Ian C. Lyon

Isotopic Compositions of Cometary Matter Returned by Stardust

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single 17O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is 16O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.

RELAX: An ultrasensitive, resonance ionization mass spectrometer for xenon

RELAX is a resonance ionization, time‐of‐flight mass spectrometer to which a cryogenic sample concentrator has been added. This has resulted in an increase in sensitivity by a factor greater than 100. The sample concentrator consists of a localized cold spot in the ion source, onto which the sample condenses, and a heating laser to release the condensed sample into the ionization region. The lifetime against detection of a sample atom is close to 20 min, which corresponds to a count rate of 1 cps from a sample of 1000 atoms, while the mass resolution is 300 (10% peak height). Sensitivity depends on the return time of sample atoms to the cold spot (10 s) and the fraction of these atoms subsequently ionized (∼1%). The minimum sample size which can be measured is limited only by blank, which is currently 2×10−15 cc STP total xenon and isotopically atmospheric (this can be attributed to the large aliquots of xenon admitted to the instrument during development, and so may be expected to decrease with time). Th...

Microbial engineering of nanoheterostructures: Biological synthesis of a magnetically recoverable palladium nanocatalyst

Precious metals supported on ferrimagnetic particles have a diverse range of uses in catalysis. However, fabrication using synthetic methods results in potentially high environmental and economic costs. Here we show a novel biotechnological route for the synthesis of a heterogeneous catalyst consisting of reactive palladium nanoparticles arrayed on a nanoscale biomagnetite support. The magnetic support was synthesized at ambient temperature by the Fe(III)-reducing bacterium, Geobacter sulfurreducens , and facilitated ease of recovery of the catalyst with superior performance due to reduced agglomeration (versus conventional colloidal Pd nanoparticles). Surface arrays of palladium nanoparticles were deposited on the nanomagnetite using a simple one-step method without the need to modify the biomineral surface, most likely due to an organic coating priming the surface for Pd adsorption, which was produced by the bacterial culture during the formation of the nanoparticles. A combination of EXAFS and XPS showed the Pd nanoparticles on the magnetite to be predominantly metallic in nature. The Pd(0)-biomagnetite was tested for catalytic activity in the Heck reaction coupling iodobenzene to ethyl acrylate or styrene. Rates of reaction were equal to or superior to those obtained with an equimolar amount of a commercial colloidal palladium catalyst, and near complete conversion to ethyl cinnamate or stilbene was achieved within 90 and 180 min, respectively.

Tracing fluid sources and interactions

Methods of evaluating possible sources and interactions of ancient and modern fluids are being significantly improved by the development of new isotopic and geochemical microprobe techniques. Applications of O, C, and S isotopes are benefiting from the development of laser ablation methods capable of 0.1 permil precision on submilligram samples and ion microprobe methods capable of permil precision on nanogram regions of rock sections. The noble gases are proving themselves useful conservative tracers of fluid processes. The ubiquitous presence of air saturated water patterns testifies to the dominance of water originating at the Earths surface, while the presence of radiogenic isotopes and 3He quantify interactions with crust and m antle respectively. The use of resonance ionisation has increased sensitivity for Xe and Kr analyses to the single atom level. Examples of fluid regimes currently under study are described.

Correlated chemical and isotopic zoning in carbonates in the Martian meteorite ALH84001

