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(U-Th)/He Ages of Phosphates from Zagami and ALHA77005 Martian Mete- Orites: Implications to Shock Temperatures

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(U-Th)/He Ages of Phosphates from Zagami and ALHA77005 Martian Mete- Orites: Implications to Shock Temperatures Accepted Manuscript (U-Th)/He Ages of Phosphates from Zagami and ALHA77005 Martian Mete- orites: Implications to Shock Temperatures Kyoungwon Min, Annette Farah, Seung Ryeol Lee, Jong Ik Lee PII: S0016-7037(16)30523-3 DOI: http://dx.doi.org/10.1016/j.gca.2016.09.009 Reference: GCA 9921 To appear in: Geochimica et Cosmochimica Acta Received Date: 28 January 2016 Accepted Date: 14 September 2016 Please cite this article as: Min, K., Farah, A., Lee, S.R., Lee, J.I., (U-Th)/He Ages of Phosphates from Zagami and ALHA77005 Martian Meteorites: Implications to Shock Temperatures, Geochimica et Cosmochimica Acta (2016), doi: http://dx.doi.org/10.1016/j.gca.2016.09.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. 1 (U-Th)/He Ages of Phosphates from Zagami and ALHA77005 Martian Meteorites: 2 Implications to Shock Temperatures 3 4 Kyoungwon Min1*, Annette Farah1†, Seung Ryeol Lee2 and Jong Ik Lee3 5 1Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA 6 2Korea Institute of Geosciences and Mineral Resources, Daejeon, Korea 7 3Korea Polar Research Institute, Incheon, Korea 8 † Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA 9 10 *Corresponding Author 11 Tel) 352-392-2720 12 Email) [email protected] 13 1 14 Abstract 15 Shock conditions of martian meteorites provide crucial information about ejection dynamics and original 16 features of the martian rocks. To better constrain equilibrium shock temperatures (Tequi-shock) of martian 17 meteorites, we investigated (U-Th)/He systematics of moderately-shocked (Zagami) and intensively 18 shocked (ALHA77005) martian meteorites. Multiple phosphate aggregates from Zagami and 19 ALHA77005 yielded overall (U-Th)/He ages 92.2 ± 4.4 Ma (2σ) and 8.4 ± 1.2 Ma, respectively. These 20 ages correspond to fractional losses of 0.49 ± 0.03 (Zagami) and 0.97 ± 0.01 (ALHA77005), assuming 21 that the ejection-related shock event at ~3 Ma is solely responsible for diffusive helium loss since 22 crystallization. For He diffusion modeling, the diffusion domain radius is estimated based on detailed 23 examination of fracture patterns in phosphates using a scanning electron microscope. For Zagami, the 24 diffusion domain radius is estimated to be ~2-9 µm, which is generally consistent with calculations from 25 isothermal heating experiments (1-4 µm). For ALHA77005, the diffusion domain radius of ~4-20 µm is 26 estimated. 27 Using the newly constrained (U-Th)/He data, diffusion domain radii, and other previously estimated 28 parameters, the conductive cooling models yield Tequi-shock estimates of 360-410 °C and 460-560 °C for 29 Zagami and ALHA77005, respectively. According to the sensitivity test, the estimated Tequi-shock values 30 are relatively robust to input parameters. The Tequi-shock estimates for Zagami are more robust than those 31 for ALHA77005, primarily because Zagami yielded intermediate fHe value (0.49) compared to 32 ALHA77005 (0.97). For less intensively shocked Zagami, the He diffusion-based Tequi-shock estimates (this 33 study) are significantly higher than expected from previously reported Tpost-shock values. For intensively 34 shocked ALHA77005, the two independent approaches yielded generally consistent results. Using two 35 other examples of previously studied martian meteorites (ALHA84001 and Los Angeles), we compared 36 Tequi-shock and Tpost-shock estimates. For intensively shocked meteorites (ALHA77005, Los Angeles), the He 37 diffusion-based approach yield slightly higher or consistent Tequi-shock with estimations from Tpost-shock, and 38 the discrepancy between the two methods increases as the intensity of shock increases. The reason for the 2 39 discrepancy between the two methods, particularly for less-intensively shocked meteorites (Zagami, 40 ALHA84001), remains to be resolved, but we prefer the He diffusion-based approach because its Tequi-shock 41 estimates are relatively robust to input parameters. 42 Keywords: Zagami, ALHA77005, Martian Meteorite, (U-Th)/He, Shock Temperature 43 3 44 Introduction 45 Shock impact is one of the most prominent dynamic events to have occurred after the formation 46 of any planetary body in our solar system (Wetherill, 1975; Melosh, 1984). Such near-surface episodes 47 cause an instantaneous temperature increase in the ejected materials, followed by rapid cooling. 