Cathodoluminescence imaging and titanium thermometry in metamorphic quartz

Transcription

1 J. metamorphic Geol., 2009, 27, doi: /j x Cathodoluminescence imaging and titanium thermometry in metamorphic quartz. S. SPEAR AND D. A. WARK* Department of Earth and Environmental Science, Rensselaer Polytechnic Institute, Troy, NY USA ABSTRACT Cathodoluminescence (CL) of quartz from metamorphic rocks representing a range of conditions from the garnet grade to the migmatite grade reveals a variety of textures, that is, a function of metamorphic grade and deformation history. Ti concentrations, determined by electron microprobe and ion microprobe, generally correlate with CL intensity (blue wavelengths), and application of the Ti-in-quartz thermometer (TitaniQ) reflects the temperature of quartz growth or recrystallization, and, in some settings, modification by diffusion. Quartz from garnet grade samples is not visibly zoned, records temperatures of C, and is interpreted to have recrystallized during fabric formation. Quartz grains from staurolite grade samples are zoned in CL with markedly darker cores and brighter rims, some of which are interpreted to have been produced by the dominant staurolite-producing reaction, whereas others are interpreted as having formed by diffusion of Ti into quartz rims. Quartz from the matrix of kyanite and sillimanite grade samples are generally unzoned, although locally displays slightly brighter rims (higher Ti); quartz inclusions within garnet and staurolite have distinctly brighter rims, which are interpreted as having been produced by diffusive exchange with the host mineral. Quartz from migmatite grade samples displays highly variable CL intensity, which is dependent on the location of the grain. Matrix grains in melanosomes are largely unzoned or rarely zoned with darker cores. Leucosome quartz is strongly zoned with bright cores and dark rims and is interpreted as having formed during crystallization of the melt. Locally within the leucosome is observed oscillatory-zoned quartz, which is interpreted as a subsolidus recrystallization to achieve strain relaxation. Quartz inclusions within garnet or plagioclase crystals often show bright domains separated by zones of dark CL. These enigmatic textures possibly reflect local melting fluxed by fluid inclusions. Temperatures calculated from the Ti in quartz thermometer are a function of the metamorphic grade of the sample, the textural setting of the quartz, the reaction history and the deformation history of the rock. The TitaniQ temperatures can be used to constrain the conditions at which various metamorphic processes have occurred. Key words: cathodoluminescence; quartz paragenesis; Ti thermometry; TitaniQ. INTRODUCTION Quartz is a ubiquitous mineral in metamorphosed pelites yet it does not display major element zoning. So, unlike garnet, plagioclase and other metamorphic minerals that do display major element zoning, it has previously been difficult to infer the growth history of individual quartz crystals in typical metamorphic schists. However, quartz is known to display appreciable cathodoluminescence (CL), and CL in quartz has been investigated by a number of workers in attempts to constrain its paragenesis (e.g. Sprunt et al., 1978; Seyedolai et al., 1997; Penniston-Dorland, 2001; Mu l- ler et al., 2002; Rusk & Reed, 2002; Rusk et al., 2006). Although numerous explanations for the intensity and colour of CL in quartz have been put forward (e.g. Smith & Stenstrom, 1965; Ramseyer et al., 1988; Go tze *Present address: General Electric Global Research Center, 1 Research Circle, Niskayuna, NY 12309, USA et al., 2001; Landtwing & Pettke, 2005; Rusk et al., 2006), recently Wark & Spear (2005) have demonstrated that the intensity of CL in quartz in the wavelength region of 415 nm is proportional to the concentration of Ti (see also Rusk et al., 2006). urthermore, the recently calibrated Ti in quartz (TitaniQ) thermometer of Wark & Watson (2006) demonstrates the correlation of Ti concentration in quartz with the temperature of crystallization in samples that contain rutile. Therefore, it is possible using the blue (e.g. 415 nm) CL emissions in quartz to obtain an approximation of the temperature of quartz rutile equilibration in a schist, although, as discussed below, there are other factors besides temperature that must be considered. This paper represents a first attempt to examine the systematics of CL zoning in quartz in a sequence of schists through a succession of metamorphic grades. The goal is to first report any systematics that are observed with metamorphic grade and to then correlate the zoning with temperature using the TitaniQ thermometer. 187

