Baecker B

Cartwright JA

Cordier C

Folco L

Ott U

Trieloff M

English

Ott U.

FOLCO, LUIGI

Archivio della Ricerca - Università di Pisa

NOBLE GAS INVENTORY OF MICROMETEORITES FROM THE TRANSANTARCTIC MOUNTAINS. B. Baecker1,4, C. Cordier2, L. Folco3, M. Trieloff4, J. A. Cartwright1 and U. Ott1, 1Abteilung Biogeochemie, Max-Planck-Institut für Chemie, Joh.-Joachim-Becher-Weg 27, 55128 Mainz, Germany. E-mail: b.baecker@mpic.de 2ISTerre, Université de Grenoble, CNRS, F-38041 Grenoble, France. 3Dipartimento di Scienze della Terra, Universi-tà di Pisa, Via S.Maria 53,56126 Pisa, Italy. 4Institut für Geowissenschaften, Universität Heidelberg, Im Neuenhei-mer Feld 236, 69120 Heidelberg, Germany.  Introduction: We have initiated a comprehensive survey of noble gases in micrometeorites (MMs) at the MPIC, Mainz. While the lighter noble gases (especially neon) are generally dominated by solar wind contribu-tions, the inventory of the heavy primordial noble gases is not well characterized. We hope to fill this gap, with a focus on the diagnostic isotopic composition of xen-on. Here, we present initial results obtained on “large” MMs from Victoria Land, Transantarctic Mountains. These were collected during a PNRA (Programma Na-zionale delle Ricerche in Antartide, Italy) expedition on top of the Miller Butte micrometeorite traps #45 b and c [1]. Our future noble gas research will include material from other collections, e.g. CONCORDIA [2].  Samples: We selected 51 samples for our study: 26 cosmic spherules (CS), 11 scoriaceous (Sc) and 14 unmelted (Un), in the ~600-800 µm size range (Fig. 1). They were split into aliquots, of which one remained in Siena for SEM and microprobe analyses and the other was taken to the MPIC Mainz for noble gas analysis.  Fig. 1: Micrographs of MMs from the Transantarctic moun-tains. A) cosmic spherule, B) scoriaceous and C) unmelted. Analytical procedures: Noble gases were released using laser heating (CO2 laser), cleaned with SAES getters and separated into He/Ne, Ar, Kr and Xe frac-tions using activated charcoal. The analyses were per-formed using our high sensitivity Noblesse (Nu Instru-ments) noble gas mass spectrometer with multi-ion-detection (8 channeltrons) [3]. Results and Discussion: At present, we have ob-tained noble gas results for three CS, four Sc and four Un MMs. Our results are summarized in Table 1 and Figures 2-4. Neon: Noble gas abundances in CS are low, and Ne isotopic compositions are indistinguishable from air (Fig. 2). Solar wind (SW)-type and spallogenic contri-butions are evident in Sc and Un MMs, however, with the latter particularly prominent in Sc 45b08 (Fig. 2). The highest concentration of cosmogenic 21Ne of ~0.86 x 10-8 cc/g found in this sample (Table 1) corresponds to a cosmic ray exposure (CRE) age of ~0.75 Ma, if the combined GCR + SCR production rate for very small particles at 1 AU from [4] is used. The highest meas-ured concentration of trapped (solar) 20Ne of ~3.46 x 10-6 cc/g in Un X1 is in the lower range of those ob-served in MMs from Dome Fuji [4], but clearly higher than in CS (Table 1; Table 4 in [13]).   Fig. 2. Three isotope plot of 20Ne/22Ne vs. 21Ne/22Ne for our data and data from [4]. Also plotted for comparison are SW [5], SEP-Ne [6], Earth atmosphere (EA) [7], HL [8], Ne-E[9], GCR [10] and the compositions of SCR - calculated from Mg, Al, Si at 0.50-10.0 g/cm² [11] using CM chondrite composition [12]. Table 1. Ne, Ar, Kr and Xe results for CS, Sc and Un MMs. Noble gas amounts (*) in 10-12 cc units, concentrations (#) in 10-8 cc/g. CRE-ages are calculated using the combined GCR+SCR production rates at 1AU from [4]. Uncertainties are given in parentheses. Corrected for blank.  Samples Size [µm] Weight [µg] 22Ne (*) 20Ne/22Ne 21Ne/22Ne 21Necos  (#) 20Netrap (#) CRE ages (Ma) 40Ar/36Ar 36Ar/132Xe 84Kr/132Xe 132Xe (*) CS45c16 465x324 105.1 (1) 0.10 (4) 10.4 (2.5) 0.041 (10) n.a. 1.0 (5) n.a. 345 (63) n.a. n.a. n.a. CS45c21  720x556 337.9 (2) 0.31 (3) 9.88 (62) 0.035 (4) n.a. 0.9 (1) n.a. n.a. n.a. n.a. n.a. Sc45b08  654x374 119.3 (1) 1.88 (4) 5.23 (7) 0.558 (12) 0.858 (26) 7.4 (5) 0.746 (23) 260 (6) 44 (3) 5.4 (6) 0.080 (4) Sc45b13  547x455 137.6 (1) 3.45 (6) 12.12 (19) 0.033 (1) 0.007 (6) 30 (2) 0.006 (5) 217 (5) 62 (5) 3.6 (3) 0.405 (31) Sc45b22  268x191 9.2 (1) 0.40 (3) 11.16 (49) 0.074 (5) 0.194 (24) 49 (5) 0.169 (21) 45 (35) n.a. n.a. n.a. Sc45c27  521x337 38.0 (1) 0.62 (4) 10.35 (34) 0.063 (5) 0.019 (3) 6.4 (6) 0.017 (3) 225 (7) 77 (48) 4.0 (2.4) 0.0078 (47) Un45b09  626x557 126.9 (1) 7.95 (13) 10.48 (16) 