Appearance and Exsolution of CaFe-rich Olivines

Ella V. Sokol (sokol@uiggm.nsc.ru)¹, Valery M. Kalugin (kalugin@uiggm.nsc.ru)², Victor V. Sharygin (sharygin@uiggm.nsc.ru)¹ & Elena N. Nigmatulina³

¹ Institute of Mineralogy, Koptuyga pr. 3, Novosibirsk, 630090, Russia

² Institute of Geology, Koptuyga pr. 3, Novosibirsk, 630090, Russia

³ United Institute of Geology, Geophysics and Mineralogy, Koptuyga pr. 3, Novosibirsk, 630090, Russia

Crystallization of Ca-Fe olivines is possible only under low-pressure strong reduced conditions from Ca,Fe-rich silica-undersaturated melts (Gustafson, 1972). The scarcity of combination of these factors explains the fact that so far olivines of a CaFeSiO₄-Fe₂SiO₄ series were not found out neither in terrestrial, nor in extraterrestrial environment. The crystallization of Ca-Fe-Mg olivines from basic melts was established only in angrites (Prinz et al., 1977).

The wide development of olivines of the CaFeSiO₄-Fe₂SiO₄ series was originally found by us in technogeneous parabasalts from burned spoil-heaps of the Chelyabinsk coal basin (the South Ural, Russia) (Sokol et al., 1998). These rocks have intermediate composition between high-alumina basalts and ferro-basalts. The rock-forming anorthite, clinopyroxene, and olivine (Fo_57-20) create a typical doleritic texture. Along with fluor-apatite, titanomagnetite, hedenbergite, the main mineral of mesostasis is sharply zonal olivine (CaO=1.4-26 wt.%; MgO= 1-8 wt.%). Its core (CaO<8 wt.%) look as homogeneous but rim is frequently represented by symplectite aggregates.

Compositions of members of lamella-matrix pairs are Fa_55-65La_45-35 and Fa_80-90La_10-20 (Fo_1-5). In the phase diagram for Ca-Mg-Fe olivine exsolution (Davidson, Mukhopadhyay, 1984) they approach the CaFeSiO₄-Fe₂SiO₄ side. Conodes for these pairs turn around in complete correspondence with the experimentally established solvus surface. Previously investigated Ca-olivine pairs from angrites have essentially higher MgO content (Prinz et al., 1977; Mikouchi et al., 1995).

The occurrence of olivines of the CaFeSiO₄-Fe₂SiO₄ series in parabasalts is stipulated by increased contents of CaO and FeO in initial melt, low oxygen fugacity (fO₂<10^-9 bar) and accumulation of iron in residual liquid. The crystallization of parabasalts was completed by formation of homogeneous Ca-rich fayalite (T<1100°C). The fayalite-kirschsteinite symplectites are the product of its decomposition at the temperatures 980-850°C. Frequency and sizes of lamellae are governed by CaO content in original olivine.

Davidson PM & Mukhopadhyay DK, Contrib. Mineral. Petrol, 86, 256-263, (1984).

Gustafson WI, J. Petrol, 15, 455-496, (1972).

Mikouchi T, et al, Amer. Mineral, 80, 585-592, (1995).

Prinz M, et al, Earth Planet. Sci. Lett, 35, 317-330, (1977).

Sokol EV, Volkova NI & Lepezin GG, Eur. J. Mineral, 10, 1003-1014, (1998).

Supercritical Behaviour in Pegmatites

John R. Sowerby (john.sowerby@uni-bayreuth.de) & Hans Keppler (hans.keppler@uni-bayreuth.de)

Bayerisches Geoinstitut, Universitaet Bayreuth, D95440 Bayreuth, Germany

The most widely accepted model of pegmatite formation is that of Jahns and Burnham, (1969), which emphasises the importance of coexisting aqueous fluid and melt in the process. However, recent work has shown that coexisting melt and aqueous fluid will form a single supercritical phase at high temperatures and pressures (Shen and Keppler, 1997; Bureau and Keppler, 1999). As the presence of such components as F can alter the behaviour of granitic melts, this present study was designed to examine if supercritical behaviour could be important in the petrogenesis of pegmatite bodies.

A reconnaissance study was performed on a glass composed of 66.5wt% albite, 19.5wt% NaF, 9.5wt% B₂O₃ and 5wt% water, synthesised at 2 kbars and 1100°C for three days in a rapid quench TZM autoclave. Further albitic glasses were synthesised containing 3.5wt% water and either 5wt% or 10wt% of excess Na, F, B or Li at 1.5 kbar, 1100°C for seven days.

Compared to the results of Shen and Keppler (1997) on pure albite the presence of excess Na, F, and B have a dramatic effect on the critical curve. For the reconnaissance study, at reasonable pressures for the emplacement of pegmatites, the critical temperature for the system has been lowered by at least 600°C, and the curve has a different slope. Results obtained from the glasses containing the single components show that all three affect the critical curve, with the presence of 5wt% F and B lowering critical temperatures by around 150°C. For the samples containing 5 and 10wt% Na, the critical temperature is lowered, and the curve displays a different slope. This suggests that such processes should be important in the early stages of pegmatite genesis, and that the model of Jahns and Burnham should be modified to reflect this.

Jahns RH & Burnham CW, Econ. Geol., 64, 843-864, (1969).

Shen AH & Keppler H, Nature, 385, 710-712, (1997).

Bureau H & Keppler H, Earth Plan. Sci. Lett., 165, 187-196, (1999).

Stability of Clinohumite in Mantle Compositions and its Importance for Subduction of Water into the Transition Zone

Roland Stalder (stalder@erdw.ethz.ch) & Peter Ulmer (pulmer@erdw.ethz.ch)

Institut für Mineralogie & Petrographie, Sonneggstrasse 5, ETH Zentrum, 8092 Zürich, Switzerland

Likely candidates to store water in the Earth's mantle are serpentine and dense hydrous magnesian phases (DHMP), that have been reported from high pressure experiments. The importance of Ti-clinohumite, a DHMP observed in nature is thought to be limited to high Ti-contents (i.e., extremely fertile lherzolites). We performed high pressure experiments between 5 and 14 on a synthetic serpentine composition. Run products were analysed by electron microprobe, Raman-spectroscopy and XRD. In most charges 4 or 5 phases were found, belonging to two or three stable sub-assemblages. Although clinohumite was found in many of these sub-assemblages over a much wider P and T range than previously reported, it occurred often only in minor amounts. This could be one reason for the underestimation of its stability field in previous studies. Our results indicate that clinohumite is stable with enstatite (though replacing the join forsterite + fluid) up to 1050°C between 6 and 14 GPa. At higher temperatures the break-down reaction produces forsterite + fluid/melt (if a distinction between fluid and melt can be made at all). The high pressure stability of clinohumite + enstatite is limited to 13-14 GPa, where they are replaced by forsterite + phase E or wadsleyite + phase E. Clinohumite forms between 600 and 850°C from forsterite + phase A.

The stability of hydrous phases over a wide range up to the solidus has major consequences for the water budget inside the Earth. It has been suggested earlier that water in the mantle can only be transported into the transition zone in cold slabs (colder than 600°C at 200 km depth), since no hydrous phase was known to be stable in the mantle at higher temperatures. The broad stability field of clinohumite is filling this gap and therefore subduction of water into the transition zones is concluded to be a ubiquitous process.