David Ronald Bell

Nelson Mandela University | NMMU · Department of Geosciences

The Saltpeterkop Complex (SPKC), Northern Cape, South Africa, contains carbonatite, potassic trachyte, olivine melilitite and ultramafic lamprophyre magmas that were erupted through the crystalline basement of the Proterozoic Namaqua-Natal mobile belt at ~74 Ma and occur primarily as shallow intrusive features. Isolated subvolcanic intrusions and e...

Abstract ii Large (cm-sized) kaersutite amphibole and Ca-augite crystals (megacrysts) occur as inclusions in a pyroclastic breccia at a locality known as Silver Dam, located on the flanks of the Salpeterkop volcano, south-east of, Sutherland, South Africa. The volcanic-complex consists of, melilititic, trachytic and carbonate eruptives, and their h...

Flow in the sub-lithospheric mantle is increasingly invoked as a mechanism to explain both modern and past surface topography, but the importance of this phenomenon and its influence at different localities are debated. Southern Africa is an elevated continental shield proposed to represent dynamically supported topography. However, this region is...

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 ra...

Water and trace element contents were measured by FTIR and laser ablation-ICPMS on minerals from peridotite xenoliths in kimberlites of the Kaapvaal craton from Finsch, Kimberley, Jagersfontein (South Africa), Letseng-La-Terae, and Liqhobong (Lesotho) mines. The peridotites record a wide range of pressure, temperature, oxygen fugacity, and metasoma...

New Pb-Sr-Nd-Hf isotope data for Cr-poor and Cr-rich clinopyroxene megacrysts from southern African kimberlites strongly support a genetic link between megacrysts and kimberlites and further indicate that the isotopic differences between the two are largely explained by assimilation-fractional crystallization processes that occur as sub-lithospheri...

We measured hydrogen concentrations in 12 olivines using secondary ion mass spectrometry (SIMS and NanoSIMS), cross-calibrated against Fourier transform infrared (FTIR) spectroscopy and nuclear reaction analysis (NRA). Five of these samples are routinely used for calibration in other laboratories. We assess the suitability of these olivines as stan...

The trace elements of high-Mg carbonatitic high-density fluids (HDFs) trapped in six fibrous diamonds from Siberia exhibit patterns that are highly similar to those of Group I kimberlites, but are slightly more fractionated. The patterns of both are similar to the average pattern of post-Archaean xenoliths from the sub-continental lithospheric mant...

We have measured the Li isotopic composition in terrestrial pyroxene megacrysts by SIMS and MC-ICPMS to assess the role of matrix effects and their potential influence on the analysis of martian pyroxenes.

The accurate analysis of trace concentrations of hydrogen in NAMs is a long-standing problem, with wide-ranging implications in geology and planetology. SIMS and FTIR are two powerful and complementary analytical tools capable of measuring concentrations down to levels of less than 1 ppm H2O. Both methods, however, are subject to matrix effects and...

Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180-250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigi...

A mineral chemistry and whole-rock major element, platinum group element (PGE), and Re^Os isotope dataset is presented for a large suite of mantle xenoliths from 13 kimberlites that erupted through the Proterozoic mobile belts surrounding the Archean Kaapvaal craton of South Africa. The peridotites have compositions that are unusually infertile com...

Abstract– Olivine from the Fukang meteorite, like that from many other pallasites, contains distinctive arrays of parallel, straight, tubular inclusions. They differ in their extension and linearity from those in terrestrial olivines. They comprise approximately 1% of the total volume. Most have lens-shaped cross-sections, but some are rounded. The...

Post-eruptive diffusive fractionation of lithium isotopes: Implications for mantle and volcanic processes

We report here Li, B and Be distributions in pyroxenes of three shergottites (Shergotty and Zagami, and SaU 005) and the nakhlite Yamato 000593 to assess the role of magmatic degassing, igneous fractionation and subsolidus diffusion processes in their petrogenesis.

The Bondoc mesosiderite contains one large pyroxenite nodule, which we suggest is an intact fragment of the mafic crust from a differentiated parent body.

