Magnetite-rutile symplectite in ilmenite records magma hydration in layered intrusions

Electron backscatter diffraction

Thin sections from the analyzed samples were polished with 0.05 μm colloidal silica for 3 h to allow EBSD analysis. SEM imaging and EBSD analysis were conducted on a Tescan MIRA3 Field Emission SEM, housed in the Microscopy & Microanalysis Facility (John de Laeter Centre) at Curtin University, Perth, Western Australia, and on a FEI Quanta 450 field emission gun SEM housed in the State Key Laboratory and Geological Process and Mineral Resources (GPMR) of China University of Geosciences (Wuhan). The EBSD measurement was performed with an accelerating voltage of 20 kV and a working distance of ~20 mm. Electron backscatter patterns (EBSPs) were automatically collected and indexed over a regular grid with a 290 nm step size by using the Oxford Aztec 4.1 software. The CHANNEL 5+ software was used for plotting color-coded maps and the low hemisphere equal area pole figures of the indexed mineral. Noise reduction was performed by using a “wildspike” correction and a five-neighbor zero solution extrapolation.

3D FIB-EDS tomography

The 3D tomography was performed using a Helios G4 Dual Beam Workstation at the Thermo-Fisher Scientific Inc., Shanghai. A selected volume was extracted from the area of interest using the focused Ga-ion beam (acceleration voltage 30 kV) for 3D reconstruction. The chemical analyses of Fe, Ti, and O were carried out using acceleration voltage 8 kV and beam current 13 nA. The energy-dispersive X‑ray spectroscopy (EDS) analysis was performed in mapping mode to investigate the two-dimensional distributions of Fe, Ti, and O. Serial cross-section slices were produced by cutting the selected volume using focused Ga-ion beam, with a distance of 50 nm between slices, and an EDS mapping was collected for every three milling steps. The scripting routine was performed automatically with the “Auto slice and view 5.0” software. After data collection, the 2D image sequences were aligned, cropped, and stacked into a 3D microstructure image. A total 3D volume of 31.6 × 11.8 × 13.25 μm³ with a voxel pixel of 11.53 × 11.53 × 50 nm³ was reconstructed for further analysis.

Major petrographic features of the Xinjie layered intrusion

The Xinjie intrusion is one of several layered intrusions in the Panzhihua-Xichang region in SW China (Fig. 1a). The intrusion is a northwest-southeast striking, sill-like body approximately 7.5 km long, 1–1.5 km wide, and 1.2 km thick, and is divided from the base upward into a marginal zone and three lithological cycles (Units I, II, and III) (after Wang et al. 2008). Units I and II contain different modal proportions of olivine, clinopyroxene, plagioclase, and Fe-Ti oxides, forming interlayered wehrlite, olivine gabbro, olivine clinopyroxenite, clinopyroxenite, and melagabbro (Fig. 1b). Wehrlite and olivine clinopyroxenite in Units I and II display similar texture, and contain <10 vol% cumulus and intercumulus Fe-Ti oxides that are scattered in the rocks (Figs. 2a and 2b). Hydrous silicates (e.g., amphibole and biotite) are scarce in Units I and II. Unit III is mainly composed of gabbro with < 30 vol% Fe-Ti oxides (Figs. 2c and 2d), but it hosts two thick (40–50 m thick) and one thin (~4 m thick) oxide gabbro layers that contain 40–70 vol% Fe-Ti oxides (Fig. 1b).

Figure 1

Geological background and lithological characters of the Xinjie intrusion. (a) A schematic geological map of the Xinjie intrusion in the Emeishan Large Igneous Province (ELIP) in SW China. (b) A stratigraphic column that cuts through the Xinjie intrusion showing the major rock types, distribution of Fe-Ti oxides, and exsolution types in ilmenite in the intrusion. Note there are three types of exsolution textures in ilmenite, including type-I (ilmenite-hematite intergrowth), type-II (magnetite-rutile symplectite), and type-III (magnetite exsolution).

Figure 2

The occurrences of silicates and Fe-Ti oxides from different lithological units of the Xinjie intrusion. (a) Anhedral and euhedral Fe-Ti oxides as interstitial phases among clinopyroxene in Unit I, transmitted light. (b) Elongated and anhedral Fe-Ti oxides as interstitial phases among silicates in Unit II, transmitted light. (c) Cumulus Fe-Ti oxides in the Fe-Ti oxide gabbro at the bottom of Unit III, transmitted light. (d) Anhedral and euhedral Fe-Ti oxides, amphibole and biotite as interstitial phases among plagioclase in Unit III. Ol = olivine; Cpx = clinopyroxene; Pl = plagioclase; Amp = amphibole; Bt = biotite.

