Kimberlite Dikes Research Articles

Alkali-carbonate melts in the cratonic mantle evidenced by a wehrlite xenolith from the Majuagaa kimberlite, West Greenland

Carbonate melts are critically important for the deep carbon cycle, mantle melting, redox reactions, and transport of highly incompatible elements. The presence of carbonate melts in the cratonic mantle has been inferred from experimental studies, metasomatic transformations, and melt/fluid inclusions in xenoliths, kimberlites, and diamonds. However, the exact composition of such melts is difficult to determine due to their ephemeral nature and highly reactive properties. Once formed, they migrate away from the source and react with silicate mantle minerals, especially orthopyroxene, causing mantle metasomatism. Wehrlite is one of the products of interaction between the carbonate melt and peridotitic mantle and hence is an excellent candidate for locating in situ carbonate melts. Here, we report petrological, geochemical, and melt inclusion data for a garnet wehrlite xenolith in the Majuagaa kimberlite dike, West Greenland. The xenolith, which last equilibrated with the mantle at 4.5 GPa and 1000 °C, contains abundant melt pools composed of dolomite, calcite, serpentine, spinel, apatite, and phlogopite. Although the original magmatic mineralogy was largely destroyed by low-temperature alteration, remnants of the crystallized carbonatitic melt are preserved as primary melt inclusions in the liquidus Ti-Mg-Fe spinel. These melt inclusions, composed of carbonates, alkali carbonates, periclase/brucite, and minor halides, K-sulfide, apatite, and phlogopite, are the first direct evidence for in situ alkali-carbonate melt in the deep cratonic mantle. Compositionally, they are very similar to primary Na-dolomite melt found in experiments and in fluid inclusions within diamonds.

Xenocrystic zircon in Central New York State Kimberlites record Neoproterozoic disturbance of Grenville Basement rocks

Zircon grains from two central New York State (USA) Mesozoic kimberlite dikes intruding Middle and Upper Devonian sedimentary rocks were analyzed for UPb and LuHf isotopes, and trace elements, and were derived from different parts of the continental crust and/or upper mantle. Zircon from the Six Mile Creek (SMC) kimberlite has variable physical properties and “felsic” chondrite normalized REE patterns, and yields dates (321–1732 Ma) compatible with entrainment of sedimentary xenoliths during accent through the upper crust. In contrast, zircon from the Taughannock Creek (TAG) kimberlite have near concordant UPb dates that are distributed between intercept ages of 572 ± 25 and 960 ± 20 (2σ) Ma (MSWD = 0.61) with homogeneous LuHf systematics and 176Lu/177Hf ratios <0.0001. Examination in CL reveal TAG zircon grains vary in internal zoning and structures, but they are geochemically and isotopically similar. Their evolution includes early Neoproterozoic (ca. 950 Ma) granulite-facies metamorphism in equilibrium with garnet growth (flat to negative sloping HREE patterns and Ti-in-zircon temperatures >900 °C) during the final assemblage of the supercontinent Rodinia, followed by subsequent Pb-loss, resulting in a discordant array and decoupling of the UPb and LuHf isotopic systems prior to entrainment. Concordant zircon UPb dates, anchoring the lower intercept, yield an age of 596 ± 7 (2σ) Ma for the timing of Pb-loss. This most likely occurred in a zone of rifting and mafic magmatism beneath Central New York and Pennsylvania during the opening of the Iapetian Ocean.

Archean to Paleoproterozoic crustal evolution of the southern Yangtze Block (South China): U–Pb age and Hf-isotope of zircon xenocrysts from the Paleozoic diamondiferous kimberlites

