b'Don Emersons best of Exploration GeophysicsFeaturemagnetic petrology. Clark (1997) tabulated magnetic propertiesoccurs in the divalent ferrous state at higher oxygen fugacities. of rock-forming minerals, and reviewed general aspects ofIn silica-bearing systems the ferrous iron is incorporated magnetic petrophysics and magnetic petrology. mainly into silicate minerals. With increasing oxygen fugacity, Magnetic properties of rocks reflect the partitioning of iron iniron occurs in both the divalent and trivalent states and the rock between strongly magnetic oxides and/or sulphidesis incorporated into magnetite as well as silicates. At still and weakly magnetic phases (silicates, carbonates etc.). Thishigher oxygen fugacities, iron occurs in the ferric state and is partitioning depends on chemical composition, oxidationincorporated into haematite. Note that the relative terms low and high fO 2depend strongly on temperature (T). At 500C ratio of the iron, and petrogenetic conditions. Thus a host ofan oxygen fugacity of 10-15 bar is strongly oxidising for most geological factors influence magnetic properties and simplisticminerals, but at 1000C the same fO 2would correspond to very correlations between magnetic properties and lithotype arereducing conditions.generally unreliable. It is dangerous to extrapolate empirical correlations between mapped geology and magnetics inIn the system Fe-O-SiO 2 , the fayalite-magnetite-quartz (FMQ) one area to another area, ignoring changes in depositionalbuffer marks the lower oxygen fugacity limit for the stability environment, metamorphic grade or structural setting. of magnetite and the haematite-magnetite (HM) buffer marks For the purposes of subsequent discussion, an informalthe upper oxygen fugacity limit (Figure 1). The corresponding classification scheme, based on rock susceptibility (k) is used.reactions are:Igneous rocks are classified as Fe 2 SiO 4+ O 2 = Fe 3 O 4+ SiO 2 (FMQ)fayalitemagnetite quartz1. diamagnetic (DIA) if k 0, 4Fe 3 O 4+ O 2 = 6Fe 2 O 3(HM)2. paramagnetic (PM) if 0 k 1260106 SI (100 G/Oe), magnetitehaematite3. weakly ferromagnetic (WFM) if 1260106 SIk 3770106SI(300 G/Oe), Whether or not magnetite is precipitated from an igneous melt 4. moderately ferromagnetic (MFM) if 3770106 SIk that is cooling along a particular T-fO 2path depends on the 37,700106 SI (3000 G/Oe), overall composition of the melt. For example, substitution of 5. strongly ferromagnetic (SFM) if kk 37,700106 SIMg for Fe in silicate minerals stabilises them to higher oxygen (3000G/Oe). fugacity (Frost and Lindsley 1991). In particular, addition of Mg reduces the activity of fayalite in olivine, thereby shifting Diamagnetic igneous intrusions are extremely rare. Thethe equilibrium in the FMQ reaction to the left. As a result, approximate magnetite contents corresponding to thesmall amounts of magnetite and quartz react to produce ferromagnetic classes are: 0.02 vol % to 0.1 vol % for WFMfayalite, thereby partially restoring the fayalite activity, plus intrusions, 0.1 vol % to 1 vol % for MFM intrusions and greateroxygen, which increases the oxygen fugacity. Thus the olivine-than 1 vol % for SFM intrusions. Rocks that have susceptibilitiesmagnetite-quartz buffer is displaced upwards from FMQ low enough to fall into the paramagnetic class contain atand the stability field of magnetite is restricted. At higher most trace amounts of ferromagnetic (sensu lato) minerals,Mg contents, this simple picture is complicated by reaction such as magnetite or monoclinic pyrrhotite. In these rocks, theof Mg-rich olivine with quartz to produce orthopyroxene + measured susceptibility is generally dominated by contributionsmagnetite. Thus, as shown in Figure 2, at high temperatures from paramagnetic minerals. Because paramagnetic mineralsthe oxygen fugacity of the Mg-rich Fe-O-SiO 2 -MgO system do not carry any remanent magnetisation, the remanentis defined by either a quartz-orthopyroxene-magnetite magnetisation of PM intrusions is very weak. Ferromagneticbuffer curve (if the melt is saturated in quartz) or an olivine-intrusions, on the other hand, may carry significant remanence. orthopyroxene magnetite buffer curve (if the melt is olivine-The concept of oxygen fugacityStandard textbooks on petrology treat the concept of oxygen fugacity in a geological context. Oxygen fugacity (fO 2 ) is measured in units of pressure and is formally defined as the chemical activity of oxygen. Apart from a small correction due to departures from ideal gas behaviour, fO 2is equal to the partial pressure of oxygen gas. It should be noted that the abundance of free oxygen is vanishingly small in magmas and hydrothermal fluids. Nevertheless, fO 2is a well-defined thermodynamic variable that can be controlled in the laboratory and can be deduced from mineral assemblages. Frost (1991b) has recently clarified some common misconceptions about oxygen fugacity and given an unusually clear and succinct treatment of the subject.Iron, which is the fourth most abundant element in the Earths crust, exists in three oxidation states: metallic (Fe0), ferrous (Fe2+) and ferric (Fe3+) iron. Oxygen fugacity is a variableFigure 1.Plot of oxygen fugacity, expressed as log 10(fO 2 ), versus temperature that strongly influences the propensity for iron to occur in ashowing the relative stabilities of the various oxidation states of iron in the system Fe-Si-O (after Frost 1991b). Below the quartz-iron-fayalite (QIF) buffer particular oxidation state. At very low oxygen fugacities, suchiron is present as Fe0; between IQF and the fayalite-magnetite-quartz (FMQ) as in the Earths core, in some serpentinised ultramafic rocks,buffer iron occurs in the ferrous (Fe2+) oxidation state; between FMQ and the and in a few exceptionally reduced lavas that have reacted withhaematite magnetite (HM) buffer iron occurs in both ferrous and ferric (Fe3+) carbonaceous material, iron occurs as the native metal. Ironoxidation states; and above HM iron is in the ferric state.45 PREVIEW APRIL 2020'