Susan J. Webb

Diamonds sampled by plumes from the core-mantle boundary

Diamonds are formed under high pressure more than 150 kilometres deep in the Earth’s mantle and are brought to the surface mainly by volcanic rocks called kimberlites. Several thousand kimberlites have been mapped on various scales, but it is the distribution of kimberlites in the very old cratons (stable areas of the continental lithosphere that are more than 2.5 billion years old and 300 kilometres thick or more) that have generated the most interest, because kimberlites from those areas are the major carriers of economically viable diamond resources. Kimberlites, which are themselves derived from depths of more than 150 kilometres, provide invaluable information on the composition of the deep subcontinental mantle lithosphere, and on melting and metasomatic processes at or near the interface with the underlying flowing mantle. Here we use plate reconstructions and tomographic images to show that the edges of the largest heterogeneities in the deepest mantle, stable for at least 200 million years and possibly for 540 million years, seem to have controlled the eruption of most Phanerozoic kimberlites. We infer that future exploration for kimberlites and their included diamonds should therefore be concentrated in continents with old cratons that once overlay these plume-generation zones at the core–mantle boundary.

New way to map old sutures using deformed alkaline rocks and carbonatites

By using a recent compilation of African alkaline igneous rocks and carbonatites, we show that nearly 90 % (28 of 32 occurrences) of nepheline syenite gneisses and deformed carbonatites are concentrated within known or inferred Proterozoic suture zones. Given the well-established intracontinental rift setting for these rocks and the likely continental collisional setting for their subsequent deformation, we suggest that deformed alkaline rocks and carbonatites (DARCs) represent the products of two well-defined parts of the Wilson cycle. DARCs mark the places where vanished oceans have opened and then closed. We further postulate that DARCs taken into the mantle lithosphere to ∼100 km depths at collision could provide source material for later alkaline magmatism. This possibility could account for the observation of recurrent alkaline magmatic activity over hundreds of million years in provinces such as that of southern Malawi.

Magmatic stratigraphy in the Bushveld Northern Lobe: continuous geophysical and mineralogical data from the 2950 m Bellevue drillcore

We present a large database of geophysical, petrological and mineralogical measurements for the ~3000 m Bellevue borehole through the entire Upper Zone (UZ) and about half of the Main Zone (MZ) of the Northern Lobe of the Bushveld Complex, South Africa. Magnetic susceptibilty readings were taken every 2 cm (n = 109,360) and densities were measured on average every 1.7 m (n = 2252). Petrographic data and microprobe analyses (n = 14,160) were obtained for plagioclase, mafic silicates, Fe-Ti oxides, amphibole and biotite in 502 samples throughout the entire sequence of layered mafic cumulates. The Bellevue UZ, as marked by the first appearance of cumulus magnetite, is ~1190 m thick (corrected for mean dip of 17.5°), which is less than UZ thicknesses in the Eastern and Western Lobes. A prominent 4 m thick pyroxenite horizon occurs ~390 m below the UZ-MZ boundary, but we show on the basis of mineralogy that this horizon cannot be correlated with the well known Pyroxenite Marker (PM) of the Eastern and Western Bushveld Complex. If the PM is indeed absent in the Northern Lobe, then a substantial portion, perhaps 500 m of the uppermost MZ may be missing; possible causes include non-deposition ( e.g. due to syn-magmatic upwarping or diapirism) or removal (e.g. due to emplacement of UZ magmas). The Bellevue drillcore penetrated only about half of the MZ (total dip-corrected thickness ~1270 m), and the lowermost ~200 m contains unusual olivine-bearing (or troctolitic) horizons that are atypical of MZ rocks elsewhere in the Bushveld Complex. These troctolites have mineral compositions as primitive as those of the upper Critical Zone (CZ), suggesting that they might represent a sliver of CZ rocks dismembered by intrusion of MZ magmas. Alternatively, they may represent an intrusive sill of syn- or post-Bushveld age, or merely a mineralogically unusual horizon in otherwise typical MZ lithologies. Mineral compositions show broad normal fractionation upwards, with plagioclase An78→21, opx (and inverted pigeonite) En 80→26, cpx Mg 86→27, olivine Fo 78→74 (in the lowermost troctolitic horizon) and Fo 59→06 (in the UZ olivine ferrodiorites). There are, however, numerous prominent reversals and discontinuities in mineral compositions, some of which are likely related to magma additions. The extensive dataset of mineral compositions allows the estimation of a new fractionation trend for the Bushveld Complex. On an En-An diagram, the Bushveld trend is shifted toward more An-rich plagioclase at equivalent Mg# of coexisting pyroxenes relative to those for Kiglapait or Skaergaard. This is attributed to the relative paucity in Bushveld of augite, which has a high fractionating power for Ca/Na in evolving liquids. Magnetic susceptibility data clearly reveal the presence of the UZ-MZ boundary. MZ cumulates have susceptibilty values <0.05 SI units, and generally <0.02 SI units. Above the UZ-MZ boundary, susceptibilty varies enormously, from anorthosites (<0.1 SI units) to magnetitites (to almost 5 SI units), and there is excellent correlation between susceptibility and lithology, in many cases to a resolution of <5 to 10 cm. Anorthositic rocks, especially in the MZ, commonly show higher susceptibilty than surrounding polyphase cumulates, due to intercumulus and/or dust-like inclusions of magnetite. Density data reveal surprising cyclicity in the Main Zone on the scale of 50 to 200 m, with progressively increasing density upwards in individual layered units, reflecting gradual increase in modal colour index from 0 to 10% to 50 to 60%. In some cases the upward density increases are correlated with broad reversals in chemical fractionation trends (e.g. upward increases in Mg# of pyroxenes), arguing against simple fractionation. We suggest that such layers may represent blending zones in which dense liquids and/or crystals from new magma additions drain downwards into the existing cumulate pile. MZ cumulates, therefore, may have been constructed by successive, compositionally different magmatic influxes, implying the existence of a sub-Bushveld magmatic staging chamber.

