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OH-defect content in detrital quartz grains as an archive for crystallisation conditions
Sedimentary Geology 307 (2014) 1–6
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OH-defect content in detrital quartz grains as an archive for crystallisation conditions Roland Stalder Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52f, A-6020 Innsbruck, Austria
a r t i c l e
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Article history: Received 12 March 2014 Received in revised form 9 April 2014 Accepted 10 April 2014 Available online 18 April 2014 Editor: J. Knight Keywords: Quartz OH-defects IR spectroscopy Provenance
a b s t r a c t Infrared spectra of 433 oriented detrital quartz grains from different large sedimentary reservoirs worldwide are evaluated with respect to their OH-defect concentration (expressed as wt ppm water), which varies between 0 and 160 wt ppm for individual grains. OH-defect contents, averaged over each individual sample, range between 5 and 20 wt ppm defect water, with a mean value around 10 wt ppm. Sand samples tend to have lower average OH-defect contents than older sandstones deposited earlier from a similar source region. Three of the most OHdefect rich grains from our data set were observed in the oldest and the most strongly cemented sandstone investigated (1400 Ma old Dala sandstone from Dalarna/Sweden), suggesting that OH-defects are barely affected during diagenetic conditions even over geological time scales. Furthermore, the average OH-defect content of each sample is well correlated to its maturity. Combined with results from high-pressure experiments in different model systems, the new results may provide an indirect method to estimate the source rock inventory of the upper crust (igneous, hydrothermal or metamorphic) and depth of the sampled crustal section. Results further suggest that provenance analyses of mature sediments and sedimentary rocks could benefit from the analysis of OH-defect content in quartz. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Quartz is a stable mineral in most environments of the Earth's crust and therefore is a major constituent of many igneous, metamorphic and sedimentary rocks and the second most abundant mineral in the Earth's crust (Ronov and Yaroshevski, 1969). Amongst the rock forming minerals quartz is most resistant towards mechanical and chemical weathering and therefore the most abundant mineral in the sand fraction of clastic sediments and sedimentary rocks. Primarily, quartz mostly crystallizes in igneous systems such as granites, but a considerable amount may later be annealed under regional or thermal metamorphism, where impurities of trace metals and defect protons equilibrate according to the thermochemical factors of (1) pressure, (2) temperature and (3) chemical environment (Wark and Watson, 2006; Stalder and Konzett, 2012). Consequently, some metal impurities such as Al and Ti have been calibrated as a geobarometer or geothermometer, respectively (Dennen et al., 1970; Wark and Watson, 2006; Thomas et al., 2010; Huang and Audétat, 2012). Metal impurities in quartz are related to cathodoluminescence emission properties and are thus valuable for discrimination of the petrogenetic origin of the quartz (Matter and Ramseyer, 1985; Götze et al., 2001; Boggs et al., 2002; Müller et al., 2003; Augustsson and Reker, 2012).
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http://dx.doi.org/10.1016/j.sedgeo.2014.04.002 0037-0738/© 2014 Elsevier B.V. All rights reserved.
Another interesting information source with respect to the petrogenetic origin is the incorporation of trace protons as point defects, such as (1) the incorporation of defect protons by substitution of Si4+ by 4H+ (also referred to as “hydrogarnet” substitution, Doukhan and Trepied, 1985; Stalder and Konzett, 2012), (2) coupled substitutions involving Na+ and Li+ (Bambauer, 1961), (3) coupled substitutions involving Al3+ (Kats, 1962; Aines and Rossman, 1984), or (4) coupled substitutions involving B3+ (Miyoshi et al., 2005; Thomas, 2008). A powerful method to detect OH-defects down to the level of parts per million (ppm) is infrared (IR) spectroscopy (Aines and Rossman, 1984), and technical innovations such as the application of focal plane array (FPA) detectors enable spatial resolution down to a few μm (Prechtel and Stalder, 2010). A significant problem for the quantification of OH-defects and trace impurities is the fact that natural quartz usually contains water as fluid inclusions (Bambauer, 1961; Müller and Koch-Müller, 2009), which therefore render the analysis of OH-defects and other chemical impurities difficult. In contrast, polarised spectroscopic measurements on oriented grains are able to eliminate the anisotropic OH-defect signal from the isotropic water signal from the fluid inclusions. This procedure is possible, since the dipoles of nearly all OH-defects in quartz are aligned ||no enabling the distillation of the proper OH-defect signal by subtracting two polarised measurements (Stalder and Konzett, 2012; Stalder and Neuser, 2013). In order to get an overview of average detrital quartz in the Earth's crust, in this study some large sedimentary reservoirs were sampled
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R. Stalder / Sedimentary Geology 307 (2014) 1–6
and their quartz grains were measured by IR spectroscopy. In comparison to the time-consuming process of direct sampling of numerous individual localities, this strategy is analogous to early attempts to estimate the average chemical composition of the continental crust, where glacial deposits were used as a natural sampling procedure (e.g., Goldschmidt, 1933). An advantage to direct sampling from exposed geological bodies and drill cores is that the chosen sampling method is less biased by the accessibility of individual geological bodies and furthermore provides data from sources that have been long since eroded. Results therefore serve as archive for formation conditions of quartz over a long geological history, and are interpreted with respect to several aspects that may contribute to the observed spectrum of defect contents, in particular mechanical weathering, diagenesis, and thermophysical conditions under which the grains initially crystallized. 