Analysis of Mn-Bearing Lawsonite Occurring in Meta-Siliceous Rocks in Hakoishi Serpentinite Mélange of Kurosegawa Belt, Central Kyushu, Japan

340 Journal of MineralogicalM. Ibuki, and S. Petrological Ohi, A. Tsuchiyama Sciences, and Volume T. Hirajima 105, page 340─ 345, 2010

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

* ** ** * Masaru Ibuki , Shugo Ohi , Akira Tsuchiyama and Takao Hirajima

*Department of Geology and Mineralogy, Division of Earth and Planetary Sciences, Graduate school of Science, Kyoto University, Kyoto 606-8502, Japan **Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan

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

2+ 3+ - (Mn Mn6 SiO12), and quartz, indicating that the rock had metamorphosed under high Oxygen fugacity (fO ) (~ 20 2 - - < log fO < 5) at the abovementioned P T conditions. The Oxygen fugacity led to the conversion of all the iron 2 into ferric compounds and some of the manganese into its trivalent form. As a result, the lawsonite was found to 3+ 3+ contain a significant amount of (Mn + Fe ), substituting ~ 5-11 mol% of Al in the ideal lawsonite formula.

- - Lawsonite in meta siliceous rocks metamorphosed under low fO state (~ log fO < 20), and metabasites collect- 2 2 3+ 3+ ed from the same mélange contained lesser amount of (Mn + Fe ), 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, 3+ 3+ CaFe2 Si2O7(OH)2∙H2O or CaMn2 Si2O7(OH)2∙H2O on the basis of the observed substitution. The extremely

- high fO state that results in the conversion of all the iron into ferric compounds along with the Mn rich local 2 chemical compositions would necessarily enhance (Mn3+ + Fe3+) ↔ Al substitution in lawsonite.

Keywords: Lawsonite, Manganese meta-siliceous rocks, Blueschist facies, Kurosegawa Belt

INTRODUCTION in the ideal composition mentioned above (e.g., Deer et al., 1992). Therefore, we can determine the decomposi- It is widely accepted that subducting slabs contain hy- tion depths of lawsonite and the amount of dehydrated drous minerals that release H2O in deep subduction zones fluids released from it by combining thermodynamic cal- (>10 km depth) through dehydration reactions involving culations (e.g., Schmidt and Poli, 1998) and model geo- transformation of hydrous assemblages into less hydrous therms in the subduction zone (e.g., Peacock and Wang, or anhydrous assemblages along with the production of + 1999).

- H2O (e.g., Schmidt and Poli, 1998). Among hydrous min- Recently, we discovered a unique lawsonite in meta erals that occur naturally in subducting hydrated slabs, siliceous rocks found in the Hakoishi serpentinite mé- lawsonite, with an ideal formula of CaAl2Si2O7(OH)2H2O lange of the Kurosegawa Belt, Central Kyushu, Japan. can serve as a source of a considerable amount of H2O The lawsonite was opaque and reddish when observed (~ 12 wt% per mole) at up to ~ 5 GPa and at ~ 400-500 under plane polarized light. The lawsonite was studied by °C through its decomposition reactions (e.g., Evans, 1990; both microprobe analysis and X-ray diffraction. These Schmidt and Poli, 1998). In fact, lawsonite is one of the analyses indicated the existence of an unknown endmem- 3+ diagnostic minerals of blueschist/eclogite facies formed in ber of the lawsonite group of minerals, CaFe2 Si2O7(OH)2∙H2O 3+ cold subduction zones (e.g., Hacker et al., 2003). Further- or CaMn2 Si2O7(OH)2∙H2O. In this study, we report the more, most of the lawsonite in blueschist typically occurs mode of occurrence, chemical composition, and X-ray doi:10.2465/jmps.100615a diffraction data of this endmember and discuss its miner- S. Ohi, [email protected] Corresponding author alogical significance. Analysis of Mn-bearing lawsonite from a meta-siliceous rock 341

