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Age of the Dakhleh impact event and implications for Middle Stone Age archeology in the Western Desert of Egypt
Earth and Planetary Science Letters 291 (2010) 201–206
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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / e p s l
Age of the Dakhleh impact event and implications for Middle Stone Age archeology in the Western Desert of Egypt Paul R. Renne a,b,⁎, Henry P. Schwarcz c, Maxine R. Kleindienst d, Gordon R. Osinski e, John J. Donovan f a
Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA, 94709, USA Department of Earth and Planetary Science, University of California, Berkeley, CA, 94720, USA School of Geography and Earth Sciences, McMaster University, Hamilton, ON, Canada L8S 4K1 d Department of Anthropology, University of Toronto at Missisauga, 3359 Missisauga Road N., Missisauga, ON, Canada L5L 1C6 e Departments of Earth Sciences/Physics and Astronomy, University of Western Ontario, 1151 Richmond Street, London, ON, Canada N6A 5B7 f CAMCOR Microanalytical Facility, University of Oregon, Eugene, OR 97403-1241, USA b c
a r t i c l e
i n f o
Article history: Received 12 October 2009 Received in revised form 2 January 2010 Accepted 6 January 2010 Available online 27 January 2010 Editor: R.W. Carlson Keywords: impact meteorite geochronology argon Dakhleh Middle Stone Age
a b s t r a c t Dakhleh Glass comprises a suite of chemically distinctive and heterogeneous glassy rocks that occur over an area of ca. 400 km2 in and around the Dakhleh Oasis in central western Egypt. Previous studies establish a meteorite impact origin for the Dakhleh Glass. No impact crater has yet been found, suggesting an airburst origin. The Dakhleh Glass-forming impact event occurred during the Middle Stone Age time of occupation, but the timing of this event has not been well established. 40Ar/39Ar incremental heating of three aliquots from a sample of Dakhleh Glass yield data that can be ascribed to quenched glass which efficiently purged radiogenic 40Ar inherited from the target rocks. One of the aliquots yielded data suggestive of an undegassed clast of target material, but these are easily resolved. The age of the impact event is determined from a compositionally filtered subset of the data that yield an isochron age of 145 ± 19 ka. © 2010 Elsevier B.V. All rights reserved.
1. Introduction As recently reviewed by Jourdan et al. (2009), the ages of terrestrial meteorite impact events are only rarely well-constrained. The paucity of reliable ages hinders accurate estimates of the terrestrial mass flux, and limits the ability to correlate specific impact events with possible consequences at high levels of confidence in all but a few cases. In this brief contribution we present 40Ar/39Ar data that serve to date one such event whose age had previously been poorly constrained. These results also serve to constrain the ages of Middle Stone Age lithic artifacts found stratigraphically below the impact event horizon. Glassy material of unusual geochemical composition was first reported by Kleindienst et al. (1999) in the Dakhleh Oasis region, in the Western Desert of Egypt, and further described by Schwarcz et al. (2008). In the area of the southwestern Teneida Paleobasin (Fig. 1) where the dated sample was collected, small remnant knolls of eroded lakebeds overlie an uneven eroded surface of red mudstones of the Mut Fm. resting on sandstones of the Taref Fm. (Schwarcz et al., 2008: ⁎ Corresponding author. Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA, 94709, USA. E-mail address: [email protected] (P.R. Renne). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.01.014
Locality 390). The old surface underlying the lake deposits here bears heavily aeolized Earlier Stone Age artifacts, estimated to be at least 300,000 years old, but no lithics characteristic of the Middle Stone Age. The so-called “Dakhleh Glass” occurs as widely distributed surface lags and in situ within the Pleistocene lake deposits (Kieniewicz, 2007). Recent field studies show that the Dakhleh Glass is distributed over an area of ∼40 × 10 km with ∼140 individual locations having been documented (Osinski et al., 2007, 2008). Dakhleh Glass is composed of irregular masses ∼2–50 cm in dimension, comprising highly vesicular glassy rocks with bulk CaO concentrations of 13–21wt.%, commonly containing abundant but highly variable proportions of clinopyroxene crystals of variable compositions (En33–52Fs0–14Wo46–56; Osinski et al., 2007, 2008), having skeletal and/or dendritic quench textures. Rare plagioclase crystallites have also been documented in some samples (Osinski et al., 2007, 2008). The unusual and highly variable chemistry and mineralogy (including lechatelierite and spheroidal pyrrhotite), textural characteristics, distribution and absence of local volcanic activity documented by Osinski et al. (2007, 2008) led to the conclusion that the Dakhleh Glass was derived from rapid melting and quenching of sedimentary rocks similar to those found in the region, due a meteorite impact event. Neither an impact crater nor products of
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shock metamorphism (e.g., shatter cones, planar deformation features) have yet been identified; these and other characteristics of the material and its context led Osinski et al. (2008) to speculate that it might have been formed in an airburst event similar to those described by Wasson (2003) and Boslough and Crawford (2008). We use the term “impact” henceforth in this paper to refer to collision of a meteorite with either the terrestrial atmosphere or lithosphere, either case being potentially capable of producing sufficient heat to melt surface rocks and/or debris. 2. Existing age constraints and relations to archeology Overlying and underlying archeological materials were interpreted by Kleindienst (Osinski et al., 2007; Smith et al., 2009) to constrain the age of the Dakhleh Glass-forming event as being between N150 and 70 ka. The older age limit is based on the typology of Middle Stone Age lithic artifacts in lakebeds that underlie lagged surface deposits of Dakhleh Glass by 3–4 m in the northeastern Teneida Paleobasin (Churcher et al., 1999; Kleindienst, 2004: Locality 374). In recent years, the origination of the Middle Stone Age has been shown to be older than historically believed, in some instances older than 280 ka (e.g., Deino and McBrearty, 2002; Morgan and Renne, 2008) and it remains unclear whether characteristics of various aggregates attributed to the Middle Stone Age can be interpreted as having geochronologic significance. Hence, the older limit for the Dakhleh Glass-forming event posed by underlying Middle Stone Age artifacts is of uncertain value, and indeed the results presented herein serve conversely to constrain the age of the artifacts. The oldest generalized Middle Stone Age assemblage in the Kharga–Dakhleh region dates by uranium-series to over ca. 200 ka (Kleindienst et al., 2008), in a period of more humid climate attested by dated tufa deposits on the Libyan Plateau escarpments at both oases (cf. Smith et al., 2004). Younger generalized Middle Stone Age aggregates appear to post-date ca. 140 ka at Kharga (Smith et al., 2007). A summary of some 40Ar/39Ar experiments was presented by Osinski et al. (2007) and Schwarcz et al. (2008), who cited isochron ages for four aliquots ranging from 106 ± 40 to 282 ± 121 ka, and interpreted a weighted mean age of 122 ± 40 ka as a best estimate for the age of the Dakhleh Glass. Argon isotope data were not presented, nor were the confidence levels of uncertainties given. Three of the four aliquots yielded isochrons with atmospheric 40Ar/36Ar intercepts, but one yielded a significantly subatmospheric value of 239.9 ± 0.4, which is difficult to explain. Osinski et al. (2007) cautioned that “… the 40Ar/39Ar data are preliminary and not entirely satisfactory”. 3. Sample characterization and analytical methods The sample (MRK-DAK13) selected for this study was found as lag in a topographic corridor between yardangs of sandstone of the Taref Fm., at 25°26′57.5″ N latitude and 29°11′09.8″ E longitude. This location (Fig. 1) is about 300 m WSW of Locality 390, Occurrence 2. The sample is approximately 3 cm× 3 cm× 2 cm in dimension. As seen in Fig. 2, compared with material described by Osinski et al. (2007) it is of typical vesicularity and is composed chiefly of diopsidic clinopyroxene crystallites and glass. Osinski et al. (2007) reported significant variation in glass composition both between and within fragments of Dakhleh Glass. Thus, because this specific sample was not analyzed by Osinski et al. (2007), we obtained additional electron microprobe data directly from this sample in order to inform interpretation of the 40Ar/39Ar results from the same sample.
