Coordinated Analyses of Hayabusa particles RB-CV-0234, RB-QD04-0039, and RA-QD02-0310:
Constraints on asteroid Itokawa formation from sulfides and silicates
Devin L. Schrader1 and Thomas J. Zega2
1
Buseck Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 852871404, USA (devin.schrader@asu.edu),
2Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA (tzega@arizona.edu)
Introduction: Sulfides are important phases in planetary materials because their compositions, textures, and crystal
structures can be used to constrain oxygen fugacity of formation, shock stage, and aqueous-, thermal-, and cooling-histories [e.g.,1–8]. The pyrrhotite-group sulfides are largely nonstoichiometric and have a range of compositions (typically 0≤x≤0.125, but x
can be ≤0.2; i.e., FeS [troilite] to Fe0.8S) and distinct crystal structures (polytypes). The stoichiometric end members are 2C(troilite; FeS, hexagonal) and typically 4C (Fe7S8, monoclinic) pyrrhotite. There are also pyrrhotites of intermediate compositionswith 0≤x≤0.2 (all hexagonal), including the non-integral NC-pyrrhotites and the integral 5C (Fe9S10), 6C (Fe11S12), and 11C(Fe10S11) pyrrhotites [e.g., 9–11]. Geothermometry of pyrrhotite-pentlandite intergrowths in meteorites shows that most formed
via primary cooling from high temperature (i.e., chondrule cooling in the protoplanetary disk) or thermal alteration on the parent
body [e.g., 12–14]. Sulfides in the LL4 to LL6 chondrites typically equilibrated ≤230°C, consistent with formation during coolingafter thermal metamorphism [2]. Analyses of Hayabusa particles identified asteroid 25143 Itokawa as LL4–6 chondrite material(~10% LL4 and ~90% LL5–6) [e.g., 15–18] that was thermally metamorphosed between ~780 and 840°C [15]. Itokawa particles
were found to record shock stages between S2 and S4, with most particles around S2 [19,20]. Sulfides in Hayabusa particles[e.g., 15,21,22] may record additional and/or complementary information on theformation conditions of asteroid Itokawa.
We performed coordinated analyses of the Hayabusa particles RB-CV-0234,
RB-QD04-0039, and RA-QD02-0310, allocated in Hayabusa AO5. Preliminary
analysis by JAXA’s Hayabusa examination team via scanning electron
microscopy (SEM) determined that particle RB-CV-0234 (25.9 μm in diameter)
consists of FeS, Fe, FeNiS, and CuS, making it an ideal target potentiallycontaining both pyrrhotite group sulfides [(Fe,Ni,Co,Cr)1– xS] and the rare Ni-richsulfide, pentlandite [(Fe,Ni,Co,Cr)9S8]. In addition, JAXA’s Hayabusa
examination team determined that particle RB-QD04-0039 (52 μm in diameter)
consists of olivine, low-calcium pyroxene, and troilite, and that particle RAQD02-0310 (127 μm in diameter) consists of olivine, plagioclase, and troilite.
These two Hayabusa particles provided excellent opportunities to identify
Figure 1. Composite TEM X-ray element map(RGB=FeSNi) of RB-CV-0234 FIB Section;
sulfides with petrographic context and to analyze their adjoining silicate
where po = pyrrhotite and pn = pentlandite.
minerals, unlike RB-CV-0234 that only contained sulfides. Here, we comparedin situ analyses of these particles to the results from LL3–6 chondrite silicates and sulfides [e.g., 1,2], with the goal of further
constraining the formation and alteration conditions of asteroid Itokawa. Preliminary analyses from RB-CV-0234 and RB-QD040039 were presented in [23,24,25].
Samples and Analytical Procedures: We mounted RB-CV-0234, RB-QD04-0039, and RA-QD02-0310 each on epoxy
bullets, using a method modified from [21], and microtomed them in preparation for analysis and sample extraction using theFEI Helios NanoLab 660 focused-ion-beam-SEM (FIB-SEM) at the University of Arizona (UAz). X-ray element maps and highresolution images of the microtomed RB-CV-0234 and RB-QD04-0039 were obtained with the FIB-SEM prior to extraction ofa ~10 × 10 μm sections from each particle, which were then thinned to electron transparency (<100 nm) following the methodsof [26]. The FIB sections were then analyzed using the 200 keV aberration-corrected Hitachi HF5000 scanning transmission
electron microscope (S/TEM) at UAz. After a FIB section of the sulfide from RB-QD04-0039 was obtained, RB-QD04-0039
was analyzed on the Cameca SX-100 electron microprobe (EPMA) at UAz. Microtome preparation of RA-QD02-0310 did notreveal sulfides for further structural analyses via TEM, but the compositions of its silicates were determined via EPMA at UAz.
Results: FIB-SEM X-ray element maps showed that the microtomed surface of RB-CV-0234 consisted entirely of pyrrhotite.
However, X-ray element maps of the extracted FIB section, determined via TEM analysis, revealed a single large grain ofpyrrhotite and a single smaller grain of pentlandite (4.8 × 1.3 μm) at the bottom of the section (Fig. 1). Selected-area electrondiffraction (SAED) patterns of the pyrrhotite and pentlandite grains index to 2C pyrrhotite (troilite) and pentlandite along the[110] zone axis (i.e., they are crystographically oriented), respectively. The compositions of RB-CV-0234 via quantitative

energy-dispersive X-ray spectroscopy [27] are 35.2 wt.% S and 64.8 wt.% Fe for pyrrhotite, and 32.4 wt.% S, 49.4 wt.% Fe, and18.2 wt.% Ni for pentlandite. The microtomed surface of RB-QD04-0039 showed a single sulfide grain, as well as olivine(Fa28.1–29.5, Fa28.9±0.5 [1σ]; n = 7) and low-calcium pyroxene (Fs23.8–24.9, Fs24.3±0.6, Wo1.1±0.1 [1σ]; n = 3) (Fig. 2). A
FIB section of the sulfide grain in RB-QD04-0039 revealed it to be troilite. The microtomed surface of RA-QD02-0310 showeda grain of olivine (Fa28.9–31.3, Fa30.2±0.8 [1σ]; n = 12), with two chrome-spinels.