Deep mantle structure as a reference frame for movements in and on the Earth

Significance Since the Pangea supercontinent formed about 320 million years ago, plumes that sourced large igneous provinces and kimberlites have been derived from the edges of two stable thermochemical reservoirs at the core–mantle boundary. We test whether it is possible to maintain this remarkable surface-to-deep Earth correlation before Pangea through the development of a new plate reconstruction method and find that our reconstructions for the past 540 million years comply with known geological and tectonic constraints (opening and closure of oceans, mountain building, and more). These results have important implications for Earth history, including the style of mantle convection in the deep past and the long-term stability of mantle reservoirs. Earth’s residual geoid is dominated by a degree-2 mode, with elevated regions above large low shear-wave velocity provinces on the core–mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to the Earth’s surface, correlate with the reconstructed positions of large igneous provinces and kimberlites since Pangea formed about 320 million years ago. Using this surface-to-core–mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander, we have developed a model for absolute plate motion back to earliest Paleozoic time (540 Ma). For the Paleozoic, we have identified six phases of slow, oscillatory true polar wander during which the Earth’s axis of minimum moment of inertia was similar to that of Mesozoic times. The rates of Paleozoic true polar wander (<1°/My) are compatible with those in the Mesozoic, but absolute plate velocities are, on average, twice as high. Our reconstructions generate geologically plausible scenarios, with large igneous provinces and kimberlites sourced from the margins of the large low shear-wave velocity provinces, as in Mesozoic and Cenozoic times. This absolute kinematic model suggests that a degree-2 convection mode within the Earth’s mantle may have operated throughout the entire Phanerozoic.

Gravity modeling of Bushveld Complex connectivity supported by Southern African Seismic Experiment results

Recent gravity modelling of the Bushveld Complex indicates that the western and eastern limbs of the Bushveld Complex are connected at depth. The model predicts a downwarp in the Moho beneath the Bushveld Complex, ensuring observed Airy isostatic balance is achieved. By constraining a new Bouguer gravity model with published Vibroseis results, crustal thicknesses determined using the receiver function method, and seismic velocity modelling of the crust from the Southern African Seismic Experiment; we demonstrate that the connected model of the Bushveld Complex is consistent with all available data. Crustal thicknesses determined from receiver functions indicate that the depth to the Moho thickens from a value of ~35 to ~40km in the southern Kaapvaal craton to ~50km beneath the central region of the Bushveld Complex. This seismologically determined Moho varies significantly from that calculated from Airy isostatic balance based solely on topography as a load in this region. The corresponding crustal velocity model, determined from inverting the receiver function results for Bushveld Complex stations, also indicates a thick crust and delimits a ~6km thick high velocity zone in the upper 10km of crust attributed to the presence of the Bushveld Complex. Comparison of the seismic crustal model with drill core data on the mafic rocks of the Bushveld Complex suggests a correspondence between high seismic velocities and high densities in the upper crust. Both the gravity model and the seismological results imply a density contrast of about 0.30mg/m3 at the crust/mantle boundary beneath the Bushveld Complex. We also find that the Moho transition beneath the Bushveld Complex is significantly broader than that beneath the rest of the Kaapvaal craton, outside of the Limpopo Belt. By constraining the modelling of the gravity data with these seismological results, outcropping geology and published Vibroseis profiles, we show that the dense mafic units of the western and eastern Bushveld Complex can be interpreted as having originally been emplaced as a connected sheet (or series of connected sheets), which has subsequently been deformed and faulted. The seismological results of the Kaapvaal project support the interpretation of a connected Bushveld Complex.

Late Jurassic age for the Morokweng impact structure, southern Africa

Abstract A roughly 70 km diameter circular feature buried beneath the Kalahari sands in South Africa is revealed on regional aeromagnetic maps. Boreholes drilled into the centre of the structure intercept a ∼ 250 m thick sheet of quartz norite, interpreted as an impact melt, which overlies brecciated and shock metamorphosed basement granite. Zircons recovered from the quartz norite, yield U-Pb ages of 145 ± 0.8 Ma, and biotites provide Ar-Ar ages of 144 ± 4 Ma. These data provide strong evidence for the occurrence of a Late Jurassic impact crater (the Morokweng impact structure) ∼ 100 m beneath the surface.

