Kimberlite Eruption Research Articles

Picroilmenites in Yakutian kimberlites: variations and genetic models

Abstract. Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modeling, shows long compositional trends for the ilmenites. These are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite minerals from stage to stage and splitting of the magmatic system in the final stages. High-pressure (5.5–7.0 GPa) Cr-bearing Mg-rich ilmenites (group 1) reflect the conditions of high-temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1–10/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb > 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) trace the high-temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10–100)/PM with La / Ybn ~ 10–25, similar to those derived from kimberlites, with high-field-strength elements (HFSE) peaks (typical megacrysts). A series of similar patterns results from polybaric Assimilation + fractional crystallization (AFC) crystallization of protokimberlite melts which also precipitated sulfides (Pb < 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high-Cr content (group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr-rich ilmenites are typical of veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. Ilmenites which have the highest trace element contents (1000/PM) have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet–spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn–Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.

High-Fe (Mg, Fe)O inclusion in diamond apparently from the lowermost mantle

The only direct samples of Earth's lower mantle are rare inclusions in diamonds. The most abundant are (Mg, Fe)O, showing highly magnesian compositions. Here we report a transmission electron microscopy study of an exceptional inclusion with composition ∼(Mg0.35Fe0.65)O, with 5–7 vol% nanometer-sized magnesioferrite spinel precipitated on dislocations and the interface with diamond. The magnesioferrite apparently precipitated as a high-pressure polymorph from the (Mg, Fe)O host that contained an extraordinarily large concentration of Fe3+. Paradoxically, the crystal also contains blebs of Fe–Ni alloy. These observations plus the recently documented very low seismic velocities of high-Fe (Mg, Fe)O and other experimental results suggest that this inclusion may have originated in one of the ultra-low velocity zones at the base of the mantle and come to the surface through diapiric upwelling culminating in kimberlite eruption.

Repeated kimberlite magmatism beneath Yakutia and its relationship to Siberian flood volcanism: Insights from in situ U–Pb and Sr–Nd perovskite isotope analysis

We report combined U–Pb ages and Sr–Nd isotope compositions of perovskites from 50 kimberlite occurrences, sampled from 9 fields across the Yakutian kimberlite province on the Siberian craton. The new U–Pb ages, together with previously reported geochronological constraints, suggest that kimberlite magmas formed repeatedly during at least 4 episodes: Late Silurian–Early Devonian (419–410 Ma), Late Devonian–Early Carboniferous (376–347 Ma), Late Triassic (231–215 Ma), and Middle/Late Jurassic (171–156 Ma). Recurrent kimberlite melt production beneath the Siberian craton – before and after flood basalt volcanism at 250 Ma – provides a unique opportunity to test existing models for the origin of global kimberlite magmatism.The internally consistent Sr and Nd isotope dataset for perovskites reveals that the Paleozoic and Mesozoic kimberlites of Yakutia have distinctly different initial radiogenic isotope compositions. There exists a notable increase in the initial 143Nd/144Nd ratios through time, with an apparent isotopic evolution that is intermediate between that of Bulk Earth and Depleted MORB Mantle. While the Paleozoic samples range between initial 87Sr/86Sr of 0.7028–0.7034 and 143Nd/144Nd of 0.51229–0.51241, the Mesozoic samples show values between 0.7032–0.7038 and 0.51245–0.51271, respectively. Importantly, perovskites from all studied Yakutian kimberlite fields and age groups have moderately depleted initial εNd values that fall within a relatively narrow range between +1.8 and +5.5.The perovskite isotope systematics of the Yakutian kimberlites are interpreted to reflect magma derivation from the convecting upper mantle, which appears to have a record of continuous melt depletion and crustal recycling throughout the Phanerozoic. The analyzed perovskites neither record highly depleted nor highly enriched isotopic components, which had been previously identified in likely plume-related Siberian Trap basalts. The Siberian craton has frequently been suggested to represent a type example of an association between kimberlite eruptions and flood basaltic volcanism within a single large igneous province (LIP), but our new extensive age and isotopic tracer constraints do not support a genetic link between these contrasting types of mantle-derived magmatism.

