Analysis of Mn-bearing lawsonite occurring in meta-siliceous rocks in Hakoishi serpentinite melange of Kurosegawa Belt, Central Kyushu, Japan

作者：Ibuki Masaru; Ohi Shugo^*; Tsuchiyama Akira; Hirajima Takao

来源：Journal of Mineralogical and Petrological Sciences, 2010, 105(6): 340-345.

DOI：10.2465/jmps.100615a

• Lawsonite
• Manganese meta-siliceous rocks
• Blueschist facies
• Kurosegawa Belt

Mn-bearing lawsonite was discovered in meta-siliceous rocks metamorphosed under the lawsonite-blueschist facies formed at less than 350 degrees C and 0.8-1.0 GPa in the Hakoishi serpentinite melange of the Kurosegawa Belt, Central Kyushu, Japan. The lawsonite deposits were accompanied by those of hematite, braunite (Mn2+Mn63+SiO12), and quartz, indicating that the rock had metamorphosed under high Oxygen fugacity (fo(2)) (similar to-20 < logfo(2) < -5) at the abovementioned P-T conditions. The Oxygen fugacity led to the conversion of all the iron into ferric compounds and some of the manganese into its trivalent form. As a result, the lawsonite was found to contain a significant amount of (Me3+ + Fe3+), substituting similar to 5-11 mol% of Al in the ideal lawsonite formula. Lawsonite in meta-siliceous rocks metamorphosed under low fo(2) state (similar to logfo(2) < -20), and metabasites collected from the same melange contained lesser amount of (Mn3+ + Fe3+), substituting similar to 3-5 mol%. The amount of Sr and Ba in the analyzed lawsonite were below the detection limit of energy-dispersive microprobe analysis, similar to <0.1 wt%. This indicated the existence of an unknown endmember of the lawsonite group of minerals, CaFe23+Si2O7(OH)(2)center dot H2O or CaMn23+Si2O7(OH)(2)center dot H2O on the basis of the observed substitution. The extremely high fo(2), state that results in the conversion of all the iron into ferric compounds along with the Mn-rich local chemical compositions would necessarily enhance (Mn3+ + Fe3+) <-> Al substitution in lawsonite.

• 出版日期2010-12