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The geochemistry and oxidation state of podiform chromitites from the mantle section of the Oman ophiolite: A reviewData 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
Ionic radiiDefinition 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).