Fluorine is a volatile constituent of magmas and hydrous mantle minerals. Fluorine's ionic radius is similar to that of OH, but its lower diffusivity renders it less subject to disturbance. Recent studies by Hervig and Bell (2005 Fall AGU) and Guggino et al. (2007 Fall AGU) demonstrated that trace F, like H, is incorporated into nominally anhydrous...

We report major, trace and volatile element compositions for seven coated diamonds from Kankan, Guinea. Four diamonds trapped microinclusions with high-Mg carbonatitic high-density fluids (HDFs) and three carry silicic to low-Mg carbonatitic HDFs. Mineral inclusions and nitrogen aggregation in the cores indicate growth at ∼ 5 GPa, 1100–1200 °C. Sig...

We use a nuclear microbeam technique (Elastic Recoil Detection Analysis or ERDA) to measure the H content in 2 rhyolitic glasses, 4 olivines, 4 orthopyroxenes, and 7 clinopyroxenes. These samples have been characterized previously for their CH absorption spectra by infrared (FTIR) spectroscopy. We use ERDA and FTIR data to calibrate the infrared mo...

The association of diamond with subcalcic garnet harzburgite (SGH) in the roots of Archean cratons is a powerful empirical tool in diamond exploration, but is not fully understood. A key question is whether or not the unusual subcalcic, high-Mg# character of SGH is created during the diamond-forming event itself. Here I combine geochemical data on...

Kimberlites are igneous rocks and the attempts to unravel their mechanisms of genesis and evolution have drawn heavily on classical methods of igneous petrology. Here I review some important differences between kimberlites and other mafic or silicic magmas and examine their impact on our understanding of kimberlite. Four factors are considered. At...

Microinclusions in fibrous diamonds carry high density fluids (HDF) with compositions that vary between silicic, carbonatitic and saline end-members. Combining EPMA and FTIR data we can estimate the composition of each end-member. The silicic end-member composition (Udachnaya) comprises ~90 wt% silicates, 4% carbonate, 4% water and minor apatite. T...

Lithium abundances and isototpic compositions from three ureilites are reported. Spot analyses reveal spatial correlations between abundances and isotopic compositions with features such as cracks, grain boundaries and the ubiquitous reduced rims.

High-spatial resolution analysis of light element isotope variations by secondary-ion mass spectrometry (SIMS) has numerous applications in geochemistry and cosmochemistry. Recent attention has focused on 7 Li/ 6 Li variations in magmatic phenocrysts to infer the volatile degassing history of their parent magmas, and on minerals from mantle samples...

High-density fluids (HDFs, either melts or supercritical fluids) are trapped as myriads of sub-micrometer inclusions during the growth of fibrous diamonds and represent the medium in which these diamonds grew. Their chemical properties shed light not only on the genesis of fibrous diamonds, but perhaps on the formation of monocrystalline diamond as...

Li isotope data are presented for olivine and other minerals from pallasites, the Allende carbonaceous chondrite, the El Gouanem ureilite, and the Pasamonte eucrite. The calibration of significant matrix effects in SIMS analysis of 7Li/6Li is discussed.

We present SIMS analyses of the lithium isotopic composition of the silicate phases in the El Gouanem ureilite. The data indicate that the phases are the residue of a partial melting event.

Fourier transform infrared (FTIR) spectroscopy is the most widely applied technique for measuring hydrogen in nominally anhydrous minerals (NAMs) and silicate glasses. FTIR is rapid, sensitive, widely available and gives information on the bonding environment of H-bearing species. H determination relies on the Beer-Lambert law and therefore require...

Lithium is the lightest of the lithophile elements, with high solid-state diffusivity (e.g. Giletti and Shanahan, 1997). Recent studies in synthetic and natural systems suggest that isotopic fractionation of lithium accompanies diffusion (Richter et al., 2003; Lundstrom et al., 2005; Beck et al., 2006; Teng et al., 2007; Jeffcoate et al., 2007). Li...

The Cr-poor megacryst suite is ubiquitous in kimberlites, but its origins are not well understood, including details of the relationship to kimberlite magma. Major and trace element and isotope data for >2000 megacrysts from ~50 kimberlites in southern Africa vary systematically with respect to host kimberlite type, geographic location and tectonic...

