EFFECTS OF NONCONSERVED DENOMINATORS ON PEARCE ELEMENT RATIO DIAGRAMS

Pearce element ratios (PER's) have ''conserved '' denominators which h ave not participated in the material transfer processes that cause che mical variations in rocks. Theoretically, there is no truly conserved element (constituent) which can be used as a PER denominator because i n every material transfer process all constituents have non-zero conce ntrations in the phases that are being transferred. Thus, constituents used as denominators of PERs may have undergone at least a small amou nt of material transfer. This communication investigates the degree to which a non-conserved PER denominator changes the trend of data produ ced by a material transfer process from that produced by the same proc ess but plotted on a PER diagram with a truly conserved denominator. A n equation is developed that utilizes the partition coefficient as the measure of the degree of involvement of the denominator constituent i n the phase undergoing transfer. This equation is examined to determin e how the magnitude and direction of a PER diagram data trend change w ith increasing involvement of the denominator constituent in the trans ferring phase. A set of plagioclase fractionation examples are present ed which use different elements as PER denominators and consider the e ffects that small amounts of these elements in the plagioclase structu re will have on the data trend, as a function of the element partition coefficient between crystal and melt. Results demonstrate that the di rection of change in slope of a material transfer data trend is a func tion of the initial relative magnitudes of the numerator constituents on the PER diagram. Additionally, if the amount of involvement of a PE R denominator in a separating phase is very small relative to the amou nt of the numerator constituents in the separating phase, there is no significant change in the data trend caused by material transfer on a PER diagram. Moreover, if the denominator constituent substitutes for a numerator constituent in the phase undergoing transfer, the intercep t of the trend of the data may not converge to zero when there is a la rge partition coefficient, as would be expected from theory. Thus, sta tistical tests to determine if a PER denominator is conserved, which e valuate whether the intercept is significantly different from zero, ma y not be very powerful because a large amount of denominator variation is necessary before the intercept of a data trend is forced through t he origin, if at all.