OH-defects in detrital quartz grains: Potential for application as tool for provenance analysis and overview over crustal average

Sedimentary Geology 294 (2013) 118–126

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OH-defects in detrital quartz grains: Potential for application as tool for provenance analysis and overview over crustal average Roland Stalder a,⁎, Rolf Dieter Neuser b a b

Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52f, A-6020 Innsbruck, Austria Institut für Geologie, Mineralogie & Geophysik, Ruhr-Universität Bochum, Universitätsstraße 150, D-44780 Bochum, Germany

a r t i c l e

i n f o

Article history: Received 11 March 2013 Received in revised form 22 May 2013 Accepted 26 May 2013 Available online 5 June 2013 Editor: J. Knight Keywords: Quartz Water content Provenance IR spectroscopy Cathodo-luminescence

a b s t r a c t OH-defects of 95 detrital quartz grains from 4 localities in North-west Germany (2 North Sea beach sands, one Triassic sandstone, and one Carboniferous sandstone) were studied with infrared (IR) microscopy. By applying novel analytical strategies, the water contribution of fluid and mineral inclusions was minimised and the amount of water incorporated as OH-point defects was quantified. The defect water concentration in all studied quartz grains ranges between 0 and 50 wt. ppm H2O with a mean value around 10 wt. ppm. Interestingly, grains from the investigated sandstones exhibit in average nearly three times higher defect water concentrations (18 wt. ppm) than the grains from the North Sea (6.5 wt. ppm). Quartz grains with extreme undulose extinction always exhibit low defect water contents and water-rich grains usually show small undulosity, but also grains with low defect water and low undulosities are common. IR spectra of the detrital quartz grains were compared to reference spectra from samples of known localities and rock types in order to identify potential sources from which the quartz grains were sampled. Most detrital quartz grains exhibit IR signature typical for granites (showing an Al-specific band at 3378 cm−1) and regional metamorphic rocks, but also absorption bands typical for pegmatites and hydrothermal quartz (showing a Li-specific band at 3480 cm−1) are observed. In contrast, IR signatures typical for high-pressure origin (i.e., hydrogarnet substitution with an absorption band at 3585 cm−1) and for tourmaline-bearing rocks (showing a B-specific band at 3595 cm−1) are subordinate to insignificant. In view of the large scatter of defect water between individual quartz grains the strategy presented here offers an option to estimate the average defect water content of quartz in the Earth's crust. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Quartz is the second most abundant mineral in the Earth's crust and occurs in a broad variety of rocks and unconsolidated sediments. In contrast to all other abundant rock forming minerals, quartz does not form solid solutions and is considered as a pure mineral. However, trace amounts of chemical impurities may be incorporated as defects in the crystal lattice, most often as combination of mono(H+, Li+, and Na+) and trivalent (Al3+, Fe3+, and B3+) cations (Bambauer, 1961, 1963; Kats, 1962). In this context the importance of H+-incorporation has early been noted, since the formation of OH-defects influences the deformation style, referred to as hydrolytic weakening (Griggs and Blacic, 1965; Griggs et al., 1966; Doukhan and Trepied, 1985). The incorporation of OH is revealed by infrared (IR) absorption bands (Bambauer, 1963; Aines and Rossman, 1984), where specific substitutions or point defects can be distinguished, e.g., (1) Si4+ → Al3+ + H+ resulting in an absorption triplet at ⁎ Corresponding author. E-mail address: [email protected] (R. Stalder). 0037-0738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sedgeo.2013.05.013

