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Oxygen Isotopic Compositions of Asteroidal Materials Returned from Itokawa by the Hayabusa Mission

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Science  26 Aug 2011:
Vol. 333, Issue 6046, pp. 1116-1119
DOI: 10.1126/science.1207776

Abstract

Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves have not been established. We measured, using secondary ion mass spectrometry, oxygen isotopic compositions of rock particles from asteroid 25143 Itokawa returned by the Hayabusa spacecraft. Compositions of the particles are depleted in 16O relative to terrestrial materials and indicate that Itokawa, an S-type asteroid, is one of the sources of the LL or L group of equilibrated ordinary chondrites. This is a direct oxygen-isotope link between chondrites and their parent asteroid.

Mineral compositions of asteroids are inferred from visible and near-infrared reflectance spectroscopy. The spectroscopic similarity between some asteroids and meteorites suggests that meteorites come from asteroids and allows indirect assessments of asteroid-meteorite connections and inferences regarding chemical compositions of asteroids (1). Of the ~40,000 meteorites we know of, only 14 have had their pre-impact orbits ascertained (2). The aphelia of these 14 orbits are located within the Main Asteroid Belt between Martian and Jovian orbits, which is consistent with an asteroidal origin. However, even the parent asteroids of these 14 meteorites have not been identified.

The taxonomy of meteorites largely has been based on the whole-rock chemical and oxygen isotopic compositions. Each meteorite group, and probably each planet, has a characteristic chemical composition and a unique oxygen isotopic composition (3, 4). The origin of oxygen isotopic variations in the solar system is thought to be an isotope-selective photodissociation of carbon monoxide that occurred before planet formation (57). The unique oxygen isotopic composition of a planet is thought to be produced by a combination of gas-dust chemistry and accretion physics in the solar nebula (6, 8). The Earth and the Moon—the only bodies for which we have measurements—have similar oxygen isotopic compositions within an uncertainty of ±0.016 per mil (‰) [2 SD (2σ)] (9, 10). The determination of an oxygen isotopic composition of an asteroid or a planet therefore would provide an indisputable means to clarify mechanisms of planet formation in the solar nebula and to connect an asteroid or a planet to a specific meteorite group.

The Hayabusa spacecraft made two touchdowns on the surface of asteroid 25143 Itokawa on 20 and 26 November 2005 JST and successfully collected grain particles from the surface of the asteroid. Itokawa is classified as an S-type asteroid. As inferred from reflectance spectrometry, it consists of materials similar to primitive achondrites or ordinary chondrites (11), which can be distinguished by their oxygen isotopic compositions (4). Previous near-infrared reflectance spectroscopy by Hayabusa suggests that the asteroid’s surface has an olivine-rich mineral assemblage that is potentially similar to that of LL5 or LL6 chondrites, with different degrees of space weathering (12). The major mineral assemblage of the sample grains collected by Haybusa is olivine, pyroxene, plagioclase, iron sulfide, and iron-nickel metal (13). The grain sizes are less than 150 μm (mostly less than several tens of micrometers), and crystal sizes in the grain are less than 80 μm (mostly less than 20 μm) (14).

We used the Hokudai isotope microscope system (15) to determine oxygen isotopic compositions of minerals in 28 of these grains, corresponding to measurements of 19 olivine crystals, 7 orthopyroxene crystals, and 7 plagioclase crystals (table S4). The results include multiphase measurements within a grain: analysis for the coexisting olivine-orthopyroxene-plagioclase system in grains RA-QD02-0010 and RA-QD02-0030 and analysis for the coexisting olivine-plagioclase system in grain RA-QD02-0031 (Fig. 1).

Fig. 1

Measurement spots for oxygen isotope analysis. An optical microscope image after measurements is superimposed on the backscattered electron image before measurements. Primary ion beam craters are indicated with dashed circles. The spatial resolution (~10 μm) was sufficient to measure an object with coexisting minerals and to allow analyses free of contamination from the respective minerals. Ol, olivine; Opx, orthopyroxene; Pl, plagioclase; Chr, chromite; Tr, troilite.

