Kimberlite Magma Research Articles

Morphology and surface features of olivine in kimberlite: implications for ascent processes

Abstract. Most kimberlite rocks contain large proportions of ellipsoidal-shaped xenocrystic olivine grains that are derived mainly from disaggregation of peridotite. Here, we describe the shapes, sizes and surfaces of olivine grains recovered from kimberlite lavas erupted from the Quaternary Igwisi Hills volcano, Tanzania. The Igwisi Hills kimberlitic olivine grains are compared to phenocrystic olivine, liberated from picritic lavas, and mantle olivine, liberated from a fresh peridotite xenolith. Image analysis, scanning electron microscopy imagery and laser microscopy reveal significant differences in the morphologies and surface features of the three crystal populations. The kimberlitic olivine grains form smooth, rounded to ellipsoidal shapes and have rough flaky micro-surfaces that are populated by impact pits. Mantle olivine grains are characterised by flaked surfaces and indented shapes consistent with growth as a crystal aggregate. Phenocrystic olivine exhibit faceted, smooth-surfaced crystal faces. We suggest that the unique shape and surface properties of the Igwisi Hills kimberlitic olivine grains are products of the transport processes attending kimberlite ascent from mantle source to surface. We infer that the unique shapes and surfaces of kimberlitic olivine grains result from three distinct mechanical processes attending their rapid transport through the thick cratonic mantle lithosphere: (1) penetrative flaking from micro-tensile failure induced by rapid decompression; (2) sustained abrasion and attrition driven by particle–particle collisions between grains within a turbulent, volatile-rich flow regime; and (3) higher-energy particle–particle collisions producing impact cavities superimposed on decompression structures. The combination of these processes during the rapid ascent of kimberlite magmas is responsible for the distinctive ellipsoidal shape of olivine xenocrysts found in kimberlites worldwide.

High water contents in the Siberian cratonic mantle linked to metasomatism: An FTIR study of Udachnaya peridotite xenoliths

The processes that control water distribution in nominally anhydrous minerals from peridotites are twofold. Melt depletion will remove water while metasomatism can potentially add water to these minerals. These processes can lead to a wide range of outcomes in water contents, which in turn could play a role in mantle rheology and long-term cratonic root stability. To examine these complexities, water concentrations in minerals from well-characterized peridotites from the Udachnaya kimberlite in the central Siberian craton were analyzed by FTIR. The peridotites span a complete top to bottom cross-section of typical cratonic lithospheric mantle (2–7GPa and 700–1400°C). Diffusion modeling of water content profiles across olivine grains shows that water loss during decompression is limited to the 100μm rims of olivines; the cores preserved their mantle water contents. Water contents range from 6 to 323ppm wt H2O in olivine, 28–301ppm H2O in orthopyroxene (opx), 100–272ppm H2O in clinopyroxene (cpx) and 0–23ppm H2O in garnet. Melting modeling cannot reproduce the high water contents of cratonic mantle peridotites and any potential partial melting trend must have been erased by later events. The water contents of minerals, however, are correlated with modal abundances of clinopyroxene and garnet, bulk rock FeO, TiO2 and SiO2 as well as with light and middle rare earth elements in clinopyroxene and garnet. These relationships are best interpreted as interaction of residual, melt-depleted peridotites with silicate melt, which produced modal and cryptic metasomatism. Importantly, the water enrichment in the Siberian cratonic mantle took place prior to kimberlite magmatism and eruption. Water addition by metasomatism occurred from pressures >4GPa all the way to the base of the cratonic root below central Siberia, but was limited to shallower levels (<5GPa) in the Kaapvaal cratonic lithosphere. The difference in olivine water contents at the deepest levels of the Kaapvaal (<5ppm H2O) and Siberian (6–323ppm H2O) cratonic roots may be linked to oxygen fugacity and resulting fluid speciation or, alternatively, to reaction with different metasomatic agents. Calculated viscosities for the deepest Udachnaya samples are similar to those inferred for the asthenosphere. If these xenoliths are representative of the deep cratonic lithosphere, water is not as important a parameter as previously thought in the strength of cratonic lithosphere, otherwise the cratonic root beneath Udachnaya would have been delaminated. Alternatively, the metasomatic xenoliths may not be representative of the Siberian cratonic root and kimberlites preferentially sample cratonic mantle lithosphere material located near, and metasomatized by, melt conduits, which served as channels for upward migration of water-rich melts and fluids including kimberlites. In that case, the cratonic root overall still may have relatively low water contents, which in addition to its less metasomatized (more refractory) and thereby buoyant nature, still play a role in making it strong enough to resist delamination by the surrounding asthenosphere.

