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Extreme Deuterium Excesses in Ultracarbonaceous Micrometeorites from Central Antarctic Snow

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Science  07 May 2010:
Vol. 328, Issue 5979, pp. 742-745
DOI: 10.1126/science.1184832

Abstract

Primitive interplanetary dust is expected to contain the earliest solar system components, including minerals and organic matter. We have recovered, from central Antarctic snow, ultracarbonaceous micrometeorites whose organic matter contains extreme deuterium (D) excesses (10 to 30 times terrestrial values), extending over hundreds of square micrometers. We identified crystalline minerals embedded in the micrometeorite organic matter, which suggests that this organic matter reservoir could have formed within the solar system itself rather than having direct interstellar heritage. The high D/H ratios, the high organic matter content, and the associated minerals favor an origin from the cold regions of the protoplanetary disk. The masses of the particles range from a few tenths of a microgram to a few micrograms, exceeding by more than an order of magnitude those of the dust fragments from comet 81P/Wild 2 returned by the Stardust mission.

The light element isotopic compositions of undifferentiated interplanetary material provide insights into the physicochemical processes that took place in the coldest regions of the early solar system. Large deuterium excesses are expected in the solid component(s) of comets because their water and HCN molecules exhibit D/H ratios from 2 to 15 times the terrestrial value, respectively (1). However, isotopic measurements of fragments of comet 81P/Wild 2 returned by the Stardust mission show moderate D/H ratios that do not exceed three times the terrestrial value, possibly indicating a substantial alteration during impact capture process (2). By contrast, large D/H ratios have been observed as micrometer-sized hot spots in organic matter of interplanetary dust particles (IDPs) (3, 4) or primitive meteorites (5, 6). These D excesses may have been inherited from the cold molecular cloud that predated the protosolar nebula (3, 5, 7) or may be the result of a local process that occurred in the cold outer regions of the protoplanetary disk (8, 9). The nature of the D-rich hot spots and their relationship to the organic matter bulk composition remain a matter of debate (6, 10). Organic matter in meteorites is sparse and disseminated in the matrix, with a maximum bulk concentration on the order of a few weight percent (wt %) (11). It is mainly accessible as the acid-insoluble component (IOM) remaining after demineralization of large amounts (grams) of primitive meteorites.

Large numbers of Antarctic micrometeorites (AMMs), which are IDPs with sizes ranging from 20 to 1000 μm, can be recovered from the Antarctic ice cap (12). Here, we describe AMMs obtained from the melting and sieving of 3 m3 of ultraclean snow that fell in the vicinity of the French-Italian CONCORDIA station at Dome C (75°S, 123°E) between 1955 and 1970 (i.e., before human activities started in the area) (13). Because they were recovered from snow rather than ice, they did not endure the mechanical stress or aqueous alteration that are typical of both meteorites and AMMs recovered from Antarctic ice (14). Within the unmelted AMM population exhibiting a fine-grained, fluffy texture with no evidence for substantial heating during atmospheric entry (i.e., vesicles and/or magnetite shell), we report mineralogical and isotopic studies of two particles (particles 19 and 119) characterized by exceptionally high carbon content (Fig. 1A), referred to as ultracarbonaceous Antarctic micrometeorites (UCAMMs) (15). Their carbonaceous component is present in the form of organic matter and represents 48% and 85% of the analyzed area of particles 19 and 119, respectively (13). The sizes of particles 19 and 119 before fragmentation were 80 μm by 50 μm and 275 μm by 110 μm, respectively.

Fig. 1

(A) Backscattered scanning electron micrograph of particle 119. The carbon-rich areas appear dark (arrows); the bright inclusions are dominated by Fe-Ni sulfides and silicates. (B) High-resolution TEM image of particle 19. (C) Bright-field TEM image of particle 19. The lacey carbon film (13) is indicated as black arrows; the crystalline phases are Mg-rich olivines (ol), Mg-rich pyroxenes (px), and Fe-Ni sulfides (S); OM, organic matter. Glassy aggregates (GEMS candidates) are highlighted in black squares (13).