The abbreviated mineral names used below are layers mainly consisted of braunite; the red layers consist- mainly according to Kretz (1983) and Whitney and Evans ed of the unique lawsonite, quartz, hematite, and braunite (2010), except for braunite, which is abbreviated as Br. (up to 50 μm in diameter) with trace amounts of apatite, titanite, albite and K-feldspar (Fig. 1b). The unique law- GEOLOGICAL OUTLINE AND sonite was opaque and reddish under plane polarized SAMPLE LOCALITY light, but microprobe analysis and X-ray diffraction proved that it belonged to the lawsonite group of miner- The studied sample was collected from the Hakoishi ser- als. Backscattered images clearly showed that the unique 2 pentinite mélange, exposed in a 2 × 10 km area with NE- lawsonite occurred as fan-shaped polycrystalline aggre- SW trend, from the Kurosegawa belt in the Yatsushiro gates consisting of columnar crystals with abundant fine- area, Kumamoto, Japan (Ibuki et al., 2008). Our studies grained inclusions of sub-micron-sized hematite and suggested that the lithotype of the Hakoishi serpentinite braunite (Figs. 1b and 1c). mélange primarily consisted of lawsonite-blueschist and Electron microscope analysis of lawsonite was car- metasiliceous rocks with trace amounts of serpentinite ried out carefully while avoiding the fine-grained opaque and metagabbro, although Saito et al. (2005) noted that inclusions (Figs. 1b and 1c) using a Hitachi S3500H scan- the major lithotype is serpentinite. However, our study re- ning electron microscope equipped with EDAX®, an ener- veals that the serpentinite was mainly recognized at unit gy-dispersive X-ray analytical system at the Kyoto Uni- boundaries along both in the northern and southern edges versity. The accelerating voltage and beam current were of the Hakoishi mélange, but is was scarce within the mé- maintained at 20 kV and 500 pA, respectively. Both natu- lange. We concluded that the intercalation between the ral and synthetic materials were used as standards, and lawsonite-blueschist and meta-siliceous rocks in the Ha- ZAF correction was carried out for the X-ray intensity koishi serpentinite mélange led to the formation of a 10- data processing. km long coherent block (Ibuki et al., 2008). The braunite in the OT19 sample is expected to in- Lawsonite-blueschist mainly consists of lawsonite, duce excessive oxidation at least during the blueschist fa- sodic amphibole, chlorite, quartz, and carbonate along cies metamorphism, under which manganese would either with small amounts of titanite, clinopyroxene, phengite, occur in divalent or trivalent form (e.g., Fig. 15 of Motta- albite, apatite, pumpellyite, and iron sulfides. The peak na, 1986). It is well known that only a small amount of pressure-temperature (P-T) conditions for the lawsonite- iron, typically occurring in the form of ferric compounds, blueschist were estimated to be 0.7-0.9 GPa and <350 °C is contained in the lawsonite as mentioned in the literature on the basis of the common occurrence of lawsonite + so- (e.g., Deer et al., 1992: Baur, 1978). Therefore, all iron dic amphibole + chlorite assemblage (Brown, 1977; Liou and manganese found in lawsonite was tentatively consid- et al., 1985; Evans, 1990). The lawsonite-blueschist and ered to be trivalent for EPMA data in this study. The metasiliceous rocks are almost devoid of later-stage over- unique lawsonite in the OT19 sample contained a signifi- printing; therefore they retain the mineral assemblage at cant amount of Mn3+, ranging from 0.10 to 0.20 with an the peak P-T conditions. average of 0.14 for 8-oxygen basis, although lawsonite in Meta-siliceous rocks in the study area exhibit a vari- another meta-siliceous rock sample, OT9B, which con- ety of colors such as white, red, pink, pale green, pale yel- tained hematite but no braunite, contained a scarce low, dark brown, and black, depending on the modal amount of Mn3+, ranging from 0.05 to 0.07 with an aver- amount of their constituent minerals (Ibuki et al., 2010). age of 0.05 for 8-oxygen basis (Table 1). Lawsonite in The details of the composition are not presented in this metabasites from the same mélange is almost devoid of report. Therefore, the following description is solely about Mn (Table 1). This reflects the scarcity of the total amount a sample, OT19, containing the unique lawsonite. of manganese in basaltic rocks relative to the amount of manganese in meta-siliceous rocks. 3+ 3+ PETROGRAPHY AND MINERALOGY The Al-(all Fe + all Mn ) variation diagram (for 8-oxygen basis, Fig. 2) shows a plot of the occurrence of The unique lawsonite was discovered in a sample, OT19 the lawsonite in meta-siliceous rocks along a line of (Al + 3+ 3+ that had intercalated black and red layers having a thick- all Fe + all Mn ) = 2.0, corresponding to a tie-line 3+ ness of the order of a few millimeters in a pinkish matrix among ideal lawsonite, ideal hennomartinite [SrMn2 Si2 - 3+ (Fig. 1a). The matrix mainly consisted of quartz, Na py- O7(OH)2∙H2O], and noélbensonite [BaMn2 Si2O7(OH)2∙

