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Matchan, EL, Honda, M., Barrows, TT, Phillips, D Matchan, E. L., Honda, M., Barrows, T. T., Phillips, D., Chivas, A. R., Fifield, L. K. and Fabel, D. (2018) Production of 21 Ne in depth- profiled olivine from a 54 Ma basalt sequence, Eastern Highlands (37° S), Australia. Geochimica et Cosmochimica Acta, 220, pp. 276-290. (doi:10.1016/j.gca.2017.09.035) This is the author’s final accepted version. There may be differences between this version and the published version. You are advised to consult the publisher’s version if you wish to cite from it. http://eprints.gla.ac.uk/151811/ Deposited on: 17 November 2017 Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk *Manuscript 1 Production of 21Ne in depth-profiled olivine from a 54 Ma basalt 2 sequence, Eastern Highlands (37° S), Australia 3 Erin L. Matchana, Masahiko Hondab, Timothy T. Barrowsc, David Phillipsa, Allan R. 4 Chivasd, L. Keith Fifielde, Derek Fabelb, 1 5 aSchool of Earth Sciences, The University of Melbourne, Parkville, VIC 3010, Australia.. 6 bResearch School of Earth Sciences, The Australian National University, Canberra, ACT 0200, 7 Australia. 8 cGeography, College of Life and Environmental Sciences, University of Exeter, Exeter, Devon, EX4 9 4RJ, UK. 10 dGeoQuEST Research Centre, School of Earth and Environmental Sciences, University of Wollongong, 11 NSW 2522, Australia. 12 eDepartment of Nuclear Physics, Research School of Physics and Engineering, The Australian 13 National University, Canberra, ACT 0200 Australia. 14 1 Now at: Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK. 1 15 Abstract 16 In this study we investigate the cosmogenic neon component in olivine samples from 17 a vertical profile in order to quantify muogenic 21Ne production in this mineral. 18 Samples were collected from an 11 m thick Eocene basalt profile in the Eastern 19 Highlands of southeastern Australia. An eruption age of 54.15±0.36 Ma (2) was 20 determined from 40Ar/39Ar step-heating experiments (n=6) on three whole-rock 21 samples. A 36Cl profile on the section indicated an apparent steady state erosion rate 22 of 4.7 ± 0.5 m Ma-1. The eruption age was used to calculate in situ produced 23 radiogenic 4He and nucleogenic 3He and 21Ne concentrations in olivine. Olivine 24 mineral separates (n=4), extracted from the upper two metres of the studied profile, 25 reveal cosmogenic 21Ne concentrations that attenuate exponentially with depth. 21 26 However, olivine (Fo68) extracted from below 2 m does not contain discernible Ne 27 aside from magmatic and nucleogenic components, with the exception of one sample 28 that apparently contained equal proportions of nucleogenic and muogenic neon. 29 Modelling results suggest a muogenic neon sea-level high-latitude production rate of 30 0.02 ± 0.04 to 0.9 ± 1.3 atom g-1 a-1 (1σ), or <2.5% of spallogenic cosmogenic 21Ne 31 production at Earth’s surface. These data support a key implicit assumption in the 32 literature that accumulation of muogenic 21Ne in olivine in surface samples is likely to 33 be negligible/minimal compared to spallogenic 21Ne. 34 Keywords: cosmogenic 21Ne; noble gases; olivine; muons; cosmogenic 36Cl 35 1. Introduction 36 Bombardment of minerals in the Earth’s crust by secondary cosmic radiation gives 37 rise to a variety of nuclear reactions. The end products of these reactions are known as 38 secondary in situ terrestrial cosmogenic nuclides, the study of which has many useful 39 geological applications (Gosse and Phillips, 2001; Dunai, 2010). The reactions that 40 produce cosmogenic nuclides are classified into three categories: spallation, muon 41 interactions and neutron capture. 42 At depths exceeding about three metres, muon interactions become the major 43 cosmogenic nuclide production mechanism (e.g. Lal, 1987; Brown et al., 1995). The 44 dominant type of muon-induced reaction producing most cosmogenic nuclides down 45 to ca. 100 hg cm-2 (equivalent to a depth of 30 m for rock density of 3 g cm-3) is 46 nuclear capture of negative slow muons, below which fast-muon induced reactions 47 dominate (e.g. Heisinger et al. 2002a,b). In rock surfaces at sea level, the contribution 2 48 of muons to total cosmogenic nuclide production ranges from minor, (e.g. 1-4 % for 49 10Be and 26Al production in quartz; Brown et al., 1995; Heisinger and Nolte, 2000), to 50 as much as 10% (e.g. 36Cl production in calcite; Stone et al., 1998). Muogenic 51 production of noble gas isotopes is relatively poorly understood because nuclear 52 cross-sections have not been measured and there are fewer empirical measurements. 