Presolar Grains in the CM2 Chondrite Sutter's Mill

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Presolar grains in the CM2 chondrite Sutter's Mill

ARTICLE · JUNE 2014 DOI: 10.1111/maps.12289

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Yangting Lin Qing-Zhu Yin Chinese Academy of Sciences University of California, Davis

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Jianchao Zhang Peter Jenniskens Chinese Academy of Sciences SETI Institute

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Available from: Yangting Lin Retrieved on: 10 September 2015 Meteoritics & Planetary Science 1–9 (2014) doi: 10.1111/maps.12289

Presolar grains in the CM2 chondrite Sutter’s Mill

Xuchao ZHAO1, Yangting LIN1*, Qing-Zhu YIN2, Jianchao ZHANG1, Jialong HAO1, Michael ZOLENSKY3, and Peter JENNISKENS4,5

1Key Laboratory of the Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, 19 Beituchengxi Rd., Beijing 100029, China 2Department of Earth and Planetary Sciences, University of California at Davis, Davis, California 95616, USA 3ARES, NASA Johnson Space Center, Houston, Texas 77058, USA 4SETI Institute, Mountain View, California 94043, USA 5NASA Ames Research Center, Moffett Field, California 94035, USA *Corresponding author. E-mail: [email protected] (Received 22 July 2013; revision accepted 20 February 2014)

Abstract–The Sutter’s Mill (SM) carbonaceous chondrite is a regolith breccia, composed predominantly of CM2 clasts with varying degrees of aqueous alteration and thermal metamorphism. An investigation of presolar grains in four Sutter’s Mill sections, SM43, SM51, SM2-4, and SM18, was carried out using NanoSIMS ion mapping technique. A total of 37 C-anomalous grains and one O-anomalous grain have been identified, indicating an abundance of 63 ppm for presolar C-anomalous grains and 2 ppm for presolar oxides. Thirty-one silicon carbide (SiC), five carbonaceous grains, and one Al-oxide (Al2O3) were confirmed based on their elemental compositions determined by C-N-Si and O-Si-Mg-Al isotopic measurements. The overall abundance of SiC grains in Sutter’s Mill (55 ppm) is consistent with those in other CM chondrites. The absence of presolar silicates in Sutter’s Mill suggests that they were destroyed by aqueous alteration on the parent asteroid. Furthermore, SM2-4 shows heterogeneous distributions of presolar SiC grains (12–54 ppm) in different matrix areas, indicating that the fine-grained matrix clasts come from different sources, with various thermal histories, in the solar nebula.

INTRODUCTION grain searches. Previous studies have suggested that different secondary processes (aqueous alteration, The Sutter’s Mill (SM) carbonaceous chondrite thermal metamorphism) may preferentially destroy (CM2) fell in California on April 22, 2012. Weather presolar mineral phases (presolar silicate/oxide, SiC) Doppler radars detected the falling meteorites, resulting (e.g., Huss et al. 2003; Floss and Stadermann 2012). in a fast recovery of some stones (Fries and Fries 2010; Here, we used NanoSIMS ion imaging to investigate the Jenniskens et al. 2012). The first stones found (SM 1-3) presolar inventories of different SM fragments, for were recovered before rain and have minimal terrestrial comparison with the presolar grain abundances in weathering. Petrographic studies indicate that Sutter’s Murchison and other primitive chondrites, and to gain Mill is a regolith breccia, composed of CM2 lithologies a better understanding of the effects of secondary with various degrees of aqueous alteration and thermal processes on the preservation of presolar grains. metamorphism (Jenniskens et al. 2012; Zolensky et al. 2012). Previous studies of acid residues show that some SAMPLES AND EXPERIMENTAL CM2 chondrites (e.g., Murchison, Murray) contain abundant presolar material, mainly presolar SiC grains The fragments analyzed in this study are SM43, (e.g., Zinner 2013). Sutter’s Mill contains abundant fine- SM51, SM2-4 (a subsample of SM2), and SM18 (for grained matrix areas, which are suitable for presolar the find locations in the strewn field, see Jenniskens

