COMPOSITIONAL VARIATIONS AND CRYSTALLIZATION HISTORY OF SOME PYROXENES FROM THE POÇOS DE CALDAS ALKALINE MASSIF (MG-SP).

G.A.R. Gualda & S.R.F. Vlach
Departamento de Mineralogia e Petrologia, IG-USP

Texture and composition of zoned pyroxene crystals found in agpaitic nepheline syenites from the Poços de Caldas Alkaline Massif have been studied in order to understand crystallization processes operating during crystal growth.

Textural features have been explored using optical microscopy while compositional studies have been performed with a JEOL-JXA8600 microprobe at the Inst. Geociências. Scanning backscattered electrons images (COMPO) were obtained under beam conditions of 15 kV acceleration, 20 nA current and minimum diameter. Crystals showing the most complex zoning patterns were selected for quantitative chemical studies. Almost 250 complete analysis (WDS) – in five profiles (15-30 µm step) – were obtained under a 5 µm beam diameter in order to minimize light element migration.

Spectral lines and analyzed elements were Ka (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K) and La (Zr). Counting times range from 20s for the major to 120s for the minor and trace elements. Synthetic and natural compounds were chosen as standards. Matrix corrections were made with the PROZA program. Estimated errors (2s ) are below 1% and 5% for major and minor elements respectively, and up to 50% for some trace elements. Fe²⁺ and Fe³⁺ were estimated from total measured values assuming stoichiometric occupation of the crystallographic sites. Detailed profiles (~1 µm step) were obtained using the same instrumental conditions, except for the beam diameter, that was set to the minimum. In these, five elements, one for each spectrometer, were quantified in order to keep the spectrometers in fixed positions. Such a procedure leads to less accurate absolute values but gives the best relative results for zoning studies of minerals with high contents of light elements.

The pyroxene crystals studied have compositions close to that of pure aegirine [NaFe³⁺Si₂O₆], with varying amounts (up to 30%) of the augite component [Ca(Fe²⁺,Mg,Al)(Si,Al)₂O₆]. Most important chemical variations are explained through coupled substitution mechanisms as [Ca]^M2+[Fe²⁺,Mg, Mn]^M1 Û [Na, K]^M2+[Fe³⁺, Al]^M1. Ti and Mn may appear as the component [Na(Ti,Fe²⁺,Mn)Si₂O₆].

The analysis of the resulting chemical zoning patterns showed in Figure 1 reveals at least two contrasting stages in the growth history of these crystals:

1. The inner portion has compositions restricted to the aegirine-augite join and shows smooth variations, with increasing amounts of the aegirine molecule from core to rim. Crystals fill the spaces between other minerals, always accommodating to them;

2. The outer regions, instead, show enrichment in Ti (and to a lesser extent in Mn). Sharp compositional variations are typical and higher degree changes make it possible to identify various growth cycles (Figure 2); these patterns are referred to as oscillatory. These portions show evidence of development through destruction of the original minerals and textures.

The properties of the inner and most abundant portions of the crystals are compatible with late-stage crystallization, under decreasing temperature, from the melt that originated these rocks. On the other hand, the oscillatory zoning in the outer regions indicate a more dynamic environment, in which higher diffusivities prevailed, which, together with textural observations, indicate crystallization as overgrowths, from an aqueous supercritical fluid, after the melt has vanished.