6 Genetic significance of CL structures and trace element distribution
6.3 Secondary CL structures and processes resulting in modification
Quartz may show a number of secondary structures made visible in CL which formed after growth of the host crystal by retrograde processes. The secondary CL structures are principally formed by a number of processes: 1) micro-cataclasis followed by healing (dissolution-precipitation), 2) diffusion, and 3) a-radiation. These processes result in the modification of defect centres in the quartz lattice or in the formation of new quartz. The secondary structures may show reduced or lower defect centre contents or reversely defect centres are induced. The secondary CL structures have been grouped according their defect centre content.
Secondary CL structures with reduced defect centres
Secondary structures with reduced defect centre content are healed veinlets, healed irregular domains, and patchy halos of secondary quartz around fluid inclusions. They are distinguished by weak red-brown or no CL. This weak CL is indicative of lower contents of activator elements and/or intrinsic defects.
Trans-granular veinlets and irregular domains of up to several hundreds micrometres wide have been observed. The veinlets are completely healed, or they contain fluid inclusions.
Some samples (Megaquartz, Schellerhau Granite) show different veinlet generations of (Fig.
6.6e, f). Thin concentric and radial, healed micro-cracks are typical for rhyolitic quartz phenocrysts (Fig. 6.2a, b). In this case the cracks are interpreted as thermally induced contraction cracks formed due to rapid cooling during effusion. Dissolution-precipitation (healing) results in veinlets and domains of secondary quartz along micro shear zones, grain edges, grain and subgrain boundaries (§ 10.7).
Fig. 6.6 SEM-CL images of secondary CL structures in quartz. a – Non-luminescent spots (easily visible in the cycle) in quartz of the Flossenbuerg Granite. b – Patchy halos of secondary quartz around fluid inclusions connected by sub-parallel healed micro cracks (Flossenbuerg Granite). c - Combination of SEM-CL and BSE images showing the residual porosity of the former fluid inclusions (black “holes”; Flossenbuerg Granite). d – Healed veinlet in the quartz of the Sunset Hills Granite/Lachlan Fold Belt (see chapter 10). The wider veinlet is not completely healed. e – The hydrothermal megaquartz shows a dense network of healed cracks and domains of different ages. Three populations can be distinguished: 1) healed domains with weak CL following fluid inclusion trails. 2) Bright luminescent, linear trails some of which overprint the population (1). 3) Linear, thin cracks healed with weak luminescent quartz. f – Light circular halo around a zircon. The radius of halo is ~40 µm. The halo is subdivided into a brighter inner zone (~25 µm) and a outer zone (~15 µm) (Schellerhau
Particularly in quartz formed at high temperatures patchy halos of secondary quartz around fluid inclusions were observed. These and similar structures have been observed since about one decade (Frentzel-Beyme, 1989; Behr, 1989; Kerkhof and Müller, 1999). In the following part the characteristics and formation mechanics of the halos are discussed in more detail. The understanding of the formation of this structures is important for the interpretation of fluid inclusion data. Magmatic quartz crystals typically contain red to red-brown luminescent, patchy halos of secondary quartz around fluid inclusions, connected by healed sub-parallel trans-granular micro-cracks of <5 µm wide (Fig. 6.6c). This structures are frequent in granitic quartz and less common in rhyolitic phenocrysts. The secondary quartz has the same crystallographic orientation as the host quartz indicating. The CL intensity of the secondary quartz increases during 2-10 min electron radiation using high beam power densities >10+4 W/cm2 (Fig. 6.2d). The halos of secondary quartz are depleted in trace elements (Li, Al, K, and Ti) compared to the host quartz (see § 10.7). Sometimes, the host crystal around the secondary quartz is enriched particularly in Fe and Ti (Fig. 5.11c). These elements are probably released from the secondary quartz.
Decrepitation experiments of fluid inclusions with halos of secondary quartz showed that they hold anomalously high fluid pressures on heating (Müller, 1995). Bodnar et al. (1989) showed that the internal pressure required to initiate decrepitation of fluid inclusions in quartz is inversely related to inclusion size according to the equation: internal pressure (kbar) = 4.26 • D-0.423, where D is the inclusion diameter in microns. Figure 6.7 shows this relationship between decrepitation pressure and fluid inclusion diameter (line) within a range (hatched area) after Bodnar et al. (1989). The data measured for the pegmatite quartz of Pleystein and Kreuzstein and Pfahl quartz (Oberpfalz, Germany) plot in the hatched area. On the other hand the data of the Rozvadov Granite (Oberpfalz, Germany) show extremely high decrepitation pressures. The fluid inclusions of this rock exhibit halos of secondary quartz, whereas the pegmatites and Pfahl quartz do not.
The specific features of secondary quartz around fluid inclusions allow us to establish a model for the mechanism of fluid inclusion decrepitation and modification, whereby three stages can be distinguished (Fig. 6.8).
Fig. 6.7 Relationship between decrepitation pressure and inclusion diameter after Bodnar et al. (1989).
According to Bodnar et al. (1989) data should plot in the hatched area like the fluid inclusions of the pegmatites and the Pfahl quartz form the Oberpfalz/Germany. The fluid inclusions in the quartz of the Rozvadov Granite showing halos of secondary quartz around fluid inclusions hold anomalously high fluid pressures on heating (Müller, 1995).
In the first stage mass decrepitation of fluid inclusions (micro-crack formation) may occur.