Crustal zircon xenocrysts from mantle-derived magmatic rocks have the potential to probe the deep crust. Here we present integrated U–Pb dating and Hf-isotope analyses of zircons from a Paleozoic diamondiferous kimberlite dike in the Maping area of Zhenyuan County (southeastern Guizhou Province), with implications for the tectonothermal evolution of unexposed continental crust beneath the southern Yangtze Block, South China. All zircons (n = 236) show a wide range of U–Pb ages between Mesoarchean and middle Carboniferous. Among them, 96 zircons with 90–110% concordance yield concordant ages from 2942 ± 8 Ma to 342 ± 2 Ma, and form major age peaks at ∼2.9 Ga, ∼2.6 Ga and ∼2.0 Ga. The overwhelming majority of zircons are dominated by Mesoarchean–Paleoproterozoic U–Pb ages regardless of their concordance degrees. The zircon populations mainly consist of magmatic zircons, with minor metamorphic grains (Th/U < 0.10). The youngest magmatic zircon (Th/U = 0.43) with a well concordant 206Pb/238U age of 342 ± 2 Ma (M1-16) is interpreted as the maximum emplacement age of the Maping diamondiferous kimberlites. Most zircons with pre-eruption ages are considered to be xenocrysts, and they may be derived from the deep-seated continental crust through which kimberlite host magmas have passed. Their U–Pb ages and Hf isotopic compositions suggest the possible existence of a highly evolved Archean basement beneath the southern Yangtze Block, South China, which is much older than its known surface rocks. A lot of magmatic zircon xenocrysts reveal complex Precambrian crustal evolution in the southern Yangtze Block. These processes involved the important growth of continental crust at 2.6–2.5 Ga, and in the meantime, crustal reworking could be intermittently proceeding at 3.0–2.6 Ga. In addition, a group of xenocrystic zircons are identified to be of metamorphic origin, indicating that the proposed Archean basement beneath the southern Yangtze Block likely experienced a metamorphic event around 2.0 Ga. This geologically significant episode is consistent with the well-developed coeval metamorphism in other places of the Yangtze Block (e.g., Kongling Terrane), which has been considered to link to the assembly of the Paleoproterozoic Nuna/Columbia supercontinent. Our zircon data implies that the unexposed Archean basement beneath the southern Yangtze Block was affected by multiple thermal activities.

Precambrian lithosphere beneath Hudson Bay: A new geological model based on the Hudson Bay Lithospheric Experiment (HuBLE), Canadian Shield

The oval-shaped basin of Hudson Bay occurs near the center of the round-oval Archaean crustal domain of the North American continent. This paper presents models of the geological structure and evolution of the subcontinental lithospheric mantle underlying Hudson Bay and surrounding tectonic provinces based on geological interpretations of regional geological and geophysical data and results of seismic tomography investigations that have been conducted under the Hudson Bay Lithospheric Experiment. The experiment was aimed at lithospheric processes directly related to the origin of the North American craton and the Hudson Bay basin. Hudson Bay is located directly above the lithospheric keel of North America. The geological history demonstrates systematic “renovation” of the basin: (1) origin and evolution of the Neoarchaean Lake Minto basin (~2.75 Ga); (2) accumulation of the Palaeoproterozoic volcanic–sedimentary filling of the epicontinental basin, relics of which is preserved on its passive margins (2.03–1.87 Ga); (3) origin of Ordovician–Late Devonian sedimentary sequence whose maximum thickness reaches 2.5 km; and (4) the development of Late Jurassic–Miocene sediment-filled ring-shaped trough immediately above the lithospheric keel. The Hudson Bay basin occurs above the lithospheric keel in compliance with thermomechanical model of ascending plume. Tomography studies have not detected evidence of either production or transformation of the lithosphere in the Palaeoproterozoic, which are implied by the model of the United Plates of America. Interpretations of tomography data reveal a vertical axial zone in the lithosphere beneath Hudson Bay, which extends from the lithosphere–asthenosphere boundary to the base of the crust or, perhaps, even to the present day surface. The zone is made up of relatively light low-velocity igneous rocks, probably a swarm of kimberlite dikes or pipes. At 2.75 Ga, the North American continent was a single continental mass with Hudson Bay at its center.