2. Samples and methods 2.1. Samples Samples analysed here were derived from two cemented sandstones, two friable sandstones, three desert sands, and one beach sand (Table 1). Samples originate from three different continents (Europe, Africa and Australia) and all reflect extended sedimentary reservoirs that are/ were supplied by multiple sources. With respect to their sedimentation age, the selected samples can be considered as representative of the age distribution of global sediments (Veizer and Jansen, 1985). Modal mineral compositions were determined by X-ray powder diffraction combined with Rietveld refinement with Topas (Bruker). The samples from Germany have previously been reported in a different context (Stalder and Neuser, 2013) and are considered here for comparison. 2.2. Sample preparation and infrared (IR) spectroscopy Samples from cemented sandstones were prepared as double polished thick sections. Samples from desert sands were prepared as double polished grain mounts. In order to produce wafers with properly exposed plateaux on both sides and with convenient thickness for IRspectroscopy, the 250–500 μm fraction was selected for preparation (in all three desert sands the 125–250 μm fraction was the most abundant). After preparation, samples were between 80 and 130 μm thick, each mount contained approximately 500 sand grains with random orientations. IR spectra were recorded at room temperature in transmission mode using a Bruker Vertex 70 FTIR spectrometer, coupled to a Hyperion 3000 microscope equipped with a liquid nitrogen-cooled MCT-detector, a globar light source, a KBr beamsplitter and a wiregrid IR-polariser and two polarisers for visible light. IR measurements were performed on selected quartz crystals cut parallel to the crystallographic c-axis from the thick sections or grain mounts according to their
interference colour. Measurements were performed with a spectral resolution of 2 cm−1 in the 550 to 7500 cm−1 range. Two measurements were performed on each grain, with the electric field vector E of the polarised radiation parallel to no, and with E parallel to ne on exactly the same spot by turning the polarizer by 90°. After data acquisition, spectral information of all measured grains was used to (1) omit minerals other than quartz, (2) check for the appropriate crystal orientation, and (3) eliminate the IR-signal for fluid inclusions by subtracting the spectra (E||no − E||ne). Step (1) was rarely necessary, since minerals other than quartz usually were recognized prior to the IR measurement based on their optical properties. For steps (1) and (2) the wavenumber range of the lattice overtones (between 1200 and 2200 cm−1) was applied. The number of successfully measured grains is given in Table 1. Subtracted spectra (no − ne) were corrected by a linear baseline between 3250 and 3610 cm−1. In a very few cases, when B-related OHabsorption bands (exhibiting absorption ||no and ||ne) were observed, spectra were corrected in the 3585–3605 cm−1 range, taking into account the total dipole intensity is defined as 2o + e (Stalder and Neuser, 2013). No fitting procedure was applied to the observed absorption bands, instead the defect water content (expressed as wt ppm water) was derived by multiplying the absorbance for each measured channel with an individual extinction coefficient according to Libowitzky and Rossman (1997). 3. Results and discussion 3.1. Systematics of OH-defects in detrital quartz Average IR spectra for all samples are shown in Fig. 1. In all samples the absorption triplet around 3375 cm−1, caused by coupled substitution involving Al3+ (Aines and Rossman, 1984), is the most prominent band. The absorption band at 3475 cm−1, caused by LiOH defects (Bambauer, 1961), is also clearly observed in some samples. B-related OH-defects (at 3595 cm−1) and hydrogarnet substitution (3585 cm−1) are not relevant for the total OH-defect concentration, but are detected in some single grains. Combined with the results from experimentally grown quartz crystals (Paterson, 1986; Stalder and Konzett, 2012), the overall low occurrence of the hydrogarnet substitution in detrital quartzes is in accord with the interpretation that quartz crystals of initially high-pressure origin (N 5 kbar) are of minor occurrence in sedimentary rocks. In contrast, the significant concentration of Li-defect may be used to estimate the proportion of late stage magmatic quartzes such as hydrothermal quartzes, typically derived from shallow crustal levels. OH-defect concentrations calculated for individual grains were arbitrarily subdivided into groups of 5 ppm steps (Fig. 2). Average OH-defect contents were calculated from averaged spectra (E||no − E||ne) of all grains from the respective sample, the resulting values varying between 5 and 20 wt ppm (Table 1). Based on these data, the average OH-defect content was
Table 1 Sample descriptions and OH-contents.
1 2 3 4 5 6 7 8 1
Sample
Sedimentation Locality period
Latitude
Longitude
N (crystals) Average OH content Range OH-content Modal mineralogy (wt ppm water)1 (wt ppm water)1 %Qz %Fs %Mi Other phases2
Subarkose Qz-arenite Subarkose Subarkose Beach sand Desert sand Desert sand Desert sand
Proterozoic Cambrian Carboniferous Triassic Quaternary Quaternary Quaternary Quaternary
61°04.7′ N 3 30°35′ N 51°23.8′ N 51°36.6′ N 53°43′ N 5 33°25.1′ N 24°44.3′ S 25°13.2′ S
13°37.3′ E 3 35°19′ E 7°03.0′ E 10°00.7′ E 7°14′ E 5 9°01.7′ E 15°18.4′ E 131°13.8′ E
70 80 8 29 58 33 75 80
Dalarna/Sweden Wadi Arabah/Jordan Essen/Germany 4 Reyershausen/Germany 4 North Sea/Germany 4 Douz/Tunisia Sossusvlei/Namibia Amadeus Basin/Australia
10.8 8.1 18.4 17.8 6.8 5.3 11.6 7.5
0–77 0–24 0–46 0–50 0–38 0–33 0–42 0–155
87 81 61 64 99 64 80 93
6
7
11 25 1 1 20 5
19 10
Cc, Go, (Ka) Chl (Hem) Gyp, Cc, (Do) (Il), (Hem)
Calculated using calibration of Libowitzky and Rossman (1997). 2 Cc = Calcite, Chl = Chlorite, Do = Dolomite, Fs = Feldspar, Go = Goethite, Gyp = Gypsum, Hem = Hematite, Il = Ilmenite, Ka = Kaolinite, Mi = Mica. Bold = N20%, () = very minor. 3 Approximate coordinates. Sample is from the area of Mångsbodarna and was derived from the collection of the Swedish Museum of Natural History/Stockholm. 4 See Stalder and Neuser (2013). 5 Average for several samples in the range 53°41′–53°45′ and 6°59′–7°29′; see Stalder and Neuser (2013).
