Jacob Adetunji

New Mössbauer measurements of Fe 3+/ΣFe in chromites from the mantle section of the Oman ophiolite: Evidence for the oxidation of the sub-oceanic mantle

Abstract Room temperature Mössbauer and electron-probe measurements of Fe3+/ΣFe in chromite from the mantle section of the Oman ophiolite define two groups of samples: a low Fe3+/ΣFe group (with Fe3+/ΣFe = 0.21-0.36) have cr# = Cr/(Cr + Al) in the range 0.49-0.75, whereas a smaller more geographically localized high Fe3+/ΣFe group (with Fe3+/ΣFe = 0.71-0.78) have a more restricted range of cr# ratios of 0.72-0.75. The low Fe3+/ΣFe chromitites have very variable Fe3+/ΣFe ratios. They are thought to have crystallized from melts that have interacted with depleted mantle and thereby acquired their variable Fe3+/ΣFe ratio. The high Fe3+/ΣFe chromitites are restricted to one small area of the mantle and their high oxidation state is thought to be post magmatic. They are either the product of later heating, related to melt flux or interaction with a later oxidising melt. A difference in oxygen fugacity between the MORB-depleted harzburgite host, which is at the quartz-fayalite-magnetite (QFM) buffer and the later chromite-bearing melts (QFM + 2) implies that there is a real difference in the oxidation state of the MORB and arc-magma sources.

Trevorite: Ni-rich spinel formed by metasomatism and desulfurization processes at Bon Accord, South Africa?

Abstract 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.

Mineralogical and geochemical characterisation of warm-water, shallow-marine glaucony from the Tertiary of the London Basin

Abstract 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 awarm, 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 somematuration 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.

Characterization of pyrrhotite in Cu-Ni-ore bodies from mines in Botswana by Mössbauer spectroscopy, X-ray diffraction, and thermomagnetometry

Cu-Ni ore bodies from five Botswana mines were investigated, using a combination of X-ray diffraction, thermomagnetic measurements, Mossbauer spectroscopy, and Inductively Coupled Plasma-Atomic Absorption Electro-optic Spectroscopy (ICP-AES). The four dominant phases in the ores were found to be pyrrhotite, magnetite, pentlandite, and chalcopyrite. Mossbauer spectroscopy shows that pyrrhotite is the dominant Fe-bearing phase in the samples with 75 % in B-S1, 84 % in B-S2, and 80 % in B-S4 and was selected for detailed characterization. Our results show that the three first techniques provide an excellent approach to estimate the degree of polytype intermixing in pyrrhotite. Accordingly, the ores can be grouped into three types: A hexagonal (or peak-type), with a 5c stacking arrangement; a monoclinic (or Weiss-type), with a 4c stacking arrangement; and a mixture of monoclinic and hexagonal types. A low-temperature study of the monoclinic form shows that the low-temperature magnetic transition close to 35 K has features similar to the Verwey transition in magnetite induced by Jahn-Teller distortion due to orbital order-disorder transition. (Less)

A Mössbauer study of steel and its ore originating from Northern Iran.

Room temperature Mossbauer spectroscopic analyses of iron ore from Northern Iran shows that it contains four components of iron bearing oxides with magnetite as the major component. The other three components, hematite, goethite and lepidocrocite are either oxidised or weathering products of the magnetite. The goethite was fitted with a sextet indicating it is well crystalline. The results of the elemental composition analysis show that Al and Ca are present in appreciable quantity to constitute the main elements partially substituting for Fe in the non-stoichiometric magnetite in the ore sample.