Petrographic and geochemical features of a suite of eclogite xenoliths from the Rietfontein kimberlite that erupted through probable Proterozoic crust west of the Kaapvaal Craton in the far Northern Cape region of South Africa, are described. Group II eclogites dominate the suite both texturally and chemically, but can be subdivided into biminerali...

We present new Fourier Transform Infrared Spectroscopy (FTIR) and ion microprobe/secondary ion mass spectrometry (SIMS) analyses of (1)H in 61 natural and experimental geological samples. These samples include 8 basaltic glasses (0.17 to 7.65 wt% H(2)O), 11 rhyolitic glasses (0.143 to 6.20 wt% H(2)O), 17 olivines (similar to 0 to 910 wt. ppin H(2)O...

Because of its large thickness and thermal relaxation time, Archean lithosphere cannot be in thermal equilibrium with the instantaneous rate of heat production in the lithospheric mantle and heat supplied to its base. Comparison of xenolith (P,T) data with time-dependent thermal models allows constraints on lithosphere thickness, in situ heat produ...

This is a reconnaissance study of Li isotope variations within olivine of various types in the Allende CV3 chondrite in order to shed further light on the cosmochemical behavior of Li and explore its potential as a geochemical tracer for the origin of met

Fluorine contents in a wide range of naturally-occurring olivine grains were determined by secondary ion mass spectrometry (SIMS) using a Cs+ primary beam, detection of negative secondary ions and an auxiliary electron gun for charge neutralization. A range of minerals and glasses containing 3 to 1300 ppm F were used to calibrate the secondary ion...

Carbon storage mechanisms in the upper mantle are understood in theory but C-bearing minerals are rarely observed in mantle rocks. Our observations on Archean low temperature harzburgites from Kimberley, South Africa, suggest that carbonate is more common in Archean subcontinental lithospheric mantle (SCLM) than previously recognized. Catastrophic...

Textural evidence in a composite garnet harzburgite mantle xenolith from Kimberley, South Africa, suggests metasomatism of a severely melt-depleted substrate by a siliceous, volatile-rich fluid. The fluid reacted with olivine-rich garnet harzburgite, converting olivine to orthopyroxene, forming additional garnet and introducing phlogopite, and smal...

Coarse-grained, granular spinel lherzolites xenoliths from the Premier kimberlite show evidence of melt extraction and metasomatic enrichment, documenting a complex history for the shallow mantle beneath the Bushveld complex. Compositions of orthopyroxene, clinopyroxene and spinel indicate equilibration within the spinel–peridotite facies of the up...

We present new SIMS calibrations of H concentrations in basalt (0.17 to 7.37 wt% H2O, n = 13), rhyolite (0.3 to 6.2 wt% H2O, n = 10), and quartz (4 wt% H2O, n =1) glasses, and olivine (~0 to 243 ppm H2O, n = 5), opx (~0 to 263 ppm H2O, n = 5), cpx (~0 to 490 ppm H2O, n = 6), pyrope garnet (~0 to 194 ppm H2O, n = 8) minerals using the CAMECA 6f ion...

Li isotope heterogeneity is recorded from intra-mineral to regional scales in olivine from terrestrial mantle xenoliths. d7Li variations with a range of 25 per mil result from magmatic mixing, metasomatic, transport and alteration effects.

Fluorine is the lightest halogen, and has an ionic radius very similar to OH. Based only on this observation, substitution of fluorine into nominally anhydrous minerals (NAMs) might be expected to be similar to that of hydrogen, but there have not been detailed comparisons between OH and F in the same samples. To explore this connection and to look...

Variations in 7/6Li of mantle-derived rocks are a potentially powerful tracer of mantle geodynamic processes. Analysis of 7/6Li by secondary ion mass spectrometry (SIMS) avoids surface contamination and permits assessment of sample heterogeneity at high spatial resolution. We performed Li abundance and isotope analysis by SIMS of mantle-derived oli...

Secondary ion mass spectrometry (SIMS) is an extremely sensitive instrument for Li (up to ~4% of the atoms of Li sputtered from minerals are detected). While precision is high, the accuracy of Li isotope analyses by SIMS is critically dependent on the homogeneity of bulk-analyzed standards. We have used the Cameca ims 3f and 6f SIMS at Arizona Stat...