The rocks of Unit III generally contain 2–5 vol% hydrous silicates, which are locally gathered and closely associated with Fe-Ti oxides (Fig. 2d).

The exsolution textures in both cumulus and intercumulus ilmenite can be divided into three types, which are distributed unevenly along the profile throughout the intrusion (Fig. 1b). The ilmenite in Units I and II is generally homogeneous in BSE images and only displays local hematite lamellae (type-I, Figs. 3a and 3b). The ilmenite in Unit III commonly contains symplectitic intergrowth of magnetite-rutile (type-II, Fig. 3c) and is closely associated with hydrous silicates (Figs. 3d and 3e). Both type-I and type-II intergrowths are observed in the ilmenite of the melagabbro at the bottom of Unit III, which was then selected to investigate in this study. In addition, the ilmenite in the Fe-Ti oxide gabbro layer of Unit III contains magnetite exsolution (type-III, Fig. 3f).

Figure 3

BSE images of ilmenite grains hosting different types of exsolution textures in the Xinjie intrusion. (a) Euhedral ilmenite as inclusions in silicates showing no exsolution (type-I) in Unit I. (b) Elongated ilmenite showing well-oriented hematite lamellae (type-I) in Unit II. (c) Occurrence of magnetite-rutile symplectite (type-II) in ilmenite grain from the leucogabbro in Unit III. (d) Biotite coexisting with plagioclase and ilmenite hosting magnetite-rutile symplectite (type-II) from the olivine gabbro at the bottom of Unit III. (e) Amphibole coexisting with plagioclase, titanomagnetite, and ilmenite hosting magnetite-rutile symplectite (Type-II) from the olivine gabbro at the bottom of Unit III, note the skeletal titanomagetite (left) formed by subsolidus reaction. (f) Occurrences of magnetite (type-III) in different ilmenite grains coexisted with cumulus titanomagnetite in the Fe-Ti oxide gabbro at the bottom of Unit III. Ilm = ilmenite; Hem = hematite; Tmt = titanomagnetite; Sym = symplectite; Mag = magnetite.

Appearance of magnetite-rutile symplectite and ilmenitehematite intergrowth

Magnetite-rutile symplectites in ilmenite comprise micro- to nano-scale anhedral magnetite and rutile (Fig. 4a). The dendritic rutile tends to pinch outward and is truncated by magnetite (Fig. 4b). The symplectites have discrete and irregular boundaries with the host ilmenite (Fig. 4c). Nanoscale hematite is evenly distributed (Fig. 4d) and oriented parallel to the (0001) planes of the host ilmenite.

Figure 4

Textures in ilmenite from the melagabbro of Unit III in the Xinjie intrusion, sample X-327. (a) Magnetite-rutile symplectite in ilmenite (Ilm). (b) Vermicular rutile (Rt) occurs as a core and magnetite (Mag) presents as connected matrix in the magnetiterutile symplectite. (c) Miniscule magnetite and rutile present near the boundary of the magnetiterutile symplectite. (d) Well-oriented nanoscale hematite (Hem) lamellae parallel to the (0001) plane of the host ilmenite.

Most rutile grains are enveloped by continuous magnetite and show dendritic shapes in the 3D images (Fig. 5a and Online Materials^[1] movie OM1). The dendritic rutile appears isolated in the 2D backscattered electron (BSE) images but is actually interconnected in the 3D morphology (Fig. 5b and Online Materials^[1] movie OM2). Magnetite appears as a connected matrix in the symplectite (Fig. 5c and Online Materials^[1] movie OM2).

Figure 5

Three-dimensional morphologies of magnetite-rutile symplectite in ilmenite. (a) Typical occurrence of rutile, magnetite, and hematite in ilmenite. (b) Dendritic rutile and isolated rutile grains, note that the dendritic rutile is interconnected. (c) Magnetite surrounding large dendritic rutile grain. (d) Nanoscale, lens- like hematite lamellae homogeneously distributed in ilmenite.

Massive lens-like hematite lamellae have sharp contacts with the host ilmenite, forming ilmenite-hematite intergrowth (Fig. 5d).