Rapid erosion of the Southern African Plateau as it climbs over a mantle superswell

AbstractWe present new sedimentary flux data confirming that a large pulse of erosion affected the Southern African Plateau in the Late Cretaceous and is likely to be related to a major uplift episode of the plateau. This short phase of erosion (i.e., less than 30 Myr in duration) has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift: given its size, the rise of the African superplume is likely to have lasted much longer. Here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent‐wide drainage reorganization, leading to substantial denudation in a relatively short amount of time. The amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the Karoo sedimentary and volcanic rocks and the underlying basement. We also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse, suggesting that climate may have also played a role in triggering the denudation. The diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are, however, better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling.

Linking continental deep subduction with destruction of a cratonic margin: strongly reworked North China SCLM intruded in the Triassic Sulu UHP belt

The collision between the North and South China cratons in Middle Triassic time (240–225 Ma) created the world’s largest belt of ultrahigh-pressure (UHP) metamorphism. U–Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ≥3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (≥3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.

Mantle eclogites and garnet pyroxenites – the meaning of two-point isochrons, Sm–Nd and Lu–Hf closure temperatures and the cooling of the subcratonic mantle

The Earthʼs mantle is a huge metamorphic complex which undergoes permanent changes ruled by plate tectonics. It also has enclaves underneath Archean crust which are exempt from the convecting mantle since at least 2.5 Ga. Since then, this mantle may cool slowly as a result of diminishing heat input from the asthenosphere, of declining heat production from radioactive decay and of denudation of the crust. Under such circumstances, two point garnet–clinopyroxene isochrons from mantle xenoliths (here garnet pyroxenites and eclogites) reflect either cooling ages or eruption ages depending on whether the mantle portion under consideration was below or above the closure temperature of a radiogenic system. Available literature data from the Slave and Kaapvaal craton for the Sm–Nd and Lu–Hf isotope systems and our own new data from Bellsbank (Kaapvaal) provide a whole range of two-point isochron ages from younger than the kimberlite eruption age to early Proterozoic. The meaning of ages other than the kimberlite eruption age is unclear. We use here a compilation of Sm–Nd and Lu–Hf two-point isochron age data from this study and the literature, to assess the meaning of such isochrons. This is achieved by plotting the temperature of last equilibration as derived from the Fe–Mg exchange between garnet and clinopyroxene versus the two-point isochron age. There is a low temperature alignment for both systems of increasing age with decreasing temperature and an alignment around the kimberlite eruption ages at high temperatures. We interpret the intersect between the low temperature limb and the kimberlite eruption age as closure temperature which gives about 920 °C for the Lu–Hf system and about 850 °C for the Sm–Nd system.The differences of the cooling ages between the two isotope systems for individual samples combined with the closure temperatures from this study are used to deduce the cooling of the Slave and Kaapvaal subcratonic mantle from the early Proterozoic until today. We deduce that the subcratonic mantle cooled since 2 Ga from a geothermal gradient of 46 mW/m2 to 38 mW/m2 at 120 Ma ago with a rate of around 0.1 °C/Ma. This value is in agreement with previous estimates which were based on various geophysical and geological constraints, heat production by radioactive elements and diffusivity of radiogenic elements.

Age and evolution of the lower crust beneath the western Churchill Province: U–Pb zircon geochronology of kimberlite-hosted granulite xenoliths, Nunavut, Canada

