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An International Journal of Solubility of Ti in Andradite in Upper Mantle

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An International Journal of Solubility of Ti in Andradite in Upper Mantle Per. Mineral. (2001), 70, 1, 99-110 http://go.to/permin An InternationalJournal of � � PERIODICO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, ArchaeometJ:y and Cultural Heritage Solubility of Ti in andradite in upper mantle conditions: preliminary .results ANDREA 0RLAND01* and DANIELE BORRINr2 1 C.N.R.-C.S. Minerogenesi e Geochimica Applicata, Via G. La Pira 4, I-50121 Firenze, Italy 2 Dipartimento di Scienze de lla Terra, Universita de gli Studi di Firenze, Via G. La Pira 4, I-50121 Firenze, Italy Submitted, July 2000- Accepted, Februrary 2001 ABSTRACT. -Piston cylinder experiments were 1,5-2,0 GPa). 11 ruolo della fugacita di ossigeno nel carried out, aimed at determining Ti solubility in controllo dell'ingresso del Ti nell'andradite non puo andradite in upper mantle conditions (P=1.5-2.0 comunque essere stabilito dal momento che gli GPa, T=1200-1300°C). Starting materials were Ti­ esperimenti al pistone cilindro sono stati eseguiti undersaturated and Ti- saturated (with respect to 1 senza controllare questa variabile. Il principale atm) synthetic andradites. From unreversed meccanismo che controlla l'ingresso del Ti nella experiments at 1.5 and 2.0 GPa, the maximum Ti struttura del granato appare essere la sostituzione contents in andradites were determined as 1.0-1.4 sciorlomitica mentre la sostituzione morimotoitica atoms per formula unit (a.p.f.u.), compared with 1.5- sembra avere una certa importanza solo nell'ambito 1.9 a.p.f.u. at 1 atm. Oxygen fugacity was not delle andraditi sottosature in Ti. controlled in experimental runs. The main mechanism controlling the entry of Ti in the garnet KEY WORDS: Synthetic garnets, titanium andradite, structure at high pressure appears to be schorlomitic high pressure experiments, piston cylinder. substitution, whereas some morimotoitic substitution may occur in Ti- undersaturated andradites. INTRODUCTION RIASSUNTO. -In questo lavoro sono riportati i risultati di alcuni esperimenti preliminari effettuati al pistone - cilindro e volti a determinare la solubilita Andradite (Ca3Fe2Si30 12) is commonly del Ti nelle andraditi in condizioni corrispondenti al found in subsilicic alkaline igneous rocks, mantello superiore (P=1,5-2,0 GP a, T= 1200- where it may also occur in its Ti-bearing 13000C). I materiali di partenza sono rappresentati varieties, melanite (Ti02 <8 wt%) and da andraditi sintetiche rispettivamente sottosature e sature in Ti (a 1 atm). A 1,5 e 2,0 GPa il massimo schorlomite (Ti02 >8 wt%; Howie and contenuto di Ti nelle andraditi diminuisce rispetto Woolley, 1968). Ti-andradite is also found in alle condizioni nelle quali la pressione e uguale a 1 metamorphic rocks which formed in oxidising atm; gli atomi di Ti per unita di formula conditions (e.g., Huckenholz and Yoder, 1971) diminuiscono infatti da 1,5-1,9 (a 1 atm) a 1,0-1,4 (a and as a characteristic phase in hydrothermal alteration assemblages (e.g., Lang et al. 1995). *Corresponding author, E-mail: [email protected] In the potassic Roman magmatic province 100 A. 0RLANDOand D. BoRRINI (Italy), melanite occurs as phenocrysts and as a reduction of significant amounts of Ti4+. groundmass phase in phonolitic trachytic Russell et al. (1999) recently proposed that the ignimbrites, phonolites and foidites from mechanisms of Ti entry into andradites allow a Monte Vulture volcano (De Fino et al., 1986; distinction between igneous and hydrothermal Caggianelli et al., 1990; Melluso et al., 1996). garnets and evaluation of the f02 and a sio 2 The parental magmas of these garnet-bearing conditions attending their formation. rocks probably derived from a mantle source, possibly metasomatised by a subducting slab EXPERIMENTAL DESIGN AND PROCEDURES active during the Tertiary (Civetta et al., 1978; Peccerillo, 1985; Conticelli and Peccerillo, Starting material 1992). In such a scenario, it is important to understand how the intensive variables (P, T, In this work, two different starting materials f02) control the chemistry of phases such as Ti­ with bulk compositions of Ti- saturated andradite. (Ca3Fe2Ti1.5Si1.5012) and Ti- undersaturated The thermal stability of Ti-andradite was (Ca3Fe2Ti0.75Si2 .25012) andradite with respect to investigated at atmospheric pressure by 1 atm (Huckenholz, 1969) were chosen. They Huckenholz (1969). In that study, the were synthesised in the following way. First, maximum solubility of Ti in andradite was gels were prepared partially according to the reported to be approximately 1.5 Ti atoms per procedure of Ito and Frondel (1967), the main formula unit in the range 1 000-1300°C. At difference consisting in the silica source; in our higher Ti contents, Ti- saturated andradite procedure, silica-free gels were prepared and coexists with perovskite (CaTi03) and hematite mixed in appropriate amounts together with (Fe203). pure natural quartz (>99.9 wto/o