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Rutile and Its Applications in Earth Sciences

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Published: Mon, 26 Feb 2018


Rutile is the most common naturally occurring titanium dioxide polymorph and is widely distributed as an accessory mineral in metamorphic rocks ranging from greenschist to eclogite and granulite facies but is also present in igneous rocks, mantle xenoliths, lunar rocks and meteorites. It is one of the most stable heavy minerals in the sedimentary cycle, widespread both in ancient and modern clastic sediments.

Rutile has a wide range of applications in earth sciences. It is a major host mineral for Nb, Ta and other high field strength elements, which are widely used as a monitor of geochemical processes in the Earth’s crust and mantle. Great interest has focused recently on rutile geochemistry because rutile varies not only by bulk composition reflected, for instance, in its Cr and Nb contents but also by the temperature of crystallisation, expressed in the Zr content incorporated into the rutile lattice during crystallisation. Rutile geochemistry and Zr-in-rutile thermometry yield diagnostic data on the lithology and metamorphic facies of sediment source areas even in highly modified sandstones that may have lost significant amounts of provenance information. Rutile may therefore serve as a key mineral in sediment provenance analysis in the future, similar to zircon, which has been widely applied in recent decades. Importantly, rutile from high-grade metamorphic rocks can contain sufficient uranium to allow U-Pb geochronology and (U-Th)/He thermochronology. Furthermore, in situ Lu-Hf isotope analysis of rutile permits insights into the evolution of the Earth’s crust and mantle. Besides that, rutile is also of great economic importance because it is one of the favoured natural minerals used in the manufacture of white titanium dioxide pigment, which is a major constituent in various products of our daily life. Heavy mineral sands containing a significant percentage of rutile are therefore the focus of exploration worldwide.

This paper aims to provide an overview of the applications of rutile in earth sciences, based on a review of data published in recent years. After giving a summary of various rutile-bearing lithologies, the focus lies on rutile geochemistry, Zr-in-rutile thermometry, O isotope analysis, U-Pb geochronology, (U-Th)/He thermochronology and Lu-Hf isotope analysis. A final outline of the economic importance of rutile highlights the demand for further rutile-related research in earth sciences.

Keywords: Rutile; Mineral geochemistry, Geothermometry; Oxygen isotopes; Geochronology; Thermochronology; Lu-Hf isotopes

1. Introduction

With an estimated TiO2 concentration of about 0.7 wt.% (Rudnick and Fountain, 1995), titanium is the ninth most abundant element of the Earth’s continental crust. The most important titanium minerals are rutile (TiO2), ilmenite (FeTiO3) and titanite (CaTiSiO5) ( 1). Rutile is an accessory mineral in a variety of metamorphic and igneous rocks and occurs as a detrital mineral in clastic sedimentary rocks. Although the main formula of rutile is TiO2, there are commonly several possible substitutions for titanium, for example, Al, V, Cr, Fe, Zr, Nb, Sn, Sb, Hf, Ta, W and U (e.g. Graham and Morris, 1973; Hassan, 1994; Fett, 1995; Murad et al., 1995; Smith and Perseil, 1997; Rice et al., 1998; Zack et al., 2002; Bromiley and Hilairet, 2005; Scott, 2005; Carruzzo et al., 2006). Variations in the geochemical composition are host rock specific and allow the rutile source to be traced and the chemical and physical properties during rutile formation to be characterised.

Great interest has focused on rutile geochemistry, because rutile is a major host mineral for high field strength elements (HFSE), amongst others Nb and Ta, which are widely used as geochemical fingerprints of geological processes such as magma evolution and subduction-zone metamorphism (e.g. Foley et al., 2000, Rudnick et al., 2000). For many years, the significance of Nb and Ta concentrations and Nb/Ta values of crustal and mantle rocks and the Earth’s hidden suprachondritic Nb/Ta reservoir have been the subject of a lively debate (e.g. Green, 1995; Foley et al., 2000; Rudnick et al., 2000; Kalfoun et al., 2002; Zack et al., 2002; Xiao et al., 2006; Miller et al., 2007; Aulbach et al., 2008; Baier et al., 2008; Bromiley and Redfern, 2008; Schmidt et al., 2009).