Petrogenesis of Mantle Polymict Breccias: Insights into Mantle Processes Coeval with Kimberlite Magmatism

Mantle polymict breccias sampled by kimberlite magmas are complex mixtures of mantle minerals and rock clasts, cemented together by olivine, phlogopite, orthopyroxene, ilmenite, rutile and sulphides. Because of the kimberlite-like texture (i.e. mineral clasts of diverse origin and composition set in a magmatic matrix) and the large geochemical heterogeneity preserved in polymict breccias, these rocks are believed to derive from primitive or precursor kimberlite magmas. Therefore, the study of such xenoliths can provide constraints on the processes occurring in the mantle during the early stages of kimberlite ascent, and possibly on the composition of kimberlite melts. To constrain the petrogenesis of these unusual rocks, we have studied two samples of polymict breccia from the Bultfontein kimberlite (Kimberley, South Africa) and compared our results with published data for other polymict breccias. The most abundant phase in the matrix of the studied samples is olivine with a narrow range in Mg# (∼88–89), but variable Ni–Mn–Ca contents. Similar compositions are characteristic of magmatic olivine in the Bultfontein and nearby De Beers kimberlites. Orthopyroxene is the dominant phase in the matrix of polymict breccias surrounding clinopyroxene clasts, which, like the other silicate mineral clasts, are highly resorbed. The matrix orthopyroxene exhibits variable compositions, with significant enrichment in Ca, Na, Cr, Sr, Ba and light rare earth elements in the grains adjacent to clinopyroxene. The other main matrix phases (phlogopite, ilmenite and rutile) also display variable compositions. Matrix olivine hosts primary carbonate-rich inclusions similar to those observed in polymict breccia ilmenite. These inclusions were previously interpreted as an alkali-carbonate melt trapped during ilmenite growth. This alkali-carbonate melt may represent the parental melt to the matrix minerals of the polymict breccias. The variable composition of the matrix minerals is attributed to rapid, small-scale (centimetre to millimetre) variations in the melt composition owing to clast dissolution, possibly coupled with wall-rock assimilation, closely followed by fast cooling. Partial digestion of silicate porphyroclasts increased the Si content of the matrix melt, thus allowing crystallization of orthopyroxene. Further arguments in favour of a genetic relationship between polymict breccias and kimberlite magmas are provided by (1) similar Hf isotope compositions of polymict breccia ilmenite and South African kimberlites, (2) overlapping olivine compositions in polymict breccias and the host Bultfontein kimberlite, and (3) the occurrence of alkali-carbonate inclusions in polymict breccia and kimberlite minerals. Polymict breccias are interpreted as failed kimberlite intrusions, which metasomatized the magmatic conduit through which subsequent pulses of kimberlite magmas ascended. These wall-rock interactions would limit reactions between later pulses of kimberlite melt and mantle wall-rocks, thus enhancing the ability of kimberlite magmas to reach the surface.

Qualitative Analysis of Mafic Dyke Swarms and Kimberlites from Morphological and Geophysical Signatures, NW of Proterozoic Cuddapah Basin, Eastern Dharwar Craton

Abstract Widespread distribution of mafic dykes and scanty occurrence of ultrabasic intrusives of kimberlitic affinity around Proterozoic Cuddapah basin, parts of Eastern Dharwar craton of south India has been the focus of attention since their discovery, to understand the structural fabric in relation to their emplacement in geological time. Satellite Imagery, geomorphological, geophysical and radiometric age data of Narayanpet area, northwest of Cuddappah basin, have clearly displayed the alignments and structures of geological significance, such as deep seated fault / fracture / shear zones, stratigraphic / lithological contacts, basic / ultrabasic intrusives and younger granites etc,. Based on the field observations such as emplacement of mafic dykes, their cross cutting relationship, study of morphological and geophysical signatures, inferred linears drawn from satellite imagery, aeromagnetic and gravity maps are arranged in a chronological order. A system of long, narrow and widely spaced dykes trending NW-SE direction conformable to gneissic foliation, typically associated with migmatites in the southwestern part of the study area are the oldest. Followed by E-W dykes, cut across by the sparsely distributed dykes associated with NW-SE and N-S features and in turn off set by dykes of NE-SW trends are the youngest. Kimberlites of Narayanpet area, belongs to hypabysal facies, which are essentially controlled by E-W to ENE-WSW deep seated fault / fracture zone, their intersection with NW-SE, NE-SW to N-S trends, which may have been reactivated during Proterozoic period as indicated by the intrusion of mafic dykes (~2270 to 1701 Ma) and emplacement of kimberlitic magmatism (~1300 to 1100 Ma) suggesting different intrusive episodes. Kimberlite pipes of Narayanpet field, falls in an ellipsoid form trending WNW-ESE direction in the northern part of the area, associated with radial drainage / topographic high and a gravity low. In addition, physical properties such as density and magnetic susceptibilities of mafic dykes and kimberlites, their geophysical signatures, emplacement of kimberlites at the close vicinity of mafic dykes or at their intersections have also been discussed.