Zircon from granulite-facies xenoliths can preserve unique geodynamic records of deep and mostly inaccessible levels of the continental crust within Archean cratons. We present the first such records from beneath two Archean crustal blocks that were affected by multiple episodes of regional-scale Proterozoic metamorphism, and are located within the western Churchill Province, Nunavut, Canada. Zircon U–Pb geochronology and trace element analysis were carried out on samples that originated in the deep crust at the time of kimberlite eruption, as confirmed with Raman spectroscopy. Zircon was recovered from four metabasites (grt–cpx–plg) from the Rankin Inlet area of the Chesterfield block, and five metabasites (grt–cpx–plg±hbl±opx) and three metatonalite/anorthosites (plg–cpx–plg±hbl±opx) from the Repulse Bay area of the Rae craton. SEM-CL imaging reveals several types of growth zoning diagnostic of igneous and metamorphic zircon crystallization. The latter are related to local high-temperature recrystallization and micro-fracturing, and are spatially associated with zones of apparent Pb-loss and/or intracrystalline Pb-variability. Evidence for the formation of Paleoarchean/Mesoarchean to Neoarchean crust is preserved in oscillatory zoned, igneous cores having 207Pb/206Pb ages of ca. 3.5Ga and ca. 3.0–2.6Ga. Two periods of Paleoproterozoic metamorphism and zircon growth followed, with the oldest episode at ca. 2.0–1.9Ga in the Chesterfield block. Metamorphic zircon rims from all twelve xenoliths have an age range of ca. 1.75 to 1.70Ga. The trace element compositions of the metamorphic rims differ from those of older igneous and metamorphic cores, with the rims characterized by lower ΣHREE contents and low (Lu/Gd)N, suggesting that garnet crystallization occurred syn- to pre-rim growth at ca. 1.75Ga. The Archean and Paleoproterozoic ages correspond broadly to the upper crustal histories of the western Churchill Province; however, the prevalence of ca. 1.75–1.70Ga metamorphic rims indicates regional-scale metamorphic reworking of the lower crust across the region, during a period of intracratonic extension and associated magmatism. Regional magmatic underplating and/or remobilization of the lithosphere at ca. 1.75–1.70Ga, to an extent not widely appreciated, represents a terminal stage in western Churchill cratonization, including stabilization of reworked Archean crust relative to underlying diamondiferous mantle.

Kimberlite (U-Th)/He dating links surface erosion with lithospheric heating, thinning, and metasomatism in the southern African Plateau

Documenting the surface response to processes at depth is a central challenge in continental tectonics. Kimberlites bear an unusual record of both surface and deep processes through their downrafted crustal clasts, pipe erosion levels, and mantle xenolith suites. Here we combine new apatite (U-Th)/He (AHe) data from four South Africa kimberlites ranging in emplacement age from ca. 143 Ma to ca. 74 Ma with a wealth of other geologic information from the pipes to resolve the timing, patterns, and causes of erosion across ∼200 km of the southern African Plateau. Comparison of the AHe dates with the kimberlite eruption ages constrains a strong regional unroofing pulse ca. 117–90 Ma, which was most intense from ca. 100 to 90 Ma. This unroofing phase is synchronous with warming, thinning, and metasomatism of the mantle lithosphere recorded by xenocrysts and peridotite xenoliths within these same kimberlites. These thermochemical changes could be related to still deeper dynamic mantle processes. Our AHe data appear to directly document the erosional response to surface uplift induced by these various events. The AHe results also detect as much as 1.5 km of Cenozoic unroofing that is spatially centered on a proposed mid-Tertiary paleotributary to the Orange River, suggesting drainage patterns as the cause. Our results not only tightly refine the surface history of the interior of the southern African Plateau, but show that kimberlite AHe dating can tie cratonic erosion phases with contemporaneous thermochemical changes in the mantle and thereby point to causative links.

Two episodes of fluid migration in the Kaapvaal Craton lithospheric mantle associated with Cretaceous kimberlite activity: Evidence from a harzburgite containing a unique assemblage of metasomatic zirconium-phases

