The nature, origin and modification of insoluble organic matter in chondrites, the possibly interstellar source of Earth's C and N

Abstract

All chondrites accreted ~3.5 wt.% C in their matrices, the bulk of which was in a macromolecular solvent and acid insoluble organic material (IOM). Similar material to IOM is found in interplanetary dust particles (IDPs) and comets. The IOM accounts for almost all of the C and N in chondrites, and a significant fraction of the H. Chondrites and, to a lesser extent, comets were probably the major sources of volatiles for the Earth and the other terrestrial planets. Hence, IOM was both the major source of Earth's volatiles and a potential source of complex prebiotic molecules. Large enrichments in D and ¹⁵N, relative to the bulk solar isotopic compositions, suggest that IOM or its precursors formed in very cold, radiation-rich environments. Whether these environments were in the interstellar medium (ISM) or the outer Solar System is unresolved. Nevertheless, the elemental and isotopic compositions and functional group chemistry of IOM provide important clues to the origin(s) of organic matter in protoplanetary disks. IOM is modified relatively easily by thermal and aqueous processes, so that it can also be used to constrain the conditions in the solar nebula prior to chondrite accretion and the conditions in the chondrite parent bodies after accretion. Here we review what is known about the abundances, compositions and physical nature of IOM in the most primitive chondrites. We also discuss how the IOM has been modified by thermal metamorphism and aqueous alteration in the chondrite parent bodies, and how these changes may be used both as petrologic indicators of the intensity of parent body processing and as tools for classification. Finally, we critically assess the various proposed mechanisms for the formation of IOM in the ISM or Solar System.

Figure 1.

The variations in bulk IOM H and N elemental and isotopic compositions within and between chondrite groups (updated from Alexander et al., 2010). TL is Tagish Lake, WIS is WIS 91600, Sem is Semarkona (LL3.0), DOM is DOM 08006 (CO3.0) and Kaba is a CV 3.1.

Figure 2.

Images of an area of QUE 99177 (CR2) matrix: (a) a secondary electron scanning electron micrograph, (b) a N isotope ratio map, (c) a C isotope ratio map, and (d) a H isotope ratio map. The isotopic maps were all measured by NanoSIMS. A relatively large ¹⁵N- and D-rich “vein” of organic matter is visible. Arrow in (c) indicates a highly ¹³C-enriched presolar SiC grain with no obvious spatial relationship to the organic matter.

Figure 3.

Coordinated in situ microanalyses of organic matter in QUE 99177 (CR2). (a) A bright field STEM image mosaic of a FIB section cut through the organic-rich vein in Figure 2, which appears to be an aggregate of nanoglobules (see inset). Figures (b) and (c) are bright field STEM images of organic inclusions in another QUE 99177 FIB section, a small aggregate (“pocket”) of nanoglobules and a carbonaceous particle with a fluffy texture, respectively. Figures d) and e) are C-XANES and N-XANES spectra, respectively, of organic features indicated in the STEM images compared to the average spectra of IOM extracted from the same meteorite. The XANES measurements reveal heterogeneity in functional-group chemistry on a μm scale. There is a much stronger nitrile peak associated with this vein than in the fluffy IOM and bulk extracted IOM.

Figure 4.

Bright-field TEM images of microtomed sections of IOM residues from (a) Bells, and (b) the primitive 5b Tagish Lake lithology. The images include solid and hollow nanoglobules, and finer grained ‘fluffy’ material.

Figure 5.

The H isotopic compositions vs. C number for the products of RuO₄ oxidation degradation of Orgueil (Remusat et al., 2005a) and Murchison (Huang et al., 2007) IOM. The Murchison data are for straight (Str.) and branched (Br.) monocarboxylic acids, and the Orgueil data are for straight and branched dicarboxylic acids. The monocarboxylic acids are side chains to aromatic moieties, the dicarboxylic acids are linkages between aromatic moieties.

Figure 6.

A total ion chromatogram for pyrolysates with molecular weights of benzene and higher produced by flash pyrolysis GCMS of a Murchison IOM residue at 600°C. Roughly 25–30 wt.% of the sample was lost as volatile, low molecular weight material, much of this being CO, CO₂, H₂O, SO₂ and H₂S. The remainder of the sample formed a char. For the pyrolysates with molecular weights of benzene and higher shown here, the bulk (ca. 80 % of the ion intensity) are in an unresolved organic matter (UOM), sometimes also referred to as humpane, that produces the nearly continuous humped background. Superimposed on the UOM are many sharp peaks (totaling ca. 20 % of the ion intensity), but even the most intense of these, naphthalene, only accounts for ~1.5 % of the total ion intensity in this chromatogram and a much smaller fraction of the material in the IOM residue.

