The cooling rate of the El gygytgyn impact glass

Transcription

1 Meteoritics & Planetary Science 48, Nr 7, (2013) doi: /maps The cooling rate of the El gygytgyn impact glass Ulrike RANTZSCH 1*, Thomas HABER 2, Detlef KLIMM 3, and Gert KLOESS 1 1 Institute of Mineralogy, Crystallography and Materials Science, Leipzig University, Scharnhorststrasse 20, Leipzig D-04275, Germany 2 Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia 3 Leibniz Institute for Crystal Growth, Max-Born-Strasse 2, Berlin D-12489, Germany * Corresponding author. rantzsch@uni-leipzig.de (Received 08 June 2011; revision accepted 29 May 2013) Abstract The thermal history of an impact glass from the El gygytgyn impact structure, central Chukotka, Russia has been investigated with the method of relaxation geospeedometry. The cooling rate of the melt fragment has been quantified by heat capacity (c p ) measurements using differential scanning calorimetry. Cycles of thermal treatments applied to the melt fragment sample result in a set of unique kinetic parameters, which were then used to model heat capacity curves. The Tool Narayanaswamy Moynihan approach was used to quantify the natural cooling rate from changes in the normalized heat capacity curves. Cooling rates ranging from 0.06 to 0.40 K min 1 were found by modeling the structural relaxation within the glass transition interval. Because of the absence of thermal reheating, a single natural annealing process is thought to have been responsible for the modeled cooling rate, even though a heat buffering process that affects the cooling history might have been involved. The El gygytgyn impact glass cooled significantly slower than previously studied tektites and faster than most volcanic glasses. The calculated viscosity of log g = Pa s at the glass transition temperature T peak g (1020 K) supports this assumption. Consequently, a slow cooling process is accompanied by a high viscosity g. INTRODUCTION Numerous investigations carried out in the last decades demonstrated that the quantification of the enthalpy relaxation of natural glasses is an effective approach to obtain cooling rates (e.g., Gottsmann and Dingwell 2001, 2002; Gottsmann et al. 2004). This method has been discussed in detail for volcanic glasses and tektites by Dingwell (1995) and Wilding et al. (1995, 1996a), but there is a lack of data concerning impact glasses. Field-based measurements of the natural quench rates of impact glasses are not available. Modeling of the cooling rate across the glass transition interval allows cooling rates to be estimated by monitoring the relaxation process through measurement of the change of enthalpy (DH) using differential scanning calorimetry (DSC) measurements. Studies of the glass relaxation processes are of particular importance in describing cooling processes in nature. In this paper, we present modeled cooling rates applicable to the El gygytgyn impact glass from the Chukotka region, Russia. Comparing these cooling rates obtained for the El gygytgyn impact glass with cooling rates of volcanic glasses and tektites, this study adds information to their thermal histories. The study presents the first quantification of the cooling rate of an impact glass by applying the method of geospeedometry. GEOLOGICAL BACKGROUND The El gygytgyn impact glass is the product of an impact event that occurred in the Chukotka region, Russia. The age of this event was determined to be Ma by K-Ar dating (Gurov et al. 1979). Analyses of El gygytgyn impact glasses yielded an age of Ma by 40 Ar- 39 Ar dating (Layer 2000). A recent revision of the terrestrial impact database did not list the age of the crater as being well constrained 1351 The Meteoritical Society, 2013.