In this study we combine textural evidence with mineral chemical, mineral structural and isotopic data in an attempt to reconstruct the history of metasomatic events recorded in a spinel-harzburgite xenolith which was sampled by a Cretaceous kimberlite of the central Kaapvaal Craton, South Africa. Two episodes of interaction between the host rock and silicate-rich high-density fluids can be distinguished which together gave rise to a uniquely complex assemblage of LILE–HFSE-rich phases including zircon together with both monoclinic (baddeleyite) and cubic (tazheranite) zirconium oxide, srilankite and a new Mn–Fe-rich member of the pyrochlore-group of phases. The pre-metasomatic assemblage is olivine+orthopyroxene+Cr-spinel. Subsequent modal metasomatism formed phlogopite+K-richterite+crichtonite-group (lindsleyite–mathiasite) phases+Nb–Cr-rich rutile+srilankite+zircon+Fe–Ni-sulfide. K-richterites are strongly zoned in Ca, Na, Fe and Cr with up to 2.3wt.% Cr2O3 which is the highest Cr content reported so far for K-richterite. SIMS U–Pb dating of the zircons yields ages in the range from 81±2 to 91±2 (2σ) Ma which are indistinguishable from emplacement ages of Cretaceous kimberlites in the Kimberley area. The age spread is interpreted as a result of minor re-setting of the U–Pb isotopic system. The coincidence between zircon and kimberlite eruption ages further supports a temporal and genetic link between Cretaceous kimberlite activity and hydrous potassic metasomatism in the central Kaapvaal Craton lithosphere and limits the residence time in the mantle of metasomatized peridotites to <~5–10Ma. Thermobarometry of the harzburgite yields 750–760°C at 3GPa with a redox state of +0.9 to +1.5 log units relative to FMQ. Infiltration of a hot and alkali-rich (kimberlitic?) high-density fluid with aSiO2 lower than that defined by olivine+orthopyroxene into the cool metasomatized peridotite led to partial breakdown of K-richterite, crichtonite-group phases, zircon and sulfides and formed Al–Ti–Ni-rich spinel+K–Cr priderite+Nb-rich titanite+baddeleyite+tazheranite+Mn–Fe-betafite. In addition, a second generation of Ni-poor and Mn-rich olivine formed along with Ti-rich K-richterite, phlogopite and clinopyroxene. Both tazheranite and Mn–Fe-betafite were unambiguously identified with electron microprobe analysis combined with selected area diffraction using FIB–TEM. Mn–Fe-betafite is new member of the pyrochlore group of phases and the tazheranite from this study represents the first occurrence reported from an upper mantle environment.

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.

The provenance of sub-cratonic mantle beneath the Limpopo Mobile Belt (South Africa)

Petrological, whole rock major element and mineral chemical analysis of mantle xenoliths from the Venetia kimberlite pipes (533Ma) in South Africa reveals an apparently stratified cratonic mantle beneath the Central Zone of the Limpopo Mobile Belt (LMB) that separates the Kaapvaal and Zimbabwe Cratons. Combined pressure–temperature (P–T) data and petrographic observations indicate that the mantle consists of an upper layer of Low-T coarse-equant garnet+spinel lherzolite (~50 to ~130km depth). This layer is underlain by a region of mixed garnet harzburgites and garnet lherzolites that are variably deformed (~130 to ~235km depth). An equilibrated geotherm did not exist at the time of kimberlite eruption (533Ma) and a localised heating event involving the introduction of asthenospheric material to the High-T lithosphere below 130km is inferred.Low-T garnet–spinel lherzolites are highly melt depleted (40% on average). In contrast, the High-T lithosphere (mostly at diamond stable conditions) consists of a mixed zone of variably sheared and melt depleted (30% on average) garnet harzburgite and mildly melt depleted (20% on average) garnet lherzolite. The chemistry of the High-T xenoliths contrasts with that of minerals included in diamond originating from the same depth. Inclusions suggest diamond crystallisation in a more melt depleted lithosphere than represented by either Low- or High-T xenoliths. High-T xenoliths are proposed to represent formerly melt depleted lithosphere, refertilised by asthenosphere-derived melts during the diapiric rise of a proto-kimberlitic melt pocket. This process is coupled to the positive temperature perturbation observed in the High-T xenoliths and may represent a common process in the lower lithosphere related to localised but intense tectono-magmatic events immediately preceding kimberlite eruption.The presence of clinopyroxene, garnet and abundant orthopyroxene in the Low-T lherzolite implies a history of melt depletion followed by metasomatic addition of Si–Al–Ca, forming high-temperature orthopyroxene from which clinopyroxene and garnet exsolved. Si enrichment is a characteristic feature of the majority of the Kaapvaal Craton to the south of the LMB but not of the Zimbabwe Craton to the north, implying a Kaapvaal origin. The provenance of the High-T lithosphere beneath the LMB is less well constrained as it is intensely modified by kimberlitic magmatism and diamond inclusion chemistry does not show significant systematic variation across the cratons. The presence of rare, mildly silica enriched high-temperature harzburgites suggests that a Kaapvaal origin for the entire lithosphere beneath the LMB is most likely.