Si02). The Since no data exist on the solubility of Ti in mixtures were kept in a Pt crucible at 1100°C andradite at high pressures and temperatures, in air for about 30 days, with periodical some preliminary data in upper mantle regrinding and powder X-ray diffraction conditions are reported in this paper. They give (XRD) checks. Hematite XRD peaks were some insights into the potential of garnet as a initially the most intense ones, but their Ti reservoir in P-T conditions where magmas intensity diminished with the number of may be generated. heating cycles. Syntheses were considered The structural position of Ti within andradite complete when hematite could no longer be and its valence state are still subject of debate detected by XRD. The products were mounted (e.g., Locock et al., 1995; Malitesta et al., in epoxy resin and analysed by electron 1995; Ambruster et al., 1998): this issue will microprobe. Both materials consist mainly of not be addressed here, although the Ti-andradite with minor amounts of experimental results might contribute to better pseudobrookite (Fe2Ti05) solid solution (Table understanding of the substitution mechanisms 1). The Ti- undersaturated starting material also occurring in andradite. Moreover, although contained traces of rutile (Ti02) and titanite several authors (e.g., Manning and Harris, (CaTiSi05), whereas minor glass (Si02=45-50 1970; Huckenholz et al., 1976; Huggins et al., wt%, Ti02=10-12 wto/o, Fe0*=15-16 wto/o, 1976; Kiihberger et al., 1989) report the Ca0=3-5 wt%) and traces of perovskite were 3 presence of Ti + in Fe-rich garnet at relatively present in the Ti- saturated starting material high f02, we do not think it useful to discuss (Table 1). The grain size of phases were less 3 here the possible presence of Ti + in our than 30 !Jm. Unfortunately, electron synthetic garnets. Not only is there no need to microprobe analyses revealed that the starting 3 invoke the presence of Ti + in recasting materials had suffered some contamination by electron microprobe analyses, but the synthesis Na20 and Al203; thus, garnets from Ti­ conditions were such as to exclude the undersaturated and Ti- saturated starting Solubility ofTi in andradite in upper mantle conditions: preliminary results 10 1 WC plug Stainless steel plug Pyrophyllite �-�NaCI Alumina disk "'"i:--� Graphite Fig. 1 -Sketch of assembly used for piston cylinder experiments. Width = 12.7 mm. 102 A. 0RLANDOand D. BORRINI 0 0 G" - e � ::s P(GPa) ::s G" 1.52 7.8 1.50 6.4 � c::: 0 :;::; 0 ·c:: 1.48 - 4.9 0. 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 28 Fig. 2 XRD spectra of products of calibration experiments performed at T=l000°C. Dashed lines: ferrosilite peaks; top: positions and relative intensities (in brackets). Solubility ofTi in andradite in upper mantle conditions: preliminary results 103 TABLE 1 Ideal chemical compositions and structural formulas of starting materials. Bulk composition Ideal formula Products Ti- undersaturated Ca3Fe2Ti 0_75Si2.250 12 Ti-andradite, pseudobrookite, rutile, titanite Ti- saturated Ca3Fe2Ti 1.soSi 1.soO12 Ti-andradite, pseudobrookite, perovskite, glass materials contained up to 0.4 wt% (Na20) and occurred·due to the subsolidus and significantly 0.2 wt% (Al203) and up to 1.3 wt% (Na20) and oxidising conditions of the experiments. 0.7 wt% (Al203) respectively. Pressure calibration Experimental procedures Pressure was calibrated at 1000°C using the Experiments were performed in a 0.5" piston 2FeSi03 (ferrosilite) = Fe2Si04 (fayalite) + cylinder at the Department of Earth Sciences, Si02 (quartz) transition (1.41 GPa, Bohlen et University of Florence. The instrument was al., 1980). Calibration experiments were modified with respect to the original design of performed using a mixture of pure synthetic Boyd and England (1960), being equipped with fayalite (kindly provided by P. Ulmer, ETH, double-acting hydraulic cylinders of 100 and Zurich), quartz and ferrosilite, the latter phase 75 tonnes capacity; the lower cylinder (75 being present in very small amounts. The XRD tonnes) loads the high-pressure piston and the spectra (collected on a PW 1830 Philips upper one provides extra force to end-load the diffractometer, Cu anticathode, 40 kV, 20 mA, pressure chamber. counting times = 0.5 sec per 0.02°/28) of the Starting materials were loaded inside Pt products of four calibration experiments are capsules (5 mm long, O.D.=3.0 mm, I.D.=2.8 shown in fig. 2. Ferrosilite growth began mm) and their ends were arc-welded shut. The within the range 1.48-1.50 GPa, indicating that assembly consisted of a NaCl/pyrex pressure a friction of 5-6% must be taken into account medium, a graphite heater, and crushable when calculating the real pressure on the alumina spacers (fig. 1 ). Temperature was capsules. measured by a Pt 100-Pt90Rh 10 (type S) Pressure calibration and all experiments were thermocouple, separated from the capsule by a performed using the «cold piston-in» thin alumina disk in order to prevent capsule technique: the load was increased up to the piercing.
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