Impurity uptake during quartz crystallisation in silicate melts

The rhyolites (samples 4, 5, 10, 11, 15) as well as a number of the sampled granites (samples 6, 7, 8, 14) contain euhedral quartz phenocrysts showing a CL-contrasted complex growth pattern (Fig. 5.11, 6.1, 6.2a-f, 7.6; Plate 1). The euhedral phenocrysts of granites are recognisable only by using CL because they are overgrown and embedded in a homogeneous anhedral quartz phase. The existence of euhedral quartz phenocrysts in granites showing CL-contrasted growth textures is currently not known to be common and was described in only a few cases (Frentzel-Beyme, 1989; Seltmann, 1994; D’Lemos et al., 1997; Müller and Behr, 1997). The questions arises if this granitic phenocrysts are comparable phenocrysts occurring in rhyolites and if they represent a similar crystallisation environment.

A main result of the trace element analysis is that the stable blue CL of the phenocrysts correlates with high Ti concentrations and that the variation of Ti is mainly responsible for the contrasting of the magmatic zoning of the quartz phenocrysts (see chapter 7 and 9). Caused by its high field strength (F = 1.04) Ti⁴⁺ can substitute Si only at high temperatures. High Ti concentrations in macroscopically rutile-free quartz generally indicate formation temperatures

=500°C (Blankenburg et al., 1994). High Ti concentrations are also typical for quartz in granulites (Kerkhof and Müller, 1999). To understand the variation of the Ti concentration in quartz phenocrysts the growth textures were classified according their structure. The classification of growth zoning is necessary to distinguish between zoning caused by self-organised growth and zoning caused by physico-chemical changes of external factors such as temperature, pressure and magma composition (e.g. Bottinga et al., 1966; Allègre et al., 1981;

Shore and Fowler, 1996). The zoning caused by external factors is of great interest for the reconstruction of the crystallisation history of felsic melts (magma storage, ascent, mixing, emplacement, and cooling rate) which will applied in the in the chapters 7-10.

Plate 1 CL images of magmatic quartz. a – Quartz of the Flossenbuerg Granite (sample 12) showing a weak red-brown CL and a dense network of healed cracks. b – Edge of a zoned quartz phenocryst with blue CL overgrown by red-brown luminescent matrix quartz (Schellerhau Granite, sample 6). c – Quartz phenocryst of the Schellerhau Granite with complex growth pattern overgrown by red-brown luminescent anhedral matrix quartz. d - Zoned quartz phenocryst of the Teplice Rhyolite (sample 5). Resorption of the phenocryst surface causes the truncation of pre-existing growth zones. Note the small quartz fragments around the phenocryst which indicate a mechanical abrasion during transport in the melt. e - Zoned quartz phenocryst of the Teplice Rhyolite.

f – Zoned quartz phenocryst of the Teplice Rhyolite with red-brown luminescent core. g – Edge of a zoned quartz phenocryst of the Weinheim Rhyolite (sample 15) with a melt inclusion. The zoning fits the shape of the melt inclusion (black with red rim). h - Zoned quartz phenocryst twin of the Weinheim Rhyolite which exhibits resorpted red-brown luminescent cores, melt inclusions (black), and healed cracks (pink lines).

Fig. 6.1 CL images of rhyolitic quartz phenocrysts recorded with a JEOL CLD40 R712 detector. a – Phenocryst from the Weinheim Rhyolite (sample 15). b – Phenocryst from the Beucha Rhyolite (sample 11). Note the anhedral, unzoned, bright quartz phase which overgrows the resorpted core with complex zoning pattern.

Fig. 6.2 CL and BSE images of magmatic quartz. The CL images were recorded with the S20 Extended detector. a – Phenocryst of the Wachtelberg Rhyolite showing a red-brown, Ti-depleted core. b – Phenocryst of the Schoenfeld Rhyolite with bright blue CL. c – BSE image of quartz in the Eibenstock Granite. d – CL image of the same area as (c) showing an euhedral zoned core overgrown by an anhedral, red luminescent quartz phase without zoning.The CL intensity of secondary quartz have been turned from low CL intensity to high CL intensity after 5 min electron radiation (bright patchy areas within the crystal). e - BSE image of quartz in the Ramberg Granite. f - CL image of the same area as (e) showing a very weakly contrasted phenocryst (bright) with faint growth zoning. The granitic quartz in figures d and f exhibits a number of secondary CL structures.

