• 57Fe Mössbauer spectroscopy investigations of iron oxidation states in the Harmattan dust nutrient contribution to West African soils

      Adetunji, Jacob; University of Derby (Elsevier, 2014-09-09)
      A variety of investigations have been carried out on Harmattan dust over many decades demonstrating the continuing importance of the Harmattan dust phenomenon. The investigations have included elemental enrichment factors, mineralogical nutrient input through dust deposition on the soil, meteorological studies, etc. Harmattan dust is important, not only for its impact on radio communication and low visibility in the shipping lanes over the Atlantic, but also on the livelihood and health of people living in countries over which the dust-laden Harmattan wind blows. However, so far, the aspect of nutrient mineral deposition on the soil has not been thoroughly investigated and requires attention, since the majority of people living in West Africa rely heavily on agriculture. It is therefore relevant to know the useful nutrients in the Harmattan dust deposited on soils of the region. This study is therefore aimed at determining the ferric-ferrous ratio of the iron-bearing minerals contained in the Harmattan dust, so their nutritional contribution can be considered. The Mössbauer technique is a powerful tool for studying the ferric-ferrous ratio and has therefore been used, for the first time, to determine the oxidation states of iron in the dust samples. The results of the analysis show that the Harmattan dust is seriously deficient in ferrous iron, which is the more soluble Fe-ion, needed in the soil for healthy crops and plants in general.
    • Archaean chromitites show constant Fe 3+ /ΣFe in Earth's asthenospheric mantle since 3.8 Ga

      Rollinson, Hugh; Adetunji, Jacob; Lenaz, Davide; Szilas, Kristoffer; University of Derby; University of Trieste; Geological Survey of Denmark and Greenland (Elsevier, 2017-03-29)
      Theoretical and planetary studies show that the Earth’s upper mantle is more oxidised than it should be. The mechanism by which this took place and the timing of the oxidation is contested. Here we present new Mössbauer spectroscopy measurements of the ionic ratio Fe3+/(Fe3++Fe2+) in the mineral chromite hosted in mantle-derived melts to show that there is no change in mantle Fe3+/(Fe3++Fe2+) ratio before and after the oxidation of the Earth’s atmosphere at ca. 2.4 Ga and over Earth history from 3.8 Ga to 95 Ma. Our finding supports the view that the oxidation of the asthenospheric mantle was very early and that the oxygenation of the Earth’s atmosphere was not directly coupled to mantle processes.
    • Chromite in the mantle section of the Oman Ophiolite: Implications for the tectonic evolution of the Oman Ophiolite

      Rollinson, Hugh; Adetunji, Jacob; University of Derby (Wiley, 2015-12)
      Chromite in the Oman ophiolite is located in the mantle section of the ophiolite sequence and forms abundant small podiformdeposits throughout the length of the ophiolite (Rollinson, 2005).The Oman ophiolite has an exposed mantle section of ca 10 000 km2, and contains ca 200 chromitite bodies. Most are less than 10 000 tonnes and a only a few are >30 000 tonnes (Boudier and Al-Rajhi, 2014). We have examined these deposits in eight different areas of the ophiolite (Figure 1, Rollinson and Adetunji, 2013a), two of which we have studied in great detail – in WadiRajmi in the north of Oman (Rollinson, 2008) and atMaqsad in the south(Rollinson and Adetunji, 2013b).
    • Comment on ‘Podiform chromitites do form beneath mid-ocean ridges’ by Arai, S. and Miura, M.

      Rollinson, Hugh; Adetunji, Jacob; University of Derby (Elsevier, 2016-06)
    • Determination of Fe3+/ΣFe ratios in chrome spinels using a combined Mössbauer and single-crystal X-ray approach: application to chromitites from the mantle section of the Oman ophiolite