Kimberlite eruptions as triggers for early Cenozoic hyperthermals

AbstractThe early Cenozoic experienced at least three short but major hyperthermals associated with disruptions of the global carbon cycle. The largest among those, the Paleocene‐Eocene thermal maximum, was associated with a negative carbon isotopic excursion of ~ 2.5‰ that appears to be best explained by the thermal dissociation of methane hydrates due to an initial period of warming. The cause of the initial warming has been attributed to a massive injection of carbon (CO2 and/or CH4) into the atmosphere; however, the source of the carbon is as yet unknown. The emplacement of a large cluster of kimberlite pipes at ~56 Ma in the Lac de Gras region of northern Canada may have provided the carbon that triggered early warming in the form of exsolved magmatic CO2. Our calculations indicate that the estimated 900–1100 Pg of carbon required for the initial ~3°C of ocean water warming associated with the Paleocene‐Eocene thermal maximum could have been released during the emplacement of a large kimberlite cluster. The coeval ages of two other kimberlite clusters in the Lac de Gras field and two other early Cenozoic hyperthermals indicate that CO2 degassing during kimberlite emplacement is a plausible source of the CO2 responsible for these sudden global warming events.

P‐wave tomography of eastern North America: Evidence for mantle evolution from Archean to Phanerozoic, and modification during subsequent hot spot tectonism

The unique physical and chemical properties of cratonic lithosphere are thought to be key to its long‐term survival and its resistance to pervasive modification by tectonic processes. Study of mantle structure in southeast Canada and the northeast US offers an excellent opportunity to address this issue because the region spans 3 billion years of Earth history, including Archean formation of the Superior craton and younger accretion of terranes to eastern Laurentia during the Proterozoic Grenville and Phanerozoic Appalachian orogenies. Trending NW–SE through each of these terranes is the track of the Great Meteor hot spot, which affected the region during the Mesozoic. Here we study mantle seismic velocity structure beneath this region of eastern North America using tomographic inversion of teleseismic P‐wave relative arrival‐times recorded by a large‐aperture seismograph network. There are no large‐scale systematic differences between Superior and Grenville mantle wave speed structure, which may suggest that tectonic stabilization of cratons occurred in a similar fashion during the Archean and Proterozoic. Cratonic lithosphere is largely thought to be resistant to modification by hot spot processes, in contrast to younger terranes where lithospheric erosion and significant magmatism are expected. Low velocities beneath the regions affected by the Great Meteor hot spot are broadest beneath the Paleozoic Appalachian terranes, indicating pervasive modification of the lithosphere during magmatism. The zone of modification narrows considerably into the Proterozoic Grenville province before disappearing completely in the Archean Superior craton, where the surface signature of Mesozoic magmatism is limited to kimberlite eruptions.

Rapid magma ascent and short eruption durations in the Lake Natron–Engaruka monogenetic volcanic field (Tanzania): A case study of the olivine melilititic Pello Hill scoria cone

The Pello Hill scoria cone displays widely different characteristics dependent on which side of the cone is studied. This follows as a direct result of a strong depositional asymmetry (i.e., the crater rim varies between 14 and 111m in height), preferentially depositing material to the northwestern side of the construct. This is interpreted to reflect sedimentation from a sustained eruption plume (with prevailing winds at the time of the eruption coinciding with the direction of maximum deposition). The scoria deposits on this side of the cone form relatively fine-grained and well-sorted deposits which are laterally continuous over distances of 10's of meters. To all other sides of the vent, deposits are characterized by coarse-grained lenticular deposits rich in mantle xenoliths, occasionally showing inverse grading (consistent with an origin as ballistic ejecta and slight reworking down-slope as grain-flows). The pyroclastic textures are dominated by a moderate vesicularity (~40vol. %) in combination with smooth, fluidal, outer surfaces. Two other features that stands out in comparison with “normal” scoria cones, these are defined by the absence of: (i) inward dipping layers into the crater area, and (ii) no agglutination/welding features can be found in any of the exposed outcrops.Calculated magma ascent rates yields values between 8.5 and 36.0m s–1, which is similar to that previously reported for kimberlitic magmas. Building on these ascent rates, and the volume of the pyroclastic construct (~5×106 m3 DRE), the eruption duration is estimated to less than 6hours (for any vent area larger than 28m2). Therefore, it may not have taken longer time than 9hours from the time the magma started to ascend from upper-mantle depth (90km) and the point at which the eruption halted.Overall, the olivine melilititic Pello Hill scoria cone displays many characteristics that can also be found in the vent-facies deposits of kimberlite eruptions. Therefore, further detailed studies of well-preserved, CO2-rich, olivine melilitite eruptions (such as the provided by the landforms within the LNE-MVF and Pello Hill) could provide a tool to understand the emplacement dynamics of the vent-facies of kimberlitic eruptions.

