Ba-Ti-rich oxymicas from olivine melanephelinites of the Udokan lava field, Siberia, Russia: chemistry and substitutions
Sharygin V.V.
V.S.Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia
The first data about the presence and chemistry of Ba-Ti mica in olivine melanephelinites from oldest (14 Ma) volcanoes of the Udokan lava field (Ingamakit, Munduzhyak, Peremychka, Lurbun) were documented by Stupak (1987) and Litasov (1992). These rocks contain phenocrysts of olivine (Fo75-85), augite (Mg# - 0.95-0.85) and groundmass consisting of olivine (Fo70-75), Ti-rich augite (Mg# - 0.85-0.80), Ti-magnetite (TiO2 - 12-24 wt.%), oxyfluorapatite (F - 2-3.5 wt.%), Ba-Ti-phlogopite, nepheline, leucite, ilmenite and rare K-feldspar, glass and post/late magmatic calcite. It should be noted that Ba-Ti-rich micas were previously described in olivine nephelinites and related rocks around the world (Mansker et al., 1979; Edgar, 1992; Zhang et al., 1993; Seifert, Kampf, 1994; Henderson, Foland, 1996; Shaw, Penczak, 1996; Greenwood, 1998; Kogarko et al., 2005; Krivdik, Mikhailov, 2007). Here we present new data on chemistry and substitutions of Ba-Ti-micas from the Udokan olivine melanephelinites.
Ba-Ti-phlogopite forms xenomorphic grains in the groundmass and rarely occurs as daughter phase of olivine-hosted melt inclusions. According to EMPA and SIMS data micas studied contain low H2O (0.1-0.3 wt.%), Li (30-50 ppm) and high BaO (6.7-11.5 wt.%), TiO2 (10-13.3 wt.%), F (to 3 wt.%), Nb (460-920 ppm), Cr (180-420 ppm), Sr (940-1970 ppm), Zr (60-150 ppm) and REE (100-120 ppm) (Table 1). In general, most mica grains do not show essential zonation in chemical composition. The core-to-rim variations in rare grains indicate the enrichment in K2O and depletion in BaO.
Table 1. Chemical composition (EMPA+SIMS, wt.%) of Ba-Ti-phlogopites from the Udokan olivine melanephelinites.
|
Sample |
Lv-18 |
|
Le23/1 |
|
Le23/2 |
|
Le24/1 |
|
Le24/2 |
|
Le26 |
|
|
|
core |
rim |
core |
rim |
core |
rim |
core |
rim |
core |
rim |
core |
rim |
|
SiO2 |
30.95 |
31.19 |
31.03 |
32.82 |
31.49 |
32.82 |
31.20 |
31.81 |
31.26 |
31.92 |
31.93 |
33.46 |
|
TiO2 |
13.25 |
12.96 |
11.52 |
11.27 |
11.96 |
11.66 |
12.26 |
12.37 |
12.89 |
12.77 |
12.75 |
12.46 |
|
ZrO2 |
0.01 |
|
0.01 |
|
0.02 |
|
0.02 |
|
0.02 |
|
0.01 |
|
|
Nb2O5 |
0.17 |
|
0.21 |
|
0.26 |
|
0.16 |
|
0.13 |
|
0.15 |
|
|
Cr2O3 |
0.11 |
|
0.05 |
|
0.06 |
|
0.08 |
|
0.12 |
|
0.05 |
|
|
Al2O3 |
14.55 |
14.57 |
13.99 |
13.65 |
13.86 |
13.74 |
13.70 |
13.27 |
13.63 |
13.45 |
13.51 |
13.46 |
|
Nd2O3 |
0.01 |
|
0.01 |
|
0.01 |
|
0.01 |
|
0.02 |
|
0.01 |
|
|
Fe2O3 |
2.11 |
2.88 |
0.57 |
2.09 |
1.82 |
4.14 |
0.15 |
1.84 |
0.58 |
0.55 |
0.48 |
3.19 |
|
FeO |
11.18 |
10.20 |
11.15 |
9.84 |
10.51 |
8.90 |
12.23 |
11.13 |
12.74 |
12.45 |
11.68 |
9.67 |
|
MnO |
0.09 |
0.08 |
0.08 |
0.06 |
0.07 |
0.08 |
0.07 |
0.10 |
0.09 |
0.08 |
0.07 |
0.06 |
|
MgO |
10.67 |
10.90 |
12.51 |
12.42 |
12.43 |
12.51 |
11.65 |
11.31 |
11.26 |
10.88 |
12.10 |
