The aeromagnetic mineralogy of igneous rocks

Magnetic minerals in igneous rocks are dominated by phases in the system FeO–Fe2O3–TiO2 and the system Fe–Ni–S. Complex solid solutions within both systems yield divergent magnetic signatures, which are dependent on primary mineralogy, on deuteric cooling, and on post-depositional alteration of the host rock. Intrinsic parameters such as temperature, oxygen fugacity, and sulfur fugacity, have effective buffering capacities, which influence the ratios Fe2+:Fe3+; (Fe2+ + Fe3+):Ti; Fe:S; and (Fe + Ni):S. These ratios are highly sensitive to environmental conditions and correlate with magnetic property measurements. In addition, bulk chemistry plays a major role, and for igneous rocks a striking pattern emerges in which oxygen fugacity is related to SiO2 (i.e., from basic to acid suites) regardless of total pressure. Acidic suites (granites and rhyolites) are more highly oxidized than basic suites (gabbros and basalts); hence. ulvöspinel (Fe2TiO4)-rich and ilmenite (FeTiO3)-rich constituents are characteristic of the latter, whereas magnetite (Fe3O4)-rich and hematite (αFe2O3)-rich components are typical of the former as primary precipitated oxides. Deep-seated equilibration results in exsolution and in low-intensity states of oxidation, and this is in contrast to extrusive suites in which intense states of high temperature oxidation are prevalent. Olivine decomposition in extrusive rocks, and the formation of magnetite solid-solution members in plagioclase and pyroxene in plutonic rocks, yield single domain particles, which in some instances are the dominant magnetic carriers. Sulfide minerals are rare in continental extrusive suites, are more abundant in oceanic basalts and in hypabyssal minor intrusives, and peak in plutonic environments.Within the context of deep crustal magnetic anomalies, and in the determination of Curie isotherms, it is proposed that the primary and secondary mineralogy of plutonic ultramafic rocks be carefully re-evaluated based on current models of serpentinization. Metallic iron and metal alloys of Fe–Ni–Co–Cu are widely recognized in these rock types, and because these minerals have elevated Curie temperatures (620–1100 °C) the depth detection limits of aeromagnetic anomalies need not be constrained by the accepted value of 580 °C, the Curie temperature of magnetite.