MgO enrichment

Eased on two main lines of evidence, Niu et al. (1997) concluded that abyssal peridotites are the end products of melt extraction followed by variable amounts of olivine crystallization. First, in their set of reconstructed compositions they found that model fractional and batch melt extraction trends could not reproduce major and minor element variations in their data set. Most importantly, they found that melt extraction models failed to account for the strong positive correlation between FeO and MgO, as well as incompatible minor-element concentrations. Specifically, at a given Na20 or Ti02 content, abyssal peridotites are enriched in MgO relative to model melt extraction residues. Niu et al. (1997) showed that these compositional anomalies can be reconciled by a model of melt extraction followed by olivine crystallization, with more MgO-enriched samples having more accumulated olivine. If correct, this model has important implications for understanding melt extraction at oceanic ridges, and it has recently been the focus of re-evaluation. 

The basal zone of the Basement sill on Roadend Nunatak Fig. 13.16 is enriched in MgO (Kibler 1981), presumably because of the presence of three pyroxenes including hypersthene. This zone is not present in the Peneplain sill which has a uniformly low MgO concentration of less than 7.0%. The Mg-enrichment of the Basement sill on Roadend Nunatak is similar to the MgO enrichment of the hypersthene-bearing sills in the ice-free valleys of southern Victoria Land reported by Gunn (1962, 1966) which was later attributed by Marsh and Philipp (1996) to flowage differentiation. 

A similar range of doloma-based products exists for cement applications, with a high proportion containing zirconia additions (up to 2% ZrOa) because of the high risk of failure due to thermal shock in the operation. For the burning zone the products are based on doloma, but for the conditions in the transition zone MgO-enriched grades are required (60-80% MgO). 

Other Processes. Dead Sea Periclase (DSP, Mishor Rotem, Israel) converts magnesium chloride into MgO by spray-roasting, then hydrates the MgO to Mg(OH)2. The Mg(OH)2 is washed and dmm filtered. DSP purchases the brine from Dead Sea Works, which collects and stores enriched brine from the southern margins of the Dead Sea (77). 

The neutral alteration is mainly characterized by increasing enrichments in MnO, F02O3, and MgO and losses of Na20 (Fig. 3) towards the ore in association with the presence of a quartz. 

Marker diagrams show depletions in Al, Ca, Ti, Fe, Mn, Mg, and P with Si02 enrichment indicative of fractionation of phenocryst phases noted above. However, major elements plotted as a function of MgO display linear trends that... 

Where naturally occurring isotopes of the cation are not suitable, then enrichment of the surface cations with nonzero-nuclear-spin isotopes is a powerful technique, e.g., 95Mo03 on A1203, Si02, or MgO (108). Often, the presence of a superhyperfine interaction leads to spectra of low resolution and it is useful to increase both the intensity of the spectra and the resolution. Second or even higher derivative spectra can be used to enhance the resolution and in some cases secondary reactions (121) have been shown to increase the intensity. 

There is little doubt that the Tuscany mafic magmas have been subject to fractional crystallisation, mixing and crustal assimilation (e.g. Conticelli 1998). However, their high MgO, Ni and Cr concentrations, whose values are close to those of primary mantle melts, exclude that the mafic magmas with different enrichments in potassium and incompatible elements can be derived from each other by any common evolution process. Therefore, it has been concluded that the variable petrological and geochemical compositions of mafic rocks in Tuscany basically result from anomalous and heterogeneous mantle sources (Peccerillo et al. 1987). 

Fig. 4.16. Variation diagrams of MgO vs. selected major and trace elements and 87Sr/86Sr ratios for Sabatini volcanics. Open triangles and full triangles represent groups of rocks with different degrees of enrichments in incompatible elements (see text for explanation).

The alkaline mafic volcanics are less enriched in silica and more enriched in incompatible trace elements than oversaturated tholeiitic rocks with similar MgO contents. REE show smooth and fractionated patterns (Fig. 9.7). Mantle normalised incompatible element patterns of mafic rocks generally display a small upward convexity with moderate enrichments in Ta and Nb. The large majority of the mafic volcanic rocks exhibit positive spikes of Ba and Pb, which are small or absent in the southern outcrops of Capo Ferrato, Guspini and Rio Girone (Fig. 9.7). Silicic volcanics exhibit a large range of incompatible element concentrations. Sr- and Nd-isotopic... 

In addition, the predicted orders in eqs (7.18) and (7.19) do not take into account phase equilibrium relationships involving coexisting minerals. For example, olivine should always be enriched in Fe2+ ions relative to orthopyroxene, according the the CFSE data plotted in fig. 7.6. Moreover, studies of the system MgO-FeO-Si02 at atmospheric pressure (Bowen and Schairer, 1935) showed that magnesium olivine crystallizes first and becomes increasingly enriched in iron before pyroxene commences to crystallize. The olivine main-... 

Che et al. observed the formation of surface Q species by EPR upon dosing 170-enriched O2 molecules on pyridine-pretreated CaO (250), MgO, and SrO (148). For the three oxidic systems the splitting of the 2p r orbitals by the surface crystal field has been measured from the gzz component of the g tensor, and a dependence on the polarizing tendency of the surface cation (q/r) was found (148). It was inferred that the donor sites producing the Oj molecular ions are the coordinatively unsaturated surface O2- ions. Fs-centers in CaO have been investigated by EPR (251). 

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