Compositions of magnetite-rutile symplectite and ilmenite-hematite intergrowth

The EMPA results indicate that rutile in the magnetiterutile symplectite contains 95.97 to 98.63 wt% TiO₂ and 1.38 to 3.59 wt% FeO. Magnetite in the symplectite contains 33.58 to 35.63 wt% FeO, 59.50 to 63.70 wt% Fe₂O₃, and 2.72 to 4.68 wt% TiO₂ (Table 1). The mineral mode of rutile in the symplectite is ~40 wt% (~45 vol%), and magnetite is ~60 wt% (~55 vol%) so that the symplectite is estimated to contain 21.90 wt% FeO, 36.82 wt% Fe₂O₃, and ~41.19 wt% TiO₂ in bulk composition (Table 1).

The ilmenite-hematite intergrowth contains 41.39–43.05 wt% FeO, 47.40–49.50 wt% TiO₂, and 6.76–10.94 wt% Fe₂O₃ in bulk composition (Table 2). Given that the hematite lamellae mainly contain Fe and O based on the scanning transmission electron mode with energy-dispersive spectrometer (STEM-EDS) mapping (Online Materials^[1] Fig. OM1), the variation of Fe₂O₃ is likely related to the uneven distribution of nano-scaled hematite lamellae in the intergrowth.

Crystallographic orientation of minerals

The host ilmenite exhibits consistent crystallographic orientation (Figs. 6a and 6e). The majority of magnetite in the symplectite shares a common {111}_Mag plane and a set of corresponding <110>_Mag directions on the common {111}_Mag plane. In detail, there is an angular variation of ~4.5° across the magnetite (Fig. 6b). Similarly, the majority of rutile shares a common {100}_Rt plane and corresponding in-plane <001>_Rt + <011>_Rt directions with an angular variation of ~16° (Fig. 6c). The lattice orientations of a single rutile and magnetite grain record the progressive variation of up to ~1.6 and ~3, respectively (Fig. 6d). Note that the shared {100}_Rt and {111}_Mag planes fall into the same area as the (0001)_Ilm plane, and the shared <001>_Rt and <011>_Rt directions and <110>_Mag directions also fall into the same area as the <1010>_Ilm directions (Figs. 6e–g). It is likely that the crystallographic orientations of the majority of magnetite and rutile are controlled by the host ilmenite. Therefore, there is an orientation relationship among the magnetite-rutile symplectites and the host ilmenite, such that {100}_Rt//{111}_Mag//(0001)_Ilm and (<011>_Rt + <001>_Rt)//<110>_Mag//<1010>_Ilm.

Figure 6

Microstructure and orientation for the major phases in the magnetite-rutile symplectite and host ilmenite constructed from EBSD data. (a) Phase-color map of magnetite (Mag, green), rutile (Rt, fuchsia) and host ilmenite (Ilm). (b) Magnetite lattice orientation variations to 4.5° from the red cross (TC_Mag, texture component for magnetite). (c) Rutile lattice orientation variations to 16° from the red cross (TC_Rt, texture component for rutile). (d) Grain reference orientation deviation angle (GROD angle) showing the deviation angle from the average orientation of a rutile grain and its surrounding magnetite. (e–g) Lower hemisphere equal area projection patterns of host ilmenite, magnetite matrix, and vermicular rutile, colored with their phase colors in a. The circles and triangles indicate the parallel planes and directions of different minerals, respectively. Note: the data on each model indicate the periodic distance of every four oxygen atoms along <1010>_Ilm, <1010>_Hem, <110>_Mag, and <011>_Rt + <001>_Rt, respectively.

The crystallographic projections of both the host ilmenite and hematite lamellae are parallel to each other along the (0001) plane (Robinson et al. 2002). The lattice fringes at the ilmenitehematite interface run straightly across all the directions on the high-resolution transmission electron microscopy (HRTEM) images (Online Materials^[1] Fig. OM2).

Thermo-dynamic factors controlling the subsolidus transformation of Ilm_ss

Ilmenite-hematite intergrowths are commonly interpreted as a subsolidus transformation product of Ilm_ss (Robinson et al. 2002). The HRTEM images reveal that the ilmenite and hematite have the same crystallographic orientation and form highly coherent interfaces in the intergrowth (Online Materials^[1] Fig. OM2), which can be attributed to their crystallographic similarity (Robinson et al. 2002). The irregular morphologies of magnetite and rutile indicate that the two minerals crystallized concurrently. In addition, the orientation relationships between the magnetite, rutile, and host ilmenite indicate that their orientations are inherited from the Ilm_ss precursor (Figs. 6e–g). Thus, the ilmenite-hematite intergrowth and the magnetite-rutile symplectite represent two types of transformation products of an Ilm_ss precursor.