Table 1. Total areas mapped and isotopes measured residues have close-to-solar 12C/13C ratios; such grains for each sample. were also identified based on combined NanoSIMS ion 12 28 Sample Area images of C and Si in this study. ID (lm2) Isotopes measured Following the identification of presolar grains, 12 13 16 17 12 14 additional imaging measurements were carried out to SM43 16,200 C , C , O , O , C N , 12 13 12 14 12 15 12C15N determine the C-N-Si ( C , C , C N , C N , 28 29 30 SM51 20,700 12C, 13C, 16O, 17O, 18O, 28Si Si , Si , Si ) isotopic compositions for most of 28,800 1H, 2H, 12C, 13C, 16O, 17O, the C-anomalous grains (SM43, SM51, and SM2-4); 18O these measurements can also provide elemental SM2-4 20,100 12C, 13C, 16O, 17O, 18O, 12C14N, information, for example, to distinguish SiC from 28Si carbonaceous grains. As most of the target presolar 12 13 16 17 18 28 SM18 6900 C , C , O , O , O , Si grains are less than 500 nm in size, all C-N-Si isotopic measurements were done by scanning an approximately et al. 2012). The polished sections of these four 1 pA focused Cs+ primary ion beam over 2 9 2 lm2 fragments were first examined using optical microscope (642 pixels) areas centered on the grains of interest. and a scanning electron microscope (SEM). Synthetic SiC and Si3N4 grains (0.5–1.0 lm) were used Representative fine-grained matrix areas in each section as isotopic standards for instrumental tuning and were selected as targets for presolar grain searches. The normalization of the isotopic ratios. For the single Cameca NanoSIMS 50L at the Institute of Geology and O-anomalous grain identified from SM2-4, we also carried Geophysics, Chinese Academy of Sciences (CAS) was out a close-up measurement (5 9 5 lm2, 1282 pixels) to used to locate areas with isotopic anomalies. Matrix better define its isotopic and elemental compositions. The areas of 12 9 12 lm2 were first presputtered using a three O isotopes and 28Si were measured along with relatively high beam current (approximately 100 pA) to 24Mg16O, 26Mg16O, and 27Al16O. Synthetic spinel and remove the carbon coating on regions of interest. The Al2O3 were used as isotopic standards. measurements were done in raster imaging mode by scanning an approximately 1 pA focused Cs+ primary RESULTS ion beam (approximately 100 nm diameter) over 10 9 10 lm2 (2562 pixels) matrix areas within the Petrography presputtered regions; however, the H-C-O imaging measurements were done within 30 9 30 lm2 (2562 The Sutter’s Mill chondrite is a regolith breccia, pixels) matrix areas. Negative secondary ions of 6 or 7 composed of CM2 lithologies with various degrees of isotopes (as detailed below and in Table 1) were aqueous alteration and thermal metamorphism collected simultaneously, along with secondary electrons (Jenniskens et al. 2012; Zolensky et al. 2012). Of the (SE). Each measurement consisted of 5–10 frames, with four samples surveyed in this study, SM43 shows the analysis times of approximately 10 min for each frame. highest degree of aqueous alteration. Petrographic The individual frames were added together to form a observations indicate that most spinel grains in the Ca- single image measurement after correction for possible Al-rich inclusions in SM43 have been transformed into position shift. All analyses were carried out in chained calcite and dolomite (Fig. 1a). In contrast, the spinel analysis mode, with automatic stage movement to grains in the Ca-Al-rich inclusions in SM2-4 are barely subsequent matrix areas, following a predefined grid altered (Fig. 1b), suggesting that they experienced less pattern on the sample. The total area of matrix aqueous alteration. Moreover, SM2-4 contains analyzed in four sections was 92,700 lm2. The imaging abundant oldhamite (CaS) (Fig. 1c) and fine-grained area and isotopes measured for each sample are shown silicate matrix (Fig. 1d). Zolensky et al. (2012) found in Table 1. Isotopic compositions were normalized by that Na and K are distributed homogeneously in the assuming the average composition of surrounding matrix of SM2-4, which also suggests that it might have matrix area to be solar. Grains were considered to be not experienced aqueous alteration. SM51 and SM18 presolar if one of their isotopic compositions deviated also contain abundant fine-grained matrix material, with from the average surrounding material by more than 5r degrees of aqueous alteration intermediate between and the anomaly was present in at least three SM43 and SM2-4. consecutive image frames. The measurement and data- processing procedures are discussed in detail by Floss NanoSIMS and Stadermann (2009a) and Zhao et al. (2013). Based on previous studies (e.g., Alexander 1993; Hoppe et al. From the NanoSIMS isotopic mapping, we 1994, 1996), some SiC grains identified from acid identified 37 C-anomalous grains and one O-anomalous Presolar grains in Sutter’s Mill 3

