Diamond Geology

Type Ib–Dominant Mixed-Type Diamond with Cuboctahedral Growth Structure: A Rare Diamond Formation

Kyaw Soe Moe and Paul Johnson
GIA, New York

Type Ib–dominant mixed-type diamonds (Ib-IaA) can be formed by multiple growth events (Titkov et al., 2015; Smit et al., 2018). In this study, we report on a 0.41 ct Fancy Dark brown gem­quality diamond that formed in a single growth event. It is a type Ib-IaA with a C defect (single-substitutional nitrogen atom) concentration up to 21 ppm. The Fourier-transform infrared (FTIR) peaks of the H1a and H1b defects (figure 1, left) suggest that this diamond was irradiated and annealed to achieve a Fancy color grade. The cuboctahedral structure can be observed in the DiamondView images (figure 1, right), which show reddish orange fluorescence in the {100} sector caused by NV centers and green fluorescence in the {111} sector caused by H3 defects. Irradiation treatment helps us to see the cuboctahedral structure. However, the presence of an amber center suggests that it did not undergo HPHT treatment.

Cuboctahedral growth structure can be usually observed in HPHT diamonds that are grown in a laboratory with a fast growth rate (i.e., less than one week per carat) under P-T conditions of 5–6 GPa and 1300–1500°C. A natural diamond that grows rapidly in a single growth event would also possess mixed cuboid and octahedral forms.

Time and temperature are critical in the aggregation of nitrogen atoms in diamond. Nitrogen atoms are initially incorporated as single atoms during diamond formation. Our sample was rich in C defects associated with A defects (a pair of nitrogen atoms). This suggests that the process of nitrogen aggregation was interrupted by the diamond’s ascent to shallower depths in the mantle at lower temperatures by rapid tectonic exhumation (Smit et al., 2018). The cuboctahedral growth structure of this diamond indicated that, prior to ascending to shallower depths of the mantle, it grew in a few days in the diamond stability field (i.e., 150–200 km in depth within the earth) in a single growth event.

REFERENCES

Smit K.V., D’Haenens-Johansson U.F.S., Howell D., Loudin L.C., Wang W. (2018) Deformation-related spectroscopic features in natural type Ib-IaA diamonds from Zimmi (West African craton). Mineralogy and Petrology, pp. 1–15, http://dx.doi.org/10.1007/s00710-018-0587-6

Titkov S.V., Shiryaev A.A., Zudina N.N., Zudin N.G., Solodova Y.P. (2015) Defects in cubic diamonds from the placers in the northeastern Siberian platform: Results of IR microspectrometry. Russian Geology and Geophysics, Vol. 56, No. 1-2, pp. 354–362, http://dx.doi.org/10.1016/j.rgg.2015.01.026

Evidence of Subducted Altered Oceanic Crust into Deep Mantle from Inclusions of Type IaB Diamonds

Tingting Gu¹, John Valley², Kouki Kitajima², Michael Spicuzza², John Fournelle², Richard Stern³, Hiroaki Ohfuji⁴, and Wuyi Wang^1
1GIA, New York
²Department of Geoscience, University of Wisconsin, Madison
³Canadian Centre for Isotopic Microanalysis (CCIM), Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton
⁴Geodynamics Research Center (GRC), Ehime University, Matsuyama, Japan

Nitrogen is one of the most common impurities in diamond, and its aggregation styles have been used as criteria for diamond classification. Pure type IaB diamonds (with 100% nitrogen in B aggregation) are rather rare among natural diamonds. The occurrence of the B center is generally associated with high temperature and a long residence time of the host diamond, which would potentially provide information on the earth’s deep interior. Seawater circulation is the unique process that shapes the surface of our planet and potentially has a profound effect on its interior due to slab subduction.

In about 50 type IaB diamonds with detectable micro-inclusions submitted to GIA for screening, we found that more than 70% of them contained a typical mineral assemblage from the sublithosphere. Jeffbenite (TAPP), majorite garnet, enstatite, and ferropericlase have been observed, which could be retrograde products of former bridgmanite. CaSiO₃-walstromite with larnite and titanite is the dominant phase present in approximately 40% of all diamond samples. Direct evidence from oxygen isotope ratios measured by secondary ion mass spectrometry, or SIMS, (δ¹⁸O_VSMOW in the range +10.7 to +12.5‰) of CaSiO₃-walstromite with coexisting larnite and titanite that retrograde from CaSiO₃-perovskite suggest that hydrothermally altered oceanic basalt can subduct to depths of >410 km in the transition zone. Incorporation of materials from subducted altered oceanic crust into the deep mantle produced diamond inclusions that have both lower mantle and subduction signatures. Ca(Si,Al)O₃-perovskite was observed with a high concentration of rare earth elements (>5 wt.%) that could be enriched under P-T conditions in the lower mantle. Evidence from ringwoodite with a hydroxide bond, coexisting tuite and apatite, precipitates of an NH₃ phase, and cohenite with trace amounts of Cl imply that the subducted brines can potentially introduce hydrous fluid to the bottom of the transition zone. In the diamonds with subducted materials, the increasing carbon isotope ratio from the core to the rim region detected by SIMS (δ¹³C from –5.5‰ to –4‰) suggests that an oxidized carbonate-dominated fluid was associated with recycling of the subducted hydrous material. The deep subduction played an important role in balancing redox exchange with the reduced lower mantle indicated by precipitated iron nanoparticles and coexisting hydrocarbons and carbonate phases.

Origin of Rare Fancy Yellow Diamonds from Zimmi (West Africa)

Karen V. Smit¹, Ulrika F.S. D’Haenens-Johansson¹, Daniel Howell², Lorne C. Loudin¹, and Wuyi Wang^1
1GIA, New York
²Department of Geoscience, University of Padua, Italy, and Diamond Durability Laboratory, New York

Type Ib diamonds from Zimmi, Sierra Leone, have 500 My mantle residency times whose origin is best explained by rapid tectonic exhumation after continental collision to shallower depths in the mantle prior to kimberlite eruption (Smit et al., 2016). Here we present spectroscopic data for a new suite of Zimmi sulfide-bearing type Ib diamonds that allow us to evaluate the link between their rare Fancy yellow colors, the distribution of their spectroscopic features, and their unusual geological history. Cathodoluminesence (CL) imaging revealed irregular patterns with abundant deformation lamellae, associated with the diamonds’ tectonic exhumation (Smit et al., 2018). Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV^0/− centers, and H3 (NVN⁰). The brownish yellow to greenish yellow colors observed in Zimmi type Ib diamonds result from visible absorption by a combination of isolated nitrogen and deformation-related vacancy clusters (Smit et al., 2018). Color-forming centers and other spectroscopic features can all be attributed to the unique geological history of Zimmi type Ib diamonds and their rapid exhumation after formation.

REFERENCES

Smit K.V., Shirey S.B., Wang W. (2016) Type Ib diamond formation and preservation in the West African lithospheric mantle: Re-Os age constraints from sulphide inclusions in Zimmi diamonds. Precambrian Research, Vol. 286, pp. 152–166, http://dx.doi.org/10.1016/j.precamres.2016.09.022

Smit K.V., D’Haenens-Johansson U.F.S., Howell D., Loudin L.C., Wang W. (2018) Deformation-related spectroscopic features in natural type Ib-IaA diamonds from Zimmi (West African craton). Mineralogy and Petrology, pp. 1–15, http://dx.doi.org/10.1007/s00710-018-0587-6