2 188. S. SPEAR AND D. A. WARK An important part of this study is the comparison of quartz growth histories with the predicted sequence of quartz crystallization histories as inferred from thermodynamic calculations. Equally important is the textural setting in which quartz is seen to recrystallize and the implications of this for understanding the process of metamorphic recrystallization. This latter point may not appear to be self-evident but consider that it is simple to infer (in most cases) that as new garnet (or any other porphyroblast) grows, material is added to the rims of existing crystals. The exact proportionation of new material onto existing crystals is not understood, but can be inferred by the examination of, for example, Mn zoning in garnet. Quartz, however, does not form porphyroblasts, so the means by which quartz recrystallizes are not obvious, and cannot be inferred from any simple measurement of zoning. It is thus a goal of this paper to demonstrate the utility of CL coupled with TitaniQ thermometry for constraining quartz paragenesis and providing insights into the mechanisms of metamorphic recrystallization. METHODS Cathodoluminescence imaging was performed using both the JEOL 733 and the Cameca SX-100 electron microprobes at Rensselaer Polytechnic Institute. The JEOL 733 is equipped with an Oxford Instruments (now Gatan) ÔMini-CLÕ detector mounted in the transmitted light finger port. This configuration requires the sample to be located at the 31-mm working distance setting so that the sample is below the detector. The Cameca SX-100 is equipped with a Gatan Mono-CL detector, which has a parabolic mirror with a hole to allow the electron beam to pass through, and this assembly is attached to the end of a light finger. The CL detector is equipped with external red, green and blue filters that can be used to generate colour RGB images by combining multiple images, and a spectrometer to examine the emission spectra (see Wark et al., 2007, for a more detailed description of the Mono-CL system). A major difference between the CL configuration on the JEOL 733 and that on the Cameca SX-100 is the field of view of the image. Because the electron beam passes through a 1-mm diameter hole in the parabolic mirror in the Mono-CL configuration on the SX-100, the maximum raster size is limited to approximately several hundred microns. No such restriction applies to the configuration on the 733 and images can be collected at the minimum magnification with a raster size of several millimetres. An accelerating voltage of 15 kv and beam currents (measured on a araday cup) of 5 50 na were used. Images were collected by rastering the beam across the desired area and collecting light for an appropriate dwell time. Most of the samples examined contain plagioclase and other phases (e.g. sillimanite) that luminescence far more than quartz. Inasmuch as there is a finite time for CL emission to decay away after the beam leaves a spot, it was necessary to pick a dwell time that would be sufficiently long so that artefacts such as streaking would not appear. Experiments with dwell times revealed that 0.1 ms was too short but that 0.5 ms would produce an image with only minor smearing from adjacent brightly luminescent phases. Most images were collected with dwell times of ms and a few were collected with dwell times as long as 2 ms pixel )1. Titanium analyses were performed on both the JEOL 733 and the Cameca SX-100 electron probes, and on the IMS-3f ion microprobe at Woods Hole Oceanographic Institute. On the JEOL-733, two PET crystals were used to collect counts simultaneously at 15 kv with a beam current of 50 na and counting times of s, which yielded a precision of approximately ±12 ppm. On the Cameca SX-100, four PET crystals, each of which has nearly twice the count rate as on the 733, were used simultaneously under similar operating conditions, which increased the precision to 6 ppm. On both instruments, silicon was also analysed to ensure that each spot was, in fact, a quartz grain (along line traverses, occasionally another phase or a crack would be encountered). Reported precisions are based on cumulative counting statistics. Analyses on the IMS-3f were carried out using a primary oxygen beam, a sample current of 5 10 na and a spot size of roughly 10 lm. Counts were measured on the 48 Ti and 30 Si peaks, and the 48 Ti 30 Si ratio was calibrated against a reference material that had been analysed by electron microprobe using the methods described above, thus ensuring an internally consistent data set. Precision of analyses using the IMS-3f based on counting statistics is estimated to be around ±0.2 ppm. Thermodynamic calculations Thermodynamic calculations were carried out using Program Gibbs (Spear et al., 1991; rpi.edu/metapetaren/software/gibbsweb/gibbs.html) and the thermodynamic database of Spear, Pattison and Cheney (. S. Spear, D. R. M. Pattison, J. T. Cheney, unpublished data, available at the above URL or on request from the first author; Pattison et al., 2002) with enthalpy of e and Mn chlorite adjusted to provide chlorite garnet equilibria consistent with natural occurrences. RESULTS All samples examined in this study are from the first authorõs collection and were chosen to be typical of metamorphosed pelites found in the New England metamorphic belt, with an additional sample of orthoamphibole rock. The study includes samples from the garnet, staurolite, kyanite, sillimanite and migmatite metamorphic zones, and several samples

3 CATHODOLUMINESCENCE IMAGING AND TITANIUM THERMOMETRY IN METAMORPHIC QUARTZ 189 were examined from most grades (Table 1). All samples have been the subjects of previous studies, thus providing context for this new work (see reference list in Table 1). Sample descriptions and mineralogy are given in Table 1 along with latitude and longitude values that locate samples to within 1 m; maps showing the locations of the samples can be found in the referenced publications. All samples were examined using CL-imaging, backscattered electron (BSE) imaging, and with the optical microscope. The nature of the luminescence observed in quartz varies with metamorphic grade, as will be described in detail below. irst, however, some general features about CL intensity in quartz will be described. General observations and CL-imaging anomalies It was noticed during the course of this study that the electron beam causes damage in quartz that affects luminosity. This can occur as either spot damage or raster damage and depends on the intensity and duration of beam exposure. The damage occurs most strongly in the red region of the spectrum, and some examples can be seen in ig. 1. igure 1(a) is a CL image of a garnet crystal that had been mapped for trace elements at a sample current of 2000 na and shows a rectangular area of beam damage. igure 1(b) shows a region of quartz adjacent to a garnet that had also been mapped at high current, but in this case the mapping was performed using a focused beam and the damage occurs on a grid. Also visible in ig. 1(b) is the beam damage from five spots where Ti analyses were made during the course of this study. Although these are extreme examples, minor beam damage has also been observed at currents of na, where most of the work reported here was performed. It is important to recognize the beam damage because of the potential for spurious information to be retrieved from damaged areas of quartz. ortunately, the beam damage is not a major concern in this study of quartz zoning for several reasons. irst, the CL intensity is only a qualitative measure of relative Ti concentrations, so if a sample is uniformly damaged by beam exposure, the relative CL intensities from Ti variability still are apparent. Second, the beam damage occurs most prominently in the red part of the spectrum, as can be seen comparing ig. 1(d,e). Using a blue filter effectively eliminates all evidence of beam damage so that only the CL caused by Ti is visible. Third, the response of the photomultiplier tube to various CL wavelengths must be considered. It was discovered by the comparison of images taken using the Mini-CL on the JEOL-733 and the Mono-CL on the Cameca SX-100 that the Mono- CL has a much more even response to the visible light spectrum whereas the Mini-CL is far more sensitive to blue light. or example, ig. 1(b,c) was taken on similar areas of the same sample using the two systems. The beam damage (which occurs in the red part of the spectrum) shows up clearly on the white light image using the Mono-CL (ig. 1b) but is not visible at all on the image taken with the Mini-CL detector (ig. 1c). It was also discovered during the course of this investigation that there appeared to be anomalous luminescence in samples examined using the Mono-CL system. This can be seen by comparing ig. 1(b,c) or (d,e). Garnet should not luminesce at all, yet in ig. 1(b) the garnet is quite bright. No such artefact is observed using the Mini-CL system on the JEOL-733 (ig. 1c). Comparison of ig. 1(d,e) reveals that this anomalous luminescence occurs predominantly in the blue part of the spectrum, and appears to be a function of the average atomic weight of the sample (e.g. proportional to BSE intensity). It was eventually discovered that a fine layer of vacuum oil had coated the mirror used in the Mono-CL system, and this oil luminesces strongly in the blue part of the spectrum. It is suggested that back-scattered electrons from the sample, the amount of which are proportional to the mineral density, are activating this oil causing the anomalous blue luminosity. ortunately, it is still easy to see the variations in CL intensity in quartz in the blue part of the spectrum (e.g. ig. 1e). Garnet grade samples igure 2 shows the variability in CL zoning in quartz observed in the sample suite from garnet grade samples. In general, there is little to no observable CL zoning in samples of this grade, and there is no observable difference in CL intensity among quartz grains of different textural settings (e.g. inclusions within garnet, in the matrix foliation, in pressure shadows). In some samples (e.g. 96 2, ig. 2c,d), quartz of different luminosity is observed but each separate grain is homogeneous and it is believed that the difference is due to different crystal orientations or dislocation densities rather than different compositions. or example, in ig. 2(c), quartz with two different levels of brightness can be seen in the pressure shadow of the adjacent garnet (see arrows). An image at higher magnification and contrast, however, (ig. 2d) reveals the brighter quartz in the lower magnification picture appears to have less of a mottled texture than nearby grains, suggesting a different state of annealing between the two types of crystals. urthermore, quartz in microlithons of crenulated samples shows no obvious distinction from quartz inclusions in garnet or quartz in mica domains (e.g. sample TM-549; ig. 2e). The lack of obvious CL zoning appears to persist through the upper garnet grade. Titanium concentrations in quartz from samples B- 53 and TM-549 are shown in ig. 3. Measured Ti concentrations are the same within the resolution of the electron microprobe (6 ± 6 ppm) (ig. 3b), with the exception of two analyses of 13 and 17 ppm on the right side of the grain. Although there are too few data

