Alkali in phlogopite and amphibole and their effects on phase relations in metasomatized peridotites: a high-pressure study |
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| Authors: | P Fumagalli S Zanchetta S Poli |
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| Institution: | 1.Dipartimento di Scienze della Terra,Università degli Studi di Milano,Milan,Italy;2.Dipartimento di Scienze Geologiche e Geotecnologie,Università degli Studi di Milano Bicocca,Milan,Italy |
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| Abstract: | Subsolidus phase relations for a K-doped lherzolite are investigated in the model system K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O 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]XIISi1K?1Al ?1 IV , where v]XII is interlayer vacancy). Mixed layering or solid solution relations between high-pressure phlogopites and the 10 Å phase, Mg3Si4O10(OH)2 nH2O, 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. |
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