The development of growth zoning during magmatic crystallisation is described by a number of models (e.g. Sibley et al., 1976; Anderson, 1984; Fowler, 1990) that have been derived from the growth zoning of plagioclase. We apply these models to the crystallisation of magmatic quartz having a very similar growth zoning as plagioclase. Below an interpretation of growth textures in quartz phenocrysts is given, based on our observations and the present level of knowledge, to explain the variations of impurity uptake in magmatic quartz. The compilation of growth textures is illustrated in Fig. 7.1 (chapter 7).

During crystallisation of a mineral four processes are competing and the overall growth rate-controlling process is the slowest one: 1) the reactions occurring at the crystal-melt interface, 2) the bulk diffusion of components in the melt close to the interface, 3) the production and dissipation of the latent crystallisation heat at the interface, and 4) the relative flow of the melt with respect to the interface. The latter two are not critical since heat diffusivities are one to several orders of magnitude higher than mass diffusivities (Dowty, 1980) and the crystal settling effects in a magma chamber are small for viscous silicate melts. Therefore, processes 1) and 2) control are the main parameters controlling the crystal growth rate. A crystal can grow only if the thermodynamic variables for the formation of that phase exceed the equilibrium conditions. This overstepping (undercooling, overheating, supersaturation) provides energy by which nuclei are formed and crystal growth is sustained. The nuclei provide sinks to which the crystallising components diffuse. The distance over which elements are transported by diffusion depends on the diffusion rate and time.

The microscopic topography of individual growth zones is a relic crystal-melt interface and is indicative for disturbances of growth and diffusion rates during crystallisation. These parameters depend on the melt composition, crystal transport in the melt (e.g., convection), the ascent velocity of the melt, and pressure- and temperature variations. These criteria regulate the type and quantity of trace elements, few of which are CL-activators. Thickness and frequency of the zones are directly related to the physical and chemical melt properties.

The diffusion rate in the melt controls the compositional variation and width of the zoning.

The higher the diffusion rates, the less are the compositional differences in trace element content in the quartz. Concentration gradients develop in the melt when the growth rate exceeds the diffusion rate, and in the solid when the reaction rate of crystals with the liquid is lower than the growth rate (Sibley et al., 1976). The development of growth zoning during magmatic crystallisation was described by a number of models (e.g. Sibley et al., 1976;

Anderson, 1984) that have been derived from the zonal structure of plagioclase. We apply

these models to the crystallisation of magmatic quartz, which shows very similar zonal growth as plagioclase.

Major discontinuities in the zoning (50 - 1000 µm width) being non-periodic and showing a significant change of the luminescence colours result in physico-chemical changes of external factors (“extrinsic” according to Shore and Fowler, 1996) such as temperature, pressure and magma composition (Bottinga et al., 1966; Allègre et al., 1981). Bottinga et al. (1966) defined the non-periodic zones as compositional zoning, and Allègre et al. (1981) called them stepped zoning. Depending on the type of the physico-chemical change (degassing, magma mixing or ascent) the trace element concentrations, mostly represented by variations of Ti and sometimes of Al, show an abrupt change (see chapter 7).

The fine oscillatory zoning (2 - 20 µm width) within the step zones can be explained by a self-organised (“intrinsic” according to Shore and Fowler, 1996) diffusion-controlled mechanism on the crystal-melt boundary layer and plays a role exists in a number of models proposed by several authors (Sibley et al., 1976; Haase et al., 1980; Allègre et al., 1981; Loomis, 1982;

Simakin, 1984; Pearce, 1993). Oscillatory growth zones form very slowly, at low degrees of undercooling and oversaturation under near-equilibrium conditions. This is possible only when the crystallising system on the solid-liquid interface is not disturbed, i.e. thus the melt should not convect (Allègre et al., 1981). The self-organisation in the crystal-melt reaction zone and boundary layer can be explained by the following model (e.g. Allègre et al., 1981;

Fig. 6.3): Saturation of silica in the reaction zone increases the quartz growth rate. The increasing growth rate results in the decrease of silica concentration if the growth rate exceeds the diffusion rate of silica. Simultaneously, quartz-foreign elements are accumulated in the reaction zone and boundary layer. The high growth rate favours the incorporation of

Fig. 6.3 Model system showing schematic concentrations of relevant species versus distance from the surface of quartz crystal in melt. The very low Na⁺ and K⁺ gradient is due to its high diffusion coefficient (after Hess

energy. The growth rate will slow down when quartz growth is so fast that silica becomes depleted in the reaction zone and boundary layer. Consequently, the diffusion rate becomes the dominant crystal growth controlling process. The growth rate starts to rise again as soon as the silica in the reaction zone has been recovered. Investigations on natural plagioclase by Greenwood and McTaggart (1957) and Wiebe (1968) confirm that oscillation zones of crystals grown from the same melt and formed by self-organising character of processes cannot be correlated. Physical or chemical changes in the bulk magma are not required to develop oscillation zones. The self-organised growth causes only slight variations of Ti upatke.

