NanoSIMS sulfur isotopic analysis at 100 nm scale by imaging technique.

NanoSIMS has been widely used for in-situ sulfur isotopic analysis (32S and 34S) of micron-sized grains or complex zoning in sulfide in terrestrial and extraterrestrial samples. However, the conventional spot mode analysis is restricted by depth effects at the spatial resolution < 0.5-1 µm. Thus sufficient signal amount cannot be achieved due to limited analytical depths, resulting in low analytical precision (1.5‰). Here we report a new method that simultaneously improves spatial resolution and precision of sulfur isotopic analysis based on the NanoSIMS imaging mode. This method uses a long acquisition time (e.g., 3 h) for each analytical area to obtain sufficient signal amount, rastered with the Cs+ primary beam of ∼100 nm in diameter. Due to the high acquisition time, primary ion beam (FCP) intensity drifting and quasi-simultaneous arrival (QSA) significantly affects the sulfur isotopic measurement of secondary ion images. Therefore, the interpolation correction was used to eliminate the effect of FCP intensity variation, and the coefficients for the QSA correction were determined with sulfide isotopic standards. Then, the sulfur isotopic composition was acquired by the segmentation and calculation of the calibrated isotopic images. The optimal spatial resolution of ∼ 100 nm (Sampling volume of 5 nm × 1.5 µm2) for sulfur isotopic analysis can be implemented with an analytical precision of ∼1‰ (1SD). Our study demonstrates that imaging analysis is superior to spot-mode analysis in irregular analytical areas where relatively high spatial resolution and precision are required and may be widely applied to other isotopic analyses.

Young asteroidal fluid activity revealed by absolute age from apatite in carbonaceous chondrite.

Chondritic meteorites, consisting of the materials that have formed in the early solar system (ESS), have been affected by late thermal events and fluid activity to various degrees. Determining the timing of fluid activity in ESS is of fundamental importance for understanding the nature, formation, evolution and significance of fluid activity in ESS. Previous investigations have determined the relative ages of fluid activity with short-lived isotope systematics. Here we report an absolute 207Pb/206Pb isochron age (4,450±50 Ma) of apatite from Dar al Gani (DaG) 978, a type ∼3.5, ungrouped carbonaceous chondrite. The petrographic, mineralogical and geochemical features suggest that the apatite in DaG 978 should have formed during metamorphism in the presence of a fluid. Therefore, the apatite age represents an absolute age for fluid activity in an asteroidal setting. An impact event could have provided the heat to activate this young fluid activity in ESS.

Biomineralization mediated by anaerobic methane-consuming cell consortia.

Anaerobic methanotrophic archaea (ANME) play a significant role in global carbon cycles. These organisms consume more than 90% of ocean-derived methane and influence the landscape of the seafloor by stimulating the formation of carbonates. ANME frequently form cell consortia with sulfate-reducing bacteria (SRB) of the family Deltaproteobacteria. We investigated the mechanistic link between ANME and the natural consortium by examining anaerobic oxidation of methane (AOM) metabolism and the deposition of biogenetic minerals through high-resolution imaging analysis. All of the cell consortia found in a sample of marine sediment were encrusted by a thick siliceous envelope consisting of laminated and cementing substances, whereas carbonate minerals were not found attached to cells. Beside SRB cells, other bacteria (such as Betaproteobacteria) were found to link with the consortia by adhering to the siliceous crusts. Given the properties of siliceous minerals, we hypothesize that ANME cell consortia can interact with other microorganisms and their substrates via their siliceous envelope, and this mechanism of silicon accumulation may serve in clay mineral formation in marine sedimentary environments. A mechanism for biomineralization mediated by AOM consortia was suggested based on the above observations.