In general, the Fe-Ti oxides have distinctly different close-packed frameworks for their oxygen atoms; hematite, ilmenite, and rutile have “hexagonal close packing” frameworks, whereas magnetite has a “cubic close packing” framework. Hematite and ilmenite have oxygen atoms closely packed or nearly close-packed on the basal (0001) plane and along the <1010> direction (Figs. 7a and 7b). Magnetite has oxygen atoms packed on the {111}_Mag and along the <110>_Mag (Fig. 7c). Rutile has oxygen atoms packed on the {100}_Rt and along the <011> _Rt + <001> _Rt (Fig. 7d). In this study, the inherited orientations of the magnetite-rutile and ilmenite-hematite intergrowths show that their oxygen atom frameworks are aligned consecutively along the interfaces of the two intergrowths.

Figure 7

Space filling models showing the symmetries of oxygen atom frameworks of (a) ilmenite, (b) hematite, (c) magnetite, and (d) rutile on their specific orientations.

The subsolidus transformation of Ilm_ss is thermodynamically determined by the total Gibbs free energy change (ΔG) in a Fe-Ti oxide system, which can be expressed as:

where ΔGv refers to the Gibbs free energy change of phase transformation, ΔGs refers to the interfacial energy change due to new interface formation, and ΔG_ξ refers to the interfacial strain energy change due to interface lattice misfit. Therefore, ΔGv is denoted as the driving force of the transformation, whereas ΔGs and ΔG_ξ are denoted as the energy barriers of the transformation (Smith 1948).

As the assemblage of magnetite + rutile are thermodynamically equivalent to that of ilmenite + hematite (Lindsley 1991), the transformation of Ilm_ss into the magnetite-rutile symplectite and ilmenite-hematite intergrowth would have the same ΔGv. Both ΔGs and ΔG_ξ are determined by the interfacial properties of the Fe-Ti oxides transformed from the Ilm_ss, and in turn the interfacial properties of the Fe-Ti oxides are mainly related to the symmetry and orientation of the oxygen atom framework in each of the Fe-Ti oxides (Feinberg et al. 2004; Wenk et al. 2011). The consecutive oxygen atom frameworks of the two intergrowths, as shown in Figure 7, indicate that they share coherent or semicoherent interfaces (Hammer et al. 2010; De Yoreo et al. 2015). In this case, the ΔGs can be treated as zero; the energy barrier ΔG_ξ is then the key to determine the transformation path of Ilm_ss.

The ΔG_ξ can be estimated by the lattice misfit of the oxygen atom framework (δ) at their interfaces (Feinberg et al. 2004; Wenk et al. 2011).

The lattice misfit of the oxygen atom framework along the interface of hematite and ilmenite can be estimated using the oxygen atomic spacing of two minerals, i.e., δ_Ilm-Hem = (a_Ilm – a_Hem)/ a_Ilm, where a_Ilm and a_Hem refers to the oxygen atomic spacing of ilmenite and hematite, respectively. The δ_Ilm-Hem is then estimated to be ~1% (Figs. 7a and 7b). Likewise, the lattice misfit of the oxygen atom framework along the magnetite-rutile interface (δ_Rt-Mag) is estimated to be ~9.8% (Figs. 7c and 7d). The relatively high δ_Rt-Mag value would increase the lattice misfit at the interface of magnetite and rutile so that they need to adjust their orientations subtly during coarsening. The intra-grain and inter-grain orientation variations of magnetite and rutile (Figs. 6b–d) could produce high ΔG_ξ to hinder the transformation of Ilm_ss to the magnetite-rutile intergrowth. In contrast, the low δ_Ilm-Hem value makes the formation of ilmenite-hematite intergrowth energetically favorable when the temperature falls below the solvus of Ilm_ss on subsolidus cooling, in accordance with its high frequency in natural occurrence.