Besides that, the Cr and Nb contents of rutile allow discrimination between various rutile source lithologies such as metapelitic rocks (e.g. mica-schists, paragneisses, felsic granulites) and metamafic rocks (e.g. eclogites, mafic granulites) (Zack et al., 2004b; Triebold et al., 2007; Meinhold et al., 2008). Furthermore, the incorporation of Zr into the rutile crystal lattice has a strong dependence on temperature and pressure, which has allowed the development of Zr-in-rutile geothermometers (Zack et al., 2004a; Watson et al., 2006; Tomkins et al., 2007). Several studies have already shown that Zr-in-rutile thermometry is an attractive method to calculate high-precision temperatures of high-grade and ultrahigh-grade metamorphic rocks (e.g. Spear et al., 2006; Zack and Luvizotto, 2006; Baldwin and Brown, 2008; Luvizotto and Zack, 2009). Moreover, rutile geochemistry combined with Zr-in-rutile thermometry can help to constrain the sediment provenance and has therefore already been applied successfully to sedimentary strata from the Precambrian to the Holocene worldwide (e.g. Zack et al., 2004b; Stendal et al., 2006; Triebold et al., 2007; Meinhold et al., 2008; Morton and Chenery, 2009).

Rocks exposed on the Earth’s surface are prone to weathering and erosion. These processes have continuously modified the Earth’s surface for probably more than 3.9 billion years producing clastic sediments. Some of the key pieces of evidence for that may record detrital zircons as old as 4.4 Ga from metaquartzites and metaconglomerates of the Yilgarn Craton, Western Australia (Mojzsis et al., 2001; Wilde et al., 2001). Clastic sediments are composed of various minerals (e.g. quartz, feldspar, mica) and lithic fragments and additionally contain a minor amount of heavy minerals. The sediment composition is primarily affected by the mineralogy of the primary source rock and by a complex set of parameters such as weathering, transport, deposition and diagenesis that modify the sediment during the sedimentary cycle (e.g. Morton, 1985; Morton and Hallsworth, 1999).

Besides whole-rock petrography and heavy-mineral analysis, geochemical studies of whole rock and specific detrital minerals are powerful tools in provenance characterisation. Understanding clastic sediment provenance is important for exploration of mineral resources, basin analysis and palaeotectonic reconstructions. The most common heavy minerals such as zircon, tourmaline, garnet and chrome spinel have long been used as provenance indicators by virtue of their geochemical and isotope signatures (e.g. Morton, 1991; von Eynatten and Gaupp, 1999; Morton et al., 2004, 2005; Mange and Morton, 2007). The exception is rutile, which received only minor attention until 2002 (Götze, 1996; Preston et al., 1998, 2002). However, rutile geochemistry, geothermometry and geochronology yield diagnostic data on source-rock lithology and metamorphic facies even in highly modified sandstones that may have lost significant amounts of provenance information. Rutile may therefore serve as a key mineral in sediment provenance analysis in the future, similar to zircon, which has been widely applied in recent decades.

Studying the geochemical and physical parameters of rutile is not only of scientific value. Rutile is an economically important mineral because of its use in the manufacture of white titanium dioxide pigment (Stanaway, 1994; Korneliussen et al., 2000), which is a component in products of our daily life such as paint, paper, plastics, toothpaste and sunscreen cream (Pearson, 1999; Carp et al., 2004; Gambogi, 2008). Mineral sands containing large percentages of rutile are therefore a focus of exploration worldwide (e.g. Goldsmith and Force, 1978; Force, 1991; Pirkle et al., 2007; Schlüter, 2008).