0.129 (2) 0.644 (18) 65 (4) 0.560 (16) 66 (3) 59 (5) 2.3 (2) 0.103 (8) Un45b18  253x235 5.3 (1) 0.36 (3) 11.86 (56) 0.041 (4) 0.074 (25) 81 (9) 0.064 (22) 193 (8) n.a. n.a. n.a. Un45c35  428x245 29.0 (1) 3.00 (9) 11.22 (26) 0.044 (1) 0.164 (26) 116 (8) 0.143 (23) 2.27 (75) 143 (6) 0.6 (1) 0.305 (11) UnX1  427x315 35.0 (1) 12.00 (17) 10.12 (21) 0.050 (1) 0.730 (75) 346 (21) 0.635 (65) 54 (1) 1718 (175) 3.8 (9) 0.010 (1)  1824.pdf43rd Lunar and Planetary Science Conference (2012)   Fig. 3. Three isotope plot of 129Xe/132Xe vs. 136Xe/132Xe showing our data compared to Murchison (measured at MPIC 2011), SW [14] and Q-Xe [15]. Isotope ratios shown in per mil-deviations from air Xe [16] composition.    Fig. 4. Elemental ratio plot of 36Ar/132Xe vs. 84Kr/132Xe for our data and data from [4], featuring also SW [17], EA [18] and Q (P1) [15].  Argon, krypton and xenon: As evident from the low 40Ar/36Ar ratios in most of our MMs (Table 1), the primordial Ar isotopes are dominated by an extraterres-trial component, with air contamination making a smaller contribution. While previous research [1, 19] suggested that large MMs show relationship to ordi-nary rather than carbonaceous chondrite composition, the low 40Ar/36Ar ratios are not consistent with ordinary chondrites, where 40Ar/36Ar ratios are typically much higher than in air (see [20]). The lowest ratio (2.27  0.75) is observed in the unmelted MM 45c35 (Table 1), which also contains Xe with an isotopic composi-tion close to the “planetary” P1 (Q) component [15], and this is typically found in bulk CM chondrites like Murchison (Fig. 3). Note that this is the first precise Xe analysis in MMs, and it indicates a relation to meteor-ites of the CM/CI type. In the elemental ratio plot 36Ar/132Xe vs. 84Kr/132Xe (Fig. 4), 45c35 plots in the carbonaceous chondrite region. In contrast, the other samples (except SW-rich MM Un X1) are character-ized by low 36Ar/132Xe and high 84Kr/132Xe (compared to meteorites), which is typical for contamination by fractionated air, e.g. [21]; [22]. Air contamination of the samples is also highlighted by Fig. 3, where these MMs show a composition consistent with fractionated air (though, fractionated SW cannot be ruled out based on the isotopic data alone). In general, air contamina-tion from adsorption or attributed to weathering is not accompanied by noticeable isotopic fractionation (e.g. [18]; [23]), and to our knowledge, experimental work leading to isotopic effects has always included ionized noble gases ([24]; [25]). Further work is needed to ascertain the origin of the apparently mass fractionated Xe observed here to determine whether it was acquired during residence on Earth or during passage through the atmosphere. References: [1] Rochette P. et al. (2008) PNAS, 105, 18206-18211. [2] Duprat J. et al. (2007) Adv. Space Res., 39, 605-611. [3] Ott U. et al. (2010) 73rd Ann. Met. Soc. Meeting (Abs. #5096). [4] Osawa T. and Nagao K. (2002) MAPS, 37, 911-936. [5] Heber et al. (2009) GCA, 73, 7414–7432. [6] Benkert J.-P. et al. (1993) J. Geophys. Res., 98, 13147-13162. [7] Eberhardt P. et al. (1965) Z. Naturforschung, 20a, 623–624. [8 ] Huss G. R. and Lewis R. S. (1994) Me-teoritics, 29, 811-829. [9] Lewis et al. (1994) GCA, 58, 471-494. [10] Garrison et al. (1995) Meteoritics, 30, 738-747. [11] Reedy R. C. (1992) LPS XXIII (Abs. #1133). [12] Wasson J. T. and Kallemeyn G. W. (1988) Phil. Trans. R. Soc. Lond., A325, 535-544. [13] Osawa T. et al. (2010) MAPS, 45, 1320-1339. [14] Pepin R.O. et al. (1995) GCA, 59, 4997-5022. [15] Busemann H. et al. (2000) MAPS, 35, 949-973. [16] Basford, J.R. et al. (1973) Proc. 4th Lunar Sci. Conf., GCA Suppl. 4, 1915-1955. [17] Vogel et al. (2011) GCA, 75, 3057-3071. [18] Ozima M. and Podo-sek F.A. (2001) Noble Gas Geochemistry, Cambridge Univ. Press. [19] Dobricǎ et al. (2011) MAPS, 46, 1363-1375. [20] Schultz L. and Franke L. (2004) MAPS, 39, 1889-1890. [21] Scherer P. et al. (1994) Noble Gas Geochemistry and Cosmochemistry, 43-53, TERRAPUB, Tokyo. [22] Schwenzer S. P. (2007) LPS XXXVIII (Abs. #1150). [23] Podosek F. A. et al. (1981) GCA, 45, 2401–2415. [24] Nichols Jr. R. H. et al. (1992) Meteoritics, 27, 555-559. [25] Hohenberg C.M. et al. (2002) MAPS, 37, 257-267. 1824.pdf43rd Lunar and Planetary Science Conference (2012)

Noble Gas Inventory of Micrometeorites from the Transantarctic Mountains

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Noble Gas Inventory of Micrometeorites from the Transantarctic Mountains

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