- roxene, braunite, and hematite with trace amounts of ti- H2O]. However, the plot deviates from a tie line between tanite and apatite. The matrix had a weak foliation defined ideal lawsonite and ideal braunite. Sr and Ba were care- by the arrangement of braunite and hematite. The black fully identified in the analyzed lawsonite. Even a minute 342 M. Ibuki, S. Ohi, A. Tsuchiyama and T. Hirajima

Figure 1. (a) Polished slab of reddish lawsonite (Lws)-bearing meta-siliceous rock (OT19). Red layer is mainly composed of quartz, Lws, he- matite (Ht), and braunite (Br). (b) Microphotograph of reddish Lws in a red layer. Plane polarized light. (c) Backscattered image of reddish Lws is shown in a green box in Figure 1b. Scale bars are shown in each figure. Abbreviations: Ap, apatite; Tit, titanite; Phn, Phengite; Qtz, quartz; Ab, albite.

- Sr Kα peak at 14 kV was not visible and the minute peaks 0.01°, and the range of diffraction angles (2θ) was 8.00

- - - around 4.5 5.0 kV better fit Ti Kα/Kβ as opposed to Ba L 155.00°. The peak profile of OT19 showed diffraction lines. Therefore, the amount of Sr and Ba in the studied patterns of quartz, hematite, albite, lawsonite, and small lawsonite can be considered to be below the detection amounts of mica and chlorite. The highest X-ray peak limit of microprobe analysis using the energy-dispersive was indexed by quartz (101). Hematite (104), albite (002), X-ray detector. and lawsonite (114), which are the strongest indices of In addition to EPMA analysis, X-ray diffraction each mineral, showed reflections with intensities of I = analysis of the powdered-rock samples OT19 and OT9B 17, I = 6, and I = 4, respectively, when the quartz (101) was carried out using the multiple-detector system of intensity was normalized as I = 100. The peak profile of

- Toraya et al. (1996) at the BL 4B2 beam line of the Pho- OT9B showed the diffraction patterns of quartz, hematite, ton Factory at the High Energy Accelerator Research Or- lawsonite, and small amounts of mica and chlorite. Hema- ganization in Japan for the determination of unit cell di- tite (104) and lawsonite (114) showed reflections with in- mensions of lawsonite. The wavelength of the X-ray tensities of I = 10 and I = 7, respectively, when the quartz beam was 1.196990 Å, the step interval for scanning was (101) intensity was normalized as I = 100. The X-ray dif- Analysis of Mn-bearing lawsonite from a meta-siliceous rock 343

Table 1. Representative microprobe data of lawsonite (Lws) and braunite (Br)

All iron was assumed as ferric, because of the association with braunite. n.d., not determined. Mn2+/Mn3+ ratio of braunite was evaluated as total cations = 8 from EPMA data.

Figure 3. X-ray-powder diffraction patterns of OT19 and OT9B for the 2θ range of 25-29°. The diffraction peaks are labeled with 3+ 3+ Miller indices; Ht, hematite; Qtz, quartz, and other Miller indices Figure 2. Al-all (Mn + Fe ) (for 8-oxygen basis) plot for ana- lyzed lawsonite and braunite along with the ideal compositions without abbreviations refer to lawsonite from OT19 (solid line) of lawsonite (Lws), quartz (Qtz), hennomartinite (Hen), ilvaite and OT9B (light solid line). The 2θ. positions for representative (Ilv), and braunite (Br). Miller indices of synthetic lawsonite obtained by Pawley et al. (1996) are indicated by vertical dotted lines. fraction patterns for the 2θ. range between 25° and 29° and (221) for both samples are shown in Fig 3. For com- are shown in Figure 3. parison, the same diffraction indices calculated from the Lawsonite (114), (022), (204), (310), (312), (220), data for synthetic lawsonite (Pawley et al., 1996) are indi- 344 M. Ibuki, S. Ohi, A. Tsuchiyama and T. Hirajima

- cated by a vertical dotted line in Figure 3. Lawsotnite dif- ganilvaite have P lattice structure of Pbnm and P21/a, re- fractions with large l indices, such as (114) and (204), spectively. were found to have similar 2θ positions in OT19 and However, lawsonite with trace amounts of iron was OT9B (Pawley et al., 1996), whereas those with large k known to exist in blueschists all over the world, e.g., Da- indices, such as (022), (220) and (221), have distinctly vis and Pabst (1960). Oxygen fugacity (fO ) plays an im- 2 different 2θ positions from each other. portant role in the substitution as described below. Motta- The unit-cell dimensions of the lawsonite in OT19 na (1986) modified an oxidation state diagram proposed were calculated to be a = 8.802(2) Å, b = 5.8781(9) Å, by Abs-Wurmbach et al. (1983), which concerns the sta- and c = 13.123(2) Å and those in OT9B were calculated bility field of minerals containing Mn4+, Mn3+, Mn2+, Fe3+, 2+ to be a = 8.799(1) Å, b = 5.8641(5) Å, and c = 13.1241(8) and Fe at around 300-600 °C at 10 kbar. According to