R. Stalder / Sedimentary Geology 307 (2014) 1–6
Fig. 1. IR spectra (E||no − E||ne averaged spectra for all samples). For identification all data points are labelled with numbers (Table 1, first column). The two subarkose sandstones from Germany (from Stalder and Neuser, 2013) are treated as one sample due to their very similar properties.
calculated as an average of all analysed grains and as average of all samples. Both values are in accord with an average of 10 ± 1 wt ppm water (Fig. 3). In all investigated samples the most abundant group is the waterpoor fraction (0–5 wt ppm water). The percentage of this group corresponds well with the average OH-defect contents determined from the averaged spectra (Fig. 3), indicating that the differences in average OH-defect content are not significantly biased by the counting statistics, as the OH-defect content is reflected by the percentage of low-defect grains and is only insignificantly affected by the counting statistics of the defect-rich grains. Average OH-defect water contents are also estimated from a broken power law distribution, in good agreement with the values determined from the average spectra, yielding comparable results as calculated from the averaged spectra (Fig. 4). A simple correlation between OH-defect content and sedimentation age is not obvious. It has previously been observed that quartz from igneous rocks contains significantly higher amounts of OH-defects than quartz annealed under metamorphic conditions (Müller and Koch-Müller, 2009; Stalder and Neuser, 2013). In this context the equal distribution in defect-poor (b 5 ppm defect water) and defect-rich (N5 ppm defect water) quartz grains reflects the average composition of the continental crust, in which quartz from igneous rocks on the one hand, and non-igneous rocks (sedimentary and metamorphic) on the other, occur in an approximate proportion of 1:1 (Wedepohl, 1995). 3.2. Monitor for weathering, diagenesis or crustal sampling? An important observation from our data set is that differences in average OH-defect content from one large reservoir to another do occur. In this context, several observations may be significant: (1) Samples with a higher modal proportion of quartz (amongst the detrital grains) have on average lower OH-defect contents (Fig. 5), (2) on average quartz grains from sandstones tend to show higher average OH-defect contents than grains from recent sands, (3) the oldest and most compacted sandstone investigated here (1400 Ma old Dala sandstone from Dalarna/Sweden) contained three of the four most OH-defect-rich grains and fewer OHdefect-poor grains than average. Based on the first observation, one seemingly apparent factor is the maturity of the sediment. Possible processes behind this correlation may be transport, weathering and chemical diffusion. It is, however, very unlikely that quartz grains partly lose their OH-defects by chemical diffusion during weathering, transport, and sedimentation, because the
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relevant parameters – time and temperature – are much less efficient to affect the OH-defect content than during diagenesis, where temperatures are higher and time spans longer. In addition, if diffusion were a relevant mechanism for OH-defect loss, larger detrital grains would show more OH-defect contents, which is not observed (Stalder and Neuser, 2013). Though, weathering and transport could be a relevant factor, if we assume that defect-poor quartz grains were more resistant to chemical and/or mechanical weathering, they would preferentially survive in the more coarse-grained fraction, which was vaguely observed by tendency in Stalder and Neuser (2013). By contrast, if the OH-defect content would only reflect the maturity, the characterisation method elaborated in this study would not be very useful, since the maturity can be determined by much easier and more established methods, such as microscopy or X-ray powder diffraction. Another factor that may have to be taken into consideration is diagenesis. Generally, the mobility of defect protons depends on temperature and, therefore, should be higher during diagenesis than during weathering and transport. At igneous and high-grade metamorphic conditions defect protons are highly mobile and allow equilibration even on laboratory time scales (e.g., Rovetta et al., 1986), but low temperature diffusion data applicable for diagenesis and low-grade metamorphic conditions are not available. From the thermodynamical point of view, the formation of defects is also generally favoured by high temperature, and it is not expected that high levels of OH-defects can form at low temperature and low (water) pressure. The exact equilibration concentration of the different OH-defects at low temperature is unknown, but it is estimated that this value is well within the “defectpoor” fraction. If OH-defects are incorporated at diagenetic conditions, at best the data points that plot in the left of Fig. 4 may show a tendency to be shifted to higher values, leading to a steeper trend there. This tendency is vaguely observed for the sandstone from Dalarna/Sweden, but this feature could also reflect the characteristic of the original detrital grains. Furthermore, if diagenesis affected the OH-defect inventory of the defect-poor grains, it should also affect the defects in the defectrich grains towards equilibrium (i.e., low concentration), which is not observed. Therefore, it seems rather improbable that diagenesis exerts a major control over the observed spectra of OH-defects in detrital quartz. Lastly, the influence of provenance and sampling of the original crustal level has to be discussed. If the North Sea beach sand and the Dala sandstone from Dalarna/Sweden are compared, the material for both sedimentary reservoirs is mainly derived from multiple sources of Proterozoic (1.8 Ga old) rocks from the Scandinavian subcontinent (Pulvertaft, 1985; Schüttelhelm and Laban, 2005), in the case of the North Sea mixed with a minor component from the Caledonian orogen and some river discharge from the South. However, the quartz grains in the Dala sandstone have on average significantly higher OH-defect contents and significantly fewer defect-poor grains. Apart from the aforementioned factors of weathering, transport, and diagenesis, the difference between both samples is the different level of crustal section that was sampled. In the case of the Dala sandstone, the upper section of a continental crust with comparable young igneous rocks was sampled, in the case of the North Sea, the deep part (approximately 10–15 km depth according to the present metamorphic grade in surface rocks, Sjöström and Bergman, 1998; Väisänen et al., 2000) of the old Fennoscandian continental shield and high pressure metamorphic rocks from the Caledonian orogen were sampled. This interpretation is consistent with experimental results from granitic systems, where low pressure favours generation of the most frequently observed OHdefect (Stalder and Konzett, 2012; Baron et al., submitted for publication). In the case of the North African–Arabian samples, the Cambrian sandstone from Jordan belongs to the most voluminous siliciclastic body ever deposited on continental crust, and was originally supplied via the Gondwana superfan system (Meinhold et al., 2013) from Neoproterozoic terranes and a more distal source farther to the south
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Fig. 2. Histograms showing the OH-defect contents in detrital quartz grains. Values for the N50 ppm fraction: Sweden: 53, 66 and 77 ppm, Australia: 155 ppm.