      Lenaz, Davide; Adetunji, Jacob; Rollinson, Hugh; University of Derby (Springer, 2014-01-03)
      We present the results of a comparative study in which we have measured Fe3+/ΣFe ratios in chromites from mantle chromitites in the Oman ophiolite using Mössbauer spectroscopy and single-crystal X-ray diffraction. We have compared these results with ratios calculated from mineral stoichiometry and find that mineral stoichiometry calculations do not accurately reflect the measured Fe3+/ΣFe ratios. We have identified three groups of samples. The majority preserve Fe3+/ΣFe ratios which are thought to be magmatic, whereas a few samples are highly oxidized and have high Fe3+/ΣFe ratios. There is also a group of partially oxidized samples. The oxidized chromites show anomalously low cell edge (a0) values and their oxygen positional parameters among the lowest ever found for chromites. Site occupancy calculations show that some chromites are non-stoichiometric and contain vacancies in their structure randomly distributed between both the T and M sites. The field relationships suggest that the oxidation of the magmatic chromitites took place in association with a ductile shear zone in mantle harzburgites. Primary magmatic Fe3+/ΣFe ratios measured for the Oman mantle chromitites are between 0.193–0.285 (X-ray data) and 0.164–0.270 (Mössbauer data) and preserve a range of Fe3+/ΣFe ratios which we propose is real and reflects differences in the composition of the magmas parental to the chromitites. The range of values extends from those MORB melts (0.16 ± 0.1) to those for arc basalts (0.22–0.28).
    • Explainer: what dust from the Sahara does to you and the planet

      Adetunji, Jacob; University of Derby (The Conversation Trust (UK), 2016-04-21)
    • The geochemistry and oxidation state of podiform chromitites from the mantle section of the Oman ophiolite: A review

      Rollinson, Hugh; Adetunji, Jacob; University of Derby (Elsevier, 2015-02)
      Data are presented for mantle podiform chromitites from eight localities over 350 km strike length of the Oman ophiolite. Chromitite compositions form a continuum from cr# = 0.501 to 0.769, although this conflates a number of different magmatic ‘events’. The Oman mantle chromitites record a wide range of Fe3 +/ΣFe ratios (as determined by Mössbauer spectroscopy) extending from low values (close to those of MORB) to values higher than currently found in arc magmas and calculated oxygen fugacities for the chromites are about 1.8 log units above the QFM buffer, higher than found in the MORB source. Calculated TiO2 and Al2O3 contents for the parental melts to the Oman chromitites show that they had low TiO2 contents (0.23–0.96 wt.%) but a range of Al2O3 contents (11.8–15.8 wt.%). The variable Al2O3 content implies a range of parental magma compositions, probably formed at different temperatures, and the range of TiO2 compositions indicates that some melts were modified by reaction during their transit through the mantle. The range of compositions observed is not consistent with either a MORB or Arc source but is thought to reflect a range of melts derived from a compositionally evolving source during subduction initiation in a forearc environment
    • Highly refractory Archaean peridotite cumulates: Petrology and geochemistry of the Seqi Ultramafic Complex, SW Greenland

      Szilas, Kristoffer; van Hinsberg, Vincent J.; McDonald, Iain; Næraa, Tomas; Rollinson, Hugh; Adetunji, Jacob; Bird, Dennis; Stanford University; McGill University; Lund University; et al. (Elsevier, 2017-06-06)
      This paper investigates the petrogenesis of the Seqi Ultramafic Complex, which covers a total area of approximately 0.5 km2. The ultramafic rocks are hosted by tonalitic orthogneiss of the ca. 3000 Ma Akia terrane with crosscutting granitoid sheets providing an absolute minimum age of 2978 ± 8 Ma for the Seqi Ultramafic Complex. The Seqi rocks represent a broad range of olivine-dominated plutonic rocks with varying modal amounts of chromite, orthopyroxene and amphibole, i.e. various types of dunite (s.s.), peridotite (s.l.), as well as chromitite. The Seqi Ultramafic Complex is characterised primarily by refractory dunite, with highly forsteritic olivine with core compositions having Mg# ranging from about 91 to 93. The overall high modal contents, as well as the specific compositions, of chromite rule out that these rocks represent a fragment of Earth’s mantle. The occurrence of stratiform chromitite bands in peridotite, thin chromite layers in dunite and poikilitic orthopyroxene in peridotite instead supports the interpretation that the Seqi Ultramafic Complex represents the remnant of a fragmented layered complex or a magma conduit, which was subsequently broken up and entrained during the formation of the regional continental crust. Integrating all of the characteristics of the Seqi Ultramafic Complex points to formation of these highly refractory peridotites from an extremely magnesian (Mg# ~ 80), near-anhydrous magma, as olivine-dominated cumulates with high modal contents of chromite. It is noted that the Seqi cumulates were derived from a mantle source by extreme degrees of partial melting (>40%). This mantle source could potentially represent the precursor for the sub-continental lithospheric mantle (SCLM) in this region, which has previously been shown to be ultra-depleted. The Seqi Ultramafic Complex, as well as similar peridotite bodies in the Fiskefjord region, may thus constitute the earliest cumulates that formed during the large-scale melting event(s), which resulted in the ultra depleted cratonic keel under the North Atlantic Craton. Hence, a better understanding of such Archaean ultramafic complexes may provide constraints on the geodynamic setting of Earth’s first continents and the corresponding SCLM.
    • Ionic radii

