10:45 〜 12:15
[SMP26-P07] Lithium behavior in middle crustal rocks: In-situ isotope geochemistry of staurolite in the Unazuki Schist, Japan
キーワード：リチウム同位体、十字石、泥質変成岩、宇奈月片岩、飛騨山脈
Lithium and its isotopes (6Li and 7Li) have been considered to be the powerful geochemical tracer to decipher the fluid-involved geological processes. Although numerous geochemical studies have been focused on the systematics in subduction zones, thus far, little is known in other tectonic settings. To better understand the fluid-involved crustal processes, we applied in-situ Li geochemistry to staurolite and coexisting minerals in medium-pressure (so-called "Barrovian-type'') metapelites.
We investigate staurolite-bearing highly aluminous metapelites of the Unazuki area in the Hida Mountains of Japan as vital samples to evaluate regional geochemical trends during progressive metamorphism. The Unazuki Schist is divided into four mineral zones in order of increasing metamorphic grade (Zone I to IV). In-situ analyses of staurolites from three zones (I, II, and III) found: (1) there is no systematic correlation between the Li abundances (24–73 µg/g) and the δ7Li values (from –1.8 to +12.6‰); (2) the δ7Li values in each sample vary from +6.7 to +8.8‰ (Zone I), from +5.5 to +6.5‰ (Zone II), from +6.4 to +12.6‰ (Zone II/III boundary), and from –1.8 to +0.3‰ (Zone III); (3) there are no systematic δ7Li trends in a progressive metamorphic change; (4) both Li abundances and δ7Li values increase in the order of staurolite > biotite > garnet. The apparent inter-mineral Li isotope fractionation values are from ~+3 to ~+8‰ between staurolite and biotite (Δ7LiSt–Bt) and from ~+7.2 to ~+18‰ between staurolite and garnet (Δ7LiSt–Grt). 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 Δ7LiSt–Grt value is larger than the Δ7LiSt–Bt value. The orders of the Δ7Li (Δ7LiSt–Grt > Δ7LiSt–Bt) and the δ7Li (staurolite > biotite > garnet) values can be explained by the different Li coordination numbers in the structure of staurolite (four-fold), biotite (six-fold), and garnet (six or eight-fold). In general, intracrustal fluid flows during orogenic processes enhance the transportation of various elements in continental crust; consequently, the fluid flows may cause Li isotope fractionation. However, the observed wide variations of both Li concentration and δ7Li values of staurolites in the Unazuki Schist cannot be simply explained by Rayleigh distillation nor fluid-rock interaction processes. Instead, it is more likely that the Li–δ7Li variations reflect their different bulk-rock compositions, which derive from diverse sedimentary protoliths. Nevertheless, we postulate that staurolite has a potential to control the geochemical behavior of Li in middle-to-lower continental crustal rocks of sedimentary origin.
We investigate staurolite-bearing highly aluminous metapelites of the Unazuki area in the Hida Mountains of Japan as vital samples to evaluate regional geochemical trends during progressive metamorphism. The Unazuki Schist is divided into four mineral zones in order of increasing metamorphic grade (Zone I to IV). In-situ analyses of staurolites from three zones (I, II, and III) found: (1) there is no systematic correlation between the Li abundances (24–73 µg/g) and the δ7Li values (from –1.8 to +12.6‰); (2) the δ7Li values in each sample vary from +6.7 to +8.8‰ (Zone I), from +5.5 to +6.5‰ (Zone II), from +6.4 to +12.6‰ (Zone II/III boundary), and from –1.8 to +0.3‰ (Zone III); (3) there are no systematic δ7Li trends in a progressive metamorphic change; (4) both Li abundances and δ7Li values increase in the order of staurolite > biotite > garnet. The apparent inter-mineral Li isotope fractionation values are from ~+3 to ~+8‰ between staurolite and biotite (Δ7LiSt–Bt) and from ~+7.2 to ~+18‰ between staurolite and garnet (Δ7LiSt–Grt). 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 Δ7LiSt–Grt value is larger than the Δ7LiSt–Bt value. The orders of the Δ7Li (Δ7LiSt–Grt > Δ7LiSt–Bt) and the δ7Li (staurolite > biotite > garnet) values can be explained by the different Li coordination numbers in the structure of staurolite (four-fold), biotite (six-fold), and garnet (six or eight-fold). In general, intracrustal fluid flows during orogenic processes enhance the transportation of various elements in continental crust; consequently, the fluid flows may cause Li isotope fractionation. However, the observed wide variations of both Li concentration and δ7Li values of staurolites in the Unazuki Schist cannot be simply explained by Rayleigh distillation nor fluid-rock interaction processes. Instead, it is more likely that the Li–δ7Li variations reflect their different bulk-rock compositions, which derive from diverse sedimentary protoliths. Nevertheless, we postulate that staurolite has a potential to control the geochemical behavior of Li in middle-to-lower continental crustal rocks of sedimentary origin.