THE DISTRIBUTION OF SR AND BA BETWEEN ALKALI FELDSPAR PLAGIOCLASE AND GROUNDMASS PHASES OF PORPHYRITIC TRACHYTES AND PHONOLITES

Data for the distribution of Sr and Ba between the feldspar phenocrysts and groundmasses of porphyritic volcanic rocks are given. It is assumed that the composition of the bulk rock represents an original liquid composition and the composition of the groundmass represents that of a residual liquid. Distribution ratios of the type, element in feldspar/element in original liquid, have been determined. These ratios for Sr and Ba in plagioclase vary from 3.27 to 4.31 and 0.72 to 1.09 respectively. Ratios for sanidine vary from 1.20 to 2.80 for Sr and 1.17 to 8.95 for Ba. Anortho-clase feldspars show distribution ratios intermediate between those of sanidine and plagioclase. The Ba Sr ratios of the sanidines are shown to be dependent on the crystallisation history of the feldspars. Thus where plagioclase is the first feldspar phase to separate the Ba Sr ratio of the liquid increases and the more plagioclase to crystallise the higher the Ba Sr ratio becomes. This is reflected in the composition of the sanidine when it starts to crystallise alongside plagioclase. © 1969.