Fig. 1. Backscattered electron (BSE) images of: a) a Ca-Al-rich inclusion mainly composed of calcite and dolomite in SM43; b) a Ca-Al-rich inclusion in SM2-4 dominated by spinel; c) abundant oldhamite (CaS) grains in SM2-4, along with fine-grained matrix material; d) fine-grained matrix area in SM2-4. grain. All nitrogen isotopic anomalies are correlative not be relocated in C-N-Si measurements are not with C-anomalous grains. No hydrogen anomalous included in Figs. 2 and 3, including two grains from areas were found. SM18 and two grains from SM51. Most of the SiC grains (27/31) have 12C/13C ratios Presolar SiC and Carbonaceous Grains of 22.5–99.2, and 14N/15N ratios of 123–881, in Of 37 C-anomalous grains identified in this study, accordance with mainstream grains (Fig. 2 and Zinner 31 are SiC and 5 are carbonaceous grains. The SiC and 2013). Although N and Si isotopic compositions are not carbonaceous compositions were distinguished based on available for four SiC grains, their C isotopic the 12C and 28Si images obtained from the C-N-Si compositions (12C/13C = 35.1–72.6) still suggest a isotopic mapping. All 31 SiC grains have comparable classification as mainstream grains (Table 2). ion concentrations both in 12C and 28Si ion images, Mainstream SiC grains are generally thought to have whereas the other five grains only show high originated in the winds of roughly solar-metallicity low- concentrations of 12C. The remaining grain (SM51A-13) mass AGB stars (e.g., Zinner 2013). Two grains, M11A-2 was identified from H-C-O measurements and could not and SM51A-30, are highly enriched in 13C, with 12C/13C be found in the subsequent C-N-Si measurements, ratios lower than 10, suggesting that they are type which leaves its elemental composition unknown. The A + B grains (Zinner 2013). Two other grains, M11C-3 isotopic data, along with size information and possible and 51C-13, have 12C/13C ratios of 14.3, slightly higher mineral phases of these grains, are listed in Table 2. than the lower limit of 10 for type A + B grains; Figures 2 and 3 show the C-N and Si isotopic plots of however, we still consider them to be type A + B, most SiC grains. Note that four SiC grains that could because signal dilution from surrounding isotopically 4 X. Zhao et al.