Transformation of magnetite-rutile symplectite from Ilm_ss precursor

The textural relationship shown in 3D images indicates that the dendritic rutile in the magnetite-rutile symplectite is likely the first phase exsolved from the Ilm_ss, and predated the matrix magnetite (Figs. 5b and 5c). The bulk composition of the symplectite is reconstructed to have ~37 wt% Fe₂O₃ (Table 1), much higher than that for coexisting ilmenite-hematite intergrowth (~8.5 wt% Fe₂O₃, Table 2), indicating that the formation of the symplectite is related to the oxidation state rather than the isochemical decomposition of the Ilm_ss precursor. The exsolution of rutile is ascribed to the subsolidus oxidation of Fe²⁺ to Fe³⁺ in Ilm_ss (Southwick 1968), which can be expressed as Fe₂O₃·5Fe₂TiO_{3 high Ti-Ilmss} + O₂ = 3Fe₂O₃·Fe₂TiO_{3 low Ti-Ilmss} + 4TiO_{2 Rt}. The earlier exsolved rutile can act as a crystal seed and significantly lower the energy barrier needed for coarsening by absorbing Ti⁴⁺ in the Ilm_ss. The exsolved rutile also creates an interface with the Ilm_ss. The lattice misfit (δ_Rt-Ilm ≈ 6.9%) at the rutile-Ilm_ss interface may cause segregation of Fe³⁺ from the Ilm_ss to the interface (Zhang and Zhang 2020), resulting in Fe³⁺ enrichment at the interface. The Ti⁴⁺ loss and Fe³⁺ enrichment along the rutile-Ilm_ss interface facilitate the growth of anhedral magnetite along dendritic rutile (Fig. 5a). Therefore, the transformation of Ilm_ss to the magnetite-rutile symplectites stems from the exsolution of rutile in Ilm_ss, which is intrinsically attributed to f_O2 elevation during subsolidus cooling.

T-f_O2 trend for transformation of magnetite-rutile symplectite from Ilm_ss

The transformation paths of Ilm_ss on the subsolidus T-f_O2 trends depend on the contents of Fe-Ti oxides and components of coexisting silicates and fluids (e.g., H₂O and CO₂) in host rocks (Frost 1991). When the rocks are buffered by anhydrous silicates (e.g., clinopyroxene and olivine), Ilm_ss follows a slightly oxidizing T-f_O2 trend (QUILF, Fig. 8) and transforms to ilmenite-hematite intergrowths when the temperature falls below the solvus (Harrison et al. 2000). When the rocks contain hydrous silicates (e.g., amphibole and biotite), Ilm_ss follows a steeply oxidizing T-f_O2 trend, e. g., the cooling trend of KUIlB (Fig. 8), and transforms to the magnetite-rutile symplectite.

Figure 8

The diagram of Δlog f_O2 (FMQ) vs. T showing the isopleths of Fe-Ti oxide solid solution and the cooling trend of KUIlB buffer (biotiteilmenite-feldspar-ulvöspinel) (modified after Frost 1991 and Harrison et al. 2000). Oxygen fugacity and temperature determined by QUILF-95 at P = 5 kbar (Online Materials^[1] Table OM2); Usp10 refers to solid solution of ulvöspinel₁₀-magnetite₉₀ (in molar fraction) and Ilm70 refers to ilmenite₇₀- hematite₃₀, and so on; Δlog f_O2 refers to the FMQ buffer; “a” refers to the crystallization T-f_O2 condition for Ilm_0.85Hem_0.15 coexisting Usp_0.45Mag_0.55, “b” refers to the T-f_O2 condition for “oxy-exsolution” of Usp_0.45Mag_0.55, and “c” refers to the T-f_O2 condition for transformation of Ilm_0.85Hem_0.15 to magnetite-rutile symplectite and ilmenite-hematite intergrowth.

In this study, the bulk composition of the Ilm_ss for the investigated sample of the Xinjie intrusion is estimated to be Ilm_0.85Hem_0.15 (Table 2), and coexisting titanomagnetite is Usp_0.45Mag_0.55 (Online Materials^[1] Table OM1). Our modeling results indicate that Ilm_0.85Hem_0.15 and Usp_0.45Mag_0.55 can crystallize simultaneously at 952 °C and FMQ+0.51 (point “a” in Fig. 8). The Ilm_0.85Hem_0.15 may have experienced two-stage transformation along the subsolidus T-f_O2 trends. Increasing f_O2 of KUIlB would induce “oxy-exsolution” of Usp_0.45Mag_0.55 at ~825 °C (point “b” in Fig. 8), and the oxidized Ilm_0.85Hem_0.15 at ~550 °C would form the magnetite-rutile symplectite (point “c” in Fig. 8) and Ti-rich solid solution of Ilm_0.92Hem_0.08. The Ilm_0.92Hem_0.08 is then decomposed into the ilmenite-hematite intergrowth on subsequent cooling (Fig. 8). This can well explain why the two intergrowths could occur in the same ilmenite grains. We infer that the Ilm_ss with higher Ti content than Ilm_0.85Hem_0.15 tends to transform to the magnetiterutile symplectites at a temperature above 550 °C when the system is buffered by hydrous silicates (Fig. 8).