where my Dad worked in the 30s. The main incline shaft. Comparatively large operation. California has a fascinating mis mash of old mines

[Axenolith]

THAT is a cool pic.

Is the Idaho-Maryland still moving forward up there or did it putter out w/1100-1200 dollar gold?

You know the Grandview got all grated over last I was up there, was bummed, I wanted to get a gas meter and check it out some.

[JimSEA]

As far as I know, the Idaho Maryland is closed but being “looked at” by a small Canadian company. Any reopening seems very doubtful.

[T-Bone Texan]

I explored an old mine in Death Valley. It had not been barred off.

It was amazing and scary what our ancestors did.

[Axenolith]

They drilled the hell out of it, last time I talked to one of the companies principles they were held up by NEPA/CEQA crap. The “review” time was going to be 4 years plus (which is total bull****).

The water in that particular mine is clean enough for direct discharge to Wolf Creek under NPDES permit with little to no treatment.

There’s still a lot of gold in there (as is the case with most of the old CA mines. Companies plan was to put a 5000 foot deep spiral decline in and expand the resource via drilling during operations.

[JimSEA]

The governments, state and local, are blocking a lot of mine projects. BHP and Rio Tinto, through Resolution Copper is trying to open a massive porphyry copper deposit in Arizona. Not much luck with environmentalists, Native Americans opposing the project. My interest is in my father and I worked for Magma Copper at this Location in Superior, AZ and another Magma mine at San Manuel, AZ. The Resolution project is basically below the mill fault and the old Magma Mine. They are at a mile plus depth and want to block cave the deposit. (A short history of the old mine: https://arizonadailyindependent.com/2015/07/19/history-of-the-magma-mine-superior-arizona/ )

San Manuel Mine lunch hole in my time - no I'm not in the photo but a couple of friends and children of friends are there.

[JimSEA]

If you have any interest in the Magma Mine: AZGS - http://repository.azgs.az.gov/uri_gin/azgs/dlio/1277