13:45 〜 15:15
[MIS13-P04] On staurolite as a geochemical tracer: A preliminary report from middle-pressure metapelites
キーワード：十字石、泥質変成岩、リチウム同位体、微量元素、中圧型変成作用
Staurolite in metapelites is an index mineral to define a mineral zone along with the metamorphic field gradient of the medium-pressure metamorphic terranes (i.e., staurolite zone: Barrow, 1893). Staurolite exhibits some unique features with respect to Li and Li isotopes. It has been known that staurolite is enriched in Li over coexisting minerals (Dutrow et al., 1986). Although most silicate minerals incorporate Li into the octahedral site, staurolite incorporates it into the tetrahedral site (Dutrow, 1991). Based on these characteristics of Li in staurolite, an experimental study defined Li isotope fractionation between staurolite and aqueous fluid (Wunder et al., 2007). The study showed that 7Li prefers staurolite compared to the fluid (Δ7LiSt–fluid [= δ7Listaurolite – δ7Lifluid] > 0). This result for staurolite contrasts with that for other most silicate minerals (Δ7Limineral–fluid < 0). Despite these unique Li isotope features of staurolite, there is no systematic study on the Li isotope composition of staurolite in natural rocks. In this contribution, we show trace elements and Li isotope characteristics of staurolite and coexisting minerals in middle-pressure metapelites from six localities: Unazuki Schist, Hida Mountains, Japan; Takanuki metamorphic rock, Abukuma Mountains, Japan; Contact aureole of the Bushveld Igneous Complex, South Africa; Littleton Formation Schist, New Hampshire, USA; Gassetts Schist, Vermont, USA; Eastern Tibetan Plateau, China).
The staurolite from six different localities shows high concentrations of Li (~92–913 µg/g), Zn (~223–4350 µg/g), and Ga (~42–200 µg/g). Overall, staurolite contains a very small amount of rare earth elements and does not have any clear correlation between the two elements. The high Li, Zn, and Ga concentrations are consistent with previous studies (Dutrow et al., 1986; Hammerli et al., 2016; Hietanen, 1969). Regardless of the localities, the δ7Li value increases in the order of staurolite > biotite > garnet. The apparent inter-mineral Li isotope fractionation value between staurolite and garnet (Δ7LiSt–Grt) is larger than that between staurolite and biotite (Δ7LiSt–Bt). Based on the crystallographic theory that the heavy 7Li generally tends to be incorporated into a mineral with a lower Li coordination site in its structure, the orders of the δ7Li (staurolite > biotite > garnet) and the Δ7Li (Δ7LiSt–Grt > Δ7LiSt–Bt) values can be explained by the different Li coordination numbers in staurolite, biotite, and garnet. Both the Δ7LiSt–Grt and Δ7LiSt–Bt values do not correlate with the estimated metamorphic temperature although the isotope fractionation between two phases generally shows temperature dependence.
Although the Li in staurolite in metapelites seems to be highly controlled by bulk-rock compositions (see another abstract of Iwaki et al. in this conference), some common geochemical trends serve as premises for more detailed investigation. Especially geochemical characterization of staurolite that formed in different bulk-rock compositions (e.g., aluminous meta-igneous rocks) and different metamorphic conditions (e.g., high- to ultrahigh-pressure metamorphism and ultrahigh-temperature metamorphism) is required.
Reference
Barrow, 1893. Quart. J. Geol. Soc. London 49, 330–358. https://doi.org/10.1144/GSL.JGS.1893.049.01-04.52
Dutrow, 1991. Am. Mineral. 76, 42–48.
Dutrow et al., 1986. Contrib. Mineral. Petrol. 94, 496–506. https://doi.org/10.1007/BF00376341
Hammerli et al., 2016. Contrib. Mineral. Petrol. 171, 36. https://doi.org/10.1007/s00410-016-1239-7
Hietanen, 1969. Am. J. Sci. 267, 422–456. https://doi.org/10.2475/ajs.267. 3.422
Wunder et al., 2007. Chem. Geol. 238, 277–290. https://doi.org/10.1016/j.chemgeo.2006.12.001
The staurolite from six different localities shows high concentrations of Li (~92–913 µg/g), Zn (~223–4350 µg/g), and Ga (~42–200 µg/g). Overall, staurolite contains a very small amount of rare earth elements and does not have any clear correlation between the two elements. The high Li, Zn, and Ga concentrations are consistent with previous studies (Dutrow et al., 1986; Hammerli et al., 2016; Hietanen, 1969). Regardless of the localities, the δ7Li value increases in the order of staurolite > biotite > garnet. The apparent inter-mineral Li isotope fractionation value between staurolite and garnet (Δ7LiSt–Grt) is larger than that between staurolite and biotite (Δ7LiSt–Bt). Based on the crystallographic theory that the heavy 7Li generally tends to be incorporated into a mineral with a lower Li coordination site in its structure, the orders of the δ7Li (staurolite > biotite > garnet) and the Δ7Li (Δ7LiSt–Grt > Δ7LiSt–Bt) values can be explained by the different Li coordination numbers in staurolite, biotite, and garnet. Both the Δ7LiSt–Grt and Δ7LiSt–Bt values do not correlate with the estimated metamorphic temperature although the isotope fractionation between two phases generally shows temperature dependence.
Although the Li in staurolite in metapelites seems to be highly controlled by bulk-rock compositions (see another abstract of Iwaki et al. in this conference), some common geochemical trends serve as premises for more detailed investigation. Especially geochemical characterization of staurolite that formed in different bulk-rock compositions (e.g., aluminous meta-igneous rocks) and different metamorphic conditions (e.g., high- to ultrahigh-pressure metamorphism and ultrahigh-temperature metamorphism) is required.
Reference
Barrow, 1893. Quart. J. Geol. Soc. London 49, 330–358. https://doi.org/10.1144/GSL.JGS.1893.049.01-04.52
Dutrow, 1991. Am. Mineral. 76, 42–48.
Dutrow et al., 1986. Contrib. Mineral. Petrol. 94, 496–506. https://doi.org/10.1007/BF00376341
Hammerli et al., 2016. Contrib. Mineral. Petrol. 171, 36. https://doi.org/10.1007/s00410-016-1239-7
Hietanen, 1969. Am. J. Sci. 267, 422–456. https://doi.org/10.2475/ajs.267. 3.422
Wunder et al., 2007. Chem. Geol. 238, 277–290. https://doi.org/10.1016/j.chemgeo.2006.12.001