3310, 3374 and 3440 cm−1 (Kats, 1962; Bambauer, 1963), (2) LiOH point defects resulting in an absorption band at 3480 cm−1 (Kats, 1962; Aines and Rossman, 1984), (3) Si4+ → 4H+, also known as hydrogarnet substitution, resulting in an absorption band at 3585 cm−1 (Paterson, 1986; Stalder and Konzett, 2012), and (4) Si4+ → B3+ + H+ resulting in an absorption band at 3595 cm−1 (Müller and Koch-Müller, 2009 and own unpublished results from high pressure syntheses). As the formation of the different types of defects is dependent on pressure, temperature and chemical system, IR spectra could principally serve as an archive of formation conditions of individual quartz grains of unknown origin, or as provenance tool for sediments and sandstones. However, the largest portion of OH in many natural quartz crystals is incorporated as molecular H2O (Bambauer, 1961; Aines et al., 1984; Müller and Koch-Müller, 2009), i.e. fluid inclusions, that appear as very broad absorption feature in the IR spectra and interfere with the OH-defect absorption bands. In order to separate absorption features of molecular water and OH-defects described above, a novel strategy was applied (Stalder and Konzett, 2012), requiring polarised IR measurements on oriented crystal sections.

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Table 1 Analysed sedimentary quartz samples (analytical error or standard deviations in parenthesis). Sample locality1

Juist Langeoog Reyershausen Essen 1 2 3

Latitude

53°40.9′ 53°45.3′ 51°36.6′ 51°23.8′

Longitude

6°59.1′ 7°29.0′ 10°00.7′ 7°03.0′

Geological formation

Analysed grains

Holocene/beach sand Holocene/beach sand Lower Triassic/Buntsandstein (BSS) Upper Carboniferous/Westphalian sandstone (WSS)

29 29 29 8

Max. water content2

Average water content

Grain size

Thickness of wafer3

wt. ppm (H2O)

wt. ppm (H2O)

mm

μm

38 22 50 46

8 (10) 5 (6) 17 (14) 19 (20)

0.30 0.51 0.22 0.10

(4) (2) (5) (5)

(0.08) (0.11) (0.05) (0.04)

92 (18) 134 (39) 75 (15) 47 (12)

See Fig. 1. Using the calibration of Libowitzky and Rossman (1997). Analytical error for single grain analysis was estimated to be ±10%. After preparation for IR measurement.

420 nm for Ti-related defects (Müller et al., 2002, 2003), and at 620–650 nm for non-bridging oxygen holes (Götze et al., 2001) have been identified. In this study quartz crystals from 95 detrital quartz grains from sands and sandstones from 4 different localities in Northern Germany (Table 1, Fig. 1) were characterised by IR spectroscopy, and the water content of each individual grain was determined. IR spectra of the detrital quartz grains were compared to IR spectra of 10 quartz crystals from natural rocks with known provenance (Table 2). The characteristics of IR spectra of 8 selected detrital quartz grains with different defect water content and origin were further compared to CL spectra recorded on the same grains, in order to evaluate whether both methods furnish analogous or complementary information. The novel approach is to be applied as tool for provenance analysis for quartz-rich sediments. 2. Sample localities Fig. 1. Map of Northern Germany with overview of sample localities. J = Juist, L = Langeoog, R = Reyershausen (see Table 1). Major cities and rivers are shown for orientation.

An overview over sample localities for sediments and sedimentary rocks reported here are shown in Fig. 1. Grid references for sample locations are given in Table 1.

In contrast to IR spectroscopy, cathodoluminescence (CL) spectroscopy has since long been recognised as a powerful tool to characterise detrital quartz grains (Zinkernagel, 1978; Matter and Ramseyer, 1985; Richter et al., 2003), but unambiguous identification of quartz provenance based on CL alone has been difficult (Boggs et al., 2002). Specifically, CL can be used to discriminate between volcanic, plutonic, metamorphic and hydrothermal quartz. Despite some overlap between these groups, CL is a reliable method, unless major input from plutonic and pegmatitic sources and/or high-T metamorphic rocks occurred (Augustsson and Reker, 2012). Thus, a combination of several methods, such as observation of optical and CL properties, is more successful (Bernet and Bassett, 2005). The CL colour of the irradiated quartz crystal is defined by the combination of intrinsic colour and impurity defects that generate emission bands. Specifically, emission bands for Al-related defects at 380–390 nm and around 500 nm (Götze et al., 2001; Götte et al., 2011), at

