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Ice flow of the last glaciation in the Buffalo Head Hills kimberlite field of northern Alberta is reconstructed from landform interpretations and clast orientations for the purpose of aiding kimberlite exploration in the region. The paucity of bedrock outcrop and the absence of preserved striae and other erosional ice-flow indicators on the soft Cretaceous marine sediments inhibit detailed interpretations on glacial flow chronology. Poorly developed bedrock drumlins on the Buffalo Head Hills and erosional ice-flow indicators preserved on the kimberlite outcrops indicate southwestward ice flow during the maximum extent of ice during the last glaciation. During the deglaciation of northern Alberta, later phases of ice flow were controlled by lobes of surging ice, which surged into proglacial lakes. West of the Buffalo Head Hills, the maximum phase of southwest flow was followed by southeastward ice movement of the Peace River ice lobe. Similarly, east of the Buffalo Head Hills, the maximum phase of ice flow was superceded by a south-southwest ice advance of the Wasbasca ice lobe. Dans le but d'aider l'exploration pour de la kimberlite dans la region, l'ecoulement glaciaire dans le champ de kimberlite des collines Buffalo Head du nord de l'Alberta est reconstruit a partir d'interpretations des formes de terrain et de l'orientation des clastes. Le peu d'affleurements de roches du socle et l'absence de preservation de stries et d'autres indicateurs formes par l'erosion glaciaire sur les tendres sediments marins du Cretace nuisent a l'interpretation detaillee de la chronologie de l'ecoulement glaciaire. Des drumlins mal developpes sur le socle des collines Buffalo Head et les indicateurs d'erosion glaciaire preserves sur les affleurements de kimberlite indiquent un ecoulement glaciaire vers le sud-ouest durant l'etendue glaciaire maximale de la derniere glaciation. Durant la deglaciation du nord de l'Alberta, des phases plus tardives d'ecoulement glaciaire etaient controlees par des lobes de glace en crue; ces lobes avancaient et reculaient dans des lacs proglaciaires. A l'ouest des collines Buffalo Head, la phase maximale d'ecoulement vers le sud-ouest a ete suivie d'un mouvement du lobe glaciaire Peace River vers le sud-est. De meme, a l'est des collines Buffalo Head, la phase maximale d'ecoulement glaciaire a ete supplantee par une avancee du lobe de glace Wasbasca vers le sud-sud-ouest. [Traduit par le Redaction], Introduction Surficial geology mapping and regional ice-flow studies in the Buffalo Head Hills, northern Alberta, was recently conducted by the Alberta Geological Survey (AGS) in collaboration with the Geological Survey [...]

Magmas of all compositions, including kimberlites, may undergo both magmatic and phreatomagmatic fragmentation during emplacement. In this contribution we assess the extent of phreatomagmatism at different stages of the pipe formation for the Victor Northwest kimberlite (northern Ontario, Canada). Detailed drill core and petrographic observations of all volcanic facies within the pipe suggest emplacement in two volcanic cycles, each consisting of three repeated eruptive phases. Pyroclastic kimberlite formed at the start of both cycles is characterized by the presence of fine-grained, poorly sorted deposits containing broken olivine crystals, angular country rock fragments, accretionary lapilli, and variably vesicular irregular-shaped juvenile pyroclasts. All observations indicate these deposits formed by phreatomagmatism. Subsequent clastogenic coherent kimberlite deposits were formed as a result of Hawaiian fire-fountaining during the second phase of both cycles. These phases were followed by mass wasting into the partly filled crater. The evolution from phreatomagmatic to magmatic fragmentation is commonplace in volcanic systems, and our study provides the first indication that the evolution from a phreatomagmatic to magmatic eruption style is also present in kimberlite volcanoes, suggesting kimberlite volcanism is not significantly different from more common basaltic to rhyolitic systems. In addition, this research indicates that high fragmentation intensities recorded by the presence of broken olivines, accretionary lapilli, abundant (angular) country rock clasts, and retention of ash are important textural criteria for recognition of phreatomagmatic kimberlite deposits. Des magmas de toutes compositions, incluant les kimberlites, peuvent etre sujets a une fragmentation magmatique et phreatomagmatique durant leur mise en place. Dans le present article, nous evaluons l'etendue du phreatomagmatisme a differentes etapes de la formation de la cheminee pour la kimberlite Victor Nord-Ouest (nord de l'Ontario, Canada). Des observations detaillees petrographiques et de carottes de forage de tous les facies volcaniques a l'interieur de la cheminee suggerent une mise en place durant deux cycles volcaniques, chacun comprenant trois phases eruptives repetitives. La kimberlite pyroclastique formee au debut des deux cycles est caracterisee par la presence de depots a grains fins, mal tries, et elle contient des cristaux brises d'olivine, des fragments angulaires de la roche encaissante, des lapillis accretionnaires et des roches pyroclastiques juveniles de forme irreguliere comportant des vesicules de tailles variables. Toutes les observations indiquent que ces depots se sont formes par phreatomagmatisme. Des gisements subsequents de kimberlite coherente clastogene ont ete formes depuis des fontaines de lave de type hawaien au cours de la seconde phase des deux cycles. Ces phases ont ete suivies de mouvements de masse dans le cratere partiellement rempli. Une evolution de fragmentation phreatomagmatique a magmatique est assez frequente dans les systemes volcaniques et notre etude fournit la premiere indication que revolution d'un style d'eruption phreatomagmatique a magmatique se retrouve aussi dans les volcans kimberlitiques, suggerant que le volcanisme kimberlitique ne differe pas de maniere significative des systemes plus communs d'evolution basaltique a rhyolitique. De plus, la presente recherche indique que les intensites elevees de fragmentation, enregistrees par les olivines brisees, les lapillis accretionnaires, les abondants clastes (angulaires) de roche encaissante et la retention des cendres, constituent des criteres de texture importants pour reconnaitre les gisements de kimberlite phreatomagmatique. [Traduit par la Redaction], Introduction Despite abundant research on kimberlite pipes, their actual formation mechanisms are intensely debated. Many authors suggest that kimberlite pipes are formed by magmatic processes, with a lesser proportion of [...]