Ultramafic xenoliths from the 1.15 Ga Certac kimberlite, eastern Superior Craton

ABSTRACT Xenoliths and xenocrysts of mantle material from kimberlite dikes located underground at the Certac Au mine, Québec, in the eastern Superior Craton, were studied in terms of the major element composition of their constituent minerals. The kimberlite was dated at 1151 ± 46 Ma by the U-Pb perovskite method. This suite thus provides a rare glimpse into the Mesoproterozoic mantle of the Superior Craton. Two parageneses of mantle material unrelated to the kimberlite magmatism occur: (1) an olivine + ilmenite ± magnetite association characterized by relatively Fe-rich olivine (Mg# = 0.68–0.84) and ilmenite enriched in Mg and Cr (4–13 wt.% MgO, Cr2O3 up to 3 wt.%), and (2) spinel peridotite characterized by Mg-rich olivine (Mg# = 0.91–0.94). The Fe-rich association is interpreted as a magmatic cumulate likely unrelated to the kimberlite. No mantle-derived garnet occurs in the xenoliths or as xenocrysts. The presence of Cr-rich spinel (Cr# = 0.84–0.98) in high temperature (860–953 °C) chromite peridotite indicates bulk compositions too depleted in Al for garnet to be stable, although geothermometry suggests they equilibrated at depths corresponding to garnet stability (90–131 km, depending on the geothermal gradient). Alternatively, the presence of phlogopite in two of the three high temperature (i.e., deepest) chromite peridotites suggests the absence of garnet and presence of low-Al chromite may have been caused by metasomatism from a K-rich fluid that replaced garnet with phlogopite + clinopyroxene ± chromite. Less depletion at shallower depths is indicated by a chromite (Cr# = 0.60) dunite that equilibrated at 831 °C and a low temperature (752 °C) Mg-Al-spinel lherzolite.

The new Massadou diamondiferous kimberlite field in Guinea

A new kimberlite field, called Massadou, has been discovered in southeastern Guinea near Macenta city. The field consists of numerous ~1 m thick kimberlite dikes with low diamond contents; altogether 16 dikes have been found so far. Mineralization occurs along a 600 m wide zone distinct in satellite images, which is oriented in the same way as the K4 kimberlite reported by Huggerty. The Massadou kimberlite is covered by a thick laterite weathering profile. Main kimberlite indicator minerals found in the area are pyrope, chromite, and ilmenite. The latter occurs as zoned grains with a high-Fe core (hemoilmenite) surrounded by a parallel-columnar aggregate in the rim. The aggregate has a composition of ordinary kimberlitic Mg ilmenite and results from interaction of hemoilmenite with the kimberlite melt. The kimberlite age is estimated as 140—145 Ma by analogy with the surrounding fields. The dikes independent products of kimberlite magmatism in the Guinea-Liberia shield rather than being roots of pipes as interpreted by Skinner (2004). Therefore, the erosion cutout is moderate, and there are no reasons to expect the presence of large and rich diamond placers.

Positively oriented trigons on diamonds from the Snap Lake kimberlite dike, Canada: Implications for fluids and kimberlite cooling rates

Positively oriented trigons on diamonds from the Snap Lake kimberlite dike, Canada: Implications for fluids and kimberlite cooling rates

‘Premier’ evidence for prolonged kimberlite pipe formation and its influence on diamond transport from deep Earth

Volcanic pipes, or maar-diatreme volcanoes, form during explosive eruptions of mantle-derived magmas near Earth's surface. Impressive examples are the carrot-shaped, downward-tapering structures formed by kimberlite magmas. Kimberlites originate from >150 km depth within Earth's mantle beneath thick continental roots, away from tectonic plate margins. Kimberlite pipes can be significant diamond deposits, and the complex architecture revealed during exploration and mining is ascribed to repeated magma injections leading to multiple eruptions. Repeated magmatic pulses cause diatremes to widen and grow downward, forming kilometer-sized subterranean structures. However, the time-resolved evolution of kimberlite pipe systems is largely unknown. We present the first U/Pb perovskite ages for newly discovered kimberlite dikes (1139.8 +/- 4.8 Ma) that cut through the volcaniclastic infill of the Premier kimberlite pipe (1153.3 +/- 5.3 Ma) at Cullinan Diamond Mine, South Africa. The ages reveal that renewed kimberlite volcanic activity occurred, at a minimum, 3 m.y. after the main pipe formation. This finding suggests that the largest kimberlite pipes, and maar-diatreme volcanoes in general, may be magmatically active for several millions of years, which conflicts with this volcanism being described as monogenetic' at millennia time scales. Exemplified by Tier-1 diamond deposits on the Kaapvaal craton, long-lasting kimberlite volcanic activity may be an important factor in growing large diatremes, plus enabling effective transport of mantle cargo from the diamond stability field to Earth's surface.