Recently, the search for secondary standards of natural rutile to assess measurement methods and demonstrate the quality of acquired geochemical data (Luvizotto et al., 2009b) has also been a focus of scientific study. Hence, the scope of this paper is to provide an overview of the applications of rutile in earth sciences, introduced with an outline of the physical properties and geochemical composition of rutile and a brief overview of various source lithologies for rutile. The latter part is relative brief because Force (1991) already gave a comprehensive overview of specific rutile sources, except for extraterrestrial material.

The current paper has the following structure: Section 1 states the objectives of this work; Section 2 presents a review of the physical properties and geochemical composition of natural rutile and other TiO2 polymorphs; Section 3 provides an overview of possible source lithologies including metamorphic rocks, igneous rocks, mineralisation, sedimentary rocks and extraterrestrial rocks; Section 4 discusses the applications of rutile geochemistry and Zr-in-rutile thermometry. Both methods are evaluated in unravelling specific source characteristics and in use as a chemostratigraphic indicator. Furthermore, oxygen isotope analysis using the quartz-rutile mineral pair is discussed. In the past, powerful new analytical methods such as in situ U-Pb and Hf isotope analyses have been developed, which significantly contribute to unravelling the geological history of natural rutile and its host lithologies and are therefore addressed here too. Finally, Section 5 summarises the economic importance of rutile and shows the need for further rutile-related scientific research.

2. Physical properties and geochemical composition

2.1. General description

Abraham Gottlob Werner created the name rutile (Ludwig, 1803), which he asigned to a mineral originally known as “red schorl”. The first description of “red schorl” is commonly attributed to Romé de l’Isle (1783); however, von Born (1772), as pointed out by Papp (2007), already mentioned “red schorl” a few years earlier. Klaproth (1795) used “red schorl” (rutile) for the description of the element titanium, which he named after the Titans of Greek mythology. Note that William Gregor originally discovered titanium (which he named menackanite) in ilmenite in 1791 (Trengove, 1972).

Although it has long been thought that Horcajuelo (also called Cajuelo) in the province of Burgos in Spain is the type locality of rutile, a thorough study by Papp (2007) has recently revealed that the type locality of rutile is Revúca in Slovakia.

The name “rutile” is derived from the Latin rutilus because of the deep red colour observed in some specimens in transmitted light. Rutile can be translucent or opaque. Yellowish and brownish colours are also very common. A rarity is natural rutile with blue colour, which has only been described so far as needle-like inclusions in garnet from ultrahigh-pressure metasedimentary rocks of the Greek Rhodope Massif (Mposkos and Kostopoulos, 2001). In reflected light rutile with bluish colour has been reported from several meteorites (El Goresy, 1971). The blue colour of meteoritic rutile may be due to a stoichiometric deficiency of oxygen in the rutile structure (El Goresy, 1971). Experiments on synthetic rutile have shown that blue colours occur in rutile samples grown or annealed under reducing conditions (Bromiley and Hilairet, 2005). Khomenko et al. (1998) demonstrated that the colour of blue rutile is mainly due to intervalence charge transfer between Ti3+ and Ti4+ on adjacent interstitial and octahedral sites (see Bromiley and Hilairet, 2005).

Rutile has a density of 4.23 g cm-3, but can range up to 5.50 g cm-3 (Deer et al., 1992), and thus belongs to the heavy mineral suite, which are minerals with density >2.8 g cm-3. Rutile is commonly diamagnetic (non-magnetic), and therefore, it can easily be separated from the paramagnetic (weakly magnetic) and ferromagnetic (strongly magnetic) heavy mineral fraction using the Frantz isodynamic separator (Buist 1963a). However, rutile can also contain a high proportion of Fe making it magnetic, and hence it could remain in the magnetic heavy mineral fraction (Buist, 1963a; Hassan, 1994). Bramdeo and Dunlevey (2000) mentioned anomalous behaviour of detrital rutile (ascribed to the substitution of cations other than Ti in the rutile crystal lattice) during industrial magnetic and electrostatic mineral separation, which can cause rutile loss. Recent studies show that Co-implanted rutile can become ferromagnetic (Akdogan et al., 2006). The melting point of pure rutile is around 1825-1830 °C (MacChesney and Muan, 1959; Deer et al., 1992). Note that synthetic rutile can be produced by heating a solution of TiCl4 to 950 °C or by heating anatase to above 730 °C (Deer et al., 1992).