Fig. 1. Summary geologic map showing locations of Dakhleh glass occurrences, after Osinski et al. (2007) and Schwarcz et al. (2008). Approximate location of the lagged sample analyzed in this study (MRK-DAK13) is indicated by the tip of the black arrow, and is distinct from the adjacent unfilled star representing both in situ and lag material at locality 390, occurrence 2.
Table 1. Data were acquired on a Cameca SX-100 electron microprobe at the CAMCOR facility of the University of Oregon. The instrument is equipped with 5 tunable wavelength dispersive spectrometers. Operating conditions were 40° takeoff angle, and a beam energy of 15 keV. The beam current was 20 nA, and the beam diameter was 5 μm. Elements were acquired using Kα peaks and analyzing crystals LIF for Ca and Fe, LLIF for Ti, Mn, Cr, Ni, and Fe, LPET for Si, Cl and K, and TAP for Na, P, Al, and Mg. The standards were TiO2 synthetic for Ti, MnO synthetic for Mn, NiO synthetic for Ni, NBS K-411 mineral glass for Fe, Ca10(PO4)6Cl2 (halogen corrected) for P and Cl, Nepheline (partial analysis) for Al and Na, Diopside (Chesterman) for Ca, Si, and Mg, Orthoclase MAD-10 for K, and Chromite (UC # 523-9) for Cr. The counting time was 20 s for Ti, Cl and Mn, 40 s for Si, Al, Cr, and Ni, 80 s for Na, 90 s for Ca, and 120 s for Fe, P, Mg, and K. The intensity data were corrected for Time Dependent Intensity (TDI) loss (or gain) using a self calibrated correction for Na, Si, Al, Ca, and Cr. The small dimensions of glass interstices precluded defocusing the electron beam to more than ∼5 μm without exciting adjacent crystals. The TDI correction extrapolates count-rate as a function of time back to zero time, using a linear fit in log–lin space for all elements except Na, which utilized a 2nd order polynomial fit to deal with the “hyper-exponential” character of the Na loss at these beam sizes. The off peak counting time was 12 s for Ti, 20 s for Mn, Cr, and Ni, and 30 s for K and P. Off Peak correction method was linear for Ti, Mn, K, Cr, and Ni, and exponential for P. The MAN background intensity data was calibrated and absorption corrected for Na, Si, Al, Mg, Ca, and Fe (Donovan and Tingle, 1996). Unknown and standard intensities were corrected for dead time. Standard intensities were corrected for standard drift over time. Interference corrections were applied to Mg for interference by Ca, and to Fe for interference by Mn, and to P for interference by Ca (Donovan et al., 1993).
3.1. Electron microprobe analysis 3.2. Compositional analyses were made at locations shown in Fig. 2. Each “unknown” represented by the boxes shown in Fig. 2 comprises 4 to 21 spot analyses, which yielded data summarized in Appendix
40
Ar/39Ar dating
Sample preparation and analysis were conducted at the Berkeley Geochronology Center (BGC). A portion of the sample was crushed
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with an agate mortar and pestle, and sieved into size fractions. While a concentration of glass was desired, the fine spatial scale of pure glass domains is sufficiently small as to be subject to severe 39Ar recoil effects if separated and irradiated as a pure concentrate. The 250 to 425 μm size fraction was selected for analysis, and was rinsed ultrasonically for 3 minutes in 5% HF, then distilled water, dried, and irradiated for 1.0 hr. in the Cd-shielded CLICIT facility at the Oregon State University TRIGA reactor along with the Alder Creek sanidine (ACs) standard (Nomade et al., 2005). The standards were analyzed by total fusion with a focused 5 W beam from a CO2 laser. The unknowns were analyzed by incremental heating of 3 aliquots, each of ∼ 25 mg, with a CO2 laser using an integrator lens to distribute the beam energy over a 20 mm2 area. All gas fractions were subjected to 180 s of purification with a SAES getter and a cryocooled cold finger maintained at ∼−130 °C. Ion beams were measured on two MAP 215 magnetic sector mass spectrometers (MAP I and MAP III, as specified in Appendix Table 2), both using magnetic field switching to cyclically measure 36–40Ar isotopes in 10 cycles of peak-hopping on Balzers electron multipliers operated in analog mode. Laser heating, extraction line operation and mass spectrometry were fully automated. Backgrounds were measured between every 1–3 samples, and average values (±σ) were used to correct the ion beam intensity data. Mass discrimination was monitored by automated analysis of air pipettes interspersed with the unknowns and correction was made using a power law relationship (Renne et al., 2009). Discrimination values (per atomic mass unit) of 1.0075 ±0.00151 (N= 19) and 1.0030 ±0.0020 (N= 8) were determined for runs on MAP I and MAP III, respectively, using 40Ar/ 36 Ar =295.5 for air (Steiger and Jäger, 1977). A more recent determination of atmospheric 40Ar/36Ar (Lee et al., 2006) is believed to be more accurate, but as shown by Renne et al. (2009) the impact on age calculations for the experiments reported herein is negligible. The Ar isotope data, corrected for backgrounds, mass discrimination, and radioactive decay, are given in Appendix Table 2. The J-value, a neutron-fluence related measure of the conversion of 39 K to 39Ar, was determined from analysis of 6 single crystals (595– 841 μm) of ACs using an age of 1.193 Ma (Nomade et al., 2005). The weighted (by inverse variance) mean value of 40Ar*/39ArK was used to calculate J = (1.296 ± 0.004) × 10− 4 (±0.33%). Corrections for interfering Ar isotopes from Ca, K, and Cl were those reported by Renne et al. (2005) and Renne et al. (2008). Ca/K and Ca/Cl were determined from 37ArCa/39ArK and 37ArCa/38ArCl using the relationships Ca/K = (1.92 ± 0.05) × (37ArCa/39ArK) and Ca/Cl = (0.122 ± 0.007) × (37ArCa/ 38 ArCl), derived from data reported by Jourdan and Renne (2007) for the Hb3gr standard.
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though tending towards higher Ca/K and Ca/Cl values. As shown in Fig. 3, the Ca/K varies from 1.5 to 6, and Ca/Cl varies from 5 to 12. Two anomalous points with Ca/K N 6 and Ca/Cl N 25 also have Mg concentrations 5–10 times higher than average, and we infer that these reflect contributions from diopside microlites. The compositional heterogeneity of the glass phase in this sample, at the cm scale, is further evidence of an impact melt origin of the Dakhleh Glass. Some heterogeneity may be explained by locally variable degrees of crystallization leading to variable enrichment of the residual liquid in incompatible elements. However, neglecting the three points with MgON 4 wt.% (suspected of reflecting a contribution from diopsidic clinopyroxene) there is no suggestion of a negative correlation between incompatible and compatible elements such as K2O and MgO, as would be expected for a fractional crystallization mechanism. 4.2.
40
Ar/39Ar dating
Apparent age spectra (Fig. 4) corresponding to the three incremental heating experiments are markedly different. Aliquot
Fig. 2. Transmitted light photomicrograph of the sample analyzed. White subspherical regions are vesicles. Numbered boxes refer to loci of electron microprobe analyses as shown in Appendix Table 1. Exterior dimensions of white boxes (outside of sample image) labeled 425 μm and 250 μm show bracketing size ranges of sieved size fractions used for 40Ar/39Ar experiments.
