Quelques exemples d’utilisation

La NanoSIMS est un instrument extrêmement versatile. Toutefois, il faut bien comprendre que beaucoup d’analyses ne sont possibles qu’après une période d’intense développement analytique impliquant typiquement l’analyse de nombreux standards adaptés afin d’évaluer la précision et la reproductibilité des analyses. La précision analytique doit être établie pour chaque type d’analyses et d’échantillons. Nous engagerons donc la plateforme sur ce type de développement uniquement si cet effort est justifié scientifiquement et réalisable dans le cadre du projet déposé. Cette décision sera prise en fonction des priorités établies par un comité de pilotage de l’instrument.

En règle générale, l’imagerie de forts contrastes chimiques et isotopiques à l’échelle submicrométrique est relativement simple avec la NanoSIMS. Nous proposons ci-dessous quelques comptes-rendus d’analyses effectuées sur notre instrument (en anglais). D’autres types d’analyses menées sur NanoSIMS sont décrits sur les sites internet d’autres laboratoires équipés de NanoSIMS.

Les exemples sont sélectionnés de manière à illustrer les possibilités et les limites de l’analyse par NanoSIMS. Enfin, la NanoSIMS est capable de former des images par balayage d’un faisceau primaire sur une zone d’intérêt carrée. Alternativement les données peuvent être collectées le long de lignes entre deux points de coordonnées fixées. L’orientation peut être décidée librement. L’analyse ponctuelle est également possible. Le choix de la méthode d’analyse dépendra en tout état de cause de la question scientifique posée et de la précision recherchée.

Fossilized Cells in ~850 My old Chert

Scientific rationale. The search for earliest life on Earth has extended to Archean organic remains that are relatively poorly preserved and considerably more difficult to interpret than the delicately permineralized microfossils known from many Proterozoic deposits. Thus, recent efforts have been directed towards finding biosignatures that can help distinguish poorly preserved fragments of microfossils from either pseudofossils or abiotic organic materials that might be formed hydrothermally or in extraterrestrial processes. We were the first to combine NanoSIMS element maps with optical microscopic imagery in an effort to develop a new method for assessing biogenicity. They showed that the ability to simultaneously map the distribution of 'organic' elements (such as carbon, nitrogen, and sulfur) and compare those element distributions with well recognized, cellularly preserved fossils could provide significant new insights into to the origin of organic materials in ancient sediments.

NanoSIMS analyses: We have used the NanoSIMS to map sub-micron scale distributions of carbon, nitrogen, sulfur, silicon, and oxygen in organic microfossils and laminae from a ~850 My old chert from the Bitter Springs Formation of Australia. The data provide clues about the original chemistry of the microfossils, the silicification process, as well as biosignatures of specific microorganisms and microbial communities. Chemical maps of fossil unicells and filaments reveal distinct wall- and sheath-like structures enriched in C, N and S, consistent with their accepted biological origin. Surprisingly, organic laminae, previously considered to be amorphous, also exhibit filamentous and apparently compressed spheroidal structures defined by strong enrichments in C, N and S (data not shown). By analogy to data from the well-preserved microfossils, these structures are interpreted as being of biological origin, most likely representing densely packed remnants of microbial mats. Because the preponderance of organic matter in Precambrian sediments is similarly "amorphous," our findings open a large body of generally neglected material to in situ structural, chemical, and isotopic study. Our results also offer new criteria for assessing biogenicity of problematic kerogenous materials and thus can be applied to assessments of poorly preserved or fragmentary organic residues in early Archean sediments and any that might occur in meteorites or other extraterrestrial samples.

Spheroidal organic microfossils in a polished thin section of chert from the ~ 0.85 Ga Bitter Springs Formation. A: Optical photomicrograph in transmitted light. B-F: NanoSIMS element maps of the same area as in (A). Arrows show corresponding cells in the different figures. Scale in (A) applies to all. ¹²C = carbon; ¹²C¹⁴N = nitrogen measured as CN^- ion; ³²S = sulfur; ²⁸Si = silicon; ¹⁸O = oxygen.

NanoSIMS images of a wall contact between two spheroidal microfossils in chert from the Bitter Springs Formation. A and B: Relatively low magnification maps. C-F: high resolution maps. White rectangle in (A) shows area of high-resolution images in (C-F). Arrows in (E and F) tie locations of the silicon globules in (F) with corresponding locations on the carbon map (E). Dotted white ovals in (E and F) are reference areas to tie the two images for comparison. ¹²C = carbon; ¹²C¹⁴N = nitrogen measured as CN^- ion; ³²S = sulfur; ²⁸Si = silicon.

Stable isotope analysis: organic matter isolated from primitive meteorites

Scientific rationale. Organic materials are the main carriers of C in primitive meteorites and are believed to be the main carrier of C to planetary bodies, such as the Earth. The survival of primitive organic material in chondritic meteorites provides a wealth of information about the origin of isotopic anomalies in the early Solar System. Large isotopic anomalies have been detected for hydrogen (i.e. the D/H ratio), C (¹²C/¹³C), N (¹⁵N/¹⁴N) and O (¹⁷O/¹⁶O and ¹⁸O/¹⁶O) and the scientific debate is raging over exactly where these anomalies were created. Two different sites of origin for these isotope anomalies might be distinguished: 1) the interstellar medium, prior to formation of the Solar System, or 2) the Solar System itself. If these isotopic anomalies are indeed created in the Solar System itself, it will radically affect the way isotopic anomalies in extra terrestrial materials are interpreted.

NanoSIMS analyses: With the NanoSIMS we have obtained isotopic maps of insoluble organic materials isolated from primitive chondritic meteorites. The NanoSIMS records simultaneously images of up to five different isotopes. For example, we often analyze simultaneously ¹²C^-, ¹³C^-, ¹²C¹⁴N^- and ¹²C¹⁵N^-. By forming ratios between these images, maps of isotopic ratios are obtained, as illustrated in the figure below. It can be seen that the insoluble organics from primitive meteorites, such as Murchison, contain 'hotspots' of highly anomalous isotopic composition, some of which are indicated by white arrows. Isotopic anomalies up to tens of thousands of permil relative to the terrestrial standard materials are not unusual in this material. Interestingly, regions of heavy hydrogen isotopic composition do not systematically coincide with regions characterized by heavy N or C isotopes. This lack of correlation between the different isotope anomalies makes it a challenge to construct models for origin. Several different processes seem to be required.

NanoSIMS maps of isotopic variation in insoluble organic matter from the Murchison carbonaceous chondrite. Maps of H, C and N isotopic composition are obtained from the same region of the sample. The O isotopic map shown here was obtained in another location on the sample. Each region is about 40 by 40 micrometers in dimension and each pixel is about 150 nanometers on each side. Isotopic anomalies are easily visible as bright spots, indicating heavy isotopic compositions. The NanoSIMS is the perfect tool for this type of analyses, where large isotopic anomalies have to be imaged with extremely high spatial resolution. The isotopic 'hotspots' illustrated here are typically micron-sized and essentially impossible to image with any other analytical technique.