2.2. Crystal structure

In nature, titanium dioxide mainly occurs in three structural states: rutile, anatase and brookite ( 2). Together they form the TiO2 end-member of the ternary system FeO-Fe2O3-TiO2 ( 3). Rutile crystallises in the tetragonal space group P42/mnm and is the most common phase, with unit cell parameters a = 4.594 Å and c = 2.959 Å for pure rutile (Baur, 1956) (Table 1). Natural rutile, however, commonly contains various trace elements such as Nb, V and Fe, as outlined in Section 2.3, and hence has distinctly higher unit cell parameters (e.g. ÄŒerný et al., 1999). The structure of pure rutile is shown schematically in 4. In the unit cell, each Ti4+ ion is surrounded by six oxygens at the corners of a slightly distorted, regular octahedron while each oxygen surrounded by three Ti4+ ions is lying in a plane at the corners of an approximately equilateral triangle (Deer et al., 1992; Baur, 2007). Baur (2007) gives a more detailed crystallographic description.

Rutile is a high-pressure and high-temperature polymorph and is isostructural with stishovite, a high-pressure silica polymorph ( 5). Stishovite is a major phase in subducting oceanic crust under lower mantle conditions (Ono et al., 2001) and has also been described from impact-related rocks (Chao et al., 1962) and meteorites (Sharp et al., 1999). The behaviour of rutile at high and ultrahigh pressures therefore offers an analogy to explore post-stishovite phase transitions (El Goresy et al., 2001; Bromiley et al., 2004). The low-temperature polymorphs of titanium dioxide are anatase (tetragonal) and brookite (orthorhombic). Under relatively low temperatures and pressures, rutile is metastable with respect to anatase when the TiO2 crystal size is less than ~14 nm, because the surface energy of rutile is then much higher than that of anatase (see Smith et al., 2009 for discussion).

Besides rutile, anatase and brookite, there are at least three additional TiO2 polymorphs (Table 1). The TiO2(B) polymorph has a structure closely related to that of VO2(B) (Marchand et al., 1980), the TiO2(II) polymorph has an α-PbO2-type structure (Simons and Dachille, 1967) and the highly metastable TiO2(H) polymorph has a hollandite-type structure (Latroche et al., 1989). For several decades, the last two were only known from synthetic polymorphs. However, Hwang et al. (2000) described an epitaxial (about 8 nm thick) slab of the TiO2(II) polymorph between twinned rutile bicrystals as inclusion in garnet of diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany, which was the first natural TiO2(II) polymorph found on Earth. This occurrence was explained by Hwang et al. (2000) to have formed during ultrahigh-pressure prograde metamorphism close to the rutile-α-PbO2 phase boundary in the diamond stability field at depths of at least 130 km. The rocks containing this TiO2(II) polymorph may have equilibrated at pressures in excess of 70 kbar (Whithers et al., 2003) ( 5). Another natural TiO2(II) polymorph was reported in omphacite from a coesite-bearing eclogite in the Dabie Mountains, China (Wu et al., 2005). Wu et al. (2005) suggested subduction of continental material to depths of over 200 km. El Goresy et al. (2001) discovered a natural shock-induced TiO2(II) polymorph from shocked garnet-cordierite-sillimanite gneiss clasts of the Ries crater, Germany. Its petrographic setting is entirely different from that of the epitaxial slab in the Saxonian diamondiferous gneisses (see description in El Goresy et al., 2001 for further details). Another natural shock-induced TiO2(II) polymorph has been identified in breccias from the Chesapeake Bay impact structure in Virginia, U.S.A. (Jackson et al., 2006).

