Alkali in phlogopite and amphibole and their effects on phase relations in metasomatized peridotites: a high-pressure study

• Published in: Contributions to mineralogy and petrology Vol. 158; no. 6; pp. 723 - 737
• Main Authors: Fumagalli, P., Zanchetta, S., Poli, S.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.12.2009; Springer Nature B.V
• Subjects: Earth and Environmental Science; Earth Sciences; Geology; Low temperature; Mineral Resources; Mineralogy; Original Paper; Petrology; Ca-amphibole; K-doped lherzolites; Metasomatism; Phlogopite; Subduction zones; 10 Å phase
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-009-0407-4

Subsolidus phase relations for a K-doped lherzolite are investigated in the model system K 2 O–Na 2 O–CaO–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O at 1.5–6.0 GPa and 680–1,000°C. Phlogopite is ubiquitous and coexists with Ca-amphibole up to 3.2 GPa and 900°C. High-pressure phlogopites show a peculiar mineral chemistry dependent on pressure: e.g., at 5.5 GPa and 680°C, excess of Si (up to 3.4 apfu) coupled with deficiency in Al (as low as 0.58 apfu) and K + Na (as low as 0.97 apfu), suggest a significant amount of a talc/10 Å phase component ([ v ] XII Si 1 K −1 Al −1 IV , where [ v ] XII is interlayer vacancy). Mixed layering or solid solution relations between high-pressure phlogopites and the 10 Å phase, Mg 3 Si 4 O 10 (OH) 2 n H 2 O, are envisaged. Phlogopite modal abundance, derived by weighted least squares, is maximum at high-pressure and relative low-temperature conditions and therefore along the slab–mantle interface (10.3 ± 0.7 wt.%, at 4.8 GPa, 680°C). In phlogopite-bearing systems, Ca-amphibole breaks down between 2.5 and 3.0 GPa, and 1,000°C, through the water conservative reaction 5(pa + 0.2 KNa −1 ) + 17en + 15phl = (10di + 4jd) + 5py + 12fo + 20(phl + 0.2 talc ), governed by bulk composition and pressure-dependent variations of K/OH in K-bearing phases and as a result, it does not necessarily imply a release of fluid.