4. Results 4.1. Glass compositions Electron microprobe data are given in Appendix Table 1. The elements analyzed in the glasses sum to between 95.1 and 99.8 wt.% oxides, with the majority of sums above 98%. Analysis of two glasses (NBS K-411 and K-412) as secondary standards yielded totals of 99.70 and 99.80 element wt.% respectively, with less than 0.25 element wt.% difference from published values for the elements analyzed. Thus we conclude that the low totals in MRK-DAK13 glass are real. Fe contents are far too low to account for difference if all Fe is Fe+ 3. We conclude that the glass contains 0–4 wt.% H2O, C and/or other elements not analyzed. Osinski et al. (2007) reported up to 0.34 wt.% SO3 in some samples, as well as discrete apparently immiscible globules of CaCO3. Glass compositions in MRK-DAK13 overlap the range reported previously (Osinski et al., 2007) for other Dakhleh glass samples, 1
Uncertainties here and throughout are given at the level of one standard deviation (σ) unless stated otherwise.
Fig. 3. Ca/K vs. Ca/Cl elemental ratios for MRK-DAK13 interstitial glass measured by electron microprobe. Glass compositions reported by Osinski et al. (2007) are shown for comparison.
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36012-1 shows pronounced discordance with a somewhat saddleshaped spectrum, whereas aliquots 36012-2 and 36012-3 yield flat, 100% concordant spectra with identical plateau ages of 151 ± 10 ka. The minimum model (i.e., based on the assumption of atmospheric initial 40Ar/39Ar) age of 178 ± 45 ka of the 8 W step in the 360121spectrum is similar to the plateau ages manifest in the other two aliquots. All three age spectra reveal plateaux in Ca/K and Ca/Cl ratios in steps extracted at low laser power, with both of these ratios climbing monotonically after ∼ 30–50% 39Ar released. This is straightforwardly interpreted to reflect initial degassing of glass with relatively high Ar diffusivity, followed by an increasing contribution of 37ArCa derived chiefly from diopside microlites. Somewhat surprisingly, there is no evidence of spuriously young ages at higher extraction temperatures resulting from (1) 39ArK recoiled from the glass into the diopside (e.g., Turner and Cadogan, 1974; Huneke and Smith, 1976) or (2) recoil fractionation of 37ArCa (strongly recoiled) from 36ArCa (weakly recoiled) (Jourdan et al., 2007a). The average plateau values of Ca/K (2.14 ± 0.07) and Ca/Cl (9.45 ± 0.27) compare favorably with the composition of interstitial glass measured by the electron microprobe, as shown in Fig. 3. The fact that the apparent age plateaux for aliquots 36012-2 and 36012-3 extend to gas fractions with Ca/Cl significantly higher than the glass phase indicates that the diopside microlites and other crystalline phases in these aliquots contain no detectable Ar with isotopic composition different from atmospheric. Thus we conclude that aliquot 36012-1, with the discordant age spectrum, must contain inherited 40Ar derived from incomplete degassing of some of the parent rock that was melted to produce the Dakhleh Glass. Though deemed improbable, it is conceivable that the extraneous material arose from laboratory contamination. Cast on isotope correlation diagrams (Fig. 5), the data from aliquots 36012-2 and 36012-3 define well-constrained isochrons that are straightforwardly interpreted as mixing lines between initial trapped components of atmospheric composition (initial 40Ar/ 36 Ar = 296.3 ± 1.6 and 295.3 ± 1.5, respectively) and K-correlated radiogenic components whose ages are 140 ± 20 ka and 150 ± 20 ka (respectively). Four of the steps from aliquot 36012-1, corresponding to Ca/Cl between 8.9 and 9.7 define a similar isochron of 150 ± 70 ka. An isochron (Fig. 6(a)) fit to 30 points from all 3 aliquots, excluding only the 6 markedly discordant steps from aliquot 36012-1, yields an age of 143 ± 14 ka, initial 40 Ar/ 36 Ar = 296.6 ± 1.0, MSWD = 0.99, probability = 0.48. To minimize the possibility of bias, an isochron was fit to data from all three aliquots whose Ca/K and Ca/Cl ratios are consistent with the glass compositions measured directly by electron microprobe from the same sample. This compositional filter admits 22 steps, and the isochron (Fig. 6(b)) derived from them has an age of 145 ± 19 ka, with initial 40Ar/ 36 Ar = 296.3 ± 1.3, MSWD = 1.2, probability = 0.22. The compositional filter employed here would not necessarily exclude inherited Ar from small amounts of material, especially if compositionally similar to the glass, but in concert with the statistical evidence for a binary mixture of Ar components, the validity of this isochron age seems unequivocal.
5. Discussion and conclusions One of the challenges to 40Ar/39Ar dating of glassy impact melt rocks, exemplified by the Tswaing crater in South Africa (Jourdan et al., 2007b), is incomplete degassing of radiogenic 40Ar accumulated in the target rock(s) prior to impact. If such inherited 40Ar is homogeneously distributed at the atomic scale in a glass, it may be difficult or impossible to fractionate it from post-impact ingrown radiogenic 40Ar by laboratory heating, hence inherited argon may be undetectable in isotope correlation plots.
Fig. 4. Apparent age, Ca/K and Ca/Cl spectra for three individual aliquots of MRKDAK13. (a) Aliquot 36012-01. No plateau is defined; (b) Aliquot 36012-02 and (c) Aliquot 36012-03 yield age plateaux including 100% of cumulative 39Ar released.
Whether Ar diffusion in impact melts is sufficiently fast to enable extensive isotopic equilibration with the atmosphere is difficult to evaluate in most cases because: (1) the thermal history of the melt at the spatial scale of interest is poorly constrained; (2) Ar diffusivities, and their temperature dependence, in diverse melt compositions are not well known. Moreover, water contents of impact melts, which can increase Ar diffusivity by more than an order of magnitude per wt.% in silicate liquids (Zhang et al., 2007), are generally not known as a function of temperature or time. A further complication is that impact melts may involve the coalescence of small melt droplets, hence the lengthscale for diffusion may evolve over a short critical time interval. As a consequence of these uncertainties, modeling the degassing of the Dakhleh glass and its progenitor melt is not sufficiently well constrained to permit meaningful predictions as to whether or not inherited Ar is expected to be present in significant concentrations. However, several qualitative features of the Dakhleh Glass, contrasting with impact melt rocks having demonstrable inherited Ar (e.g., Tswaing), would tend to disfavor age bias from inherited Ar: (1) the relatively young age (Cretaceous to Holocene) and low K-contents of the inferred target materials (Osinski et al., 2007), thereby producing lower concentrations of 40Ar in the melting environment; (2) the relatively low SiO2 and high H2O contents, which would tend to enhance Ar diffusivity.
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Fig. 6. Isochron diagrams for (a) steps from all 3 aliquots. Shaded ellipses are excluded from isochron regression; (b) steps from all 3 aliquots meeting compositional criteria of glass (see text). Error envelopes are shown (shaded) for isochrons.
Fig. 5. Isochron diagrams for three individual aliquots of MRK-DAK13. (a) Aliquot 36012-01 (shaded ellipses are excluded from isochron regression); (b) Aliquot 3601202 and (c) Aliquot 36012-03 isochrons include all steps. Error envelopes are shown (shaded) for isochrons.