Because the mineral polymorphs of TiO2 cannot be distinguished by their geochemical composition other identification methods such as X-ray diffraction (Spurr and Myers, 1957; Raman and Jackson, 1965) and reflected light microscopy (Mader, 1980) have to be used. The most reliable technique to identify mineral polymorphs is laser micro-Raman spectroscopy. A compilation of Raman bands for the three major structural TiO2 polymorphs is shown in 6. Rutile can be identified by bands at wavenumbers 143, 247, 447 and 612 cm-1 (Porto et al., 1967; Tompsett et al., 1995) and anatase by bands at wavenumbers 144, 197, 400, 516 and 640 cm-1 (Ohsaka et al., 1978). Brookite is characterised by several bands. Strong bands are at wavenumbers 153, 247, 322 and 636 cm-1 (Tompsett et al., 1995).

2.3. Crystal chemistry

Presently, analysis of rutile can be routinely performed by sophisticated techniques such as electron microprobe (EMP), proton microprobe (PIXE) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) so that differences in geochemical composition can easily be identified ( 7). The disadvantage of the LA-ICP-MS technique is that it makes a crater often several tens of micrometres in diameter whereas the first two techniques are nondestructive to the sample, but they have higher detection limits (especially EMP) for the concentration of certain elements compared with LA-ICP-MS facilities. For very tiny rutile crystals analysed in thin sections, it is recommended that the Si content is measured in order to identify beam interferences with adjacent silicate minerals (Carruzzo et al., 2006; Baldwin and Brown, 2008). Smith and Perseil (1997) considered Si substitution in rutile as “highly contentious at non-ultra-high pressure” and thus if Si is detected in rutile of non-ultrahigh-pressure rocks it is likely to be due to analytical error, microinclusions, or introduction into defective parts of the crystal structure (Smith and Perseil, 1997). Moreover, very fine zircon lamellae can occur in rutile (Schmitz and Bowring, 2003; Downes et al., 2007), which may cause elevated Zr contents if such zircon lamellae-rich rutiles are analysed. Thus, rutile measurements with Si contents >300 ppm showing abnormally high Zr contents should be excluded from the data set (see Zack et al., 2004a; Luvizotto and Zack, 2009 for further discussion). Rutiles from diamondiferous kyanite-bearing eclogites can contain elevated Al contents, which can be explained by several tiny rods of corundum (Sobolev and Yefimova, 2000). Ilmenite and magnetite may also occur as tiny lamellae in rutile. In general, if such mineral rods are smaller than the electron beam size a mixture of them and rutile is analysed. Literature data show a wide range of certain trace elements, in particular Al and Fe, in rutile from crustal and mantle eclogites, which most likely reflects the presence of corundum and ilmenite lamellae (Sobolev and Yefimova, 2000).

Most naturally occurring rutile corresponds to the general formula TiO2, with titanium occurring as Ti4+. Note that titanium also exists in different states of oxidation: besides Ti4+, there are also Ti3+, Ti2+ and Ti0 (MacChesney and Muan, 1959). In rutile, there are commonly several possible substitutions for titanium such as hexavalent (W6+, U6+), pentavalent (Nb5+, Sb5+, Ta5+), tetravalent (Zr4+, Mo4+, Sn4+, Hf4+, U4+), trivalent (Al3+, Sc3+, V3+, Cr3+, Fe3+, Y3+) and divalent (Fe2+, and to a lesser degree Mg2+, Mn2+, Zn2+) cations (Graham and Morris, 1973; Brenan et al., 1994; Hassan, 1994; Fett, 1995; Murad et al., 1995; Smith and Perseil, 1997; Rice et al., 1998; Zack et al., 2002; Bromiley and Hilairet, 2005; Scott, 2005; Carruzzo et al., 2006). For instance, Fe3+ and Nb5+ incorporation (Fe3+ + Nb5+ ↔ 2Ti4+) maintaining charge neutrality can compensate for substitution of tetravalent Ti4+ ions. Several further examples can be found in the literature (e.g. Urban et al., 1992; Michailidis, 1997; Smith and Perseil, 1997; Rice et al., 1998; Scott and Radford, 2007). Substitution of Ti4+ in the rutile crystal lattice is based on the ionic radius and ionic charge of the substituted cation ( 8). Cathodoluminescence (CL) and backscattered electron (BSE) images of polished rutile grains can reveal complex oscillatory zonation patterns or patchy zoned crystals (e.g. Michailidis, 1997; Carruzzo et al., 2006; Birch et al., 2007), which indicate variations in the geochemical composition in the rutile crystal lattice. Thus, a single-spot analysis may not be representative for the bulk composition of a rutile grain. That is particularly critical for calculating the crystallisation temperatures using Zr-in-rutile thermometry and for in situ isotope analysis.