Simultaneous decrepitation of fluid inclusions is assumed, because in the present samples the halos are connected with one generation of healed micro-cracks. Mass decrepitation may be induced by differences between fluid pressure and lithostatic pressure e.g. during uplift (isothermal decompression). The α/β-transition causes an anisotropic contraction of 0.86 vol.% vertical to the c-axis and 1.3 vol.% parallel to the c-axis and induces stress within individual grains (e.g. Blankenburg et al. 1994) and subsequently may also trigger mass decrepitation. Dissolution-precipitation initiated by shearing leads to the healing of the micro cracks.
In the second stage the defect-poor quartz grows at the cost of the host quartz and releases or replaces defect centres. The formation of defect-poor quartz at the cost of the defect-rich host quartz is explained by the displacing of atoms along the phase boundary of the quartz with higher defect density so that the atoms fit to the lattice of the quartz with low defect density (e.g. Passchier and Trouw, 1998; Stünitz, 1998). This results in local displacement of the phase boundary (between the new quartz and the host quartz) and the growth of the more pure crystal at the cost of more disordered neighbour. The process reduces the internal free energy of the crystals involved and causes the release and replacement of defect centres, which are enriched in the grow front.
In conclusion, the formation of secondary quartz around fluid inclusions is explained by a reconstituting process of the host crystal and represents a structural transformation, whereby the crystallographic orientation is preserved.
Fig. 6.8 Model of formation of halos of secondary quartz around fluid inclusions. a – first stage: mass decrepitation of fluid inclusions (micro-crack formation). b – second stage: defect-poor quartz grows at the cost of the host quartz and releases or replaces defect centres.
Secondary CL structures with induced defect centres
Secondary CL structures with induced defect centres are halos around radioactive inclusions, diffusion rims along grain boundaries and micro-cracks, and non-luminescent spots in granitic quartz. Natural a–radiation and trace element diffusion cause the modification and formation of defect centres in quartz.
Some samples show round halos around radioactive inclusions with pinkish/yellowish white CL (Fig. 6.6b). Natural a-radiation of radioactive micro-inclusions (zircon, monazite) results in damage of the crystal structure (metamictisation). The radius of halos is typically ~40 µm.
Each halo is subdivided into a brighter inner zone (~25 µm) and an outer zone (~15 µm). The radius of ~40 µm corresponds to the interaction radius of the a-particles in quartz (Owen, 1988). Similar bright CL was also observed in hydrothermal quartz (sample 1) along opeb cracks, indicating that quartz lattice damage was produced by a-radiation from radioactive components of the migrating fluid.
In magmatic quartz diffusion rims at grain boundaries (grain boundary alteration) frequently occur at the grain contacts between quartz and plagioclase or biotite (see § 10.7). The diffusion rims show the enrichment of Fe, Al, and K. Fe may diffuse up to 400 µm into the
defect centres
a b
quartz. The Fe enrichment at the grain boundaries not always results in the change of the CL colour (Fig. 5.11c). This observation may be explained by the fact that Fe occurs as divalent and trivalent ions. The diffusion rims in magmatic quartz are explained by solid state diffusion of trace elements at high (sub-solidus) temperatures.
Particularly in granitic quartz radiation-induced non-luminescent spots were observed. Weak luminescent spots up to 5 µm in size developed during electron radiation in granitic quartz (sample 7, 8, 12, and 14; Fig. 6.6a). The radiation-induced spots are interpreted as aggregations of aqua complexes with a gel-like disordered structure, i.e. regions with a high local concentration of H2O and substitutional Al with one or more OH groups attached in a locally disordered network. Stenina et al. (1984) described similar spots developed during electron radiation and identified these structures as amorphous (non-crystalline) micro-areas using TEM imaging. The electron radiation results in the release of molecular water from the aqua complexes (see § 2.2). H+ ions migrate along weak donor-acceptor and hydrogen bonds of the aqua complex defects. As a consequence oxygen may migrate due to the radiation-triggered redistribution of Si-O and Mm+-O bonds (Stenina et al., 1984; Gritsenko and Lisitsyn, 1985). At the defective places the following reaction occurs: O2- + 2H+ ? H2O.
Here water bubbles may develop. Heggie (1992) showed that microcracks by producing radicals facilitate the migration of water. In this case “water” moves through SiO2 matrix in the dissociated form. Heat-treatment experiments (500°C for several hours) showed that the aqua complexes homogenised into the lattice and structural water converts to non-bound form and creates micro bubbles (e.g. Griggs, 1967; Brunner et al., 1961; Bambauer et al., 1969;
McLaren et al., 1983; Stenina et al., 1984). These bubbles of heat-treated quartz arise as a result of the thermal-induced break down of 2[SiO3]---O-H-H bonds and the following H+ migration along the weak bonds of the lattice. The gel like defect structures were incorporated during crystallisation. The frequent occurrence of these spots in anhedral granitic quartz (matrix quartz) reflects “wet” conditions during crystallisation whereas rhyolitic and granitic quartz phenocrysts does not show spots indicating “dry” crystallisation conditions.
In summary, trace elements in quartz are largely redistributed during retrograde processes.
Non-luminescent spots and secondary quartz around fluid inclusions are frequently in granitic matrix quartz and less common or lacking in rhyolitic phenocrysts. The frequent occurrence of spots caused by gel-like defect structures in anhedral granitic matrix quartz reflects “wet”
conditions during crystallisation, whereas rhyolitic and granitic quartz phenocrysts does not show spots indicating “dry” crystallisation conditions. Rhyolitic phenocrysts are mostly fluid
inclusion free, whereas granitic phenocrysts frequently show halos of secondary quartz around fluid inclusions as a result of a overprinting by late-magmatic voliatiles.