The meteorite ALH84001, a sample of the ancient martian crust, contains small quantities (∼1%) of strongly chemically zoned carbonate. High spatial resolution (10 μm) ion microprobe analyses show that the chemical zoning is strongly correlated with variations in oxygen isotope ratios. Early formed Ca,Fe-rich cores have δ18O ∼ 7‰ increasing to 22‰ SMOW in the more Mg-rich outer cores and magnesite rims. Isolated areas of ankerite appear to be isotopically lighter with δ18O ∼ 1‰. The large range in δ18O requires a significant range in either fluid isotopic composition, or temperature, or both, in the course of the deposition sequence. Our data are inconsistent with formation of the zoned carbonates by closed system Rayleigh fractionation. There is no unique interpretation of the oxygen data, but the recent observation of existence of Δ17O excesses in the carbonate appears to rule out models which involve high temperature isotopic exchange with silicate. Comparison with terrestrial analogues suggests that ALH84001 carbonates formed in a hydrothermal system with T<∼400°C, and which, at least in the early stages of formation, may have involved water with δ18O < 0‰ SMOW. The later stages of deposition probably occurred at temperatures below 150°C, a conclusion which does not preclude the co-existence of thermophilic bacteria; temperatures during earlier stages of deposition are less likely to have been hospitable to bacteria.

A conspicuous clay ovoid in nakhla: Evidence for subsurface hydrothermal alteration on mars with implications for astrobiology

Abstract A conspicuous biomorphic ovoid structure has been discovered in the Nakhla martian meteorite, made of nanocrystalline iron-rich saponitic clay and amorphous material. The ovoid is indigenous to Nakhla and occurs within a late-formed amorphous mesostasis region of rhyolitic composition that is interstitial to two clinopyroxene grains with Al-rich rims, and contains acicular apatite crystals, olivine, sulfides, Ti-rich magnetite, and a new mineral of the rhoenite group. To infer the origin of the ovoid, a large set of analytical tools was employed, including scanning electron microscopy and backscattered electron imaging, wavelength-dispersive X-ray analysis, X-ray mapping, Raman spectroscopy, time-of-flight secondary ion mass spectrometry analysis, high-resolution transmission electron microscope imaging, and atomic force microscope topographic mapping. The concentric wall of the ovoid surrounds an originally hollow volume and exhibits internal layering of contrasting nanotextures but uniform chemical composition, and likely inherited its overall shape from a preexisting vesicle in the mesostasis glass. A final fibrous layer of Fe-rich phases blankets the interior surfaces of the ovoid wall structure. There is evidence that the parent rock of Nakhla has undergone a shock event from a nearby bolide impact that melted the rims of pyroxene and the interstitial matter and initiated an igneous hydrothermal system of rapidly cooling fluids, which were progressively mixed with fluids from the melted permafrost. Sharp temperature gradients were responsible for the crystallization of Al-rich clinopyroxene rims, rhoenite, acicular apatites, and the quenching of the mesostasis glass and the vesicle. During the formation of the ovoid structure, episodic fluid infiltration events resulted in the precipitation of saponite rinds around the vesicle walls, altered pyrrhotite to marcasite, and then isolated the ovoid wall structure from the rest of the system by depositing a layer of iron oxides/hydroxides. Carbonates, halite, and sulfates were deposited last within interstitial spaces and along fractures. Among three plausible competing hypotheses here, this particular abiotic scenario is considered to be the most reasonable explanation for the formation of the ovoid structure in Nakhla, and although compelling evidence for a biotic origin is lacking, it is evident that the martian subsurface contains niche environments where life could develop.

Isotopic fractionation during secondary ionisation mass spectrometry: Crystallographic orientation effects in magnetite

Isotopic and elemental fractionation occurs as an inevitable and often frustrating factor in SIMS although the potential and demonstrated capabilities of ion probes make the solution of such problems well worth attaining. This paper describes a source of isotopic fractionation which may arise from the crystallographic structure of the sample. This effect, observed in magnetite, is potentially a source of error in acquiring reproducible isotopic ratios during SIMS of magnetite although no similar effect has been observed during several years of measurements on quartz, olivine or carbonates. The measurements were acquired using an Isolab 54 ion probe by rotating the sample about an axis normal to the surface and measuring 18O/16O ratios as a function of angle. The primary ion beam impacts the sample at an angle of 45° so that rotation of the sample should bring lattice planes approximately into and out of coincidence with the primary beam. Repeated 18O/16O measurements from magnetite crystals as a function of orientation show maxima in measured 18O/16O values which can be correlated with channelling along lattice planes. The measured ratio has been observed to increase by approximately 7‰ as a function of angle, variations which are very significant compared with isotopic fractionations produced by geological processes. The maxima correlate well with the [1 1 0], [1 0 1] and [0 1 1] lattice vectors of magnetite and are interpreted as channelling of the primary ions along the lattice vectors. Since it is extremely difficult, if not impossible, to determine the crystallographic orientation of magnetite in thin section, we believe that it is therefore necessary to measure magnetite samples in at least two orientations (separated by an angle that is not equal to the difference between two channelling orientations) against a standard in a known orientation.

