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The upper thermal stability of chlorite + quartz: an experimental study in the system MgO‐Al 2 O 3 ‐SiO 2 ‐H 2 O
Author(s) -
MASSONNE H.J.
Publication year - 1989
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1989.tb00619.x
Subject(s) - talc , kyanite , cordierite , chlorite , quartz , mineralogy , analytical chemistry (journal) , enstatite , geology , materials science , chemistry , ceramic , metallurgy , chromatography , physics , meteorite , astronomy , chondrite
Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H 2 O, chlorite + quartz = talc + cordierite + H 2 O, and talc + kyanite + quartz = cordierite ± H 2 O by reversed runs in the system MgO‐Al 2 O 3 ‐SiO 2 ‐H 2 O (MASH). These reaction curves intersect at an invariant point (IP1) at P H 2 O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H 2 O at water pressures above 6.4 kbar shows a negative dP/dT , with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H 2 O at water pressures is clearly positive. The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg 4.85 Al 1.15 [Al 1.15 Si 2.85 O 10 ](OH) 8 over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg 2.94 Al 0.06 [Al 0.06 Si 3.94 O 10 ](OH) 2 at P H 2 O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit at P ,H 2 O = 21 kbar. As a consequence of the new experimental data, the existing phase topologies of the MASH‐system and K 2 O‐MASH‐system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H 2 O = chlorite + cordierite are stable. For this reason, the development of medium‐ to high‐temperature metamorphic rocks compositionally approximating the MASH‐system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe 2+ into magnesian chlorite extends its stability field in the presence of quartz by 10°‐15°C in comparison with pure Mg‐chlorite.