Experimental Study of the Formation of Ba–Cr Titanates in the Fluid-Bearing Chromite–Rutile/Ilmenite System at Т = 1000–1200°С and Р = 1.8–5.0 GPa

Abstract

The results of experimental study of crystallization of chromium-bearing Ba-titanates (redlegeite, lindsleyite, and hawthorneite) in the chromite–rutile/ilmenite system in the presence of the H₂O–CO₂–BaCO₃ fluid at pressures of 1.8, 3.5, and 5.0 GPa and temperatures of 1000–1200°C, simulating the formation of these phases through metasomatism of peridotite of the upper mantle, are reported. Experiments showed the formation of Ba–Cr-titanates in the entire pressure range studied and the possibility of joint crystallization of titanates. However, redledgeite is formed only in the Fe-poor chromite–rutile–H₂O–CO₂–BaCO₃ system, whereas minerals of the magnetoplumbite group are preferably crystallized in the system with ilmenite. Lindsleyite was not detected at a pressure of 1.8 GPa. A positive dependence of the Cr content in titanates on pressure was shown. The Raman spectra of redlegeite, lindsleyite, and hawthorneite are presented.

Mantle metasomatism is the process of transformation of mantle rocks under the influence of external fluids and melts, regardless of their origin and composition [1]. In most cases, this process is expressed in the formation of minerals atypical of mantle rocks, such as amphiboles, phlogopite, apatite, various carbonates and sulfides, titanite, ilmenite, rutile, and other rarer minerals. The consistent and regular formation of assemblages with these minerals indicates that metasomatism of mantle rocks proceeds in several stages with an increasing effect, rather than being a one-stage process. It is primarily expressed in an increase in the activity of K and/or Na, which at the initial stages of the process is due to the active decomposition of Al-rich phases (garnet, spinel) with the formation of phlogopite. Intensification of metasomatism results in further reactions of pyroxenes with the appearance of potassium richterite [2]. The formation of phases in which K and Na (and LILEs) are bound to Cr, Ti, and Fe³⁺ usually indicates the most advanced stages of metasomatic transformations [2]. The mineral indicators of these stages include chromium-bearing titanates enriched in K and Ba (in some cases, Na and Ca), HFSEs, LREEs, U, and Th. These are the priderite-group minerals of the hollandite supergroup [7], namely the solid solutions K(Ti₇Cr)O₁₆ (K–Cr priderite)–K(Ti₇Fe³⁺)O₁₆ (priderite)–Ba(Ti₆Cr)O₁₆ (redledgeite)–Ba(Ti₆Fe³⁺)O₁₆ (Ba priderite); crichtonite-group minerals, namely the solid solution K(Ti, Cr, Fe,..)₂₁O₃₈–Ba(Ti, Cr, Fe,..)₂₁O₃₈ (mathiasite–lindsleyite, hereinafter LIMA) and minerals of the magnetoplumbite group, namely the solid solution K(Ti, Cr, Fe,..)₁₂O₁₉–Ba(Ti, Cr, Fe,..)₁₂O₁₉ (yimengite–hawthorneite, hereinafter HAYI). These minerals are described in metasomatized peridotite in association with phlogopite, potassium richterite, and low-alumina clinopyroxene, where garnet is absent (or occurs only as relics), and spinel is characterized by a high Mg# and Cr content (see Table 1 in [3]). Their formation is explained by the reactions of peridotite with fluids or melts with a low SiO₂ activity, enriched in alkalis and LILEs [4–6]. The close associations of titanates with chromite, ilmenite, and rutile convince us that these minerals are the sources of Cr and Ti for the formation of titanates. Sometimes various K–Ba titanates coexist or replace each other, depending on the degree of metasomatism.

Experimental data on the stability of K–Ba titanates include a few works [9, 10]. The synthesis of K–Ba priderite containing Fe³⁺ and Fe²⁺ from mixtures of oxides and simple titanates showed that this mineral was stable at pressures of 3.5 and 5 GPa up to temperatures of ~1500°С [9]. Experiments on the synthesis of Ba–Cr priderite are not known. Hawthorneite and lindsleyite were synthesized in the TiO₂–ZrO₂–Cr₂O₃–Fe₂O₃–MgO–BaO–K₂O system at 7–15 GPa and temperatures of 1300–1500°С [10].