In the case of Dakhleh, it is clear from the extensive vesiculation that a discrete vapor phase was present upon quenching, though it is unclear whether this was exsolved from the melt or was rather mechanically admixed during the rapid melting event. In either case, any inherited Ar remaining in the melt would be expected to partition into the vapor phase. The size distribution of the vesicles extends well into the grain size fraction analyzed in our 40Ar/39Ar experiments, and whatever Ar was present in them would inevitably have been represented to some extent in the step heating analyses. Given that the isochron intercepts betray no evidence for inherited Ar with supra-atmospheric 40Ar/36Ar, we conclude that the vapor phase represented by the vesicles contained Ar whose isotopic composition was dominated by air. After deletion of outliers suspected of arising from undegassed clast(s), analysis of 40Ar/39Ar step heating data yields statistically valid isochrons whether or not the regression includes compositions restricted to those of the glass phase. At a penalty of ∼ 30% in precision, the result corresponding to the glass is preferred. Thus the impact event that produced the Dakhleh Glass is inferred to have occurred at 145 ± 19 ka, consistent with the typological aspects of the underlying Older Middle Stone Age archeology. The relatively large uncertainty in this age encompasses the range of variations arising
from updated values of systematic variables such as those related to the standard and 40K decay constants (e.g., Kuiper et al., 2008 and references therein). For example, the calibration presented by Kuiper et al. (2008) yields an age of 146 ka. Potential consequences of the impact event on the Middle Stone Age people in the Dakhleh region are a topic of ongoing interest (e.g., Smith et al., 2009). The age of the impact is compatible with the likely age of underlying artifacts in the Kellis and Teneida paleobasins. Thus, it is plausible that people inhabited the area immediately prior to the impact. It can be reasonably hypothesized that the consequences of the impact may have displaced people from the area for an as-yet undetermined interval of time. A reliable age for the Dakhleh glass provides a useful chronostratigraphic datum for future archeological and paleoenvironmental studies in the Dakhleh region. This is especially welcome because chronometrically-dateable materials related to Pleistocene lithics are scarce. The areal extent of impact-related debris has yet to be determined, although presumably it will prove larger than the ca. 400 km2 area in which mesoscopic material has been found to date. Acknowledgements P.R.R. gratefully acknowledges support from the Ann and Gordon Getty Foundation, and from the U.S. National Science Foundation (grants SBR-9601592 and BCS-0715465) for the BGC 40Ar/39Ar lab facilities used in this study. Members of the Dakhleh Oasis Project, ‘DG Working Group’ particularly thank Leslie and Tony Mills for facilitating fieldwork over the years, and Charles Churcher, Jennifer
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Smith, Johanna Kieniewicz, and Albert Haldemann for various contributions. Field research on Dakhleh glass has been funded by the Social Sciences and Humanities Research Council of Canada, the Natural Sciences and Engineering Research Council of Canada, the National Geographic Society, the University of Toronto, and the Dakhleh Trust. Constructive advice from D.F. Mark, R.W. Carlson and an anonymous reviewer helped improve the manuscript and are appreciated. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi: 10.1016/j.epsl.2010.01.014. References Boslough, M.B., Crawford, D.A., 2008. Low-altitude airbursts and the impact threat. Int. J. Impact Eng. 35, 1441–1448. Churcher, C.S., Kleindienst, M.R., Schwarcz, H.P., 1999. Faunal remains from a Middle Pleistocene lacustrine marl in Dakhleh Oasis, Egypt: palaeoenvironmental reconstructions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 154, 301–312. Deino, A.L., McBrearty, S., 2002. 40Ar/39Ar dating of the Kapthurin Formation, Baringo, Kenya. J. Hum. Evol. 42, 185–210. Donovan, J.J., Tingle, T.N., 1996. An improved mean atomic number correction for quantitative microanalysis. J. Microsc. 2, 1–7. Donovan, J.J., Snyder, D.A., Rivers, M.L., 1993. An improved interference correction for trace element analysis. Microbeam Anal. 2, 23–28. Huneke, J.C., Smith, S.P., 1976. The realities of recoil: 39Ar out of small grains and anomalous patterns in 40Ar–39Ar dating. Geochim. Cosmochim. Acta Suppl. 7, 1987–2008. Jourdan, F., Renne, P.R., 2007. Age calibration of the Fish Canyon sanidine 40Ar/39Ar dating standard using primary K–Ar standards. Geochim. Cosmochim. Acta 71, 387–402. Jourdan, F., Matzel, J.P., Renne, P.R., 2007a. 39Ar and 37Ar recoil ejection during neutron irradiation of sanidine and plagioclase. Geochim. Cosmochim. Acta 71, 2791–2808. Jourdan, F., Renne, P.R., Reimold, W.U., 2007b. The problem of inherited 40Ar* in dating impact glass by the 40Ar/39Ar method: evidence from the Tswaing impact crater (South Africa). Geochim. Cosmochim. Acta 71, 1214–1231. Jourdan, F., Renne, P.R., Reimold, W.U., 2009. An appraisal of the ages of terrestrial impact structures. Earth Planet. Sci. Lett. 286, 1–13. Kieniewicz, J.M. 2007. Pleistocene Pluvial Lakes of the Western Desert of Egypt: Paleoclimate, Paleohydrology, and Paleolandscape Reconstructions, Ph.D. Thesis, Department of Earth and Environmental Sciences, St. Louis, Washington University. Kleindienst, M.R., 2004. Strategies for studying Pleistocene archeology based upon surface evidence: first characterization of an older Middle Stone Age unit, Dakhleh Oasis, Egypt. In: Bowen, G.E., Hope, C.A. (Eds.), The Oasis Papers 3: Proceedings of the Third International Conference of the Dakhleh Oasis Project. Dakhleh Oasis Project monograph, vol. 14. Oxbow Books, Oxford, pp. 1–42. Kleindienst, M.R., Churcher, C.S., McDonald, M.M.A., Schwarcz, H.P., 1999. Geography, geology, geochronology, and geoarchaeology of the Dakhleh Oasis Region: an interim report. In: Chuecher, C.S., Mills, A.J. (Eds.), Reports from the Survey of Dakhleh Oasis, Western Desert of Egypt, 1977–1987. Dakhleh Oasis Project Monograph, vol. 2. Oxbow Books, Oxford, pp. 1–54.
Kleindienst, M.R., Schwarcz, H.P., Nicoll, K., Churcher, C.S., Frizano, J., Gigengack, R.W., Wiseman, M.F., 2008. Water in the desert: first report on uranium-series dating of Caton–Thompson's and Gardner's “classic” Pleistocene sequence at Refuf Pass, Kharga Oasis. In: Wiseman, M.F. (Ed.), The Oasis Papers 2. Proceedings of the Second International Conference of the Dakhleh Oasis Project. Dakhleh Oasis Project Monograph, vol. 12. Oxbow Books, Oxford, pp. 25–54. Kuiper, K.F., Deino, A., Hilgen, F.J., Krijgsman, W., Renne, P.R., Wijbrans, J.R., 2008. Synchronizing the rock clocks of Earth history. Science 320, 500–504. Lee, J.-Y., Marti, K., Severinghaus, J.P., Kawamura, K., Yoo, H.-S., Lee, J.B., Kim, J.S., 2006. A redetermination of the isotopic abundances of atmospheric Ar. Geochim. Cosmochim. Acta 70, 4507–4512. Morgan, L.E., Renne, P.R., 2008. Diachronous dawn of Africa's Middle Stone Age: new 40 Ar/39Ar ages from the Ethiopian Rift. Geology 36, 967–970. Nomade, S., Renne, P.R., Vogel, N., Deino, A.L., Sharp, W.D., Becker, T.A., Jaouni, A.R., Mundil, R., 2005. Alder Creek Sanidine (ACs-2): a Quaternary 40Ar/39Ar dating standard tied to the Cobb Mountain geomagnetic event. Chem. Geol. 218, 319–342. Osinski, G.R., Schwarcz, H.P., Smith, J.R., Kleindienst, M.R., Haldemann, A.F.C., Churcher, C.S., 2007. Evidence for a ∼200–100 ka meteorite impact in the Western Desert of Egypt. Earth Planet. Sci. Lett. 253, 378–388. Osinski, G.R., Kieniewicz, J., Smith, J.R., Boslough, M.B.E., Eccleston, M., Schwarcz, H.P., Kleindienst, M.R., Haldemann, A.F.C., Churcher, C.S., 2008. The Dakhleh Glass: product of an impact airburst or cratering event in the Western Desert of Egypt? Meteorit. Planet. Sci. 43, 2089–2106. Renne, P.R., Knight, K.B., Nomade, S., Leung, K.-N., Lou, T.