4 190. S. SPEAR AND D. A. WARK Table 1. Sample locations and descriptions. Sample number Location Latitude Longitude Rock type ormation name Grade Mineralogy e Ti oxide texture Reference for sample B-53 Eastern VT ) Phyllite Littleton Garnet Garnet + chlorite + biotite + muscovite + quartz + plagioclase + ilmenite + rutile 93 19A Eastern VT ) Phyllite Littleton Garnet Garnet + chlorite + biotite + muscovite + quartz + plagioclase + ilmenite + rutile 96 2 Harpswell Neck, ME ) Phyllite Jewel Garnet Garnet + chlorite + biotite + muscovite + quartz + plagioclase + ilmenite + rutile + monazite + apatite TM-549 Eastern VT ) Phyllite Gile Mountain Garnet Garnet + chlorite + biotite + muscovite + quartz + plagioclase + ilmenite + rutile + apatite + monazite + chalcopyrite B-38B Western NH ) Schist Littleton Staurolite Garnet + biotite + staurolite + muscovite + quartz + plagioclase + ilmenite + rutile + apatite B-52A Eastern VT ) Schist Littleton Staurolite Garnet + biotite + staurolite + muscovite + quartz + plagioclase + ilmenite + monazite + apatite 88 44B Western NH ) Schist Littleton Staurolite Garnet + biotite + staurolite + muscovite + quartz + plagioclase + ilmenite + rutile + zircon + monazite D Eastern VT ) Schist Littleton Kyanite Garnet + biotite + staurolite + kyanite j Eastern VT ) Schist Ammonoosuc Volcanics muscovite + quartz + plagioclase + ilmenite + zircon + apatite + monazite Kyanite Anthophyllite + gedrite + chlorite + plagioclase + quartz + rutile B-78 Gilsum, NH ) Schist Rangeley Sillimanite Garnet + biotite + sillimanite + muscovite + quartz + plagioclase + ilmenite + zircon + xenotime + monazite B-14P all Mtn, NH ) Schist Rangeley Migmatite (Sil + ) B-9E all Mtn, NH ) Schist Rangeley Migmatite (Sil + ) LM-1B Gilsum, NH ) Schist Rangeley Migmatite ( + Crd) LM-1C1 Gilsum, NH ) Schist Rangeley Migmatite ( + Crd) Garnet + biotite + sillimanite + muscovite + quartz + plagioclase + ilmenite + rutile + spinel + apatite + monazite Garnet + biotite + sillimanite + muscovite + quartz + plagioclase + ilmenite + rutile zircon + apatite + xenotime + monazite Garnet + biotite + sillimanite + muscovite + quartz + plagioclase + ilmenite + rutile + monazite Garnet + biotite + sillimanite + muscovite + quartz + plagioclase + ilmenite + rutile + monazite + zircon + pyrite + pyrrhotite Minor rutile associated with ilmenite Pyle & Spear (1999), Pyle et al. (2001), Spear et al. (2002, 2008) Large ilmenite with inclusions of rutile, chlorite, apatite Pyle & Spear (1999, 2000), Pyle et al. (2001), Spear et al. (2002, 2008) Ilmenite in garnet and matrix. No rutile Daniel & Spear (1998), Spear & Daniel (2001) Ilmenite in garnet and matrix. Rutile in matrix only Menard & Spear (1994), Pyle & Spear (1999, 2000), Pyle et al. (2001), Spear et al. (2002) Ilmenite and rutile in garnet and in matrix Pyle & Spear (1999, 2000), Pyle et al. (2001), Spear et al. (1990, 2008) Ilmenite abundant in matrix and inside garnet. No rutile Pyle & Spear (1999), Pyle et al. (2001), Spear et al. (1995, 2002) Ilmenite and rutile in matrix lorence & Spear (1993), lorence et al. (1993) Ilmenite abundant inside garnet and in matrix. No rutile Spear & Rumble (1986) Abundant rutile in matrix Spear (1982) Ilmenite exsolved from biotite and appears retrograde. No rutile Ilmenite + rutile in garnet and in mica clots. No ilmenite in matrix Rutile abundant. Associated with ilmenite and in matrix. Ilmenite abundant in garnet Ilmenite in melanosomes. Minor rutile associated with ilmenite (possibly retrograde) Rutile surrounding ilmenite in matrix and as inclusions in ilmenite Spear et al. (1995), Pyle & Spear (1999, 2000), Pyle et al. (2001), Spear et al. (2002, 2008) Spear et al. (1990, 1995), Pyle & Spear (1999), Pyle et al. (2001), Spear et al. (2002, 2008), Kohn et al. (1997) Spear et al. (1990, 1995, 2002), Kohn et al. (1997) Spear et al. (1995), Pyle & Spear (1999), Pyle et al. (2001), Spear et al. (2002), Pyle et al. (2005), Spear et al. (2008) Spear et al. (1995), Pyle & Spear (1999), Pyle et al. (2001), Spear et al. (2002), Pyle et al. (2005), Spear et al. (2008)

