Kimberlites

The Alfeu-I kimberlite is an intrusive volcanic chimney in Suite Pinheiro Machado of the Batholith Pelotas domain, which occurs in the eastern portion of the Rio Grande do Sul Shield (ESRG). The body has a semicircular shape with a diameter ≈20m. Textural and primary structural features and mineralogy observed in the rocks of this body indicate that at current exposure level can be related to diatreme facies.
The Alfeu-I rock is composed of phenocrysts and megacrystals / macrocrystals of varied composition (mica, spinel, garnet, ilmenite and pyroxene) in interstitial matrix with accreted clasts (lapilli peletal) and lithic, as mantle autoliths and xenoliths and surrounded rocks. In the optical microscope have serial inequigranular texture, composed of mica, spinel, garnet and ilmenite macrocrystals; phlogopite phenocrysts and rarely clinopyroxene and olivine microphenocrystals, surrounded by brown color rocks with aphanitic matrix. The field data and petrographic observations allowed classifying the kimberlite Alfeu-I as: intrusive and coherent kimberlite (ICK) (Smith et al 2008); diatreme facies of kimberlite tuff (Clement; Skinner, 1985); or volcanoclastic kimberlite (Mitchell, 1986; 1995). As Mitchell (1995), the body is a kimberlite Group II type, as shown by plenty of macro and phenocrysts of phlogopite.
Despite the petrography and mineralogy of Alfeu-I are compatible to kimberlites Group II type, the mineral chemistry indicates transitional points between Group I and II kimberlite and lamproites. It means that the petrogenesis of Alfeu-I is similar to other potassic “magmas from metassomatized mantle” without any strong affinity with a particular clan or group. And demonstrates that the Alfeu I-body shares petrological and geochemical characteristics that can be related to these three types of rocks (kimberlites Group I and II and lamproites). The findings based on the review and investigation of mineral chemistry compositions pointed to: (i) transitional between kimberlites Group I -Group II - and lamproite; (ii) related to a mantle spinel lherzolitic associated with pyroxenites and eclogites; (iii) enriched in shallower depths common to "kimberlitic” magmas; (iv) with minor indications of a deep, remaining mantle lherzolite garnet facie. These findings indicate that the magmatism of the Serra Geral formation is closely related to the alkaline magmatism and recent Brazilian southern portion.

Exsolution lamellae in pyroxene and garnet porphyroblasts in pyroxenite xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlites (Siberian Craton) reveal a diverse suite of exsolved phases, including oxides (spinels, ilmenite, rutile, and chromite), pyroxene, and garnet. Textural characteristics suggest that exsolved phases progressively increased in volumetric proportions, and in some cases, the bulk xenoliths transformed from a lithology dominated by coarse grains (i.e. > 2 cm; megacrystalline) to a significantly finer-grained texture (i.e. < 1 cm).These exsolved lamellae are the result of a complex and protracted sub solidus history following magmatic crystallization. Equilibrium pressure–temperature estimates place these xenoliths at low-to-moderate pressure–temperature conditions (690–910°C and 2.0–4.5 GPa) in the lithospheric mantle at the time of entrainment in the kimberlite. However, reconstructed compositions of initial pyroxene and garnet crystals suggest that this suite of pyroxenites formed at considerably higher temperatures and pressures that, in some instances, may have approached the majorite stability field. Pyroxenites that do not contain primary garnet may have been derived from shallower depths.Progressive exsolution in these pyroxenites is of importance inasmuch as such processes can permit localized changes in rheological properties and may also accommodate strain within portions of lithospheric mantle. Because most xenolith studies focus on peridotites and eclogites, the pyroxenite sample suite studied in this work represents an important contribution towards a greater understanding of the Siberian lithospheric mantle.