Middle Paleozoic basaltic and kimberlitic magmatism in the northwestern shoulder of the Vilyui Rift, Siberia: relations in space and time

A Middle Paleozoic tectonothermal event in the eastern Siberian craton was especially active in the area of the Vilyui rift, where it produced a system of rift basins filled with Devonian–Early Carboniferous volcanics and sediments, as well as long swarms of mafic dikes on the rift shoulders. Basalts occur mostly among Middle Devonian sediments and are much less spread in Early Carboniferous formations. The dolerite dikes of the Vilyui–Markha swarm in the northwestern rift border coexist with the Mirnyi and Nakyn fields of diamond-bearing kimberlites. The voluminous dikes and sills intruded before the emplacement of kimberlites. The Mir kimberlite crosscuts a dolerite sill and a dike in the Mirnyi field, while a complex dolerite dike (monzonite porphyry) cuts through the Nyurba kimberlite in the Nakyn field. Thus, the kimberlites correspond to a longer span of Middle Paleozoic basaltic magmatism. The basalts in Middle Paleozoic sediments have faunal age constraints, but the age of dolerite dikes remains uncertain. The monzonite porphyry dike in the Nyurba kimberlite has been dated by the 40Ar/39Ar method, and the obtained age must be the upper bound of the dike emplacement. The space and time relations between basaltic and kimberlitic magmatism were controlled by Devonian plume–lithosphere interaction.

Eitelite in sheared peridotite xenoliths from Udachnaya-East kimberlite pipe (Russia) ? a new locality and host rock type

For the first time eitelite Na 2 Mg(CO 3 ) 2 was observed as a daughter phase in the melt inclusions in olivine from one of the deepest known mantle rocks sampled by kimberlite magma – sheared peridotite xenoliths (190 – 230 km), taken from the Devonian (~370 Ma) Udachnaya-East kimberlite pipe (Siberian craton, Russia). Eitelite was identified by confocal Raman spectroscopy and confirmed by energy-dispersive X-ray spectroscopy. Raman spectra of eitelite in the melt inclusions are characterized by a very strong band at 1105 cm −1 attributed to CO 3 2− symmetric stretching, and weaker bands at 207–208 and 260–263 cm −1 due to lattice vibration. Our findings of eitelite in the melt inclusions entrapped by olivine of mantle xenoliths indicate that this rare carbonate can crystallise from primitive mantle-derived alkaline carbonatite melt.

A Record of Paleoproterozoic Subduction Preserved in the Northern Slave Cratonic Mantle: Sr–Pb–O Isotope and Trace-element Investigations of Eclogite Xenoliths from the Jericho and Muskox Kimberlites

