Stability field of the high-pressure assemblage talc + phengite and two new phengite barometers
Massonne, Hans-Joachim; Schreyer, Werner
European Journal of Mineralogy Volume 1 Number 3 (1989), p. 391 - 410
published: Jul 27, 1989
manuscript accepted: Feb 24, 1989
manuscript received: Jul 28, 1988
ArtNo. ESP147050103010, Price: 29.00 €
Abstract In the system K2O-MgO-Al2O3-SiO2-H2O (KMASH) the high-temperature and low-pressure stability limits for the assemblage talc + phengite have been investigated by reversed experiments at pressures between 9 and 25 kbar, bracketing the reactions: talc + phengite = phlogopite + kyanite + quartz + H2O chlorite + phlogopite + quartz = talc + phengite ± H2O The univariant curve of the latter reaction has a dP/dT slope of about 0, whereas the curve of the other reaction shows an almost constant slope of 85 bar/°C. The curves intersect at an invariant point at 10.8 kbar and 610 °C. In addition, the compositional variability of phengite coexisting with talc + kyanite + quartz/coesite or with talc + phlogopite + quartz has been studied experimentally in the KMASH-system by synthesis runs in the range 15 to 40 kbar and 550 to 800 °C resulting in two new phengite barometers. Using the Si content p.f.u. of phengite as a chemical parameter, the following equations have been derived: Si = 0.02146 P - 0.0001904 T + 3.025 and Si = 0.00809 P - 0.0000523 T + 3.298 (P in kbar, T in °C), for phengite of the limiting assemblage with talc + kyanite + quartz or talc + kyanite + coesite, respectively (tks-phengite barometer). In phengite coexisting with talc + phlogopite + quartz (tps-phengite barometer), the Si content also increases with rising synthesis pressure. However, the corresponding Si isopleths have dP/dT slopes steeper than those of phengite coexisting with talc + kyanite + quartz. From the available experimental data, a topology for the system K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) is derived for medium-grade metamorphic conditions between 5 to 18 kbar considering chloritoid and garnet in addition to the KMASH phases. The most important features of the KFMASH topology are: (1) the stability fields of chloritoid and of biotite + kyanite do not overlap in the presence of phengite, quartz and H2O ; (2) in the presence of garnet, the low-pressure limit of talc + phengite is shifted towards pressures of about 12.5 kbar compared to about 11 kbar in the KMASH-system ; (3) the high-temperature limit of talc + phengite lies at temperatures about 30 °C lower than in the KMASH - system. The proposed KMASH topology is confirmed by assemblages found in metapelitic high-pressure rocks from various localities. The Si content of phengite was used to estimate metamorphic pressures for natural rocks exhibiting the limiting assemblages studied here. The pressure estimates obtained agree fairly well with those of the previous investigators.