Evolution of the South African mantle—a case study of garnet peridotites from the Finsch diamond mine (Kaapvaal craton); Part 2: Multiple depletion and re-enrichment processes

Recalculated whole rock major-, trace-element and isotope compositions from mineral analyses of Finsch mantle peridotites indicate a complex history of the South African sub-continental lithospheric mantle (SA-SCLM). According to the Lu–Hf whole rock isotopic data last depletion coupled with the final cratonization of the Kaapvaal craton underneath Finsch occurred at ~2.6Ga. The data of this study indicate several depletion and enrichment processes during that early history of Finsch SCLM. Calculated degrees of partial melting using major and rare earth element (REE) compositions show that all depletion episodes together have removed more than 40% of melt from the SA-SCLM.As indicated by elevated Hf concentrations and subchondritic Lu/Hf, many Finsch peridotites were metasomatically overprinted between the last and earlier depletion stages. Silica enrichment, expressed in high modal abundance of orthopyroxene in the majority of the samples likely took place at that early stage. The last coupled melting and re-enrichment event (at ~2.6Ga) resulted in complete re-equilibration of trace element and Hf isotope signatures at the whole rock scale and thus, erased most signs of previous events. After the final depletion and before the final kimberlite eruption, the Finsch mantle suffered from further metasomatic overprints. Metasomatism is expressed in the enrichment of light REE (LREE) and some large ion lithophile elements (e.g. Sr, Rb, Th). Additionally, the Nd isotope signatures of all samples as well as the Hf isotope compositions of some samples were modified.Late enrichment is documented by a modally metasomatized sample that contains rutile and sulfide. This sample is characterized by strong enrichment of high-field-strength elements and LREE and was probably overprinted with a sulfide bearing silico-carbonatitic melt with high contents of Ti, Fe and Ni. This melt likely originated from an enriched mantle that was isolated for a long time during which it developed a very unradiogenic Hf isotope signature, characteristic for this type of metasomatism.

Evolution of the South African mantle — A case study of garnet peridotites from the Finsch diamond mine (Kaapvaal craton); part 1: Inter-mineral trace element and isotopic equilibrium

A thorough assessment of inter-mineral equilibrium is essential for the understanding of trace element partitioning and also for the interpretation of isotopic data. Here we investigated high temperature (~1200°C and 6GPa) garnet peridotite xenoliths from the Kaapvaal craton (Finsch mine, South Africa), with exceptionally well equilibrated mineral major element compositions, for their trace element and isotopic inter-mineral equilibrium. Trace element compositions for all major mineral phases, i.e. olivine, orthopyroxene (opx), clinopyroxene (cpx) and garnet, were analysed by laser ablation inductively coupled plasma mass spectrometry (ICP-MS). Garnet, cpx and opx of selected samples were analysed for their Sm–Nd and Lu–Hf isotope compositions by multi collector ICP-MS.Most important mineral characteristics include: a) olivines from most samples are enriched in high-field-strength elements relative to other incompatible trace elements. Their lithium content correlates negatively with Mg#, indicating a depletion signature; b) all other minerals are depleted in heavy and middle rare earth elements (H- and M-REE) and enriched in light REE and large ion lithophile elements. This implies a complex history of depletion and metasomatic overprint for the Finsch cratonic mantle; c) orthopyroxene has similarly shaped trace element patterns as cpx, with one to two orders of magnitude lower abundances; and d) both, garnet and cpx, display variable, mostly positive εHf coupled with moderate variations in εNd.Trace element partitioning between garnet/cpx, cpx/opx and garnet/opx, displays only a weak pressure and temperature dependency. However, equilibrium partitioning of most trace elements between garnet and cpx shows a strong compositional dependency, i.e. on the Cr- (and Ca-) content of the garnets. Garnet–cpx partition coefficients follow a second grade polynomial correlation with Cr2O3 of garnet, whereby high chromium garnets (Cr2O3>6wt.%) have generally higher partition coefficients for all trace elements than low Cr garnets. Potentially, enhanced trace element compatibility in high-Cr garnets may be related to the formation of a knorringite component and its influence on the garnet crystal structure. However, some elements, especially the quadrivalent Zr and Hf and the small trivalent HREE, show increased scatter indicating deviation from complete inter-mineral equilibrium.Garnet–cpx Sm–Nd isochrons of all analysed samples are consistent with the age of the Finsch kimberlite eruption (~118Ma). This further demonstrates complete isotopic equilibration of Sm and Nd between garnet and cpx for these mantle samples. In contrast to Sm–Nd, the Lu–Hf garnet–cpx isochrons display an age range between 60 and 570Ma, indicating Lu–Hf disequilibrium. Most probably slow diffusion of Hf and Lu, in the absence of a melt or fluid, resulted in incomplete Lu–Hf equilibration and the preservation of an “old” radiogenic Hf in garnet and cpx.

Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania)

The Igwisi Hills volcanoes (IHV), Tanzania, are unique and important in preserving extra-crater lavas and pyroclastic edifices. They provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes. Cosmogenic 3He dating of olivine crystals from IHV lavas and palaeomagnetic analyses indicates that they are Upper Pleistocene to Holocene in age. This makes them the youngest known kimberlite bodies on Earth by >30 Ma and may indicate a new phase of kimberlite volcanism on the Tanzania craton. Geological mapping, Global Positioning System surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices, craters and lavas. The volcanoes stand <40 m above the surrounding ground and are comparable in size to small monogenetic basaltic volcanoes. Pyroclastic cones consist of diffusely layered pyroclastic fall deposits comprising scoriaceous, pelletal and dense juvenile pyroclasts. Pyroclasts are similar to those documented in many ancient kimberlite pipes, indicating overlap in magma fragmentation dynamics between the Igwisi eruptions and other kimberlite eruptions. Characteristics of the pyroclastic cone deposits, including an absence of ballistic clasts and dominantly poorly vesicular scoria lapillistones and lapilli tuffs, indicate relatively weak explosive activity. Lava flow features indicate unexpectedly high viscosities (estimated at >102 to 106 Pa s) for kimberlite, attributed to degassing and in-vent cooling. Each volcano is inferred to be the result of a small-volume, short-lived (days to weeks) monogenetic eruption. The eruptive processes of each Igwisi volcano were broadly similar and developed through three phases: (1) fallout of lithic-bearing pyroclastic rocks during explosive excavation of craters and conduits; (2) fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent; and (3) effusion of degassed viscous magma as lava flows. These processes are similar to those observed for other small-volume monogenetic eruptions (e.g. of basaltic magma).

A Microscopy and FTIR and PL Spectra Study of Polycrystalline Diamonds from Mengyin Kimberlite Pipes

The results of a microscopy and FTIR and PL spectra study of the natural polycrystalline diamonds from the Mengyin kimberlite pipes show that they can be classified as the euhedral faceted polycrystalline diamonds (EFPCDs) and anhedral rounded polycrystalline diamonds (ARPCDs). Different diamond grains or their points were formed in different conditions or processes. They were not formed in diamond nucleation stage, but in the diamond growth period. They probably originated from the relatively deeper mantle and were formed in the environment like the peridotitic (P) type diamond single crystals. The EFPCDs did not undergo a remarkable dissolution process during their formation and were possibly fast formed shortly before the kimberlite eruption. The ARPCDs not only were formed at a higher temperature than the EFPCDs but also underwent a notable dissolution process and had been stored relatively longer in the mantle. Fluids or melts probably participated in the formation of the ARPCDs or modified them during the period of their storage in the mantle.

