Xenolithic Fe,Ni Metal in Polymict Ureilite Meteorites

50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1622.pdf

XENOLITHIC FE,NI METAL IN POLYMICT UREILITE METEORITES. Y. Boleaga1 and C.A. Goodrich2. 1Dept. of Geology, The City College of New York (City University of New York), 160 Convent Ave, New York, 2 NY 10031 USA ([email protected]); Lunar & Planetary Institute, USRA, Houston, TX 77058 USA.

Introduction: Ureilite meteorites are ultramafic Internal textures show no Widmanstättan patterns typi- rocks that come from the mantle of a differentiated cal of iron meteorites. asteroid. They are the second largest achondritic mete- Grain 5 in NWA 10657_003 (Fig. 2a,b) consists of orite group and consist of olivine and pyroxene with metal (lighter gray) surrounded by an assemblage of minor elemental carbon, sulfide and metal [1-4]. In this enstatite grains (dark rounded grains), Cr-rich troilite, study we focus on polymict ureilites, which are brecci- and graphite. Figures 2 d-f show plots of Ni (wt.%) as that represent regolith on a ureilitic asteroid [3-6]. versus Co (wt.%), Si (wt.%) and P (wt.%), respectively They contain xenoliths from impactors including for grain 5 compared with the same metals as Fig. 1. fragments of ordinary chondrites (OC), enstatite chon- a) b) 20µm drites (EC), carbonaceous chondrites (CC), Rumuruti-

type chondrites (R-type), and angrites. Profile 2 In this study, we searched for xenoliths of iron meteorites in polymict ureilites. Xenoliths of iron meteorites have not previously been reported in polymict Ureilitic 200µm ureilites. An absence of such metal materials could material

mean that silicate materials preferentially survive dur- Ni 1.4 Ureilite metal Iron meteorite 45 ing impacts due to their physical properties compared 1.2 Kamacite EC type 3, 4-6 OC type 3, 4-6 40 DaG999_r2 Grain 1 1.0

with those of metal. Alternatively, iron meteorite as- ) 35 d) 0.8

wt.% 30

teroids may not have been present where and when ( c) 0.6 Ni 25

ureilitic regolith was forming. Co (wt.%) 0.4 Taenite 20

0.2 We studied the distribution of Fe,Ni-rich grains and 15 associated assemblages in polymict ureilites Northwest 10 0.0 5 -0.2 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 0 5 10 15 20 25 30 35 Africa (NWA) 10657 and Dar al Gani (DaG) 999. Ni (wt.%) Distance (microns) 1.4

6 Kamacite Methods: We studied sections NWA 10657_003 1.2 Kamacite 5 and DaG 999_r2. Reflected light microscopy was done 1.0

4 f) at the Lunar and Planetary Institute (LPI). The JEOL e) 0.8

3 8530F field emission electron microprobe (EMPA) at 0.6 Si (wt.%)

Taenite P (wt.%) 2 0.4 Johnson Space Center (JSC) was used to obtain back- Taenite scattered electron images (BEI), X-ray element maps, 1 0.2 and quantitative analyses. For analysis of metals and 0 0.0 0 5 10 15 20 25 30 35 Ni (wt.%) 0 5 10 15 20 25 30 35 40 45 50 55 60 sulfides, Fe, Ni, Co, Cr, P, Si, S and Mg were meas- Ni (wt.%)

ured using 40 nA beam current and 15 keV accelerat- Figure 1. a) BEI of grain 1 in DaG 999_r2; b) BEI ing potential. The obtained data were compared with showing location of profile 2 (4 µm steps) crossing the data in the literature for metal in OC, EC, iron meteor- rims, the lighter gray, (Ni rich); c) Distance (µm) vs. ites, and ureilites. We identified 8 metal rich clasts, 2 Ni (wt.%) of profile 2 shown in 1b; d) Ni (wt.%) vs. Co in DaG 999 and 6 in NWA 10657. (wt.%); e) Ni (wt.%) vs. Si (wt.%); f) Ni (wt.%) vs. P Results: We describe grain 1 from DaG 999_r2 and grain 5 from NWA 10657_003 in detail. Grain 1 in (wt.%). The data for grain 1 are shown by the orange DaG 999_r2 (Fig.1a,b) consists of metal (lighter gray) circles. Ureilite metal data [7], iron meteorite data [8], partially surrounded by sulfide (darker gray). Figure 1c EC data [9-11]. OC data [12-16]. shows Ni content along the profile marked in Fig. 1b. Grain 5 consists of kamacite. The Ni vs. Co plot Figures 1 d-f show plots of Ni (wt.%) versus Co shows that grain 5 compositions overlap those of EC, (wt.%), Si (wt.%) and P (wt.%), respectively for grain OC and ureilite metal. The Ni vs. Si, P plots show that 1 compared with metal in various meteorites. grain 5 compositions overlap mainly EC metal. Grain 1 consists mostly of kamacite, with thin rims The internal structure of grain 5 shows an inter- trending into the taenite field. The Ni vs. Co, and Ni growth texture (Fig. 2b). However, compositional pro- vs. Si plots show that grain 1 compositions overlap files show no variation across this structure (Fig. 2c), those of iron meteorites and OCs. However, P contents suggesting that it is probably martensite [17-19]. are higher than those in either OC or iron meteorites. Grain 6 in NWA 10657_003 is similar to grain 5 in compositions, as well as associated minerals (enstatite,

