John Verhoogen

Oxidation of Iron-Titanium Oxides in Igneous Rocks

Iron-titanium oxides in the compositional range magnetite-ulvospinel-ilmenite are likely to undergo oxidation during cooling in the presence of magmatic gases or air. The mineralogy and magnetic properties of igneous rocks depend on the initial composition of the oxide phases, the composition (particularly with regard to sulfur) and amount of gas phase, and the relative rates of cooling, unmixing, and oxidation. Tlfe oxides in an igneous rock may have a more complicated chemical and magnetic history than a deceptively simple mineralogy might lead one to suspect. Several mechanisms for self-reversal of the remanent magnetization are described. Approximate free energies of formation have been calculated for ulvospinel and maghemite. Partial pressures of oxygen have been calculated for some oxidation reactions involving ulvospinel. The equilibrium between members of the ilmenite-hematite and ulvospinel-magnetite series has been investigated.

Phase Changes and Convection in the Earth's Mantle

A phase change may hinder or enhance convection, depending on its characteristics. Univariant transformations such as may occur in the mantle constitute a barrier to convection unless the motion starts at some distance above or below the transition level; an initial temperature gradient in excess of the adiabatic value is also required. Multivariant transformations only require, in the transformation zone, an initial gradient slightly greater than the adiabatic value for a homogeneous layer. The effect on convection of transformation rates is not likely to be serious.

Thermal regime of the Earth's core

The outer core is assumed to consist of iron and sulfur, with a small amount of potassium that generates heat by radioactive decay of sim||pre|40 K. Two cases are considered, corresponding respectively to a high rate of heat production (Q = 2 · 1012 cal./sec, about 0.1% K), and to a low rate (Q = 2 · 1011 cal./sec). The temperature at a depth of 2800 km in the mantle is taken to be 3300°K (Wang, 1972). The temperature Tc at the core-mantle boundary depends on whether or not a density gradient in the lowermost layer D″ of the mantle prevents convection in that layer. In the first case, and for high Q, Tc = 4500–5000°K. In the second case, or for low Q, Tc ≈ 3500°K. The heat-conduction equation is used to calculate the temperature Ti at the inner-core boundary in the absence of convection. For high Q, Ti − Tc ≈ 1600°K; for low Q, Ti − Tc ≈ 160°K. Corresponding temperature gradients at r = rc and r = ri are listed in Table I. The adiabatic gradient at the top of the core is calculated by the method of Stewart (1970). It strongly depends on the parameters (ρ0, c0, γ0, etc.) that characterize core material at low pressure. Stewart has drawn graphs that allow the selection of sets of parameters that are consistent with seismic velocities and a given density distribution in the core. Some acceptable sets of parameters are listed in Table II. Many sets yield temperatures Tc in the range 3500–5000°K; some give an adiabatic gradient steeper than the conductive gradient and are compatible with convection; others do not. Since properties of FeS melts remain unknown, there is at present no way of selecting any set in preference to another. Properties of the FeS system at low pressure suggest the possible appearance of immiscibility at high temperature in liquids of low sulfur content; accordingly, the inner-core boundary is thought to represent equilibrium between a solid (FeNi) inner core and a liquid layer containing only a small amount of sulfur; layer F in turn is in equilibrium with another liquid (forming layer E) containing more sulfur, and slightly less dense, than F. The temperature Ti at the inner-core boundary is about 6000–6500°K for high Q and Tc ≈ 4500–5000°K. It is consistent with Alders (1966) and Leppaluotos (1972) estimates of the melting point of iron at 3.3 Mbar, but not with that of Higgins and Kennedy (1971).