3-1 Anhydrous high-temperature experiment at atmospheric pressure
The experimental results at atmospheric pressure are listed in Table 3-1. The range of experimental temperature is between 1310 oC and 1125 oC. Liquidus temperature of the gabbroic melt at atmospheric pressure is determined as ~1303 oC, whereas the solidus temperature is estimated to be ~1120 oC and the melting interval is ~183 oC. The crystallization sequence is: the iron-titanium oxide crystallizes at 1303 oC; pyroxene appears at 1184 oC; and plagioclase is present at 1162 oC. The crystallization sequence of the BIC melt at atmospheric pressure is shown in Fig. 3-1.
39
Table 3-1. Run products at atmospheric pressure.
*Gl = glass; FTO = Fe-Ti oxide; Pl = plagioclase; Px = pyroxene Run
40
Fig. 3-1. The crystallization sequence of the BIC melt at atmospheric pressure.
41
3-1-1 SEM analysis
Mineral phases with different compositions will appear different grayscale in the back-scattered electron image (BEI). It is not only very helpful for us to identify the phases, textures and mineral composition but also can be solid bases before EPMA work. The highest temperature (1310 oC) of experiment run is filled with glass (Fig.
3-2). The glass is gray in BEI. Iron-titanium oxide is present at 1300 oC. It is eubhedral and white in BEI (Fig. 3-3). Pyroxene appears at 1180 oC. It is subhedral to anhedral and gray in BEI (Fig 3-4). Plagioclase appears at 1159 oC. It is subhedral to euhedral and black in BEI (Fig. 3-5). In the run of 1125 oC, there is a trace amount of glass (Fig.
3-6). The solidus of the BIC gabbro at atmospheric pressure is therefore deduced to be 1120 oC.
42
Gl
Gl FTO
Fig. 3-2. The BEI of run BM-023 (1310 oC). Gl: glass.
Fig. 3-3. The BEI of run BM-005 (1300 oC). FTO: iron-titanium oxide. Gl: glass.
43
Fig. 3-4. The BEI of run BM-026 (1180 oC). FTO: iron-titanium oxide, Gl: glass, Px:
pyroxene. The pyroxene is mainly subhedral to anhedral and usually has inclusion FTO.
Gl
FTO
Px
44
Fig. 3-5. The BEI of run BM-027 (1159 oC). FTO: iron-titanium oxide, Gl: glass, Pl:
plagioclase, Px: pyroxene. FTO is tiny, less than 20 μm, and clustered together.
Plagioclase is mainly in lath shape. Pyroxene is subhedral to anhedral.
Gl
FTO
Px
Pl
45
Fig. 3-6. The BEI of run BM-007 (1125 oC). FTO: iron-titanium oxide, Gl: glass, Pl:
plagioclase, Px: pyroxene. The glass is trace. The whole space is mostly occupied by the crystals. The temperature is close to the solidus.
Gl
FTO Px
Pl
46
3-1-2 Mineral composition of the synthetic phases
The major elements of synthesized minerals were analyzed by electron probe micro-analyzer (EPMA) at Academia Sinica. The trace elements of synthesized minerals were analyzed by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) at National Taiwan University.
3-1-2-1 Major elements 3-1-2-1-1 Fe-Ti oxide mineral
The compositions of the synthesized iron-titanium oxides are listed in Table 3-2. It is the near liquidus mineral. The calculated cation exchange FeO content of iron-titanium oxide at atmospheric pressure ranges from 34.92 wt % to 36.56 wt %, Fe2O3 content ranges from 37.94 wt % to 48.43 wt %, the TiO2 content ranges from 10.70 wt % to 16.78 wt %. At the lowest temperature, the iron-titanium oxide exsolves into two mineral phases, one is Ti-rich spinel and the other is similar to ilmenite.
47
Table 3-2. The average compositions of the synthesized Fe-Ti oxide at atmospheric pressure of this study.
Run No. BM-005 BM-026 BM-027 BM-007-a BM-007-b
a Standard deviations of the mean.
b Mg# = 100 × Mg/(Mg＋Fe2+).
48
3-1-2-1-2 Pyroxene
The synthesized pyroxene compositions are listed in Table 3-3. The wollastonite (Wo) component of the synthesized pyroxenes ranges from 45.2 mole % to 46.2 mole %;
Enstatite (En) component ranges from 34.8 mole % to 41.5 mole %; and the ferrosilite (Fs) component ranges from 13.3 mole % to 19.1 mole %. They are classified as diopside and augite (Fig. 3-7). The TiO2 content ranges from 0.28 wt % to 1.37wt %.
The tFeO content ranges from 8.36 wt % to 11.48 wt %.
3-1-2-1-3 Plagioclase
The compositions of synthesized plagioclases are listed in Table 3-4. Most of their An components are from 48.6 mole % to 67.9 mole % and correspond to andesine and labradorite (Fig. 3-8). The Al2O3 component ranges from 27.23 wt % to 27.32 wt %.