Wavy surfaces are sometimes observed between the straight-bordered growth zones. They are characterised by convex inlets which are opposed to the growth direction. In contrast to resorption surfaces which are resulted in sharp truncation of the regular zoning and rounded crystal corners (e.g. Shore and Fowler, 1996) the inlets of the wavy surface are much smaller and extend maximal 20 µm in the growth zones and do not cut older zones or round-off corners. The subsequent growth zones are rectilinear bordered again and parallel to the euhedral crystal habit. This feature is in contrast to the wavy zoning which is described for plagioclase by Pearce and Kolisnik (1990), where the subsequent growth zones keep the wavy structure. The wavy zones of quartz are interpreted as a small scale diffusion front caused by rapid variation of the melt temperature and/or composition leading to changes of CL properties developed during crystal growth.

Discussions about the distinction of resorption surfaces and growth impediments in quartz phenocrysts have been controversial (Kozlowski, 1981; Harris and Anderson, 1984).

Laemmlein (1930) first recognised and described lobate depressions at the quartz crystal surface as growth impediments. This result is in accordance with our CL observations which show clearly that the zoning around lobate depressions, mineral-, and melt inclusions adapts to the shape of the impediments. In contrast, resorption surfaces cut pre-existing growth zones. Growth impediments are caused by immiscible liquids, vapor bubbles, molten sulfide or fluid-rich melt droplets which stick on the crystal surface, hinder the crystal growth and result in lobate depressions and entrapments (Kozlowski, 1981; Donaldson and Henderson, 1988; Lowenstern, 1995). The resorption (melting) of quartz surfaces is due to SiO2 -undersaturation of the melt that may be caused by increase in temperature, isothermal depressurisation or magma mixing. Crystals may undergo rounding due to chemical interaction (melting) and mechanical abrasion during transport in the melt. The occurrence of resorption which results in strong rounding of the quartz crystals is in accordance with the

rapid ascent of granitic melts by dyke formation as found by Holtz and Johannes (1994) and Johannes and Holtz (1996).

The presence of skeletal (dendritic) growth indicates supercooling and the consequent decrease of the diffusion/growth rate. Here, the compositional gradient on the crystal-melt interface develops, but the planar crystal-melt interface becomes unstable and changes to cellular and dendritic growth. This type of crystal growth results in skeletal (dendritic) crystal morphologies (Kirkpatrick, 1981; Fowler, 1990).

The growth pattern in rhyolitic and granitic quartz phenocrysts are similar indicating a similar crystallisation environment. Flick (1984; 1987) describes rhombohedral a-quartz phenocrysts from the Weinheim Rhyolite (sample 15). Assuming a typical solidus of a rhyolitic melt of 900-950°C, a crystallisation pressure of at least 13 kbar is necessary for a-quartz crystallisation (Flick 1987). This pressure corresponds with a formation depth of phenocrysts in this rhyolite of about 40 km assuming a geothermal gradient of 20/25°C/km. Thomas (1992) calculated the depth of quartz phenocryst crystallisation of granites of the Erzgebirge (e.g. Eibenstock Granite) of up to 21 km provided by microthermometric studies of silicate melts. Our observations and the two calculations of the crystallisation depth show that euhedral quartz phenocrysts in rhyolites as well as granites exhibiting blue CL-contrasted growth zoning represent a low to mid-crustal crystallisation environment.

Like shown above except for the oscillatory zoning all growth textures are caused by physico-chemical changes of external factors such as temperature, pressure and magma composition which result in the abruptly change of Ti concentration. In § 9.8 is shown that the Ti content in quartz phenocrysts increases with increasing growth rate. Quartz phenocrysts frequently show a Ti depletion in the red/red-brown luminescent crystal core (sample 5, 6, 10, 15) indicating a slow growth rate during the early crystallisation stage (Fig. 6.2a).