Paleoproterozoic eclogite xenoliths from the Jericho and Muskox kimberlites located in the northern Slave craton, Canada, display geochemical and isotopic evidence of a recycled oceanic lithosphere origin. The eclogites comprise a diverse suite of kyanite- and diamond-bearing and bimineralic xenoliths that have a wide range of geochemical compositions. Calculated whole-rock eclogite compositions are broadly basaltic to picritic, ranging from 5 to 13 wt % MgO. Calculated whole-rock trace-element compositions for the eclogites show three overall REE patterns: the first pattern resembles that of oceanic gabbros with prominent positive Eu anomalies and flat heavy REE; the second pattern is similar to the first but displays marked light REE depletion that is interpreted to record partial melt loss; the third pattern lacks Eu anomalies and instead displays both gradually increasing REE contents from La to Lu similar to boninitic melts and ‘humped’ middle REE patterns similar to those observed in eclogites formed by reaction with subduction fluids. Several eclogites have garnet and/or clinopyroxene δ 18 O values >6‰, which coupled with the trace-element systematics of all the eclogites is interpreted to reflect derivation from recycled oceanic lithosphere protoliths. Lead isotope data for eclogite clinopyroxenes form a broad array that extends from highly radiogenic isotope compositions similar to the host Jericho kimberlite to an intersection with the Stacey–Kramers terrestrial two-stage Pb evolution curve at c. 2·2 Ga. The slope of this data array defines a secondary Pb isochron with an associated age of 1·7 ± 0·3 Ga. Although it is possible that the Pb isotope array is actually a mixing line between the eclogite protolith and the host Jericho kimberlite, both the secondary isochron age and the c. 2·2 Ga Stacey–Kramers model age are consistent with Paleoproterozoic protolith oceanic crust formation. Eclogite xenoliths and eclogitic diamond inclusions from the Slave craton almost exclusively yield c. 2 Ga ages, which are broadly compatible with formation during subduction associated with the c. 1·9–1·8 Ga Wopmay orogeny at the western margin of the Slave craton. Trends in the Sr–Pb isotopic compositions of the eclogite clinopyroxene show that seawater alteration of the oceanic crust protoliths was an important process during protolith formation, and cryptic metasomatism further increased the Pb and Sr isotope values. This metasomatism is probably associated with the Jurassic kimberlite magmatism; however, contributions from the Mesoproterozoic Mackenzie event cannot be completely excluded.

Faulting in the lithosphere: the 35th anniversary of the Irkutsk school of tectonophysics S. I. Sherman, K. Zh. Seminsky, S. A. Bornyakov

Faulting in the lithosphere: the 35th anniversary of the Irkutsk school of tectonophysics S. I. Sherman, K. Zh. Seminsky, S. A. Bornyakov

"Clifford's Rule" and the geodynamics of kimberlite magmatism

The Clifford rule emphasizes the association of kimberlite magmatism with the crust areas of Archean and Early Proterozoic age. Though there are diamantiferous and highly productive primary diamond deposits outside Archean cratons, the Clifford rule is valid for most cases. However, from the geodynamics viewpoint the rule is not consistent because the kimberlites have a deep origin, and crustal structures can not affect the formation of protokimberlite melts. The extended Clifford rule associates the primary diamonds deposits not with the Archean crust but with the thick depleted lithosphere. It is geodynamically justified and has almost no exceptions. Besides, with such an interpretation of the Clifford rule the association of the kimberlite magmatizm with the ancient structures becomes self-evident because a lithosphere characteristic of Archean cratons could not be formed later due to a decrease of the potential temperature of the convecting mantle since Archean. Therefore the cooling of the convecting mantle is in fact the primary physical reason underlying the validity of the Clifford rule.

Using perovskite to determine the pre-shallow level contamination magma characteristics of kimberlite

It remains difficult to obtain reliable geochemical signatures of uncontaminated kimberlite magma from bulk rock studies due to the combined effects of crustal assimilation and element mobility during post-emplacement alteration processes. Groundmass perovskite (CaTiO3), a typical accessory phase, from Orapa (Botswana) and Wesselton (South Africa) kimberlites has been used to evaluate the isotope and trace element composition of the pre-contamination magmas and the effects of shallow level contamination. In-situ trace element signatures of Orapa and Wesselton perovskite grains are broadly similar and unaffected by crustal contamination. Single grain 87Sr/86Sr isotope ratios of perovskite from Orapa (0.7030–0.7036) are less scattered than bulk rock analyses (0.7063–0.7156), which are variably affected by contamination and late stage alteration. Initial 87Sr/86Sr isotope ratios of perovskite (0.7044–0.7049) from Wesselton overlap with published whole rock studies on fresh hypabyssal kimberlites (0.7042–0.7047). The limited intra-kimberlite variation in Sr isotope ratios recorded by the perovskite are unlikely to be due to crustal contamination as the calculated liquid compositions in equilibrium with the perovskite analysed typically have >1500ppm Sr, and most common crustal lithologies underlying these kimberlites have relatively low Sr contents and are not highly radiogenic. Calculated pre-shallow level contamination magma compositions for Orapa and Wesselton have significantly fractionated LREE and highly variable non-smooth trace element patterns. Initial Sr and Nd isotope ratios of both kimberlites fall on the mantle Nd–Sr array with enriched Sr and slightly depleted Nd signatures, similar to Group I kimberlites. Overall, the trace element and isotopic composition of Orapa and Wesselton kimberlites are similar to the reported Group I kimberlites from southern Africa, which are derived by very low degrees of partial melting from a LREE depleted metasomatised sub-continental lithospheric mantle (SCLM) source.

