GEOCHEMICAL COMPOSITION OF MAGNETIC IRON-OXIDE GRAINS IN CIRCUM-ARCTIC SEDIMENTS

Nine magnetic Fe-oxide minerals were analyzed from the 45-250μm size fraction (Fe, Ti, O, Mn, Mg, Si, Al, Ca, Ni, V, Cr, Ta, Nb, & Zn) by electron probe microanalysis in more than 440 samples from nearshore and coastal deposits around the entire Arctic Ocean. These Fe minerals include fresh and altered ilmenite, magnetite, titanomagnetite, hematite, chromite, and exsolved mineral phases of ilmenite-hematite-magnetite where one of the three phases dominated. Forty-one source areas were determined by clustering lithic grain counts (>250μm) from the same samples and establishing statistically different compositional clusters. These source areas are 50-650 km in length along the Arctic coast. The Fe grain compositions of each of >37,000 analyzed grains were then matched with all grains of the same mineral from the entire circum-Arctic. Three standard deviations about the mean of more than 50 replicate analyses for each mineral is used as the acceptable range in each element for a match. The results indicate that generally the source areas contain unique compositions of each Fe mineral (<10% incorrect matches, i.e., matches to grains other than from the same source area). Matches of grains sampled from the nearshore to grains from other source areas reveal important information concerning the mixing of sediments by sea ice. Sea ice entrains sediment from one area and can deposit it down-drift in another. Drift patterns can transport ice-rafted sediment several thousands of kilometers across the Arctic Ocean. Most other incorrect matches were due to similar source rocks in adjacent areas. The unique combination of elements in these Fe minerals results from the amount of substitutions accepted by these minerals. For example, magnetite can accept up to 40% by weight of several elements for Fe. Thus the large number of possible compositions and the stability of these Fe minerals make them ideal for use in source determination. In addition, the Fe minerals show some important concentrations of economic minerals such as altered ilmenite (Ti ore) and chromite (Cr ore).