53 Constraining the muogenic component is imperative because the muon flux scales 54 with altitude differently to the fast neutron flux (e.g. Lal 1988; Dunai, 2000), and 55 muogenic production could be important for studies of longer exposure involving 56 greater sample depths, and burial history (e.g. Balco and Shuster, 2009; Braucher et 57 al., 2013). 58 In this paper we investigate the muogenic component of 21Ne production in olivine. 59 We analyse depth-profiled samples from an Eocene basalt flow exposed in a quarry in 60 the Eastern Highlands of southeastern Australia. Whole-rock 36Cl analyses were 61 conducted on the same profile as an independent measure of the exposure history. 62 1.1. Cosmogenic neon production in olivine 63 In surface samples, it is generally assumed that the muogenic contribution to the 64 cosmogenic neon budget is insignificant compared to the spallogenic component (e.g. 65 Gillen et al., 2010; Schimmelpfennig et al., 2011). A major limitation for validating 66 this assumption is that nuclear reaction cross-sections and probability factors have not 67 been experimentally determined for muon-capture reactions on elements producing 68 isotopes of neon. Therefore, these values currently have to be estimated from proxy 69 reactions assumed to have similar energy profiles (e.g. Fernandez-Mosquera et al., 70 2010). 21 21 71 In olivine ((Mg,Fe)2SiO4), cosmogenic Ne ( Nec) is considered to be dominantly 72 produced by the spallation reaction 24Mg(n,)21Ne, with a minor component derived 73 from reactions on Si (~20%) and trace amounts from reactions on Al, Na, Ca and Fe 74 (e.g. Masarik and Reedy 1995; Leya et al., 1998; Masarik 2002). Although numerous 3 3 75 studies have investigated cosmogenic He ( Hec) production in olivine (see review by 21 76 Goehring et al., 2010), relatively few have considered Nec production in this 21 77 mineral. This is likely primarily due to the lower Nec production rate in olivine of 78 ca. 45 atom g-1 a-1 (dependent on Mg-content; Poreda and Cerling 1992) at sea-level -1 -1 3 79 high-latitude (SLHL), compared to 123 ± 4 atom g a SLHL for Hec (1σ; Goehring 3 80 et al., 2010). It is also stressed that measurement of neon isotopes is more challenging 21 81 than measurement of helium isotopes. In the case of Nec analyses of Quaternary 82 samples, olivine separates in the order of a gram are typically required given 83 conventional mass spectrometer sensitivities (e.g. Amidon et al., 2009; Gillen et al., 21 84 2010; Espanon et al., 2014). Studies investigating Nec production in pyroxene are 21 -1 -1 85 less common, with Nec SLHL production rates of approximately 20–50 atom g a , 86 dependent on elemental composition and production rate calculation methods (e.g. 87 Schaefer et al., 1999,2000; Niedermann et al. 2007). 88 1.2. Deconvolving the cosmogenic neon signature 89 As summarised by Graham (2002), the neon isotopic compositions of both Ocean 90 Island Basalts (OIB) and Mid-Ocean Ridge Basalts (MORB) occupy a broad 91 distribution, but 21Ne/20Ne values generally overlap with the atmospheric composition 92 because atmospheric neon generally overwhelms mantle neon in these samples. 93 Therefore, the trapped 21Ne/20Ne ratio in olivine can be assumed to be 21 20 94 indistinguishable from the atmospheric composition in most cases (( Ne/ Ne)air = 95 0.00296 0.7% (Eberhardt et al., 1965)), avoiding the need for sample-intensive 96 crushing experiments as routinely applied in characterising cosmogenic He in these 97 samples. 21 98 In situ produced nucleogenic neon ( Nenucleo) is an important consideration for rocks 21 99 with crystallisation ages older than a few 100 ka. Nenucleo is produced predominantly 100 by the reaction 18O(,n)21Ne and to a lesser extent 24Mg(n,)21Ne (Wetherill, 1954). 101 In both cases, and n are produced as a result of local decay of U and Th. 102 Nucleogenic production of 20Ne and 22Ne under normal mantle conditions is 103 considered to be insignificant (Yatsevich and Honda, 1997). The production rate of 20 20 -21 3 -1 -1 104 Nenucleo is extremely low (e.g. Nenucleo ~2 10 cm STP g a , assuming a 105 typical U concentration in olivine of 2 ppm; Yatsevich and Honda, 1997). This results 20 20 - 106 in negligible amounts of Nenucleo compared to a typical total Ne content of >1 10 107 11 cm3 STP g-1 in olivine (e.g. Gillen et al., 2010; Schimmelpfennig et al., 2011). 108 Similarly, since the 21Ne/20Ne production ratio of cosmogenic Ne components in 109 olivine is ~1 (Schafer et al., 1999), the contribution of cosmogenic 20Ne can be 110 ignored. Therefore, all 20Ne in a sample may be regarded as magmatic, atmospheric or 111 their mixture. Using the elemental composition of the sample and following the 4 21 112 approach of Ballentine and Burnard (2002), the more significant Nenucleo contents 113 may be calculated (see section 4.4). 114 2.
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