(By Artemieva I.M.)
A new method for modeling the thermal structure of the lithosphere, termed thermal isostasy, is presented and used to calculate the lithosphere thickness and to predict geothermal heat flux in Europe and the adjacent offshore regions. The method is based on analysis of topography deviations from the expected correlation between the Moho depth and topography. Anomalous topography is interpreted as caused by thermal anomalies in the lithosphere and by variations in the lithosphere thermal thickness. The new method allows for recovering the lithosphere thermal structure with a high lateral resolution and for testing the results by comparing the predicted and measured heat flux.
The results for Europe, constrained by the regional seismic crustal model EUNAseis, demonstrate that the lithosphere thermal structure is controlled by both the geodynamic setting and the lithosphere age. The lithosphere of the East European craton ranges in thickness from ca. 140-180 km in the paleorifts to 200-220 km on average with local lithosphere roots down to ca. 300 km depth, such as in the south-central Proterozoic Finland, and local thinning to<100 km in the Peri-Caspian Basin and the Mezen rift system. Lithosphere thinning to 70-100 km beneath southern Norway may have caused its recent uplift. The craton to noncraton transition at depth roughly follows the geological boundary but with up to 200 km lateral deviations, which indicate reworking of the cratonic lithosphere along the craton edge. Paleoproterozoic – early Paleozoic Gondwana massifs of western Europe are distinct in having cold and thick lithosphere, typically 120-180 km thick, up to 200 km thick in the London-Brabant Massif, and only ca. 100 km in the Iberian Massif. The European Cenozoic Rift System is marked by a thin (< 80 km) and hot lithosphere that forms a linear belt from southern France to the North Sea. Lithosphere thinning to 50-60 km thickness beneath the Pannonian and Po basins may indicate the onset of oceanization. Cenozoic orogens of western Europe have lithospheric roots down to ca. 200-250 km associated with subducting lithospheric slabs, while a 80 km thick lithosphere beneath the Vrancea zone is in agreement with slab delamination. The Arctic shelf of the Barents Sea has a thick cratonic lithosphere with a sharp transition from a 120-150 km thick lithosphere of the western Barents Sea to a 175-230 km thick lithosphere of the eastern Barents Sea. The block with a ca. 150 km thick lithosphere in the North Atlantic region between the Aegir paleo-spreading and Norway may represent a continental terrane within the oceanic domain. The upper mantle temperature beneath the Bay of Biscay and the Black Sea is similar to the cratonic and is anomalously cold down to a 200 km depth.

[by Cherepanova Y. and Artemieva I.M.]

Using free-board modeling, we examine a vertically-averaged mantle density beneath the Archean–Proterozoic Sibe-rian Craton in the layer from the Moho down to base of the chemical boundary layer (CBL). Two models are tested: in Model 1 the base of the CBL coincides with the LAB, whereas in Model 2 the base of the CBL is at a 180 km depth. The uncertainty of density model is b 0.02 t/m 3 or b 0.6% with respect to primitive mantle. The results, calculated at in situ and at room temperature (SPT) conditions, indicate a heterogeneous density structure of the Siberian lithospheric mantle with a strong correlation between mantle density variations and the tectonic setting. Three types of cratonic mantle are recognized from mantle density anomalies. 'Pristine' cratonic regions not sampled by kimberlites have the strongest depletion with density deficit of 1.8–3.0% (and SPT density of 3.29–3.33 t/m 3 as compared to 3.39 t/ m 3 of primitive mantle). Cratonic mantle affected by magmatism (including the kimberlite provinces) has a typical density deficit of 1.0–1.5%, indicative of a metasomatic melt-enrichment. Intracratonic sedimentary basins have a high density mantle (3.38–3.40 t/m 3 at SPT) which suggests, at least partial, eclogitization. Moderate density anomalies beneath the Tunguska Basin imply that the source of the Siberian LIP lies outside of the Craton. In situ mantle density is used to test the isopycnic condition of the Siberian Craton. Both CBL thickness models indicate significant lateral variations in the isopycnic state, correlated with mantle depletion and best achieved for the Anabar Shield region and other intracratonic domains with a strongly depleted mantle. A comparison of synthetic Mg# for the bulk lithospheric mantle calculated from density with Mg# from petrological studies of peridotite xenoliths from the Siberian kimber-lites suggests that melt migration may produce local patches of metasomatic material in the overall depleted mantle.

(by Artemieva I.M.)

