PEARCE.PLOT: a Turbo-Pascal program for the analysis of rock compositions with Pearce element ratio diagrams

Abstract

Pearce element ratios can be used to test specific hypotheses concerning igneous differentiation processes. Pearce element ratio diagrams can be employed to recognize cogenetic rock analyses, the minerals involved in differentiation, and the compositions and proportions of the involved phases. Petrologic hypotheses can be tested by examining the trend of data on a variety of Pearce element ratio diagrams. Hypotheses are rejected on the basis that the data lack correlation, or the best fit line has a zero intercept, or where the slope of the best fit line does not equal the slope predicted by mineral stoichiometry. To facilitate the analysis of petrologic hypotheses with Pearce element ratios we present the following interactive, graphics-supported Turbo-Pascal computer program. The program PEARCE.PLOT reads interactively identified data files and allows the user to select specific samples from data files, assign analytical errors to oxide analyses, and generate and print a variety of Pearce element ratio diagrams. Each plot has options for error representation, labeling of data, and fitting lines to the data. Attributes of the program are illustrated with two data sets from Kilauea volcano, Hawaii. The 1955 and 1960 eruptions of Kilauea are shown to be derived from a single magma which has undergone variable sorting of olivine, plagioclase, and augite. Differentiation of the 1955 lavas and the early 1960 lavas was characterized by subequal amounts of all three (micro-) phenocrysts, whereas the late 1960 eruptions owed their chemical variation mainly to olivine and subordinate amounts of plagioclase and augite. Pearce element ratios illustrate visually and graphically most of the same conclusions reached by conventional mass-balance calculations. However their real strength lies in their ability to analyze extended differentiation trends in terms of mineralogy and mineral proportions. Additionally, they form a rigorous basis for rejecting ill-conceived hypotheses.