A resonance ionization mass spectrometer for xenon

A time-of-flight resonance ionization mass spectrometer is described which has been used to measure isotopic ratios of the nine naturally occurring xenon isotopes, with a precision of less than 0.5% for the major isotopes, in samples of 10-12 cm3 stp of xenon. The precision is essentially limited by counting statistics. A linear mass discrimination up to 0.4% per u, favouring the light isotopes, is exhibited and may be a velocity effect at the detector. No resonance effects nonlinear in mass are seen. The sensitivity of the instrument, which has a detection limit of 3*104 atoms for a particular isotope of xenon, agrees well with theoretical predictions and is comparable to that of the best electron bombardment instruments. Use of lasers with a higher duty cycle and/or a cold finger to concentrate the sample in the ionizing volume could increase the sensitivity by up to two or three orders of magnitude. Interferences due to the non-resonant ionization of hydrocarbons can be measured by tuning off resonance but are negligible in normal operation.

Interstellar dust laser explorer: A new instrument for elemental and isotopic analysis and imaging of interstellar and interplanetary dust

We present the performance characteristics of a time-of-flight secondary ion mass spectrometer designed for 157 nm laser postionization of sputtered neutrals for high sensitivity elemental and isotopic analyses. The instrument was built with the aim of analyzing rare element abundances in micron to submicron samples such as interstellar grains and cometary dust. Relative sensitivity factors have been determined for secondary ion mass spectrometry which show an exponential dependency against the first ionization potential. This allows elemental abundances to be measured with errors below 25% for most major elements. The accuracy for isotope ratios, where isotopes can be resolved from isobaric interferences, is usually limited only by counting statistics. In laser secondary neutral mass spectrometry, the spatial and temporal overlaps between the laser and sputtered neutral atoms are modeled and predictions of total detection efficiency and isotopic and elemental fractionation are compared with experimental data. Relative sensitivity factors for laser-ionized secondary neutrals from a stainless steel standard are found to vary less than 3% above saturation laser pulse energy enabling more accurate quantification.

Oxygen isotopic composition of carbonate in the nakhla meteorite: implications for the hydrosphere and atmosphere of mars

Abstract We have located small areas of siderite within the mesostasis of the Nakhla meteorite. High concentrations of Mn (up to ∼50% rhodochrosite) and elevated D/H ratios indicate that the siderite is not a terrestrial alteration product. The isotopic composition of oxygen in the siderite has been determined with the Manchester ISOLAB 54 ion microprobe. The siderite has δ 18 O = +34 ± 1‰, which is higher than any other martian carbonate yet reported. If the δ 18 O value is the result of equilibration with water at 2 /H 2 O produced during the degassing of Mars, and not modified subsequently isotopically. Formation from a water rich fluid at >60°C requires that the fluid was heavy isotope enriched relative to fluids produced during planetary degassing. An enrichment of 8–15‰ is consistent with theoretical outgassing models that are able to account for enhancements of Δ 17 O in martian alteration products. Estimated deposition temperatures would be raised to 80–170°C. The effect of a global scale fixation of martian CO 2 as carbonate operates in the opposite direction and could lead to a reduction in δ 18 O of the martian hydrosphere of a few permil.