containing a larger proportion of older rocks that were transported over very long distance (Kolodner et al., 2006; Morag et al., 2011). The Sahara desert sand is mainly recycled material from Phanerozoic sandstones from North Africa and exposed basement to the south, modified by loss of fine-grained material by aerial transport. If the aforementioned interpretation concerning the mechanical weathering is correct, the fine-grained fraction is enriched in defect-rich grains, and therefore
the defect-rich grains in the remaining coarser fractions become preferentially depleted. Alternatively, if the influence of mechanical weathering was not the dominant factor, the low average defect content probably would indicate which crustal section was sampled, in particular that the average sand grain from the Sahara was equilibrated at greater depths (more deeply eroded continental surface) than the investigated Cambrian sandstone from Jordan. It is noteworthy that the
R. Stalder / Sedimentary Geology 307 (2014) 1–6
Fig. 3. Correlation of the percentage of “low-defect water” grains versus average OH-defect pffiffiffi content. Error is calculated as NN . Data points are labelled with numbers according to Table 1 (first column). The two subarkose sandstones from Germany (from Stalder and Neuser, 2013) are treated as one sample due to their very similar properties.
grains from the Cambrian sandstone from Jordan show a pattern very similar to the characteristics of all analysed grains (Figs. 2–4), justifying the assumption that the grains analysed are representative of the Earth's crust. Only with respect to maturity, the Jordan sandstone is not representative for all of the analysed samples (Fig. 5). Another sample that can be considered representative for all grains (here analysed) is the dune sand from Sossusvlei/Namibia (Figs. 2–5). Ultimately, it is derived from the drainage of the Orange River and transported northwards by longshore currents in the South Atlantic (Garzanti et al., 2012). In its drainage area of nearly 106 km2, Neoarchean metavolcanic rocks, Proterozoic meta-volcanics and -sediments and Phanerozoic sediments and basalts are exposed (de Villiers et al., 2000; Becker et al., 2006). As part of the Namib desert, its material can be considered as crustal average for southern Africa. In contrast to the Namib desert, the material in the Amadeus Basin in Australia was predominantly derived from a more confined source, the Proterozoic Musgrave complex (Camacho et al.,
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Fig. 5. Compositional maturity (plotted as Mica and Feldspar content) against average OHpffiffiffi defect content (error calculated as NN). Data points are labelled with numbers according to Table 1 (first column).
2002). Here again, the low average OH-defect content probably reflects the high proportion of metamorphic rocks from mid-crustal levels. Even if several different interpretations are possible from the current data set (and probably several factors contribute to the observed patterns), it is illustrated that the method outlined here is promising as a novel tool to characterise quartz in siliciclastic samples. If the detrital quartz fraction was selectively modified by mechanical weathering, this would mean that a distinct group of quartz grains selectively left the detrital inventory, and any study based on the analysis of quartz (even if other methods such as cathodoluminescence (CL) are used) would be biased. Future work has to be concentrated on a series of samples of a selected detrital sediment system from different stratigraphic units and with different properties (such as grain size and maturity) in order to fully understand the interplay of the different processes that contribute to the evolution of OH-defects in detrital quartz grains.
Fig. 4. Double-logarithmic plot of % grains against defect water content. For the most water-rich and the most water-poor sample a broken power law analysis was performed, yielding very similar results (broken lines: Tunisia: 4 wt ppm, Germany: 17 wt ppm) as calculated from the average spectra. Contents below 1 ppm are not plotted due to large uncertainty.
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4. Conclusions In this study a novel tool to characterise the quartz inventory in siliciclastic sediments based on the distribution of the OH-defect content is presented. In detail, it is observed that (1) Significant differences even between different large and supposedly well-mixed reservoirs exist. (2) Single detrital grains contain up to 160 wt ppm water as OHdefects. This amount has hitherto been only observed in quartz crystals from water-saturated high-pressure experiments. (3) Detrital quartz grains contain on average 10 wt ppm water as OH-defects. (4) The average OH-defect content in detrital quartz shows a fair correlation to the modal quartz proportion in the detrital grains, suggesting that defect-rich grains are less resistant to mechanical weathering. Furthermore it is hypothesized that (5) The OH-defect contents of quartz may be used as an information source for the sampled crustal level. Acknowledgements The author gratefully acknowledges Peter Mirwald, Ralf Tappert, Burkhard Schmidt and Marzena Kohut for providing samples from Wadi Arabah, Amadeus Basin, Sossusvlei and Douz, respectively. Daniela Schmidmair is thanked for support with XRD analyses and Franz Gartner for preparing thick sections and grain mounts. Guido Meinhold is thanked for valuable comments and communicating about his work. Two anonymous reviewers are thanked for constructive suggestions. References Aines, R.D., Rossman, G.R., 1984. Water in minerals? A peak in the infrared. Journal of Geophysical Research 89, 4059–4071. Augustsson, C., Reker, A., 2012. Cathodoluminescence spectra of quartz as provenance indicators revisited. Journal of Sedimentary Research 82, 559–570. Bambauer, H.U., 1961. Spurenelementgehalte und γ-Farbzentren in Quarzen aus Zerrklüften der Schweizer Alpen. Schweizerische Mineralogische und Petrographische Mitteilungen 41, 335–369. Baron, M.A., Stalder, R., Konzett, J., Hauzenberger, C.A., 2014. Formation conditions of quartz recorded by OH-point defects — experimental and analytical approach. Physics and Chemistry of Minerals (submitted for publication). Becker, T., Schreiber, U., Kampunzu, A.B., Armstrong, R., 2006. Mesoproterozoic rocks of Namibia and their plate tectonic setting. Journal of African Earth Sciences 46, 112–140. Boggs, S., Kwon, Y.I., Goles, G.G., Rusk, B.G., Krinsley, D., Seyedolali, A., 2002. Is quartz cathodoluminescence color a reliable provenance tool? A quantitative examination. Journal of Sedimentary Research 72, 408–415. Camacho, A., Hensen, B.J., Armstrong, R., 2002. Isotopic test of a thermally driven intraplate orogenic model, Australia. Geology 30, 887–890. De Villiers, S., Compton, J.S., Lavelle, M., 2000. The strontium isotope systematics of the Orange river, Southern Africa. South African Journal of Geology 103, 237–248. Dennen, W.H., Blackburn, W.H., Quesada, A., 1970. Aluminium in quartz as a geobarometer. Contributions to Mineralogy and Petrology 27, 332–342. Doukhan, J.C., Trepied, L., 1985. Plastic deformation of quartz single crystals. Bulletin de Mineralogie 108, 97–123.