2.1. North Sea beach sands (Juist and Langeoog) Both samples were collected at the seaside during low-tide close to the high-tide shoreline. Apart from some shell fragments they consisted of 99% quartz. Both samples are well-sorted and the grain sizes (determined by examination under the microscope) exhibit a nearly Gaussian distribution of 300 ± 80 μm and 510 ± 110 μm for the sand sample from Juist and Langeoog, respectively (standard deviation given as 1σ, 140 grains of each sample were measured). Juist and Langeoog are part of the East Frisia island chain, belonging to the Wadden Sea barrier island system consisting of Holocene dune and beach sands (Streif, 2004). The current sedimentary material of the North Sea is a mixture of several components, dominated by fluvial and glacial components from Scandinavia and (to a lesser extent) from the British Isles, with contributions from the large Middle European river systems Elbe, Rhine and Thames (Schüttenhelm and Laban, 2005).

Table 2 Reference quartzes from known localities (standard deviations in parenthesis); water concentrations were calculated using the calibration of Libowitzky and Rossman (1997). Sample locality

Host rock

Number of analysed grains

Västervik/Sweden Åland/Finland Großer Inselsberg/Germany Bozen/South Tyrol Tauern Window/Austria Larvik/Norway Schneeberg Complex/South Tyrol Kreuzeck Group/Austria Pfitsch valley/South Tyrol Brazil

Götemar granite Rapakiwi granite Rhyolite Rhyolite/quartz porphyry Quartzite Quartzite Tourmaline-bearing pegmatite Tourmaline-bearing quartz eclogite Tourmaline-bearing gneiss Hydrothermal quartz/rock crystal

3 3 3 3 3 3 2 2 1 1

Water content wt. ppm (H2O) 5 (1) 16 (3) 12 (1) 1.4 (0.5) 1.6 (0.5) 1.5 (0.5) 4.3 (0.5) 3.2 (0.1) 2 15

Reference Friese et al. (2012)

Hoschek et al. (2010)

Konzett et al. (2012)

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0.7

bedding (Brinkmann, 1933), grain size decrease (Leggewie et al., 1977) and geochronology of detrital micas (Paul et al., 2008), the source from the Buntsandstein sediments is mostly from late Palaeozoic rocks of the Bohemian Massif and the Vindelician High located in the south.

0.6 Gneiss/Pfitsch Pegmatite/Schneeberg Eclogite/Kreuzeck

2.3. Upper Carboniferous sandstone (Essen)

rock crystal/Brazil

0.4

0.3

Granite/Åland Rhyolite/Inselsberg Granite/Västervik Rhyolite/Bozen

absorbance / mm

0.5

0.2

0.1 Quartzite/Tauern Quartzite/Larvik

4000

3800

3600

3400

0 3200

wavenumber (cm-1) Fig. 2. IR-spectra of some reference quartz crystals from known localities (Table 2). All spectra show the difference of two polarised measurements (E||no–E||ne) for elimination of molecular water. For the two spectra that exhibit the B-related OH-absorption band at 3595 cm−1, the contribution of the IR spectrum recorded ||ne in the range 3585–3605 cm−1 was added, yielding the spectral component (2o + e) / 2. Spectra are normalised to thickness and offset for clarity. Vertical dotted lines from left to right represent B-, Li- and Al-related OH-absorption bands, respectively.