48 Constraining shock P-T conditions and post-shock cooling paths of meteorites is crucial in at least the 49 following three aspects: (1) understanding ejection processes, (2) evaluating pre-shock features in the 50 meteorites, and (3) testing possible transfer of viable life among different planets. Tremendous effort has 51 been devoted to studying the physical conditions of the shock events (Ahrens and Gregson, 1964; Ahrens 52 et al., 1969; Stöffler, 1971; Melosh, 1989; Stöffler et al., 1986, 1991; Stöffler and Langenhorst, 1994). 53 The most solidly established method to constrain shock P-T involves comparing microscopic textures of 54 meteorites with those of artificially shocked terrestrial rocks (summarized in Stöffler et al., 1988). Using 55 the equation of state, shock pressures can be converted to corresponding “post-shock temperatures (Tpost- 56 shock),” which represent temperature increases (∆T) during the shock relative to the pre-shock 57 temperatures. The shock conditions determined for martian meteorites are summarized in Nyquist et al. 58 (2001) and re-evaluated by Artemieva and Ivanov (2004) and Fritz et al. (2005). 59 An alternative way to estimate the shock T conditions is using radioisotopic systems that are 60 sensitive to temperature. This approach can provide absolute temperature conditions of the shock event 61 instead of T increases (∆T) that can be constrained through the texture-based approach. 40Ar/39Ar method 62 has been used for martian meteorites because of the rapid diffusion of Ar in maskelynite (or feldspar), the 63 major K-bearing mineral phase in the meteorites. The 40Ar/39Ar ages of all martian meteorites are much 64 older than their times of ejection, because ejection-related shock (1) caused a limited effect on the 65 diffusive loss of Ar (Bogard et al., 1979; Ash et al., 1996; Turner et al., 1997; Bogard and Garrison, 1999; 66 Shuster and Weiss, 2005; Walton et al., 2007; Bogard and Park, 2008), and (2) implanted atmospheric Ar 67 into the target materials (Bogard and Johnson, 1983; Bogard et al., 1984, 1986; Becker and Pepin, 1984; 68 Park et al, 2014). The resulting 40Ar/39Ar age spectra can be forward-modeled to provide a set of thermal 4 69 histories, including the shock T conditions of ejection (Weiss et al., 2002), as well as pre-ejection thermal 70 histories (Shuster and Weiss, 2005; Cassata et al., 2010). The (U-Th)/He thermochronometer is more 71 sensitive to temperatures than 40Ar/39Ar, allowing for characterization of short-term, high-T events, such 72 as shock impacts on the surface of planetary bodies (e.g., Mars: Schwenzer et al., 2007, 2008), or 73 wildfires that have occurred on the Earth’s surface (Mitchell and Reiners, 2003; Reiners et al., 2007). In 74 addition to the high sensitivity to temperatures, the (U-Th)/He method has other merits in constraining 75 shock T conditions of martian meteorites: (1) U- and Th-rich minerals (e.g., phosphates) are common in 76 many martian and other meteorites; (2) the applicable age range of the method spans from the beginning 77 of the solar system (4.5 Ga; Min et al., 2003) to relatively modern human history (AD 79; Aciego et al., 78 2003); and (3) He concentration in the martian atmosphere is negligible (Owen et al., 1977), therefore it is 79 unnecessary to consider atmospheric He contamination for (U-Th)/He age determinations, in contrast to 80 Ar implantation during impact (Bogard and Johnson, 1983; Bogard et al., 1984, 1986; Becker and Pepin, 81 1984). 82 Although the high diffusivity of He can provide a means to describe the transient episode, it poses 83 a significant problem for (U-Th)/He thermochronology because the resulting ages from various meteorites 84 are frequently scattered and differ from the expected formation ages (Strutt, 1908, 1909, 1910). Before 85 the late 1980s, when detailed He diffusion properties became known, it was difficult to quantitatively 86 evaluate the meanings of (U-Th)/He ages. Another problem related to (U-Th)/He dating of meteorites is 87 contamination by other sources of 4He, particularly cosmogenic 4He (Baur, 1947). In the early history of 88 (U-Th)/He application to meteorites, the cosmogenic 4He correction was not properly included in age 89 calculations, resulting in old ages (Paneth et al., 1930; Arrol et al., 1942). Because these two contrasting 90 problems (one tending to yield younger, and the other older ages) were combined, the resulting ages were 91 commonly scattered and considered “unreliable” (summarized in Min, 2005). With these problems in 92 mind, multiple studies were performed for a range of meteorites: iron meteorites (Paneth et al., 1930, 93 1952), ordinary chondrites (Heymann, 1967; Wasson and Wang, 1991; Alexeev, 1998), and martian 5 94 meteorites
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