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OH-defect content in detrital quartz grains as an archive for crystallisation conditions Roland Stalder Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52f, A-6020 Innsbruck, Austria
a r t i c l e
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Article history: Received 12 March 2014 Received in revised form 9 April 2014 Accepted 10 April 2014 Available online 18 April 2014 Editor: J. Knight Keywords: Quartz OH-defects IR spectroscopy Provenance
a b s t r a c t Infrared spectra of 433 oriented detrital quartz grains from different large sedimentary reservoirs worldwide are evaluated with respect to their OH-defect concentration (expressed as wt ppm water), which varies between 0 and 160 wt ppm for individual grains. OH-defect contents, averaged over each individual sample, range between 5 and 20 wt ppm defect water, with a mean value around 10 wt ppm. Sand samples tend to have lower average OH-defect contents than older sandstones deposited earlier from a similar source region. Three of the most OHdefect rich grains from our data set were observed in the oldest and the most strongly cemented sandstone investigated (1400 Ma old Dala sandstone from Dalarna/Sweden), suggesting that OH-defects are barely affected during diagenetic conditions even over geological time scales. Furthermore, the average OH-defect content of each sample is well correlated to its maturity. Combined with results from high-pressure experiments in different model systems, the new results may provide an indirect method to estimate the source rock inventory of the upper crust (igneous, hydrothermal or metamorphic) and depth of the sampled crustal section. Results further suggest that provenance analyses of mature sediments and sedimentary rocks could benefit from the analysis of OH-defect content in quartz. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Quartz is a stable mineral in most environments of the Earth's crust and therefore is a major constituent of many igneous, metamorphic and sedimentary rocks and the second most abundant mineral in the Earth's crust (Ronov and Yaroshevski, 1969). Amongst the rock forming minerals quartz is most resistant towards mechanical and chemical weathering and therefore the most abundant mineral in the sand fraction of clastic sediments and sedimentary rocks. Primarily, quartz mostly crystallizes in igneous systems such as granites, but a considerable amount may later be annealed under regional or thermal metamorphism, where impurities of trace metals and defect protons equilibrate according to the thermochemical factors of (1) pressure, (2) temperature and (3) chemical environment (Wark and Watson, 2006; Stalder and Konzett, 2012). Consequently, some metal impurities such as Al and Ti have been calibrated as a geobarometer or geothermometer, respectively (Dennen et al., 1970; Wark and Watson, 2006; Thomas et al., 2010; Huang and Audétat, 2012). Metal impurities in quartz are related to cathodoluminescence emission properties and are thus valuable for discrimination of the petrogenetic origin of the quartz (Matter and Ramseyer, 1985; Götze et al., 2001; Boggs et al., 2002; Müller et al., 2003; Augustsson and Reker, 2012).
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http://dx.doi.org/10.1016/j.sedgeo.2014.04.002 0037-0738/© 2014 Elsevier B.V. All rights reserved.
Another interesting information source with respect to the petrogenetic origin is the incorporation of trace protons as point defects, such as (1) the incorporation of defect protons by substitution of Si4+ by 4H+ (also referred to as “hydrogarnet” substitution, Doukhan and Trepied, 1985; Stalder and Konzett, 2012), (2) coupled substitutions involving Na+ and Li+ (Bambauer, 1961), (3) coupled substitutions involving Al3+ (Kats, 1962; Aines and Rossman, 1984), or (4) coupled substitutions involving B3+ (Miyoshi et al., 2005; Thomas, 2008). A powerful method to detect OH-defects down to the level of parts per million (ppm) is infrared (IR) spectroscopy (Aines and Rossman, 1984), and technical innovations such as the application of focal plane array (FPA) detectors enable spatial resolution down to a few μm (Prechtel and Stalder, 2010). A significant problem for the quantification of OH-defects and trace impurities is the fact that natural quartz usually contains water as fluid inclusions (Bambauer, 1961; Müller and Koch-Müller, 2009), which therefore render the analysis of OH-defects and other chemical impurities difficult. In contrast, polarised spectroscopic measurements on oriented grains are able to eliminate the anisotropic OH-defect signal from the isotropic water signal from the fluid inclusions. This procedure is possible, since the dipoles of nearly all OH-defects in quartz are aligned ||no enabling the distillation of the proper OH-defect signal by subtracting two polarised measurements (Stalder and Konzett, 2012; Stalder and Neuser, 2013). In order to get an overview of average detrital quartz in the Earth's crust, in this study some large sedimentary reservoirs were sampled
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and their quartz grains were measured by IR spectroscopy. In comparison to the time-consuming process of direct sampling of numerous individual localities, this strategy is analogous to early attempts to estimate the average chemical composition of the continental crust, where glacial deposits were used as a natural sampling procedure (e.g., Goldschmidt, 1933). An advantage to direct sampling from exposed geological bodies and drill cores is that the chosen sampling method is less biased by the accessibility of individual geological bodies and furthermore provides data from sources that have been long since eroded. Results therefore serve as archive for formation conditions of quartz over a long geological history, and are interpreted with respect to several aspects that may contribute to the observed spectrum of defect contents, in particular mechanical weathering, diagenesis, and thermophysical conditions under which the grains initially crystallized. 