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

Abstract

Mn-bearing lawsonite was discovered in meta-siliceous rocks metamorphosed under the lawsonite-blueschist facies formed at less than 350 °C and 0.8-1.0 GPa in the Hakoishi serpentinite mélange 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 (f_O2) (∼ -20 < log f_O2 < -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 (Mn³⁺ + Fe³⁺), substituting ∼ 5-11 mol% of Al in the ideal lawsonite formula. Lawsonite in meta-siliceous rocks metamorphosed under low f_O2 state (∼ log f_O2 < -20), and metabasites collected from the same mélange contained lesser amount of (Mn³⁺ + Fe³⁺), substituting ∼ 3-5 mol%. The amount of Sr and Ba in the analyzed lawsonite were below the detection limit of energy-dispersive microprobe analysis, ∼ <0.1 wt%. This indicated the existence of an unknown endmember of the lawsonite group of minerals, CaFe₂³⁺Si₂O₇(OH)₂·H₂O or CaMn₂³⁺Si₂O₇(OH)₂·H₂O on the basis of the observed substitution. The extremely high f_O2 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.