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

摘要

Isotopic and elemental fractionaltion 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 so 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 ~(18)O/~(16)O ratios as a function of angle. The primary ion beam impacts the sample at an angle of 45°so that ration of the sample should bring lattice planes approximately into and out of coincidence with the primary beam. Repeated ~(18)O/~(16)O measurements from magnetite crystals as a function of orientation show maxima in measured ~(18)O/~(16)O 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 crystabllographic 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.