Тезисы международной конференции
Abstracts of International conference
Interaction of haemoilmenite with kimberlitic magma at 2GPa and 1100 °C
Nikolenko E.I. Afanasiev V.P. Zhimulev E.I. Chepurov A.I.
V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia
Picroilmenite in kimberlite has quite a complex composition which is a solid solution of the ilmenite, geikielite, and hematite components with variable minor contents of Cr2O3 (0.01–12 wt.%) and Al2O3 (0.4–0.7 wt. %, rarely above 1 wt.%). Most kimberlites contain high magnesium (MgO > 6 wt. %) and high titanium (TiO2 > 43 wt. %) picroilmenite, paramagnetic at room temperature, with the hematite component ranging from 5 to 30 mol.%. The ilmenite with less than 6 wt.% MgO and 30-43 wt.% TiO2 is ferrimagnetic at room temperature [Garanin et al., 1984]. With its low content of MgO (6-17 mol.% Gei) and high Fe2O3, it belongs to FeTi03 - Fe2O3 solutions that produce haemoilmenite [FeTiO3]x[Fe2O3]1–x. The latter is compositionally proximal to ulvospinel but differs from it in texture and optic properties [Nikolenko, Afanasiev, 2007; Rozova et al., 1980].
Haemoilmenites from kimberlite dikes in the Massadou field (Guinea Republic) and in the pipes of Katoka (Angola), and Dachnaya, Mir, and Yagodka (Yakutia) have the hematite component as high as 31-48 mol.%, and FeO in the range 52-66 wt.% [Nikolenko, Afanasiev, 2007; Rozova et al., 1980; Haggerty 1991]. In Yakutia this haemoilmenite is found commonly in pipes from the Malaya- Botuobia district and farther to the north in haloes of kimberlite indicator minerals along the Vilyui-Markha system of deep faults [Afanasiev et al., 2001].
Studying the genetic features of haemoilmenite is difficult because it is rarely found in mantle-derived ilmenite parageneses which may have implications for the conditions of its origin and existence in the Earth’ silicate material, and for the mineral formation processes at great depths.
We expected to obtain, in laboratory experiments, zoned ilmenite similar in texture and composition to its naturally occurring kimberlite-borne counterparts. Preconditioning of samples for the experiments included selection of ferrimagnetic ilmenites from a mineral concentrate of the Massadou (Guinea) kimberlites and separation of the grain core rich in hematite with an UZDN-1 ultrasonic dispersant. The composition of kimberlite from the middle of the Poiskovaya pipe (Upper Muna field, Yakutia), which was freed from xenoliths and then powdered, was analyzed by XRF scanning at the Institute of the Earth’s crust (Irkutsk).
The experiments were performed on a high-pressure pressless apparatus "split sphere" type BARS [Chepurov et al., 1997]. The sample was placed in a Pt capsule, arc-welded, and compressed into a pile of high-melting oxides (MgO, ZrO2). Pressures were calibrated at room temperature against Bi and PbSe phase transitions. Temperatures were monitored with a PtRh 30/6 thermocouple placed in the heater center. After heating, the samples were quench cooled. At that stage of the tests, the pressure was 2.0±0.25 GPa and the temperature 1200 - 1350 ±200С (Table 1).
The second series of tests yielded a haemoilmenite with its zonation similar to that in natural kimberlite-hosted haemoilmenite from Guinea and Yakutia, though with a thin reaction rim. Increasing the exposition time from 30 to 60 min caused almost no effect on the rim thickness, but it thickened up from 100 to 200-250 micron as the time reached 600 min.
Table 1. Experimental conditions.
SEM imagery (LEO 1430 VP) has shown a zonation in the synthetized ilmenite identical to that in natural African and Yakutian kimberlite-hosted ilmenites (Fig. 1). However, although being identical in the rim structure, the natural and synthetic ilmenites have different compositions. Normally a rim of this kind differs in high Mg, Ti, and Cr but TiO2 enrichment is absent from the rim of the synthetic sample, possibly because of originally low Ti contents in the starting kimberlite. In further tests, metallic titanium specially added to the capsule bound the oxidizing components to maintain a highly reducing environment.
X-ray spectrometry (JEOL JXA 8100) after the first series of tests has demonstrated MgTiO3 growth in the rim at constant Fe2O3 and highlighted the role of homovalent isomorphic substitution (Mg2+↔Fe2+).
In the second series of tests, a reaction of high-Fe starting ilmenite with Mg- and Ti-rich kimberlite melt has resulted in a complete substitution of the original grain. The newly formed ilmenite has greater MgTiO3/Fe2O3 enrichment and an invariable content of FeTiO3, which is evidence of heterovalent isomorphism (Mg2+, Fe2+)+Ti4+↔2Fe3+. The synthetic ilmenite has a nonuniform fine-grained structure and, according to its composition, is a high-Mg high-Ti paramagnetic picroilmenite.
The experiments indicate that rims in ferrimagnetic picroilmenite may result from a reaction with kimberlite melt at 1100±20 0C and 2.0±0.25 GPa.
The large difference (tens of percent) in the Hem component between the core and the rim is evidence that the core is xenogenic and originated in an environment more reducing than the kimberlite melt.
As one may hypothesize proceeding from the experimental results, ferromagnetic picroilmenite can form, among other ways, through a reaction of an initial high-Fe material with kimberlite magma. Ferrimagnetic ilmenite typical of the Malaya Botuobia kimberlites is a product of an incomplete reaction and is more compositionally proximal to the original mineral. The origin of the latter mineral which paramagnetic ilmenite substitutes for remains so far unclear.
The study was supported by grant MK-908.2011.5 from the President of the Russian Federation
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