Diamonds and Their Mineral Inclusions, and What They Tell Us: A Detailed “Pull-Apart” of a Diamondiferous Eclogite

For the first time, three-dimensional, high-resolution X-ray computed tomography (HRXCT) of an eclogite xenolith from Yakutia has successfully imaged diamonds and their textural relationships with coexisting minerals. Thirty (30) macrodiamonds (greater than or equal to1 mm), with a total weight of just over 3 carats, for an ore grade of some 27,000 ct/ton, were found in a small (4 x 5 x 6 cm) eclogite, U51/3, from Udachnaya, Based upon 3-D imaging, the diamonds appear to be associated with zones of secondary alteration of clinopyroxene (Cpx) in the xenolith, The presence of diamonds with secondary minerals strongly suggests that the diamonds formed after the eclogite, in conjunction with metasomatic input(s) of carbon-rich fluids. Metasomatic processes are also indicated by the nonsystematic variations in Cpx inclusion chemistry in the several diamonds. The inclusions in the diamonds vary considerably in major- and trace-element chemistry within and between diamonds, and do not correspond to the minerals of the host eclogite, whose compositions are extremely homogeneous. Some Cpx inclusions possess +Eu anomalies, probably inherited from their crustal source rocks. The only consistent feature for the Cpx crystals in the inclusions is that they have higher K2O than the Cpx grains in the host. The delta C-13 compositions are relatively constant at -5 parts per thousand both within and between diamonds, whereas delta N-15 values vary from -2.8 parts per thousand to -15.8 parts per thousand. Within a diamond, the total N varies considerably from 15 to 285 ppm in one diamond to 103 to 1250 ppm in another. Cathodoluminescent imaging reveals extremely contorted zonations and complex growth histories in the diamonds, indicating large variations in growth environments for each diamond. This study directly bears on the concept of diamond inclusions as time capsules for investigating the mantle of the Earth. If diamonds and their inclusions can vary so much within this one small xenolith, the significance of their compositions is a serious question that must be addressed in all diamond-inclusion endeavors.