Olivine is the dominant component in coherent kimberlite rocks and related pyroclastic rocks. Quantitative characterization of olivine crystals in kimberlite rocks may be used to better understand kimberlite emplacement and eruption. Here, we construct the first complete olivine crystal size distribution (CSD) for magmatic or coherent kimberlite using two-dimensional image analysis techniques with a new method for scale-integration. Crystal size and frequency data are collected from polished slabs and thin sections, normalized to the largest scale of observation, and stereologically corrected to create a complete CSD covering the full range of observed olivine crystal sizes. Resulting CSDs show the following properties for olivine in crystal-rich interiors of intrusive coherent kimberlite in the A154N kimberlite pipe, Diavik, Northwest Territories: (1) 0.03-10.0 mm in diameter size range; (2) 46.7-51.2% modal crystal content; (3) modal% dominance by crystals >1 mm; (4) frequency dominance by crystals Measured size ranges and modal crystal content are consistent with previous reported data. Relative frequency and area ratios between crystals of different sizes and shape properties differ from previous assessments. Olivine CSDs are consistent with a power-law distribution over at least two orders of magnitude, and statistical parameters (power-law exponents, Inman parameters) suggest that apparent characteristic ranges are independent of crystal frequencies, modal abundance (area%), or sampling locale. These properties suggest olivine crystals may be a single population with a shared paragenesis, and not the end-product of a combination of two populations of olivine. On this basis, olivine CSDs and population parameters have the following uses: (1) to compare and discriminate among kimberlite facies and deposit types, and (2) to test models for olivine paragenesis. Keywords: Olivine, kimberlite, crystal size distribution, image analysis, emplacement, diamond DOI: 10.2138/am.2010.3277

Siberian chromite included in diamond, kimberlite, and spinel peridotite as well as Cr-spinel from garnet-spinel peridotite have been studied by single-crystal X-ray diffraction and electron microprobe analysis. Cell edges and oxygen positional parameters, u, of chromite in diamond and in kimberlite are comparable (cell edge, 8.3249-8.3390 [Angstrom]; u, 0.26175-0.26213). The structural parameters of chromite in the spinel peridotite are similar to those of chromite grains from ophiolitic complexes, and those of Cr-spinel from garnet-spinel peridotite are comparable to those of Cr-spinel in Iherzolitic mantle xenoliths. With the exception of the chromite in garnet-spinel peridotite, all analyzed spinels have a high Cr content. Recasting the chemical analyses according to spinel stoichiometry reveals negligible or no [Fe.sup.3+]. Chrome spinel may be present in heavy concentrates derived from serpentinized mafic and ultramafic rocks as the sole surviving primary mineral and, as such, their particular structural and chemical parameters may represent a new prospecting tool for discriminating the ultramafic source. Keywords: Siberia, diamond, chromite, crystal chemistry

Group II (143-117 Ma) and Group I kimberlites (108-74 Ma) intrude across the southwest boundary of the Kaapvaal craton, sampling the same volume of the subcontinental lithospheric mantle (SCLM) in two time slices. Major and trace element analyses of 3699 peridotitic garnet xenocrysts were used to construct paleogeotherms for 17 kimberlite localities, and to place each garnet at its depth of origin. The Ti contents of each garnet and the calculated [X.sub.Mg] of its coexisting olivine were projected onto a southwest-northeast section across the craton boundary, and splines were used to interpolate between the virtual boreholes in each age group. The sections show that the cratonic SCLM extends at least 75 km southwest of the mapped craton boundary, suggesting a dipping contact. Marked differences between the time-slice sections show that between 117 and 108 Ma the SCLM on both sides of the craton boundary was heated and chemically refertilized by infiltrating asthenosphere-derived melts, thinning the depleted layer by ~40 km. The thermal and geochemical changes record a significant tectonothermal event that may be related to changes in the stress field associated with opening of the South Atlantic, or with mantle upwelling. Keywords: South Africa, lithosphere evolution, lithosphere structure, mantle xenocrysts, mantle garnets.

Abstract: New Rb-Sr phlogopite and U-Pb perovskite age determinations for 12 of 38 known ultramafic bodies in the Buffalo Head Hills area of north-central Alberta show that kimberlitic magmatism occurred [...]

The depth of origin of kimberlite magmas remains controversial. Hypothesized sources include the asthenosphere, the lower part of the upper mantle, and the lower mantle. Based on Sr-Nd isotope systematics, kimberlites are divided into two groups. Group 1 kimberlites have Sr-Nd isotope compositions similar to bulk earth. These compositions are consistent with a kimberlite source that is either a relatively unfractionated ocean-island basalt (OIB)--like source, or a mixture of depleted mid-oceanic-ridge basalt and enriched mantle components. In order to distinguish between these two possibilities, we performed noble gas analyses on olivine crystals separated from extremely fresh kimberlites from South and West Greenland. We detected OIB-like (Loihi-type) high [sup.3]He/[sup.4]He ratios of up to 26.6 [R.sub.A] in kimberlites. This clearly indicates that group 1 kimberlite magmas have a source similar to that of mantle plumes, implying that the source of kimberlite magma is in the lower mantle. Keywords: kimberlite, source, ocean-island basalt, [sup.3]He/[sup.4]He ratio, lower mantle.