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Abstract:
Introduction:
Anhydrous interplanetary dust particles are one of the least altered ancient solar system materials.
Their bulk chemical compositions match those of CI chondrites within a factor of 2-3, except for carbon which is
enriched in both anhydrous and hydrated IDPs by ~4x CI [1]. Hydrous IDPs often show 16 O-poor isotopic composi-
tions, likely due to the interaction with isotopically heavy H 2 O [2]. Anhydrous IDPs are interpreted to originate from
comets from the outer solar system [3]. Oxygen isotopic data for these materials is scarce, but results point to a wider
range of compositions [4]. This heterogeneity might be a result of mixing between a 16O-rich and a 16
O-poor reservoir
formed via self-shielding and photodissociation of CO [5]. Here, we report bulk oxygen isotopic compositions of three
anhydrous IDPs in context of their mineralogical composition and discuss possible origins of isotopic heterogeneity.
Experimental: Bulk chemical composition and mineralogy of 70 nm thin ultramicrotomed sections of three an-
hydrous IDPs (L2099A7, L2099A8, and L2071AB1) were analyzed using a Thermo Scientific Titan Themis G3 60-
300 TEM equipped with a four-quadrant energy-dispersive X-ray detector (Super-X G2) at University of Münster.
Bulk oxygen isotopic compositions were measured using a NanoSIMS 50 at the Max-Planck-Institut für Chemie in
Mainz. A focused Cs + ion beam (~1 pA, ~100 nm) was rastered over a field of view varying from 5 x 5 to 6 x 6 μm2,
depending on particle size for 15 layers. Negative secondary ions of 16O, 17 O, 18O, 12 C14
N, and 28
Si were collected on
electron multipliers. The 16OH contribution to the 17O peak was 1-2 ‰. As a standard, a matrix region of meteorite
CR2 Queen Alexandra Range (QUE) 99177 was used, whose oxygen isotopic composition was measured by [6].
Results and Discussion: Particles A7 and A8 are both fine-grained, consisting mostly of small, 100 nm-sized
equilibrated aggregates (EA). Both particles exhibit discontinuous magnetite rims with thicknesses up to 50 nm in A7
and 100 nm in A8 on the outsides. In Particle AB1, the upper part of the particle consists of a few EAs, while the
lower part contains lots of small GEMS grains with 100-200 nm diameter clustered together. Magnetite rims are nearly
absent, the rare rims reaching maximum 20 nm thickness, indicating that AB1 is of more primitive nature than the
other two IDPs [1]. Furthermore, it contains several diffuse GEMS-like areas, identified by an amorphous silicate
groundmass with small nano-inclusions of Fe,Ni-metal and Fe-sulfides. All three particles are subsolar for all major
element/Si ratios. Mean S/Si of particles A7 and A8 is 0.046 and 0.048, respectively, while it is an order of magnitude
higher in AB1 (0.184). This reflects the higher degree of thermal alteration in A7 and A8, resulting in the loss of
volatile S and oxidation of Fe-sulfides and FeNi-metal to magnetite as present in particle rims. A8 is rather 16
O-poor
with δ17 O SMOW = 7.8 ± 3.4 ‰ and δ18O SMOW = 9.5 ± 3.1 ‰ (1σ), followed by A7 with δ17O SMOW = − 0.8 ± 4.6 ‰ and
δ18O SMOW = −2.2 ± 3.4 ‰. In contrast to that, AB1 has the most 16O-rich bulk composition with δ17 O SMOW = −24.2 ±
5.4 ‰ and δ18O SMOW = −25.3 ± 3.6 ‰. All investigated IDPs plot slightly above the CCAM line as previously reported
for other anhydrous IDPs [4], maybe due to a contribution from circumstellar dust from AGB stars enriched in 17
O
[7]. The 16O-rich composition of AB1 is similar to the most 16 O-rich anhydrous IDP U2015D21 measured by [8] which
is dominated by GEMS [9] and the GEMS-rich IDP GM4-2 reported by [10]. Nevertheless, it is unlikely that GEMS
are the carrier for the 16O-rich composition of AB1 because most GEMS have O isotopic compositions indistinguish-
able from terrestrial values, although errors on these analyses are large [11]. The isotopic heterogeneity is best ex-
plained by contribution of grains from different regions in the protoplanetary disk, sampling different O isotope res-
ervoirs, created by self-shielding and photodissociation of CO. This process resulted in 16O-rich CO gas and 16
O-poor
H 2O that froze as ice-mantles onto dust grains [12]. Alternatively, the higher degree of heating in A7 and A8 could
have influenced isotopic fractionation, because higher δ17,18 O values are reported from the regions of the atmosphere
where small particles are heated and oxidized, as observed for the thermal alteration of cosmic spherules [13].
Acknowledgements: We would like to thank NASA Astromaterials Acquisition & Curation Office for providing IDP samples
and DFG for funding this project (VO1816/5-1).
References: [1] Bradley J.P. (1988) Geochimica et Cosmochimica Acta 52:889-900. [2] Keller L.P. and Snead
C.J. (2021) LPS LII, Abstract #2389. [3] Bradley J.P. and Brownlee D.E. (1986) Science 231:1542-1544. [4] Starkey
N.A. and Franchi I.A. (2013) Geochimica et Cosmochimica Acta 105:73-91. [5] Yurimoto H. and Kuramoto K. (2004)
Science 305:1736-1766. [6] Schrader D. et al. (2014) Earth and Planetary Science Letters 407:48-60. [7] Nittler L.
and Gaidos E. (2012) Meteoritics and Planetary Science 47:2031-2048. [8] Nguyen A.N. et al. (2021) MAPS LXXXIV,
Abstract #2609. [9] Nguyen A.N. et al. (2015) LPS XLVI, Abstract #2868. [10] Aléon J. et al. (2009) Geochimica et
Cosmochimica Acta 73:4558-4575. [11] Keller L.P. and Messenger S. (2011) Geochimica et Cosmochimica Acta
75:5336-5365. [12] Krot A. et al. (2020) Science Advances 6. [13] Clayton R.N. et al. (1986) Earth and Planetary
Science Letters 79:235-240