Kimberlite discoveries in NW Liberia: Tropical exploration & preliminary results

This report is brief in context and rich in unexpected discovery. With >2km of erosion, kimberlite models predict the near-complete removal of pipes with exposures to the pipe-root-zones of dikes. Exploration in NW Liberia has, indeed, uncovered eight kimberlite dikes (~10m wide) but also an en echelon pipe, comparable in size to the Kimberley pipe and De Beers' pipe in South Africa. Discoveries are in a narrow 200–300m wide valley of extraordinary thick bush, undergrowth, and organic overburden. Ilmenite and co-existing leucoxene were used as diagnostic tracers for detecting hard rock kimberlite in this tropical terrane. Micro-diamonds show that the redox state of ilmenite is a potentially useful proxy as an index for macro-diamond preservation. The tectonic control of kimberlites is complex, with diverse lithologies. Discoveries include a well-defined regional trend for kimberlite dikes along paleo-fracture zones, Precambrian in age (Liberia Trend), coupled with kimberlite dikes on the craton that are traced to Mesozoic oceanic transform faults (the Sierra Leone Trend). Although long predicted, this is the first report of kimberlite dike-trends in Liberia that are similar in orientation to those in Sierra Leone. An explosive blow on a Liberia-Trend dike demonstrates a similarity to the dynamics attendant in rich (50–500cpht) diamond-bearing dikes in Sierra Leone, and in South Africa of comparable age. The potentially high grade dikes, along with the pipe (~500×50m), now more reasonably accounts for the enormous number of alluvial diamonds (blood and non-conflict), recovered over more than seven decades, downstream from the discovery cluster.A neglected region since the classic work by Bardet (1974), and with few contributions on Liberia since then, an update is considered timely, particularly in the context of discoveries of diamond-bearing kimberlite.

Mantle Xenocrysts From the Masontown, Pennsylvania Kimberlite: An Ordinary Mantle With Si-Enriched Spinel

A hypabyssal kimberlite dike in southwestern Pennsylvania (USA), emplaced through Proterozoic basement and Phanerozic cover, contains a xenocryst and xenolith assemblage typical of material sampled within the subcontinental lithosphere, including xenocrysts of Cr-rich pyrope, magnesiochromite, Cr-rich diopside, and peridotite xenoliths. Temperatures and depths of equilibration of the clinopyroxene (840 oC and 130 km to 1350 oC and 170 km) indicate some sampling in the field of diamond stability. Diamonds have not been reported, however, and the chemistry of the garnet (lherzolite, Cr-poor megacryst, and Group II eclogite) and spinel (<56.0 wt.% Cr2O3) are consistent with diamond absence and the off-craton tectonic setting of the kimberlite. An unusual feature of this suite is that, unlike most mantle xenolith/xenocryst spinel, some of those from Masontown have an unusually high silica content (to 0.59 wt.% SiO2). The significance of the high silica content is unclear, but may be related to an ultrahigh-pressure precursor chromite polymorph with a calcium ferrite structure, which can accommodate Si in solid solution.

