N94- 35428 LPSC JCVF 403 / SOLAR PROTON PRODUCED NEON IN SHERGOTI'ITE METEORITES D. H. Garrison 1, M. N. Rao 1,and D. D. Bogard 2 (1Lockheed-ESC and 2Code SN1, NASA, Johnson Space Center, Houston, TX 77058) Cosmogenic radionuclides produced bynear-surface, nuclear interactions of energetic solar protons ('10-100 MeV) have been reported in several lunar rocks and a very few small meteorites. We recently documented the existence and isotopic compositions of solar-produced, or SCR Ne in two lunar rocks [I, 2]. Here we present the first documented evidence for SCR Ne ina meteorite, ALI-I77005, which was reported to contain SCR radionuclides [3]. Examination of literature data for other shergottltes suggest that they may also contain a SCR Ne component. Existence of SCR Ne in shergottites may be related to a martian origin. Solar-Produced Ne inALH77005: To resolve SCR Ne produced near the surface of ALH77005 from galactic (GCR) Ne expected to dominate at depths below a few era, we made temperature extractions of 7 samples from different shielding depths. A three-isotope Ne correlation plot (Fig. 1) indicates that most of the Ne iscosmogenic incomposition; only the first (350°C) extractions, releasing asmall fraction of the total Ne, plot outside of Fig. 1and along the trend lines that connect with atmospheric composition. The cosmogenic 21Ne/22Ne ratio inALH77005 appears variable, however, and gives values of 0.71-0.78 (for 20Ne/22Ne <1.0). These ratios are less than those shown by typical chondrites, e.g., the 31H-chondrites plotted on the same 21Ne/22Ne scale inFig. 1[4]. The range of 21Ne/22Ne ratios shown by30of these chondrites (0.82-0.94) spans much of the range that ispredicted to occur from significant variations in GCR shielding depths [5]. High 3He/21Ne ratios (ALH77005 is "8) and 21Ne/22Ne ratios below -0.8 are very difficult to obtain in chondrites from GCR shielding alone [5]. We conclude that ALH77005 contains, in addition to a GCR component, a SCR Ne component having the calculated composition (Fig. 1)for energetic solar protons over a shielding range of0.5-10 g/era 2 [6]. This isthe first description of SCR Ne ina meteorite. In addition to having lower 21Ne/22Ne ratios, those ALH77005 samples believed to have resided nearer the meteorite surface also tend to have somewhat larger concentrations of total cosmogenlc 21Necompared to more interior samples. This observation is consistent with an extra SCR component but not with GCR production as afunction of depth. Because the predicted SCR/GCR production ratio for Ne varies "1-0.1 over shielding depths of "0.7-8 g/cm 2(6), 21Ne/_'Ne in ALH77005 isexpected tovary "0.70-0.80 over the same shielding range, in agreement with measured data. We recently showed very systematic correlations in cosmogenic 21Ne concentrations, 21Ne/e2Ne ratios, and subsurface depths for samples from lunarrock 61016; these were attributed to adepth-variable SCR component [1]. One H-chondrite (Fig. 1)gave 21Ne_2Ne =0.74 and high 3He_lNe =9 indicative of irradiation under low GCR shielding [4];we suggest that this chondrite also contains a SCR Ne component. SCR Ne in Other Shergo_'tes: Literature data [7, 8, 9, 10,11, 12, 13,14,15 16, 17] of Ne released during temperature extractions of shergottites EET79001, LEW88516, Shergotty, and Zagami also show 21Nef_Ne lower than that for typical chondrites and suggest the presence of SCR Ne. A least squares fitto the EET79001 data (Fig. 2) defines cosmogenic 21Ne/22Ne =0.76 at 2°Ne/22Ne =0.85 and passes near the atmospheric composition. Data forthe other shergottites (Fig. 2 inset with the same 2tNe/e2Ne scale) show a more cosmogenic composition and indicate 21Ne/eeNe of "0.73-0.83. (One temperature extraction of Shergotty shows a chondrite-like 21Ne/22Ne of 0.88 but anunexplainably low 2°Ne_2Ne.) Measurements of cosmogenic radionuclides and tracks in shergottites suggest that they were irradiated as small objects and suffered low ablation losses averaging 1-3 cm [3, 18]. The cosmogenlc 3He/21Ne ratio forALH77005 and LEW88516 ('7-8) suggests that shielding was less than for the other three shergottites (3He/21Ne =4-6) [17], and thus a larger SCR component might be expected inALH77005. Effects of Composition: Because ALH77005 and LEW88516 have chemical compositions similar to ordinary chondrites, observed differences in 21Ne/22Ne (Fig. 1) cannot be caused by target element effects. However, because other shergottites show considerable compositional differences compared to chondrites, we examined the compositional effects on cosmogenic 21Ne/22Ne in more detail (Fig. 3). Mg yields