Sulfide inclusions modeling

Until the application of the Re-Os isotopic system to whole-rock peridotites (Section 2.05.2.7.4) the Archean and Proterozoic Pb-Pb and Sm-Nd model ages and isochrons obtained on diamonds were the chief constraints on the antiquity of the continental hthospheric mantle (Kramers, 1979 Richardson et al, 1984). Early work, most of which was on diamonds from the Kaapvaal-Zimbabwe craton because of sample availability, suggested a simple difference from Meso-Archean P-type (harzburgitic) diamonds and Proterozoic E-type diamonds (Richardson, 1986 Richardson et al, 1984, 1993). More recent Re-Os work on single sulfide inclusions from the Kaapvaal-Zimbabwe craton has underscored the importance of a Neo-Archean E-type diamond... 

Schematic modeling of the effect of sulfides First the sulfides dissolve more or less quickly, depending on their stability and of the solution corrosivity. Then dissolved sulfur containing species redeposit on the passive film close to the sulfide inclusion, promoting local film breakdown or preventing film healing after pit nucleation.

However, whatever the details of the mechanisms, pitting initiation occur more easily close to a sulfide inclusion than far from it, and the pitting sensitivity increases with the inclusion solubility. i Such a model explains why Ti containing steels (then MnS free) are more resistant that MnS containing ones, since the Ti sulfide solubility is smaller than the one of MnS. The effect of other type of sulfides confirms this view, as illustrated in the last section of this chapter. 

Several proposals have been made for the chemistry and minerals involved. The loaded dielectric model uses inclusions of iron sulfides, or other high dielectric minerals, in rocks. Higher amounts of the dielectric minerals produce lower radar emissivities and higher dielectric constants. The iron sulfides, or other dielectric minerals, are destroyed by chemical reactions with Venus atmosphere. The reactions proceed faster at lower elevations, where the temperatures are higher, so the lowest radar emissivity regions are predicted at the highest elevations, or in areas with the youngest volcanic rocks. 

The sample suite from each core can be divided into four factors on the basis of similarities in chemical compositions as defined by Q-mode factor modeling. Factor-1 samples are rich in trace metals due to adsorption onto clay, altered tuffaceous material, and (or) organic matter and precipitation as sulfides. The relatively high concentration of boron is probably related to its inclusion in authigenic feldspars. Factor-2 samples are rich in elements commonly associated with minerals of detrital or volcanic origin. These samples contain relatively high concentrations of analcime, dawsonite, and (or) potassium feldspar, all of which are associated with alteration of tuffaceous material. Siderite and ferroan or ankeritic... 

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