Large and rough diamonds from the Letseng mine in Lesotho could reveal more about the geochemistry of the Earth's deep mantle. | GIA/ Robert Weldon and Gem Diamonds Ltd.
Trapped within large diamonds, slivers of embedded iron surrounded by special gases reveal that the precious gems were formed in liquid metal — a discovery that also confirms metal-saturated conditions deep in the Earth, a new study in the 16 December issue of Science reports.
Traces of metals within diamonds may be considered a defect to a jeweler, but for scientists these inclusions can provide valuable insights into geological processes of the past.
"Inclusions give us a snapshot of the diamond growth environment," explained Evan Smith of Gemological Institute of America. "Not only can this snapshot tell us about how, where and when certain diamonds form, it can tell us about more general features of the rocks in Earth's mantle."
The mantle is the layer that lies between Earth's crust and core, and the majority of diamonds originate along the upper region of this tier. The larger and rarer rough diamonds analyzed in this study — those measuring around a centimeter or more on their longest side — formed deeper within the mantle, taking scientists' understanding of the mantle to new depths.
Attaining diamonds for scientific purposes can be a challenge, however. Large diamonds with metallic inclusions are rare, and when they do have inclusions, the "flawed" parts of the diamond are usually trimmed off or polished away, to maximize the value of a finished gemstone.
Smith and colleagues screened thousands of diamonds submitted daily to the grading operations of the Gemological Institute of America, searching for appropriately sized samples with detectable inclusions. The size of the diamonds varied, most being larger than 10 millimeters but some reaching sizes of 98 millimeters, similar to the Cullinan diamond discovered in South Africa in 1905 and subsequently cut for some of the British Crown Jewels.
Smith said, "In 38 of the 53 diamonds [we analyzed], the metallic inclusions were the only kind of inclusion present. These inclusions are a metallic mixture of iron and nickel, with carbon and sulfur, and a few additional impurities like hydrogen. Inclusions like these, providing samples of metal from Earth's deep mantle, have not been recognized before."
Close-up of a metallic inclusion in a large diamond. The inclusion is silver-colored, and is surrounded by a black, graphite-bearing crack. (Image is 1.99 mm wide) | Evan Smith
The confirmed presence of metallic iron 250 kilometers within the mantle holds important implications for understanding geochemical processes in this deep layer of the Earth, as metallic iron can chemically react with oxygen and limit its availability.
"The inclusions also tell us this iron-nickel metal can readily dissolve carbon, sulfur and other elements such as hydrogen, which is hugely important for their cycling and storage over geologic history," explained Smith.
The nature of the inclusions in these large diamonds also explains why diamond exploration and mining mostly uncovers common varieties of diamonds rather than rare large ones. Historically, geologists have often relied on "indicator minerals" — mineral grains that are associated with the occurrence of diamond — in order to find diamond deposits. Yet, indicator minerals do not correlate with the presence of these especially large and valuable diamonds. Based on this recent analysis, it appears that these large gem diamonds form in a completely different part of the mantle than most common diamonds.
Smith and colleagues will continue to analyze large diamonds, which could still yield a wealth of information. They plan to explore what other traces of elements are present in the inclusions, as well as make measurements of the isotopic composition of some elements, which could reveal exactly how the metal forms and how it interacts with rocks in the mantle. This could provide valuable insights into the deep-Earth cycling of carbon, nitrogen and water.
"It is astonishing that diamonds, the most valuable and prized of all gems, are coincidentally some of the most scientifically valuable samples of the Earth. You couldn't ask for a better vessel than diamond to trap and preserve materials from the Earth's interior," Smith said. "Each diamond has the potential to host real physical samples from a place we cannot go, from a time long since passed."