[meteorite-list] Study Hints that Ancient Earth Made Its Own Water - Geologically

[Ron Baalke]

http://news.osu.edu/news/2014/12/17/study-hints-that-ancient-earth-made-its-own-water%E2%80%94geologically/

Study Hints that Ancient Earth Made Its Own Water - Geologically

Evidence that rock circulating in the mantle feeds world's oceans even 
today

Pam Frost Gorder
Ohio State University
December 17, 2014

SAN FRANCISCO - A new study is helping to answer a longstanding question 
that has recently moved to the forefront of earth science: Did our planet 
make its own water through geologic processes, or did water come to us 
via icy comets from the far reaches of the solar system?

The answer is likely "both," according to researchers at The Ohio State 
University -  and the same amount of water that currently fills the Pacific 
Ocean could be buried deep inside the planet right now.

At the American Geophysical Union (AGU) meeting on Wednesday, Dec. 17, 
they report the discovery of a previously unknown geochemical pathway 
by which the Earth can sequester water in its interior for billions of 
years and still release small amounts to the surface via plate tectonics, 
feeding our oceans from within.

In trying to understand the formation of the early Earth, some researchers 
have suggested that the planet was dry and inhospitable to life until 
icy comets pelted the earth and deposited water on the surface.

Wendy Panero, associate professor of earth sciences at Ohio State, and 
doctoral student Jeff Pigott are pursuing a different hypothesis: that 
Earth was formed with entire oceans of water in its interior, and has 
been continuously supplying water to the surface via plate tectonics ever 
since.

Researchers have long accepted that the mantle contains some water, but 
how much water is a mystery. And, if some geological mechanism has been 
supplying water to the surface all this time, wouldn't the mantle have 
run out of water by now?

Because there's no way to directly study deep mantle rocks, Panero and 
Pigott are probing the question with high-pressure physics experiments 
and computer calculations.

"When we look into the origins of water on Earth, what we're really asking 
is, why are we so different than all the other planets?" Panero said. 
"In this solar system, Earth is unique because we have liquid water on 
the surface. We're also the only planet with active plate tectonics. Maybe 
this water in the mantle is key to plate tectonics, and that's part of 
what makes Earth habitable."

Central to the study is the idea that rocks that appear dry to the human 
eye can actually contain water - in the form of hydrogen atoms trapped inside 
natural voids and crystal defects. Oxygen is plentiful in minerals, so 
when a mineral contains some hydrogen, certain chemical reactions can 
free the hydrogen to bond with the oxygen and make water.

Stray atoms of hydrogen could make up only a tiny fraction of mantle rock, 
the researchers explained. Given that the mantle is more than 80 percent 
of the planet's total volume, however, those stray atoms add up to a lot 
of potential water.

In a lab at Ohio State, the researchers compress different minerals that 
are common to the mantle and subject them to high pressures and temperatures 
using a diamond anvil cell - a device that squeezes a tiny sample of material 
between two diamonds and heats it with a laser - to simulate conditions 
in the deep Earth. They examine how the minerals' crystal structures change 
as they are compressed, and use that information to gauge the minerals' 
relative capacities for storing hydrogen. Then, they extend their experimental 
results using computer calculations to uncover the geochemical processes 
that would enable these minerals to rise through the mantle to the surface - a 
necessary condition for water to escape into the oceans.

[Graphic]
This plate tectonics diagram from the Byrd Polar and Climate Research 
Center shows how mantle circulation delivers new rock to the crust via 
mid-ocean ridges. New research suggests that mantle circulation also delivers 
water to the oceans.

In a paper now submitted to a peer-reviewed academic journal, they reported 
their recent tests of the mineral bridgmanite, a high-pressure form of 
olivine. While bridgmanite is the most abundant mineral in the lower mantle, 
they found that it contains too little hydrogen to play an important role 
in Earth's water supply.

Another research group recently found that ringwoodite, another form of 
olivine, does contain enough hydrogen to make it a good candidate for 
deep-earth water storage. So Panero and Pigott focused their study on 
the depth where ringwoodite is found - a place 325-500 miles below the surface 
that researchers call the 'transition zone" - as the most likely region 
that can hold a planet's worth of water. From there, the same convection 
of mantle rock that produces plate tectonics could carry the water to 
the surface.

One problem: If all the water in ringwoodite is continually drained to 
the surface via plate tectonics, how could the planet hold any in reserve?

For the research presented at AGU, Panero and Pigott performed new computer 
calculations of the geochemistry in the lowest portion of the mantle, 
some 500 miles deep and more. There, another mineral, garnet, emerged 
as a likely water-carrier - a go-between that could deliver some of the 
water from ringwoodite down into the otherwise dry lower mantle.

If this scenario is accurate, the Earth may today hold half as much water 
in its depths as is currently flowing in oceans on the surface, Panero 
said—an amount that would approximately equal the volume of the Pacific 
Ocean. This water is continuously cycled through the transition zone as 
a result of plate tectonics.

"One way to look at this research is that we're putting constraints on 
the amount of water that could be down there," Pigott added.

Panero called the complex relationship between plate tectonics and surface 
water "one of the great mysteries in the geosciences." But this new study 
supports researchers' growing suspicion that mantle convection somehow 
regulates the amount of water in the oceans. It also vastly expands the 
timeline for Earth's water cycle.

"If all of the Earth's water is on the surface, that gives us one interpretation 
of the water cycle, where we can think of water cycling from oceans into 
the atmosphere and into the groundwater over millions of years," she said. 
"But if mantle circulation is also part of the water cycle, the total 
cycle time for our planet's water has to be billions of years."