b'Don Emersons best of Exploration GeophysicsFeatureFigure 3.(a) Isopleths for various titanomagnetite and ilmenite compositions, plotted infO 2- T space. The FMQ and HM buffers are shown for comparison. (b)Contrasting tholeiitic and calc-alkaline cooling trends in fO 2- T space for an initially reduced (log 10 [fO 2 ] FMQ - 1), high temperature magma that is in equilibrium with Mt15 and Ilm99. The re-equilibrated Fe-Ti oxide compositions at ~ 600C for the two cooling trends are indicated. The dashed line indicates the boundary between the stability fields of ferromagnetic (sensu lato) and paramagnetic titanomagnetites. The fields representing fO 2- T conditions recorded by Fe-Ti oxides in basic and acid extrusive rocks (Haggerty 1976) are also shown.titanomagnetite composition evolves from very ulvospinel- paramagnetic. However, if the magma is emplaced at depth rich at high temperatures to magnetite-rich at subsolidusand cools slowly, following a typical tholeiitic fO 2- T cooling temperatures. Corresponding changes in ilmenite compositiontrend as shown in Figure 3(b), the Fe-Ti oxides re-equilibrate are less pronounced. For example, a relatively oxidised meltattaining compositions ofMt80 and llm97 by ~ 600C. If this at ~ 1000C (point A in Figure 3(a)) may be in equilibrium withtitanomagnetite composition is metastably stranded upon Fe-Ti oxide compositions of 50 mole % magnetite-50 mole %further cooling, the titanomagnetite is ferromagnetic at ulvospinel (Mt50) and 85 mole % ilmenite-15 mole % haematiteambient temperature and greatly enhances the susceptibility (Ilm85). If this magma cools slowly along the Ilm85 isopleth,of the rock. Thus, ferromagnetic titanomagnetites may form which almost parallels FMQ, the ilmenite composition remainseven under relatively reducing conditions, provided the cooling unchanged, but the equilibrium titanomagnetite compositionis sufficiently slow. Figure 3(b) also shows an alternative, more at ~ 600C (point B) evolves to Mt90. oxidised, cooling trend that is characteristic of calc-alkaline The final titanomagnetite composition found in the rockmagmas. Even though fO 2decreases strong ly with falling T, depends on the initial redox state of the magma (relativelythe calc-alkaline path falls more slowly than the FMQ buffer, oxidised magmas initially crystallise titanomagnetites withso the system evolves to a relatively oxidised state that is lower Ti than more reduced magmas) and the temperature atin equilibrium with more oxidised mineral assemblages. In which the titanomagnetite composition is frozen in, whichparticular, the equilibrium Fe-Ti oxide compositions at ~600C depends on cooling rate. Rapidly cooled volcanic rocks quenchare Mt90 and Ilm85 for the calc-alkaline trend. Figure3(b) is in relatively titaniferous compositions that are metastable at lowschematic, because initial magmatic conditions and cooling temperatures. On the other hand, in slowly cooled intrusions,paths can vary substantially, but it serves to illustrate qualitative oxide mineral compositions continue to re-equilibratetrends. Initial conditions of calc-alkaline magmas are generally well below the solidus, producing titanomagnetites withmore oxidising than those of tholeiitic magmas, so the final Fe-Ti progressively lower Ti, until the increasingly sluggish kineticsoxide compositions may be even more oxidised than indicated of Fe and Ti exchange between oxide phases inhibits furtherin Figure 3(b). The fields representing fO 2- T conditions recorded change. Furthermore, slowly cooled titanomagnetites tend toby Fe-Ti oxides in basic and acid extrusive rocks (Haggerty 1976) exsolve into inter growths of magnetite-rich and ulvospinel-richare also shown in Figure 3(b).phases. Figure 4 plots the range of titanomagnetite compositions The magnetic properties of titanomagnetites depend onfound in the major types of igneous rock. Note the tendency for composition. Titanomagnetites with more than 80 moledecreasing Ti content of titanomagnetite, i.e. more magnetite-rich % ulvospinel are paramagnetic at ambient temperaturescompositions, for more felsic compositions. There is also a clear and have very low susceptibility. Compositions withtendency for lower Ti contents in titanomagnetites from intrusive less Ti are ferromagnetic sensu lato. Consider a reducedrocks than for their extrusive analogues, reflecting greater re-(log 10[fO 2 ] = FMQ - 1), high temperature magma that isequilibration during cooling for intrusive rocks. Paramagnetic in equilibrium with Mtl5 and llm99. If this magma weretitano magnetite compositions are rare and are only found in a cooled very rapidly by being extruded onto the oceanfew mafic extrusive rocks with primitive compositions. The inferred floor, for example, the quenched titanomagnetite would beprimary magnetite composition of the Skaergaard gabbros, 47 PREVIEW APRIL 2020'