Oxygen and magnesium mass-independent isotopic fractionation induced by chemical reactions in plasma.

Enrichment or depletion ranging from -40 to +100% in the major isotopes 16O and 24Mg were observed experimentally in solids condensed from carbonaceous plasma composed of CO2/MgCl2/Pentanol or N2O/Pentanol for O and MgCl2/Pentanol for Mg. In NanoSims imaging, isotope effects appear as micrometer-size hotspots embedded in a carbonaceous matrix showing no isotope fractionation. For Mg, these hotspots are localized in carbonaceous grains, which show positive and negative isotopic effects so that the whole grain has a standard isotope composition. For O, no specific structure was observed at hotspot locations. These results suggest that MIF (mass-independent fractionation) effects can be induced by chemical reactions taking place in plasma. The close agreement between the slopes of the linear correlations observed between δ25Mg versus δ26Mg and between δ17O versus δ18O and the slopes calculated using the empirical MIF factor η discovered in ozone [M. H. Thiemens, J. E. Heidenreich, III. Science 219, 1073-1075; C. Janssen, J. Guenther, K. Mauersberger, D. Krankowsky. Phys. Chem. Chem. Phys 3, 4718-4721] attests to the ubiquity of this process. Although the chemical reactants used in the present experiments cannot be directly transposed to the protosolar nebula, a similar MIF mechanism is proposed for oxygen isotopes: at high temperature, at the surface of grains, a mass-independent isotope exchange could have taken place between condensing oxides and oxygen atoms originated form the dissociation of CO or H2O gas.

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).

Reactivity of secondary phases in weathered limestone using isotopic tracers (D and 18O).

For a long time, limestone has been massively used in stone building and monuments because of its easy extraction and common presence in the landscape. On ancient monuments, mostly built in urban areas, it is exposed to urban-borne pollutants responsible for specific alteration mechanisms and weathering kinetics. Especially, the dissolution of calcite and the precipitation of new phases will affect the limestone pore network, modify the stones capillary properties, and influence the further alteration. In order to better understand these processes, an altered limestone sample from 'Tribunal Administratif' (TA) in Paris was studied. The main secondary phase was found to be syngenite, which can be explained by the location of the sample close to the soil, a potential source of K (fertilizers). This phase is more soluble than gypsum that is commonly found on altered limestone. In order to assess the reactivity of the system (limestone and new phases), oxygen and hydrogen isotopes were used to trace the transfer of water (D218O) and identify the location of the reactive areas (susceptible to alteration). For that, TA samples were exposed in a climatic chamber to relative humidity (RH) cycles (25% RH for 2.5 days and 85% RH for 4.5 days) for 2 months with a D218O vapor to simulate alteration occurring in conditions sheltered from the rain. Results have shown that the water vapor easily circulates deep in the sample and reacts preferentially with syngenite the most reactive phase (compared with calcite and quartz). This phase could evolve in gypsum when exposed to an environment different from the one resulting in its formation.