2.2. Lower Triassic sandstone, “Buntsandstein” (Reyershausen) All measured spectra from this sample are referred to as “BSS” (Buntsandstein) in all figures. The sample was derived from the Solling-sandstone, exposed in a natural outcrop between Reyershausen and Billingshausen in Lower Saxony/Germany. X-ray powder diffraction combined with Rietveld refinement with Topas (Bruker) yielded a mineral composition of 64% quartz, 25% feldspar, 10% mica and 1% hematite. The grain size (Table 1) was determined by averaging the diameter of the 29 handpicked grains and is considered as representative for the whole sample. The Sollingsandstone is the uppermost formation of the Middle Buntsandstein that is part of a large Lower Triassic sequence of continental basin deposits (Wolburg, 1968; Aigner and Bachmann, 1992). Based on cross

All measured spectra from this sample are referred to as “WSS” (Westphalian sandstone) in all figures. The sample was derived from a small outcrop of Westphalian A between the former railway station “Haus Scheppen” and the reservoir Baldeneysee. X-ray powder diffraction combined with Rietveld refinement with Topas (Bruker) yielded a mineral composition of 61% quartz, 11% feldspar, 19% mica and 9% chlorite. The grain size (Table 1) was determined by averaging the diameter of the 8 handpicked grains. Deposition occurred in a foreland basin during the peak activity of the Variscan orogeny (Ziegler, 1990). The source regions of Westphalian sandstones in the Ruhr Valley were in the south, namely the Rhenohercynian and the Central German High (Massonne, 1984). 3. Analytical techniques 3.1. Sample preparation and strategy to eliminate molecular water One of the most abundant hydrous species in quartz is molecular water hosted as fluid inclusions, which in turn may contain significant amounts of dissolved ions. Therefore, results from most chemical analysis methods represent a mixed analysis (with unknown proportions) of the OH-defects of the host crystal and fluid inclusions. In contrast, spectroscopic methods such as IR spectroscopy are able to identify (1) different hydrous species by the energy of the respective vibrations, and (2) the orientation of the OH-dipole vector if polarised measurements are performed. Since most absorption bands of OH-defects are more or less perfectly oriented ||no of quartz (as illustrated by the original IR calibration of Aines et al. (1984), where only this component was considered for water quantification), the isotropic component is represented by the IR absorbance ||ne and can be eliminated by subtraction from the IR absorbance ||no. The absorbance ||no–||ne can further be used as a tool to quantify the OH-defect content in quartz, as recently shown by Stalder and Konzett (2012). The only OH dipole vector worth mentioning that exhibits a significant contribution for ||ne is a band at 3595 cm−1 (Götte et al., 2011) which has been assigned to B-related OH-defects (Müller and Koch-Müller, 2009). If this band was observed in the IR spectrum, the contribution in the range 3585–3605 cm−1 of the IR spectrum recorded ||ne was taken into account during OH-defect quantification (note: calculation of total

Fig. 3. IR image showing the total absorbance (unpolarised measurement) in the range 3580–3680 cm−1 of grain #352 from Juist beach sand. Enhanced absorbance is caused by micro-inclusions of mica crystals. Spectra extracted from the mica-free regions (dark blue) did not exhibit OH-absorption bands. The right panel is an enlargement from the rectangle shown in the left panel.

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triple grating spectrograph (Model 275, Acton Research) by a quartz light guide. An ultra-sensitive cooled CCD detector camera (PIXIS, Princeton Instruments) collects the light at the exit of the spectrograph

