NanoSIMS Imaging of D/H Ratios on FIB Sections.

The D/H ratio imaging of weakly hydrated minerals prepared as focused ion beam (FIB) sections is developed in order to combine isotopic imaging by nanoscale secondary ion mass spectrometry (NanoSIMS) of micrometer-sized grains with other nanoscale imaging techniques, such as transmission electron microscopy. In order to maximize the accuracy, sensitivity, precision, and reproducibility of D/H ratios at the micrometer size, while minimizing the surface contamination at the same time, we explored all instrumental parameters known to influence the measurement of D/H ratios in situ. Optimal conditions were found to be obtained with the use of (i) a Cs+ ion source and detection of H- and D- at low mass resolving power, (ii) a primary beam intensity of 100 pA, and (iii) raster sizes in the range of 8-15 µm. Nominally anhydrous minerals were used to evaluate the detection limits and indicate a surface contamination level of ∼200 ppm equivalent H2O under these conditions. With the high primary intensity used here, the dwell time is not a parameter as critical as found in previous studies and a dwell time of 1 ms/px is used to minimize dynamic contamination during analysis. Analysis of FIB sections was found to reduce significantly static contamination due to sample preparation and improved accuracy compared to using polished sections embedded not only in epoxy but in indium as well. On amphiboles, the typical overall uncertainty including reproducibility is ∼20 ‰ on bulk FIB sections and ∼50 ‰ at the 1.5 µm scale using image processing (1σ).

Investigation of the Geochemical Preservation of ca. 3.0 Ga Permineralized and Encapsulated Microfossils by Nanoscale Secondary Ion Mass Spectrometry.

Observations of Archean organic-walled microfossils suggest that their fossilization took place through both encapsulation and permineralization. In this study, we investigated microfossils from the ca. 3.0 Ga Farrel Quartzite (Pilbara, Western Australia) using transmitted light microscopy, scanning electron microscopy, Raman microspectrometry, and nanoscale secondary ion mass spectrometry (NanoSIMS) ion microprobe analyses. In contrast to previous studies, we demonstrated that permineralized microfossils were not characterized by the micrometric spatial relationships between Si and C-N as observed in thin sections. Permineralized microfossils are composed of carbonaceous globules that did not survive the acid treatment, whereas encapsulated microfossils were characterized due to their resistance to the acid maceration procedure. We also investigated the microscale relationship between the 12C14N- and 12C2- ion emission as a proxy of the N/C atomic ratio in both permineralized and encapsulated microfossils. After considering any potential matrix and microtopography effects, we demonstrate that the encapsulated microfossils exhibit the highest level of geochemical preservation. This finding shows that the chemical heterogeneity of the microfossils, observed at a spatial resolution of a few hundreds of micrometers, can be related to fossilization processes. Key Words: Carbonaceous matter-Farrel Quartzite-Fossilization-NanoSIMS-Nitrogen-Permineralization. Astrobiology 17, 1192-1202.

Hydrogen isotope fractionation in methane plasma.

The hydrogen isotope ratio (D/H) is commonly used to reconstruct the chemical processes at the origin of water and organic compounds in the early solar system. On the one hand, the large enrichments in deuterium of the insoluble organic matter (IOM) isolated from the carbonaceous meteorites are interpreted as a heritage of the interstellar medium or resulting from ion-molecule reactions taking place in the diffuse part of the protosolar nebula. On the other hand, the molecular structure of this IOM suggests that organic radicals have played a central role in a gas-phase organosynthesis. So as to reproduce this type of chemistry between organic radicals, experiments based on a microwave plasma of CH4 have been performed. They yielded a black organic residue in which ion microprobe analyses revealed hydrogen isotopic anomalies at a submicrometric spatial resolution. They likely reflect differences in the D/H ratios between the various CHx radicals whose polymerization is at the origin of the IOM. These isotopic heterogeneities, usually referred to as hot and cold spots, are commensurable with those observed in meteorite IOM. As a consequence, the appearance of organic radicals in the ionized regions of the disk surrounding the Sun during its formation may have triggered the formation of organic compounds.