K2O component ranges from 0.25 wt % to 0.63 wt %.
49
Table 3-3. The average compositions of the synthesized pyroxene at atmospheric pressure of this study.
Run No. BM-026 BM-027 BM-007
Temp. (oC) 1180 1159 1125
Avg. of 3 7 8
SiO2 46.75(0.50) 48.26(0.99) 52.42(0.51)
TiO2 1.34(0.08) 1.37(0.21) 0.28(0.11)
Al2O3 5.22(0.51) 4.35(0.65) 1.13(0.82)
Cr2O3 0.00 0.00 0.00
tFeO 9.69(0.26) 8.36(1.19) 11.48(0.91)
MnO 0.15(0.01) 0.24(0.06) 0.33(0.05)
MgO 13.73(0.25) 14.60(1.17) 11.75(0.39)
CaO 22.43(0.34) 22.16(0.25) 21.73(0.86)
Na2O 0.68(0.08) 0.65(0.07) 0.79(0.26)
50
Fig. 3-7. Variations of synthesized pyroxene composition compared with those of natural pyroxene in BIC. The natural data from Shellnutt and Pang (2012). The boundary lines are based on Morimoto (1988).
51
Table 3-4. The average compositions of the synthesized plagioclase at atmospheric pressure of this study.
Run No. BM-027 BM-007
52
Fig. 3-8. Variations of synthesized plagioclase composition compared with those of natural plagioclases in BIC. The natural plagioclase data are from Shellnutt and Pang (2012). The boundary lines are drawn based on Deer (1963).
53 temperature decreased, the liquid line of descend demonstrated that SiO2, Al2O3, Na2O and K2O were enriched, whereas MgO, TiO2,and total FeO were depleted. The CaO of the residual liquids became enriched between 1305 oC and 1180 oC, and then depleted in the final stage at lower temperatures. The crystallization trend indicated that the residual melts were considerably depleted in TiO2 and total FeO when the temperature was below 1305 oC. This depletion was triggered by the extensive crystallization of the iron-titanium oxides. When the temperature decreased below 1180 oC, the CaO and MgO contents of the residual liquids became depleted as a result of the crystallization of clinopyroxene.
The variation diagrams of the glass compositions versus silica are shown in Fig 3-10. The negative correlations of CaO, TiO2, tFeO, MgO, and positive trends of Al2O3,
K2O against SiO2 are shown.The BIC residual melt has highly variable concentrations of compatible trace elements, for example, Sc (15.1 – 30.6 ppm), Ni (16.1 – 54.8 ppm), and Cr (3.8 – 26.1 ppm).
54
Table 3-5. The average compositions of the glass at atmospheric pressure of this study.
Run No. BM-023 BM-005 BM-026 BM-027 BM-007
Temp. (oC) 1310 1300 1180 1159 1125
Avg. of 165 73 6 7 4
SiO2 47.00(0.36) 46.94(0.27) 52.63(0.27) 55.15(0.8) 59.57(0.52)
TiO2 3.96(0.41) 4.09(0.12) 2.98(0.12) 2.62(0.34) 2.11(0.22)
Al2O 12.68(0.26) 12.92(0.22) 14.21(0.24) 14.14(1.13) 15.66(0.55)
Cr2O3 0.01(0.01) 0.01(0.02) 0.00 0.00 0.01(0.01)
tFeO 15.04(0.21) 14.84(0.18) 7.58(0.21) 6.50(0.31) 4.97(0.31)
MnO 0.20(0.04) 0.21(0.04) 0.21(0.04) 0.20(0.05) 0.15(0.04)
MgO 6.26(0.10) 6.24(0.10) 6.00(0.10) 5.40(0.81) 3.96(0.46)
CaO 10.06(0.12) 10.16(0.13) 11.00(0.15) 9.76(1.02) 6.31(0.59)
Na2O 3.57(0.07) 3.40(0.07) 3.98(0.07) 4.49(0.38) 4.32(0.29)
55
Fig. 3-9. Variation of glass composition versus temperature at atmospheric pressure.
56
Fig. 3-10. The residual glass composition at 1 atm compared with natural rock in BIC by Shellnutt et al. (2009, 2010). (a): Silica versus aluminum oxide contents, (b): Silica versus calcium oxide, (c): Silica versus total iron oxide, (d): Silica versus titanium dioxide, (e): Silica versus potassium oxide, (f): Silica versus magnesium oxide.
57
3-1-2-2 Trace elements
The trace elements of the synthesized pyroxenes of this study are listed in Table 3-6. The trace element compositions of the synthesized plagioclase of this study are listed in Table 3-7. The trace element compositions of the residual melt of this study are listed in Table 3-8. The concentrations of compatible elements of the residual melts are highly variable; for example: Sc (15.1 – 30.6 ppm), Ni (16.1 – 54.8 ppm), and Cr (3.8 – 26.1 ppm).
