Search for: All records

We report on the design and performance of an improved duoplasmatron ion source for secondary ion mass spectrometers. The source is designed specifically to optimize extraction of negative oxygen ions while suppressing electron extraction using a built-in magnetic asymmetry in the anode electrode. Other changes from conventional designs are (a) drilling the ion extraction aperture directly into the magnetic steel anode rather than in a refractory (nonmagnetic) metal insert, thereby eliminating a magnetic “hole” that acts to counter the desired magnetic concentration of the discharge at the aperture and (b) forming the anode into a conical shape convex toward the intermediate electrode to increase the magnetic field concentration at the extraction aperture, hence the term “Canode.” The built-in magnetic asymmetry allows the width and shape of the intermediate electrode to be varied to further optimize magnetic concentration of the discharge. Tests were performed with both ims 6f and NanoSIMS 50L instruments manufactured by Cameca Instruments, Inc. (Fitchburg, WI, USA). In the ims 6f, the Canode design gave O− primary ion currents up to a factor of five greater than the factory ion source design. In the NanoSIMS 50L, the Canode source produced a focused O− ion beam at the sample with a diameter of 50 nm, identical to the performance of the radio-frequency Hyperion ion source developed by Oregon Physics (Beaverton, OR, USA) and offered as an option by Cameca. 

Asteroid Ryugu and Ivuna-type carbonaceous meteorites may have originated from the outskirts of the Solar System. 

Abstract Oxygen 3‐isotope ratios of magnetite and carbonates in aqueously altered carbonaceous chondrites provide important clues to understanding the evolution of the fluid in the asteroidal parent bodies. We conducted oxygen 3‐isotope analyses of magnetite, dolomite, and breunnerite in two sections of asteroid Ryugu returned samples, A0058 and C0002, using a secondary ion mass spectrometer (SIMS). Magnetite was analyzed by using a lower primary ion energy that reduced instrumental biases due to the crystal orientation effect. We found two groups of magnetite data identified from the SIMS pit morphologies: (1) higher δ¹⁸O (from 3‰ to 7‰) and ∆¹⁷O (~2‰) with porous SIMS pits mostly from spherulitic magnetite, and (2) lower δ¹⁸O (~ −3‰) and variable ∆¹⁷O (0‰–2‰) mostly from euhedral magnetite. Dolomite and breunnerite analyses were conducted using multi‐collection Faraday cup detectors with precisions ≤0.3‰. The instrumental bias correction was applied based on carbonate compositions in two ways, using Fe and (Fe + Mn) contents, respectively, because Ryugu dolomite contains higher amounts of Mn than the terrestrial standard. Results of dolomite and breunnerite analyses show a narrow range of ∆¹⁷O; 0.0‰–0.3‰ for dolomite in A0058 and 0.2‰–0.8‰ for dolomite and breunnerite in C0002. The majority of breunnerite, including large ≥100 μm grains, show systematically lower δ¹⁸O (~21‰) than dolomite (25‰–30‰ and 23‰–27‰ depending on the instrumental bias corrections). The equilibrium temperatures between magnetite and dolomite from the coarse‐grained lithology in A0058 are calculated to be 51 ± 11°C and 78 ± 14°C, depending on the instrumental bias correction scheme for dolomite; a reliable temperature estimate would require a Mn‐bearing dolomite standard to evaluate the instrumental bias corrections, which is not currently available. These results indicate that the oxygen isotope ratios of aqueous fluids in the Ryugu parent asteroid were isotopically heterogeneous, either spatially, or temporary. Initial water ice accreted to the Ryugu parent body might have ∆¹⁷O > 2‰ that was melted and interacted with anhydrous solids with the initial ∆¹⁷O < 0‰. In the early stage of aqueous alteration, spherulitic magnetite and calcite formed from aqueous fluid with ∆¹⁷O ~ 2‰ that was produced by isotope exchange between water (∆¹⁷O > 2‰) and anhydrous solids (∆¹⁷O < 0‰). Dolomite and breunnerite, along with some magnetite, formed at the later stage of aqueous alteration under higher water‐to‐rock ratios where the oxygen isotope ratios were nearly at equilibrium between fluid and solid phases. Including literature data, δ¹⁸O of carbonates decreased in the order calcite, dolomite, and breunnerite, suggesting that the temperature of alteration might have increased with the degree of aqueous alteration.