2. Samples and methods 2.1. Samples Samples analysed here were derived from two cemented sandstones, two friable sandstones, three desert sands, and one beach sand (Table 1). Samples originate from three different continents (Europe, Africa and Australia) and all reflect extended sedimentary reservoirs that are/ were supplied by multiple sources. With respect to their sedimentation age, the selected samples can be considered as representative of the age distribution of global sediments (Veizer and Jansen, 1985). Modal mineral compositions were determined by X-ray powder diffraction combined with Rietveld refinement with Topas (Bruker). The samples from Germany have previously been reported in a different context (Stalder and Neuser, 2013) and are considered here for comparison. 2.2. Sample preparation and infrared (IR) spectroscopy Samples from cemented sandstones were prepared as double polished thick sections. Samples from desert sands were prepared as double polished grain mounts. In order to produce wafers with properly exposed plateaux on both sides and with convenient thickness for IRspectroscopy, the 250–500 μm fraction was selected for preparation (in all three desert sands the 125–250 μm fraction was the most abundant). After preparation, samples were between 80 and 130 μm thick, each mount contained approximately 500 sand grains with random orientations. IR spectra were recorded at room temperature in transmission mode using a Bruker Vertex 70 FTIR spectrometer, coupled to a Hyperion 3000 microscope equipped with a liquid nitrogen-cooled MCT-detector, a globar light source, a KBr beamsplitter and a wiregrid IR-polariser and two polarisers for visible light. IR measurements were performed on selected quartz crystals cut parallel to the crystallographic c-axis from the thick sections or grain mounts according to their
interference colour. Measurements were performed with a spectral resolution of 2 cm−1 in the 550 to 7500 cm−1 range. Two measurements were performed on each grain, with the electric field vector E of the polarised radiation parallel to no, and with E parallel to ne on exactly the same spot by turning the polarizer by 90°. After data acquisition, spectral information of all measured grains was used to (1) omit minerals other than quartz, (2) check for the appropriate crystal orientation, and (3) eliminate the IR-signal for fluid inclusions by subtracting the spectra (E||no − E||ne). Step (1) was rarely necessary, since minerals other than quartz usually were recognized prior to the IR measurement based on their optical properties. For steps (1) and (2) the wavenumber range of the lattice overtones (between 1200 and 2200 cm−1) was applied. The number of successfully measured grains is given in Table 1. Subtracted spectra (no − ne) were corrected by a linear baseline between 3250 and 3610 cm−1. In a very few cases, when B-related OHabsorption bands (exhibiting absorption ||no and ||ne) were observed, spectra were corrected in the 3585–3605 cm−1 range, taking into account the total dipole intensity is defined as 2o + e (Stalder and Neuser, 2013). No fitting procedure was applied to the observed absorption bands, instead the defect water content (expressed as wt ppm water) was derived by multiplying the absorbance for each measured channel with an individual extinction coefficient according to Libowitzky and Rossman (1997). 3. Results and discussion 3.1. Systematics of OH-defects in detrital quartz Average IR spectra for all samples are shown in Fig. 1. In all samples the absorption triplet around 3375 cm−1, caused by coupled substitution involving Al3+ (Aines and Rossman, 1984), is the most prominent band. The absorption band at 3475 cm−1, caused by LiOH defects (Bambauer, 1961), is also clearly observed in some samples. B-related OH-defects (at 3595 cm−1) and hydrogarnet substitution (3585 cm−1) are not relevant for the total OH-defect concentration, but are detected in some single grains. Combined with the results from experimentally grown quartz crystals (Paterson, 1986; Stalder and Konzett, 2012), the overall low occurrence of the hydrogarnet substitution in detrital quartzes is in accord with the interpretation that quartz crystals of initially high-pressure origin (N 5 kbar) are of minor occurrence in sedimentary rocks. In contrast, the significant concentration of Li-defect may be used to estimate the proportion of late stage magmatic quartzes such as hydrothermal quartzes, typically derived from shallow crustal levels. OH-defect concentrations calculated for individual grains were arbitrarily subdivided into groups of 5 ppm steps (Fig. 2). Average OH-defect contents were calculated from averaged spectra (E||no − E||ne) of all grains from the respective sample, the resulting values varying between 5 and 20 wt ppm (Table 1). Based on these data, the average OH-defect content was
Table 1 Sample descriptions and OH-contents.
1 2 3 4 5 6 7 8 1
Sample
Sedimentation Locality period
Latitude
Longitude
N (crystals) Average OH content Range OH-content Modal mineralogy (wt ppm water)1 (wt ppm water)1 %Qz %Fs %Mi Other phases2
Subarkose Qz-arenite Subarkose Subarkose Beach sand Desert sand Desert sand Desert sand
Proterozoic Cambrian Carboniferous Triassic Quaternary Quaternary Quaternary Quaternary
61°04.7′ N 3 30°35′ N 51°23.8′ N 51°36.6′ N 53°43′ N 5 33°25.1′ N 24°44.3′ S 25°13.2′ S
13°37.3′ E 3 35°19′ E 7°03.0′ E 10°00.7′ E 7°14′ E 5 9°01.7′ E 15°18.4′ E 131°13.8′ E
70 80 8 29 58 33 75 80
Dalarna/Sweden Wadi Arabah/Jordan Essen/Germany 4 Reyershausen/Germany 4 North Sea/Germany 4 Douz/Tunisia Sossusvlei/Namibia Amadeus Basin/Australia
10.8 8.1 18.4 17.8 6.8 5.3 11.6 7.5
0–77 0–24 0–46 0–50 0–38 0–33 0–42 0–155
87 81 61 64 99 64 80 93
6
7
11 25 1 1 20 5
19 10
Cc, Go, (Ka) Chl (Hem) Gyp, Cc, (Do) (Il), (Hem)
Calculated using calibration of Libowitzky and Rossman (1997). 2 Cc = Calcite, Chl = Chlorite, Do = Dolomite, Fs = Feldspar, Go = Goethite, Gyp = Gypsum, Hem = Hematite, Il = Ilmenite, Ka = Kaolinite, Mi = Mica. Bold = N20%, () = very minor. 3 Approximate coordinates. Sample is from the area of Mångsbodarna and was derived from the collection of the Swedish Museum of Natural History/Stockholm. 4 See Stalder and Neuser (2013). 5 Average for several samples in the range 53°41′–53°45′ and 6°59′–7°29′; see Stalder and Neuser (2013).