DISCOVERY OF A KIMBERLITE PIPE AND RECOGNITION OF A DIAGNOSTIC BOTANICAL INDICATOR IN NW LIBERIA

Diamond-bearing kimberlite dikes (fissures) are present in the deeply eroded Man Shield of West Africa. Small kimberlite pipes, generally less than 1 hectare in area, are known in Guinea, Sierra Leone, and Liberia. Exploration for larger bodies has been severely hampered by thick tropical vegetation, and the lack of distinct geophysical contrasts between weathered kimberlite and the nonresponsive nature of country-rock granites and granodioritic gneisses. Following several years of intense exploration in the highly active artisanal diamond district of northwestern Liberia (which was a major source of alluvial “blood diamonds”) by several large companies and the present study, we report that an elusive diamond-bearing kimberlite pipe has finally been located. A bonus to the pipe location is that an unusual botanical indicator, Pandanus candelabrum , is now recognized exclusively on the pipe and not in eluvium covering the adjacent kimberlite dikes. Plants ( Lychnis alpina ) have been widely used since medieval times for copper in Sweden, and with Haumaniastrum katangese, more recently in Africa. Other plants have evolved to physiologically stabilize heavy metals (U, Pb, Zn, Ni, Cr, Ba, Pb, Zn) in leaves and bark. Termite hills have been used in diamond exploration for kimberlitic indicator minerals (ilmenite, chromite, garnet, pyroxene) in Botswana, the United States, and Australia, but the identification of Pandanus candelabrum, with stilt-like aerial roots, is the first plant to be described that has a marked affinity for kimberlite pipes. This could dramatically change the exploration dynamics for diamonds in West Africa, as geobotanical mapping and sampling is cost-effective in tough terrain.

Comparative analysis of ore-bearing structures in Maiskoe, Markha, and Ozernoe kimberlite bodies at the Nakyn field, Yakutia

Quantitative characteristics of ore-bearing structures in kimberlite dikes and veins at the Nakyn field (Yakutia) were compared, and local tension and compression zones were identified. According to the results, dikes, veins and their swells are controlled by local strike-slip tension zones, while their morphology and dimensions are highly dependent on the tectonodynamic conditions the ore-bearing structures were formed under.

Carbonatite metasomatism of peridotite lithospheric mantle: implications for diamond formation and carbonatite-kimberlite magmatism

Mineral inclusions in diamond record its origin at different depths, down to the lower mantle. However, most diamonds entrained with erupting kimberlite magma originate in lithospheric mantle. Lithospheric U-type diamonds crystallize during early metasomatism of reduced fO2 at the IW oxygen buffer) depleted peridotite in the roots of Precambrian cratons. Evidence of the metasomatic events comes from compositions of garnets in peridotitic xenoliths and inclusions in diamonds. On further interaction with carbonatitic melt, peridotite changes its composition, while diamond no longer forms in a more oxidized environment (fO2 near the CCO buffer). Silicate metasomatism of depleted peridotite (by basanite-like melts) does not induce diamond formation but may participate in generation of group I kimberlite. Low-degree (below 1%) partial melting of metasomatized peridotite produces a kimberlite-carbonatite magmatic assemblage, as in the case of the Snap Lake kimberlite dike. Occasionally, mantle metasomatism may occur as reduction reactions with carbonates and H2O giving rise to hydrocarbon compounds, though the origin of hydrocarbons in the deep mantle remains open to discussion. Melting experiments in carbonate systems show hydrous carbonated melts with low H2O to be the most plausible agents of mantle material transport. An experiment-based model implies melting of carbonates in subducting slabs within the mantle transition zone, leading to formation of carbonatitic diapirs, which can rise through the mantle by buoyancy according to the dissolution-precipitation mechanism. These processes, in turn, can form oxidized channels in the mantle and maintain diamond growth at the back of diapirs by reducing carbon from carbonated melts. When reaching the lithospheric base, such diapirs form a source of kimberlite and related magmas. The primary composition of kimberlite often approaches carbonatite with no more than 10-15% SiO2.

The shapes of dikes: Evidence for the influence of cooling and inelastic deformation

