Fluid Inclusions in Fibrous Diamonds

Abstract

Giardini (1974, 1975) reported the composition of gases released by crushing diamonds under vacuum. Gas signals of transparent or light-yellow diamonds were similar to the system blank levels (0.2-2 × 10 −5 cm 3 STP, for an empty crushing chamber at 0 °C and 1 bar). However, the crushing of cubic and translucent diamonds at 200 °C yielded gas volumes that were an order of magnitude larger. The main gases released were H 2 O and CO 2 . Gas released by graphitization of diamonds contained mostly CO with minor H 2 and the quantities were >10 −2 cm 3 STP (Kaiser and Bond 1959;Melton and Giardini 1976), but the H 2 may be attributed to a reaction of water during graphitization (Fesq et al. 1975).Early attempts to determine the nature of the material trapped in the microinclusions by X-ray diffraction and X-ray fluorescence were not conclusive (Lonsdale and Milledge 1965;Seal 1966Seal , 1970Harris 1968). Prinz et al. (1975) found "fluffy, filamentous material" in two diamonds from the Democratic Republic of the Congo (DRC), but did not describe the diamonds. The composition obtained by electron probe microanalysis (EPMA) of this material was SiO 2 67 wt%; TiO 2 2.5 wt%; Al 2 O 3 14-17 wt%; FeO 2-3 wt%; MgO 1 wt%; Na 2 O 0.2-2.5 wt% and K 2 O 8.5-10 wt%. As mentioned below, this composition agrees roughly with that of later studies of microinclusion-bearing fibrous diamonds. Fesq et al. (1975) used instrumental neutron activation analysis (INAA) to investigate the nature of impurities in diamonds. They examined groups of diamonds (~10-20 crystals in each sample) with mineral inclusions as well as diamonds with no visible inclusions. They concluded that in addition to mineral inclusions, the diamonds also carried picritic (high-Mg basaltic) melt associated with an H 2 O + CO 2 rich component or phase that carried incompatible trace elements. The presence of a sulfide-rich phase was inferred from the correlation between Fe, Ni, Cu and Co. These impurities were found in diamonds with and without visible inclusions.INAA of coated diamonds was attempted by Kodochigov and co-workers (according to Orlov 1977) and Glazunov et al. (1967). Bibby (1979) used INAA to determine the traceelement composition of a coated diamond. He found low levels in the core, but the coat carried up to 50 ppm Fe and a few ppm of Na, K, Ba and Ce. Based on the good intercorrelation of K, Na, Ba, and the REE and their poor correlation with Sc, Cr, and Mn, he suggested the presence of carbonate microinclusions.

Mineral Inclusions in Lithospheric Diamonds

“…These authors consider that during percolation of such saline high-pressure fluids, Cr could dissolve from the peridotitic wall rocks without significantly affecting their high Cr concentrations. The ability of such saline fluids to impose their high-Cr characteristics on garnet inclusions in diamond, however, is likely limited, given that their mean Cr# (100Cr/(Cr+Al)) is only 7.5 (high-density fluid compilation of Weiss et al 2022) compared to Cr# of 38-65 for the unusually Cr 2 O 3 -rich (> 13 wt%) garnet inclusions discussed here. In contrast, enrichment of Al in cratonic peridotites is frequently observed and possibly relates to fluids derived from recycled sediments (e.g., Rapp et al 2017).…”

“…This is because the four, six or twelve sided cavities of 1 to 5 µm size contained no solid material when broken or polished through. Subsequent studies have shown this not to be the case (Weiss et al 2022). Clouds range in color from white to mid-grey with an overall shape that can be irregular but is more often distinctly cuboid or star-shaped and visually aligned with the shape of the diamond.…”

“…The opposite is true for potassic metasomatism documented in clinopyroxene, where only fairly moderate levels of enrichment are seen in xenoliths (K 2 O ≤ 0.26 wt%) whilst clinopyroxene inclusions show a tail in their frequency distribution of up to 1.68 wt% K 2 O. This type of metasomatism may relate to potassic brines that are frequently observed as highdensity fluid inclusions in fibrous diamonds (Izraeli et al 2001;Weiss et al 2022). That evidence for such metasomatism is best preserved inside diamond may relate to reactivation of potassic mantle metasomes and migration to shallower mantle in the course of transient heating events (e.g., Waters and Erlank 1988;Tappe et al 2008).…”

“…In conclusion, (1) the redox state recorded in mantle xenoliths from the main portion of the lithospheric diamond stability field falls at or near the water maximum, (2) H 2 O is the only CHO fluid species with a sufficiently low dihedral angle with olivine to readily percolate along grain boundaries and (3) aqueous fluids also are the only CHO species with the ability to dissolve incompatible trace elements in the required quantities. The high load of dissolved solids carried by aqueous fluids at the pressure-temperature conditions inside the diamond stability field justifies using the term "high-density fluid" (HDF; Klein-BenDavid et al 2007;Weiss et al 2022) instead. Research on primary fluid inclusions in fibrous, clouded and coated diamonds indicates that for peridotitic substrates, hydrous HDFs generally range from potassium-rich brines to Mg-carbonatitic, while in eclogitic substrates an evolution from saline through low-Mg carbonatitic to silicic HDFs is observed (Weiss et al 2015(Weiss et al , 2022.…”

“…The high load of dissolved solids carried by aqueous fluids at the pressure-temperature conditions inside the diamond stability field justifies using the term "high-density fluid" (HDF; Klein-BenDavid et al 2007;Weiss et al 2022) instead. Research on primary fluid inclusions in fibrous, clouded and coated diamonds indicates that for peridotitic substrates, hydrous HDFs generally range from potassium-rich brines to Mg-carbonatitic, while in eclogitic substrates an evolution from saline through low-Mg carbonatitic to silicic HDFs is observed (Weiss et al 2015(Weiss et al , 2022. The carbonatitic and silicic inclusions are transitional between hydrous HDFs and hydrous melts.…”