-P., 2005. Application of deuteron–deuteron (D–D) fusion neutrons to 40Ar/39Ar geochronology. Appl. Radiat. Isotopes 62, 25–32. Renne, P.R., Sharp, Z.D., Heizler, M.T., 2008. Cl-derived argon isotope production in the CLICIT facility of OSTR reactor and the effects of the Cl-correction in 40Ar/39Ar geochronology. Chem. Geol. 255, 463–466. Renne, P.R., Cassata, W.S., Morgan, L.E., 2009. The isotopic composition of atmospheric argon and 40Ar/39Ar geochronology: time for a change? Quat. Geochronol. 4, 288–298. Schwarcz, H.P., Szkudlarek, R., Kleindienst, M.R., Evensen, N., 2008. Fire in the desert: the occurrence of a high-Ca silicate glass near the Dakhleh Oasis, Egypt. In: Wiseman, M.F. (Ed.), The Oasis Papers 2. Proceedings of the Second International Conference of the Dakhleh Oasis Project. Dakhleh Oasis Project Monograph, vol. 12. Oxbow Books, Oxford, pp. 55–71. Smith, J.R., Giegengack, R., Schwarcz, H.P., McDonald, M.M.A., Kleindienst, M.R., Hawkins, A.L., Churcher, C.S., 2004. A reconstruction of Quaternary pluvial environments and human occupations using stratigraphy and geoarchaeology of fossil-spring tufas, Kharga Oasis, Egypt. Geoarchaeol. Int. J. 19, 407–439. Smith, J.R., Hawkins, A.L., Asmerom, Y., Polyak, V., Giegengack, R., 2007. New age constraints on the Middle Stone Age occupations of Kharga Oasis, Western Desert, Egypt. J. Hum. Evol. 52, 690–701. Smith, J.R., Kleindienst, M.R., Schwarcz, H.P., Churcher, C.S., Kieniewicz, J.M., Osinski, G.R., Haldemann, A.F.C., 2009. Potential consequences of a mid-Pleistocene impact event for the Middle Stone Age occupants of Dakhleh Oasis, Western Desert, Egypt. Quat. Int. 195, 138–149. Steiger, R.H., Jäger, E., 1977. Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362. Turner, G., Cadogan, P.H., 1974. Possible effects of 39Ar recoil in 40Ar–39Ar dating. Geochim. Cosmochim. Acta Suppl. 5, 1601–1615. Wasson, J.T., 2003. Large aerial bursts: an important class of terrestrial accretionary events. Astrobiology 3, 163–179. Zhang, Y., Xu, Z., Zhu, M., Wang, H., 2007. Silicate melt properties and volcanic eruptions. Rev. Geophys. 45, RG4004. doi:10.1029/2006RG000216.
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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / e p s l
Age of the Dakhleh impact event and implications for Middle Stone Age archeology in the Western Desert of Egypt Paul R. Renne a,b,⁎, Henry P. Schwarcz c, Maxine R. Kleindienst d, Gordon R. Osinski e, John J. Donovan f a
Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA, 94709, USA Department of Earth and Planetary Science, University of California, Berkeley, CA, 94720, USA School of Geography and Earth Sciences, McMaster University, Hamilton, ON, Canada L8S 4K1 d Department of Anthropology, University of Toronto at Missisauga, 3359 Missisauga Road N., Missisauga, ON, Canada L5L 1C6 e Departments of Earth Sciences/Physics and Astronomy, University of Western Ontario, 1151 Richmond Street, London, ON, Canada N6A 5B7 f CAMCOR Microanalytical Facility, University of Oregon, Eugene, OR 97403-1241, USA b c
a r t i c l e
i n f o
Article history: Received 12 October 2009 Received in revised form 2 January 2010 Accepted 6 January 2010 Available online 27 January 2010 Editor: R.W. Carlson Keywords: impact meteorite geochronology argon Dakhleh Middle Stone Age
a b s t r a c t Dakhleh Glass comprises a suite of chemically distinctive and heterogeneous glassy rocks that occur over an area of ca. 400 km2 in and around the Dakhleh Oasis in central western Egypt. Previous studies establish a meteorite impact origin for the Dakhleh Glass. No impact crater has yet been found, suggesting an airburst origin. The Dakhleh Glass-forming impact event occurred during the Middle Stone Age time of occupation, but the timing of this event has not been well established. 40Ar/39Ar incremental heating of three aliquots from a sample of Dakhleh Glass yield data that can be ascribed to quenched glass which efficiently purged radiogenic 40Ar inherited from the target rocks. One of the aliquots yielded data suggestive of an undegassed clast of target material, but these are easily resolved. The age of the impact event is determined from a compositionally filtered subset of the data that yield an isochron age of 145 ± 19 ka. © 2010 Elsevier B.V. All rights reserved.
1. Introduction As recently reviewed by Jourdan et al. (2009), the ages of terrestrial meteorite impact events are only rarely well-constrained. The paucity of reliable ages hinders accurate estimates of the terrestrial mass flux, and limits the ability to correlate specific impact events with possible consequences at high levels of confidence in all but a few cases. In this brief contribution we present 40Ar/39Ar data that serve to date one such event whose age had previously been poorly constrained. These results also serve to constrain the ages of Middle Stone Age lithic artifacts found stratigraphically below the impact event horizon. Glassy material of unusual geochemical composition was first reported by Kleindienst et al. (1999) in the Dakhleh Oasis region, in the Western Desert of Egypt, and further described by Schwarcz et al. (2008). In the area of the southwestern Teneida Paleobasin (Fig. 1) where the dated sample was collected, small remnant knolls of eroded lakebeds overlie an uneven eroded surface of red mudstones of the Mut Fm. resting on sandstones of the Taref Fm. (Schwarcz et al., 2008: ⁎ Corresponding author. Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA, 94709, USA. E-mail address: [email protected] (P.R. Renne). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.01.014
Locality 390). The old surface underlying the lake deposits here bears heavily aeolized Earlier Stone Age artifacts, estimated to be at least 300,000 years old, but no lithics characteristic of the Middle Stone Age. The so-called “Dakhleh Glass” occurs as widely distributed surface lags and in situ within the Pleistocene lake deposits (Kieniewicz, 2007). Recent field studies show that the Dakhleh Glass is distributed over an area of ∼40 × 10 km with ∼140 individual locations having been documented (Osinski et al., 2007, 2008). Dakhleh Glass is composed of irregular masses ∼2–50 cm in dimension, comprising highly vesicular glassy rocks with bulk CaO concentrations of 13–21wt.%, commonly containing abundant but highly variable proportions of clinopyroxene crystals of variable compositions (En33–52Fs0–14Wo46–56; Osinski et al., 2007, 2008), having skeletal and/or dendritic quench textures. Rare plagioclase crystallites have also been documented in some samples (Osinski et al., 2007, 2008). The unusual and highly variable chemistry and mineralogy (including lechatelierite and spheroidal pyrrhotite), textural characteristics, distribution and absence of local volcanic activity documented by Osinski et al. (2007, 2008) led to the conclusion that the Dakhleh Glass was derived from rapid melting and quenching of sedimentary rocks similar to those found in the region, due a meteorite impact event. Neither an impact crater nor products of
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shock metamorphism (e.g., shatter cones, planar deformation features) have yet been identified; these and other characteristics of the material and its context led Osinski et al. (2008) to speculate that it might have been formed in an airburst event similar to those described by Wasson (2003) and Boslough and Crawford (2008). We use the term “impact” henceforth in this paper to refer to collision of a meteorite with either the terrestrial atmosphere or lithosphere, either case being potentially capable of producing sufficient heat to melt surface rocks and/or debris. 