An experimental investigation on the P-T stability of Mg-staurolite in the system MgO-Al2O3-SiO2-H2O |
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Authors: | Thomas Fockenberg |
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Institution: | Ruhr-Universit?t Bochum, Institut für Mineralogie, D-44780 Bochum, Germany Tel.: ++49/0234/700-4392; Fax: ++49/0234/7094-433 e-mail: thomas.fockenberg@ruhr-uni-bochum.de, DE
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Abstract: | The pressure-temperature stability field of Mg-staurolite, ideally Mg4Al18Si8O46(OH)2, was bracketed for six possible breakdown reactions in the system MgO-Al2O3-SiO2-H2O (MASH). Mg-staurolite is stable at water pressures between 12 and 66 kbar and temperatures of 608–918 °C, requiring linear
geotherms between 3 and 18 °C/km. This phase occurs in rocks that were metamorphosed at high-pressure, low-temperature conditions,
e.g. in subducted crustal material, provided they are of appropriate chemical composition. Mg-staurolite is formed from the
assemblage chlorite + kyanite + corundum at pressures <24 kbar, whereas at pressures up to 27 kbar staurolite becomes stable
by the breakdown of the assemblage Mg-chloritoid + kyanite + corundum. Beyond 27 kbar the reaction Mg-chloritoid + kyanite + diaspore
= Mg-staurolite + vapour limits the staurolite field on its low-temperature side. The upper pressure limit of Mg-staurolite
is marked by alternative assemblages containing pyrope + topaz-OH with either corundum or diaspore. At higher temperatures
Mg-staurolite breaks down by complete dehydration to pyrope + kyanite + corundum and at pressures below 14 kbar to enstatite +
kyanite + corundum. The reaction curve Mg-staurolite = talc + kyanite + corundum marks the low-pressure stability of staurolite
at 12 kbar. Mg-staurolite does not coexist with quartz because alternative assemblages such as chlorite-kyanite, enstatite-kyanite,
talc-kyanite, pyrope-kyanite, and MgMgAl-pumpellyite-kyanite are stable over the entire field of Mg-staurolite.
Received: 16 April 1997 / Accepted: 24 September 1997 |
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