Stepwise etching of acid-resistant residues from carbonaceous chondrites yields a new, extraordinarily low He isotopic ratio for phase Q that might represent early solar nebula composition. A much lower ratio possibly characterizes He-P3 from circumstellar diamonds

Baur, H.

Busemann, H.

Wieler, R.

English

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Open Research OnlineThe Open University’s repository of research publicationsand other research outputsHelium isotopic ratios in carbonaceous chondrites:Significant for the early solar nebula and circumstellardiamonds?Conference or Workshop ItemHow to cite:Busemann, H.; Baur, H. and Wieler, R. (2001). Helium isotopic ratios in carbonaceous chondrites: Significantfor the early solar nebula and circumstellar diamonds? In: 32nd Lunar and Planetary Science Conference, 12-16 Mar2001, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1598.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.uk  HELIUM ISOTOPIC RATIOS IN CARBONACEOUS CHONDRITES: SIGNIFICANT FOR THE EARLY SOLAR NEBULA AND CIRCUMSTELLAR DIAMONDS?  H. Busemann1, H. Baur2, and R. Wieler2, 1University of Bern, Physics Institute, Space Research & Planetary Science, Sidlerstr. 5, 3012 Bern, Switzerland, (busemann@phim.unibe.ch), 2ETH Zürich, Isotope Geochemistry, Sonneggstr. 5, 8092 Zürich, Switzerland.   Introduction:  A confusingly large number of no-ble gas components has been found in meteorites, based on distinct compositions, origins, carrier phases, or release characteristics [1]. The discovery that the carriers of the primordial noble gases survive acid treatment [2] led to the identification of several circum-stellar grain types [3]. The oxidisable carbonaceous carrier “phase Q”, however, which contains most of the isotopically “normal” Ar-Xe in acid-resistant residues has not been identified yet [see 4 for references].  0 10 20 30 40 50 60 70 80 90 1000.00.51.01.52.0MurchisonAllendeGrosnajaIsnaQ + P3He-QpsCold B'veldx?He(upper limit)b. Solar system reservoirsa. Stepwise etching experimentssolar, SEPHe-HLJupiterHe-P3IsnaIsnaCold BokkeveldGrosnaja3 He/ 4He x 10 4% of released 4HeP3cosmHe-Q + He Fig. 1. 3He/4He release patterns during CSSE and 3He/4He ratios from different solar system reservoirs.   The He composition is difficult to measure. Ratios for He-Q have been given only for the Q-gas-rich Al-lende and Murchison [5,6], because He-Q is strongly depleted relative to other He components and easily overwhelmed by cosmogenic He (Hecosm). Circumstel-lar He has been measured in diamonds and SiC-rich separates. Helium in the latter is accompanied by Ne-E and appears to be a mixture of pure 4He, originating in the He shell of AGB stars, and “normal” He with 3He/4He ~2.6 x 10-4 [7]. Huss et al. found 3He/4He = (1.7 ± 0.1) x 10-4 for He-HL which is probably related to circumstellar diamonds [8] and can amount to ~80 % of the primordial He in meteorites. The 3He/4He ratio for P3, another, probably superficially bounded component in diamonds, is ≤1.35 x 10-4 [8]. The pub-lished data yield numerous 3He/4He ratios in the range (0.95-1.50) x 10-4 for mineral and density separates, stepwise heating experiments, diamond-rich separates, and acid-resistant residues of carbonaceous chondrites [e. g. 9-12]. This indicates at least one important pri-mordial He component with a correspondingly low isotopic ratio. Here, we present new estimates for Q and P3 and discuss possible implications. Experiment:  HF/HCl-resistant silicate-poor resi-dues from unequilibrated chondrites have been ana-lysed with our on line closed system stepped etching technique (CSSE) [4-6]. This allows to clearly recog-nize the release of different He components from resi-dues of the chondrites CV3 Grosnaja, CM2 Cold Bok-keveld, and CO3.7 Isna.  