The evidence for solar cosmic ray produced neon in ... · Figure 3: H (and C) in CI chondrite...
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The evidence for solar cosmic ray produced neon in shergottites, acapulcoites/lodranites, and angrites revisited R. Wieler 1 , L. Huber 1 , I. Leya 2 , J. Masarik 3 , R. Trappitsch 4 1 ETH Zürich, Earth Sciences, Zürich, Switzerland; 2 Univ. Bern, Space Res. and Planet. Sci., Bern, Switzerland; 3 Comenius Univ., Nuclear Physics, Bratislava, Slovakia; 4 Univ. of Chicago, Center for Cosmochemistry, Chicago IL, USA [email protected] SCR-Ne ubiquitous in shergottites, very rare in chondrites? ( 21 Ne/ 22 Ne) cos ratios <0.80 found in many shergottites [1, 2] (and some other rare achondrites [e. g. 3-5]), but almost in no chondrites. Explained by SCR-Ne (( 21 Ne/ 22 Ne) SCR ~0.6 [6]) in a large fraction of shergottites and other achondrites, due to low ablation losses [1-5]. This work: We revisit evidence for SCR Ne by comparing measured ( 21 Ne/ 22 Ne) cos with predictions for GCR-Ne production by Leya & Masarik [7]. References. [1] Garrison DH et al. (1995) Meteoritics 30:738 [2] Schwenzer S et al. (2007) MAPS 42:387 [3] Weigel A et al. (1999) GCA 63:175 [4] Swindle TD et al. (1998) MAPS 33:31 [5] Busemann H et al. (2006) GCA 70:5403 [6] Trappitsch R & Leya I (2014) this conference, #1894 [7] Leya I & Masarik J (2009) MAPS 44:1061 [8] Swindle TD et al. (1996) LPS XXVII:1297 [9] Welten KC et al. (2006) MAPS 41:1081 Fig. 1: Measured ( 21 Ne/ 22 Ne) cos ratios in shergottites (mostly literature data) and values modelled for GCR-Ne in r = 5 cm (black lines) and r = 10 cm (red lines) meteoroids of the respective target element composition. Green line: modelled range for r = 5 cm L chondrite Minimum modelled ( 21 Ne/ 22 Ne) GCR ratios for shergottites (Fig. 1) Minimum GCR value predicted for small chondrites (green line) consistent with observations on many chondrites [1]. Model predictions accurate. Measured data often lower than modelled minimum GCR ratios, in agreement with [1]. SCR-Ne likely explanation. However, no dependence of minimum ( 21 Ne/ 22 Ne) GCR on Mg/(Al+Si) for very small meteoroids, unlike as proposed by [1]. (#1331) Figure 2: Influence of Na on ( 21 Ne/ 22 Ne) GCR for two shergottites with high and low Mg, resp., and assumed variable Na concentrations. Ranges (from surface to center) for three meteoroid radii shown SCR-Ne in achondrites (Fig. 4) Several acapulcoites/lodranites, brachinite-related LEW88763 and at least some of the angrites shown have ( 21 Ne/ 22 Ne) cos values lower than possibly to be explained by pure GCR-Ne. Like shergottites, a remarkably large fraction of these rare achondrites very likely contains SCR-Ne, as already concluded or suspected by others [3-5]. Figure 3: H (and C) in CI chondrite matrix lead to lower ( 21 Ne/ 22 Ne) GCR ratios. Summary. Model calculations [7] confirm [1-5] that many shergottites and rare achondrites contain SCR-Ne. Sodium needs to be considered case by case but high Na cannot explain the lowest measured ( 21 Ne/ 22 Ne) cos values. A matrix effect is also not responsible for low ( 21 Ne/ 22 Ne) cos in these H- and C-poor meteorites. One reason for the preponderence of SCR-Ne in rare meteorites seems to be that for them noble gases are routinely measured even if the recovered mass is very small, i. e. atmospheric ablation is untypically low. Influence of Na (Fig. 2) High Na in some shergottites can depress ( 21 Ne/ 22 Ne) GCR . Na needs to be considered case by case. However, only in interior samples from very large meteoroids values < 0.80 are expected. Na not reason for low observed ( 21 Ne/ 22 Ne) cos in many shergottites. Matrix effect ? (Fig. 3) Flux and energy distribution of secondary cosmic-ray particles depend on bulk meteorite composition, e. g., H and C concentration. However, me- teorites considered here have low H and C. Matrix effect cannot explain low observed ( 21 Ne/ 22 Ne) cos ratios. Fig. 4: Measured ( 21 Ne/ 22 Ne) cos ratios in various achondrites (literature data) and GCR values modelled for meteoroids of three different radii. Why is SCR-Ne much more common in these rare meteorite classes than in chondrites? * Small preatmospheric size/space erosion, high solar proton flux during last few Ma? [1] * Different orbital parameters of shergottites compared to other meteorites (including nakhlites), leading to lower entry velocities [1] * Typical shergottite orbits may lead to more time close to the sun [1] * Are encounter velocities of shergottites really sufficiently low to lead to low ablation [8]? * Shergottites and acapulcoites/lodranites physically strong, less likely to fragment [4]. * Selection effect: many achondrites studied for noble gases have very small recovered masses (a few to a few ten grams), unlike chondrites. Among 26 exceptionally small chondrites from the Frontier Mountain area, two likely contain SCR-Ne [9], supporting sample size selection bias as one reason for preponderence of SCR-Ne in rare classes.
