Diamonds Show Carbon Cycles in Deep Earth

Analysis of six diamonds from the Juina region in Brazil shows that the diamonds' inclusions initially crystallized as single minerals that could only form below 700 km within Earth, but were recrystallized into multiple minerals during their trip to the surface.

diamondA research team led by Michael Walter (University of Bristol, UK) analyzed minute (one to two hundredths of a millimeter) mineral inclusions from six diamonds from the Juina region in Brazil [1]. Their analysis shows that the inclusions initially crystallized as single minerals that could only form below 700 km within Earth. However, they were recrystallized into multiple minerals during their trip to the surface—first probably from a mantle plume, then again as they erupted to the surface in kimberlite.

Four of the diamonds analyzed contained low amounts of light carbon-13, a signature not found in the lower mantle but consistent with an ocean-crust origin at Earth’s surface. “The carbon identified in other super-deep, lower mantle diamonds is chiefly mantle-like in composition,” says co-author Steve Shirey at the Carnegie Institution of Washington where the diamonds were analyzed for carbon.  “We looked at the variations or isotopes of the carbon atoms in these impurities. Carbon originating in the rock called basalt, which forms from lava at the surface, is often different from that which originated in the mantle, which does not contain the lighter compositions. The impurities from these super-deep diamonds contained very light carbon, which is likely to not have formed in the mantle. It is most consistent with an origin in the organic component found in altered oceanic crust.”

Most diamonds don’t have inclusions and come from depths less than 200 km in stiff upper mantle rock beneath the continents. In a few localities, however, researchers have found super-deep diamonds from the convecting upper and lower mantle—extending from about 35 to 2,890 kilometers deep. While inclusions in diamonds from upper mantle and transition zone depths have been consistent with surface rock origin, none to date from the lower mantle have borne the oceanic crust signature found in this study. Although scientists have long believed that the carbon cycle extends into deep Earth, the mantle’s inaccessibility has proved a challenge. Thus, with diamonds serving as Earth’s deep messengers, the current research provides the first direct evidence that slabs of ocean crust are subducted into the lower mantle and that material, including carbon, is cycled between Earth’s surface and its depths.

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