Educació emocional

5.4.1. Paragenetic classification

The fluid inclusions present within the xenolith suite from Oldoinyo Lengai show primary, secondary and pseudosecondary inclusions, which are intrinsically linked to the formation of the host minerals. The plutonic and volcanic rock units typically contain primary and pseudosecondary inclusions which are indicative of the material from which they have formed.

The primary inclusions are easily identified as they occur along the growth zones in minerals such as nepheline (Figure 5.1a, d, f). Interestingly, in Figure 5.1a and 5.1d not all of the growth zones are depicted by inclusions trails, with Figure 5.1d in particular only showing primary inclusion trails during the later stages of crystallisation. The presence of inclusions as large assemblages rather than as defined trails complicates the classification in terms of crystal growth and so are treated as primary if they occur towards the centre of the crystal but secondary or pseudosecondary if they are located near to the crystal edges.

The inclusions within the fenitic units are mainly pseudosecondary or secondary in origin if located within relict crystals from the original host rock, such as quartz (Figure 5.2d).

Secondary fluid inclusions are generally not useful in fluid inclusions studies, for example in ore petrogenesis investigations, as they do not represent samples of the fluid from which the crystal formed. However in terms of fenitisation processes, secondary trails of inclusions in relict crystals are samples of the metasomatising fluid and so perfect for investigation. Primary inclusions in relict minerals would represent the original fluids involved in the metamorphism of the country rock. In newly precipitated crystals such as nepheline or sanidine feldspar, which have formed during the fenitisation process, the inclusions are primary or pseudosecondary. These again should contain fluids which characterise the fenitising fluid or perhaps different generations of fenitising fluids if pseudosecondary.

166 5.4.2. Mono-phase, bi-phase or multi-phase inclusions.

As stated previously, no vapour bubbles or daughter crystals are visible within the inclusions of the plutonic and volcanic units of the xenolith suite. They have therefore been classified as mono-phase inclusions until further analysis is completed.

Classification of the assemblages within the fenitic units are much simpler with both multi-phase and bi-multi-phase inclusions present under higher magnification of an optical microscope.

The majority of inclusions within the fenites appear at first glance to be liquid-rich, bi-phase inclusions, containing a vapour bubble which occupies <30% of the inclusion (Figure 5.7a). The inclusions are between 15 and 50 µm in length and often contain a thick, dark rim, which is indicative of carbon dioxide. The dark rim is a result of a contrast between the refractive index (RI) of carbon dioxide (1.18) and the host mineral (quartz for example has RI = 1.55) (Shepherd et al., 1985). The vapour bubble can be seen in rapid motion around the fluid inclusion due to the excitation caused by the light of the microscope. A number of the inclusions also seem to produce negative crystal features (Figure 5.7c).

Further analysis of the bi-phase inclusions reveals that a number of them also contain solid salt crystals (discussed in section 5.5 below) which are not visible during optical examination as they are obscured by the dark rim. However it is possible to see multiple solids in some inclusions where the dark rim is absent (Figure 5.7b). The multi-phase inclusions displayed in Figure 5.7b appear to contain up to 3 solid crystals, one of which is opaque and occupies the end of the inclusion. The remainder are colourless crystals of both square and needle-like shape, contained within a colourless liquid phase.

167 Figure 5.7: Photographs of inclusions from fenitic units illustrating phases present within samples: (a) Clear liquid-rich inclusion with vapour bubbles towards the end of the inclusion;

(b) Multi-phase inclusions within fenite OLX 3 with different solid crystals within liquid inclusion; (c) large number of inclusions in assemblage of OLX 17b showing dark rims and negative crystal shapes. All appear as mono-phase inclusions

B

C

168 5.4.3. Trapping conditions

The manner in which the inclusions were trapped – homogeneously so that all inclusions represent samples of the same fluid and appear identical at first, or heterogeneously where variable compositions and phase ratios are trapped at the same time – is important in determining the timing of inclusion assemblage formation and characteristics of the solution at the time of trapping. Figure 5.8 below represents the different morphologies of inclusions that can be expected based upon the trapping conditions (Van den Kerkhof and Hein, 2001). For fluid inclusion analyses it is preferable to study assemblages of homogeneously trapped inclusions as they can provide multiple datasets for the same fluid.

The inclusions analysed within this study appear to be mainly samples of homogeneous trapping with subsequent shrinkage and saturation, generating vapour bubbles and solid crystals. There are, however, examples of heterogeneously trapped fluids from a boiling solution which can be recognised by the presence of a meniscus between the vapour and liquid. It is possible that some of the crystals may be incorporated rather than precipitated, which is another indication of heterogeneous trapping, but it is difficult to determine the difference using optical microscopy (Shepherd et al., 1985).

Figure 5.8: Illustration of inclusion morphologies depending on method of trapping.

Homogeneous trapping results in inclusions which have similar phase ratios. Upon cooling vapour bubbles and solids may appear. Heterogeneous trapping results in inclusions with variable phase ratios. (Van den Kerkhof and Hein, 2001)

169 With the inclusions now classified in terms of their timings of formation and phases present, the next step is to determine the chemical species within the inclusions. This can be completed using two different methods; the first discussed below in section 5.5, is non-destructive Laser Raman spectroscopy which can be used to determine all phases and their chemical composition. The second is microthermometry, which is a tool to determining the density of the inclusions, and shall be the focus of section 5.6.