5 CATHODOLUMINESCENCE IMAGING AND TITANIUM THERMOMETRY IN METAMORPHIC QUARTZ 191 Pl (a) (b) Qtz 1 mm 0.2 mm Ky (D,E) ig. 6c ig. 1. Cathodoluminescence images of quartz showing effects of beam damage and detector sensitivity. (a) CL image of garnet and vicinity from a staurolite grade schist (sample B-38B). Bright area is the region that had been mapped for trace elements at 2000 na beam current. Image taken on JEOL-733 using Mini-CL system, no filter. (b e) CL images of staurolite schist (sample D). (b) Bright spots are beam damage from X-ray mapping at 1000 na sample current. Arrows point to two out of five additional damage spots caused by the quantitative analysis for Ti (this study: 200 na for 20 min). Image taken on Cameca SX-100 using Mono-CL system, no filter. (c) CL image-encompassing region of image in (b) taken on JEOL-733 using Mini-CL system, no filter. Boxes show locations of images (b, d & e). Note that this image does not show evidence of beam damage, indicating that the Mini-CL is more sensitive to blue light than is the Mono-CL system. (d, e) CL images with red (d) and blue (e) filters of a quartz grain included in garnet. Note the beam damage evident in red light is not visible in blue light. Also note the apparent luminosity of garnet and ilmenite in blue light, which is an artefact caused by oil coating the parabolic mirror in the Mono- CL system. This can also be seen comparing images in (b) and (c). Abbreviations after Kretz (1983). (c) (d) Qtz Qtz 0.05 mm (e) Pl (B) Qtz Ilm 0.05 mm to be sure, it is likely that these two analyses are spurious. or example, Wark & Watson (2006) have discussed the fluorescence of Ti in nearby Ti-bearing phases by background radiation during the analysis of quartz rutile experimental run products. It is believed that these two high Ti analyses are the result of Ti fluorescence from adjacent biotite and ilmenite. Two ion probe analyses from one sample (B-53) yield 2.1 and 3.2 ppm (ig. 3a), which differ by more than the ion probe precision of around ±0.2 ppm. Although insufficient work has been performed to be conclusive, the lower Ti analysis appears to be from the centre of a grain and the higher Ti analysis is from nearer the rim, suggesting compositional zoning with growth of quartz. The TitaniQ thermometer yields temperatures of 428 and 453 C respectively, for the two ion probe analyses, which are lower than the temperature of

6 192. S. SPEAR AND D. A. WARK Qtz 3b B Pl (a) (b) 0.2 mm Qtz D (c) (d) 0.1 mm 0.05 mm Qtz (e) (f) 0.1 mm ig. 2. Cathodoluminescence images of samples of schist from the garnet grade. (a, b) Sample 93-19A. Crenulated matrix contains quartz + plagioclase + biotite + chlorite. Note in (b) some quartz grains appear to have brighter cores than rims. Box in (b) shows the location of image in ig. 3b. Circles in garnet in (a) are laser pits from LA-ICPMS analysis. (c, d) Sample Quartz in matrix, in garnet inclusions and in pressure shadows have similar luminosities. Some grains in (c) are apparently brighter in CL (see arrow), but there is no compositional difference between these and other quartz grains and it is interpreted as an effect of crystal orientation. (e, f) Sample TM-549. Well-crenulated samples shows homogeneous quartz and no apparent distinction between matrix quartz and quartz inclusions (f). Bright rectangle in the centre of (f) is the result of beam damage.

7 CATHODOLUMINESCENCE IMAGING AND TITANIUM THERMOMETRY IN METAMORPHIC QUARTZ 193 (a) (b) < C estimated for the peak of metamorphism from garnet biotite e Mg thermometry reported by Spear et al. (2002). Possible reasons for this discrepancy will be discussed later in the paper. Staurolite grade samples igure 4 contains several images of staurolite grade samples, which reveal a dramatic change from the appearance of garnet grade samples. Nearly all matrix quartz in staurolite grade samples observed in this study is zoned with darker CL cores and brighter CL rims. In addition, zoned quartz crystals were observed inside of staurolite porphyroblasts (ig. 4b) and inside of garnet (ig. 4c), although some garnet crystals contained inclusions of unzoned quartz, similar to garnet grade samples. In some locations, quartz of distinctly different textural habit was observed. or mm 0.05 mm ig. 3. Titanium concentrations (ppm) in quartz from garnet grade samples. (a) Sample B-53. Analysis by ion probe (precision 0.2 ppm). Another analysis in the same sample yielded 3.2 ± 0.2 ppm. (b) Sample 93-19A (see ig. 2b for the location of area). Analyses by electron microprobe (precision 6 ppm). With the exception of the two analyses of 13 and 17 ppm, all of these analyses must be considered statistically identical. example, quartz on the margin of staurolite (ig. 4b) is largely unzoned and has faintly darker regions along grain boundaries. A clot of quartz + biotite (ig. 5e) contains similarly unzoned quartz, although some crystals appear to display strings of brighter CL intensity of unknown origin. Titanium concentrations in quartz from staurolite grade samples measured with the electron microprobe range from nearly zero to 16 ppm (precision = ±6 ppm). Three ion probe spots shown in ig. 5(a) of 7.2 ± 0.3, 9.2 ± 0.3 and 11.4 ± 0.6 ppm, and two additional ion probe analyses of matrix quartz from the same sample of 6.6 ± 0.3 and 7.7 ± 0.3 ppm are consistent with these results. Within analytical precision, then, the darker-luminescing cores of matrix quartz grains contain Ti concentrations of 6 8 ppm, corresponding to temperatures of C, whereas the brighter-luminescing rims of quartz grains and bright quartz around garnet has concentrations ranging from 11 to 16 ppm, corresponding to temperatures of C. Unfortunately, the well-defined core to rim CL zoning observed in sample B-52A (e.g. igs 4d,e & 5d) are not reflected unambiguously in the Ti concentrations. Grain core concentrations range from 2 3 ppm to 8 10, which are difficult to distinguish statistically from the 9 16 ppm measured from near-rim locations. Additional work involving higher precision analysis is clearly warranted on this sample. The homogeneous quartz surrounding staurolite (ig. 5c) records a temperature of 495 C whereas the concentrations in the biotite quartz clot (ig. 5e) record a large range of temperatures from 580 to <490 C. Kyanite grade samples Quartz in a metapelitic schist and an orthoamphibole schist from the kyanite grade were examined (ig. 6). The metapelitic schist (sample D) contains quartz that displays two types of luminescence. Quartz in the matrix of the schist, in pressure shadows, and in quartz segregations show a range of brightness, but are generally unzoned (ig. 6a,b). Quartz inclusions within garnet are markedly zoned with dark cores and bright rims (ig. 6c). It appears that the bright rims are approximately the same brightness as the bright matrix grains. Temperatures calculated from quartz inclusions range from 450 to 505 C for the core and 530 to 555 C for the rim, similar to the range of temperatures recorded from garnet and staurolite grade samples. Matrix grains record temperatures from 470 to 570 C, the latter being close to the peak metamorphic conditions determined from garnet biotite thermometry. A second sample from the kyanite grade (sample j) is an orthoamphibole schist and contains quartz that shows core to rim brightening, core to rim darkening and relatively homogeneous grains (ig. 6d,e). Ti concentrations determined using the ion probe range