Table 2. C-anomalous grains from Sutter’s Mill. Grain Diameter (nm) 12C/13C 14N/15N d29Si/28Sia d30Si/28Sia C/Si Phases and typesb SM43 M8A-24 250 25.7 0.6 256 25 31 23 6 28 1.30 SiC-MS M9A-1 360 67.5 0.7 466 28 56 767 8 1.47 SiC-MS M9A-20 360 22.5 0.1 646 25 16 53 6 1.09 SiC-MS M11A-2 350 6.99 0.04 318 12 13 711 8 1.20 SiC-AB M11C-3 250 14.3 0.1 87.2 2.2 3 15 31 18 2.51 SiC-AB M16A-9 250 78.9 1.3 312 14 17 921 11 1.29 SiC-MS M16A-13 460 66.1 0.6 626 32 10 531 5 0.98 SiC-MS M42A-15 350 38.3 0.3 590 27 64 660 7 1.09 SiC-MS SM51 SM51A-30 300 4.87 0.04 199 17 8 12 17 14 0.79 SiC-AB 51A-6 440 67.4 2.5 282 19 15 19 17 22 0.85 SiC-MS 51C-13 310 14.3 0.2 352 40 59 19 45 23 0.96 SiC-AB 51F-10 310 52.3 1.3 301 25 38 13 55 16 0.81 SiC-MS 51F-12 380 42.5 0.7 325 26 74 11 105 14 1.05 SiC-MS 51F-31 380 59.1 1.3 458 81 53 13 65 16 0.92 SiC-MS 51E-10 410 79.9 1.8 321 18 55 12 50 14 1.12 SiC-MS 51E-24 400 35.1 2.3 1.51 SiC-MS 51F-26 340 72.6 5.0 1.05 SiC-MS SM51A-13 530 43.7 3.0 51E-30 360 18.5 0.4 4.49 C 51E-35 370 19.6 0.4 13.01 C SM2-4 M1B2-20 260 53.0 1.6 199 22 33 19 32 23 0.89 SiC-MS M12A-16-2 260 99.2 3.4 325 67 61 17 18 20 1.00 SiC-MS M12A-16-1 250 69.8 1.7 295 33 54 16 52 20 1.39 SiC-MS M1A-11 350 81.7 0.9 678 47 94 7 112 9 0.90 SiC-MS M1A-9-2 270 78.7 1.7 648 72 71 14 70 17 1.37 SiC-MS M1A-3 300 75.6 1.4 354 28 56 12 48 14 1.31 SiC-MS M1A-2 250 62.3 1.2 881 129 121 13 111 16 1.11 SiC-MS M5A-8 260 77.3 1.5 646 73 70 11 65 14 1.01 SiC-MS M5A-4 380 75.2 0.5 684 13 95 21 104 16 1.01 SiC-MS M1D-4 300 87.2 3.7 205 24 1 20 29 24 0.93 SiC-MS M1C-8 380 62.0 0.8 561 45 123 8 129 10 1.28 SiC-MS M1C-2 350 61.7 2.3 123 11 70 21 11 26 1.09 SiC-MS M1A-9-1 350 43.0 0.9 4.02 C M2A-30 300 15.8 0.5 12.16 C M5A-22 380 71.8 3.1 2.10 C SM18 M1B-2 390 48.3 0.9 1.40 SiC-MS M3B-4 1100 57.8 0.3 2.01 SiC-MS a Deviation from normal in parts per thousand: (Rmeas/Rstd 1) 9 1000. bMS = mainstream; C = carbonaceous; grain-types in italics are those for which the identiﬁcation is less certain (see the text for details). Errors are 1r.

normal material might have increased their 12C/13C grains in Murchison (3–4% in KJA + KJB: 0.2–0.5 lm; ratios above 10. The origin of type A+B grains remains Hoppe et al. 2010). unclear, and it is possible that they may come from J- Five grains were determined as carbonaceous on the type carbon stars or born-again AGB stars (Amari basis of their high concentrations of 12C in the et al. 2001). The relative proportion of A+B grains in NanoSIMS ion images. As shown in Table 2, these þ9% r the SiC inventory of SM is 64 (1 ; Gehrels 1986), or grains have relatively high C/Si ratios, and have grain þ11% 12 13 137 if we take the two grains with C/ C ratios of sizes similar to most SiC grains identiﬁed in this study. 14.3 into account, comparable to the fraction of A + B The 12C/13C ratios of these grains are ranging from 15.8 Presolar grains in Sutter’s Mill 5

