MULTIPLE TITANIUM SUBSTITUTION MECHANISMS IN BIOTITES FROM HIGH GRADE METAPELITIC XENOLITHS: A CASE STUDY FROM THE EUGANEAN HILLS

Biotites from metapelitic xenoliths trapped within trachytes from the Euganean Hills (Italy) were studied by single crystal XRD, EMPA, SIMS and Mössbauer spectroscopy in order to determine the dominant Ti incorporation mechanism. The TiO₂ content ranges from high (8.66 wt %) to lower but still important values (3.96 wt %).A combination of the Ti-Tschermak and Ti-vacancy mechanisms was preliminary suggested only based on EMPA data [1]. New SIMS measurements show that the H₂O content falls in the range 2.40 - 4.08 wt % suggesting that Ti-oxy substitution is also present althought less important than in biotites from anatectic metapelites of El Joyazo (Spain) [2]. Single crystal XRD structure analysis suggests that the Ti-oxy exchange was indeed active but is responsible only for about half of the Ti incorporation. The OH contents inferred from the structural parameters [2, 3] are in excellent agreement with those determined by SIMS. Despite the relatively similar HT conditions between Euganean Hills and El Joyazo xenoliths, the above oddity is likely to involve some petrogenetic difference. It is well known, in general, that Ti incorporation is favoured by high temperature. However, little information exists concerning other possible factors related to the petrogenetic context. In the present case, both the relatively high Ti content and the limited amount of stacking faults, as inferred from X-ray precession images, are consistent with a biotite crystallization favoured by high temperature and, possibly, high water vapour activity. For a more speculative interpretation we might recall previous conclusions [1], suggesting that the occurrence of fibrolite in the Euganean metapelitic xenoliths is related to the reaction temperature overstepping induced by the rapidity of incorporation in the trachytic melt. The re-equilibration of biotites at high temperature might have occurred in a too short time to allow complete preferred partitioning of Ti into the M2 site. This might have limited the extent of Ti off-center shift towards O4 and the proton loss.

[1] Sassi R., Mazzoli C., Spiess R., Cester T. (2004). J. Petrol., 45, 1467-1479. [2] Cesare B., Cruciani G., Russo U. (2003). Am. Miner., 88, 583-595. [3] Cruciani G. & Zanazzi P.F. (1994). Am. Miner, 79, 289-301.