The goal of the present study is to extract non-thermal signal from seismic tomography models in order to distinguish compositional variations in the continental lithosphere and to examine if geochemical and petrologic constraints on global-scale compositional variations in the mantle are consistent with modern geophysical data. In the lithospheric mantle of the continents, seismic velocity variations of a non-thermal origin (calculated from global Vs seismic tomography data [Grand S.P., 2002. Mantle shear-wave tomography and the fate of subducted slabs. Philosophical Transactions of the Royal Society of London. Series A, 360, 2475–2491.; Shapiro N.M., Ritzwoller M.H. 2002. Monte-Carlo inversion for a global shear velocity model of
the crust and upper mantle. Geophysical Journal International 151, 1–18.] and lithospheric temperatures [Artemieva I.M., Mooney W.D., 2001. Thermal structure and evolution of Precambrian lithosphere: A global study. Journal of Geophysical Research 106, 16387–16414.] show strong correlation with tectono-thermal ages and with regional variations in lithospheric thickness constrained by surface heat flow data and seismic velocities. In agreement with xenolith data, strong positive velocity anomalies of non-thermal origin (attributed to mantle depletion) are clearly seen for all of the cratons; their amplitude, however, varies laterally and decreases with depth, reflecting either a peripheral growth of the cratons in Proterozoic or their peripheral reworking. These cratonic regions where kimberlite magmas erupted show only weakly positive
compositional velocity anomalies, atypical for the “intact” cratonic mantle. A reduction in the amplitude of compositional velocity anomalies in kimberlite provinces is interpreted to result from metasomatic enrichment (prior or during kimberlite emplacement) of the cratonic mantle, implying that xenolith data maybe non-representative of the “intact” cratonic mantle.

[by Artemieva I.M. and Vinnik L.P.]

We present a new regional model for the depth-averaged density structure of the cratonic lithospheric mantle in southern Africa constrained on a 30′ × 30′ grid and discuss it in relation to regional seismic models for the crust and upper mantle, geochemical data on kimberlite-hosted mantle xenoliths, and data on kimberlite ages and distribution. Our calculations of mantle density are based on free-board constraints, account for mantle contribution to surface topography of ca. 0.5–1.0 km, and have uncertainty ranging from ca. 0.01 g/cm 3 for the Archean terrains to ca. 0.03 g/cm 3 for the adjacent fold belts. We demonstrate that in southern Africa, the lithospheric mantle has a general trend in mantle density increase from Archean to younger lithospheric terranes. Density of the Kaapvaal mantle is typically cratonic, with a subtle difference between the eastern, more depleted, (3.31– 3.33 g/cm 3) and the western (3.32–3.34 g/cm 3) blocks. The Witwatersrand basin and the Bushveld Intrusion Complex appear as distinct blocks with an increased mantle density (3.34–3.35 g/cm 3) with values typical of Pro-terozoic rather than Archean mantle. We attribute a significantly increased mantle density in these tectonic units and beneath the Archean Limpopo belt (3.34–3.37 g/cm 3) to melt-metasomatism with an addition of a basaltic component. The Proterozoic Kheis, Okwa, and Namaqua–Natal belts and the Western Cape Fold Belt with the late Proterozoic basement have an overall fertile mantle (ca. 3.37 g/cm 3) with local (100–300 km across) low-density (down to 3.34 g/cm 3) and high-density (up to 3.41 g/cm 3) anomalies. High (3.40–3.42 g/cm 3) mantle densities beneath the Eastern Cape Fold belt require the presence of a significant amount of eclogite in the mantle , such as associated with subducted oceanic slabs. We find a strong correlation between the calculated density of the lithospheric mantle, the crustal structure, the spatial pattern of kimberlites, and their emplacement ages. (1) Blocks with the lowest values of mantle density (ca. 3.30 g/cm 3) are not sampled by kimberlites and may represent the " pristine " Archean mantle. (2) Young (b 90 Ma) Group I kimberlites sample mantle with higher density (3.35 ± 0.03 g/cm 3) than the older Group II kimberlites (3.33 ± 0.01 g/cm 3), but the results may be biased by incomplete information on kimberlite ages. (3) Diamondiferous kimberlites are characteristic of regions with a low-density cratonic mantle (3.32–3.35 g/cm 3), while non-diamondiferous kimberlites sample mantle with a broad range of density values. (4) Kimberlite-rich regions have a strong seismic velocity contrast at the Moho, thin crust (35–40 km) and low-density (3.32–3.33 g/cm 3) mantle, while kimberlite-poor regions have a transitional Moho, thick crust (40–50 km), and denser mantle (3.34–3.36 g/cm 3). We explain this pattern by a lithosphere-scale (presumably, pre-kimberlite) magmatic event in kimberlite-poor regions, which affected the Moho sharpness and the crustal thickness through magmatic underplating and modified the composition and rheology of the lithospheric mantle to make it unfavorable for consequent kimberlite eruptions. (5) Density anomalies in the lithospheric mantle show inverse correlation with seismic Vp, Vs velocities at 100–150 km depth. However, this correlation is weaker than reported in experimental studies and indicates that density-velocity relationship in the cratonic mantle is strongly non-unique.