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Fig. 1. IR spectra (E||no − E||ne averaged spectra for all samples). For identification all data points are labelled with numbers (Table 1, first column). The two subarkose sandstones from Germany (from Stalder and Neuser, 2013) are treated as one sample due to their very similar properties.
calculated as an average of all analysed grains and as average of all samples. Both values are in accord with an average of 10 ± 1 wt ppm water (Fig. 3). In all investigated samples the most abundant group is the waterpoor fraction (0–5 wt ppm water). The percentage of this group corresponds well with the average OH-defect contents determined from the averaged spectra (Fig. 3), indicating that the differences in average OH-defect content are not significantly biased by the counting statistics, as the OH-defect content is reflected by the percentage of low-defect grains and is only insignificantly affected by the counting statistics of the defect-rich grains. Average OH-defect water contents are also estimated from a broken power law distribution, in good agreement with the values determined from the average spectra, yielding comparable results as calculated from the averaged spectra (Fig. 4). A simple correlation between OH-defect content and sedimentation age is not obvious. It has previously been observed that quartz from igneous rocks contains significantly higher amounts of OH-defects than quartz annealed under metamorphic conditions (Müller and Koch-Müller, 2009; Stalder and Neuser, 2013). In this context the equal distribution in defect-poor (b 5 ppm defect water) and defect-rich (N5 ppm defect water) quartz grains reflects the average composition of the continental crust, in which quartz from igneous rocks on the one hand, and non-igneous rocks (sedimentary and metamorphic) on the other, occur in an approximate proportion of 1:1 (Wedepohl, 1995). 3.2. Monitor for weathering, diagenesis or crustal sampling? An important observation from our data set is that differences in average OH-defect content from one large reservoir to another do occur. In this context, several observations may be significant: (1) Samples with a higher modal proportion of quartz (amongst the detrital grains) have on average lower OH-defect contents (Fig. 5), (2) on average quartz grains from sandstones tend to show higher average OH-defect contents than grains from recent sands, (3) the oldest and most compacted sandstone investigated here (1400 Ma old Dala sandstone from Dalarna/Sweden) contained three of the four most OH-defect-rich grains and fewer OHdefect-poor grains than average. Based on the first observation, one seemingly apparent factor is the maturity of the sediment. Possible processes behind this correlation may be transport, weathering and chemical diffusion. It is, however, very unlikely that quartz grains partly lose their OH-defects by chemical diffusion during weathering, transport, and sedimentation, because the
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relevant parameters – time and temperature – are much less efficient to affect the OH-defect content than during diagenesis, where temperatures are higher and time spans longer. In addition, if diffusion were a relevant mechanism for OH-defect loss, larger detrital grains would show more OH-defect contents, which is not observed (Stalder and Neuser, 2013). Though, weathering and transport could be a relevant factor, if we assume that defect-poor quartz grains were more resistant to chemical and/or mechanical weathering, they would preferentially survive in the more coarse-grained fraction, which was vaguely observed by tendency in Stalder and Neuser (2013). By contrast, if the OH-defect content would only reflect the maturity, the characterisation method elaborated in this study would not be very useful, since the maturity can be determined by much easier and more established methods, such as microscopy or X-ray powder diffraction. Another factor that may have to be taken into consideration is diagenesis. Generally, the mobility of defect protons depends on temperature and, therefore, should be higher during diagenesis than during weathering and transport. At igneous and high-grade metamorphic conditions defect protons are highly mobile and allow equilibration even on laboratory time scales (e.g., Rovetta et al., 1986), but low temperature diffusion data applicable for diagenesis and low-grade metamorphic conditions are not available. From the thermodynamical point of view, the formation of defects is also generally favoured by high temperature, and it is not expected that high levels of OH-defects can form at low temperature and low (water) pressure. The exact equilibration concentration of the different OH-defects at low temperature is unknown, but it is estimated that this value is well within the “defectpoor” fraction. If OH-defects are incorporated at diagenetic conditions, at best the data points that plot in the left of Fig. 4 may show a tendency to be shifted to higher values, leading to a steeper trend there. This tendency is vaguely observed for the sandstone from Dalarna/Sweden, but this feature could also reflect the characteristic of the original detrital grains. Furthermore, if diagenesis affected the OH-defect inventory of the defect-poor grains, it should also affect the defects in the defectrich grains towards equilibrium (i.e., low concentration), which is not observed. Therefore, it seems rather improbable that diagenesis exerts a major control over the observed spectra of OH-defects in detrital quartz. Lastly, the influence of provenance and sampling of the original crustal level has to be discussed. If the North Sea beach sand and the Dala sandstone from Dalarna/Sweden are compared, the material for both sedimentary reservoirs is mainly derived from multiple sources of Proterozoic (1.8 Ga old) rocks from the Scandinavian subcontinent (Pulvertaft, 1985; Schüttelhelm and Laban, 2005), in the case of the North Sea mixed with a minor component from the Caledonian orogen and some river discharge from the South. However, the quartz grains in the Dala sandstone have on average significantly higher OH-defect contents and significantly fewer defect-poor grains. Apart from the aforementioned factors of weathering, transport, and diagenesis, the difference between both samples is the different level of crustal section that was sampled. In the case of the Dala sandstone, the upper section of a continental crust with comparable young igneous rocks was sampled, in the case of the North Sea, the deep part (approximately 10–15 km depth according to the present metamorphic grade in surface rocks, Sjöström and Bergman, 1998; Väisänen et al., 2000) of the old Fennoscandian continental shield and high pressure metamorphic rocks from the Caledonian orogen were sampled. This interpretation is consistent with experimental results from granitic systems, where low pressure favours generation of the most frequently observed OHdefect (Stalder and Konzett, 2012; Baron et al., submitted for publication). In the case of the North African–Arabian samples, the Cambrian sandstone from Jordan belongs to the most voluminous siliciclastic body ever deposited on continental crust, and was originally supplied via the Gondwana superfan system (Meinhold et al., 2013) from Neoproterozoic terranes and a more distal source farther to the south
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Fig. 2. Histograms showing the OH-defect contents in detrital quartz grains. Values for the N50 ppm fraction: Sweden: 53, 66 and 77 ppm, Australia: 155 ppm.
