Abstract:
Optical investigation of the orthopyroxenes also revealed a compositional break in the differentiation sequence at the height of the Pyroxenite Marker. A reversal in the trend of iron enrichment is recorded at all four of the localities examined. At the same stratigraphic height, the orthopyroxene is poorer in iron in the north than in the south. The compositional trends of the plagioclase from the four localities resemble those of the orthopyroxene. Separated pyroxenes were analysed chemically by means of the energy dispersive analyses of x-rays (EDAX) method. Whole-rock samples were also analysed by means of EDAX. Quantitative trace-element analyses on the whole rock and on separated cumulus minerals were carried out by atomic absorption spectrometry (AAS) for the element’s barium, chromium, cobalt, copper, nickel, strontium and zinc, whereas ion exchange chromatography was used for titanium and vanadium. Variation in the chromium and nickel contents with fractionation was shown to conform with that predicted by crystal field theory. At the Pyroxenite Marker the reversal or the normal fractionation trend is borne out by the partitioning of chromium. The reversed fractionation trend, as well as the partitioning of the trace elements at the level of the Pyroxenite Marker, are inconsistent with the process of continuous fractionation within the magma chamber. The gradual addition of undifferentiated magma during crystallization of the upper part of Subzone B, followed by a larger, sudden influx of magma at the level of the Pyroxenite Marker, were probably responsible for the observed relationships.
