ISOTOPIC FRACTIONATION DURING SECONDARY IONIZATION 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 ca pabilities of ion probes make the solution of such problems well worth attaining. This paper describes a source of isotopic fractionation wh ich may arise from the crystallographic structure of the sample. This effect, observed in magnetite, is potentially a source of error in acq uiring reproducible isotopic ratios during SIMS of magnetite although no similar effect has been observed during several years of measuremen ts on quartz, olivine or carbonates. The measurements were acquired us ing an Isolab 54 ion probe by rotating the sample about an axis normal to the surface and measuring O-18/O-16 ratios as a function of angle. The primary ion beam impacts the sample at an angle of 45 degrees so that rotation of the sample should bring lattice planes approximately into and out of coincidence with the primary beam. Repeated O-18/O-16 measurements from magnetite crystals as a function of orientation show maxima in measured O-18/O-16 values which can be correlated with chan nelling along lattice planes. The measured ratio has been observed to increase by approximately 7 parts per thousand as a function of angle, variations which are very significant compared with isotopic fraction ations produced by geological processes. The maxima correlate well wit h the [1 1 0], [1 0 1] and [0 1 1] lattice vectors of magnetite and ar e interpreted as channelling of the primary ions along the lattice vec tors. Since it is extremely difficult, if not impossible, to determine the crystallographic orientation of magnetite in thin section, we bel ieve that it is therefore necessary to measure magnetite samples in at least two orientations (separated by an angle that is not equal to th e difference between two channelling orientations) against a standard in a known orientation. (C) 1998 Elsevier Science B.V.