Partitioning of H2O between olivine and carbonate–silicate melts at 6.3 GPa and 1400 °C: Implications for kimberlite formation

Partitioning of H2O between olivine and carbonate–silicate melts has been studied at 6.3 GPa and 1400 °C using a split-sphere multianvil apparatus. Olivine was synthesized in equilibrium with hydrous silicate and hydrous carbonate–silicate±chloride melts saturated with respect to one of Opx, Grt, Ms or a harzburgitic (Ol+Opx+Grt) residue and had CO2/(CO2+SiO2) molar ratios from 0 to 0.8. The concentration of H2O in olivine was determined using FTIR spectroscopy. We found that depending on the melt carbonation and saturation in equilibrium silicate phases the H2O content in olivine varied from 100 to 1500 ppm. The obtained results and data reported in Sokol et al. (2013) indicate that H2O content in olivine becomes approximately two times lower as CO2/(CO2+SiO2) molar ratios in the equilibrium melt increases from 0 to 0.4–0.8 and the crystallization media transform from hydrous silicate to hydrous carbonate–silicate (kimberlite like) melt. The estimated water partitioning between carbonate–silicate melt and nominally anhydrous mantle minerals indicates that carbonatitic melt can effectively extract water once it invades H2O-poore the peridotite. We suggest that extraction of H2O owing to the freezing point depression may provide the necessary melting degree of metasomatized peridotite source and formation of kimberlitic magma.

Factors controlling the internal facies architecture of maar-diatreme volcanoes

Most if not all kimberlite pipes show a multitude of facies types, which imply that the pipes were emplaced under an episodic re-occurrence of eruptive phases, often with inter- mittent phases of volcanic quiescence. The majority of these facies can be related to either the fragmentation behaviour of the magma during emplacement or changing conditions dur- ing sedimentation of volcaniclastic deposits, as well as their alteration and compaction after deposition. An additional factor controlling pipe-facies architecture is the degree of mobility of the loci of explosions in the explosion chambers of the root zone or root zones at the base of the maar-diatreme volcano. In a growing pipe, the root zone moves downward and, with that movement, the overlying diatreme enlarges both in size and diameter. However, during the life span of the volcano, the explosion chamber can also move upward, backintothelower diatreme,where renewedexplosionsresult inthedestructionofolderdepositsandtheirstructures.Nextto vertical shifts of explosion chambers, the loci of explosions can also move laterally along the feeder dyke or dyke swarm. This mobility of explosion chambers results in a highly com- plex facies architecture in which a pipe can be composed of several separate root zones that are overlain by an amalgam- ated, crosscutting diatreme and maar crater with several lobes. Pipe complexity is amplified by periodic changes of the fragmentation behaviour and explosivity of kimberlite mag- ma. Recent mapping and logging results of Canadian and African kimberlite pipes suggest that kimberlite magma frag- mentation ranges from highly explosive with abundant entrained country rock fragments to weakly explosive spatter-like production with scarce xenoliths. On occasions, spatter may even reconstitute and form a texturally coherent deposit on the crater floor. In addition, ascending kimberlite magma can pass the loci of earlier fragmentation events in the root zone and intrudes as coherent hypabyssal kimberlite dykes in high pipe levels or forms extrusive lava lakes or flows on the crater floor or the syneruptive land surface, respectively. This highly variable emplacement behaviour is typical for basaltic maar-diatreme volcanoes and since similar deposits can also be found in kimberlites, it can be concluded thatalsothevolcanologicalprocessesleadingtothesedeposits are similar to the ones observed in basaltic pipes.

Dispersion halos of kimberlite indicator minerals in the Siberian Platform: History and formation conditions

The formation history of dispersion halos of kimberlite indicator minerals in the Siberian Platform is described beginning from the main Middle Paleozoic productive epoch of kimberlite magmatism. The history and formation conditions of the halos are reconstructed from the ontogeny and typomorphism of indicator mineral assemblages with involvement of the geological history of the Siberian Platform. The general trend characterizing the development of indicator mineral halos is discussed. The data obtained allow adequate estimation of of the character and geological setting of indicator mineral halos for forecasting purposes.