Research Article| July 01, 2012 The shapes of dikes: Evidence for the influence of cooling and inelastic deformation Katherine A. Daniels; Katherine A. Daniels † 1School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK †E-mail: k.a.daniels@bristol.ac.uk Search for other works by this author on: GSW Google Scholar Janine L. Kavanagh; Janine L. Kavanagh 2School of Geosciences, Monash University, Clayton Campus, Wellington Road, Clayton, Victoria 3800, Australia Search for other works by this author on: GSW Google Scholar Thierry Menand; Thierry Menand 3Clermont Université, Laboratoire Magmas et Volcans, Université Blaise Pascal, BP 10448, F-63000 Clermont-Ferrand, France4Centre National de la Recherche Scientifique, UMR 6524, Laboratoire Magmas et Volcans, F-63038 Clermont-Ferrand, France5Institut de Recherche pour le Développement, R 163, Laboratoire Magmas et Volcans, F-63038 Clermont-Ferrand, France Search for other works by this author on: GSW Google Scholar J. Sparks R. Stephen J. Sparks R. Stephen 1School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Katherine A. Daniels † 1School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK Janine L. Kavanagh 2School of Geosciences, Monash University, Clayton Campus, Wellington Road, Clayton, Victoria 3800, Australia Thierry Menand 3Clermont Université, Laboratoire Magmas et Volcans, Université Blaise Pascal, BP 10448, F-63000 Clermont-Ferrand, France4Centre National de la Recherche Scientifique, UMR 6524, Laboratoire Magmas et Volcans, F-63038 Clermont-Ferrand, France5Institut de Recherche pour le Développement, R 163, Laboratoire Magmas et Volcans, F-63038 Clermont-Ferrand, France J. Sparks R. Stephen 1School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK †E-mail: k.a.daniels@bristol.ac.uk Publisher: Geological Society of America Received: 24 Apr 2011 Revision Received: 25 Aug 2011 Accepted: 05 Oct 2011 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2012 Geological Society of America GSA Bulletin (2012) 124 (7-8): 1102–1112. https://doi.org/10.1130/B30537.1 Article history Received: 24 Apr 2011 Revision Received: 25 Aug 2011 Accepted: 05 Oct 2011 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Katherine A. Daniels, Janine L. Kavanagh, Thierry Menand, J. Sparks R. Stephen; The shapes of dikes: Evidence for the influence of cooling and inelastic deformation. GSA Bulletin 2012;; 124 (7-8): 1102–1112. doi: https://doi.org/10.1130/B30537.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract We provide detailed observations on the shape of dikes from well-exposed field locations in the Isle of Rum, Scotland, and Helam Mine, South Africa. The basaltic Rum dikes crop out on a smaller scale than the Helam kimberlite dikes and have a smaller length to thickness ratio (∼100:1 Isle of Rum, ∼1000:1 Helam Mine). We compare the dike thickness field measurements with the geometry predicted by elastic theory, finding best-fit models to estimate magma overpressure and regional stress gradients at the time of dike emplacement. Most of the dike shapes fit poorly with elastic theory, being too thick at the dike ends and too narrow in the middle. Even for dikes where the model fit is acceptable, the calculated overpressures and stress gradients are very large and much larger than independent estimates based on rock strength, particularly for the small-scale basaltic dikes on Rum, where calculated overpressures average 687 MPa, and calculated stress gradients average 622 MPa m−1. The Swartruggens dikes have calculated overpressures of between 4 and 40 MPa and calculated stress gradients in the range of 15–87 kPa m−1. Dike shape can be explained by a combination of host-rock inelastic deformation prior to and coeval with magma emplacement, and by magma chilling at the dike's tapering edges, which prevented its closure as magma pressure declined during emplacement; this sequence provides the most complete explanation for the mismatches between the data and the model. The permanent wedging of the dike edges due to chilling has implications for crustal magma transport and strain response in the crust due to dike emplacement. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

THE MAJUAGAA KIMBERLITE DIKE, MANIITSOQ REGION, WEST GREENLAND: CONSTRAINTS ON AN Mg-RICH SILICOCARBONATITIC MELT COMPOSITION FROM GROUNDMASS MINERALOGY AND BULK COMPOSITIONS

The Majuagaa kimberlite dike in the Maniitsoq region, southern West Greenland, is 564 Ma old, 2.5 km long, and up to 2 m wide. It is well exposed and very fresh, allowing detailed petrographic and chemical investigations. Little or no serpentinization is observed, and primary petrographic characteristics and grain morphologies are preserved. The dike is diamondiferous and carries xenoliths (up to 30 cm), megacrysts and macrocrysts characteristic of kimberlite. On the bases of the paragenesis and compositions of groundmass phlogopite, geikielite and Mg–Ti-enriched spinel, the dike is classified as kimberlite, despite being carbonate-rich and the apparent absence of monticellite characteristic of many occurrences of bona fide hypabyssal kimberlite. Olivine megacrysts, macrocrysts and microcrysts are all xenocrystic, whereas microphenocrysts are xenocrystic microcrysts overgrown by equilibrium olivine. Most olivine (max. 37 wt.%) is found to be xenocrystic on the basis of Ni mass balance. The average bulk composition of the melt involved in the dike compares well with Group-1 kimberlite. Corrected for xenocrystic olivine and ilmenite, the melt has the composition of a silicocarbonatite and compares with experimental melts formed at very low degrees of partial melting of CO2-bearing lherzolite.