Large-ion lithophile elements (LILE)-enriched chromium titanates of the magnetoplumbite (AM 12 O 19) and crichtonite (ABC 18 T 2 O 38) groups have been recognized as abundant inclusions in orthopyroxene grains in a mantle-derived xenolith from the Udachnaya-East kimberlite pipe, Daldyn field, Siberian craton. The studied xenolith consists of three parts: an orthopyroxenite, a garnet clino-pyroxenite, and a garnet-orthopyroxene intermediate domain between the two. Within the host enstatite (Mg# 92.6) in the orthopyroxenitic part of the sample titanate inclusions are associated with Cr-spinel, diopside, rutile, Mg-Cr-ilmenite, and pentlandite. Crichtonite-group minerals also occur as acicular inclusions in pyrope grains of the intermediate domain adjacent to the orthopyroxenite, as well as in interstitial to enstatite oxide intergrowths together with Cr-spinel, rutile, and ilmenite. Yimengite-hawthorneite inclusions in enstatite contain (wt%) 3.72-8.04 BaO, 2.05-3.43 K 2 O, and 0.06-0.48 CaO. Their composition is transitional between yimengite and hawthorneite end-members with most grains exhibiting K-dominant chemistry. A distinct feature of the studied yimengite-hawthorneite minerals is a high content of Al 2 O 3 (5.74-7.69 wt%). Crichtonite-group minerals vary in compositions depending on the occurrence in the xenolith: inclusions in enstatite are moderate-high in TiO 2 (62.9-67.1 wt%), moderately Cr-rich (12.6-14.0 wt% Cr 2 O 3), Ba-or K-specific in the A site, and contain low ZrO 2 (0.05-1.72 wt%), whereas inclusions in pyrope are moderate in TiO 2 (61.7-63.3 wt% TiO 2), relatively low in Cr (8.98-9.62 wt% Cr 2 O 3), K-dominant in the A site, and are Zr-enriched (4.64-4.71 wt% ZrO 2). Crichtonite-group minerals in polymineralic oxide intergrowths show highly diverse compositions even within individual aggregates, where they are chemically dominated by Ba, Ca, and Sr. P-T estimates indicate the orthopyroxenite to have equilibrated at ~800 °C and 35 kbar. Preferentially oriented lamellae of enstatite-hosted Cr-spinel and diopside, as well as pyrope, diopside, and Cr-spinel grains developed around enstatite crystals, are interpreted to have been exsolved from the high-T Ca-Al-Cr-enriched orthopyroxene precursor. The exotic titanate compositions and observed textural relationships between inclusions in enstatite imply that the studied orthopyroxenite has undergone metasomatic processing by a mobile percolating agent afterward; this highly evolved melt/ fluid was enriched in Ba, K, HFSE, and other incompatible elements. The infiltration of the metaso-matizing liquid occurred through interstices and vulnerable zones of enstatite grains and manifested in the crystallization of titanate inclusions. It is assumed that Cr-spinel lamellae served as seeds for their nucleation and growth. The prominent textural and chemical inhomogeneity of the interstitial oxide intergrowths is either a consequence of the metasomatic oxide crystallization shortly prior to the kimberlite magma eruption or arose from the intensive modification of preexisting oxide clusters by the kimberlite melt during the Udachnaya emplacement. Our new data provide implications for the metasomatic treatment of orthopyroxenites in the subcontinental lithospheric mantle from the view of exotic titanate occurrences.

(By Artemieva I.M.)