      Rollinson, Hugh; Adetunji, Jacob; University of Derby (Springer International Publishing, 2017-09-27)
      Definition and Assumptions An ion is an atom with an electrical charge, achieved either by gaining or losing one or more electrons. The ionic radius of the ion (rion) of an atom (either a cation or anion) is a measure of the size of a spherical ion. The ionic radius is similar to but different from the atomic radius for the ionic size is dependent on the distribution of its outermost electrons and is inversely proportional to the effective nuclear charge experienced by ions. It is calculated from the internuclear distance between a cation and a neighboring anion in a lattice. Ionic radii are typically reported in picometers (pm, 1 × 10−12 m) or in the older literature as Angstroms (Å), where 1 Å = 100 pm. A typical range of ionic radii is 25–170 pm for four to eightfold coordination (see Table 1).
    • Low temperature, authigenic illite and carbonates in a mixed dolomite-clastic lagoonal and pedogenic setting, Spanish Central System, Spain

      Huggett, Jennifer; Cuadros, Javier; Gale, Andrew S.; Wray, David; Adetunji, Jacob; Natural History Museum; University of Portsmouth; University of Greenwich; University of Derby (Elsevier, 2016-06-30)
      The aim of this study was to further our understanding of the pedogenic and lacustrine modification of clay minerals. Some of these modifications are of special interest because they constitute reverse weathering reactions, rare in surface environments, and because there is not yet an accurate assessment of their global relevance in mineralogical and geochemical cycles. For this study, two sections from the Central System in Spain were selected. Both are sections through the Uppper Cenomanian-Turonian mixed clastic and carbonate succession, containing both calcite and dolomite, in the Sierra de Guadarrama. Mid-Turonian sea level fall resulted in the formation of a coastal plain environment in which extensive pedogenesis occurred around saline lagoons. The mineralogical changes that have occurred as a result of sedimentation in saline lagoons and as a consequence of pedogenesis are described. Textural relationships indicate that the dolomite cement pre-dates the calcite. Silicate minerals are represented by quartz, kaolinite, illite-smectite, illite, minor plagioclase and alkali feldspar, and trace chlorite and palygorskite. There is a positive correlation between the intensity of pedogenesis and the proportion of illite in the clay assemblage in one of the sections, indicating pedogenic illitisation. In this section, the intensity of the illitisation process increases up, reaching a maximum where pedogenesis is most intense in the middle part, and then decreases as marine influence increases towards the top of the Alcorlo Formation and the overlying marine Tranquera Formation. The clay assemblages are consistent with a slow transformation process from kaolinite to illite by way of illite-smectite, taking place under surface conditions. The illitisation process has resulted in a less Fe-rich, more Mg-, and Al-rich illite than the majority of previously documented cases in the near surface. Formation of Al-rich illite is not therefore restricted to the deep subsurface. The mechanism for low temperature illitisation involves enhanced layer charge resulting from Mg2 + substitution for Al3 + (or Fe3 +) and Fe3 + to Fe2 + reduction. Mg2 + enrichment may have occurred principally in saline lagoons or lakes, while Fe3 + to Fe2 + reduction occurred as a result of wetting and drying in a pedogenic environment. So far as it has been possible to establish, this dual mechanism has not previously been documented. This study indicates clearly that the dolomite and calcite are authigenic cements that precipitated in a clastic sediment, probably soon after deposition. Dolomitisation and Mg enrichment of the clay may have occurred at the same time. Seawater is the most probable source of Mg.
    • Mineralogical and geochemical characterisation of warm-water, shallow-marine glaucony from the Tertiary of the London Basin