2 1352 U. Rantzsch et al. (Jourdan et al. 2009). The crater is one of the bestpreserved impact structures in the world. It is a circular basin approximately 18 km in diameter and currently filled by water. Within the crater, and around the present lake, lacustrine terraces of about 80 m are present. These terraces are caused by changing levels of the lake. Within terrace deposits, shock-metamorphosed rocks and impact melt rocks are present within the crater (Gurov and Gurova 1979, 1991; Gurov and Koeberl 2004). Streams of terraces represent the main deposit for the glassy melt fragments, outside of the crater. The first research at the El gygytgyn impact crater, carried out in the 1970s and 1980s, focused on the petrography and geochemistry of the target sequences and impact products (e.g., Gurov et al. 1978, 1979), which are still a matter of interest (Adolph and Deutsch 2010). In 2009, it was the subject of a large international drilling project (by ICPD) (Koeberl et al. 2013). The meteorite impacted a sequence of volcanic rocks and tuffs predominantly of siliceous composition. The preimpact stratigraphy was determined by Gurov and Gurova (1991). At the top, a sequence of ignimbrites (250 m) followed by tuffs and lava of rhyolites (200 m) and tuffs and lava of andesites (70 m) are present. At the bottom, ash tuff and welded tuff of rhyolites and dacites ( 100 m) exist (Gurov and Koeberl 2004). The El gygytgyn impact glass investigated for this study is cm in size (Fig. 1). The impact glass is composed of black, semitransparent glass with a few mineral fragments (quartz) and cracks on the rough exterior surface. The impact glass has a preserved aerodynamically shaped form. The interior of the glassy sample is more homogeneous, with no mineral inclusions, but rare small pores. The sample is preserved with original exterior. No information about the original source location of the El gygytgyn impact glass is available. METHODS Theoretical Background of Structural Relaxation Processes as Geospeedometer Numerous studies have been published concerning the structural relaxation of a melt toward the glass transition (e.g., Davies and Jones 1953; Narayanaswamy 1971; Moynihan et al. 1974). Studies of the glass relaxation processes are of interest in describing cooling processes in nature. The theoretical and mathematical background of the application of the structural relaxation method as a geospeedometer has been already discussed in detail (e.g., Wilding et al. 1995; Fig. 1. Aerodynamically shaped El gygytgyn impact glass with quartz on surface. Gottsmann and Dingwell 2001). Here, we describe the method of geospeedometry briefly. When a melt passes through the glass transition, its physical properties change considerably. The influence of previously applied heat and cooling cycles as a memory effect on the structural behavior of glass was discussed in detail by Goldstein (1964). To understand the glass transition, the concept of the fictive temperature T f was applied and first described by Tool and Eichlin (1931). T f is used to compare the nonequilibrium structure of the glass with the structure of the liquid phase in equilibrium, e.g., to describe the change in a macroscopic property Dp (e.g., enthalpy DH) in successive thermal treatments. The relaxation process can be described using the Kohlrausch Williams Watts (KWW) function in Equation 1 to depict the relaxation property p (e.g., Moynihan et al. 1974, 1976; DeBolt et al. 1976; Scherer 1984). p ¼ p o exp t! b s 0k The degree of rearrangement of the structure of the glass (represented by the macroscopic property p) from a nonequilibrium state (with p = p 0 ) toward equilibrium after a given time (t) is determined by the relaxation time s 0k. Moreover, the constant b (0 < b < 1) specifies the nonlinear and nonexponential character of the relaxation process (e.g., Moynihan et al. 1974, 1976; DeBolt et al. 1976; Moynihan 1995). The change of a structural property like the enthalpy (DH) along a temperature path across the glass transition interval could be equalized to the change of (1)