U–Pb SHRIMP geochronology of zircon from the Catoca kimberlite, Angola: Implications for diamond exploration

We present the first age determinations of zircon from the diamondiferous Catoca kimberlite in northeastern Angola, the fourth largest kimberlite body in the world. The U–Pb ages were obtained using a Sensitive High Resolution Ion Microprobe II (SHRIMP II) on zircon crystals derived from tuffisitic kimberlite rocks and heavy-mineral concentrates from the Catoca kimberlite. The SHRIMP results define a single weighted mean age of 117.9±0.7Ma (Mean square weighted deviation MSWD=1.3). More than 90% of the results indicate a single age population. There is no evidence for variable ages within single crystals, and no diffusional profiles are preserved. These data are interpreted as the maximum age of the kimberlite eruption at Catoca. The U/Th values suggest at least two different sources of zircon crystals. These different populations may reflect different sources of kimberlitic magma, with some of the grains produced in U- and Th-enriched metasomatized mantle units. This idea is consistent with the two populations of zircon identified in this study. One population originated from a depleted mantle source with low total REE (less than 25ppm), and the other was derived from an enriched source, likely from the mantle or a carbonatite-like melt with high total REE (up to 123ppm).The tectonic setting of northeastern Angola is influenced by the opening of the south Atlantic, which reactivated deep NE–SW-trending faults during the early Cretaceous. The eruption of the Catoca kimberlite can be correlated with these regional tectonic events. The Calonda Formation (Albian–Cenomanian age) is the earliest sedimentary unit that incorporates eroded material derived from the diamondiferous kimberlites. Thus, the age of the Catoca kimberlite eruption is restricted to the time between the middle of the Aptian and the Albian. The new interpretation will be an important guide in future exploration for diamonds because it provides precise data on the age of a diamond-bearing kimberlite pulse in Angola.

Multi-stage origin of Roberts Victor eclogites: Progressive metasomatism and its isotopic effects

Xenolithic eclogites are high-pressure, high-temperature garnet–clinopyroxene rocks brought from the mantle to the surface by kimberlites and other primitive magmas. Their origins have been controversial for decades: do they represent metamorphosed subducted oceanic crust or magmatic rocks originally crystallized in the deep earth? The answer has important implications for the definition of major Earth processes.The extensive eclogite suite from the Roberts Victor kimberlite (South Africa) has previously been divided into two types (Type I and II); this study proposes five subgroups (IA, IB, IK; IIA, IIB) based on mineral assemblages and compositions. All of these eclogites were derived from depths of 170–200km, near the base of the contemporary subcontinental lithospheric mantle.The fresh, microstructurally equilibrated Type IIA eclogites are the protoliths of Type I, which were heavily metasomatized by carbonatitic–kimberlitic melts/fluids to form Types IAIBIK. All the Type I rocks show extensive textural disequilibrium, and have abundant fluid inclusions, secondary phases, and higher δ18O than Type II (IA: 5.0–8.0‰; IB: 4.1–6.8‰; IK:2.2–6.8‰; IIA=3.5‰; IIB:2.2–3.9‰). Type I are richer in LREE and LILE than Type IIA; they also are isotopically more homogeneous, with higher 87Sr/86Sr and less radiogenic Nd and Hf isotopes. Type I eclogites give gnt–cpx isochron ages (SmNd (103±10Ma); LuHf (132±16Ma)) similar to the kimberlite eruption age (128±15Ma). Type IIA and IIB eclogites have unradiogenic Sr and radiogenic Nd and Hf isotopic compositions, and give a range of Proterozoic two-mineral “ages” (SmNd: 738–1143Ma; LuHf: 1148–1544Ma), reflecting some preservation of their original isotopic compositions. Type IIB are broadly similar to Type IIA, but have lower MgO; their mutual relationships are not clear.Neither Type I nor Type II eclogites are similar to modern or Archean oceanic crust when all the chemical evidence is compared. In any case, the data from Type I eclogites cannot be used to support a “subduction model” of origin because their major- and trace-element chemistry, and radiogenic- and stable-isotope compositions, reflect the extensive metasomatism, rather than any primary characteristics.Type II eclogites are the key samples for the study of eclogite origins. Their bulk compositions and microstructures (exsolution of gnt and rutile) suggest that most or all the garnet in Type II samples might have formed by exsolution from high-Al clinopyroxenes, crystallized from mafic–ultramafic magmas in the deep mantle. Therefore, the xenolithic eclogites from Roberts Victor Mine probably were originally magmatic rocks crystallized at depth, rather than subducted oceanic crust; as a suite, they record a major metasomatic event in the deep lithospheric mantle, close to the time of kimberlite eruption.