Lithosphere thermal structure in Greenland is poorly known and models based on seismic and magnetic data are inconsistent, while growing awareness in the fate of the ice sheet in Greenland requires reliable constraints on geothermal heat flux (GHF) from the Earth's interior in the region where conventional heat flux measurements are nearly absent. The lithosphere structure of Greenland remains controversial, while its geological evolution is constrained by direct observations in the narrow ice-free zone along the coasts. The effect of the Iceland hotspot on the lithosphere structure is also debated. Here I describe a new thermal isostasy method which I use to calculate upper mantle temperature anomalies, lithosphere thickness, and GHF in Greenland from seismic data on the Moho depth, topography and ice thickness. To verify the model results, the predicted GHF values are compared to available measurements and show a good agreement. Thick (200-270 km) cratonic lithosphere of SW Greenland with GHF of ca. 40 mW/m 2 thins to 180-190 km towards central Greenland without a clear boundary between the Archean and Proterozoic blocks, and the deepest lithosphere keel is observed beneath the largest kimberlite province in West Greenland. The NW-SE belt with an anomalously thin (100-120 km) lithosphere and GHF of 60-70 mW/m 2 crosses north-central Greenland from coast to coast and it may mark the Iceland hotspot track. In East Greenland this anomalous belt merges with a strong GHF anomaly of > 100 mW/m 2 in the Fjordland region. The anomaly is associated with a strong lithosphere thinning, possibly to the Moho, that requires advective heat transfer such as above active magma chambers, which would accelerate ice basal melting. The anomaly may extend 500 km inland with possibly a significant contribution of ice melt to the ice-drainage system of Greenland.

[by Artemieva I.M. and Vinnik L.P.]

The origin of high topography in southern Africa is enigmatic. By comparing topography in different cratons, we demonstrate that in southern Africa both the Archean and Proterozoic blocks have surface elevation 500–700 m higher than in any other craton worldwide, except for the Tanzanian Craton. An unusually high topography may be caused by a low density (high depletion) of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed here to be at the lithosphere base). We use free-board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern Africa. Our analysis takes advantage of the SASE seismic experiment which provided high resolution regional models of the crustal thickness. We calculate the model of density structure of the lithospheric mantle in southern Africa and show that it has an overall agreement with xenolith-based data for lithospheric terranes of different ages. Density of lithospheric mantle has significant short-wavelength variations in all tectonic blocks of southern Africa and has typical SPT values of ca. 3.37–3.41 g/cm 3 in the Cape Fold and Namaqua–Natal fold belts, ca. 3.34–3.35 g/cm 3 in the Proterozoic Okwa block and the Bushveld Intrusion Complex, ca. 3.34–3.37 g/cm 3 in the Limpopo Belt, and ca. 3.32–3.33 g/cm 3 in the Kaapvaal and southern Zimbabwe cratons. The results indicate that 0.5–1.0 km of surface topography, with the most likely value of ca. 0.5 km, cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities and requires the dynamic (or static) contribution from the sublithospheric mantle. Given a low amplitude of regional free air gravity anomalies (ca. +20 mGal on average), we propose that mantle residual (dynamic) topography may be associated with the low-density region below the depth of isostatic compensation. A possible candidate is the low velocity layer between the lithospheric base and the mantle transition zone, where a temperature anomaly of 100–200 °C in a ca. 100–150 km thick layer may explain the observed reduction in Vs velocity and may produce ca. 0.5–1.0 km to the regional topographic uplift.

[By Artemieva I.M., Thybo H., Cherepanova Y.]

The isopycnicity hypothesis states that the lithospheric mantle of ancient platforms has a unique composition such that high density due to low lithosphere temperature is nearly compensated by low-density composition of old cratonic mantle. This hypothesis is supported by petrological studies of mantle xenoliths hosted in kimberlite magmas. However, the representativeness of the kimberlite sampling may be questioned, given that any type of magmatism is atypical for stable regions. We use EGM2008 gravity data to examine the density structure of the Siberian lithospheric mantle, which we compare with independent constraints based on free-board analysis. We find that in the Siberian craton, geochemically studied kimberlite-hosted xenoliths sample exclusively those parts of the mantle where the isopycnic condition is satisfied, while the pristine lithospheric mantle, which has not been affected by magmatism, has a significantly lower density than required by isopycnicity. This discovery allows us to conclude that our knowledge on the composition of cratonic mantle is incomplete and that it is biased by kimberlite sampling which provides a deceptive basis for the isopycnicity hypothesis

