J. M. Saxton

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.

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.

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.

The manchester isolab 54 ion microprobe

Abstract The Isolab 54 ion microprobe installed at the Department of Earth Sciences at the University of Manchester has been designed to measure isotopic abundances of elements with high precision from ions formed by secondary ionisation (SIMS), laser resonance ionisation (RIMS) or thermal ionisation (TIMS). This instrument has been extensively modified since its original installation to improve its precision and capabilities, and these improvements are detailed, as are applications for which the instrument has been used. The principal modifications were the installation and testing of a new type of ion detector (conversion dynode system, CDS), which has an ion-to-pulse conversion efficiency that has been constant to better than 2% over periods of many months, a method of integrating secondary ion currents to eliminate the effects of irreproducible instrumental isotopic fractionation, and a method of rapid electrostatic peak centring which can centre high mass resolution peaks on a detector to a precision of 2 ppm in mass in 1.2 s. Particular attention has been paid to in situ SIMS analysis of oxygen isotopes from insulating materials, where a spatial resolution of 10–20 μm with a precision of 1–2% has been achieved for both 17O/16O and 18O/16O. Analytical techniques for the measurement of carbon, nitrogen and osmium isotope ratios by SIMS are also described, together with the development of a technique for the measurement of strontium isotope ratios using laser resonance ionisation of sputtered neutral atoms.

High-precision, in situ oxygen isotope ratio measurements obtained from geological and extra-terrestrial materials using an Isolab 54 ion microprobe

We have developed an ion probe technique for the in situ measurement of oxygen isotope ratios, 18O/16O and 17O/16O, with high spatial resolution in polished thin sections of silicate minerals. This technique allows the isotopic analysis of samples as small as picomoles of material which represents only 10–7 of the sample size required by conventional fluorination oxygen isotope measurement methods and thus has wide application to the study of many terrestrial and extra-terrestrial samples that have heterogeneous oxygen isotope compositions on a scale of tens of micrometres. To illustrate the breadth of studies that are made possible by the ion probe, we report here oxygen isotope measurements from an authigenic quartz overgrowth obtained from the Penrith sandstone, UK, and measurements of magnetite grains from the Orgueil and Yamato 82162 carbonaceous chondrites which may be used to constrain possible formation mechanisms of the magnetite.

Techniques and methodology used in a mineralogical and osmium isotope study of platinum group minerals from alluvial deposits in Colombia, California, Oregon and Alaska

Abstract Platinum-group minerals (PGM) from placer deposits in Colombia, California, Oregon and Alaska were investigated with the electron microprobe, proton microprobe (μ-PIXE) and ion probe to analyse their major and trace element contents and 187Os/186Os isotopic ratios. Most of the grains in the samples investigated proved to be essentially homogeneous alloys of Pt-Fe and Os-Ir-Ru although a few of them contained inclusions of other PGM such as cooperite and laurite. Detailed analyses were undertaken on the Os-Ir-Ru alloy phases. The 187Os/186Os isotope ratios fell into a range from 1.005 to 1.156 and are consistent with data published on PGM from other placer deposits from these regions. The ratios, together with the trace element data (and in particular the low rhenium content) determined by ion probe and μ-PIXE, indicate that crustal osmium was not incorporated in the grains and that no significant evolution of the 187Os/186Os ratios occurred during their history. These data, along with mineralogical and textural evidence, are consistent with a mantle origin for the grains through ultramafic intrusions, although the data do not entirely rule out alternative interpretations.