Rutile is the dominant carrier of HFSE (e.g. Foley et al., 2000; Zack et al., 2002; Kalfoun et al., 2002). For example, in eclogites 1 modal-% of rutile can carry more than 90% of the whole-rock content for Ti, Nb, Sb, Ta and W and considerable amounts (5-45% of the whole-rock content) of V, Cr, Mo and Sn (Rudnick et al., 2000; Zack et al., 2002). Because rutile dominates the budget of Nb and Ta, Nb+Ta concentrations and Nb/Ta values of rutile should be identical to that of the host rock (Rudnick et al., 2000; Zack et al., 2002; Carruzzo et al., 2006). Schmidt et al. (2009) recently made a critical note on this subject, based on Nb/Ta zoning in eclogitic rutile. In general, the rutile structure can accommodate up to 37 wt.% Nb2O2 in solid solution (Roth and Coughanour, 1955; Tien et al., 1969). Villaseca et al. (2007) suggested that around 10-35% of the whole-rock Zr content of peraluminous granulites could be contained in rutile. The needle-like blue rutile inclusions in garnet from ultrahigh-pressure metasedimentary rocks of the Greek Rhodope Massif contain a small but significant amount of SiO2 (Mposkos and Kostopoulos, 2001) that indicates high-temperature/high-pressure (HT/HP) conditions and represents stishovite component in rutile (H.-J. Massonne, in Mposkos and Kostopoulos, 2001).

Rutile can also contain considerable amounts (tens to thousands of ppm) of H2O, structurally bounded as hydroxyl (OH), which has been reported from both synthetic rutile (Bromiley et al., 2004; Bromiley and Hilairet, 2005) and natural rutile (Rossman and Smyth, 1990; Hammer and Beran 1991; Vlassopoulos et al., 1993; Zhang et al., 2001). Vlassopoulos et al. (1993) pointed out that rutile is one of the most “hydrous” nominally anhydrous minerals (NAMs: Bell and Rossman, 1992) so far identified. Experimental investigations have shown that OH solubility in NAMs increases with pressure (e.g. Lu and Keppler, 1997; Bromiley et al., 2004; Mierdel and Keppler, 2004; Rauch and Keppler, 2004). Zhang et al. (2001) presented values of about 4,300 to 9,600 ppm H2O in rutile from eclogites of the Dabie Mountains, China. Therefore, besides pyroxene and garnet, rutile is also an important NAM to recycle water into the mantle (Zhang et al., 2001; Zheng et al., 2003; Bromiley et al., 2004).

3. Occurrences

3.1. Rutile in metamorphic rocks

Rutile is mainly formed during medium- to high-grade metamorphic processes (e.g. Goldsmith and Force, 1978; Force, 1980, 1991) ( 9), but it can also form in low-grade metamorphic rocks (e.g. Banfield and Veblen, 1991; Luvizotto et al., 2009a). Rutile in low- to medium-grade metamorphic rocks typically occurs as small (~20-100 µm), scattered grains, generally needle-like crystals, or polycrystalline aggregates. Banfield and Veblen (1991) recognised that within each sample analysed, the rutile crystals are uniform in size, but they are smaller in samples from the chlorite and chloritoid zones than in samples from the garnet and staurolite zones. The needle-like, euhedral morphology of the crystals show that rutile grew during metamorphism, rather than being detrital in origin (Banfield and Veblen, 1991). Luvizotto et al. (2009a) recently described prograde rutile crystallisation in low- to medium-grade metasedimentary rocks from the Saxonian Erzgebirge, Germany. The rutiles form polycrystalline aggregates made of fine-grained intergrowths of rutile and chlorite that replaces ilmenite ( 9). Luvizotto et al. (2009a) suggested that rutile has been derived from ilmenite breakdown due to following simplified reaction:

Ilmenite + Silicates + H2O → Rutile + Chlorite

Rutile in high-grade and ultrahigh-grade metamorphic rocks (e.g. eclogites, granulites) forms mainly single crystals in the matrix but also occurs as inclusion in other minerals such as garnet, pyroxene, amphibole and zircon ( 1). These rutile grains can show euhedral to subhedral forms with oval shapes or irregular shapes, with grain sizes from a few µm up to a few mm (Hills and Haggerty, 1989; Brenan et al., 1994; Zack et al., 2002; Huang et al., 2006; Xiao et al., 2006; Janousek et al., 2007; Chen and Li, 2008). Brenan et al. (1994) described large (1-5 mm in size), euhedral single crystals of rutile that contain fine exsolution lamellae of ilmenite from an eclogite vein sample of the Rocciavre Massif, Western Alps. Smythe et al. (2008) noted that ilmenite lamellae are abundantly present in mantle-derived rutile.

Eclogite is the major rock type under high-grade metamorphic rocks containing large percentages of rutile (Korneliussen and Foslie, 1985; Liou et al., 1998; Korneliussen et al., 2000; Zack et al., 2002; Chen et al., 2005; Huang et al., 2006; Zhang et al., 2006). Korneliussen and Foslie (1985) estimated that about 90% of the titanium in eclogites of the Sunnfjord region of the Western Gneiss Region (WGR), Norway, is hosted in rutile. Eclogite from the Western Ligurian Alps contains up to 4 vol.% of rutile (Liou et al., 1998). Eclogite is a plagioclase-free metamorphic rock composed of ≥75 vol.% of garnet and Na-rich clinopyroxene (omphacite) and mainly forms by subduction of gabbroic or basaltic rocks to great depths where rutile crystallises from Fe-Ti oxides and Ti-bearing silicates during metamorphic recrystallisation (Krogh, 1980, 1982; Korneliussen and Foslie, 1985; Liou et al., 1998; Korneliussen et al., 2000; Miller et al., 2007; Figs. 5 and 10). Eclogite has a density of up to 3.6 g cm-3 (Hills and Haggerty, 1989; Rudnick and Fountain, 1995), higher than any other crustal rock (Hacker, 1996), even peridotite, which it exceeds by 0.2-0.4 g cm-3 (Rudnick and Fountain, 1995). Experimental data suggest that rutile is the main Ti carrier in eclogite below 150 kbar under sub-solidus conditions, even in relatively Ti-poor systems (Okamoto and Maruyama, 2004). However, under ultrahigh-pressure metamorphic conditions close to the coesite-stishovite phase boundary it is more likely to be the TiO2(II) polymorph instead of rutile (Withers et al., 2003; 5). In general, the stability of rutile in subducted lithosphere is a complex function of whole-rock composition, temperature and pressure (e.g. Zhang et al., 2003; Klemme et al., 2005; Bromiley and Redfern, 2008; 10).

