The chemical zoning of augite phenocrysts in alkaline basalts from Gough Island, South Atlantic

Abstract

The chemical zoning of augite phenocrysts in alkaline basalts from the Middle Basalt Series lavas of Gough Island was studied in detail for the major and trace elements using an ion microprobe. The observations are summarized as follows: 1. (1) The augite phenocrysts do not show a simple ‘normal’ or ‘reverse’ zoning, continuous and gradational from core to rim; rather they show concentric growth zones of distinct chemistry. 2. (2) Transitions from one growth zone to another can be as sharp as the spatial resolution of this study (5–8 microns), or can also be diffused over a distance up to 50 microns. 3. (3) All transitions are characterized by combined major- and trace-element variations, i.e., when major elements change, so do trace elements and vice versa. 4. (4) There are two types of major-element variations, i.e., one involving change in Mg/Fe ratios (or the Fs content) and another involving change only in the Wo contents with constant Fs. 5. (5) The trace-element variations appear to override element compatibility, i.e., a compatible element (e.g., Cr) can vary either sympathetically or antithetically with incompatible elements (Ti and Sr). 6. (6) The magnitude of trace-element variations at a given transition over a distance of 10 microns can be as large as a factor of 2 or more. 7. (7) The variation of Cr content is not always concurrent with those of other elements, i.e., in some cases Cr remains unchanged when other elements do and in others Cr varies when other elements do not. The major-element changes at the growth zone transitions suggest that the growing phenocryst was exposed to a new liquid, which does not seem to have a direct genetic link to the pre-existing liquid via fractional crystallization of phenocrysts. The trace-element variations at the transitions are consistent with the arguments based on the major elements. If the partition coefficients remained unchanged at the transitions, the trace-element concentration ranges observed in augites require that the phenocrysts saw a much broader spectrum of magma chemistry than actually sampled as lavas. If, however, the partition coefficients varied at the introduction of new liquid, the combined changes in the liquid chemistry and the partition coefficients appear to explain the ranges of trace-element contents in augite without invoking ‘unsampled’ liquids. It also seems possible that a part of the trace-element variations is due to the non-equilibrium effects. Ubiquity of ‘multi-liquid’ growth zones with sharp transitions between them can be considered as strong and direct evidence for extensive magma mixing and hence extensive ‘open-system behaviour’ of the Gough Island magma body.