Avoidable Euler Errors – the use and abuse of Euler deconvolution applied to potential fields†

Window-based Euler deconvolution is commonly applied to magnetic and sometimes to gravity interpretation problems. For the deconvolution to be geologically meaningful, care must be taken to choose parameters properly. The following proposed process design rules are based partly on mathematical analysis and partly on experience. 1. The interpretation problem must be expressible in terms of simple structures with integer Structural Index (SI) and appropriate to the expected geology and geophysical source. 2. The field must be sampled adequately, with no significant aliasing. 3. The grid interval must fit the data and the problem, neither meaninglessly overgridded nor so sparsely gridded as to misrepresent relevant detail. 4. The required gradient data (measured or calculated) must be valid,with sufficiently low noise, adequate representation of necessary wavelengths and no edge-related ringing. 5. The deconvolution window size must be at least twice the original data spacing (line spacing or observed grid spacing) and more than half the desired depth of investigation. 6. The ubiquitous sprays of spurious solutions must be reduced or eliminated by judicious use of clustering and reliability criteria, or else recognized and ignored during interpretation. 7. The process should be carried out using Cartesian coordinates if the software is a Cartesian implementation of the Euler deconvolution algorithm (most accessible implementations are Cartesian). If these rules are not adhered to, the process is likely to yield grossly misleading results. An example from southern Africa demonstrates the effects of poor parameter choices.

The formation and evolution of Africa from the Archaean to Present: introduction

DOUWE J. J. VAN HINSBERGEN1,2*, SUSANNE J. H. BUITER1,2,3, TROND H. TORSVIK1,2,3,4, CARMEN GAINA1,2,3 & SUSAN J. WEBB4 Physics of Geological Processes, University of Oslo, Sem Saelands vei 24, NO-0316 Oslo, Norway Center for Advanced Study, Norwegian Academy of Science and Letters, Drammensveien 78, 0271 Oslo, Norway Centre for Geodynamics, Geological Survey of Norway (NGU), Leiv Eirikssons vei 39, 7491 Trondheim, Norway School of Geosciences, University of the Witwatersrand, WITS 2050 Johannesburg, South Africa

Cooling of the Bushveld Complex, South Africa: Implications for paleomagnetic reversals

Igneous rocks record the direction of the Earth’s magnetic field as they cool through their Curie temperature. The mafic magmas of the 8-km-thick Bushveld Complex of South Africa took 65 k.y. to be emplaced, 180 k.y. to solidify (to 900 °C), and a further 500 k.y. for the entire intrusion to cool below 580 °C, the Curie temperature of magnetite. Once solid, the cooling of this intrusion occurred mainly from the top downward, with slower cooling through its floor. As a result, the upper rocks cooled through their Curie temperature before those at the base; the portion 6 km below the upper contact was the last to reach the Curie temperature. Thus, the intrusion records a mainly top-down sequence of three paleomagnetic reversals starting with N (normal direction). The last two are also recorded from the base of the mafic sequence upward as it cooled through 580 °C later than the top. The lateral variations in thickness of the Bushveld Complex are important in this interpretation, because thinner sections cooled more quickly. Hence, reversals do not always correlate with stratigraphy. Specific reversals provide a cooling marker horizon that may crosscut the stratigraphic layering. The interpretation of the order and number of paleomagnetic reversals presented here differs from previous interpretations that envisage the oldest paleomagnetic directions to be recorded sequentially from the base upward, and has implications for the interpretation of paleomagnetic results from all thick intrusions, mafic and felsic.

New Palaeoproterozoic palaeomagnetic data from the Kaapvaal Craton, South Africa

Abstract Palaeomagnetic data from the well-dated 2060.6±0.5 Ma Phalaborwa Complex in South Africa (Kaapvaal Craton) are of excellent quality. High unblocking components are carried by magnetite and single polarity remanence directions (mean declination 5.0°, inclination 57.3°, α95 = 5.2°) yield a palaeomagnetic pole (latitude 27.7°N, longitude 35.8°E, A95 = 6.6°) that overlaps with existing poles from the near coeval 2054.4±1.3 Ma Bushveld Complex. The Phalaborwa and Bushveld complex poles, along with poles from the well-dated Vredefort impact (2023±4 Ma) and Post-Waterberg Dolerites (1874.6±3.9 Ma), define the most reliable poles for the Kaapvaal Craton during this time interval (c. 2060–1875 Ma) and witness low rates of Mid-Palaeoproterozoic apparent polar wander. Poorly dated NE–NNE-trending dyke swarms that intrude the Phalaborwa and Bushveld complexes both yield dual-polarity remanence components that share a common mean at the 95% confidence level. Primary palaeomagnetic poles (Phalaborwa dykes pole latitude 7.6°, longitude 12.1°, A95 = 11.8°; Bushveld dykes pole latitude 12.6°, longitude 24.1°, A95 = 10.8°) suggest that they are of the same age as the Post-Waterberg dolerites (c. 1875 Ma). They could also be as old as the Phalaborwa and Bushveld Complexes, however; high-precision geochronology is required to resolve this issue and to enlarge the number of Palaeoproterozoic key poles for the Kaapvaal Craton.