8 0.1 mm 0.1 mm 0.05 mm 0.2 mm 0.05 mm 194. S. SPEAR AND D. A. WARK 5a 5b 5c St (a) 0.2 mm (b) (c) 4e 5d St (d) 1.0 mm (e) 0.2 mm (f) ig. 4. Cathodoluminescence images of samples from the staurolite grade. (a c) Sample B-38b. (d, e) Sample B-52a. (f) Sample 88-44b. Note all matrix quartz is zoned with darker cores and brighter rims. Also note brighter quartz surrounding garnet in (a), unzoned larger quartz on margin of staurolite (b), quartz inclusions inside staurolite and garnet are zoned (b, c, f). Boxes show locations of images in other figures as indicated by labels (a) (b) (c) (d) (e) ig. 5. Titanium concentrations (ppm) in quartz from staurolite grade samples. Spots indicated by circles are electron probe analyses (precision is 6 ppm) and spots indicated by squares are ion probe analyses (precision is 0.2 ppm). (a c) Sample B- 38b. (d, e) Sample B-52a. Locations of images in (a) and (b) are from ig. 4a, image (c) is from ig. 4b, (d) is from ig. 4e and (e) is from an area to the northeast of ig. 4d. Note the lowest Ti concentrations from the quartz cores ranges from 2 to 7 ppm whereas the rim values range from 10 to 16 ppm. from 5.1 ± 0.3 to 9.2 ± 0.3 ppm, corresponding to a temperature range of C. There is a clear tendency for the rims to record a higher Ti concentration and higher temperature, although it appears that the very outer rim was not captured by an analysis. Sillimanite grade sample A sample of sillimanite schist from western New Hampshire (B-78) is illustrated in ig. 7. Quartz is patchy in CL brightness and does not display the

9 CATHODOLUMINESCENCE IMAGING AND TITANIUM THERMOMETRY IN METAMORPHIC QUARTZ (a) 1.0 mm (b) 0.2 mm (c) 0.02 mm Pl Pl (d) (e) 0.2 mm ig. 6. Cathodoluminescence images and Ti concentrations in quartz in samples from the kyanite grade. (a c) Sample d. (d, e) Sample j. Titanium concentration are in ppm. Analyses in (a c) are electron probe; analyses in (d, e) are ion probe. Box in (a) shows locations of (b) and (c) (also see ig. 1). systematic zoning observed in the staurolite grade samples. Locally (e.g. ig. 7b) quartz grain boundaries appear darker than the cores of grains. Ti concentrations, determined from electron probe analyses range from 4 8 ppm in the darker regions to ppm in the brighter regions, corresponding to temperatures of and C respectively. Migmatite grade samples Quartz from two migmatite localities in western New Hampshire were examined in this study. igures 8 & 9 show CL images and Ti concentrations respectively from sillimanite + biotite + garnet migmatites from all Mountain, New Hampshire, which experienced muscovite dehydration melting and peak conditions near 700 C and 6 kbar (Kohn et al., 1997; Spear et al., 2002). Quartz is strongly zoned in these samples and several types of zoning have been observed. Many quartz grains are zoned with brighter cores and darker rims. This is quite common in quartz grains from leucosomes (e.g. ig. 8a), and is also observed in quartz inclusions within garnet. Some quartz grains are very irregularly zoned with dark patches and anastomosing bands cutting through grains. This type of texture is most common in quartz included in garnet (e.g. igs 8b e & 9c), but is also observed in grains from leucosomes. A number of quartz grains were observed with oscillatory zoning (e.g. igs 8b,f & 9a,d), which is generally subparallel to the grain boundary in the oscillatorily zoned grain. Grains with oscillatory zoning appear to be replacing strained grains that display undulose extinction and patchy zoning. Titanium concentrations of quartz from this locality (ig. 9) are highly variable. The highest Ti concentrations were found in the brightest CL parts of quartz crystals included within garnet crystals (e.g. ig. 9b) and range from ppm, corresponding to temperatures of C. Dark parts of the same crystals have Ti concentrations ranging from ppm corresponding to temperatures of C (e.g. ig. 9b,c). The brightest parts of leucosome quartz and the oscillatory-zoned quartz contain ppm Ti (e.g. ig. 9a,d) and the darkest parts contain <6 ppm Ti, corresponding to temperatures of C and <490 C respectively. Garnet cordierite grade migmatites from sample location LM-1 (ig. 10) show some similar CL-zoning textures to the lower grade migmatites including leucosome quartz with bright cores and dark rims (ig. 10b e) and patchy zoning of grains (ig. 10f). Some leucosome grains are homogeneous (ig. 10a) and some grains from melanosomes show dark cores and brighter rims, similar to zoned crystals from the staurolite grade samples (ig. 10a, arrow). Titanium concentrations in the leucosomes range from