Physical characteristics of kimberlite and basaltic intraplate volcanism and implications of a biased kimberlite record

We assess bias in the record of kimberlite volcanism by using newly acquired size data on more than 900 kimberlite bodies from 12 kimberlite fields eroded to depths of between 0 m and >1200 m, and by a comparison with intraplate monogenetic basaltic volcanic fields. Eroded kimberlite fields are composed of pipes (or diatremes) and dikes, and within any one kimberlite field, regardless of erosion level, kimberlite bodies vary in area at Earth’s surface over 2–3 orders of magnitude. Typically 60%–70% of the bodies are <10% the area of the largest pipe in the field. The maximum size of a kimberlite pipe found in a field shows a relationship with estimated erosion levels, suggesting that the erosion level of a region could be used to predict the maximum potential size of a pipe where it intersects the surface. The data indicate that the selective removal of surface volcanic structures and deposits by erosion has distorted the geological record of kimberlite volcanism. Selective mining of preferentially large, diamondiferous kimberlite pipes and underreporting of small kimberlite pipes and dikes add further bias. A comparison of kimberlite volcanic fields with intraplate monogenetic basaltic volcanic fields indicates that both types of volcanism overlap in terms of field size, volcano number and size, and typical erupted volumes. Eroded monogenetic basaltic fields consist of dikes that fed effusive and weakly explosive surface eruptions, and diatremes (pipes) generated during phreatomagmatic eruptions, and they are structurally similar to eroded kimberlite fields. Reassessment of published data suggests that kimberlite magmas can erupt in a variety of ways and that most published data, taken from the largest kimberlite pipes, may not be representative of kimberlite volcanism as a whole. This refuels long-standing debates as to whether kimberlite pipes (diatremes) primarily result from phreatomagmatic eruptions (as in basaltic volcanism) or from volatile-driven magmatic eruptions.

Kimberlite Volcanism

Kimberlite magmas are volatile-rich, silica-poor ultrabasic magmas originating as small-degree mantle melts at depths of 150 km or greater. Alteration and entrained xenoliths obscure their original magma chemistry and properties. Kimberlite magmas decrease temperature by a few hundred degrees during ascent. Changes of melt composition can result as a function of assimilation. Stalling of kimberlite can result in fractional crystallization, loss of xenocrysts, and loss of volatiles. Multiple pulses of kimberlite magmas form several distinct geological units in the same pipe or intrusion. Kimberlite pipes form by explosive disruption and deformation of country rocks. Confinement in a pipe introduces new processes such as fluidization, dynamic sintering, and intense mixing between volcanic jets and concentrated trapped mixtures. Occurrences of extravent and crater-fill lithofacies indicate that kimberlite eruptions generate eruptive products that are similar to those produced by common magma types. Alteration is largely attributed to hydrothermal systems, diagenesis, and weathering involving external water.

Digestion Fractional Crystallization (DFC): an Important Process in the Genesis of Kimberlites. Evidence from Olivine in the Majuagaa Kimberlite, Southern West Greenland

High-precision electron microprobe data for major and trace elements (nickel, calcium and manganese) in the margins and rims of groundmass olivine grains from the Majuagaa kimberlite (sensu stricto) in southern West Greenland are presented. Despite a range of olivine core compositions defining several zoning types most of the olivine grains have similar margins. The olivine cores are considered to be xenocrysts in the kimberlite magma whereas the margins represent cognate olivine crystallized from the kimberlite melt itself. We evaluate models of olivine margin formation by fractional crystallization, fractional crystallization with simultaneous digestion of xenoliths, and diffusion. Only fractional crystallization coupled with digestion of xenocrysts (primarily orthopyroxene), with subsequent minor diffusion, can account for the observed compositional profiles in the olivine margins. We propose that the digestion crystallization process, whereby the entrained xenolithic material reacts with the kimberlite melt, is important in the evolution of kimberlite magmas. We suggest that this type of reaction gives a distinct local flavour to the parental kimberlite melt and can account for the similarities and differences between the various kimberlitic (sensu lato) magma types present in southern West Greenland and worldwide.

Deep processes and indigenous diamond deposits

Processes of formation and transportation of kimberlite magma and diamonds in the upper mantle and the Earth's crust have been considered. Analysis corresponds to the ideas of advec-tion-polymorph hypothesis. The Klifford rule has been considered, the depth of the main diamond generation and the depth of primary source of magma have been identified.

