Ilmenite-magnetite geothermometry in trondhjemites from the Scourian complex of NW Scotland

Synopsis

The crystallization history of four trondhjemite samples from the Scourian complex, NW Scotland, has been investigated using composite ilmenite-magnetite grains. A variety of compositions are present both as large- and small-scale exsolution lamellae, which can be used to unravel the complex cooling history of these rocks. The samples were collected near Upper Badcall, Sutherland, where intrusive trondhjemite sheets 1–2 m thick cut banded gabbro. The trondhjemites have a complex history that includes four stages: magmatic intrusions, granulite facies metamorphism, hydration and retrogression to amphibolite facies, and slow cooling with uplift.

Ilmenite-magnetite grains in samples HR. 49, 53, 86 display a complex exsolution pattern (fig. 1A). An original titanomagnetite exsolved into large-scale (up to 50 µm wide) ilmenite-magnetite lamellae from which have subsequently exsolved small-scale lamellae (c.4 µm wide) parallel to the earlier lamellae. The ilmenite-magnetite pairs form subhedral grains in a granoblastic aggregate of plagioclase and quartz. A little biotite overgrows some oxide grains. Ilmenite-magnetite grains in sample HR. 56 are composed of broad-zoned lamellae (fig. 1B); small-scale exsolution lamellae are absent. Silicate-grain boundaries are irregular and lower-temperature minerals (chlorite and carbonate) are more common.

The experimental results of Buddington and Lindsley (1964) allow the equilibration temperature and oxygen fugacity of coexisting ilmenite and magnetite to be determined from their chemical composition. Subsequent workers have shown that it is possible to determine liquidus temperatures and oxygen fugacity for volcanic rocks (Carmichael, 1967; Anderson, 1968a). Slowly cooled igneous and metamorphic rocks, however, have continued to equilibrate below their solidus and show a range of temperatures and oxygen-fugacity conditions (Anderson, 1968b; Duchesne, 1972; Oliver, 1978; Bowles, 1976, 1977).

This paper presents 42 new pairs of analyses made by electron-probe microanalysis, from 13 composite ilmenite-magnetite grains (Table I). Mole % ulvöspinel and R₂O₃ values have been calculated using the method of Carmichael (1967) and used to determine temperature and oxygen fugacity at equilibration, from the experimental data of Buddington and Lindsley (1964).

By using a scanning electron beam it is possible to obtain the average composition of a broad lamella that contains smaller exsolution lamellae in order to estimate its composition prior to exsolution. A −log₁₀ƒ_o2ν.T °C plot of lamellae whose original composition has been determined in this way shows that they lie on a curve slightly above the Ni-NiO buffer between 1010 and 850 °C (fig. 2). Temperatures of the order of 1000 °C are probably magmatic temperatures since they are higher than is normally recorded for granulite-facies metamorphism; 850 °C is interpreted as the blocking temperature below which diffusion was unable to occur to form large-scale lamellae. A comparison may be made between this oxygen-fugacity curve and the curves determined by Carmichael (1967) (fig. 2) for acid lavas coexisting with different phenocryst phases. If an adjustment is made for the differences in bulk composition and pressure, some correspondence between the analysed points and the curve for hydrous silicates would be expected since hornblende is the earliest Fe-bearing silicate seen in the trondhjemite. However, correspondence is not found, implying that amphibole did not control the oxygen fugacity, either because it was not the main Fe-bearing phase at magmatic temperatures, or because the oxygen fugacity was externally controlled.

After the formation of broad high-temperature lamellae Ti diffusion continued on a smaller scale (2–3 µm) so that the lower-temperature history of these grains can be considered in terms of many independent microsystems. Limited diffusion continued across the boundaries of and within early magnetite and ilmenite lamellae. The compositions of small-scale exsolution lamellae in ilmenite and magnetite hosts have been determined. Lamellae of ilmenite in magnetite from different grains define separate log ƒ_o2-T curves for different grains. Lamellae of magnetite in ilmenite equilibrated at lower temperatures and oxygen fugacities. Individual microsystems have equilibrated at different temperatures and oxygen fugacities within the same grain and similar microsystems in different grains have equilibrated at different temperatures and oxygen fugacities, suggesting that the rock itself has become a series of independent closed systems.

The compositions of phases either side of early high-temperature lamellar boundaries have been measured. Analyses from different rocks yield different oxygen-fugacity curves in the same temperature range (765 to 610 °C). Higher temperatures were obtained for grains 49/3 and 86/4, which have exsolved into broad-zoned lamellae with no small-scale exsolution. The sense of the zoning is such that R₂O₃ in ilmenite decreases as it approaches magnetite. The equilibration temperature and ƒ_o, at the grain boundary increases from the centre of the grain to the edge.

In HR. 56 ilmenite magnetite grains show broad-zoned lamellae with no late small exsolution lamellae. The sense of zoning is such that magnetite grains increase in ulvöspinel content towards ilmenite and ilmenite decreases in R₂O₃ towards magnetite (fig. 1B). Even though the grains are in disequilibrium, it is assumed that equilibrium was at least established close to the boundary between lamellae. Equilibration temperatures thus obtained are between 410 and 430 °C at an ƒ_o2 between the Ni-NiO and QFM buffers. Ilmenite-magnetite grains coexist with a biotite richer in Ti and a hornblende depleted in Fe relative to those in samples yielding higher oxide temperatures suggesting that there was continuous Fe-Ti exchange between oxides and silicates as well as between ilmenite and magnetite.