Mechanically disrupted and chemically weakened zones in segmented dike systems cause vent localization: Evidence from kimberlite volcanic systems

Deformation and alteration zones along kimberlite dikes hold clues as to how point-source volcanic vents can localize along sheet-like intrusions. Brittle deformation zones occur in host rock adjacent to kimberlite intrusions of the Swartruggens Kimberlite Dike Swarm, South Africa. Deformation includes local fracturing and brecciation and is associated with relay zones between offset dike segments. Breccia zones indicate dilation and hydraulic fracturing, and some zones along the dikes were affected by chemical corrosion, forming fresh cores surrounded by onion-skin concentric shells of altered rock. The alteration was caused by either exsolved magmatic volatiles moving in advance of the magma through the fracture, or by hydrothermal fluids. Consideration of the time scales needed for chemical corrosion of host rock requires intrusions to stall at depth prior to transport to higher crustal levels. Highly disrupted offsets could be preferred locations for explosive activity and initial vent formation as dikes approach the surface. A kimberlite pipe forms after magma breaks through to the surface; the altered zones are reamed out and fresh cores in spheroidally altered rock are incorporated into the pipe fill along with more angular country-rock material, as observed in layered volcanic breccias in kimberlite pipes at the Venetia Mine, South Africa. This model may have wider implications for the localization of conduits along dikes in other volcanic systems. Dike segmentation provides weak zones where hydrothermal fluids and magmatic volatiles can be preferentially channeled. Chemical corrosion can further weaken these zones, which may then become the locus for initial phreatic and phreatomagmatic explosions, creating shallow vents that can then channel magma to the surface during eruption.

Multiple-mineral inclusions in diamonds from the Snap Lake/King Lake kimberlite dike, Slave craton, Canada: a trace-element perspective

Multiple inclusions of minerals in diamonds from the Snap Lake/King Lake kimberlites of the southeastern Slave craton in Canada have been analyzed for trace elements to elucidate the petrogenetic history of these inclusions, and of their host diamonds. As observed worldwide, the harzburgitic-garnet diamond inclusions (DIs) possess sinusoidal REE patterns that indicate an early depletion event, followed by metasomatism by LREE-enriched, HREE-depleted fluids. Furthermore, these fluids appear to contain appreciable concentrations of LILE and HFSE, based on the increasing abundances of these elements in the olivine inclusion that occurs at the outer portion of a diamond compared to that near the core. The compositions of these fluids are probably a mixture of hydrous-silicic melt, carbonatitic melt, and brine, similar to the compositions of micro-inclusions in diamonds reported by Navon et al. (2003). Comparison between the compositions of majoritic and normal harzburgitic garnets shows that the former are more depleted in terms of major/minor elements (higher Cr#) but significantly more enriched in the REE (up to ∼10×). This characteristic may indicate the higher susceptibility for metasomatic enrichment of previously more depleted garnets. Garnets of eclogitic paragenesis show strong LREE-depleted patterns, whereas the coexisting omphacite inclusion has relatively flat light- and middle-REE but depleted HREE. Whole-rock reconstruction from coexisting garnet and omphacite inclusions indicates that the protolith of these inclusions was probably the extrusive section of an oceanic crust, subducted beneath the Slave craton.