Fig. 7.

The isotopic compositions of individual aromatic molecules produced during pyrolysis of Orgueil IOM. Squares are from Remusat et al. (2006) and triangles are from Wang et al. (2005). Open symbols are for unsubstituted molecules, and filled symbols are for substituted molecules. The solid line is the bulk composition of Orgueil IOM (Alexander et al., 2007b), and the dashed lines are the isotopic compositions of the end-member components for Orgueil IOM proposed by Remusat et al. (2006).

Fig. 8.

Comparison of CP and SP ¹³C NMR spectra of Murchison IOM (Cody and Alexander, 2005). The signal-to-noise of the SP spectrum is lower than for the CP spectrum, but within the uncertainties they are the same. This shows that the CP spectrum was collected under conditions where all functional groups have similar detection efficiencies and that, except in the nanodiamonds, little of no C is so far from a H that it is not detected by the CP technique (i.e., there are no large PAHs present).

Figure 9.

Average bulk XANES spectra of IOM from several primitive carbonaceous chondrites: Murchison (CM2), Orgueil (CI1), QUE 99177 (CR2), Tagish Lake (C2, lithology 5b) and ALH 77307 (CO3.0). The C XANES spectra show the greatest variation in functional chemistries between different meteorite groups and petrologic types, but are still very similar. As shown in the representative spectrum from Tagish Lake, the N XANES spectra from primitive IOM are relatively featureless, only containing minor spectral “shoulders” on the main N absorption edge. In addition, IOM O XANES spectra rarely show spectral features other than the main π* and σ* peaks for C=O and C-O bonds, respectively. The C XANES data are taken from De Gregorio et al. (2013), while the data for Tagish Lake are unpublished.

Figure 10.

The CP ¹³C NMR spectra of IOM from a number CM chondrites that experienced a range of degrees of alteration (Cody et al., 2008d). The samples are stacked (top to bottom) in order of increasing degree of alteration (Alexander et al., 2013). As can be seen, there is very little variation in their functional group chemistries.

Figure 11.

The variations in bulk IOM H and N isotopic compositions with petrologic type for CIs, CMs, CRs and the two most primitive Tagish Lake (C2) lithologies (after Alexander et al., 2013). There seem to be general trends of bulk IOM isotopic compositions with petrologic type. However, there are notable exceptions in the highly altered CR GRO 95577 and the more heavily processed Tagish Lake lithologies (not shown), and there is little overlap in the petrologic types of the different chondrite groups.

Figure 12.

Comparison of the NMR spectra of IOM from several Tagish Lake (C2) lithologies (Cody and Alexander, 2005; Herd et al., 2011; Alexander et al., 2014) with those for IOM from EET 92042 (CR2), Orgueil (CI1), Muchison (CM2) (Cody and Alexander, 2005) and the strongly heated CM PCA 91008 (Yabuta et al., 2010). The variation in IOM NMR spectra amongst the various Tagish Lake lithologies range from intermediate between CR and CI, to similar to the heated CM PCA 91008. This is consistent with the changes in H/C ratio of the Tagish Lake IOM samples (Figs. 1 and 13). The Tagish Lake samples demonstrate that parent body processes are capable of producing much of the range of IOM elemental compositions and functional group chemistries seen amongst chondrites.

Figure 13.

Comparison of the IOM compositions from five Tagish Lake lithologies (Herd et al., 2011; Alexander et al., 2014) and the products of hydrothermal experiments on Murchison IOM conducted at three temperatures for 72 hours by Oba and Naraoka (2009). Also shown are the IOM compositions CI, CM, CR, CV and CO chondrites (Alexander et al., 2007b). The experiments parallel the Tagish Lake IOM samples. Both the Tagish Lake samples and experimental products cover wide ranges in H/C. In the case of the Tagish Lake samples, this range is from CR-like to H/C ratios that resemble heated CMs and the least metamorphosed CVs and COs. However, the Tagish Lake and experiment trends do not reproduce the full range of H/C and H isotopic compositions found in carbonaceous and ordinary (Fig. 1) chondrites. It seems likely that neither the experimental conditions nor the conditions experienced by the Tagish Lake lithologies cover the full range of parent body conditions experienced by the chondrites.