Discussion: The sulfides in RB-CV-0234 are most consistent with it being
a sulfide grain from an LL6 chondrite, similar to the sulfides from Saint-Séverin
studied by [1,2]. We infer this because: (1) the Ni content of pentlandite isconsistent with the characteristic Ni contents of LL chondrite pentlandite [2,14];and (2) based on results from the LL3–6 chondrite sulfides we previously
studied [1,2,14], sulfide grains in Saint-Séverin (LL6, S2 [2,28]) contain asimilar pentlandite/pyrrhotite morphology (i.e., blocky pentlandite inpyrrhotite) and 2C pyrrhotite (troilite) with pentlandite [1]. The other LL
chondrites we studied contained either a distinct morphology (e.g., pentlanditelamellae) and/or multiple polytypes of pyrrhotite [1,2]. The compositions ofpyrrhotite and pentlandite in RB-CV-0234 are consistent with equilibrating<300°C, perhaps as low as 230°C, similar to that of LL4–6 chondrites [2,25].
The olivine grain in RB-QD04-0039 contains planar fractures, indicating ashock stage of at least S3 and minimum shock pressures of 5–10 GPa [29]. The
silicate compositions of both RB-QD04-0039 and RA-QD02-0310 (Fa28.9±0.5
and Fa30.2±0.8, respectively) are consistent with that of equilibrated LLchondrites, consistent with that found by [15] for Hayabusa particles. Relativeto the iron-wüstite (IW) buffer, the oxygen fugacity constraints on the formationof troilite [30] and of pentlandite [14] indicates that RB-CV-0234 and RBQD04-0039 were likely altered between approximately IW–2.2 and IW,
consistent with that found for LL5 and LL6 chondrites by [14]. Therefore, theFigure 2. (a) Back scattered electron (BSE)
morphology and chemical compositions of sulfides and silicates of RB-CVimage and composite EPMA X-ray element map(RGB=FeMgSi) of RB-QD04-0039; LCP =0234, RB-QD04-0039, and RA-QD02-0310, as well as inferred sulfide
low-calcium pyroxene.
equilibration temperature and oxygen fugacity constraints are consistent withequilibrated LL chondrites.
Acknowledgements: We thank JAXA for the loan of the Hayabusa particles used in this study through the 5th International
Announcement of Opportunity, and NASA grant NNX17AE53G (DLS PI, TJZ Co-I) for funding this research.
References[1] Schrader D. L. and Zega T. J. 2017. Abstract #6347. 80th MetSoc. [2] Schrader D. L. and Zega T. J. 2019. Geochim.
Cosmochim. Acta 264:165. [3] Arnold R. G. 1967. Can. Min. 9:31. [4] Kissin S. A. and Scott S. D. 1982. Econ. Geol. 77:1739.
[5] Raghavan V. 2004. J. Phase Equilib. 25:373. [6] Wang H. et al. 2006. J. Sulfur Chem. 27:1. [7] Harries D. and Langenhorst
F. 2013. Meteorit. Planet. Sci. 48:879. [8] Bennet M. E. and McSween Jr. H. Y. 1996. Meteorit. Planet. Sci. 31:255. [9] Morimoto
N. et al. 1975. Econ. Geo. 70:824. [10] Wang H. et al. 2006. J. Sulfur Chem. 27:1. [11] Harries D. et al. 2011. Am. Min. 96:716.
[12] Jamsja N. and Ruzicka A. 2010. Meteorit. Planet. Sci. 45:828. [13] Schrader D. L. et al. 2015. Meteorit. Planet. Sci. 50:15.
[15] Nakamura T. et al. 2011. Science 333:1113. [14] Schrader D. L. et al. 2016. Geochim. Cosmochim. Acta 189:359. [16]Noguchi T. et al. 2014. Earth, Planets, and Space 66:124. [17] Tsuchiyama A. et al. 2011. Science 333: 1125. [18] Tsuchiyama
A. et al. 2014. Meteorit. Planet. Sci. 49:172. [19] Noguchi T. et al. 2011. Science 333:1121. [20] Zolensky M. E. et al. 2012.
Abstract #1477. 43rd LPSC. [21] Berger E. L. and Keller L. P. 2015. Microscopy Today 23:18. [22] Harries D. and Langenhorst
F. 2014. Earth, Planets, and Space 66:163. [23] Schrader D. L. and Zega T. J. 2018. Hayabusa 2018, 6th Symposium on Solar
System Materials. [24] Schrader D. L. and Zega T. J. 2019a. Lunar Planet. Sci. L, Lunar Planet. Inst., Houston, #2009. [25]Schrader D. L. and Zega T. J. 2019b. Abstract #6400. 82nd MetSoc. [26] Zega T. J. et al. 2007. Meteorit. Planet. Sci. 42:1373.
[27] Zega T. J. et al. 2018. Abstract #2084. Microsc. Microanal. 24 (Suppl. 1). [28] Rubin A. E. 2004. Geochim. Cosmochim.
Acta 68:673. [29] Stöffler D. et al. 2018. Meteorit. Planet. Sci. 53:5. [30] Schrader D. L. et al. 2021. Geochim. Cosmochim. Acta
303:66.