Detrital Magnetite as a Provenance Indicator

Abstract

ABSTRACT Detrital magnetite grains carry unique petrographic and chemical fingerprints that can be used in provenance research. Petrographic analysis of 2,941 detrital magnetite grains in Holocene sands from felsic plutonic and volcanic, intermediate volcanic, mafic plutonic and volcanic and metamorphosed mafic/ultramafic parent rocks has found that 58% are polymineralic. Polymineralic detrital magnetite grains are most common in sands derived from felsic volcanic, mafic plutonic, and metamorphosed mafic/ ultramafic parent rocks. Sands from felsic volcanic parent rocks are characterized by grains with composite- or trellis-type magnetite-ilmenite intergrowths (c/(c + t) > 0.5), whereas sands from marie plutonic and metamorphosed mafic/ultramafic parent rocks are characterized by grains with exsolved pleonaste or ulvospinel. Monomineralic (homogeneous) grains are characteristic of sands derived from felsic plutonic and intermediate volcanic sources. Sands from mafic volcanic sources are characterized by a 1:1 relationship between polymineralic (grains with trellis- or composite-type magnetite-ilmenite intergrowths, (c/(c + t) < 0.5) and monomineralic grains. A ternary plot, with homogeneous grains, grains with trellis- or composite-type magnetite-ilmenite intergrowths, and grains with exsolved pleonaste or ulvOspinel as corners, successfully grouped grains from these parent rocks into fields with minimum overlap. Electron probe microanalysis of 433 homogeneous grains identified the presence of a chemical fingerprint. Step-wise discriminant function analysis found that TiO2, MgO, V2O3, and Al2O3 could best discriminate between detrital magnetite grains from felsic plutonic and volcanic, intermediate volcanic, and mafic plutonic parent rocks. Within this control set, detrital magnetite grains were correctly classified 98% of the time. However, grains from mafic volcanic and metamorphosed mafic/ultramafic sources were not chemically distinct, emphasizing the importance of integrating petrographic and chemical analyses in provenance research.