High-resolution transmission electron microscopy (HRTEM) of the UCAMMs shows a poor structural organization of the organic matter, with nanometer-sized polyaromatic layers, single or stacked in groups of two or three (Fig. 1B). This fringe length has the same scale as that reported in HRTEM studies of IOMs from Murchison and Orgueil carbonaceous chondrites (16). Combined studies of field emission gun scanning electron microscopy (FEG-SEM) and analytical TEM reveal mineral assemblages embedded within the organic matter (Fig. 1C) consisting mainly of Mg-rich silicates and Fe-Ni sulfides. These fine-grained mineral assemblages contain both amorphous and crystalline phases (Fig. 1C). Some glassy aggregates are comparable to the glass with embedded metal and sulfides (GEMS) identified in IDPs (Fig. 2) (13). The crystalline fraction of particle 19 is dominated by fine-grained clumps of olivines, pyroxenes, and iron sulfides, with mineral sizes ranging from ~15 to ~500 nm and having a pronounced frequency peak around 100 nm.

Fig. 2

(Left) Bright-field TEM image of a GEMS candidate embedded in the carbonaceous matter of particle 19. (Right) Stacked energy-dispersive x-ray spectroscopy elemental distribution maps of Mg, S + Fe (top) and Ni, S + Fe (bottom) for the GEMS candidate (black square).

Isotopic imaging on fragments of particles 19 and 119 reveals large D excesses associated with the carbon-rich areas (Fig. 3) (13). By contrast, the carbon (12C/13C) and oxygen (18O/16O) isotopic maps do not exhibit large isotopic anomalies (13). The highest D/H ratio of 4.6 (±0.5) × 10−3 [δD = 29,000 ± 3000 per mil (‰)] was measured within particle 119 (Fig. 3D). This ratio approaches that of the most D-rich hot spots observed by isotopic imaging in interplanetary material (3, 5). However, the D enrichments in the UCAMMs we analyzed have occurred over much wider areas than those reported previously in extraterrestrial materials. The areas with D/H > 10−3 (δD > 5400‰) extend over 135 to 280 μm2 and 65 to 200 μm2 in particles 19 and 119, respectively (Fig. 3, A and C) (13). In the in situ isotopic imaging of IOM from primitive chondrites, the contribution of hot spots to the bulk D/H ratio remains small (5, 6), whereas in the UCAMMs the areas with D/H ratios larger than 10 times the terrestrial value represent most of the analyzed surface (Fig. 4) (13). Because the average D/H ratio of the UCAMMs is compatible with that of D-rich hot spots reported in other interplanetary materials, it is conceivable that these hot spots may result from the fragmentation and/or alteration of a primitive organic matter component similar to that revealed by this study.

Fig. 3

NanoSIMS-50 (secondary ion mass spectrometry) isotopic and elemental maps of UCAMMs. (A and B) δD (‰) (13) (A) and C/H atomic ratio (B) of particle 19. The contour in (A) indicates a region with low D/H ratio (Fig. 4) (13). (C) δD (‰) map of particle 119. (D) Higher-magnification δD (‰) map of the zone indicated by the white rectangle in (C).

Fig. 4

Distribution of D/H versus C/H atomic ratios in particles 19 and 119 (13). The data from bulk IOM from CR (white diamonds), UOCs (black dots) (10), and the range of D-rich hot spots observed in primitive chondrites (5) (white arrow) and that from IDPs (4, 17) (gray surface) are reported.

In the D/H versus C/H atomic ratio plot, the data from particles 19 and 119 cluster along a main trend going from a first component at C/H = 1 to 2 and D/H = 0.5 × 10−3 to 1 × 10−3 up to a high C/H component with C/H ratios ranging from 3 to 7 and D/H ratios ranging from 1 × 10−3 to 1.4 × 10−3 (Fig. 4) (13). The first component corresponds to that reported in bulk IOM analyses of CR carbonaceous chondrites (10). The nature of the UCAMM high C/H component remains to be clarified, but HRTEM data establish its highly disorganized structure and therefore rule out a substantial graphitization (Fig. 1B). The isotopic and elementary ratios of IOM from unequilibrated ordinary chondrites (UOCs) (10) plot along the same main trend (Fig. 4). It has been suggested that this main trend in UOCs may be due to a parent body process (10). However, it is difficult to conceive that the main trend from the UCAMMs may result from the same process, because (i) the UCAMMs and UOCs strongly differ both chemically and mineralogically [in particular, the UOC organic matter concentration (<0.5 wt %) (10) is much lower than that observed in UCAMMs]; (ii) we do not observe any sign of thermal processing of the UCAMMs and, by contrast to the UOCs, we observe for C/H > 3 a constant D/H plateau rather than a correlated increase of D/H with C/H (fig. S2) (13); and (iii) the data presented here (Fig. 3) show that the whole range of variation of the main trend coexists within a few tens of square micrometers, whereas the UOC main trend concerns IOM residues from the bulk meteorites. Therefore, the UCAMM main trend seems compatible with the sampling of a heterogeneous organic matter reservoir.