Tab. 1. 3He/4He ratios in phase Q, Jupiter, circumstel-lar diamonds, and local interstellar cloud (LIC).  3He/4He x 104  Isna 1.23 ± 0.02 He-Q Cold Bokkeveld 1.41 ± 0.01 He-Q (+ Hecosm) Grosnaja 1.46 ± 0.01 He-Q (+ Hecosm) Allende 1.59 ± 0.04 [5] Murchison ≤ 1.33* [6] Jupiter 1.66 ± 0.05 [15] LIC 2.48 ± 0.68 [23] LIC 1.30 ± 0.76 [24] *We adopted the lowest ratio measured instead of the more conservative limit given in [6].  Results:  Fig. 1 shows the extraordinarily constant plateaux of the He isotopic ratios released during the etch runs, indicating well-defined components. The adopted ratios (Tab. 1) lie below that of Allende [5]. The differences can be attributed to 3Hecosm which in-creases the originally trapped 3He/4He ratios. Indeed, Fig. 2 shows a positive correlation of (3He/4He)Q with exposure age. Isna has an almost negligible exposure age. We thus adopt its very low 3He/4He = (1.23 ± 0.02) x 10-4 as the true ratio for He-Q. Conservatively correcting for 3Hecosm, assuming 1 % silicates in the residue, would lower this ratio by 0.1 %.  Another component with a lower ratio was released from Isna in the second part of the run (Fig. 1). The 4He and 3He abundances relative to Ar-Xe strongly increased in these steps, suggesting that this He was more likely released from circumstellar grains rather than phase Q. Solar or terrestrial gases can be excluded according to the isotopic composition of Ne-Xe in these steps. Correcting for He-Q released in the later Lunar and Planetary Science XXXII (2001) 1598.pdfHELIUM ISOTOPIC RATIOS IN CARBONACEOUS CHONDRITES:  H. Busemann et al.  steps as well with the ratio given above, we obtain 3He/4He = (0.45 ± 0.04) x 10-4 for this component, which might represent P3, because this is the least bound component in diamonds, and Ne-E indicating SiC or graphite has not been observed. Discussion:  Protosolar (ps) 3He/H and D/H ratios are important to test predictions of the Big Bang nu-cleosynthesis theory and set constraints on cosmic pa-rameters [13]. These ratios can be obtained from solar wind and meteoritic He isotopic ratios or direct meas-urements in Jupiter’s atmosphere [14,15]. A 3He/4He ratio of (1.5 ± 0.3) x 10-4 from carbonaceous chon-drites has widely been used [14]. However, as dis-cussed above, this He is largely related to circumstellar diamonds. We thus propose that He-Q better represents the gaseous protosolar cloud He. Arguments that sup-port this view are: The depletion of the light Q-gases relative to the heavier ones and solar ratios can result from trapping on ice-coated circumstellar grains in the molecular cloud [16]. Q-gases and circumstellar grains must have been homogeneously mixed in the nebula prior to formation of the first planetesimals [17]. Ex-periments simulating conditions in the molecular cloud yield residues that contain heavy noble gases in con-centrations similar to phase Q [18].  0 1 2 3 4 51.201.301.401.501.60max.max.CM2 Cold Bokkeveldx(    CM2 Murchison)CO3.7 IsnaCV3 (ox.) GrosnajaCM2 Murchison (revised)CV3 Allende(3 He/ 4He) Q(+cosm) x 104Exposure Age [Ma]  Fig. 2. (3He/4He)Q(+cosm) vs. exposure age [26,27].  