Return to Alkaline Rocks
Kimberlites are fascinating rocks derived from small-volume ultramafic magmas enriched in alkalis and volatiles (especially, K and CO2). Most kimberlites show a conspicuously inequigranular texture consisting of large crystals (macrocrysts) and/or rock fragments (xenoliths) enclosed in a much finer-grained matrix (groundmass). There seems little doubt that some of the macrocrysts are actually phenocrysts precipitated from the parental kimberlitic magma at high pressures, while the rest are probably xenocrysts formed by fragmentation of mantle rocks (peridotite, eclogite, dunite, etc.) and transported to shallow crustal levels. The macrocryst suite usually consists of olivine (forsterite), garnet (pyrope), Mg-rich ilmenite, pyroxenes (enstatite and diopside), phlogopite, and Cr-rich spinel-group minerals. Diamonds, which are extracted mostly from kimberlites, are also xenocrysts of mantle provenance. The fine-grained fabric of fresh kimberlite is made up of Mg silicates (olivine, monticellite, serpentine, diopside), calcite, apatite, oxides (spinel-group minerals, perovskite and ilmenite), and pulverized country-rock material. Progressive alteration gradually converts kimberlite into a friable mixture of serpentine, hematite, calcite and clays. Depending on the local geological and tectonic setting, kimberlitic magma may solidify below the surface as dikes and sills, erupt to form a crater filled with volcaniclastic and resedimented volcaniclastic material (and, possibly, also a tuff cone surrounding the crater), or burst explosively through hundreds of meters of crustal rocks producing a steep-sided carrot- or cylinder-shaped body (diatreme) filled with strongly fragmented and jumbled kimberlitic and country-rock material. Despite their seemingly idiosyncratic petrographic characteristics, kimberlites are very difficult to recognize. A cursory examination of hand specimens or thin sections (even if accompanied by whole-rock chemical analysis) would be insufficient because many other rock types (e.g., ultramafic lamprophyres, carbonatites and olivine lamproites) look superficially similar to kimberlites. Geologists involved in diamond exploration use a wide array of analytical tools and methods when searching for these unique and beautiful rocks.
Shown on this page are:
[Upper right] Kimberlite intrusion (greenish gray) in granite (red). Kelsey Lake diamond mine, Colorado;
[Middle right] Volcaniclastic kimberlite (dark gray in the near view) exposed by mining in the A154 pit, Diavik diamond mines (Northwest Territories, Canada); light-gray rocks in the background are Archean granites of the Slave craton;
[Lower right] Small outcrop of macrocryst-rich kimberlite dike near the Iron Mountain, Wyoming; large black spots are ilmenite macrocrysts;
[Upper left] The three major kimberlite facies: hypabyssal kimberlite containing numerous olivine macrocrysts, Lac de Gras kimberlite field, NWT (left); diatreme-facies tuffisitic kimberlite breccia containing abundant xenoliths of sedimentary country-rock, "Triple B" diatreme, Ontario (middle); resedimented volcaniclastic kimberlite (right) with abundant macrocrysts of serpentinized olivine, diopside and pyrope in a clay-rich matrix, Lac de Gras, NWT.
[Lower left] Hypabyssal-facies kimberlite from the Lac de Gras kimberlite field (NWT) as seen under the petrographic microscope in plane-polarized light (left) and crossed polars (right). Note the presence of xenoliths (X) and olivine macrocrysts (M). Xenolith CX is of crustal origin, whereas MX is a fragment of harzburgite from the Earth's upper mantle. The field of view is ~4 mm across.