Reversed-zoned olivines (Fe-richer cores compared to rims), appear to be ubiquitous in kimberlites with a wide distribution. These olivines generally comprise a subordinate population relative to the dominant normally zoned olivines. However, they are notably more abundant in the megacryst-rich mid-Cretaceous Monastery and early Proterozoic Colossus kimberlites, located on the Kaapvaal and Zimbabwe cratons, respectively. The reverse-zoned olivines at these two localities define compositional fields that are closely similar to those for two olivine megacryst populations of the Cr-poor association which have been documented in the Monastery kimberlite. This points to a genetic link between megacrysts and the reversed zoned olivines. The ubiquitous, occurrence of the Fe-rich (relative to the field for rims) olivines in kimberlites with a wide geographic distribution in turn argues for an intimate link between megacrysts and the host kimberlite. Some large olivines have inclusions of rounded Cr-rich clinopyroxenes, garnets and/or spinel, characterized by fine-scale, erratic internal compositional zoning. Olivines with such chemically heterogeneous Cr-rich inclusions are not derived from dis-aggregated mantle peridotites, but are rather linked to the Cr-rich megacryst suite. Consequently, they cannot be used as evidence that cores of a majority of kimberlitic olivines are derived from disaggregated mantle peridotites.

(By Shulgin A. and Artemieva I.M.) [open access]

We present a new model, EUNA-rho, for the density structure of the continental and oceanic upper mantle based on 3-D tesseroid gravity modeling. On continent, there is no clear difference in lithospheric mantle (LM) density between the cratonic and Phanerozoic Europe, yet an ~300-km-wide zone of a high-density LM along the Trans-European Suture Zone may image a paleosubduction. Kimberlite provinces of the Baltica and Greenland cratons have a low-density (3.32 g/cm 3) mantle where all non-diamondiferous kimberlites tend to a higher-density (3.34 g/cm 3) anomalies. LM density correlates with the depth of sedimentary basins implying that mantle densification plays an important role in basin subsidence. A very dense (3.40-3.45 g/cm 3) mantle beneath the superdeep platform basins and the East Barents shelf requires the presence of 10-20% of eclogite, while the West Barents Basin has LM density of 3.35 g/cm 3 similar to the Variscan massifs of western Europe. In the North Atlantics, south of the Charlie Gibbs fracture zone (CGFZ) mantle density follows half-space cooling model with significant deviations at volcanic provinces. North of the CGFZ, the entire North Atlantics is anomalous. Strong low-density LM anomalies (< −3%) beneath the Azores and north of the CGFZ correlate with geochemical anomalies and indicate the presence of continental fragments and heterogeneous melting sources. Thermal anomalies in the upper mantle averaged down to the transition zone are 100-150 °C at the Azores and can be detected seismically, while a <50 °C anomaly around Iceland is at the limit of seismic resolution.

Kimberlite is an igneous rock, which sometimes contains diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.70 g) diamond called the Star of South Africa in 1869 spawned a diamond rush and the digging of the open-pit mine called the Big Hole.