3 Cooling rate, El gygytgyn, geospeedometry 1353 the fictive temperature (DT f ) along this temperature path (Moynihan et al. 1976). To allow this equalization, the change of the property with temperature, in our case DH/DT = c p, has to be normalized to the value of 0 within the glass state and to the value of 1 within the supercooled liquid (e.g., Gottsmann and Dingwell 2002). For that reason, the development of T f will be generated with the approach from DeBolt et al. (1976) in Equation 2, which is used for further modeling of T f across the glass transition. Within this extended form of the KWW function (e.g., Scherer 1984), the starting temperature is represented by T 0 and the time (t) of Equation 1 is replaced by the term DT/ q, whereas DT is the temperature step and q the heating or, respectively, the quench rate. T f;m ¼ T 0 þ Xm j¼1 2 0 DT j 41 exp@ Xm k¼j DT k jq k js 0k! b 13 A5 (2) The relaxation time (s 0k ) is calculated using the Tool Narayanaswamy Moynihan approach in Equation 3 (e.g., Tool 1946; Narayanaswamy 1971, 1988; DeBolt et al. 1976). s 0k ¼ s 0 exp ndh RT k þ ð1 nþdh RT f;m 1 This modified Arrhenius equation contains a temperature-dependent (ξdh*/rt k ), and a structuraldependent part (1 ξ)dh*/rt f,m 1. Furthermore, s 0 is the characteristic relaxation time, DH* is the activation enthalpy of the relaxation process, and R is the ideal gas constant. The empirical factor ξ (0 < ξ < 1) depicts the changes in the structure, more precisely, the effect of the fictive temperature during the relaxation process (DeBolt et al. 1976; Wilding et al. 1995). A fourparameter model (parameters: s 0, ξ, DH, b) results using Equations 2 and 3. Differential Scanning Calorimetry The specific heat capacity (c p ) traces of the impact glass sample were obtained using DSC. The DSC measurements were conducted at the Leibniz Institute for Crystal Growth, Berlin, using a Netzsch DSC 449C instrument. For DSC measurements, the El gygytgyn impact glass sample was cut using a diamond coring drill. For the measurements, a pristine unaltered glass fragment from the interior of the bulk sample, with dimensions of mm, was used. To ensure an optimal fit (3) within the crucible base, the El gygytgyn impact glass sample was cut as cuboid. For all measurements, the same sample and Pt/Rh crucibles were used. The mg cuboid was placed in the crucible, which was closed with a platinum lid. The calorimeter was calibrated using a mm cuboid of a synthetic crystal of sapphire. The reference material had the same geometry as the sample to ensure compatibility. All measurements were performed under an argon/air mix to avoid oxidation of the sample and to guarantee the highest possible sensitivity of DSC. The impact glass sample underwent a series of thermal treatments with heating above the glass transition to ensure the structural relaxation. The c p -trace of the data obtained from the initial heating is influenced by the original cooling rate and therefore contains information on the natural cooling rate. This information can be obtained by modeling the natural cooling c p -path using the model described above. Furthermore, to determine the parameters for the model via some calibration curves, the sample underwent a set of thermal treatments at matching heating and cooling rates (with q 40, 20, and 10 K min 1 ). Isothermal holds at 673 K before heating and at 1473 K before cooling treatments were performed. These ensured the thermal equilibration of all components within the oven and were also needed for the transformation of the measured DSC data to c p values. Furthermore, the isothermal hold at 1473 K (well above the glass transition) ensured the complete structural relaxation of the sample. Normally, the glass transition temperature T g is rather defined as interval than one defined temperature. In DSC measurements, it is determined as extrapolated onset or as peak temperature (T peak g ) used for the concept of geospeedometry (e.g., Scherer 1984; Stevenson et al. 1995; Gottsmann and Dingwell 2002). Figure 2 shows the obtained heat capacity curves during heating as a function of temperature. For applying the four-parameter geospeedometry model, a distinct end temperature of the glass transition is necessary. The heat capacity curves are normalized in the glass area to 0 and in the supercooled liquid field to 1. The heat capacity curve of the first heating shows no distinct end of the glass transition (Fig. 2). Thus, two normalizations of the curve were performed, one for the maximum (T max = 1112 K) and the minimum (T min = 1058 K) ending temperature of the glass transition interval (normalized heat capacity curves displayed in Fig. 3). The activation enthalpy DH* and the characteristic relaxation time s 0 as starting parameters in the Tool Narayanaswamy Moynihan model (Equations 2 and 3) are calculated using the data obtained from the measurements with the known cooling and heating