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The evidence for solar cosmic ray produced neon in shergottites, acapulcoites/lodranites, and angrites revisited
R. Wieler1, L. Huber1, I. Leya2, J. Masarik3, R. Trappitsch4
1ETH Zürich, Earth Sciences, Zürich, Switzerland; 2Univ. Bern, Space Res. and Planet. Sci., Bern, Switzerland; 3Comenius Univ., Nuclear Physics, Bratislava, Slovakia; 4Univ. of Chicago, Center for Cosmochemistry, Chicago IL, USA
SCR-Ne ubiquitous in shergottites, very rare in chondrites?(21Ne/22Ne)cos ratios <0.80 found in many shergottites [1, 2] (and someother rare achondrites [e. g. 3-5]), but almost in no chondrites.Explained by SCR-Ne ((21Ne/22Ne)SCR ~0.6 [6]) in a large fraction ofshergottites and other achondrites, due to low ablation losses [1-5].
This work: We revisit evidence for SCR Ne by comparing measured(21Ne/22Ne)cos with predictions for GCR-Ne production by Leya &Masarik [7].
References. [1] Garrison DH et al. (1995) Meteoritics 30:738 [2] Schwenzer S et al. (2007) MAPS42:387 [3] Weigel A et al. (1999) GCA 63:175 [4] Swindle TD et al. (1998) MAPS 33:31 [5] BusemannH et al. (2006) GCA 70:5403 [6] Trappitsch R & Leya I (2014) this conference, #1894 [7] Leya I &Masarik J (2009) MAPS 44:1061 [8] Swindle TD et al. (1996) LPS XXVII:1297 [9] Welten KC et al.(2006) MAPS 41:1081
Fig. 1: Measured (21Ne/22Ne)cos ratios in shergottites (mostly literature data) and values modelled for GCR-Ne in r = 5 cm (black lines) and r = 10 cm (red lines) meteoroids of the respective target element composition. Green line: modelled range for r = 5 cm L chondrite
Minimum modelled (21Ne/22Ne)GCR ratios for shergottites (Fig. 1)Minimum GCR value predicted for small chondrites (green line)consistent with observations on many chondrites [1]. Modelpredictions accurate.
Measured data often lower than modelled minimum GCR ratios, inagreement with [1]. SCR-Ne likely explanation. However, nodependence of minimum (21Ne/22Ne)GCR on Mg/(Al+Si) for very smallmeteoroids, unlike as proposed by [1].
(#13
31)
Figure 2: Influence of Na on (21Ne/22Ne)GCR for two shergottites with high and low Mg, resp., and assumed variable Na concentrations. Ranges (from surface to center) for three meteoroid radii shown
SCR-Ne in achondrites (Fig. 4)Several acapulcoites/lodranites, brachinite-related LEW88763 and atleast some of the angrites shown have (21Ne/22Ne)cos values lowerthan possibly to be explained by pure GCR-Ne. Like shergottites,a remarkably large fraction of these rare achondrites very likelycontains SCR-Ne, as already concluded or suspected by others [3-5].
Figure 3: H (and C) in CI chondrite matrix lead to lower (21Ne/22Ne)GCR ratios.
Summary. Model calculations [7] confirm [1-5] that many shergottitesand rare achondrites contain SCR-Ne. Sodium needs to be consideredcase by case but high Na cannot explain the lowest measured(21Ne/22Ne)cos values. A matrix effect is also not responsible for low(21Ne/22Ne)cos in these H- and C-poor meteorites. One reason for thepreponderence of SCR-Ne in rare meteorites seems to be that for themnoble gases are routinely measured even if the recovered mass is verysmall, i. e. atmospheric ablation is untypically low.
Influence of Na (Fig. 2) High Na in some shergottites can depress (21Ne/22Ne)GCR. Na needs to be considered case by case. However, only in interior samples from very large meteoroids values < 0.80 are expected. Na not reason for low observed (21Ne/22Ne)cos in many shergottites.
Matrix effect ? (Fig. 3)Flux and energy distribution ofsecondary cosmic-ray particlesdepend on bulk meteoritecomposition, e. g., H and Cconcentration. However, me-teorites considered here have low Hand C.
Matrix effect cannotexplain low observed (21Ne/22Ne)cosratios.
Fig. 4: Measured (21Ne/22Ne)cos ratios in various achondrites (literature data) and GCR values modelled for meteoroids of three different radii.
Why is SCR-Ne much more common in these rare meteorite classesthan in chondrites?* Small preatmospheric size/space erosion, high solar proton flux during last few Ma? [1]
* Different orbital parameters of shergottites compared to othermeteorites (including nakhlites), leading to lower entry velocities [1]* Typical shergottite orbits may lead to more time close to the sun [1]* Are encounter velocities of shergottites really sufficiently low to lead to low ablation [8]?
* Shergottites and acapulcoites/lodranites physically strong, less likelyto fragment [4].
* Selection effect: many achondrites studied for noble gases have verysmall recovered masses (a few to a few ten grams), unlike chondrites.Among 26 exceptionally small chondrites from the Frontier Mountainarea, two likely contain SCR-Ne [9], supporting sample size selectionbias as one reason for preponderence of SCR-Ne in rare classes.
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