Fig. 3. Silicon isotope ratio plots of 27 SiC grains in Sutter’s Fig. 2. Plot of 12C/13C versus 14N/15N ratios showing the Mill. The dashed lines indicate solar values. The background distribution of SiC grains in Sutter’s Mill. Note that the solar data points are from the Presolar Grain Database at http:// wind analyses of Genesis samples showed that the Sun’s presolar.wustl.edu/pgd/ (Hynes and Gyngard 2009). nitrogen isotopic ratio is about 440 and different from the terrestrial ratio (Marty et al. 2011). The dashed lines indicate both values. The background data points are from the Presolar Grain Database at http://presolar.wustl.edu/ pgd/ DISCUSSION (Hynes and Gyngard 2009). SiC Abundances and Grain Distributions to 71.8, which is different from most carbonaceous The abundances of presolar grains in Sutter’s Mill grains found in CR chondrites (12C/13C >100; Floss and were calculated on the basis of the total surface areas of Stadermann 2009b; Zhao et al. 2013). These grains are the presolar grains and the total matrix area mapped. probably presolar graphite. The areas of the presolar grains were calculated based on the grain size information extracted from the Presolar Oxide NanoSIMS ion and SE images. Note that grain sizes Only one O-anomalous grain (M1B2-3, in SM2-4) determined using NanoSIMS images are generally was found in this study. M1B2-3 is approximately approximately 20% larger than those from Auger SE 400 nm in diameter, and is enriched in 17O images due to beam broadening and dilution effects; (17O/16O = 5.47 0.14 9 104) with a close-to-solar thus, we need to apply a correction to the grain sizes 18O/16O ratio (18O/16O = 2.04 0.04 9 103). Based on for more meaningful abundance calculations (see the classification of presolar oxide grains (Nittler et al. detailed descriptions in Zhao et al. 2013). Two grains 1997), this grain belongs to Group 1, with a likely from SM51 were found in the 30 9 30 lm2 H-C-O origin in a low-to-intermediate mass red giant (RG) or measurements; however, for the abundance calculation, asymptotic giant branch (AGB) star. As shown in the we only use the 35 grains found within the 10 9 10 lm2 ion images (Fig. 4), the grain has less 28Si than the measurements because different mapping sizes might surrounding matrix, suggesting that it is likely a lead to different detection efficiencies and grain presolar oxide. The subsequent close-up NanoSIMS abundances. ion-imaging measurement (16,17,18O, 28Si, 24,26Mg16O, The 35 C-anomalous grains identified within 27Al16O) of M1B2-3 shows a high abundance of 27Al16O 63,900 lm2 results in a matrix-normalized abundance of and little 24Mg16O (Fig. 4), indicating that it is an 63 10 ppm, of which the SiC abundance is 26 Al-oxide (Al2O3). No excess Mg due to the decay of 55 10 ppm (errors are based on counting statistics 26Al was found. only). If we calculate the SiC abundances for individual 6 X. Zhao et al.