2. Existing age constraints and relations to archeology Overlying and underlying archeological materials were interpreted by Kleindienst (Osinski et al., 2007; Smith et al., 2009) to constrain the age of the Dakhleh Glass-forming event as being between N150 and 70 ka. The older age limit is based on the typology of Middle Stone Age lithic artifacts in lakebeds that underlie lagged surface deposits of Dakhleh Glass by 3–4 m in the northeastern Teneida Paleobasin (Churcher et al., 1999; Kleindienst, 2004: Locality 374). In recent years, the origination of the Middle Stone Age has been shown to be older than historically believed, in some instances older than 280 ka (e.g., Deino and McBrearty, 2002; Morgan and Renne, 2008) and it remains unclear whether characteristics of various aggregates attributed to the Middle Stone Age can be interpreted as having geochronologic significance. Hence, the older limit for the Dakhleh Glass-forming event posed by underlying Middle Stone Age artifacts is of uncertain value, and indeed the results presented herein serve conversely to constrain the age of the artifacts. The oldest generalized Middle Stone Age assemblage in the Kharga–Dakhleh region dates by uranium-series to over ca. 200 ka (Kleindienst et al., 2008), in a period of more humid climate attested by dated tufa deposits on the Libyan Plateau escarpments at both oases (cf. Smith et al., 2004). Younger generalized Middle Stone Age aggregates appear to post-date ca. 140 ka at Kharga (Smith et al., 2007). A summary of some 40Ar/39Ar experiments was presented by Osinski et al. (2007) and Schwarcz et al. (2008), who cited isochron ages for four aliquots ranging from 106 ± 40 to 282 ± 121 ka, and interpreted a weighted mean age of 122 ± 40 ka as a best estimate for the age of the Dakhleh Glass. Argon isotope data were not presented, nor were the confidence levels of uncertainties given. Three of the four aliquots yielded isochrons with atmospheric 40Ar/36Ar intercepts, but one yielded a significantly subatmospheric value of 239.9 ± 0.4, which is difficult to explain. Osinski et al. (2007) cautioned that “… the 40Ar/39Ar data are preliminary and not entirely satisfactory”. 3. Sample characterization and analytical methods The sample (MRK-DAK13) selected for this study was found as lag in a topographic corridor between yardangs of sandstone of the Taref Fm., at 25°26′57.5″ N latitude and 29°11′09.8″ E longitude. This location (Fig. 1) is about 300 m WSW of Locality 390, Occurrence 2. The sample is approximately 3 cm× 3 cm× 2 cm in dimension. As seen in Fig. 2, compared with material described by Osinski et al. (2007) it is of typical vesicularity and is composed chiefly of diopsidic clinopyroxene crystallites and glass. Osinski et al. (2007) reported significant variation in glass composition both between and within fragments of Dakhleh Glass. Thus, because this specific sample was not analyzed by Osinski et al. (2007), we obtained additional electron microprobe data directly from this sample in order to inform interpretation of the 40Ar/39Ar results from the same sample.
Fig. 1. Summary geologic map showing locations of Dakhleh glass occurrences, after Osinski et al. (2007) and Schwarcz et al. (2008). Approximate location of the lagged sample analyzed in this study (MRK-DAK13) is indicated by the tip of the black arrow, and is distinct from the adjacent unfilled star representing both in situ and lag material at locality 390, occurrence 2.
Table 1. Data were acquired on a Cameca SX-100 electron microprobe at the CAMCOR facility of the University of Oregon. The instrument is equipped with 5 tunable wavelength dispersive spectrometers. Operating conditions were 40° takeoff angle, and a beam energy of 15 keV. The beam current was 20 nA, and the beam diameter was 5 μm. Elements were acquired using Kα peaks and analyzing crystals LIF for Ca and Fe, LLIF for Ti, Mn, Cr, Ni, and Fe, LPET for Si, Cl and K, and TAP for Na, P, Al, and Mg. The standards were TiO2 synthetic for Ti, MnO synthetic for Mn, NiO synthetic for Ni, NBS K-411 mineral glass for Fe, Ca10(PO4)6Cl2 (halogen corrected) for P and Cl, Nepheline (partial analysis) for Al and Na, Diopside (Chesterman) for Ca, Si, and Mg, Orthoclase MAD-10 for K, and Chromite (UC # 523-9) for Cr. The counting time was 20 s for Ti, Cl and Mn, 40 s for Si, Al, Cr, and Ni, 80 s for Na, 90 s for Ca, and 120 s for Fe, P, Mg, and K. The intensity data were corrected for Time Dependent Intensity (TDI) loss (or gain) using a self calibrated correction for Na, Si, Al, Ca, and Cr. The small dimensions of glass interstices precluded defocusing the electron beam to more than ∼5 μm without exciting adjacent crystals. The TDI correction extrapolates count-rate as a function of time back to zero time, using a linear fit in log–lin space for all elements except Na, which utilized a 2nd order polynomial fit to deal with the “hyper-exponential” character of the Na loss at these beam sizes. The off peak counting time was 12 s for Ti, 20 s for Mn, Cr, and Ni, and 30 s for K and P. Off Peak correction method was linear for Ti, Mn, K, Cr, and Ni, and exponential for P. The MAN background intensity data was calibrated and absorption corrected for Na, Si, Al, Mg, Ca, and Fe (Donovan and Tingle, 1996). Unknown and standard intensities were corrected for dead time. Standard intensities were corrected for standard drift over time. Interference corrections were applied to Mg for interference by Ca, and to Fe for interference by Mn, and to P for interference by Ca (Donovan et al., 1993).
3.1. Electron microprobe analysis 3.2. Compositional analyses were made at locations shown in Fig. 2. Each “unknown” represented by the boxes shown in Fig. 2 comprises 4 to 21 spot analyses, which yielded data summarized in Appendix
40
Ar/39Ar dating
Sample preparation and analysis were conducted at the Berkeley Geochronology Center (BGC). A portion of the sample was crushed
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with an agate mortar and pestle, and sieved into size fractions. While a concentration of glass was desired, the fine spatial scale of pure glass domains is sufficiently small as to be subject to severe 39Ar recoil effects if separated and irradiated as a pure concentrate. The 250 to 425 μm size fraction was selected for analysis, and was rinsed ultrasonically for 3 minutes in 5% HF, then distilled water, dried, and irradiated for 1.0 hr. in the Cd-shielded CLICIT facility at the Oregon State University TRIGA reactor along with the Alder Creek sanidine (ACs) standard (Nomade et al., 2005). The standards were analyzed by total fusion with a focused 5 W beam from a CO2 laser. The unknowns were analyzed by incremental heating of 3 aliquots, each of ∼ 25 mg, with a CO2 laser using an integrator lens to distribute the beam energy over a 20 mm2 area. All gas fractions were subjected to 180 s of purification with a SAES getter and a cryocooled cold finger maintained at ∼−130 °C. Ion beams were measured on two MAP 215 magnetic sector mass spectrometers (MAP I and MAP III, as specified in Appendix Table 2), both using magnetic field switching to cyclically measure 36–40Ar isotopes in 10 cycles of peak-hopping on Balzers electron multipliers operated in analog mode. Laser heating, extraction line operation and mass spectrometry were fully automated. Backgrounds were measured between every 1–3 samples, and average values (±σ) were used to correct the ion beam intensity data. Mass discrimination was monitored by automated analysis of air pipettes interspersed with the unknowns and correction was made using a power law relationship (Renne et al., 2009). Discrimination values (per atomic mass unit) of 1.0075 ±0.00151 (N= 19) and 1.0030 ±0.0020 (N= 8) were determined for runs on MAP I and MAP III, respectively, using 40Ar/ 36 Ar =295.5 for air (Steiger and Jäger, 1977). A more recent determination of atmospheric 40Ar/36Ar (Lee et al., 2006) is believed to be more accurate, but as shown by Renne et al. (2009) the impact on age calculations for the experiments reported herein is negligible. The Ar isotope data, corrected for backgrounds, mass discrimination, and radioactive decay, are given in Appendix Table 2. The J-value, a neutron-fluence related measure of the conversion of 39 K to 39Ar, was determined from analysis of 6 single crystals (595– 841 μm) of ACs using an age of 1.193 Ma (Nomade et al., 2005). The weighted (by inverse variance) mean value of 40Ar*/39ArK was used to calculate J = (1.296 ± 0.004) × 10− 4 (±0.33%). Corrections for interfering Ar isotopes from Ca, K, and Cl were those reported by Renne et al. (2005) and Renne et al. (2008). Ca/K and Ca/Cl were determined from 37ArCa/39ArK and 37ArCa/38ArCl using the relationships Ca/K = (1.92 ± 0.05) × (37ArCa/39ArK) and Ca/Cl = (0.122 ± 0.007) × (37ArCa/ 38 ArCl), derived from data reported by Jourdan and Renne (2007) for the Hb3gr standard.