0.3

0.2

3.2. Infrared spectroscopy and water quantification WSS average

Mid-infrared absorption spectra were recorded at room temperature in transmission mode using a Bruker Vertex 70 FTIR spectrometer, coupled to a Hyperion 3000 microscope using a liquid nitrogencooled MCT detector and a focal plane array detector (FPA) consisting of 64 × 64 MCT-detectors, a globar light source, a KBr beamsplitter and a wire-grid polariser. Depending on the size of the prepared crystal section, 32 to 600 scans between 550 and 7500 cm−1 were recorded on an area of 100 × 150 μm2 to 30 × 30 μm2 with a spectral resolution of 2 cm−1. On each specimen two measurements, parallel to no and ne, were performed on the same spot. After subtracting a linear background between 3630 and 3200 cm−1 from the spectral component ||no–||ne water concentration was determined using the calibration of Libowitzky and Rossman (1997) and Thomas et al. (2009), where the total hydrogen from the OH-defects is expressed as H2O concentration. Both calibrations give nearly identical results within error, except for crystals with IR bands at high wavenumbers, where the calibration of Libowitzky and Rossman (1997) furnish slightly higher water concentrations (Stalder and Konzett, 2012). For all detrital quartz grains the difference between the two calibrations was negligible. Samples containing μm-sized mineral inclusions were also inspected by IR imaging using a focal plane array (FPA) detector with a spatial pixel resolution of 2.7 μm (Prechtel and Stalder, 2010), in order to get an overview over the OH distribution in the crystal. 3.3. Optical microscopy All grains were inspected under a polarising microscope after preparation and the undulose extinction and grain size were noted. In addition, petrographic information such as grain size and the occurrence and abundance of inclusions (fluid, melt, mineral) was noted. This documentation was initially intended to define different groups of quartz grains, but later turned out to be not very helpful for the discrimination of different sources. However, the relation between the defect water concentration and the deformation (documented by the undulose extinction) of the individual grains is of particular interest, as defects water has a strong influence on the mechanical strength (Griggs et al., 1966) and may influence the time scale of mechanical fragmentation during weathering.

absorbance / mm

A

0.1

BSS average

Juist average Langeoog average

0 4000

3800

3600

3400

3200

wavenumber (cm-1) 1.5

B 1.2

WSS 348

BSS 321

0.9

Juist 298 Langeoog 07 Langeoog 13

0.6

absorbance / mm

absorbance in trigonal crystals is performed by adding the components 2o + e). However, this band was only unequivocally observed in two quartzes from tourmaline-bearing reference materials and in one detrital quartz grain from the Triassic sandstone. In order to measure spectra parallel to both refractive indices, quartz crystals were aligned parallel to the c-axis in a thermoplastic resin, and ground and polished on both sides. The sandstone samples were crumbled by hand before preparation, in order to guarantee the preservation of detrital grains. Individual grains were randomly handpicked under the microscope. In some of the reference rocks, oriented quartz crystals were identified in thick sections. Optical orientation was checked by polarising microscopy using orthoscopic (birefringence Δ = 0.009) and conoscopic illumination (flash figure). The thickness was determined using a mechanical micrometre with a precision of ± 2 μm.

121

Juist 413 Juist 408 Langeoog 402

0.3

WSS 392 BSS 323 Juist 364 Langeoog 05

3.4. Cathodoluminescence Eight selected crystals were also analysed by CL. Each grain was prepared as a separate polished thin section. After applying a thin carbon coating (b3 nm) the samples were studied using a hot-cathode CL device at 14 kV beam energy (HC1-LM, Lumic, s. Neuser et al., 1996). Photos were taken by a highly sensitive high-resolution digital microscope camera (DP73, Olympus). The CL-microscope is connected to a

3800

3600

3400

0 3200

wavenumber (cm-1) Fig. 4. IR-spectra (E||no–E||ne) of detrital quartz grains. (A) Average spectra, (B) samples with maximum and minimum defect water content of all samples. Spectra are normalised to thickness and offset for clarity. Vertical dotted lines from left to right represent B-, Liand Al-related OH-absorption bands, respectively. WSS = Westphalian sandstone, BSS = Buntsandstein.

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45

and spectra were then processed using a computer programme (WinSpec, Princeton Instruments).