The results of experiments on the formation of K–Cr end members of titanates (K–Cr priderite, matiasite, and yimengite) from chromite–rutile and chromite–ilmenite associations in the presence of H₂O–CO₂–K₂CO₃ fluid were reported earlier [3, 8]. This paper presents the results of experiments on joint crystallization of Ba–Cr end members of solid solutions of titanates (redledgeite, hawthorneite, and lindsleyite) upon interaction of these associations with H₂O–CO₂–BaCO₃ fluid at 5 GPa and 1200°С, 3.5 GPa and 1200°С, and 1.8 GPa and 1000°С.

The experiments were performed at the Korzhinskii Institute of Experimental Mineralogy, Russian Academy of Science, using the NL-13T “anvil-with-hole” apparatus at 5 GPa, NL-40 at 3.5 GPa, and a PC-40 piston-cylinder at 1.8 GPa (Table 1). Natural chromite with the composition (Mg_0.49–0.54Fe_0.50–0.54Mn_0.01–0.02Zn_0.01–0.02) (Al_0.17–0.20Cr_1.55–1.61Fe_0.10–0.22Ti_0.03–0.07)O₄ from garnet lherzolite xenolith from the Pionerskaya kimberlite pipe (Arkhangelsk region), ilmenite with the composition Fe_0.98Mg_0.01Mn_0.06Ti_0.93Al_0.01Nb_0.01O₃ represented by a xenocrystal from kimberlite of the Udachnaya pipe (Yakutia), and synthetic TiO₂ powder were used as the starting materials. The fluid component was prepared from a mixture of synthetic BaCO₃ and oxalic acid. The oxygen fugacity was not buffered in the experiments. Following the previous conclusions [3, 8], the Fe³⁺/Fe²⁺ ratio in chromite in the experimental products corresponds to Δ log f_O2 values, 1.1–1.6 logarithmic units below the FMQ buffer.

Table 1. Conditions and results of experiments on the synthesis of Ba–Cr titanates in the chromite–ilmenite/rutile system with H₂O–CO₂–BaCO₃ fluid

The composition of the phases (Tables 2, 3) was analyzed by X-ray spectral microanalysis on a Tescan Vega-II XMU scanning electron microscope equipped with an INCA Energy 450 X-ray detection and sample composition calculation system in the EDS mode at an accelerating voltage of 20 kV, a current of 400 pA, and an electron beam diameter of 157–180 nm (for chemical analysis) or 60 nm (for imaging). The Raman spectra of titanates were obtained using a Renishaw RM1000 Raman spectrometer equipped with a Leica microscope. A diode-pumped solid-state laser with a wavelength of 532 nm and a power of 20 mW was used. The spectra were recorded at 50-fold magnification for 100 s.

Table 2. Representative analyses of Ba–Cr titanates (redledgeite, hawthorneite, and lindsleyite) synthesized in experiments in the chromite–ilmenite/rutile system with H₂O–CO₂–BaCO₃ fluid

In the chromite–rutile–H₂O–CO₂–BaCO₃ system, redledgeite and lindsleyite are formed at 3.5 and 5 GPa and 1200°C (Table 2). They are associated with recrystallized chromite and rutile (Fig. 1a). Redledgeite forms individual anhedral and subhedral (suboctahedral) crystals ranging in size from a few to 40 μm (with a maximum up to 100 μm) (Fig. 1a), as well as inclusions in transformed chromite. Such forms of occurrence of redledgeite are known in natural associations (for example, in chromitite of the Verblyuzh’egorsk Massif, Russia [11]). Lindsleyite forms small anhedral grains up to 10 µm in size, with inclusions in chromite, rutile, and the newly formed redledgeite.

Fig. 1.

BSE images of run products in the systems: (a) chromite–rutile with H₂O–CO₂–BaCO₃ fluid, run Ba–Ti; (b) chromite–ilmenite with H₂O–CO₂–BaCO₃ fluid, run 3Ba-1 (Table 1). Chr, chromite; Hwt, hawthorneite; Ldy, lindsleyite; Red, redledgeite; Rt, rutile.

Hawthorneite and lindsleyite coexisting with recrystallized chromite, ilmenite, and Nb-bearing rutile are formed in the chromite–ilmenite–H₂O–CO₂–BaCO₃ system at pressures of 3.5 and 5.0 GPa and 1200°C. Hawthorneite crystallizes as euhedral octahedral and subhedral grains up to 40 µm in size, but there are individual anhedral grains larger than 100 µm as well. Lindsleyite forms anhedral inclusions in hawthorneite with a size of <10 μm (Fig. 1b). At 1.8 GPa and 1000°С, redledgeite and hawthorneite were detected in the experimental products in association with chromite, ilmenite, and a phase similar in composition to barium mica, ferrokinositalite [12] (wt %, K₂O, 0.62; BaO, 22.55; MgO, 8.71; Al₂O₃, 16.87; FeO + Fe₂O₃, 12.54; SiO₂, 23.62; Cr₂O₃, 3.55; H₂O, 3.05). Hawthorneite is characterized by angular isometric grains up to 150 µm in size, while redledgeite forms inclusions in recrystallized chromite with a size of <30 µm.