Fluid characteristics of the diamondiferous Droogeveldt Gravels, Vaal Valley, South Africa

The Late Cretaceous to early Tertiary Droogeveldt gravels, one of the more famous alluvial diamond diggings in the Vaal basin of South Africa for both grade and large stone size, were totally exploited in the earlier part of this century. The best recoveries were obtained from high elevation (1,100- to 1,130-m elev) bimodal gravels situated in narrow (2- to 20-m wide), straight-sided, bedrock depressions <1 km long, some 5 to 10 km from the current Vaal River (1,000-m elev). These gravels, forming a fining-upward profile, comprise cobble to pebble, silica-rich clasts characteristic of the current Vaal drainage basin set among now-weathered bedrock boulders derived locally from the underlying Archean Ventersdorp volcanic rocks. The gravel-filled depressions, known by the early mining community as sluits, coincide with major structural lineaments in the Ventersdorp bedrock. Although the now rare, remnant weathered profiles resemble colluvium or debris flow-type deposits, the presence of potholed and smooth, polished, but irregular bedrock footwall in the sluits as well as rounded and percussion-scarred silica clasts points to a more energetic fluvial influence. The well-jointed bedrock sidewalls of the sluits provided readily accessible boulders (oversize clasts) to these gully-like features. Thus, the exceptional diamond concentrations and large stone size in the Droogeveldt gravels are attributed to a combination of primary, fixed turbulent zones formed by the narrow bedrock gullies with attendant pothole and scour trapsites, and secondary, quasi-fixed turbulent zones associated with the locally derived boulders (oversize clasts) in a paleo-Vaal River drainage. The sluits are not eroded kimberlite dikes and the bulk of their diamonds are likely to be sourced from the Cretaceous kimberlites in the greater Kimberley area.

Cathodoluminescence Properties of CO2-Bearing and CO2-Free Diamonds from the George Creek K1 Kimberlite Dike

The cathodoluminescence (CL) properties of diamonds from the George Creek Kl kimberlite dike are discussed with reference to their complex growth, mantle residence, and deformation histories. Diamonds shown by infrared (IR) spectrometry to contain submicroscopic, highpressure CO2 inclusions are characterized by anomalous CL colors such as pink, pale violet, orange, and brown, instead of the blue colors typically exhibited by CO2-free diamonds. Complex intergrowth relationships between CO2-bearing and CO2-free diamond-growth generations are recognized from CL photomicrographs of CO2-bearing diamonds. CO2-free diamonds commonly exhibit octahedral growth zonation, and growth-sector zonation was identified in a single diamond of cubo-octahedral morphology. The distribution of platelets that form during prolonged mantle residence time is related to initial nitrogen concentration and thus is influenced strongly by growth zonation. The extreme heterogeneity and complex growth history of the George Creek diamonds...

A Palaeomagnetic Study of 143 Ma Kimberlite Dikes In Central New York State

SUMMARY A palaeomagnetic study of 143 Ma serpentinized kimberlite dikes near Ithaca, New York, yields a dual-polarity, high unblocking temperature, and high coercivity magnetization which passes the reversal test and two baked contact tests. The mean pole position (58N, 203E; A95 = 3.8, N = 7) differs from published late Jurassicearly Cretaceous North American poles currently used to define the apparent polar wander path. The angular dispersion in mean directions (&=3.5) is low but the presence of reversals argues that the Ithaca kimberlites magnetization should represent sufficient time for averaging of palaeosecular variation. Similar findings apparently typify palaeomagnetic studies of other serpentinized kimberlite, supporting the suggestion that thermo-chemical remanent magnetization in this lithology prolongs the duration of magnetization acquisition sufficiently to average secular variation per dike. A consistent but weak foliation in anisotropy of ARM parallels the N-S and vertical orientation of the Ithaca dikes, but is apparently unrelated to the northwest-down or southeast-up remanence. The Ithaca kimberlites pole may therefore record a previously undocumented sharp bend or 'cusp' at -143Ma, and the initiation of a Cretaceous and Cenozoic interval of apparent polar wander that generally follows a great circle along the 200E meridian to geographic north. A coeval (-145 Ma) kimberlite pole from southern Africa transferred to North America agrees with the Ithaca kimberlites pole position whereas reported poles from the Berriasian stratotype (southern France) and 144Ma Svalbard dolerites provide less diagnostic tests of the Ithaca kimberlites pole due in part to uncertainties in Europe-North America reconstructions.