NSB

40

4. Results

BSS

IR spectra of the reference materials from known sample localities (Table 2) are shown in Fig. 2. The most prominent absorption feature is the triplet around 3400 cm−1 that is related to the substitution of Si4+ by Al3+ and H+. Granites and rhyolites tend to show much stronger absorptions compared to quartzites, but a simple distinction between different source rocks based on the Al-related OH-band cannot be made. An unequivocal distinction between different source rocks based on other OH absorption bands cannot be achieved either, e.g. (1) the Li-related OH-absorption band that appears in the hydrothermal quartz is also clearly detected in quartz from the Götemar granite (Västervik/ Sweden), and (2) both tourmaline-bearing rocks – even though totally different in modal mineralogy (pegmatite versus eclogite) – show very similar IR spectra with a sharp B-related OH-absorption band at 3595 cm−1. In some quartz grains (e.g., the tourmaline-bearing gneiss from Pfitsch) another dominant absorption feature centred at 3620 cm−1 was observed. This feature was also observed in some detrital grains and was related to micro-inclusions of mica that were spread over the whole crystal (Fig. 3). Extraction of spectra from mica-free regions revealed no significant absorption in the whole OH-absorption range. In all reference quartz samples studied here, the total water concentration spans a range between 0 and 20 wt. ppm defect water (Table 2). IR spectra of several detrital quartzes are shown in Fig. 4. Average spectra of all four samples (Fig. 4A) are dominated by the Al-specific OH-bands. In addition, the Li-related OH-band is clearly identified in the Buntsandstein sample and to a lesser extent in the Westphalian sandstone and the beach sand from Juist. All sands and sandstones

undulose extinction (°/mm)

4.1. IR spectroscopy and defect water contents

30

25

20

15

10

5

0 0

20

40

60

H2O (wt ppm) Fig. 6. Plot of undulose extinction and defect water content of all analysed detrital quartz grains. Undulose extinction is normalised to grain size to take into account the fact that smaller grains may show only a limited range of the original undulosity. NSB = North Sea Beach, BSS = Buntsandstein, WSS = Westphalian sandstone.

contained virtually defect water-free grains (Fig. 4B), but considerable differences in average absorption (i.e., water content) occur (Fig. 4A, Table 1). Water-poor quartz grains are in all samples the most abundant group, and the sandstone samples show a tendency towards bimodal distribution (Fig. 5).

5

10

3 2

Triassic sandstone "BSS" (Buntsandstein) n=29

B N (crystals)

Carboniferous sandstone "WSS" (Westphal A) n=8

A

4

N (crystals)

WSS

35

5

1 0

0

wt ppm water

15

25

10

5

North Sea Beach Sand (Langeoog) n=29

D

20

N (crystals)

North Sea Beach Sand (Juist) n=29

C N (crystals)

45-50

40-45

35-40

30-35

25-30

20-25

15-20

10-15

5-10

0-5

45-50

40-45

35-40

30-35

25-30

20-25

15-20

10-15

5-10

0-5

wt ppm water

15 10 5

0

0 45-50

40-45

35-40

30-35

25-30

20-25

15-20

10-15

5-10

0-5

45-50

40-45

35-40

30-35

25-30

20-25

15-20

10-15

5-10

0-5

wt ppm water

wt ppm water

Fig. 5. Histogram showing the abundance of quartz grains with respect to their defect water content using the calibration of Libowitzky and Rossman (1997). (A) Carboniferous sandstone, (B) Triassic sandstone, (C) beach sand from Juist, (D) beach sand from Langeoog.

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Intensity (arbitrary units)

Langeoog 402 (3 ppmwater)

Juist #413 (12 ppm water) BSS #323 (7 ppm water)

Langeoog 13 (16 ppm water)

BSS #321 (50 ppm water)

Langeoog 07 (22 ppm water)

Juist #408 (9 ppm water)

Langeoog 05 (<1 ppm water)

300

400

500

600

700

800

wavelength (nm) Fig. 7. Cathodoluminescence (CL) spectra recorded after 1 min of electron irradiation.