- Å by least squares fitting using the PDIndexer software this diagram, f O conditions for braunite stability are de- 2 program developed by Y. Seto (Kobe University; URL; fined by following two reactions: http://www2.kobe-u.ac.jp/~seto/). There is a significant

- difference in the cell dimensions along the b axis for the 7 pyrolusite + quartz = braunite + O2 (1), two samples, i.e., the cell dimension along the b-axis for OT19 (Mn-rich type) is slightly larger than that for OT9B 2 braunite = 4 hausmannite

- - (less Mn type), although the cell dimensions along the a + 2 rhodonite/pyroxmangite + O2 (2), and c-axes are almost similar. Comparing these cell di-

- - mensions with those of synthetic lawsonite obtained by and the range is 20 < log fO < 5 at around 300 °C. In 2 Pawley et al. (1996), i.e., a = 8.790(1) Å, b = 5.840(1) Å such a highly oxidized state, all the iron is converted to

- and c = 13.133(2) Å, the difference between the cell di- ferric iron. Ferrous iron can occur at log fO < 35 at 2 mensions along the a- and c-axes for the lawsonite in this around 300 °C. study and those for the synthetic one is relatively small Lawsonite with significant amount of (Mn3+ + Fe3+) (~ 0.01 Å ), but the dimensions along the b-axis for the in sample OT19 was in close association with braunite two natural samples are significantly larger than those for and hematite, as mentioned above (Fig. 1c). Therefore, the synthetic one. A scarce but significant amount of the replacement by ilvaite or manganilvaite component in 3+ 3+ Mn -Al substitution in lawsonite would affect the such lawsonite is unlikely because they contain ferrous 3+ length of the cell dimensions along the b-axis. iron in the ideal formula. Mn -bearing lawsonite group Microprobe data for braunite in sample OT19 are endmembers, hennomartinite and noélbensonite, contain also shown in Table 1. All iron was tentatively considered Sr or Ba in the divalent cation sites, and Mn3+ replaces Al to be as trivalent, and considering the total number of cat- (Armbruster et al., 1993: Kawachi et al., 1996). However, ions, the Mn2+/Mn3+ ratio was evaluated to be 8.0 from the the studied lawsonite is almost devoid of Sr and Ba (Table EPMA data in this study. Although the total wt% is less 1). Hence, the presence of a significant amount of Mn3+ than 100 wt%, the obtained formula is mainly that of and Fe3+ by the substitution of the hennomartinite/noél- braunite. This may be caused by weathering. A small bensonite component is highly unlikely in the lawsonite amount of trivalent Mn is substituted with ferric iron sample OT19. Hypothesizing the existence of an un- 3+ 3+ 3+ [Fe /(Al + Mn + Fe ) = 0.05-0.28], and a small amount known endmember of the lawsonite group of minerals 2+ 3+ 3+ of divalent Mn is also substituted with Ca [Ca/(Ca + Mn like Ca(Mn , Fe )2Si2O7(OH)2∙H2O is one explanation. + Mg) = 0.05-0.15]. This is yet to be confirmed. As mentioned above, high oxidation states would DISCUSSION strongly influence the Mn + Fe content in lawsonite found in meta-siliceous rocks, e.g., 5-11 mol% in a braunite- It is considered that lawsonite, with a space group of bearing sample (OT19) and 3-5 mol% in a sample with- Ccmm, typically occurs with a composition similar to the out braunite (OT9B). Furthermore, bulk chemical compo- ideal composition (e.g., Deer et al., 1992). The lawsonite- sitions are also expected to have a strong effect on them. 3+ ilvaite group of minerals, hennomartinite [SrMn2 Si2O7 The bulk chemical compositions and Fe/Mn-content in (OH)2∙H2O], itoigawaite [SrAl2Si2O7(OH)2∙H2O], noél- meta-siliceous rocks are mostly derived from the original 3+ 2+ 3+ bensonite [BaMn2 Si2O7(OH)2∙H2O], ilvaite [CaFe2 Fe compositions of Fe/Mn nodules mainly composed of 2+ 2+ 2+ 3+ 4+ - Si2O7O(OH)] and manganilvaite CaFe (Mn , Fe )Fe buserite [RO∙6Mn O2∙3 4H2O: R = Ca, Mg, Cu, Ni, Zn, 4+ 3+ Si2O7O(OH) are known as endmembers (Barthelmy, 2Na], vernadite [(Mn , Fe , Ca, Na)(O, OH)2∙nH2O], and