The analytical uncertainty was determined from oxygen isotope measurements of an ordinary chondrite, Ensisheim LL6. It is ±0.7‰ (2σ) for δ17OSMOW, ±1.5‰ (2σ) for δ18OSMOW for olivine and orthopyroxene, and twice that for plagioclase, where SMOW is standard mean ocean water. The precision of Δ17OSMOW is ~±0.5‰ (2σ) for all analyses [Fig. 2 and supporting online material (SOM) text]. This precision is sufficient to distinguish most meteoritic materials known to date from terrestrial materials. However, uncertainties of δ17OSMOW and δ18OSMOW are too high to allow a precise determination of metamorphic temperatures by means of the isotopic fractionation among the minerals. Nevertheless, the mineralogical order of isotopic equilibration by thermal metamorphism on the parent body could be recognized within the measurement uncertainties. Thus, the analytical uncertainties could be applied to the Itokawa grains.

Fig. 2

Oxygen isotopic compositions of Ensisheim minerals (A and B) compared with those of a forsterite crystal from San Carlos, Arizona, USA and an anorthite crystal from Miyake-jima, Japan. Instrumental mass fractionation for each mineral is corrected by use of the reference value shown in table S3. Isotope variation defined by 2σ for each mineral phase is shown by a rectangle with a color of the corresponding symbol. Open circles on the ECL (equilibrated chondrite line) correspond to average O isotopic compositions of ordinary chondrites, LL, L, H, from top to bottom. TF, terrestrial fractionation line; Ol, olivine; Opx, orthopyroxene; Pl, plagioclase; An, anorthite; Miyake, Miyake-jima. Data are from tables S2 and S3. Δ17OSMOW = δ17OSMOW – 0.52 δ18OSMOW. A mass fractionation line of the average O isotopic composition of LL chondrite group is shown as a reference. Variations (2 σ) of whole-rock Δ17OSMOW values for H, L, and LL chondrite groups are shown to the right of (B).

The variations of Δ17OSMOW for Itokawa minerals are about ±0.5‰ (2σ) (table S4), which are equivalent to the dispersion expected from measurement uncertainties. All oxygen isotopic compositions of the minerals from Hayabusa sample return capsule plot on the upper side of terrestrial standards on a three-isotope oxygen diagram and are distributed parallel to the terrestrial mass fractionation line (Fig. 3). This indicates that the grains returned by Hayabusa are not terrestrial materials and further demonstrates that the spacecraft retrieved asteroid Itokawa’s surface materials during touchdown.

Fig. 3

Oxygen isotopic compositions of Itokawa minerals (A and B) compared to those of a forsterite crystal from San Carlos, Arizona, USA and an anorthite crystal from Miyake-jima, Japan. Isotope variation defined by 2σ for each mineral is shown by a rectangle with a color of the corresponding symbol. Open circles on the ECL correspond to average O isotopic compositions of ordinary chondrites, LL, L, H, from top to bottom. TF, terrestrial fractionation line; Ol, olivine; Opx, orthopyroxene; Pl, plagioclase; An, anorthite; Miyake, Miyake-jima. Data are from tables S2 and S4. A mass fractionation line of the average O isotopic composition of LL chondrite group is shown as a reference. Variations (2σ) of whole-rock Δ17OSMOW values for H, L, and LL chondrite groups are shown to the right of (B).

Isotopic compositions of meteorites occupy distinct regions of the oxygen three-isotope diagram according to meteorite group. The region of the Itokawa grains overlaps with those of the ordinary chondrites (16). Ordinary chondrites are subdivided into H, L, and LL chondrites. These groups also have distinct ranges of whole-rock oxygen isotopic compositions, with magnitudes of departure from the terrestrial fractionation line, Δ17OSMOW, being 0.73 ± 0.18‰ (2σ) for H-chondrite group, 1.07 ± 0.18‰ (2σ) for L-chondrite group, and 1.26 ± 0.24‰ (2σ) for LL-chondrite group (17). The ranges of Δ17OSMOW of L and LL group overlap each other, but compositions from the H group are distinct from the other groups.