Multiplying (3He/4He)Q/ps as determined above with (4He/H)ps = 0.095 ± 0.010 [19] yields (3He/H)ps = (1.16 ± 0.12) x 10-5. The present solar 3He is the sum of pro-tosolar 3He and D, because of pre-main-sequence D-burning in the Sun [14]. Subtracting (3He/4He)ps from the present 3He/4He in the solar outer convection zone [20], we obtain (D/H)ps = (2.4 ± 0.7) x 10-5 which is in agreement with earlier determinations [14] and the ratio in Jupiter’s atmosphere [15]. Thus, the latter might contain unfractionated protosolar hydrogen and Jupiter’s precursor planetesimals experienced only minor fractionations by physicochemical processes. The new (3He/4He)ps ratio is 35 % lower than the ratio measured in Jupiter’s atmosphere [15]. This points to fractionation either in the solar nebula prior to incorporation into phase Q, within the carrier itself, during accretion of Jupiter, or within its atmosphere. Helium is indeed elementally fractionated in Jupiter because of rainout in metallic H [21], but the He deple-tion is 10 % only. The icy precursor planetesimals that formed the Jovian core might also contain fractionated He. These ices must have formed at temperatures be-low 30 K and thus beyond the Pluto region of the solar system [22]. However, it is argued that these ices do not trap large amounts of He. Our (3He/4He)ps is lower than the 3He/4He in the present local interstellar cloud (LIC) as given in [23]. However, a preliminary ratio from foils directly ex-posed to the LIC [24] suggests no significant differ-ence. A ratio in the LIC higher than (3He/4He)ps would reflect the 3He production within stars and subsequent mixing into the interstellar medium during the last 4.6 Ga. The agreement of our protosolar and the new LIC ratio (Tab. 1) might support the idea that no major net 3He production or loss has occurred in the galaxy [25]. Acknowledgement:  This work was supported by the Swiss National Science Foundation. References:  [1] Swindle T. D. (1988) in: Meteor-ites and the early solar system, 535. [2] Lewis R. S. et al. (1975) Science, 190, 1251. [3] Zinner E. (1998) Ann. Rev. Earth Planet. Sci., 26, 147. [4] Busemann H. et al. (2000) Meteoritics & Planet. Sci., 35, 949. [5] Wieler R. et al. (1991) GCA, 55, 1709. [6] Wieler R. et al. (1992) GCA, 56, 2907. [7] Lewis R. S. et al. (1994) GCA, 58, 471. [8] Huss G. R. and Lewis R. S. (1994) Meteoritics, 29, 791. [9] Eberhardt P. et al. (1981) GCA, 45, 1515. [10] Zaizen S. et al. (2000), Antarct. Met. Res., 13, 100. [11] Englert P. et al. (1983) EPSL, 65, 1. [12] Reynolds J. H. et al. (1978) GCA, 42, 1775. [13] Schramm D. N. (1998) Rev. Mod. Phys., 70, 303. [14] Geiss J. (1993) in: Origin and evolution of the elements, 89. [15] Mahaffy P. R. (2000) JGR, 105, 15061. [16] Huss G. R. and Alexander Jr. E. C. (1987) Proc. 7th LPSC, E710. [17] Huss G. R. (1997) AIP Conf. Proc., 402, 721. [18] Sandford S. A. (1998) Me-teoritics & Planet. Sci., 33, A135. [19] Bahcall J. M. and Pinnsonneault M. H. (1995) Rev. Mod. Phys., 67, 781.[20] Bodmer R. and Bochsler P. (1998) Astron. Astrophys., 337, 921. [21] von Zahn U. et al. (1998) JGR, 103, 22815. [22] Owen T. et al. (1999) Nature, 402, 269. [23] Gloeckler G. and Geiss J. (1998) Space Sci. Rev., 84, 275. [24] Salerno E. et al., in preparation. [25] Rood R. T. et al. (1998) Space Sci. Rev., 84, 185. [26] Eugster O. et al. (1998) GCA, 62, 2573. Scherer P. and Schultz L. (2000) Meteoritics & Planet. Sci., 35, 145. Lunar and Planetary Science XXXII (2001) 1598.pdf

Helium isotopic ratios in carbonaceous chondrites: Significant for the early solar nebula and circumstellar diamonds?

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Helium isotopic ratios in carbonaceous chondrites: Significant for the early solar nebula and circumstellar diamonds?

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