10 196. S. SPEAR AND D. A. WARK (a) (b) ppm in the bright regions ( C) to <6 ppm in the dark regions (<495 C). A particularly notable texture is the presence of quartz inclusions within either garnet or plagioclase porphyroblasts that display a domain structure (ig. 10g,h). The bright domains display homogeneous brightness and have Ti concentrations of ppm ( C) and the dark regions contain as little as ppm (550 C). DISCUSSION mm ig. 7. Cathodoluminescence images and Ti concentrations in quartz from a sample from the sillimanite grade, New Hampshire (sample B-78). (a) Low magnification image showing the distribution of relatively unzoned and patchily zoned quartz grains. Note the large sillimanite porphyroblast, which is pseudomorphed after andalusite, displays CL zoning in the form of a chiastolite cross. (b) Higher magnification image showing the location of analysis spots and Ti concentrations in ppm. The CL of quartz discussed above shows a dramatic variation from low to high grades and sometimes even within a single sample. In order to interpret these observations, it is useful to list the various mechanisms that are believed to be responsible for the recrystallization of quartz in metamorphic settings and the development of Ti zoning. 1 Crystallization (or recrystallization) as a result of prograde or retrograde metamorphic reactions. Most metamorphic reactions will consume or produce some quartz, resulting in a net change in the modal amount of quartz in the sample. These reactions may be either discontinuous (i.e. occurring over a narrow change in metamorphic conditions) or continuous (i.e. occurring progressively and smoothly with changing conditions). New quartz produced by a metamorphic reaction can be grown as a newly nucleated grain, or added onto the rims of existing phases. 2 Recrystallization due to deformation (e.g. fabric formation). It is well known that the formation of, for example, crenulation cleavage results in the production of quartz- and mica-rich domains, which involves the transfer of SiO 2 from the mica domains into the quartz domains (e.g. Bell & Rubenach, 1983). As before, quartz grown in the quartz domains may nucleate as new phases or overgrow existing phases. 3 Precipitation from fluids. Quartz is relatively soluble in aqueous fluids as evidenced by the abundance of quartz veins in many localities. SiO 2 may be added to or removed from a metamorphic rock by an infiltrating fluid (e.g. Ague, 1991). 4 Strain release. It is well known from studies of mylonites that deformed quartz will anneal by a variety of mechanisms (e.g. Hobbs, 1968; Marjoribanks, 1976; Hippertt & Egydio-Silva, 1996). 5 Diffusion. In addition, diffusion can modify preexisting Ti distributions, which will be manifest as CL zoning and might give the false impression that quartz has recrystallized (e.g. Cherniak et al., 2007). One or more of these mechanisms may be operative in any given sample, and it is certainly possible that different mechanisms may be operating in different parts of the same sample, either simultaneously or at different times. In the discussion below, each sample will be evaluated in the context of the metamorphic paragenesis, the quartz micro textures, the strain conditions and the Ti diffusivity. Before these can be carried out, however, it is necessary to outline the possible metamorphic reactions that each sample has encountered, whether these reactions have produced or consumed quartz, and to summarize the data for Ti diffusion in quartz. Pseudosection and P T paths igure 11 is a P T pseudosection calculated for a typical pelitic bulk composition representative of the samples discussed in this paper (SiO 2 = 54.89, Al 2 O 3 = 20.22, MgO = 3.03, eo = 9.94, MnO = 0.40, CaO = 0.67, Na 2 O = 0.66, K 2 O = 5.73, plus sufficient H 2 O to keep vapour saturated except in the melting region). The diagram has been contoured for the moles of quartz (proportional to modal amount) in order to provide a theoretical framework for quartz

11 CATHODOLUMINESCENCE IMAGING AND TITANIUM THERMOMETRY IN METAMORPHIC QUARTZ 197 (d) (f ) (9a) (a) Pl (b) 1.0 mm 2.0 mm (c) (9c) (e) (d) 0.1 mm (9d) (e) (f) 0.1 mm 0.2 mm ig. 8. Cathodoluminescence images of quartz from sillimanite + biotite + garnet grade migmatites at all Mountain, New Hampshire. (a) Quartz from leucosome. Sample B-14p. (b) Garnet interior showing distribution of quartz grains. Sample B-9e. (c) Quartz inclusions inside of garnet. Box shows the location of close up in (e). (d) Close up of quartz inclusion in garnet image (b). (e) Close up of quartz crystal shown in (c). (f) Close up of quartz crystals at margin of garnet shown in (b).

12 198. S. SPEAR AND D. A. WARK (a) (b) (c) 0.2 mm (d) 0.2 mm ig. 9. Titanium concentrations in quartz from samples of migmatites at all Mountain, New Hampshire. (a) Sample B-14p (see ig. 8a for location). (b) Sample B-14p; quartz inclusions in garnet. (c) Sample B-9e; quartz inclusion in garnet (see ig. 8b,d for location). (d) Sample B-9e; oscillatory-zoned quartz (see ig. 8b,f for location). Concentrations in ppm by electron microprobe. evolution in a pelite against which to interpret the zoning observed in the CL images. Inasmuch as the molar volume of quartz varies as a function of pressure and temperature, it is not practical to contour such a diagram directly with the volume of quartz. However, using the known molar volume of every phase at each pressure and temperature, the modelled modal amount of quartz can be calculated. The diagram is especially useful as an illustration of how the amount of quartz in a typical sample changes as a function of progressive metamorphism. To facilitate the discussion further, the relative changes in the amount of quartz along several specific P T paths have been illustrated in ig. 12. Quartz is present in these pelites throughout their metamorphic evolution with the possible exception of the highest grade migmatites where quartz may be completely consumed (ig. 11). In the lowest grade samples in which garnet has not yet nucleated, the typical pelitic assemblage is chlorite + biotite + muscovite + quartz (PT location A; all assemblages may also contain plagioclase and ilmenite or rutile). Isopleths of quartz (MQtz) have positive slopes in this assemblage such that the amount of quartz will increase with increasing temperature. Once garnet nucleates (e.g. at B or G), the assemblage becomes garnet + biotite + chlorite + muscovite + quartz (e.g. C or H). Initially, there may be little change in the amount of quartz as the amount of garnet in the assemblage is, at first, vanishingly small (e.g. along path B C or G H). As the amount of garnet increases,