4 1354 U. Rantzsch et al. Fig. 2. Set of the measured heat capacity (c p ) curves as a function of the temperature for the El gygytgyn impact glass. The raw curve was generated upon first heating in the DSC at 10 K min 1. The glass transition temperature (T peak g ) for the raw curve is located at 1020 K. Curves with subsequent heating and cooling rates of 10, 20, and 40 K min 1 are displayed. A decrease of the c p -maximum in the glass transition with decreasing heating/cooling rates is obvious. Fig. 4. Relation between applied heating/cooling rates q and the obtained glass transition temperature T peak g for the calibration curves (shown as circles). The obtained fit parameters (dotted line) are used to calculate the activation enthalpy DH* and the characteristic relaxation time s 0 via Equation 4. ln jqj ln A þ DH R 1 T peak g (4) While DH* and s 0 are obtained using Equation 4 (see Fig. 4), the parameters ξ and b are quantified by adjusting modeled curves to the experimentally obtained data with known heating and cooling histories. Using these parameters, it was possible to calculate the fictive temperature T f imprinted through the natural cooling rate by adjusting the model to the normalized heat capacity curves. In a final step, the cooling path has to be modeled, which reflects the imprinted fictive temperature T f by variation of the cooling rate. Consequently, the cooling rate of the El gygytgyn impact glass is then represented by the cooling rate that leads to the imprinted fictive temperature T f. Figure 3 shows the modeled natural cooling and raw heating curves. Chemical Analysis Fig. 3. Normalized heat capacity curves with different temperatures for normalization: a) at T max = 1112 K, b) at T min = 1058 K, and the adjusted raw heating and cooling curves for the El gygytgyn impact glass. The cooling rates ranging from 0.06 to 0.40 K min 1 were modeled. history via Equation 4 where ln A = ln s 0 (Gottsmann et al. 2004). That is due to the concept of thermorheological simplicity (Narayanaswamy 1988). The homogeneous El gygytgyn impact glass sample was analyzed using electron microprobe analysis before and after the thermal treatments. These area analyses were conducted using a CAMECA SX 100 at the Institute of Mineralogy, Crystallography and Materials Science (Leipzig University) with quantitative wavelength-dispersive X-ray fluorescence analysis. The microprobe was operated at an acceleration voltage of 15 kv and a sample current of 20 na and a beam

5 Cooling rate, El gygytgyn, geospeedometry 1355 Table 1. Comparison between sample composition of the El gygytgyn impact glass before and after differential scanning calorimetry (DSC) and data by Gurov and Koeberl (2004). Values are given in wt% with the corresponding standard deviation of 20 analyses in parentheses. Major elements Pre-DSC Post-DSC diameter of 9.0 lm. The means, calculated from 20 analyses, as well as the standard deviations are presented in Table 1. The H 2 O content on a different fragment from our glass sample was determined by means of hightemperature catalytic combustion at the Geoforschungszentrum Potsdam (GFZ) using a Vario EL III instrument. Viscosity Determination at the Glass Transition Temperature T g peak The viscosity at the glass transition temperature can be linked to the temperature via the Arrhenius Andrade equation (Andrade 1930). g ¼ g 0 exp DH R T Gurov and Koeberl (2004) a SiO (0.28) (0.28) TiO (0.04) 0.35 (0.04) 0.32 Al 2 O (0.15) (0.15) 15.2 FeO* 2.68 (0.17) 2.68 (0.17) 2.46 b MnO n.d. n.d MgO 1.12 (0.04) 1.16 (0.04) 0.87 CaO 2.76 (0.09) 2.79 (0.09) 2.42 Na 2 O 2.80 (0.10) 2.61 (0.09) 2.78 K 2 O 4.07 (0.14) 4.06 (0.14) 3.99 Total c /99.17 d a Average chemical composition of impact melt rocks (n = 8) from El gygytgyn impact crater obtained by wet chemical analyses by Gurov and Koeberl (2004). b Calculated from the data for FeO and Fe 2 O 3 from Gurov and Koeberl (2004). c Value from Gurov and Koeberl (2004). d Recalculated value using calculated FeO*. Values for P 2 O 5, CO 2, H 2 O, and LOI from Gurov and Koeberl (2004), which are not shown here are included into the calculation. FeO* denotes total FeO. n.d. = not determined. The viscosity g in Pa s is described in terms of the