PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF DETRITAL MAGNETITE OF LATE WISCONSINAN TILLS IN EASTERN INDIANA AND WESTERN OHIO

Detrital magnetite, although averaging less than 1% of till volume, is a common constituent in glacial tills of eastern Indiana and western Ohio.  Because of its abundance and ease of sampling, detrital magnetite was chosen to determine its potential use in determination of the glacial tills as well as a tool to chemically fingerprint glacial sedimentary deposits for use in stratigraphic analysis.  Two sampling approaches were performed.  First, glacial till samples were collected from a vertical section in western Ohio at the location of Doty’s High Bank.  Second, glacial tills were collected from a lateral distribution of five moraines in northeast and east central Indiana.

Petrographic analysis of 946 detrital magnetite grains from eastern Indiana and western Ohio has shown that 81% of the magnetite grains are homogeneous, 15% have magnetite-ilmenite intergrowths, and 4% have exsolved phases of ulvospinel/pleonaste.  18 % of all detrital magnetite grains have some level of hematite alteration.  403 of homogeneous detrital magnetite grains were chosen for chemical analyses.  These grains were analyzed for Fe, Ti, Mg, Mn, Cr, V, Al, and Si.  The means and standard deviations of these eight elements, in weight percent oxide, are FeO (89.808 ± 7.696), TiO₂ (1.58 ± 4.99), MgO ( 0.052 ± 0.200), MnO ( 0.172 ± 0.284), Cr₂O₃ (0.194 ± 2.256), V₂O₃ ( 0.241 ± 0.245), Al₂O₃ (0.455 ± 1.234), and SiO₂ ( 0.035 ± 0.047).  In general, bivariate plots of MgO/MgO(MgO + Al₂O₃) versus TiO₂ + V₂O₃ show a bimodal distribution with 11% of the grains having a total of TiO₂ + V₂O₃ value greater than 5% and 89% of the grains having a total TiO₂ + V₂O₃ value less than 5%.

A Canadian source north of Lake Huron is suggested in previous studies based on flow directions of the Laurentide Ice Sheet.  The bedrock in this area is primarily felsic plutonic and mafic volcanic.  The petrographic and geochemical results of this study indicate this area as the source area for the detrital magnetite in eastern Indiana and western Ohio tills and support transport of ice from this region north of Lake Huron in Canada.