Diamond Crystals and Their Mineral Inclusions From the Lynx Kimberlite Dyke Complex, Central Quebec

Abstract

Lynx is a 522 Ma kimberlite dyke complex located in the Otish Mountains of central Quebec, Canada. Test sampling of the Lynx kimberlite yielded 6598 macrodiamond samples from 528 t of kimberlite. Single crystals strongly dominate the population, with only 14% of the macrodiamond portion consisting of macles and aggregates. Within the single crystals, 41% are dominated by octahedral surface features, and the remaining ones are partially resorbed tetrahexahedroida. Brown is the most common color among the tetrahexahedroida, with only 22% gray to colorless stones comprising the remainder. In contrast, the octahedra are roughly equal in numbers of brown, gray and colorless stones. A subset of twenty stones was selected in the -11 to +3 DTC (0.256 c to 0.67 c) range of circular sieve sizes on the basis of visible inclusions. These have octahedral primary growth-forms and include three macles and three aggregates of octahedra. Most of the samples are significantly resorbed; they range from octahedra with rounded corners and edges to tetrahexahedroida. Shield laminae, serrate laminae, and hillocks are the most common resorption-related surface features. The stones were cut and polished along single planes to expose mineral inclusions for analysis and to allow imaging of internal structure of the diamond relative to the inclusions. Cathodoluminescence imaging revealed deformation lamellae in most of the polished crystals. Some exhibit deformation lamellae truncated by growth or resorption zones or intersections of different crystallographic planes. Oscillatory patterns of planar growth with complex cores are most common. Inclusions, particularly of olivine, typically occur in core and early growth regions of the diamond crystals. Primary inclusions exposed by polishing are magnesian olivine, chromian diopside, chromian pyropic garnet, magnesian orthopyroxene, omphacite, and sulfide. The chromian diopside inclusions yield equilibration conditions in the range of 58-60 kbar and 1250-1280°C that correspond to conditions slightly below a 42 mW/m 2 surface heat-flow geothermal gradient. The most iron-rich olivine inclusion (mg# = 0.916) occurs with the diopside inclusions, suggesting a relatively fertile lherzolitic component of mantle at 180-190 km depth. The garnet data indicate that within the peridotite parentage, both harzburgitic (three G10, 12.4-13.7 wt.% Cr 2 O 3 , 3.7-4.4 wt.% CaO) and lherzolitic (one G9, 8.9 wt.% Cr 2 O 3 , 5.8 wt.% CaO) parageneses are present.

Pressure and Temperature Data for Diamonds

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