a considerably lower 21Nep2Ne than doAl and Si, and the Mg/Mg+ Si+Al parameter has been previously used to examine the effects of sample composition on cosmogenic 21Ne/22Ne [19, 20]. The ovalfield labeled "chondrites" shows analyses of silicate mineral separates from two chondrites [see 19],whereas the dashed line represents the variations in GCR shielding for the H-chondrite datashown in Fig. 1. Also plotted are data for three eucrites [21] and multiple depth samples of lunar rocks 68815 and 61016 [1, 2], the latter being pure anorthosite with 404 LPSC XXV SOLAR PRODUCED NEON IN SHERGOTHTF-.S: Garrison, Rao, & Bogard essentially no Mg. All samples of the two lunar rocks except those with the highest 21Ne/22Ne were shown to contain significant SCR Ne. These data suggest that GCR 21Ne/22Ne tends to correlate with Mg in silicates, but that even extreme compositional variations ranging from olivine to anorthosite cannot explain the ALH77005 data (labeled A in F'_ 3). We conclude that GCR Ne compositions generally would be limited to the shaded area of Fig. 3, and that increasing amounts of SCR Ne would move compositions to the left. Literature data for other shergottites (L, E, Z, and S) as well as Nakhla (N) and Chassigny ((3) are also shown in Fig. 3. The L and Z points are individual analyses, whereas the E and S points show the median values and total ranges of temperature extractions (Fig. 2). Although Chassigny and Nakhla give no evidence of SCR Ne, such acomponent appears to be present in LEW88516, EET79001, and possibly in some samples of Zagami and Shergotty. It anpears that all known shergottites, in contrast to chondrites, contain a cosmogenic Ne component with low 2fl_le/22Ne that cannot be explained by shielding or composition; this suggests a Mars-relatea [actor. Some evidence exists that the GCR 21Ne/22Ne ratio produced from pure Na is as low as 0.4-0.5. These shocked shergottltes might contain such a cosmogenic component produced from Na-rich salts on the martian surface and shock-lmplanted by the process that has been invoked to implant Mar_. att_a_,_hericgas_. -into . shergottites [22]. One sample of EET79001 showed variations in cosmogemc "--r_e/--r_e ot u./,t-u./v ounng temperature extractions [8, and Fig. 2], which might suggest separate release of cosmogenic Ne produced from Na compared to that produced within the silicate. Alternatively, orbital parameters of the shergottites during transit to earth may have produced more favorable conditions for SCR Ne production. Among the various possibilities are lower eccentridties or different inclinations of shergottites compared to chondrites to enhance the SCR/GCR production ratio, or smaller entry velocities into the earth's atmosphere to cause lower surface ablation of shergottites [H. Zook, pet's, comm.]. In the latter case, the longer exposure ages of nakhlites and Chassigny compared to shergottites [7] may explain the absence of this SCR Ne in the former. References: (1)Rao, Garrison,Bogard&Reedy,J. I ! OeophysR.e,.9S,7S2_1,993;(2)P.aoO, misoa,Bogntd&Reedy, 4 Fig.1 ,H4& _5 Chqndrites'i submitted G.C.A., 1993; (3) Nishiizumi, Klein, Middleton, Eimore, Kubik, & Arnold, G.C.A. 50, 1017, 1986; (4) Schultz, Weber & Begemann, G.C.A. 55, 59, 1991; (5) Graf, Baur & Signer, G.C.A. 54, 2521,1990;,(6)Reedy, LPSCxxm, 1133,1992;(7)Bogard,Nyquist &Johnson,G.C.A. 48,1723,1984;(8)Swindle,Caffee,and Z i ! =\,.t I i Hohenberg,G.C.A.50,1001,1986;(9)Becker&Pepin,G. C.A.50, "--2 993,1986;(10)Becker&Pepin,LPSCXXIV,77,1993;(11)Weinr,, z Be.cker,andPepin,E.P.S.L 77,149-158,1986;(12)Weitm,EP.S.L 91, o /, .......i ......i....._' IAL_A 7_005 55,1988;(13)Nagoa&Matsuda,11thSymp.Antarc.Met.,131,1986; (14)Nagoa, 12thSymp.Antarc.Met.,110,1986;(15)Ott,G.C.A52, \ t , 1937,1988;(16)Ott,unpublished1989;,(17)Bogard&Garrison, ....'.*...O._._.c_-!........ m" i Open = e_edor 0 i! ........i.!..i...................................................... LPSCXXIV, 139,1993;(18)Bhandari,Goswami,Jha,Sengupta,& Shukla,G.C.A.50,1023,1986;(19)Begemann&Schultz,LPSCXIX, 51,19ss;(2o)M_ &Reedy,U_SCXXIV9,37,199(231;) 0.6 0.7 0.B 0.9 21Ne /22Ne Hampel, Waenke, Hofmeistcr, Spettel, & Herzog, G._ 44, 539, 1960;,(22)Bosard&Johnum,Science221,651,1983. 0.6 ¸ I0 Sh_rgotty 0 Fig.3 ,!!_iii_iliii:;i_:!_::i 0.5 _:!iiii_ ii !:!:!:!:!:i:!:i:i 0.4 + - i _6 + 0.3- - _D = z = _:_!:i.:.:.:.:.:.:.:.:.:.:.i.:.i...".-.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.::::::::::::::::: z4 _:_ 0.2- -- - -- m;_!!._;_;_;_;_!_i_!;E_;_;_;_;_;;_i_;i_!;ii_!_!!!_!_!!i!!;!iTi_ 0.1 t i 0 0.2 0.4 0.6 0.8 0.65 0.70 0.75 0.80 0.85 0.90 0.95 21Ne /22Ne 21Ne /22Ne =