At issue is the discovery of microdiamonds within sedimentary crustal rocks in continental collision zones in Kazakhstan, Norway, China and Germany. The researchers discussed their findings today during the December meeting of the American Geophysical Union in San Francisco. A paper detailing their work recently also was submitted to the science journal Nature.
Based on geophysicists' current understanding of plate tectonics, microdiamonds, which range in size from a mere 1 micron (0.000039 of an inch) to 100 microns, aren't supposed to be found in any crustal rocks. With the exception of alluvial diamonds, which have been released from their original environment by erosion and deposited elsewhere, no diamonds of any size should be in crustal rocks. The Los Alamos/UC Riverside research team believes that the microdiamonds formed deep inside Earth's mantle.
"Most geophysicists disputed and ultimately dismissed the microdiamonds' existence when they first were discovered in Kazakhstan in the mid-1970s," said Kristin Bennett of Los Alamos' Neutron Science Division, where some of the research was performed. "However, other geophysicists have since confirmed the presence of microdiamonds in Kazakhstan and other places around the world. This is a very hot topic in geophysics right now because it could change our understanding of plate tectonics."
In plate tectonics, when two plates of Earth's outer shell collide, the heavier rock gets pushed down (subducted) back into the mantle, the only place where pressures and temperatures are high enough to form diamonds, explained study project leader Larissa Dobrzhinetskaya of UC Riverside. Volcanic eruptions later force diamonds up through the mantle and crust via narrow conduits called kimberlitic pipes.
"The microdiamonds were discovered within crustal rocks, which until recently was considered not possible because the laws of physics hold that they are too light to be subducted," she said.
To explain this geophysical enigma, Dobrzhinetskaya, UC Riverside colleague Harry Green and Los Alamos colleagues Terrence Mitchell and Robert Dickerson studied the mineral inclusions found inside microdiamonds obtained from Kazakhstan. The inclusions serve as an environmental record of the conditions under which the microdiamonds formed.
Their analyses revealed that the microdiamonds have an imperfect, skeletal crystalline structure, much like the shape of a gypsum desert rose commonly found in the Southwest. Conversely, typical diamonds have perfect crystalline structures.
"Their imperfect structure suggests that they most likely formed in an impure environment, meaning drops of fluid oversaturated with carbon and water eventually turned into diamonds in an environment laden with other minerals," Dobrzhinetskaya said. Such minerals included several types of silicates, carbonates and oxides, some of which are found only in crustal rock, she noted.
"The presence of these minerals suggests that continental collisions must have forced at least some of Earth's crustal rock down a subduction zone, where it somehow mixed with the mantle materials, then rose back to the surface over tens of millions of years, which is considered fast in terms of tectonic processes, " she said.
The researchers postulate that the mineral inclusions - and thus the diamonds -- were formed very deep inside the mantle.
"If our future analyses of the mineral inclusions' chemical compositions and crystalline structures confirm this," said Dobrzhinetskaya, "then the theory of plate tectonics will need to be reworked to account for how some crustal rocks managed to make a round trip from Earth's surface deep into the mantle and back, and under what circumstances."
Los Alamos' Center for Materials Science, Institute of Geophysics and Planetary Physics, and Neutron Science Center and UC Riverside collaborated on the project, with support from Los Alamos' Science and Technology Base Programs Office.
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