Since rutile is a major accessory mineral in eclogite and plays a major roll in the Earth’s HFSE budget (in particular Nb and Ta) (see Section 2.3), in the last decade, eclogite has received much attention regarding its whole-rock and mineral geochemical composition (e.g. Rudnick et al., 2000; Zack et al., 2002; Xiao et al., 2006; Miller et al., 2007; Schmidt et al., 2009). In addition, the processes returning extremely dense eclogite from great depths back to the Earth’s surface (e.g. Guillot et al., 2000; Neufeld et al., 2008), supplemented by geochronological studies (e.g. Baldwin et al., 2004; Glodny et al., 2005; Kylander-Clark et al., 2008), have been much discussed. Unfortunately, the original mineral assemblage of lower crustal rocks such as eclogite and eclogite facies rocks are subject to retrogression during their ascent from great depths to the Earth’s surface. For example, in eclogites and eclogite facies rocks from the Western Ligurian Alps, rutile was subsequently replaced by ilmenite and titanite along margins and cracks (Liou et al., 1998). Xiao et al. (2006) described thin titanite (10-20 μm) replacement surrounding rutile and a thin ilmenite (a few μm) rim at the rutile margin from an eclogite and nearby quartz vein, respectively, of the Dabie-Sulu ultrahigh-pressure terrane in east-central China. Note that, under conditions of rapid exhumation (as fast as subduction; Rubatto and Hermann, 2001; Baldwin et al., 2004) the original mineral assemblage is largely preserved and allows insights into the pressure, temperature and geochemical conditions present at great depths. Rutile in eclogitic (E-type) diamonds and diamondiferous eclogites (Prinz et al., 1975; Mvuemba Ntanda et al., 1982; Sobolev et al., 1997; Sobolev and Yefimova, 2000) are accessible at the Earth’s surface due to fast ascent from the Earth’s mantle as xenoliths in kimberlite pipes. The extensive studies of the composition of eclogite in recent years have significantly contributed to the increase of geochemical data for natural rutile, and thus they have opened new ways for rutile characterisation such as rutile geochemistry and Zr-in-rutile thermometry, as outlined in Section 4.

Rutile also occurs in the form of oriented, needle-like rods, known as sagenitic texture. Shau et al. (1991) described these from matrix biotite of an orthogneiss of the Tananao metamorphic complex, NE Taiwan. Van Roermund et al. (2000) described oriented, needle-like rutile rods of 2-15 µm in diameter and up to 200-300 µm in length from protogranular and porphyroclastic garnet from garnet peridotite of the WGR. Attoh and Nude (2008) recently described spindle shaped rutile rods of <3 µm in diameter, <50 µm long and arranged in a rectilinear pattern in garnet megacrystals from mafic granulites of the Panafrican Dahomeyide suture zone, SE Ghana. Oriented, needle-like rutile rods in garnet are commonly taken as an indicator for UHP metamorphism (Ye et al., 2000; Mposkos and Kostopoulos, 2001; Hwang et al., 2007; Attoh and Nude, 2008). They show crystallographically controlled growth of the exsolution rods and suggest transformation in the garnet crystal structure involving substitution of Ti4+ in the precursor garnet structure (Zhang et al., 2003). Hwang et al. (2007) investigated the origin of oriented, needle-like rutile rods in garnet from eclogitic rocks of Sulu, from diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge and from felsic granulite of Bohemia. Their results argue against a simple solid-state precipitation scenario. The authors favour a mechanism where oriented rutile needles form by cleaving and healing of garnet with rutile deposition because of the natural analogue of oriented titanite needles in biotite. Nonetheless, their study clearly shows the need for further investigation of the origin of oriented rutile needles in garnet of high-grade and ultrahigh-grade metamorphic rocks (see Griffin, 2008).

3.2. Rutile in igneous rocks and mineralisation

Additional sources of rutile can be quartz veins (e.g. Watson, 1922; Deer et al., 1992), granites (e.g. Force, 1980, 1991; Scott, 1988; Michailidis, 1997; von Quadt et al., 2005), pegmatites (e.g. Force, 1980, 1991; ÄŒerný et al., 1989; Deer et al., 1992; Okrusch et al., 2003), carbonatites (e.g. Bailey, 1961; Ripp et al., 2006; Doroshkevich et al., 2007), kimberlites and xenoliths of metasomatised peridotite (e.g. McGetchin and Silver, 1970; Dawson and Smith, 1977; Jones et al., 1982; Kramers et al., 1983; Tollo and Haggerty, 1987

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