Fig. 4. The O-Si-Mg-Al ion images and SE image of of grain M1B2-3 from SM2-4 (field of view: 5 9 5 lm2). samples, they would be 44 15, 42 15, 44 13 ppm for SM43, SM51, and SM2-4, respectively, showing no significant variation from one fragment to the next. SM18 has abundance of 155 110 ppm (based on two grains) and, within errors, is consistent with the other samples. Figure 5 shows a comparison of presolar SiC abundances among CM chondrites. The average SiC abundance of Sutter’s Mill, as well as the abundances of each individual sample, is similar to the results from other CM chondrites, such as Murchison and Murray. Presolar SiC abundances determined for Murchison using the Cameca ims 1270 + SCAPS system (Nagashima et al. 2005) are lower than the estimates based on NanoSIMS ion imaging (Davidson et al. 2009) (Fig. 5). This may be due to the relatively large primary ion beam size of the Cameca ims 1270 + SCAPS system, resulting in a lower detection efficiency for very Fig. 5. Bar graph showing the abundance comparison between small SiC grains. As shown in Fig. 5, NanoSIMS ion- Sutter’s Mill and other CM2 chondrites. The dashed line imaging measurements for SM01 and SM47 performed indicates the average presolar SiC abundance of 55 ppm for by Heck et al. (2013) show that SM47 has SiC Sutter’s Mill. Other data are from literature (Huss et al. 2003; Nagashima et al. 2005; Davidson et al. 2009; Heck et al. 2013; abundances similar to the four fragments in our study, Ott et al. 2013). whereas SM01 (the prerain sample) has a slightly lower abundance of SiC, even when the relatively large discrepancies have also been observed in CR chondrites, uncertainties are considered (21 15). The abundance and one possible explanation is that the noble gas of presolar SiC can also be estimated based on the contents could be underestimated if the SiC grains contents of noble gases (e.g., Huss et al. 2003). experienced degassing processes before and/or after However, the presolar SiC abundance estimate for accretion of the parent body (Davidson et al. 2009). SM51 based on the noble gases (3.8 0.4; Ott et al. Based on noble gas analysis of acid residues of bulk 2013) is much lower than our result (55 10). Such samples of chondrites, the matrix-normalized Presolar grains in Sutter’s Mill 7 abundances of presolar SiC decrease as the degree of thermal metamorphism increases (Huss et al. 2003); thus, the comparable SiC abundances in different CM chondrites suggest that there were no significant heterogeneities in the thermal histories of the matrix materials in these samples (Fig. 5). Although the SiC abundances of SM fragments are comparable and uniform suggesting no heterogeneous distributions among these samples, we did find heterogeneity among different matrix areas within SM2-4. Heterogeneous distributions of presolar grains have been recognized in other primitive meteorites (Floss and Stadermann 2009a, 2009b; Zhao et al. 2011). In SM2-4, we found 5 SiC grains in area M1 (6000 lm2), resulting in a matrix-normalized abundance of 54 ppm. No presolar SiC grains were found in matrix area M2 (also 6000 lm2), but we can calculate an upper limit of 12 ppm for this area based on the typical grain size (approximately 300 nm). Although the small numbers of presolar SiC grains introduce large uncertainties to the presolar grain abundances, the overall difference between area M1 and M2 still suggests a significant difference, with a higher SiC abundance in area M1. Note that the only presolar O- anomalous grain was also found in area M1. It is possible that the matrix area with the higher presolar SiC abundance experienced less secondary processing than other matrix areas (Floss and Stadermann 2009a, 2009b; Zhao et al. 2011). As discussed before, presolar SiC grains could be affected by thermal metamorphism (Huss et al. 2003). Thus, matrix areas with lower presolar SiC Fig. 6. Backscattered electron (BSE) images of: a) fine-grained abundances might have undergone higher degrees of matrix area M1 (presolar SiC-rich); b) fine-grained matrix area thermal processing. Based on the regolith texture of M2 (no presolar SiC grains were found). Both areas are in SM2-4. Sutter’s Mill, we suggest the following two possibilities: (1) the SiC-poor regolith clast could be derived from a is consistent with the abundance estimate of presolar O- different location (buried deep in the same parent body or anomalous grains for Murchison using Cameca a distinct parent body) that had experienced a higher 1270 + SCAPS system (approximately 3 ppm; degree of thermal metamorphism and part of SiC grains Nagashima et al. 2005). A recent study of Murchison have been destroyed; (2) the surface of its parent body has using NanoSIMS ion imaging (Leitner et al. 2013) has been heavily bombarded by other asteroids (as evidenced estimated a higher abundance of approximately 75 ppm by E chondrite clast found in Sutter’s Mill; Jenniskens for presolar silicate grains, based on two grains found in et al. 2012), and some regolith clasts could experience the the fine-grained rim around a chondrule. As aqueous impact-induced thermal processing that destroyed SiC alteration tends to destroy presolar O-anomalous grains selectively in localized regions. However, thermal (mainly silicates), the lower abundance of presolar processing will also result in the coarsening of grain sizes O-anomalous grains in Sutter’s Mill may suggest that it in the affected matrix areas (e.g., Floss and Stadermann has undergone higher degrees of aqueous alteration than 2012). Figures 6a and 6b are backscattered electron Murchison. The only presolar O-anomalous grain we images of the matrix areas M1 and M2, showing that there found in Sutter’s Mill was in SM2-4, consistent with are no obvious differences in grain size among these areas. earlier petrographic observations that SM2-4 has experienced less aqueous alteration than the other three Presolar Oxide and Aqueous Alteration fragments (Fig. 1; Jenniskens et al. 2012; Zolensky et al. 2012). Clasts containing highly water-sensitive The single presolar corundum identified from SM2-4 oldhamite, such as SM2-4 (Zolensky et al. 2012), must results in matrix-normalized abundances of 2 ppm for have been added after aqueous alteration occurred, all measured Sutter’s Mill samples (63,900 lm2), which probably during later asteroid impacts. 8 X. Zhao et al.

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