Titanium in phengite: a geobarometer for high temperature eclogites

Abstract

Phengite chemistry has been investigated in experiments on a natural SiO 2 -TiO 2 -saturated greywacke and a natural SiO 2 -TiO 2 -Al 2 SiO 5 -saturated pelite, at 1.5-8.0 GPa and 800-1,050°C. High Ti-contents (0.3-3.7 wt %), Ti-enrichment with temperature, and a strong inverse correlation of Ti-content with pressure are the important features of both experimental series. The changes in composition with pressure result from the Tschermak substitution (Si ? R 2? = Al IV ? Al VI ) coupled with the substitution:The latter exchange is best described using the end-member Ti-phengite (KMgTi[Si 3 Al]O 10 (OH) 2 , TiP). In the rutile-quartz/coesite saturated experiments, the aluminoceladonite component increases with pressure while the muscovite, paragonite and Ti-phengite components decrease. A thermodynamic model combining data obtained in this and previous experimental studies are derived to use the equilibrium MgCel ? Rt = TiP ? Cs/Qz as a thermobarometer in felsic and basic rocks. Phengite, rutile and quartz/coesite are common phases in HT-(U)HP metamorphic rocks, and are often preserved from regression by entrapment in zircon or garnet, thus providing an opportunity to determine the T-P conditions of crystallization of these rocks. Two applications on natural examples (Sulu belt and Kokchetav massif) are presented and discussed. This study demonstrates that Ti is a significant constituent of phengites that could have significant effects on phase relationships and melting rates with decreasing P or increasing T in the continental crust.

High-pressure Tethyan Himalaya rocks along the India-Asia suture zone in southern Tibet

“…Alternatively, HP rock exhumation and changes in slab dynamics can occur in the absence of slab breakoff (Warren et al, 2008). Future, holistic studies of endmembers (Coggon and Holland, 2002;Connolly, 2009;Auzanneau et al, 2010). This formulation incorporates the Tschermak (celadonite) and pyrophyllite substitutions to minimize the uncertainty in pressure estimates from Si-in-phengite geobarometry.…”

“…These inconsistencies and the possibility of further unconstrained substitutions in white mica necessitate caution when estimating pressure conditions with phengite geobarometry alone. However, we minimized the uncertainty in this approach by modeling Si-in-phengite isopleths in a pseudosection using an updated white mica solution model adapted from Coggon and Holland (2002) and Auzanneau et al (2010) that accounts for the Tschermak and pyrophyllite substitutions and their dependence on bulk chemistry and mineral assemblage. To critically evaluate our pressure estimates, we compare the mineral assemblages preserved in Lopu Range samples with their predicted stability fields from pseudosection modeling.…”

Coesite and diamond inclusions, exsolution microstructures and chemical patterns in ultrahigh pressure garnet from Ceuta (Northern Rif, Spain)

“…Experimental data indicate that Ti solubility in garnet increases with both P and T (Hermann and Spandler, 2008;Kawasaki and Motoyoshi, 2007;. Auzanneau et al (2010), in experiments on a natural SiO 2 -TiO 2 -saturated greywacke and a natural SiO 2 -TiO 2 -Al 2 SiO 5 -saturated pelite, at 1.5-8.0 GPa and 800-1050°C noted that the garnet composition changed drastically with P, T and coexisting assemblage. Although Na is involved in the substitution, garnet is devoid of majoritic component, even at the highest P. Maximum Ti content was determined at 4 GPa, whereas the Ti content tends to decrease at higher P. In our Ti-rich garnet the compensatory mechanism would be the presence of vacancies in VIII, since alternative substitution mechanisms (Pertermann et al, 2004) are not possible with the obtained formula.…”

Polymetamorphic evolution of the granulite-facies Paleoproterozoic basement of the Kabul Block, Afghanistan

“…This system includes TiO 2 and O 2 which allow the consideration of ilmenite within the peak assemblages and have been shown to affect the stability of biotite at high temperatures (Tajčmanová et al 2009). The solution models used for these calculations were: orthopyroxene, clinopyroxene (Powell and Holland 1999) garnet, cordierite (Holland and Powell 1998), feldspar (Benisek et al 2010), white mica (Auzanneau et al 2010 andCoggon andHolland 2002), ilmenite (White et al 2000), biotite (Tajčmanová et al 2009), and silicate-melt (White et al 2007). …”