Above the main trend, the data broadly spread toward extreme D/H ratios with D/H > 2.5 × 10−3 and C/H = 2 to 6 (Fig. 4). The D-rich hot spots in IDPs for which C/H ratios have also been reported span a large range (0 < C/H < 3) (4), whereas the hot spots from IOMs of CR2 primitive chondrites have C/H ratios limited to a more restricted zone (1 < C/H < 1.5) (5). The UCAMM extreme D/H component seems to extend the high C/H trend observed in IDPs, including particles collected during the meteor shower associated with comet 26P/Grigg/Skjellerup (17).

High D excesses observed in interplanetary materials have long been attributed to interstellar chemistry, because large D enrichments (D/H > 0.01) are observed in the gas phase of cold molecular clouds (7). However, there is a strict upper limit on the fraction of crystalline relative to amorphous silicates in the interstellar medium (<0.2% by mass) (18). If the organic matter from the UCAMMs was a direct heritage of interstellar origin, one would expect the associated minerals to be dominated by amorphous silicates, which is not the case. Quite the opposite, the organic matter of the UCAMMs contains crystalline phases typical of silicates processed within the accretion disk (19), such as those observed both in anhydrous IDPs (20) and in the fine-grained fraction of Wild 2 particles (21). Therefore, the UCAMMs cannot be considered as a direct interstellar heritage but most probably sampled material (organic matter and minerals) from the protoplanetary disk itself.

Substantial D excesses have been identified at the molecular level in IOM from the Orgueil and Murchison meteorites, supporting an exchange mechanism between the organic matter and a local gaseous D-rich reservoir within the nascent solar system (9). Numerous astronomical observations demonstrate the occurrence of deuterated molecules in protoplanetary disks, some of them exhibiting large D/H variations (0.01 < D/H < 0.1) for radial distances between 30 and 70 AU (22). The large range of D/H ratios observed in the UCAMMs may be reminiscent of the D/H gradient that once existed at several tens of astronomical units from the young Sun.

Other than the bona fide Wild 2 particles returned by the Stardust mission, the assignment of a cometary or asteroidal origin to a given interplanetary dust particle remains speculative. The unmelted nature of the UCAMMs precludes high atmospheric entry velocities usually associated with a cometary origin. However, once released from their parent body, the trajectories of dust within that size range substantially evolve (as a result of resonances with giant planets, radiation pressure, the Poynting-Robertson effect, and solar wind drag), and cometary dust can eventually have Earth-crossing orbits that are difficult to distinguish from typical asteroidal trajectories (23). Some chondritic porous anhydrous IDPs are believed to be of cometary origin, as assessed by their noble gas content, high porosity, high carbon content, and high D/H ratios (3, 24, 25). The exceptionally high carbon content of the UCAMMs equals or exceeds that of the most C-rich IDPs (26, 27) and falls in the range of the CHON particles detected in comet 1P/Halley (28). Both the crystalline and amorphous silicates in the UCAMMs are comparable to those detected in the dust of different comets (29) and are compatible with the Wild 2 material (30). From these lines of evidence, we favor a cometary origin for these particles.

Both the large sizes and the high carbon content of the UCAMMs allow the direct in situ study of an early solar system organic compound together with its associated minerals, without any chemical extraction procedure. The association of extreme D-rich organic matter with high-temperature minerals confirms that material condensed or processed at close distances from the young Sun can be efficiently transported at several tens of astronomical units (21). The UCAMMs provide unique access to the intimate association of high- and low-temperature material from the protoplanetary disk in an unprecedented state of preservation.

Supporting Online Material

www.sciencemag.org/cgi/content/full/328/5979/742/DC1

Materials and Methods

Figs. S1 to S4

Table S1

References

References and Notes

  1. See supporting material on Science Online.
  2. Supported by ANR grant 05-JC05-51407, FP6 Marie Curie Research Training Network “ORIGINS,” ANR T-Tauri Chem, INSU (PNP), IN2P3, CNES, and CNRS. We are grateful to the French and Italian polar institutes, IPEV and PNRA, for their financial and logistic support. The NanoSIMS facility was established with funding from the CNRS, Région Île de France, MESR, and MNHN. The electron microscopy work in Lille is a national facility instrument INSU-CNRS. We are grateful to M. Maurette for stimulating exchanges and pioneering the micrometeorite research in Orsay; S. Derenne, E. Jacquet, E. Quirico, and G. Slodzian for enlightening discussions; and M. Horlok for careful reading of the manuscript.
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