      Huggett, Jennifer; Adetunji, Jacob; Longstaffe, Fred; Wray, David; Natural History Museum, London; University of Derby; University of Western Ontario; University of Greenwich (Mineralogical Society, 2017-11-27)
      Glaucony is present in the Palaeocene sediments of the London Basin, from the Thanet Sand Formation to the gravel beds at the base of the Lower Mottled Beds of the Reading Formation. The Upnor Formation glaucony is a rare example of formation in warm, shallow, brackish water and this, combined with the ready availability of fresh material from boreholes, make this study important in developing our understanding of this mineral. Glaucony comprises up to 50% of the Upnor Formation, a grey to green sandstone, of variable thickness and composition, which was deposited in a warm, shallow, marine to estuarine environment, ∼55.6–56.2 Ma. Using morphological criteria, X-ray diffraction data and K+ abundance, the Upnor glaucony may be defined as evolved. The underlying shallow marine Thanet Sand contains <5% of nascent to slightly evolved glaucony. The rare earth element (REE) data for the Upnor Formation suggest more than one source for the sediment from which the Upnor glaucony formed, while the Thanet REE data are consistent with a large detrital clay component. In the Upnor Formation, the large proportion of glaucony that occurs as granule fragments rather than whole granules, and the high-energy estuarine to shallow-marine environment of deposition, are indicative of reworking. The Upnor glaucony is inferred to be intraformationally reworked, rather than derived from the Thanet Sand Formation. The glaucony may have formed in sediments deposited away from the main estuarine channel, and been subsequently reworked into higher-energy sediments. Warm seas with freshwater mixing are more typically characteristic of verdine formation than of glaucony. The shallow, brackish environment of deposition suggests that there is not a clear distinction between the environmental requirements of verdine (or odinite) and glaucony (or glauconite), as is often proposed. The highly fractured, delicate nature of some granules indicates that they have experienced some maturation in situ, after reworking. The oxygen and hydrogen isotopic compositions of Upnor Formation shark teeth and glaucony point to formation in low-salinity water at ∼23 ± 3°C, also consistent with formation in the Upnor Formation, rather than in a fully marine sediment and subsequent reworking. A higher than normal temperature of formation may have increased the rate of evolution of glaucony. Our multidisciplinary study considers many of the factors relating to depositional environment that must be considered when glaucony-rich facies are encountered in comparable palaeo-environmental settings elsewhere in the geological record.
    • Trevorite: Ni-rich spinel formed by metasomatism and desulfurization processes at Bon Accord, South Africa?

      O'Driscoll, Brian; Clay, Patricia L.; Cawthorn, R. Grant; Lenaz, Davide; Adetunji, Jacob; Kronz, Andreas; Keele University; University of Manchester; University of the Witwatersrand; Trieste University; et al. (Mineralogical Society, 2014-02-01)
      The 3.5 Ga Bon Accord Ni deposit occurs within the lowest serpentinized mafic ultramafic lavas of the Barberton Greenstone Belt (South Africa). Though now completely mined out, it comprised a suite of rare Ni-rich minerals that led to its interpretation as either an extraterrestrial body or as an oxidized fragment of Fe-Ni alloy originating from the terrestrial core. In this study, we draw on detailed petrographic observation and mineral chemical data, as well as previous work, to re-evaluate these ideas. The balance of evidence, from thin section (<1 mm) to regional (∼10s of km) scales, appears to support an alternative origin for Bon Accord, possibly as an oxidized Ni-sulfide deposit formed in association with ocean floor komatiite eruptions.
    • UK pension changes in 2015: some mathematical considerations

      Stubbs, John; Adetunji, Jacob; University of Derby (Cambridge University Press, 2016-06-14)
      This paper presents a mathematical treatment of some of the changes made to pension arrangements by the UK government in 2015. A mathematical model of a pension fund is developed based on three variables: life expectancy of pensioner, interest rates on investments and rates of inflation. The model enables a prospective pensioner to decide, at point of retirement and on the basis of predicted income streams, whether to opt for, (i) a (life) annuity or a draw down scheme, (ii) an inflation proofed (index linked) income or a fixed income and (iii) an immediate income or a deferred income. Numerical examples are provided to add clarity to the financial options available at retirement. On the basis of the numerical examples given, the paper concludes by urging caution on the part of the pensioner before taking an annuity rather than a draw down scheme, an index linked rather than a fixed income and a deferred rather than an immediate pension income. UK pension changes in 2015: some mathematical considerations.