Quartz grains with a large angular range of extinction (i.e., extreme undulosity) always exhibited low defect water contents (and vice versa), but also grains with low defect water and low undulosities are common (Fig. 6). 4.2. Cathodoluminescence For all investigated grains, either minor or no changes in luminescence colour were observed during electron irradiation. The initial CL colour is always blue. Final CL spectra recorded after about 1 min of electron irradiation reveal different broad band emissions between 400 and 750 nm (Fig. 7). A blue emission band at about 450 nm is always present and represents the intrinsic portion of the CL of quartz (the inherent CL colour of a chemically clean stoichiometric quartz without lattice defects). The emission around 700 nm is varying and increases in some samples during electron bombardment, resulting in a more violet CL colour. The strongest change from blue to a violet tint within 30 s was observed in sample 402. Samples 413, VH13 and 323 only show a weak shift in colour and samples 408, VH05, VH07 and 321 remain blue (Fig. 8). No apparent correlation between defect water content and CL emission is visible. However, this may be due to the relatively weak change in CL colour in the studied samples compared to for example hydrothermal quartz (see Richter et al., 2003, their plate 1). 5. Discussion CL and IR spectroscopy provide complementary information that can be combined to characterise individual quartz grains of unknown origin. In agreement with previous studies (Müller and Koch-Müller, 2009), IR spectra and defect water concentrations exhibit strong variations, even if specimens from the same rock-type (but from different localities) are compared. Therefore, based on the IR characteristics alone,

a simple distinction between different signatures (e.g., igneous, metamorphic, and sedimentary) cannot be made, and IR spectra of single detrital quartz grains do not reveal unequivocally their host rocks. However, grains from the same source (same rock-type and same locality) seem to have similar properties. Therefore, the IR spectrum of each individual grain is clearly a characteristic property, and adds important aspects to the characterisation of the grain. Specific information can be obtained from the Li-specific OH-absorption band at 3480 cm−1, that can be interpreted as pegmatitic or hydrothermal in origin (Müller and Koch-Müller, 2009). Further information is obtained from the presence or absence of the band at 3585 cm−1 caused by hydrogarnet substitution. From the absence or very low intensity of this band in all quartz grains investigated here, a low-pressure origin of most quartz grains in the Earth's crust can be inferred (Paterson, 1986; Stalder and Konzett, 2012). The analysis of detrital quartz may provide a fingerprint and has the potential to reveal the average defect water content of quartz from the respective source area. On the one hand, the high variability of defect water contents from the investigated sands or sandstones (Fig. 5) reflects the (not surprisingly) large number of different source rocks. On the other hand, the average composition of the investigated sand and sandstone samples is by far less variable than the properties of individual grains, which span a range in defect water content between 0 and 50 wt. ppm with an average around 10 wt. ppm. The average defect water concentration of the analysed grains of the Westphalian sandstone and the Buntsandstein is significantly higher than in the beach sand samples from Juist and Langeoog. The most probable explanation is that the different defect water content reflects the different source region of the quartz grains, i.e. mostly from the south for the investigated sandstones (Leggewie et al., 1977; Paul et al., 2008) versus mostly from the north for the North Sea beach sands (Schüttenhelm and Laban, 2005). Probably the defect water content is inherited from the crystallisation and/or metamorphism process and therefore reflects

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Fig. 8. Selected cathodoluminescence (CL) images of quartz grains recorded after 1 min of electron irradiation.

60

WSS

50

BSS Juist Langeoog

wt ppm water

40

30

20

10

0 0.01

0.1

1

the composition of the source rocks. In addition, however, there may be other factors that influence the modal distribution of defect water. Specifically it can be noted that the only obvious difference between the two beach sands is the grain size (Fig. 9). The correlation of grain size and defect water content poses the question, whether the grains are smaller because they have higher water contents or whether they have higher water contents because they are smaller. Incorporation of OH-defects under sedimentary (i.e., low temperature) conditions seems to be improbable. In contrast, a connection between deformation style (see trend of undulosity, Fig. 6), grain size and water content appears sound (Griggs et al., 1966). More detailed case studies have to be performed to answer this question unequivocally. It is also to be noted that the defect water content of the detrital grains spans a wider range than that of the quartz crystals derived from the known localities. The study of detrital quartz grains furnishes a more realistic portrait of the average water concentration in crustal quartz crystals and has the advantage that abundant sources can be characterised. The identification of these sources, however, is not straightforward, and may be even impossible because the source bodies may have since long been eroded. In this way, detrital quartz grains may be regarded as archive to display the defect water content of “extinct” rock sources.