We present new results on structure, thickness, and composition of the crust in southern Africa based on 6300 seismic receiver functions at 85 stations. Application of Hk-stacking to the entire SASE dataset and use of multi-frequency bands improve resolution substantially. We observe a highly heterogeneous crustal structure with short wavelength variations in thickness (H), Vp/Vs-ratio (composition), and Moho sharpness, which defines ~20 blocks that do not everywhere coincide with surface tectonic features. In the Zimbabwe Craton, the Tokwe block has H = 35–38 km and Vp/Vs = 1.74–1.79 whereas the thicker crust in the Tati block (H = 47– 51 km) may be related to deformation of the Archean crust along the cratonic margin. Two distinct crustal blocks with similar crustal thickness (42–46 km) but significantly different Vp/Vs-ratios are recognized in the Limpopo Belt. Extreme values of 1.90–1.94 at the dyke swarms in eastern Limpopo, and 1.84 at the Olifants River Dyke Swarm and easternmost Bushveld Intrusion Complex (BIC) indicate voluminous magmatic intrusions in the whole crust. We find no evidence for magmatic intrusions in the central (inferred) part of BIC, where the crust is thick (45–50 km) and Vp/Vs is low (1.68–1.70). This thick crustal root may have deflected rising magmas to form the two BIC lobes. Most of central Kaapvaal has thin (35–40 km) crust and Vp/Vs ~ 1.74. These characteristics are similar to the Tokwe block in Zimbabwe Craton and may indicate delamination of pre-existing lower crust, which is further supported by a very sharp Moho transition. The exposed cross-section in the Vredefort impact crater is non-representative of cratonic crust due to shallow Moho (34 km) and high Vp/Vs ~ 1.80 attributed to shock metamorphism. High Vp/Vs = 1.76 is typical of the Witwatersrand Basin, and anomalously low Vp/Vs = 1.66–1.67 marks the Kaapvaal–Kheis–Namaqua transition. Highly heterogeneous crust, both in thickness and Vp/Vs-ratio is typical of the Namaqua–Natal and Cape Fold Belts.

Geochemical data are presented for the kimberlitic Holocene Igwisi Hills volcanoes (IHV), Tanzania, which preserve
extra-crater lavas and pyroclastic rocks. Their young age and exceptional preservation enable investigation
of kimberlite magma compositions and alteration pathways of kimberlites. The IHV lavas have a variable matrix
assemblage dominated by calcite, olivine and a serpentine-like mineral (termed serpentine-X). Minor primary
groundmass phases include apatite, phlogopite, monticellite, perovskite and spinel representing late-stage
crystalisation. Secondary phases include hydrogarnet, a mixed-layer chlorite–vermiculite–montmorrilonite,
minor brucite and low-temperature oxides and clays such as goethite and jamborite. The matrix of pyroclastic
rocks is dominated by calcite with fewer groundmass phases. The parental magmas are inferred to have had
~21 wt.% SiO2, ~22 wt.% CaO, ~23 wt.% MgO and Mg# ~70. The IHV are classified as calcite kimberlites. The
total volatile concentrations of the primary melt are ~14 wt.%, which predominantly consists of CO2 although
the H2O content is also high. Whole-rock geochemical analyses indicate minor crustal contamination, lowtemperature
alteration and weathering. Pervasive serpentinisation in both lavas and pyroclastic rocks results
from low-temperature alteration induced by the circulation of meteoric waters during cooling. Serpentine-X is
potentially a newmineral and is richer in Al2O3 and FeO and poorer in SiO2 than published analyses of serpentine
minerals. These compositions are attributed to a 1:2 mixture of serpentine and hydrotalcite. We propose that
serpentine-X has replaced a reactive, late stage residual silicate glass, the existence of which helps explain the
presence of vesicular scoria (similar to glassy basaltic pyroclasts) and viscous kimberlite lavas.

(by Yulia Cherepanova, Irina M. Artemieva, Hans Thybo, Zurab Chemia)

We present a digital model SibCrust of the crustal structure of the Siberian craton (SC) and the West Siberian
basin (WSB), based on all seismic profiles published since 1960 and sampled with a nominal interval of
50 km. Data quality is assessed and quantitatively assigned to each profile based on acquisition and interpretation
method and completeness of crustal model. The database represents major improvement in coverage and
resolution and includes depth to Moho, thickness and average P-wave velocity of five crustal layers (sediments,
and upper, middle, lower, and lowermost crust) and Pn velocity. Maps and cross sections demonstrate strong
crustal heterogeneity, which correlates weakly with tectono-thermal age and strongly with tectonic setting.
Sedimentary thickness varies from 0–3 km in stable craton to 10–20 km in extended regions. Typical Moho
depths are 44–48 km in Archean crust and up-to 54 km around the Anabar shield, 40–42 km in Proterozoic
orogens, 35–38 km in extended cratonic crust, and 38–42 km in the West Siberian basin. Average crustal Vp velocity
is similar for the SC and the WSB and shows a bimodal distributionwith peaks at ca. 5.4 km/s in deep sedimentary
basins and ~6.2–6.6 km/s in parts of the WSB and SC. Exceptionally high basement Vp velocities (6.8–
7.0 km/s) at the northern border between the SC and the WSB indicate the presence of magmatic intrusions and
are proposed to mark the source zone of the Siberian LIP. The cratonic crust generally consists of three layers and
high-velocity lowermost crust (Vp ~ 7.4 km/s) is observed only locally. Pn velocities are generally ~8.2 km/s in
the SC and WSB and abnormally high (8.6–8.9 km/s) around kimberlite fields. We discuss the origin of crustal
heterogeneity and link it to regional crustal evolution.