containing a larger proportion of older rocks that were transported over very long distance (Kolodner et al., 2006; Morag et al., 2011). The Sahara desert sand is mainly recycled material from Phanerozoic sandstones from North Africa and exposed basement to the south, modified by loss of fine-grained material by aerial transport. If the aforementioned interpretation concerning the mechanical weathering is correct, the fine-grained fraction is enriched in defect-rich grains, and therefore
the defect-rich grains in the remaining coarser fractions become preferentially depleted. Alternatively, if the influence of mechanical weathering was not the dominant factor, the low average defect content probably would indicate which crustal section was sampled, in particular that the average sand grain from the Sahara was equilibrated at greater depths (more deeply eroded continental surface) than the investigated Cambrian sandstone from Jordan. It is noteworthy that the
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Fig. 3. Correlation of the percentage of “low-defect water” grains versus average OH-defect pffiffiffi content. Error is calculated as NN . Data points are labelled with numbers according to Table 1 (first column). The two subarkose sandstones from Germany (from Stalder and Neuser, 2013) are treated as one sample due to their very similar properties.
grains from the Cambrian sandstone from Jordan show a pattern very similar to the characteristics of all analysed grains (Figs. 2–4), justifying the assumption that the grains analysed are representative of the Earth's crust. Only with respect to maturity, the Jordan sandstone is not representative for all of the analysed samples (Fig. 5). Another sample that can be considered representative for all grains (here analysed) is the dune sand from Sossusvlei/Namibia (Figs. 2–5). Ultimately, it is derived from the drainage of the Orange River and transported northwards by longshore currents in the South Atlantic (Garzanti et al., 2012). In its drainage area of nearly 106 km2, Neoarchean metavolcanic rocks, Proterozoic meta-volcanics and -sediments and Phanerozoic sediments and basalts are exposed (de Villiers et al., 2000; Becker et al., 2006). As part of the Namib desert, its material can be considered as crustal average for southern Africa. In contrast to the Namib desert, the material in the Amadeus Basin in Australia was predominantly derived from a more confined source, the Proterozoic Musgrave complex (Camacho et al.,
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Fig. 5. Compositional maturity (plotted as Mica and Feldspar content) against average OHpffiffiffi defect content (error calculated as NN). Data points are labelled with numbers according to Table 1 (first column).
2002). Here again, the low average OH-defect content probably reflects the high proportion of metamorphic rocks from mid-crustal levels. Even if several different interpretations are possible from the current data set (and probably several factors contribute to the observed patterns), it is illustrated that the method outlined here is promising as a novel tool to characterise quartz in siliciclastic samples. If the detrital quartz fraction was selectively modified by mechanical weathering, this would mean that a distinct group of quartz grains selectively left the detrital inventory, and any study based on the analysis of quartz (even if other methods such as cathodoluminescence (CL) are used) would be biased. Future work has to be concentrated on a series of samples of a selected detrital sediment system from different stratigraphic units and with different properties (such as grain size and maturity) in order to fully understand the interplay of the different processes that contribute to the evolution of OH-defects in detrital quartz grains.
Fig. 4. Double-logarithmic plot of % grains against defect water content. For the most water-rich and the most water-poor sample a broken power law analysis was performed, yielding very similar results (broken lines: Tunisia: 4 wt ppm, Germany: 17 wt ppm) as calculated from the average spectra. Contents below 1 ppm are not plotted due to large uncertainty.
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4. Conclusions In this study a novel tool to characterise the quartz inventory in siliciclastic sediments based on the distribution of the OH-defect content is presented. In detail, it is observed that (1) Significant differences even between different large and supposedly well-mixed reservoirs exist. (2) Single detrital grains contain up to 160 wt ppm water as OHdefects. This amount has hitherto been only observed in quartz crystals from water-saturated high-pressure experiments. (3) Detrital quartz grains contain on average 10 wt ppm water as OH-defects. (4) The average OH-defect content in detrital quartz shows a fair correlation to the modal quartz proportion in the detrital grains, suggesting that defect-rich grains are less resistant to mechanical weathering. Furthermore it is hypothesized that (5) The OH-defect contents of quartz may be used as an information source for the sampled crustal level. Acknowledgements The author gratefully acknowledges Peter Mirwald, Ralf Tappert, Burkhard Schmidt and Marzena Kohut for providing samples from Wadi Arabah, Amadeus Basin, Sossusvlei and Douz, respectively. Daniela Schmidmair is thanked for support with XRD analyses and Franz Gartner for preparing thick sections and grain mounts. Guido Meinhold is thanked for valuable comments and communicating about his work. Two anonymous reviewers are thanked for constructive suggestions. References Aines, R.D., Rossman, G.R., 1984. Water in minerals? A peak in the infrared. Journal of Geophysical Research 89, 4059–4071. Augustsson, C., Reker, A., 2012. Cathodoluminescence spectra of quartz as provenance indicators revisited. Journal of Sedimentary Research 82, 559–570. Bambauer, H.U., 1961. Spurenelementgehalte und γ-Farbzentren in Quarzen aus Zerrklüften der Schweizer Alpen. Schweizerische Mineralogische und Petrographische Mitteilungen 41, 335–369. Baron, M.A., Stalder, R., Konzett, J., Hauzenberger, C.A., 2014. Formation conditions of quartz recorded by OH-point defects — experimental and analytical approach. Physics and Chemistry of Minerals (submitted for publication). Becker, T., Schreiber, U., Kampunzu, A.B., Armstrong, R., 2006. Mesoproterozoic rocks of Namibia and their plate tectonic setting. Journal of African Earth Sciences 46, 112–140. Boggs, S., Kwon, Y.I., Goles, G.G., Rusk, B.G., Krinsley, D., Seyedolali, A., 2002. Is quartz cathodoluminescence color a reliable provenance tool? A quantitative examination. Journal of Sedimentary Research 72, 408–415. Camacho, A., Hensen, B.J., Armstrong, R., 2002. Isotopic test of a thermally driven intraplate orogenic model, Australia. Geology 30, 887–890. De Villiers, S., Compton, J.S., Lavelle, M., 2000. The strontium isotope systematics of the Orange river, Southern Africa. South African Journal of Geology 103, 237–248. Dennen, W.H., Blackburn, W.H., Quesada, A., 1970. Aluminium in quartz as a geobarometer. Contributions to Mineralogy and Petrology 27, 332–342. Doukhan, J.C., Trepied, L., 1985. Plastic deformation of quartz single crystals. Bulletin de Mineralogie 108, 97–123.
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