13 CATHODOLUMINESCENCE IMAGING AND TITANIUM THERMOMETRY IN METAMORPHIC QUARTZ 199 Sil Pl 10D 10C (a) 1.0 mm (b) 1.0 mm (c) (d) , mm Pl G (e) 1.0 mm (f) 0.2 mm Pl (g) 0.1 mm (h) 42, mm ig. 10. Cathodoluminescence images and Ti concentrations in quartz from samples of garnet cordierite grade migmatites from west central New Hampshire (sample locality LM-1). (a) Garnet surrounded by quartz in small leucosome. Arrow points to a quartz grain in the melanosome that has darker core and lighter rim. (b) Low-magnification image of leucosome. Boxes show locations of images in (c) and (d). (c, d) Close up of areas shown in (b) with Ti concentrations in ppm. (e) Garnet crystal with quartz inclusions. Boxes show the location of close up images in (f) on the garnet margin and (g) an inclusion of quartz. (h) Inclusion of optically continuous quartz inside of a plagioclase phenocryst from sample LM-1B-1.

14 200. S. SPEAR AND D. A. WARK P (kbar) Qtz +Pl +Ilm 0.38 B Chl Ms C 0.37 DE St Ms L K M AlSi Ms 0.38 AlSi Kfs L 4 H P Q S R A 0.39 Chl Ms G IJNO AlSi Ms AlSi Kfs T ( C) ig. 11. P T diagram showing mineralassemblage stability fields for a representative pelitic bulk composition (i.e. a pseudosection). P T fields are contoured for moles of quartz in the rock (MQtz). Lines with circled letters are P T paths along which moles of quartz have been calculated and are plotted in ig Moles of quartz (M Qtz ) A Path 1 B G C H I J N O D E Path 2 Path 2 alt K L T ( C) M P Q melting ----> R R S freezing ----> Mode of quartz (vol %) ig. 12. Diagram showing the calculated amounts of quartz (MQtz) as a function of temperature along P T paths shown in ig. 11. Lettered circles indicate specific P T points in ig. 11. Volume % axis is approximate because of the dependency of molar volume on P and T. however, the amount of quartz begins to decrease as garnet grows by the dominant net transfer reaction (e.g. path C D): chlorite þ quartz ¼ garnet þ H 2 Omuscovitebiotite This reaction proceeds up grade until either staurolite or aluminosilicate (kyanite or andalusite) join the assemblage, shortly after which chlorite reacts out. In pelites that develop staurolite, the dominant isograd reaction is chlorite þ garnet þ muscovite ¼ staurolite þ biotite þ quartz þ H 2 O; which occurs over a small region of P T space (e.g. path D E or I J). Over this interval where staurolite

15 CATHODOLUMINESCENCE IMAGING AND TITANIUM THERMOMETRY IN METAMORPHIC QUARTZ 201 grows at the expense of chlorite and garnet, there is a significant, nearly discontinuous increase in the modal amount of quartz in the assemblage (ig. 12, paths D EandI J). Aluminosilicate joins the assemblage at higher grade and staurolite reacts out by a reaction such as staurolite þ muscovite þ quartz ¼ garnet þ biotite þ Al-silicate þ H 2 O Across this reaction, a small amount of quartz should be consumed (e.g. K L or N O). Above this reaction, the assemblage is garnet + biotite + AlSi + muscovite + quartz (e.g. M or path O P). Isopleths of quartz have relatively flat slopes in this field and minor amounts of quartz should be produced along isobaric heating and decompression paths (e.g. paths L M or O P). The next major reactions to be crossed are those along which muscovite breaks down. Below 4 kbar, muscovite dehydrates via the reaction muscovite þ quartz ¼ AlSi þ K-feldspar þ H 2 O Whereas above this pressure, muscovite melts via a reaction such as muscovite þ quartz ¼ AlSi þ K-feldspar þ liquid Both reactions consume a considerable amount of quartz, as can be seen by the deflection of Mqtz isopleths in igs 11 and 12 (path P Q). In the muscovite-absent subsolidus (garnet + biotite + AlSi + K-feldspar + quartz), quartz continues to be consumed by continuous reactions until the vapour-saturated melting occurs garnet þ biotite þ AlSi þ K-feldspar þ quartz þ H 2 O ¼ melt In the melting region, quartz will be consumed by melting reactions such as biotite þ AlSi þ quartz ¼ garnet þ K-feldspar þ melt or, at lower pressure biotite þ AlSi þ quartz ¼ garnet þ cordierite þ K-feldspar þ melt; until eventually all quartz will be consumed. The change in modal amount of quartz is dramatic (e.g. ig. 12, path Q R) and may result in the complete consumption of quartz in certain bulk compositions or if the temperature gets sufficiently high. It is important to remember, however, that so long as some of the melt is retained in the rock (although melt is commonly lost to some extent), then on cooling new quartz will crystallize from this melt, typically in leucosomes. Note that the freezing path shown in ig. 12 (e.g. path R S) is calculated assuming no melt is lost, so it represents a maximum amount of newly formed quartz. Also note that the above reaction produced garnet, which in some cases grows around existing quartz grains thus insulating them from further reaction. Diffusion of Ti in quartz The phase diagrams shown in igs 11 & 12 plus the derived reaction histories provide a basis for interpretation of the CL zoning observed in these samples, provided the zoning profiles have not been significantly modified by diffusion. Cherniak et al. (2007) have measured Ti diffusivity in quartz as a function of temperature and concluded that the characteristic length scale for diffusion over a 1 Myr episode at temperature (i.e. ÖDt) is on the order of 1 lm at 500 C, 10 lm at 600 C and 70 lm at 700 C. Therefore, it is expected that some diffusional modification of the Ti concentrations in quartz may have occurred and be visible in middle grade samples, and certainly within high-grade samples. Garnet grade samples Quartz in samples from the garnet grade show little zoning and Ti concentrations indicate that the quartz crystallized at C with a slight tendency for the core temperature to be lower than the rim temperature in the few grains studied. As mentioned earlier, the TitaniQ temperatures are lower than the inferred peak metamorphic temperatures from garnet biotite thermometry by as much as 50. In addition, quartz grains that define matrix crenulations (ig. 2e) have identical CL brightness to similar quartz inclusions within garnet, so it must be concluded that the recrystallization of these quartz grains either predated garnet growth or grew along with garnet. Examination of igs 11 & 12 indicate that only minor amounts of quartz is involved in either pre-garnet grade reactions or in garnet-producing reactions. Therefore, it is not possible that the quartz observed in these garnet grade samples obtained their Ti concentrations solely as a consequence of these reactions. A more plausible interpretation of the garnet grade samples is that the quartz obtained its present Ti concentrations by recrystallization during fabric formation. Samples 93-19a (ig. 2a,b) and TM-549 (ig. 2e,f) display different stages of crenulation development and the quartz in each has uniform CL brightness. In sample 96-2, garnet overgrows an earlier crenulation cleavage and, again, the quartz is uniform in CL brightness both inside and outside the garnet grains. Unfortunately, there is insufficient temperature resolution using the electron microprobe to determine whether subtle differences in Ti concentrations and hence temperature exist and extensive ion probe work would be required to provide further documentation. However, these observations all suggest that the Ti concentrations are recording the temperature at which the early fabric formed in these samples. If correct, then TitaniQ thermometry may be a way to determine