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though tending towards higher Ca/K and Ca/Cl values. As shown in Fig. 3, the Ca/K varies from 1.5 to 6, and Ca/Cl varies from 5 to 12. Two anomalous points with Ca/K N 6 and Ca/Cl N 25 also have Mg concentrations 5–10 times higher than average, and we infer that these reflect contributions from diopside microlites. The compositional heterogeneity of the glass phase in this sample, at the cm scale, is further evidence of an impact melt origin of the Dakhleh Glass. Some heterogeneity may be explained by locally variable degrees of crystallization leading to variable enrichment of the residual liquid in incompatible elements. However, neglecting the three points with MgON 4 wt.% (suspected of reflecting a contribution from diopsidic clinopyroxene) there is no suggestion of a negative correlation between incompatible and compatible elements such as K2O and MgO, as would be expected for a fractional crystallization mechanism. 4.2.
40
Ar/39Ar dating
Apparent age spectra (Fig. 4) corresponding to the three incremental heating experiments are markedly different. Aliquot
Fig. 2. Transmitted light photomicrograph of the sample analyzed. White subspherical regions are vesicles. Numbered boxes refer to loci of electron microprobe analyses as shown in Appendix Table 1. Exterior dimensions of white boxes (outside of sample image) labeled 425 μm and 250 μm show bracketing size ranges of sieved size fractions used for 40Ar/39Ar experiments.
4. Results 4.1. Glass compositions Electron microprobe data are given in Appendix Table 1. The elements analyzed in the glasses sum to between 95.1 and 99.8 wt.% oxides, with the majority of sums above 98%. Analysis of two glasses (NBS K-411 and K-412) as secondary standards yielded totals of 99.70 and 99.80 element wt.% respectively, with less than 0.25 element wt.% difference from published values for the elements analyzed. Thus we conclude that the low totals in MRK-DAK13 glass are real. Fe contents are far too low to account for difference if all Fe is Fe+ 3. We conclude that the glass contains 0–4 wt.% H2O, C and/or other elements not analyzed. Osinski et al. (2007) reported up to 0.34 wt.% SO3 in some samples, as well as discrete apparently immiscible globules of CaCO3. Glass compositions in MRK-DAK13 overlap the range reported previously (Osinski et al., 2007) for other Dakhleh glass samples, 1
Uncertainties here and throughout are given at the level of one standard deviation (σ) unless stated otherwise.
Fig. 3. Ca/K vs. Ca/Cl elemental ratios for MRK-DAK13 interstitial glass measured by electron microprobe. Glass compositions reported by Osinski et al. (2007) are shown for comparison.
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36012-1 shows pronounced discordance with a somewhat saddleshaped spectrum, whereas aliquots 36012-2 and 36012-3 yield flat, 100% concordant spectra with identical plateau ages of 151 ± 10 ka. The minimum model (i.e., based on the assumption of atmospheric initial 40Ar/39Ar) age of 178 ± 45 ka of the 8 W step in the 360121spectrum is similar to the plateau ages manifest in the other two aliquots. All three age spectra reveal plateaux in Ca/K and Ca/Cl ratios in steps extracted at low laser power, with both of these ratios climbing monotonically after ∼ 30–50% 39Ar released. This is straightforwardly interpreted to reflect initial degassing of glass with relatively high Ar diffusivity, followed by an increasing contribution of 37ArCa derived chiefly from diopside microlites. Somewhat surprisingly, there is no evidence of spuriously young ages at higher extraction temperatures resulting from (1) 39ArK recoiled from the glass into the diopside (e.g., Turner and Cadogan, 1974; Huneke and Smith, 1976) or (2) recoil fractionation of 37ArCa (strongly recoiled) from 36ArCa (weakly recoiled) (Jourdan et al., 2007a). The average plateau values of Ca/K (2.14 ± 0.07) and Ca/Cl (9.45 ± 0.27) compare favorably with the composition of interstitial glass measured by the electron microprobe, as shown in Fig. 3. The fact that the apparent age plateaux for aliquots 36012-2 and 36012-3 extend to gas fractions with Ca/Cl significantly higher than the glass phase indicates that the diopside microlites and other crystalline phases in these aliquots contain no detectable Ar with isotopic composition different from atmospheric. Thus we conclude that aliquot 36012-1, with the discordant age spectrum, must contain inherited 40Ar derived from incomplete degassing of some of the parent rock that was melted to produce the Dakhleh Glass. Though deemed improbable, it is conceivable that the extraneous material arose from laboratory contamination. Cast on isotope correlation diagrams (Fig. 5), the data from aliquots 36012-2 and 36012-3 define well-constrained isochrons that are straightforwardly interpreted as mixing lines between initial trapped components of atmospheric composition (initial 40Ar/ 36 Ar = 296.3 ± 1.6 and 295.3 ± 1.5, respectively) and K-correlated radiogenic components whose ages are 140 ± 20 ka and 150 ± 20 ka (respectively). Four of the steps from aliquot 36012-1, corresponding to Ca/Cl between 8.9 and 9.7 define a similar isochron of 150 ± 70 ka. An isochron (Fig. 6(a)) fit to 30 points from all 3 aliquots, excluding only the 6 markedly discordant steps from aliquot 36012-1, yields an age of 143 ± 14 ka, initial 40 Ar/ 36 Ar = 296.6 ± 1.0, MSWD = 0.99, probability = 0.48. To minimize the possibility of bias, an isochron was fit to data from all three aliquots whose Ca/K and Ca/Cl ratios are consistent with the glass compositions measured directly by electron microprobe from the same sample. This compositional filter admits 22 steps, and the isochron (Fig. 6(b)) derived from them has an age of 145 ± 19 ka, with initial 40Ar/ 36 Ar = 296.3 ± 1.3, MSWD = 1.2, probability = 0.22. The compositional filter employed here would not necessarily exclude inherited Ar from small amounts of material, especially if compositionally similar to the glass, but in concert with the statistical evidence for a binary mixture of Ar components, the validity of this isochron age seems unequivocal.
5. Discussion and conclusions One of the challenges to 40Ar/39Ar dating of glassy impact melt rocks, exemplified by the Tswaing crater in South Africa (Jourdan et al., 2007b), is incomplete degassing of radiogenic 40Ar accumulated in the target rock(s) prior to impact. If such inherited 40Ar is homogeneously distributed at the atomic scale in a glass, it may be difficult or impossible to fractionate it from post-impact ingrown radiogenic 40Ar by laboratory heating, hence inherited argon may be undetectable in isotope correlation plots.
Fig. 4. Apparent age, Ca/K and Ca/Cl spectra for three individual aliquots of MRKDAK13. (a) Aliquot 36012-01. No plateau is defined; (b) Aliquot 36012-02 and (c) Aliquot 36012-03 yield age plateaux including 100% of cumulative 39Ar released.