Concentrations of OH, and major and trace elements were determined in a suite of mantle-derived megacrysts that represent the crystallization products of a kimberlite-like magma at ~5 GPa and ~1400–1100°C. OH concentrations, determined by single-crystal Fourier transform infrared spectroscopy, display the following ranges (ppmw H2O): olivine 54–262...

The hydrogen contents of four natural kyanite samples were determined by 15N nuclear reaction analysis and used to calibrate an IR spectroscopic method for a more convenient quantitative H analysis of kyanite. Hydrogen is present as the OH - ion and (expressed as ppm H 2O by weight) ranges from near zero up to 230 ppm. Its content is best determine...

Xenoliths of the Cr-poor megacryst suite were formed by the crystallization of magmas in the deeper regions of subcontinental lithospheric mantle of southern Africa, and may be used to probe its compositional structure. Some 2300 pyrope garnet megacrysts from 61 kimberlites in southern Africa define four broad arrays on the basis of Cr2O3 - Mg# sys...

Differences in the viscosity of the earth's upper mantle beneath the western US (∼1018–1019 Pa s) and global average values based on glacial isostatic adjustment and other data (∼1020–1021 Pa s) are generally ascribed to differences in temperature. We compile geochemical data on the water contents of western US lavas and mantle xenoliths, compare t...

We have exploited a new data set of broad-band seismic waveforms recorded near the region of Kimberley, South Africa to place new constraints on seismic anisotropy in an area of extensive mantle modification within an Archaean cratonic setting. The Kimberley region is significant in that it represents Archaean cratonic mantle that has been studied...

We impose geologic constraints on computed tomographic structure of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 120 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based...

1] We impose geologic constraints on seismic three-dimensional (3-D) images of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 120 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates...

The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150–200 °C isobaric temperature range at 5–6 GPa confirms original interpretations o...

We impose geologic constraints on seismic 3-D images of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 100 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based on the elast...

Kimberlites from the Kaapvaal craton have sampled numerous mantle garnet lherzolites in addition to garnet harzburgites. Trace element characteristics of constituent clinopyroxenes allow two groups of garnet lherzolites to be distinguished. Trace element compositions of all clinopyroxenes are characterized by enrichment in light rare earth elements...

1] We report analyses of hydrogen abundance in experimentally annealed and natural mantle minerals using FTIR and use these data to establish calibration lines for measurement of H 2 O concentrations in olivine, pyroxenes, garnet, amphibole and mica by secondary ion mass spectrometry (SIMS). We have reduced the detection limit for H 2 O analysis by...

Olivine is an important host of hydrogen in the Earth's upper mantle, and the OH abundance in this mineral determines many important physical properties of the planet's interior. To date, natural and experimentally hydrated olivines have been analyzed by uncalibrated spectroscopic methods with large (±100%) uncertainties in accuracy. We determined...

Recent thermobarometry1 and accessory-mineral thermochronology2 studies on mantle and lower crustal xenoliths proposed a thermal pulse that affected the southern African mantle lithosphere during the Mesozoic and may relate to supercontinent breakup and associated mantle upwelling. Effects are concentrated on the margins of the Archean craton and i...

Thermobarometric studies on a limited number of off-craton peridotite suites from southern Africa (e.g., Finnerty and Boyd, 1987, Mantle Xenoliths) have shown that peridotites from Proterozoic mobile belts have higher equilibration temperatures for a given pressure than peridotites from the Archean Kaapvaal Craton. These higher temperatures have be...

There is a well-established debate regarding the role of higher mantle heat flux for accommodating the elevated average surface heat flow in the Proterozoic orogenic belts relative to the Archean cratonic regions of southern Africa1,2. Advocates of steeper off-craton lithospheric mantle thermal gradients commonly support their arguments with thermo...

Mantle xenoliths have been discovered in a Cretaceous nephelinite plug in the Damara Belt of NW Namibia, providing a new window into the thermal and compositional structure of the lithospheric mantle that separates the Congo and Kaapvaal cratons of southern Africa. The xenoliths range in size from 1.5 to 5 cm, and comprise lherzolite and harzburgit...