activation enthalpy DH*, R as ideal gas constant (8.314 J mol 1 K), and T as temperature T(K). With the Maxwell (1867) relationship (5) s 0 ¼ g 0 G 0 (6) the relaxed Newtonian shear viscosity of the melt g 0 can be calculated. G 0 is the unrelaxed elastic shear modulus of the melt with a value of log 10 (Pa) = for silicate melts (Dingwell and Webb 1989). RESULTS 1. The presented data in Table 2 are obtained as described above, the activation enthalpy DH* and s 0 through Equation 4 and the parameters b and ξ through the Tool Narayanaswamy Moynihan model. The fictive temperature T f obtained from the raw curve varies due to the two different normalizations between 836 and 861 K. These two fictive temperatures were used to calculate the natural cooling rates, which range from 0.06 to 0.40 K min The major element geochemistry throughout the El gygytgyn impact glass remains constant within error before and after DSC measurements (Table 1). The analytical error is constant throughout the microprobe measurements. 3. An H 2 O content of 0.45 wt% was determined. 4. The viscosity g is calculated throughout Equations 5 and 6 using the activation enthalpy and characteristic relaxation time given in Table 2. The El gygytgyn impact glass has a viscosity of log g = Pa s at the glass transition temperature T g peak (1020 K). DISCUSSION The obtained chemical data (Table 1) for the impact glass are comparable to the data obtained by Gurov and Koeberl (2004) for El gygytgyn impact melt rocks. Only the SiO 2 content seems to be a bit higher and the Al 2 O 3 a bit lower in our impact glass. However, it should be kept in mind that the chemical data were obtained by two different methods, EMPA (this study) and wet chemistry methods (Gurov and Koeberl 2004), and a direct comparison should only be carried out with caution. Thus, even though the chemical data suggest that our melt fragment represents an average El gygytgyn impact glass, this cannot be definitely inferred. The range of cooling rates from 0.06 to 0.40 K min 1 for the El gygytgyn impact glass is obtained by modeling. These represent the cooling history of the glass on its natural cooling path through the glass transition range. The modeled curve in Fig. 3a

6 1356 U. Rantzsch et al. Table 2. Adjusted parameters, T g peak,t f and the cooling rates for the El gygytgyn impact glass. q (K min 1 ) DH* (kj mol 1 ) s 0 (s) b ξ T peak g (K) T f (K) 0.06 (natural) * (natural) * * * * is shifted compared with the raw curve, while this is not the case in Fig. 3b. This might indicate that the normalization at 1058 K (Fig. 3b) might be closer to the end of the glass transition than the normalization at 1112 K (Fig. 3a) and therefore the value for the cooling rate might be closer to 0.4 K min 1 than to 0.06 K min 1. Compared to other ballistic fragments such as tektites, the modeled cooling rate for the El gygytgyn impact glass is slower than that of 1 10 K s 1 for tektites determined by Wilding et al. (1996b). This is due to many parameters such as the radiation of heat during the flight from the origin and the initial temperature of the tektite melt as well as the thermal conductivity through the glass rind. Volcanic glasses often have slower cooling rates (ranging from 10 K s 1 to 7 K day 1 ) than the modeled cooling rate for the El gygytgyn impact glass (e.g., Wilding et al. 1995, 1996a; Gottsmann and Dingwell 2002). The cooling processes of volcanic glasses are related to a variety of different mechanisms. The process of vesiculation can be responsible for faster cooling, e.g., up to 1 log 10 unit in rhyolitic magmas with 0.5 wt% H 2 O by degassing (Wilding et al. 1996a). A natural annealing process on the other hand leads to slower cooling and is revealed by a low fictive temperature (Wilding et al. 1996a). Even though our sample has small pores and contains 0.45 wt% H 2 O, the low fictive temperatures derived for the raw curve point to a natural annealing process. Therefore, we favor the assumption that the cooling mechanism was mainly driven by a natural annealing process. Obsidians from the Aeolian Islands have been investigated with relaxation geospeedometry (Gottsmann and Dingwell 2001). The modeled cooling rates range between 0.2 and 0.03 K min 1. Some of the quantified cooling rates are similar to those inferred here for the El gygytgyn impact glass. Even though volcanic and impact processes can hardly be compared, the similar