Experimental Modelling of Regressive Transformation of Garnets and Picroilmenites from Kimberlite Rocks

Vladimir Ya. Medvedev (med@gpg.crust.irk.ru), Larissa A. Ivanova & K. N. Egorov

Institute of the Earth's Crust, Irkutsk, 664033, Russia

Regressive transformations of barophilic minerals of kimberlites under 300-800C and 500-4000 bars with the fluid of C-O-H system with variable ratio of components have been experimentally studied. The rates of transformations of garnets and picroilmenites have been determined in the whole range of temperatures and pressures. The width of kelyphitic zones formed from these minerals lies within the range from 100 mkm to 2 mm. In terms of the phase composition the titanium-magnetites-perovskite reaction borders on picroilmenites in the system megacrysts-kimberlite (obtained experimentally) correspond to natural kelyphitic formations of ilmenites from the hosting kimberlites. It allows evaluating the duration of preserving high temperatures in pipe conditions. The investigations of compositions of newly formed phases in the Mg-Fe, Cr-Al triangle and interactions of rock-forming minerals in it permits refining the mechanisms and corresponding mineral reactions of kelyphitic formation for the system garnet-kelyphitic border-kimberlite. It can be done as the experiment allows obtaining reaction zones different in terms of the structure and phase composition and thus determining in more detail the conditions of forming natural kelyphitic borders. The study was supported by the Russian Foundation for Basic Research, grant # 98-05-65107.

The Determination of Vacancy Concentrations in Phlogopitic Micas by IR-Spectroscopy

Stefan Melzer (smelzer@gfz-potsdam.de)

GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Postdam, Germany

In order to investigate alkali-partitioning between phlogopites and fluids, (Rb,Cs,K)-phlogopites were synthesized at a temperatures of 800°C and pressures between 0.2 and 4 GPa. The run products were characterized using EMP, XRD, SEM and IR-spectroscopy. Besides the relative variations of K, Rb and Cs on the interlayer site, most synthesized phlogopites show significant vacancy concentrations of up to 0.25 based on the EMP-analyses.

In the OH-vibrational range, the IR-spectra the of phlogopites with completely occupied interlayers exhibit a broad single band around 3700 cm^-1 with distinctive shoulders. In addition, in some of the spectra an extremely low intensity band is visible at 3600 cm^-1 due to octahedral vacancies (Robert & Kodama, 1988). In all spectra from phlogopites with incompletely filled interlayers, an additional band appears at 3675 cm^-1. According to Robert and Kodama (1988), bands between 3650 and 3675 cm^-1 may be attributed to some Al on the octahedral sites. However, the amount of octahedral coordinated Al in the synthesized phlogopites is extremely low and show no correlation with the intensity of that band. XRD did not reveal other minerals, such as amphiboles and talc, with IR-bands at this wavenumber. However, the intensity of the band at 3675 cm^-1 correlates with the measured vacancy concentration. Therefore, this band may be attributed to an interlayer vacancy. The vacancy concentration on the A-site in amphiboles was determined quantitatively using IR-spectroscopy (Melzer et al., 2000). For amphiboles, tremolite was used as a standard, because the vacancy concentration in tremolite is 100%. For the determination of interlayer vacancy concentrations in phlogopite, talc should be a suitable standard. The environment of the OH-dipole in talc is almost similar to that of OH-dipoles in phlogopites next to a vacant interlayer site. The vacancy concentration derived by IR-spectroscopy agrees very well with that obtained by EMP.

Robert J-L & Kodama H, Am. J. Science, 288-A, 196-212, (1988).

Melzer S, Gottschalk M, Andrut M & Heinrich W, Eur. J. Mineral, 12, in press, (2000).

Dehydration and Metasomatism in Subduction Zone Environments: An Experimental Study on Dehydration Mechanism of Phengitic Muscovite

Stefan Melzer (smelzer@gfz-potsdam.de) & Bernd Wunder

GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

Fluids released from a subducted slab are important for subduction-related volcanism. The principles of dehydration within a descending slab, the transport processes through the mantle wedge and the trace-element and alkali-signatures in subduction-related island-arc basalts are strongly debated. Consequently, controversial models are suggested to explain these interacting processes.

The main goal of this study was to simulate fluid-induced processes occurring at the contact of a subducting slab and the overlying hotter mantle in piston-cylinder experiments. The following experimental assemblage was placed within a gold capsule of ca. 10 mm length and 4 mm in diameter: Synthetic phengite K_0.75Rb_0.05Cs_0.06Mg_0.50Al_1.53[Al_0.43Si_3.57O₁₀/(OH)₂], representing the slab material, was placed at the bottom of the capsule. Phengite was chosen, because, occurring in the sedimentary layer beyond the oceanic crust, it is proposed to control the alkali-budget within the descending lithosphere. Synthetic Fe-free forsterite and enstatite (weight-proportion 3:1) represented the simplified mantle. To avoid direct chemical contact, the slab and mantle materials were separated by a perforated gold foil. A temperature gradient of about 100 K occurred between the phengite (ca. 900°) and the mantle assemblage (ca. 1000°C).

After the experiment, performed at 3.5 GPa, Al-rich phlogopite K_0.70Rb_0.08Cs_0.09Mg_2.01Al_0.69[Al_0.93Si_3.07O₁₀/(OH)₂] and Al-containing enstatite Mg_1.90Al_0.08Si_1.98O₆ formed within the contact region. Additionally, a Cs-rich, almost Mg-free amorphous phase (65wt% SiO₂; 9wt% Al₂O₃; 2wt% K₂O; 1wt% Rb₂O; >3wt% Cs₂O), which is proposed to represent the quenched fluid, was detected by EMP-analyses. The remaining phengite K_0.82Rb_0.07Cs_0.05Mg_0.55Al_1.53[Al_0.64Si_3.36O₁₀/(OH)₂] is more Cs-poor and enriched in Rb relative to the starting material. According to the Rb-K-Cs-partitioning between phengite and fluid (Melzer & Wunder, 2000), the fluid expelled due to the partial dehydration of phengite, must be relatively enriched in Cs and depleted in Rb. The infiltration of such a fluid into the overlying mantle material may than alter the alkali-contents of the newly formed phlogopites.

Melzer S & Wunder B, Geology, submitted, (2000).