Whether Ar diffusion in impact melts is sufficiently fast to enable extensive isotopic equilibration with the atmosphere is difficult to evaluate in most cases because: (1) the thermal history of the melt at the spatial scale of interest is poorly constrained; (2) Ar diffusivities, and their temperature dependence, in diverse melt compositions are not well known. Moreover, water contents of impact melts, which can increase Ar diffusivity by more than an order of magnitude per wt.% in silicate liquids (Zhang et al., 2007), are generally not known as a function of temperature or time. A further complication is that impact melts may involve the coalescence of small melt droplets, hence the lengthscale for diffusion may evolve over a short critical time interval. As a consequence of these uncertainties, modeling the degassing of the Dakhleh glass and its progenitor melt is not sufficiently well constrained to permit meaningful predictions as to whether or not inherited Ar is expected to be present in significant concentrations. However, several qualitative features of the Dakhleh Glass, contrasting with impact melt rocks having demonstrable inherited Ar (e.g., Tswaing), would tend to disfavor age bias from inherited Ar: (1) the relatively young age (Cretaceous to Holocene) and low K-contents of the inferred target materials (Osinski et al., 2007), thereby producing lower concentrations of 40Ar in the melting environment; (2) the relatively low SiO2 and high H2O contents, which would tend to enhance Ar diffusivity.
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Fig. 6. Isochron diagrams for (a) steps from all 3 aliquots. Shaded ellipses are excluded from isochron regression; (b) steps from all 3 aliquots meeting compositional criteria of glass (see text). Error envelopes are shown (shaded) for isochrons.
Fig. 5. Isochron diagrams for three individual aliquots of MRK-DAK13. (a) Aliquot 36012-01 (shaded ellipses are excluded from isochron regression); (b) Aliquot 3601202 and (c) Aliquot 36012-03 isochrons include all steps. Error envelopes are shown (shaded) for isochrons.
In the case of Dakhleh, it is clear from the extensive vesiculation that a discrete vapor phase was present upon quenching, though it is unclear whether this was exsolved from the melt or was rather mechanically admixed during the rapid melting event. In either case, any inherited Ar remaining in the melt would be expected to partition into the vapor phase. The size distribution of the vesicles extends well into the grain size fraction analyzed in our 40Ar/39Ar experiments, and whatever Ar was present in them would inevitably have been represented to some extent in the step heating analyses. Given that the isochron intercepts betray no evidence for inherited Ar with supra-atmospheric 40Ar/36Ar, we conclude that the vapor phase represented by the vesicles contained Ar whose isotopic composition was dominated by air. After deletion of outliers suspected of arising from undegassed clast(s), analysis of 40Ar/39Ar step heating data yields statistically valid isochrons whether or not the regression includes compositions restricted to those of the glass phase. At a penalty of ∼ 30% in precision, the result corresponding to the glass is preferred. Thus the impact event that produced the Dakhleh Glass is inferred to have occurred at 145 ± 19 ka, consistent with the typological aspects of the underlying Older Middle Stone Age archeology. The relatively large uncertainty in this age encompasses the range of variations arising
from updated values of systematic variables such as those related to the standard and 40K decay constants (e.g., Kuiper et al., 2008 and references therein). For example, the calibration presented by Kuiper et al. (2008) yields an age of 146 ka. Potential consequences of the impact event on the Middle Stone Age people in the Dakhleh region are a topic of ongoing interest (e.g., Smith et al., 2009). The age of the impact is compatible with the likely age of underlying artifacts in the Kellis and Teneida paleobasins. Thus, it is plausible that people inhabited the area immediately prior to the impact. It can be reasonably hypothesized that the consequences of the impact may have displaced people from the area for an as-yet undetermined interval of time. A reliable age for the Dakhleh glass provides a useful chronostratigraphic datum for future archeological and paleoenvironmental studies in the Dakhleh region. This is especially welcome because chronometrically-dateable materials related to Pleistocene lithics are scarce. The areal extent of impact-related debris has yet to be determined, although presumably it will prove larger than the ca. 400 km2 area in which mesoscopic material has been found to date. Acknowledgements P.R.R. gratefully acknowledges support from the Ann and Gordon Getty Foundation, and from the U.S. National Science Foundation (grants SBR-9601592 and BCS-0715465) for the BGC 40Ar/39Ar lab facilities used in this study. Members of the Dakhleh Oasis Project, ‘DG Working Group’ particularly thank Leslie and Tony Mills for facilitating fieldwork over the years, and Charles Churcher, Jennifer
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Smith, Johanna Kieniewicz, and Albert Haldemann for various contributions. Field research on Dakhleh glass has been funded by the Social Sciences and Humanities Research Council of Canada, the Natural Sciences and Engineering Research Council of Canada, the National Geographic Society, the University of Toronto, and the Dakhleh Trust. Constructive advice from D.F. Mark, R.W. Carlson and an anonymous reviewer helped improve the manuscript and are appreciated. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi: 10.1016/j.epsl.2010.01.014. References Boslough, M.B., Crawford, D.A., 2008. Low-altitude airbursts and the impact threat. Int. J. Impact Eng. 35, 1441–1448. Churcher, C.S., Kleindienst, M.R., Schwarcz, H.P., 1999. Faunal remains from a Middle Pleistocene lacustrine marl in Dakhleh Oasis, Egypt: palaeoenvironmental reconstructions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 154, 301–312. Deino, A.L., McBrearty, S., 2002. 40Ar/39Ar dating of the Kapthurin Formation, Baringo, Kenya. J. Hum. Evol. 42, 185–210. Donovan, J.J., Tingle, T.N., 1996. An improved mean atomic number correction for quantitative microanalysis. J. Microsc. 2, 1–7. Donovan, J.J., Snyder, D.A., Rivers, M.L., 1993. An improved interference correction for trace element analysis. Microbeam Anal. 2, 23–28. Huneke, J.C., Smith, S.P., 1976. The realities of recoil: 39Ar out of small grains and anomalous patterns in 40Ar–39Ar dating. Geochim. Cosmochim. Acta Suppl. 7, 1987–2008. Jourdan, F., Renne, P.R., 2007. Age calibration of the Fish Canyon sanidine 40Ar/39Ar dating standard using primary K–Ar standards. Geochim. Cosmochim. Acta 71, 387–402. Jourdan, F., Matzel, J.P., Renne, P.R., 2007a. 39Ar and 37Ar recoil ejection during neutron irradiation of sanidine and plagioclase. Geochim. Cosmochim. Acta 71, 2791–2808. Jourdan, F., Renne, P.R., Reimold, W.U., 2007b. The problem of inherited 40Ar* in dating impact glass by the 40Ar/39Ar method: evidence from the Tswaing impact crater (South Africa). Geochim. Cosmochim. Acta 71, 1214–1231. Jourdan, F., Renne, P.R., Reimold, W.U., 2009. An appraisal of the ages of terrestrial impact structures. Earth Planet. Sci. Lett. 286, 1–13. Kieniewicz, J.M. 2007. Pleistocene Pluvial Lakes of the Western Desert of Egypt: Paleoclimate, Paleohydrology, and Paleolandscape Reconstructions, Ph.D. Thesis, Department of Earth and Environmental Sciences, St. Louis, Washington University. Kleindienst, M.R., 2004. Strategies for studying Pleistocene archeology based upon surface evidence: first characterization of an older Middle Stone Age unit, Dakhleh Oasis, Egypt. In: Bowen, G.E., Hope, C.A. (Eds.), The Oasis Papers 3: Proceedings of the Third International Conference of the Dakhleh Oasis Project. Dakhleh Oasis Project monograph, vol. 14. Oxbow Books, Oxford, pp. 1–42. Kleindienst, M.R., Churcher, C.S., McDonald, M.M.A., Schwarcz, H.P., 1999. Geography, geology, geochronology, and geoarchaeology of the Dakhleh Oasis Region: an interim report. In: Chuecher, C.S., Mills, A.J. (Eds.), Reports from the Survey of Dakhleh Oasis, Western Desert of Egypt, 1977–1987. Dakhleh Oasis Project Monograph, vol. 2. Oxbow Books, Oxford, pp. 1–54.
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