4+ 3+ - 2007). Among these, hennomartinite, itoigawaite and noé- todorokite [(Na, Ca, K)2(Mn , Mn )6O12∙3 4.5H2O] (In- lbensonite have a space group of Ccmm. Ilvaite and man- ternational Mineralogical Association does not authorize Analysis of Mn-bearing lawsonite from a meta-siliceous rock 345 the use buserite, Usui, 2002) along with the mixing ratio 123-136. between Fe/Mn nodules and siliceous ooze during the Davis, G.A. and Pabst, A. (1960) Lawsonite and pumpellyite in glaucophane schist, north Berkeley Hills, California. Ameri- subduction process. In fact, the abundant fine-grained can Journal of Science, 258, 689-704. quartz aggregates with an ellipsoidal shape, ca. less than 1 Deer, W.A., Howie, R.A. and Zussman, J. (1992) An Introduction nd mm in long dimension, in the pinkish matrix of sample to the Rock-Forming Minerals (2 Edition), Longman Pub OT19 explain the origin of the siliceous ooze in the stud- Group. ied sample. Although it is difficult to determine the bulk Evans, B.W. (1990) Phase relations of epidote-blueschists. Lithos, 25, 3-23. composition of the meta-siliceous rocks found in the Hacker, B.R., Abers, G.A. and Peacock, S.M. (2003) Subduction study area, the composition of the various intercalated factory 1. Theoretical mineralogy, densities, seismic wave colored layers makes the Mn content in sample OT19 ap- speed, and H2O contents. Journal of Geophysical Research, pear higher than that in sample OT9B. This can be attrib- 108, B1, 2029, doi:10.1029/2001JB 001127. uted to the fact that the major opaque minerals in OT19 Ibuki, M., Fujimoto, Y., Takaya, M., Miyake, A. and Hirajima, T. (2008) Howieite in meta-manganese siliceous rocks of Kuro- are both hematite and braunite, but only hematite in segawa belt, western Kyushu, Japan. Journal of Mineralogi- OT9B. These abovementioned facts suggest that the high- cal and Petrological Sciences, 103, 365-370. - er fO state and Mn rich local chemical compositions 2 Ibuki, M., Ohi, S., Tsuchiyama, A. and Hirajima, T. (2010) H2O 3+ 3+ would necessarily enhance (Mn + Fe ) ↔ Al substitu- storages in meta-manganese siliceous rocks of the lawsonite tion in the lawsonite crystalline structure. blueschist facies. Abstract of annual meeting of JpGU, SMP055-05, Chiba, Japan, 2010. Kawachi, Y., Coombs, D.S. and Miura, H. (1996) Noélbensonite. ACKNOWLEDGMENTS A new BaMn silicate of the lawsonite structure type, from Woods mine, New South Wales, Australia. Mineralogical This is a part of the master thesis of the first author from Magazine, 60, 369-374. Kyoto University. He would like to express sincere thanks Kretz, R. (1983) Symbols for rock-forming minerals. American - to the staff and students of the Earth Material Science Mineralogist, 68, 277 279. Liou, J. G., Maruyama, S. and Cho, M. (1985) Phase equilibria Group of Kyoto University. The authors are also deeply and mineral parageneses of metabasite in low-grade meta- indebted to Mr. Y. Fujimoto for a kind offer of his sample morphism. Mineralogical Magazine, 49, 321-333. collection and to Prof. A. Yoshiasa, and the anonymous Mottana, A. (1986) Blueschist-facies metamorphism of manganif- reviewer for their constructive comments that helped im- erous cherts: A review of the alpine occurrences. Geological - prove the earlier draft. This study was partly supported by Society of America Memoir, 164, 267 299. Pawley, A.R., Redfern, S.A.T. and Holland, T.J.B. (1996) Volume - the Japan Society for the Promotion of Science (Grant in behavior of hydrous minerals at high pressure and tempera- Aid for Science Research Nos. 17204047 and 21109004) ture: I. Thermal expansion of lawsonite, zoisite, clinozoisite st and both the 21 century COE project (KAGI21) and the and diaspore. 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