George D. Guthrie Jr.
Geology/Geochemistry, MS D469, Los Alamos
Los Alamos, NM 87545, USA
David R. Veblen
Department of Earth and Planetary Sciences, The
Johns Hopkins University
Baltimore, MD 21218, USA
Oded Navon and George R. Rossman
Department of Geological and Planetary
Pasadena, CA 91125-2500, USA
Transmission and analytical electron microscopies were used to characterize the turbid coats on two diamonds from Zaire. The coats contained euhedral cavities (generally < 0.5 mm) that are believed to represent decrepit fluid inclusions. Crystals (generally < 0.2 mm) were sometimes found in the cavities, but they were never observed to fill the cavities entirely. Cavities that appeared to be intact typically contained several solid inclusions and an amorphous material with a low average atomic weight. The crystals in such cavities were able to move under a condensed electron beam, suggesting that the amorphous material was a liquid and not a glass.
Using compositional analysis and electron diffraction, five minerals were identified as daughter crystals in the cavities: apatite, high-Ca carbonate, low-Ca carbonate, mica, and quartz. Coesite and olivine were not observed in any of the cavities. Compositional analysis of some crystals indicated that other minerals (e.g., amphibole) were present as daughter crystals; however, electron diffraction data were insufficient to identify them unambiguously.
Since these inclusions are believed to have been trapped during the growth of the diamond coats, it may be possible to constrain the environment under which the coats grew, assuming that the daughter minerals precipitated from the trapped fluid and that the fluid inclusions have not re-equilibrated. Coexisting magnesite-like and dolomite-like carbonates and silica constrain XCO2 of the fluid to greater than 0.4. The presence of quartz is consistent with the coats developing at lower pressures and temperatures than the cores they surround; alternatively, quartz grew from a glass or a high-F, high-T silica polymorph (coesite) when the inclusions re-equilibrated in the quartz stability field.
Earth and Planetary Science Letters, 105 (1991) 1-12