50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1622.pdf

sulfides including niningerite). Grain 5 and 6 are close with derivation mostly from ECs and OCs. Grain 5 and to one other in the section, which may indicate that 6 from NWA 10657_003 have metal compositions and they are derived from a common EC impactor. associated minerals (enstatite, Cr-troilite, graphite, niningerite) very similar to those of metal-sulfide- b) 10µm a) 110µm En silicate assemblages in ECs [4, 9-11 and 20]. Grain 1 En Profile 5 s 38 pts from DaG 999_r2 and grains 1 and 2 from NWA Metal 5 µm spacing 10657_003 have metal compositions and associated minerals (kamacite partially surrounded by sulfide and En Ni-enriched rims) similar to assemblages in OCs [12- En En 16]. However, the concentration of Ni decreases from rim to center (Fig. 1c) in these grains, instead of in- Ni 1.4 8 Kamacite creasing as it typically does in an OC [14]. This may 1.2 d) be due to a reaction with a gaseous environment which 7 c) 1.0

) Ureilite metal 0.8 changed the rims when impact occurred.

wt.% Iron Meteorites ( EC Type 3, 4-6 6 Ni 0.6 NWA10657-003 Grain 5 OC Type 3, 4-6 It is notable that half of the metal grains we studied Co (wt.%) 0.4 Taenite appear to be derived from ECs, since previous observa- 5 0.2 tions have shown that EC material is rare in typical 0.0 4 0 20 40 60 80 100 120 140 160 180 200 0 5 10 15 20 25 30 35 40 45 50 55 60 polymict ureilites [21]. EC material is very abundant in Distance (microns) Ni (wt.%)

6 1.4 the Almahata Sitta (AhS) anomalous polymict ureilite Kamacite Kamacite 5 1.2 [4,14], which could mean that AhS and typical

1.0 polymict ureilites have different origins. Our observa- 4 f) e) 0.8 3 tions suggest that EC material may be common in 0.6 Si (wt.%) 2 P (wt.%) typical polymict ureilites after all. This possibility 0.4 should be studied in more depth. 1 Taenite 0.2 Taenite Acknowledgements: We thank Michael Weisberg 0 0.0

0 5 10 15 20 25 30 35 40 45 50 55 60 0 5 10 15 20 25 30 35 40 45 50 55 60 Ni (wt.%) for data on enstatite chondrites and Kent Ross for as- Ni (wt.%) sistance with EMPA at JSC. This work was supported Figure 2. a) BEI of grain 5 in NWA 10657_003; b) by the LPI summer intern program. BEI showing location of profile 2 (5 µm steps,) cross- References: [1] Goodrich C.A. (1992) Meteoritics 27, ing across α structure; c) Distance (µm) vs. Ni (wt.%) 2 327–352. [2] Mittlefehldt D. W. et al. (1998) In Planetary of profile 2 shown in 2b; d) Ni (wt.%) vs. Co (wt.%); Materials. RIM 36, 4-1 to 4-195. [3] Downes H. et al. e) Ni (wt.%) vs. Si (wt.%); f) Ni (wt.%) vs. P (wt.%). (2008). Geochimica et Cosmochimica Acta 72, 4825–4844. Data for grain 5 are represented by the orange circles. [4] Goodrich C. A. et al. (2015) Meteoritics & Planetary Data for other meteorites from same sources as Fig. 1. Science 50, 782–809. [5] Cohen B. A. et al. (2004) Geo- chimica et Cosmochimica Acta 68, 4249–4266. [6] Goodrich Grains 1 and 2 of NWA 10657_003 are similar to C. A. et al. (2004) Chemie der Erde 64, 283–327. [7] grain 1 of DaG 999_r2 in composition and associated Goodrich C. A. et al. (2013) Geochimica et Cosmochimica Acta 112, 340–373. [8] Goldstein J. I. et al. (2017) Geo- minerals (kamacite with Ni-enriched rims partially chimica et Cosmochimica Acta 200, 367–407. [9] Weisberg surrounded by sulfide). M. K. and Kimura M. (2012) Chemie der Erde 72, 101–115. Discussion: The abundance of metal grains found [10] Horstmann M. et al. (2014) Geochimica et Cosmo- in sections DaG 999_r2 and NWA 10657_003 was chimica Acta 140, 720–744. [11] Brearley A. J. and Jones R. small. Also, the metal grains we did observe were H. (1998) In Planetary Materials. RIM 36. pp. 3-1 to 3-398. much smaller than previously described xenoliths in [12] Rubin A. E. (1990) Geochimica et Cosmochimica Acta polymict ureilites. None of the Fe,Ni metal grains that 54, 1217–1232. [13] Afiattalab F. and Wasson J. T. (1979) we found appeared to be derived from iron meteorites. Geochimica et Cosmochimica Acta 44, 431–446. [14] Although we had two possible hypotheses, that iron Reisener R. J. and Goldstein J. I. (2003) Meteoritics & Plan- etary Science 38, 1679–1696. [15] Reed S. J. B. (1964) Na- meteorite asteroids may not have been present when ture 204, 374–375. [16] Zanda B. et al. (1994) Science 265, and where the polymict ureilite regolith formed, or that 1846-1849. [17] Buckwald Vagn F. (1975) In Handbook of an absence of such metal materials could mean that Iron Meteorites, “Chapter 9: The Minerals and Structural silicate materials preferentially survive during impacts Components of Iron Meteorites.” [18] Jones F. W. and due to their physical properties compared with those of Pumphrey W. I. (1949) Journal of the Iron and Steel Institute metal, our results did not test either one and only 163, 121–131. [19] Allen N. P. and Earley C. C. (1950) showed the absence of iron meteorite xenoliths. Other Journal of the Iron and Steel Institute 166, 281–288. [20] Keil K. (1968). Journal of Geophysical Research, Atmos- types of studies would need to be done to distinguish th between the two hypotheses. pheres 73, 6945-6976. [21] Goodrich C.A. et al. (2015) 78 MSM, #5018. The observed grains have properties consistent