Unequilibrated chondrites consist of minerals having highly variable Δ17OSMOW values. Two mechanisms can homogenize the Δ17OSMOW among minerals: metamorphism and melting (18). Minerals of equilibrated chondrites become homogenized to a Δ17OSMOW value by metamorphism toward the whole-rock oxygen isotopic composition, with variability decreasing in the order of metamorphic grades from type 4 to 6.

The Δ17OSMOW values for Itokawa are 1.46 ± 0.41‰ (2σ) for olivine, 1.57 ± 0.62‰ (2σ) for orthopyroxene, and 1.15 ± 0.51‰ (2σ) for plagioclase (table S4). The observed variations among the minerals are within analytical uncertainties of our measurements. The mean Δ17OSMOW for minerals from Itokawa, 1.39 ± 0.36‰ (2σ), coincides with that of LL or L chondrite groups but is clearly distinguished from H chondrites (Fig. 3B). The small variation of Δ17OSMOW demonstrates that the Itokawa minerals were equilibrated during metamorphism.

The variations of δ18OSMOW of orthopyroxene and plagioclase from Itokawa are similar to those measured from the Ensisheim LL6 chondrite. The range of variation in δ18OSMOW of Itokawa olivine is greater than that of Ensisheim olivine and is as large as those of Itokawa plagioclase. The large variation for Itokawa olivine could be attributed to instrumental mass fractionation relating to irregularities of the sample surface owing to the small size of the grains. Nevertheless, the isotopic relationship among olivine, orthopyroxene, and plagioclase shows that the oxygen isotopes fractionated under equilibrium between coexisting phases. Degrees of the isotopic fractionation among minerals are slightly larger in Itokawa materials than in Ensisheim. The larger isotopic fractionation among the minerals may indicate that the metamorphic temperature was lower in Itokawa material than in Ensisheim.

The metamorphic temperature would be determined by means of the oxygen isotopic fractionation among minerals. The plagioclase-olivine, orthopyroxene-olivine, and plagioclase-orthopyroxene temperatures are calculated to be 600, 650, and 720°C, respectively, through application of an oxygen isotope thermometer (19). The estimated temperatures from 600 to 720°C for Itokawa are lower than those for LL6, L6, and L5 chondrites and higher than for a L4 chondrite (16).

On the basis of this equilibration and the small variation of Δ17OSMOW, the petrographic type of Itokawa is equivalent to type 4-6 in the LL or L chondrite group. The Itokawa material is compatible with an LL4-6 chondrite classification if we combine the oxygen isotope data with the results of the chemical compositions of minerals (13).

The oxygen isotopic composition of asteroid Itokawa thus provides unequivocal evidence that ordinary chondrites come from S-type asteroids.

Supporting Online Material

www.sciencemag.org/cgi/content/full/333/6046/1116/DC1

Materials and Methods

SOM Text

Figs. S1 to S3

Tables S1 to S4

Reference (20)

References and Notes

  1. Martian meteorites are thought to be martian rocks, but definitive proof will require a direct measurement of the oxygen isotopic composition of Mars.
  2. All samples collected by the Hayabusa spacecraft and analyzed here have been characterized by means of x-ray microtomography, x-ray diffraction analysis, x-ray fluorescence analysis, scanning electron microscopy, and electron probe microanalysis before isotope measurements were made (13). For our analysis, chemically equilibrated grains were prepared. Chemically less equilibrated grains described in (13) have not been included because a precise chemical characterization was still in process, and they were not ready for this study. The less equilibrated grains were not common in the Hayabusa sample return capsule (13). We mounted each grain at the center of an epoxy disk and polished the surface according to the processes established for the preliminary examination. We coated a thin layer of gold with a thickness of 60 nm on the samples for secondary ion mass spectrometry.
  3. Acknowledgments: We thank the Hayabusa sample curation team and the Hayabusa project team for close cooperation. This study was supported by the Monka-sho grant (H.Y.) and by the NASA Muses-CN/Hayabusa Program (M.E.Z.).
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