BULK MAJOR AND TRACE ELEMENTAL COMPOSITION OF AN AGGREGATE SAMPLE FROM
ASTEROID BENNU. P. Koefoed
1
, K. Wang (王昆)
1
,
C. M. O’D. Alexander
2
, J. A. Barrat
3
, P. Haenecour
4
, J. J.
Barnes
4
, A. N. Nguyen
5
, H. C. Connolly Jr.
4,6,7
, D. S. Lauretta
4
,
1
McDonnell Center for the Space Sciences, Dept. of
Earth, Environmental, & Planetary Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis,
2
Earth and Planets Laboratory, Carnegie Institution for Science, Wash-
ington, DC, USA,
3
Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Plouzané, France,
4
Lunar and Planetary Laboratory, University of Arizona, Arizona, USA,
5
Astromaterials Research and Exploration
Science Division, NASA Johnson Space Center, Houston, Texas, USA,
6
Department of Geology, School of Earth and
Environment, Rowan University, Glassboro, NJ, USA,
7
Department of Earth and Planetary Science, American Mu-
seum of Natural History, New York, NY, USA.
Introduction: On September 24, 2023, NASA’s
OSIRIS-REx mission returned a capsule to Earth carry-
ing material from asteroid Bennu. This event was the
first time a U.S. mission delivered pristine samples from
an asteroid and is the largest asteroid sample return to
date. As these samples represent some of the oldest,
most primitive, and pristine materials available to us,
and which originate from a known and well-studied as-
teroid [1], they allow us a rare opportunity to gain a bet-
ter understanding of the formation and evolution of our
solar system.
Key to understanding the material returned from as-
teroid Bennu is establishing its bulk chemical composi-
tion. Previous studies have shown that each chondrite
group has a distinct elemental composition [2,3]. For the
carbonaceous chondrites specifically, each group exhib-
its a distinct pattern of moderately and highly volatile
elemental depletions, relative to CI chondrites [2,3]. CI
chondrites are considered the most primitive chondrite
group and broadly represent the solar photosphere com-
position [4]. The two most striking features of these de-
pletion patterns is that the moderately volatile element
depletions increase with decreasing 50% condensation
temperature and then plateau out at abundances that
roughly correlate with matrix abundance [2,3].
Due to these distinctive patterns, bulk elemental
composition has become an important classification tool
for establishing the different chondrite groups and the
connections between them [5,6]. As such, determining
the bulk chemical composition of the Bennu aggregates
will help to test two mission hypothesis: “Bennu’s bulk
elemental composition reflects that of its main parent
asteroid and is similar to the composition of the Sun,
with depletions in moderately to highly volatile ele-
ments” and “Bennu’s dominant lithologies are compa-
rable in bulk mineralogy, petrology, and composition to
the most aqueously altered carbonaceous chondrites”
[7]. Furthermore as the carbonaceous and non-carbona-
ceous chondrites are thought to have likely formed in
the inner and outer protoplanetary disk, respectively
(e.g., [8]), determining the bulk chemical compositions
of the Bennu aggregates will also help test the major
mission hypothesis that “Bennu's parent body formed
beyond the snow line by accretion of material in the pro-
toplanetary disk” [7].
Regarding chondrite formation, observed elemental
patterns have been successfully used to model how the
mixing of volatile-rich and volatile-poor chondritic
components can produce the observed carbonaceous
chondrites groups [2,9]. They have also been significant
in investigating how volatilization processes influenced
chondrite formation [2,6,10]. As such, establishing bulk
chemical compositions of the pristine Bennu samples is
vital for understanding the asteroid, and solar system
formation. To begin this processes, we analyzed the
bulk major and trace elemental compositions of Bennu
aggregates.
Samples: The elemental analyses were undertaken
on aggregate sample OREX-803015-0 (20.66 mg of
<1mm-sized particles), which was separated from the
parent sample OREX-800033-0. Additionally, nine car-
bonaceous chondrite fall samples (14 – 69 mg) were run
alongside OREX-803015-0 to both monitor data quality
and provide direct comparisons to the Bennu aggregate.
These nine chondrite analog samples consisted of two
samples of Orgueil (CI1), Tagish Lake (C2-ung.), Tarda
(C2-ung.), Winchcombe (CM2), Murchison (CM2),
Lancé (CO3.5), Vigarano (CV3), and Karoonda (CK4).
Analytical Methods: Dissolution of all samples
was undertaken on a hotplate using concentrated HF and
HNO
3
at a 3:1 ratio, with HCl and H
2
O
2
also used to
remove fluorides and organics, respectively. All major
and trace elemental analyses were conducted using a
Thermo Fisher iCAP Qc ICP-MS. Linear calibrations
were done using a synthetic CM chondrite standard run
at multiple dilution factors (BIR-1 and BHVO-2 were
also run alongside to monitor the CM chondrite stand-
ard). A 5 ppb internal standard of Re+Rh was run
throughout the analysis session to correct for instrument
drift. Samples were run three times each at total dilution
factors of ~5,000 and ~50,000. The major element data
were calculated using the ~50,000 dilution factor anal-
yses, while the trace element data were calculated using
the ~5,000 dilution factor analyses.
2264.pdf55th LPSC (2024)
Results and Discussion: The nine chondrite analog
samples measured here show good agreement with the
literature data (e.g., [2,11]). All fifty-four of the ele-
ments analyzed for the Bennu aggregate sample lie
close to the average CI chondrite [4, 11], and thus the
solar photosphere composition, indicating that Bennu’s
bulk elemental composition is similar to that of the Sun
and some of the most aqueously altered carbonaceous
chondrites. Furthermore, the Bennu aggregate sample is
distinguishable from the CM, CO, CV, CK and the two
ungrouped carbonaceous chondrites (Tagish Lake and
Tarda) that were analyzed at the same time. Compared
to samples returned from asteroid Ryugu [12,13], this
Bennu aggregate sample appears similar in elemental
composition, yet without the small refractory element
enrichments seen in Ryugu. Overall, these results sug-
gest that Bennu is indeed a carbonaceous asteroid that
formed beyond the snow line by accretion of material in
the protoplanetary disk.
Acknowledgments: This material is based upon
work supported by NASA under Contract
NNM10AA11C issued through the New Frontiers Pro-
gram. We are grateful to the entire OSIRIS-REx Team
for making the return of samples from Bennu possible.
We also thank Natural History Museum (UK), Smith-
sonian National Museum of Natural History, and
Muséum National d'Histoire Naturelle for providing
meteorite samples.
References: [1] Lauretta D.S. et al. (2015) MAPS,
50, 834-849. [2] Braukmuller N. et al. (2018) GCA, 239,
17-48. [3] Wasson J.T. & Kallemeyn G.W. (1988)
Philos. Trans. Royal Soc. A, 325, 535-544. [4] Lodders
K. (2003) APJ, 591, 1220-1247. [5] Krot A.N. et al.
(2014) in Tre. Geochem. 2
nd
ed., 1-63. [6] Scott E.R.D.
& Krot A.N. (2014) in Tre. Geochem. 2
nd
ed., 65-137.
[7] Lauretta D.S. et al. (2023) arXiv 2308.11794. [8]
Savage P. S. Et al. (2022) Icarus, 386, 115172. [9]
Alexander C.M.O’D. (2019) GCA, 254, 277-309. [10]
Alexander C.M.O’D. (2005) MAPS, 40, 943-965. [11]
Barrat J.A. (2012) GCA, 83, 79-92. [12] Nakamura E. et
al. (2022) Japan Acad. Ser. B, 98, 227-282. [13]
Yokoyama T. et al. (2023) Science, 379, eabn7850.
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