Kimberlite magmas from the Kimberley area of South Africa have sampled two main types of phlogopite-rich mafic xenoliths which represent deep mantle segregations from highly alkaline melts. The first group corresponds to the MARID rocks characterised by the mineral association mica (phlogopite)-amphibole (K-richterite)-rutile-ilmenite-clinopyroxene...

As radiogenic isotope compositions of Cr-poor megacrysts from kimberlite demonstrate that the megacrysts and kimberlite hosts are genetically related (megacrysts are likely deep-seated liquidus phases), the oxygen isotope compositions of Cr-poor megacrysts are indicative of the δ18O of the host kimberlites and their mantle source regions. Oxygen is...

Evidence for the nature and timing of processes responsible for the formation and consolidation of the ancient mantle roots to continents is commonly obscured by more recent mantle events. In this study we apply a number of analytical approaches to a suite of highly depleted garnet harzburgite xenoliths from the Bultfontein kimberlite, South Africa...

Whole rock and mineral major and trace element compositions were determined by XRF, electron microprobe and solution- and laser ablation ICP-MS on a modally diverse suite of 15 micaceous xenoliths from the Kimberley kimberlites. The aim was to better define the chemical characteristics of the MARID and "glimmerite" suites of micaceous xenoliths and...

Major element analyses, mineral chemistry data and Re-Os model ages have been determined on a suite of 50 garnet- and spinel-facies lherzolite, harzburgite and pyroxenite xenoliths from 9 Mesozoic kimberlite localities in western South Africa, mainly located on the Proterozoic Namaqua Natal mobile belt terrane. These off-craton peridotites are gene...

New and published mineral analyses of peridotite xenoliths from kimberlites of western South Africa and southern Namibia were used to compute pressures and temperatures of origin with standard mineral thermobarometers. These data are combined with results of crustal thermochronology studies to construct a model for dynamic thermal evolution of this...

Lower crustal xenoliths exhumed in Cretaceous kimberlites of the Kaapvaal Craton of southern Africa provide samples of a region of the lithosphere amenable to quantitative isotopic thermochronology, which may in turn provide insight into secular changes in the thermal structure of the underlying mantle. Deep crustal garnet granulite parageneses fro...

The common, nominally anhydrous minerals of the Earth’s mantle are an important reservoir for hydrogen in the Earth’s interior due to their ability to incorporate hydroxyl in trace quantities. We present measurements of the isotopic composition (D/H ratio) of hydrogen from upper-mantle derived garnet, enstatite and augite employing a two-step extra...

To understand the origin, modification, and preservation of continents on Earth, a multidisciplinary study is examining the crust and upper mantle of southern Africa. Xenoliths of the mantle brought to the surface by kimberlites show that the mantle beneath the Archean Kaapvaal craton is mostly melt-depleted peridotite with melt extraction accompan...

To calibrate infrared (IR) spectroscopy for quantitative analysis of trace structural OH in specific minerals, we have determined concentrations of H in pure separates of mantle derived pyrope garnet (56 ± 6 ppm H_2O by weight), augite (268 ± 8, ppm H_2O, and enstatite (217 ± 11 ppm H_2O) by manometry after heating the samples and extracting H_2 ga...

D/H ratios are, in principle, useful in characterizing reservoirs of mantle hydrogen and as tracers of volatile transfer processes in Earth's interior. In practice, however, interpretation of isotopic measurements on mantle derived H is complicated by surface processes such as contamination and degassing which may alter the primary D/H ratio. Altho...

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. This thesis is an investigation of the infrared spectroscopic characteristics, abundance, stability and partitioning behavior of hydroxyl (OH) in minerals from the Earth's upper mantle. Using manometric and nuclear reaction analysis t...

New experiments suggest that olivine, probably the most abundant mineral in the Earth to a depth of 400 km, can incorporate substantial amounts of hydrogen under water-rich, high-pressure conditions. This work, reported by Bai and Kohlstedt on page 672 of this issue, addresses two basic questions facing those looking at the role of water in the evo...

166 garnets of dominantly mantle origin were analyzed for OH content by infrared (IR) spectroscopy. IR spectra in the 3400–3700 cm-1 region display consistent absorption patterns attributable to OH structurally bound within the garnet crystal, occasionally contaminated by low intensity OH absorptions from microscopic inclusions. The principal struc...