Ion Probe Oxygen Isotope Analysis of Quartz and Magnetite from the Biwabik Iron Formation, Minnesota

Abstract

The oxygen isotopic compositions of coexisting quartz and magnetite have previously been measured, using conventional fluorination techniques, in samples from a number of Precambrian iron formations (e.g. Perry and Bonnischen, 1966; Perry et al., 1973). Their aim was to utilize equilibrium mineral pair geothermometry to infer temperatures of metamorphism and diagenesis of both recrystallized and relatively unaltered Precambrian chemical sediments. The present study has acquired in situ, high spatial resolution, ion probe oxygen isotope measurements of samples from the relatively undeformed Biwabik Iron Formation, Minnesota, in conjunction with detailed petrographic observation. This had the aim of testing the extent of isotopic equilibrium between this mineral pair to investigate the interplay of depositional signatures and modifications due to diagenetic recrystallization of initial precipitates. Methods . Detailed petrographic observation and compositional analyses were obtained of seven samples from cores 2, 5 and 7 from the Biwabik Iron Formation of the Mesabi Deep Drilling Project (details in Pfleider et al., 1968). Oxygen isotope analyses of these samples were then obtained using the Isolab 54 ion microprobe at the University of Manchester, UK (Saxton et al., 1996). The 1So/160 ratios were determined by secondary ionization mass spectrometry using a Cs + primary beam and the 81SOsMow values determined by calibration with standard materials (Lyon et al., 1995). Analysis transects of between 11 and 19 spots were made across quartz and magnetite in the selected areas within the chosen samples with crater sizes ranging from 20 to 60 Ixm in diameter and a few gm in depth. Results and discussion. Petrographic observation at high resolution identified magnetite as a late stage mineral, generally with a subhedral to euhedral crystal habit from 10 to 100 gm, which exhibited a cross-cutting relationship with all the other constituent phases. It was present in a number of textural associations including granule rims, replaced granules, disseminated crystals and crystalline aggregates. Quartz was present as a microcrystalline matrix mineral between the disseminated and aggregated magnetite crystals. Mean values for quartz and magnetite within each sample were calculated to enable the determination of the oxygen isotopic fractionation (AQM) between this coexisting mineral pair. These values are shown in Table 1. Mean quartz values range from 18.3 to 24.2 %0 and mean magnetite values range from -10.7 to -3 .6 %0. The calculated isotopic fractionations range from 25.6 to 33.6 %0. Our data have been plotted in Fig. 1 in conjunction with previously published oxygen isotope analyses of the quartz-magnetite mineral pair from the Biwabik Iron Formation (Perry and Bonnischen, 1966; Perry et al, 1973). Oxygen isotope ratios of quartz and magnetite from the metamorphosed Duluth contact zone and the transition zone between this and the relatively undeformed iron formation are shown by open circles (Perry and Bonnischen, 1966). Data from samples taken from the same cores as the present study (Perry et al., 1973) are plotted as solid diamonds and lie in a steeply dipping array towards lower quartz and higher magnetite values than the present results. An average fractionation of 24%0 was determined from this data. This is almost 10%o lower than the highest fractionation shown by the present study (open diamonds). This can be explained by a comparison of ion microprobe analysis and the conventional fluorination techniques that were used to generate the other data sets. Firstly, ion probe analysis eliminates the effects of contamination of