16 202. S. SPEAR AND D. A. WARK the temperature at which major deformation events occur in low-grade metamorphic rocks as well as the temperature interval (and time interval if a heating rate can be assumed) over which recrystallization occurs. Staurolite grade samples Quartz from samples of the staurolite grade is distinctive in that nearly all grains have rims with higher Ti concentrations than cores. A plausible interpretation is that the quartz rims were produced by the reaction chlorite þ garnet þ muscovite ¼staurolite þ biotite þ quartz þ H 2 O rom ig. 12, it is clear that this reaction (path D Eor I J) produces a sudden increase in the amount of quartz in the sample. The absolute amount of quartz produced, as well as the relative change in mode of quartz, is a function of the specific bulk composition, so the amounts shown in igs 11 & 12 should only be construed as representative. As a close approximation, the modal amount of quartz produced is 40% of the amount of staurolite produced. or example, if 10 vol.% staurolite is grown, then 4 vol.% quartz will also be grown. This volume change will be added to pre-existing quartz and thus the total modal change will depend on the bulk rock SiO 2 content and the amount of quartz already present. In the staurolite grade samples studied here (B-38, B-52a & 88 44), there is modal per cent staurolite, which requires a production of 6 10 modal per cent quartz. The total amount of quartz in the sample is modal per cent, so adding 6 10 modal per cent is significant and should readily be observed. One interpretation of the CL imaging of these samples is that the brighter rims seen on many quartz grains and locally around the margins of garnet represent this new quartz formed on the crossing of the staurolite isograd. This interpretation is generally supported by the TitaniQ temperatures calculated from the rims of staurolite-bearing samples of C, which is similar to the temperature of the staurolite-in reaction in these rocks (e.g. ig. 11). The one staurolite-bearing sample that is not as convincingly zoned with respect to quartz temperatures is B-52A. Rim Ti concentrations in this sample average 10 ppm or so, which corresponds to a temperature of 530 C, which is probably too low for the staurolitein reaction. As mentioned earlier, higher precision analyses on this sample are clearly warranted. The cores of quartz grains from staurolite grade samples record temperatures of C. These temperatures are slightly higher than those obtained from garnet grade samples but not greatly so. Although there is no way to be certain, it is suspected that the cores of the quartz grains were produced by processes similar to quartz grains from the garnet grade samples; that is, a fabric-producing event such as crenulation development. Although a metamorphic origin for the growth of quartz on pre-existing grains is a plausible interpretation of the observations from the staurolite grade samples, certain additional observations suggest the story may be somewhat more complex. In particular, quartz inclusions within staurolite (ig. 4b,f) and garnet (ig. 4c) are zoned with increasing Ti concentrations towards their rim. It is not possible that this increase in Ti results from the same process that produced the increase in Ti on the rims of the matrix quartz in sample B-52a (ig. 4d,e) because (i) the garnet predates the staurolite-producing reaction and (ii) zoned crystals are present within staurolite itself. The only plausible interpretation is that these quartz crystals have experienced diffusional modification of the original Ti concentrations at conditions near the metamorphic peak. Both staurolite and garnet contain more Ti than does quartz at these temperatures. or example, garnet from sample B-38b contains at least 70 ppm Ti (.S. Spear, unpublished LA-ICPMS data). Although the partitioning of Ti between quartz and garnet has not been quantified, it is consistent with other partitioning studies suggesting that the distribution coefficient will approach a value of 1.0 with increasing temperature. Therefore, a quartz grain with 6 ppm Ti trapped inside of a garnet with 70 ppm Ti will attempt to increase its Ti content by exchange and diffusion as temperature increases. The magnitude of the diffusion penetration distance is on the order of a few tens of microns at most, consistent with Ti diffusivities at 600 C for a few million years (Cherniak et al., 2007). Of course, it is also possible that diffusion has modified the rims of matrix quartz grains as well, although in many places the Ti increase is observed along quartz quartz boundaries, so it is unclear where the increase in Ti activity at the rim of the quartz grain would have originated. urthermore, in sample B-52a, the sharp CL boundaries between quartz cores and rims argue for a limited diffusion in this sample. Kyanite and sillimanite grade samples Quartz from the kyanite and sillimanite grade samples shows some evidence of progressive recrystallization of quartz with increasing temperature in the form of higher Ti rims. This is especially noteworthy in the orthoamphibole schist (68-432j), although the quartzproducing reaction in this assemblage is not known. Most matrix quartz in these samples appears to be relatively homogeneous, however, suggesting either growth or recrystallization at nearly constant (near peak) temperature, or homogenization due to diffusion. or a peak metamorphic temperature of C and metamorphic durations of Myr, diffusive length scales are on the order of lm. Considering the size of the homogeneous matrix quartz grains is up to several millimetres, it is not possible for diffusion to account for this homogeneity and recrystallization must be called upon.