The stability of ortho- and clinopyroxenes, olivine, and garnet in kimberlitic magma

It is generally accepted that the composition of ultrabasic nodules and their quantitative proportions do not significantly change during their transportation with kimberlitic magma to the Earth’s surface. We performed an experimental study of the relative stability of olivine, garnet, and pyroxenes in kimberlite melt at high pressure and temperatures (4 GPa, 1300–1500 °C). The study has shown that the loss in weight of minerals and, correspondingly, the rate of their dissolution in kimberlite melt differ considerably. The following sequence of the dissolution rates of minerals has been established: Cpx ≥ Opx > Gar > Ol. Pyroxenes are characterized by the most rapid dissolution, and olivine is the most stable mineral. The assumption is made that clinopyroxenites and websterites disintegrate more rapidly than dunites and lherzolites in kimberlitic magma.

Provenance of zircon xenocrysts in the Neoproterozoic Brauna Kimberlite Field, São Francisco Craton, Brazil: Evidence for a thick Palaeoproterozoic lithosphere beneath the Serrinha block

The 642 Ma-old Brauna Kimberlite Field is located on the northeastern sector of the São Francisco Craton (Serrinha block) and is one of the rare Neoproterozoic kimberlitic events in South America. Zircon xenocrysts from the volumetric most important kimberlite pipes Brauna 03, Brauna 07 and Brauna 04 were used as a tool to identify different components of the lithosphere beneath the northeast region of the São Francisco craton. A composite kimberlite sample of eight representative and different drill holes and three samples of the host rocks (Nordestina granodiorite) were sampled for SHRIMP geochronology. The results were compared with precise U–Pb age data for the regional rocks, i.e. the Archaean basement and the Palaeoproterozoic Rio Itapicuru greenstone belt. Samples from the Nordestina granodiorite gave three different ages: 2155 Ma in the western part of the batholith, 2139 Ma in its central part, and 2132 Ma in its eastern part. Zircon 207Pb/206Pb ages of the Brauna kimberlite zircon grains spread over the timespan 2107–2223 Ma and indicate four age groups at 2105 ± 3 Ma, 2138 ± 7 Ma, 2166 ± 5 Ma, and 2198 ± 4 Ma. Source rocks for the three former age groups can be found in the Rio Itapicuru greenstone belt, including zircon xenocrysts from the Nordestina granodiorite, whereas the latter age group has not yet been reported in the Serrinha block. The new zircon ages show that only rocks of the Palaeoproterozoic Rio Itapicuru greenstone belt and of a hidden 2.17–2.20 Ga crust were sampled by the kimberlite magma during its ascent through the lithosphere. It is proposed that there is none or a few Archaean crust beneath the kimberlite emplacement area, hence implying a thick Palaeoproterozoic lithosphere for this portion of the São Francisco craton.

Rapid ascent conditions of diamond-bearing kimberlitic magmas: Findings from high pressure–temperature experiments and finite element modeling

This paper deals with the problem of rapid ascent mechanism of kimberlite magmas with a multi-directional approach: 1) the kinetics of diamond–graphite transition; 2) settling velocity of diamond phenocrysts in magmas and 3) formation of ruptures required for magma ascent with a high speed. Based on the diamond–graphite transition, we present an estimate of the ascent rates from high pressure and temperature experiments using Walker-type multi-anvil apparatus. The experiments were conducted with diamond, placed within a synthetic kimberlitic assemblage, keeping an initial pressure and temperature of 6GPa and 1350°C, respectively. It was observed that the volume fraction of diamond to graphite conversion strongly depended on the ascent rates. Using electrical resistivity and X-ray diffraction studies, we measured the degree of graphitization as a function of the ascent rate (u). For u<3m/s, diamond underwent almost complete graphitization (conversion>90%), whereas it remained nearly intact (conversion<10%) when u>10m/s. Our theoretical calculations of the settling velocity of mantle xenoliths again confirm that diamond can exist when u>3m/s. We performed numerical experiments with finite element (visco-elastic) models to analyze the dynamics of tensile failure at the tip of magma pools, leading to dilatational vertical fractures for magma transport. Considering the tensile strength of mantle in the order of 0.5kb, our models show this failure process as a function of the critical shape (Ar: ratio of vertical and horizontal dimensions) and density contrast (Δρ) of magma pools. The critical Δρ is estimated nearly 200kg/m3 when Ar is very large (>4).