12.42 |
|
CaO |
0.02 |
0.13 |
0.01 |
0.10 |
0.01 |
0.04 |
0.02 |
0.04 |
0.00 |
0.04 |
0.09 |
0.11 |
|
BaO |
9.37 |
9.14 |
11.20 |
8.98 |
10.58 |
8.60 |
11.46 |
9.21 |
10.08 |
9.77 |
9.83 |
6.68 |
|
SrO |
0.23 |
|
0.11 |
|
0.11 |
|
0.15 |
|
0.15 |
|
0.12 |
|
|
Na2O |
0.77 |
0.73 |
0.57 |
0.57 |
0.53 |
0.40 |
0.63 |
0.62 |
0.57 |
0.55 |
0.64 |
0.53 |
|
K2O |
5.33 |
5.34 |
5.10 |
5.63 |
5.40 |
6.31 |
4.87 |
5.72 |
5.40 |
5.48 |
5.43 |
6.71 |
|
Li2O |
0.01 |
|
0.02 |
|
0.02 |
|
0.02 |
|
0.02 |
|
0.02 |
|
|
F |
1.28 |
1.29 |
1.91 |
1.80 |
1.73 |
1.78 |
1.75 |
1.83 |
1.63 |
1.63 |
1.70 |
1.80 |
|
H2O |
0.11 |
|
0.29 |
|
0.21 |
|
0.17 |
|
0.21 |
|
0.19 |
|
|
Total |
100.24 |
99.41 |
100.35 |
99.23 |
101.10 |
100.97 |
100.59 |
99.25 |
100.80 |
99.56 |
100.76 |
100.55 |
|
O=F2 |
0.54 |
0.54 |
0.80 |
0.76 |
0.73 |
0.75 |
0.74 |
0.77 |
0.69 |
0.69 |
0.72 |
0.76 |
|
Total |
99.70 |
98.87 |
99.54 |
98.47 |
100.37 |
100.22 |
99.86 |
98.48 |
100.11 |
98.88 |
100.04 |
99.79 |
Note: Lv - Munduzhyak volcano; Le - Ingamakit volcano. ZrO2, Nb2O5, Cr2O3, Nd2O3, SrO, Li2O and H2O were analyzed by SIMS, others - by EMPA. FeO and Fe2O3 are calculated by stoichiometry for formula on the basis of 16 cations and 24 anions.
The complete chemical compositions (with H2O and F) show the following formulas: K1.5Ba0.5(Mg,Fe)4.5Ti1.5 [Al2.5Si5.5O20]O3F - K1.25Ba0.75(Mg,Fe)4.5Ti1.5[Al3Si5O20]O3.25F0.75 - K1.5Ba0.5(Mg,Fe)4Fe3+0.5Ti1.5[Al2.75Si5.25O20]O3.25F0.75 and indicate that studied micas are related to intermediate members of the theoretical solid solution K(Mg,Fe)6[Al2Si6O20]F4 - KBa(Mg,Fe)4Ti2[Al3Si5O20]O4 via complex isomorphic substitution K+2(Mg,Fe2+)+2Si+4F → Ba+2Ti+Al+4O. In general, composition of the Udokan mica approaches oxykinoshitalite KBa(Mg,Fe)3.5Fe3+Ti1.5 [Al3Si5O20]O3F0.5(OH)0.5 (Kogarko et al., 2005) differing in lower Ba, Fe3+, H2O and higher K and F. In addition to the main substitution, K+2Si+(Mg,Fe2+)+F ╚ Ba+2Al+Ti+O and K+2Fe3++O ╚ Ba+2(Mg,Fe2+)+F are not excluded in the Udokan micas. The latter scheme explains the positive correlation of Ba with F, Fet and antipathetic relations of Ba with Mg, K (Fig. 1), showing the incorporation of octahedral Fe3+ in the mica structure. The above substitutions strongly suggest that specific change in chemical composition does not lead to the appearance of vacancy sites in the structure of the Udokan micas.
Figure 1. Compositional variations (in apfu) for Ba-Ti-micas from the Udokan olivine melanephelinites.
Note: 1 - Ingamakit volcano; 2 - Munduzhyak volcano; 3 - Peremychka volcano. Formula is calculated on the basis of 16 cations.
Like apatite, the Udokan Ba-Ti-phlogopite contains very low H2O what strongly illustrates that melanephelinite magma and its derivates were virtually dry. The presence of Fe3+ in mica possibly indicates the gradual increasing in fO2 during evolution of melanephelinite melt.
This study was financially supported by RFBR (grant 08-05-00134).
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