grain size (mm) Fig. 9. Defect water concentrations in quartz grains from the analysed sandstones and beach sands versus grain size. The average value for each sample is shown as large symbol. Averaged values cluster in two groups, probably reflecting different source regions of the beach sand (mostly from Scandinavia) and the sandstones (mostly from Central Europe).

5.1. Novel tool and potential benefit The novel strategy to eliminate molecular water enables the determination of defect water in quartz. In this study it is applied for

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the first time to detrital quartz grains, and significant differences with respect to the nature and amount of the OH defects are revealed. The novel technique furnishes complementary information to other state-of-the-art techniques such as chemical analysis of accessory minerals (Morton et al., 2004; Zack et al., 2004; Richter et al., 2006; von Eynatten and Dunkl, 2012) and CL on quartz grains. The fact that no correlation between water content and CL colour was revealed, underlines the potential and sensitivity of IR spectroscopy to add news aspects for the characterisation of quartz grains of unknown origin. The method described here has the advantage that it is concerned with the most common detrital mineral, and therefore can be applied to sediments and sedimentary rocks that (1) do not contain heavy minerals, (2) contain no distinct or (3) only unstable heavy minerals. The resources needed to undertake this analysis are also reasonable: running cost of an optical microscope and FTIR are very low and are widely accessible. The preparation takes 20 min per grain and can easily be carried out by students after a short introduction and practical exercise. The novel strategy is not restricted to loose sediments and friable sandstones. Cemented sandstones can be prepared as doubly polished thick sections (ideally several sections – cut in different directions – are prepared for each specimen), where the grains with suitable orientation are easily identified by their interference colour and can be selectively measured by IR spectroscopy. In this way, also syntaxial overgrowths could be measured separately and the time-consuming process of crystal alignment and preparation would be skipped. The last point is actually also applicable to loose samples, if a larger amount of grains is embedded in epoxy and prepared as polished section. 6. Conclusions (1) The water content of quartz crystals (hosted as OH-defects) in the Earth's crust is highly variable and ranges between 0 and at least 50 ppm (wt.) (2) OH-defects in quartz crystals can serve as archive for their formation conditions, and infrared (IR) spectroscopy furnishes information that is complementary to other techniques such as cathodoluminescence (CL) spectroscopy. (3) The assignment of an individual quartz crystal to a source rock-type based on the infrared absorption spectrum alone is not unequivocal, because similar rocks (from different localities) may exhibit strong differences in OH-defect content. (4) Infrared (IR) spectra of quartz crystals from sedimentary samples exhibit a large variety that is in sharp contrast to the monotonous IR-spectra from one single source (same geological body). (5) Sedimentary reservoirs show distinct characteristics in their modal distribution of the OH-defect content of their quartz crystals. Acknowledgements Richard Tessadri and Daniela Schmidmair are thanked for support with XRD analyses and Rietveld analysis and Christoph Spötl and Karl Krainer for valuable discussion. Sabine Schremmer and Ellen Kessler conscientiously prepared the CL thin sections with precision. The manuscript benefited from constructive comments of Jasper Knight, Hilmar von Eynatten and an anonymous reviewer. References Aigner, T., Bachmann, G.H., 1992. Sequence-stratigraphic framework of the German Triassic. Sedimentary Geology 80, 115–135. Aines, R.D., Rossman, G.R., 1984. Water in minerals? A peak in the infrared. Journal of Geophysical Research 89B, 4059–4071. Aines, R.D., Kirby, S.H., Rossman, G.R., 1984. Hydrogen speciation in synthetic quartz. Physics and Chemistry of Minerals 11, 204–212.

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