cooling rates lead to the question whether those processes can lead to similar conditions affecting the cooling process. For example, volcanic glasses can experience reheating by subsequent lava flows or heat buffering by the adjacent volcanic layers, resulting in low cooling rates (Gottsmann et al. 2004). The question is arising whether a melt fragment can, under impact conditions, also experience reheating or heat buffering? Even the formation of a complex impact crater on Earth is, in geological time scale, an instant event that takes place in less than five minutes (Collins et al. 2012). In this short time, the sample could not have undergone a complete cooling through the glass transition phase and subsequent reheating into the glass transition phase. Thereby, a falsification effect of the cooling history of the sample by reheating during the impact event can be ruled out. Thus, we can assume a quasilinear cooling process during the glass transition, represented by the modeled cooling rate. However, the cooling process of the sample might have been buffered by the burial under further impact material, which could explain the rather slow cooling rate. Unfortunately, no detailed information about the original deposit of the El gygytgyn impact glass is available, which could illuminate the possibility of a buffering effect. Even though a relationship between the viscosity and the calorimetric glass transition and the quench rate is known (Stevenson et al. 1995), the viscosity still is primarily a function of the composition of the melt. A change of the chemical composition during the DSC measurements could not be addressed by the model used for calculating the cooling rate. The undertaken EMPA analysis ensured that the composition of the melt fragment remained constant through the DSC analysis (Table 1). The present study revealed a viscosity of log g = Pa s at the glass peak transition temperature T g (1020 K) for the El gygytgyn impact glass with a cooling rate ranging between 0.06 and 0.40 K min 1. Other quench rate studies for volcanic glasses revealed varying cooling rates of several orders of magnitude (Wilding et al. 1995, 1996a, 2000). With varying cooling rates for peak natural glasses, the glass transition temperature T g varies as well. As the viscosity g is temperature dependent (Equation 5), slow cooling processes, which are accompanied by lower T peak g values, thus have higher viscosities g at T peak g. Scherer (1984) proposed this assumption already for synthetic melts. These

7 Cooling rate, El gygytgyn, geospeedometry 1357 relationships are directly comparable to our calculated viscosity data for the El gygytgyn impact glass. The knowledge of the viscosity at the glass transition temperature T peak g adds information of the thermal history, in our case to the relatively slow annealing process of this natural impact glass compared with tektites. CONCLUSIONS The calorimetric data yield the basis for the determination of the natural cooling rate within the glass transition phase for the El gygytgyn impact glass. Due to the sample-specific kinetic parameters achieved through heat capacity measurements, the natural quench rate was determined. The cooling rate, ranging from 0.06 to 0.40 K min 1, is achieved by modeling the structural relaxation within the glass transition interval. We assume that the cooling rate of the El gygytgyn impact glass reflects a natural annealing process. The calculated viscosity of log g = Pa s at the glass transition temperature T g peak (1020 K) reflects a simple conductive heat loss. Consequently, a slow cooling process is accompanied by a high viscosity g. The modeled cooling rate for the El gygytgyn impact glass is slower than for tektites and faster than those of most volcanic glasses. The assumed cooling process by natural annealing might have been slowed down by heat buffering through burial under further impact material. Consequently, the method of geospeedometry used here can be applied to the El gygytgyn impact glass to model the initial cooling rate. The method is appropriate for glasses where the structural relaxation within the glass transition range is large. Further DSC measurements of different impact glasses would be necessary to generalize the applied method. Acknowledgments We thank Rudolf Naumann from the GFZ Potsdam for conducting the water analysis. 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