Minor and Trace Element Partitioning between Peridotite Phases and Carbonatitic Melts

John Adam (john_adam@one.net.au)¹ & Trevor Green²

¹ 408 Princes Highway, Blakehurst, NSW 2221, Australia

² Dept Earth & Planetary Sciences, Macquarie University, NSW 2109, Australia

To examine whether direct partial melting of mantle peridotite is likely to produce carbonatitic melts with the minor and trace element characteristics of natural carbonatites, we used a laser ablation microprobe coupled to an ICPMS to analyse concentrations of Ga, K, Rb, Cs, Sr, Ba, Ti, Zr, Hf, Nb, La, Ce, Sm, Ho, Yb and Lu in experimentally produced peridotite phases (including garnet, clinopyroxene and amphibole) and quenched magnesiocarbonatite melt. The experiment was conducted at 1050°C and 2.5 GPa on a synthetic composition designed to duplicate natural peridotite melting. Crystal/melt partition coefficients determined from the experiment were used to calculate minor and trace element concentrations in a hypothetical carbonatitic melt equilibrated with an amphibole-bearing garnet lherzolite of primitive mantle composition. The melt is strongly enriched in alkaline earths and rare earths relative to high field strength elements and alkalis, and plots within the compositional range of natural carbonatites. Many of these characteristic are also common to spinel lherzolite xenoliths from western Victoria, Australia, that are thought to have been affected by carbonatite metasomatism. In all of these respects, the hypothetical composition is carbonatite like, and demonstrates that melts with the minor and trace element characteristics of natural carbonatites can be generated by partial melting of peridotite. In spite of this, the compositions of natural carbonatites are so variable that factors in addition to peridotite melting (such as crystal fractionation, wall rock reactions, and variations in source chemistry) are needed to account for their full compositional range.

A Numerical Model for the Calculation of Phase Equilibria in Calc-alkaline Magmas

Renat Al'meev (almeev@geokhi.ru) & Alexei Ariskin (ariskin@geokhi.ru)

Vernadsky Institute, Kosygin St.,19, Moscow, 117975, Russia

We present results of calibrations of a new phase equilibria model calculating melting-crystallization relationships in hydrous and anhydrous calc-alkaline systems, ranging from high-alumina basalts to andesites. This model is the further development of the COMAGMAT program [1,2]which simulates magma fractionation in a wide range of pressures and oxygen fugacities.

For the purpose to modernize the basic model a set of empirical mineral-melt equations for Oliv, Plag, Aug, Opx and Pig have been calibrated. More than 25 experimental studies conducted at 1 atm to 15 kbar and dry to water saturated conditions were included in the processing. Comparison of the calculated crystallization temperatures and mineral compositions with those obtained in experiments indicate a good accuracy of ±10-15°C and 1-2 mol%. The calculated liquid lines of descent are also coincident with experimental data.

This new model was used to study the genesis of clearly defined calc-alkaline series of the Bezymianny volcano (Eastern Kamchatka). Using a typical HAB as a parental magma, a set of calculations simulating fractional crystallization at the range of pressures 2-5 kbar has been carried out. During simulations oxygen fugacity was varied from NNO to NNO +1, whereas initial water content was about 2-3 wt.%. Results of these calculations demonstrate that an early crystallization of Mt ± Hbl is necessary to generate petrochemical trends corresponding to those observed in the Bezymianny volcano lavas.

Ariskin AA, Barmina GS, Frenkel MYa & Nielsen RL, Computers & Geosciences, 19, 1155-1170, (1993).

Ariskin AA, J. Volcanol. Geotherm. Res, 90, 115-162, (1999).

Rehydration Mechanism in Zeolite Brewsterite: Reversibility of the T-O-T Breaking and Tetrahedral Cation Migration

Alberto Alberti (alberti@dns.unife.it)¹, Giovanna Vezzalini (giovanna@unimo.it)², Simona Quartieri (simonaq@unimo.it)³ & Giuseppe Cruciani (cru@dns.unife.it)¹

¹ Istituto di Mineralogia, C.so Ercole I d'Este 32, 44100 Ferrara, Italy

² Dipartimento di Scienze della Terra, via S.Eufemia 19, 41100 Modena, Italy

³ Dipartimento di Scienze della Terra, Salita Sperone 31, 98166 S.Agata di Messina, Messina, Italy

A single crystal of zeolite brewsterite (space group P21/m, ideal formula (Sr,Ba)Al₂Si₆O16·5H₂O), heated in an evacuated capillary for 24 hours at 280°C, shows a strong decrease of the unit cell volume (about 10%) and the statistical breaking of one T-O-T bridge of the 4-ring of the 4254 PBU (T1-O7-T2 according to Alberti et al. 1999 and Sacerdoti et al. 2000). T1 cation migrates to a new tetrahedral site, which shares three vertices with the previously occupied one; the fourth vertex is on the mirror plane and joins two adjacent layers. In about half of cases a new T-O-T bridge is formed, whereas in the remaining 50% of cases the oxygen atom is unshared to form an OH group. T2 migrates to a new site, which is 5-coordinated to three oxygens of the previously occupied tetrahedron. The exchangeable cations, which occupy only one extraframework site in the untreated brewsterite, spread over several sites. This crystal, held at room conditions for one month, does not show any noticeable structural difference with respect to the heated one. This work reports the X-ray diffraction study of the same brewsterite crystal held at room conditions for three years. The unit cell volume resulted very similar to that of the untreated sample. Some interesting features characterize this new, re-expanded phase: 1. The T1 and T2 cations, migrated in the new polyhedra and not involved in the formation of new T-O-T bridges, regain their original position. 2. The other T1 cations remain in their position, preserving the new T-O-T bridge; in this case the bridging oxygen shifts on the mirror plane allowing a re-expansion of the channel system. 3. Consequently, an almost complete rehydration occurs, with the water molecules reassuming the original positions. 4. The extraframework cations migrate in the original site occupied in the untreated form.

Alberti A, Sacerdoti M, Quartieri S, Vezzalini G, Phys. Chem. Minerals, 26, 181-186, (1999).

Sacerdoti M, Vezzalini G, Quartieri S., J. Conf. Abs., 5, 90, (2000).