ABSTRACT Geochemical data are presented for the kimberlitic Holocene Igwisi Hills volcanoes (IHV), Tanzania, which preserve extra-crater lavas and pyroclastic rocks. Their young age and exceptional preservation enable investigation of kimberlite magma compositions and alteration pathways of kimberlites. The IHV lavas have a variable matrix assemblage dominated by calcite, olivine and a serpentine-like mineral (termed serpentine-X). Minor primary groundmass phases include apatite, phlogopite, monticellite, perovskite and spinel representing late-stage crystalisation. Secondary phases include hydrogarnet, a mixed-layer chlorite-vermiculite-montmorrilonite, minor brucite and low-temperature oxides and clays such as goethite and jamborite. The matrix of pyroclastic rocks is dominated by calcite with fewer groundmass phases. The parental magmas are inferred to have had ~ 21 wt % SiO2, ~ 22 wt % CaO, ~ 23 wt % MgO and Mg# ~ 70. The IHV are classified as calcite kimberlites. The total volatile concentrations of the primary melt are ~ 14 wt %, which predominantly consists of CO2 although the H2O content is also high. Whole-rock geochemical analyses indicate minor crustal contamination, low-temperature alteration and weathering. Pervasive serpentinisation in both lavas and pyroclastic rocks results from low-temperature alteration induced by the circulation of meteoric waters during cooling. Serpentine-X is potentially a new mineral and is richer in Al2O3 and FeO and poorer in SiO2 than published analyses of serpentine minerals. These compositions are attributed to a 1:2 mixture of serpentine and hydrotalcite. We propose that serpentine-X has replaced a reactive, late stage residual silicate glass, the existence of which helps explain the presence of vesicular scoria (similar to glassy basaltic pyroclasts) and viscous kimberlite lavas.

by Artemieva I.M. and Vinnik L.P. . The origin of high topography in southern Africa is enigmatic. By comparing topography in different cratons, we demonstrate that in southern Africa both the Archean and Proterozoic blocks have surface elevation 500–700 m higher than in any other craton worldwide, except for the Tanzanian Craton. An unusually high topography may be caused by a low density (high depletion) of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed here to be at the lithosphere base). We use free-board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern Africa. Our analysis takes advantage of the SASE seismic experiment which provided high resolution regional models of the crustal thickness. We calculate the model of density structure of the lithospheric mantle in southern Africa and show that it has an overall agreement with xenolith-based data for lithospheric terranes of different ages. Density of lithospheric mantle has significant short-wavelength variations in all tectonic blocks of southern Africa and has typical SPT values of ca. 3.37–3.41 g/cm 3 in the Cape Fold and Namaqua–Natal fold belts, ca. 3.34–3.35 g/cm 3 in the Proterozoic Okwa block and the Bushveld Intrusion Complex, ca. 3.34–3.37 g/cm 3 in the Limpopo Belt, and ca. 3.32–3.33 g/cm 3 in the Kaapvaal and southern Zimbabwe cratons. The results indicate that 0.5–1.0 km of surface topography, with the most likely value of ca. 0.5 km, cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities and requires the dynamic (or static) contribution from the sublithospheric mantle. Given a low amplitude of regional free air gravity anomalies (ca. +20 mGal on average), we propose that mantle residual (dynamic) topography may be associated with the low-density region below the depth of isostatic compensation. A possible candidate is the low velocity layer between the lithospheric base and the mantle transition zone, where a temperature anomaly of 100–200 °C in a ca. 100–150 km thick layer may explain the observed reduction in Vs velocity and may produce ca. 0.5–1.0 km to the regional topographic uplift.