Abstract

Samples representing consecutive 2-cm-thick slabs of a drill core through the UG-2 chromitite layer were analyzed for platinum-group elements. The results, when plotted against height, reveal an interesting pattern, with sequences that follow exponential curves of the type Y = ZX k , where Y equals the concentration of the element in samples taken at a distance X from the base of the layer, and Z is a constant. The data fit the curves with correlation coefficients of better than 0.90, and levels of significance less than 0.1 percent, which shows that the fit cannot be due to chance but that a process following or approximating an exponential equation must have been involved. The most probable such process is fractionation according to the Rayleigh law. It is postulated that these sequences that follow the Rayleigh law formed by bottom crystallization when the magma was in motion. Calculations indicate that the elements supplied to the site of crystallization were withdrawn from a surprisingly small thickness of magma and that their supply was not limited by diffusion.Between the sequences that follow the Rayleigh law are sequences that probably crystallized from stagnant magma, so that the supply of elements to the site of crystallization was diffusion controlled. In these sequences the ratios between the different platinum-group elements change progressively.It is argued that these results reveal that the platinum-group minerals crystallized very early in UG-2 layer-type deposits and show a pattern of element behavior conforming closely with that of a classical compatible element which has a very large bulk distribution coefficient. Therefore, the well-known processes of enrichment applicable to such elements during processes of melting and crystallization could have been operative.This one example is not sufficient to disclaim the possibility that platinum-group elements can also behave like incompatible elements under different conditions, as is assumed by many authors in this field.

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