Experimental Studies of Carbonatite Problems: The Origin and Differentiation of Carbonatite Magmas

Abstract

The experimental data reported by Wyllie and Tuttle (1960a, b) confirm that melts with a variety of compositions can precipitate calcite through a wide pressure range and through a wide temperature interval, and that these melts persist down to temperatures of the order of 600°C, which agrees reasonably well with temperatures inferred for the emplacement of natural carbonatites. This is regarded as verification for the magmatic origin of carbonatites where the field relationships are consistent with magmatic origin. However, neither the experimental data nor the field and petrographic studies provide reliable estimates of the compositions of natural carbonatite magmas at the time of intrusion. This is clear from the variety of hypotheses to be found in the geological literature as well as in the earlier chapters of this book. The composition and nature of a carbonatite magma depends upon the processes involved in its formation. In this chapter are described the results of experiments designed to elucidate the petrogenic relationships among carbonatite magmas and the alkaline igneous rocks associated with carbonatites in the field, and to test various hypotheses of origin which have been proposed on the basis of petrological studies and inferences. In addition to results related to these central objectives, the phase relationships in some of the systems studied illustrate several ways in which the crystallization of carbonatite magmas could proceed, and they indicate that some of the observed sequences of emplacement of carbonatites could be explained by differentiation processes occurring within a crystallizing carbonatite magma. Most of the material in this chapter represents a progress report of an extended programme at the Pennsylvania State University which has been supported by the National Science Foundation since 1961 (Grant No. NSF-G-19588). This support is gratefully acknowledged. Thanks are due also to the Department of Scientific and Industrial Research for its support of the work in systems containing P_2O_5 which was conducted at Leeds University between 1959 and 1962. The chapter covers research completed by September 1963.

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