The compositions of the synthetic chromium-bearing Ba-titanates vary slightly in the products of individual runs (Fig. 3), except for hawthorneite synthesized at 5 GPa. This phase shows a Ti + Fe ↔ Cr trend of isomorphism. This trend coincides with the trend towards an increase in the content of Cr in this phase with pressure from 3.5 to 5 GPa (Fig. 3). The Cr content in lindsleyite and redledgeite shows a direct dependence on pressure, especially for the phases synthesized at 3.5 and 5 GPa (Fig. 3). However, in contrast to hawthorneite, an increase in the Cr content in these phases is accompanied by an increase in the Fe content, according to the scheme Ti ↔ Fe + Cr. Thus, with increasing pressure, all Ba titanates tend to be enriched in chromium. These results are consistent with the data [10] for pressures of 7–15 GPa, which show a poor negative dependence of the Ti content on pressure for the phases of the hawthorneite–yimengite series, and negative relationship between Ti + Mg and Cr for the phases of the lindsleyite–mathiasite series, which indirectly indicates a positive correlation of the Cr content in titanates with pressure.

Fig. 2.

Raman spectra of redledgeite (red line), lindsleyite (green line), and hawthorneite (blue line). Peak values are indicated in the text.

Fig. 3.

Ti–Fe–Cr diagram (apfu) illustrating compositional variations of synthetic redledgeite, lindsleyite, and hawthorneite. The arrows indicate the shift of points with increasing pressure.

The presence of Ba titanates was confirmed by Raman spectroscopy. Intense peaks in the spectrum of redledgeite (red spectrum in Fig. 2) at 179, 354, and 693 cm^–1 and peaks of lower intensity at 276 and 549 cm^–1 are close to the peaks in the spectra of synthetic K–Cr priderite [13].

The shift of the first peak in the redledgeite spectrum towards higher wavenumbers may be due to the presence of the heavier Ba cation instead of K. The Raman spectrum of lindsleyite (green line in Fig. 2) is characterized by intense peaks at 213, 275, 384, and 679 cm^–1 and a shoulder characteristic of all titanates at ~580 cm^–1. Intense peaks at 204, 353, and 685 cm^–1 in the spectrum of hawthorneite (blue line in Fig. 2) are comparable to the peaks in the spectra of Al-rich yimengite from xenolith garnet lherzolite from the Obnazhnaya kimberlite pipe, Yakutia [3]. The shift of the latter peak to lower wavenumbers in synthetic hawthorneite compared to natural yimengite is associated with a higher content of Ti.

Thus, experiments in the chromite–rutile/ilmenite–H₂O–CO₂–BaCO₃ systems show that Ba–Cr titanates are formed in a wide pressure range of 1.8–5.0 GPa. Compared with the data obtained for K–Cr titanates [3, 13], the new experiments indicate that the hollandite-group minerals, K–Cr priderite and redledgeite, are formed only in the low-iron systems chromite–rutile–H₂O–CO₂–BaCO₃/K₂CO₃, while the magnetoplumbite-group minerals crystallize more preferentially in the ilmenite-bearing systems. No crichtonite-group minerals were detected at pressures of <3 GPa. As a result of the experiments, pairs of titanate phases (redledgeite + hawthorneite and hawthorneite + lindsleyite) were obtained, similar to the associations in the chromite–rutile/ilmenite–H₂O–CO₂–K₂CO₃ systems [3, 13]. This supports the possibility of joint formation of titanates through metasomatism of peridotite of the upper mantle in the presence of fluids or melts containing K and Ba (and other LILEs).

The positive dependence of the Cr content in Ba titanates on pressure (Fig. 3) may be considered as a relative depth marker for natural associations including K–Ba titanates. However, among K-titanates, such a dependence was revealed only for magnetoplumbite-group minerals [3, 13]. Therefore, to use this effect, further study of the K and Ba isomorphism and their influence on the isomorphism of Cr, Ti, and Fe in titanates in the presence of H₂O–CO₂–K₂CO₃–BaCO₃ fluids with a variable K/Ba ratio is required.