The evolutions of the trace elements versus silica of the residual melt are illustrated in SiO2-Rb, SiO2-Ba, SiO2-Nd, SiO2-Zr diagrams (Fig 3-11). As the SiO2 content of the residual glass increased, the amount of Ba, Rb, Nd, and Zr increased.
The variations of trace elements of glass compositions versus temperature are plotted in Fig. 3-12. As the temperature decreased, the line of descend demonstrated that Nd, Sr, Zr, Nb, Rb, and Ba were enriched. It is consistent with the crystallization of the iron-titanium oxides and pyroxene. Those elements became depleted along with the crystallization of the plagioclase. The Ni and Cr of the residual liquids became depleted between 1305 oC and 1159 oC because of the crystallization of the iron-titanium oxides and pyroxene. They became enriched in the final stage as the plagioclase crystallized at lower temperatures.
58
59
60
61
Fig. 3-11. The residual glass trace elements composition at atmospheric pressure of this study compared with natural rock in BIC (black star is GS04-026 data) by Shellnutt et al.
(2008). Silica versus Rb, Ba, Nd and Zr (symbols as in Fig. 3-10).
62
Fig. 3-12. Variation of trace elements of glass compositions versus temperatures at atmospheric pressure.
63
3-2 Anhydrous high pressure high temperature experiment at 1 GPa
The experimental results at 1 GPa are listed in Table 3-9. The range of experimental temperature is between 1240 oC and 1000 oC. Liquidus temperature of the gabbroic melt is determined to be ~1220 oC, whereas the solidus temperature of it is estimated to be ~980 oC and the melting interval is determined to be ~240 oC. The crystallization sequence of the gabbroic melt is: the iron-titanium oxide and pyroxene appears at 1220 oC; and plagioclase is present at 1130 oC. The crystallization sequence of the BIC melt at 1 GPa is shown in Fig. 3-13. The phase relationship of the gabbroic melt at atmospheric pressure and 1 GPa are plotted in a pressure-temperature diagram Fig. 3-14.
3-2-1 SEM analysis
The highest temperature (1240 oC) of experiment run is filled with glass (Fig.
3-15). Iron-titanium oxide and pyroxene crystallizes at 1220 oC. The glass is gray, iron-titanium oxide is subhedral and white, and pyroxene is subhedral and gray in BEI (Fig. 3-16). Plagioclase appears at 1130 oC. It is subhedral and darker than the phases mentioned above in BEI (Fig. 3-17). In the run at 1000 oC, there is a trace amount of glass (Fig. 3-18). The solidus of the BIC gabbro at 1 GPa is, therefore, deduced to be 980 oC.
64
Table 3-9. Run products at 1 GPa.
*FTO = Fe-Ti oxide; Gl = glass; Pl = plagioclase; Px = pyroxene.
Run
65
Fig. 3-13. The crystallization sequence of the BIC melt at 1 GPa.
Fig. 3-14. Phase relations of a BIC gabbro at 0 to 1.0 GPa.
66
Fig. 3-15. The BEI of run BM-H20 (1240 oC). Gl: glass. This is the highest temperature run of the experiments at 1 GPa. The glass is homogeneous.
Fig. 3-16. The BEI of run BM-H16 (1140 oC). FTO: iron-titanium oxide, Gl: glass, Px:
pyroxene. Pyroxene is subhedral to anhedral.
67
Fig. 3-17. The BEI of run BM-H09 (1120 oC). FTO: iron-titanium oxide, Gl: glass, Pl:
plagioclase, Px: pyroxene. Plagioclase is mainly in lath shape. Pyroxene is subhedral to anhedral.
Fig. 3-18. The BEI of run BM-H05 (1000 oC). FTO: iron-titanium oxide, Gl: glass, Pl:
plagioclase, Px: pyroxene. Pyroxene is subhedral to anhedral. The glass is of trace amount.
68
3-2-2 Mineral composition of the synthetic phases
The major elements of synthesized minerals were analyzed by the electron probe micro-analysis (EPMA) at Academia Sinica. The chemistry of the synthesized minerals will be discussed as follows:
3-2-2-1 Fe-Ti oxide
The compositions of the synthesized iron-titanium oxide at 1 GPa are listed in Table 3-10. It is the near liquidus mineral. The FeO of the iron-titanium oxides at 1GPa ranges from 22.73 wt % to 35.91 wt %, Fe2O3 component ranges from 16.24 wt % to 44.81 wt %. At lower temperatures (1000 oC), the iron-titanium oxides are exsolved into two (a and b) types. The FeO component of a-type iron-titanium oxides ranges from 21.55 wt % to 33.49 wt %, Fe2O3 component ranges is from 14.37 wt % to 39.54 wt % and TiO2 component of a-type ranges from 29.76 wt % to 46.43 wt %, whereas the FeO component of b-type ranges from 33.90 wt % to 34.96 wt %, Fe2O3 component ranges is from 39.67 wt % to 46.24 wt %.
69
Table 3-10. The average compositions of the synthesized Fe-Ti oxide at 1 GPa of this study.
Run No. BM-H16 BM-H09 BM-H05-a BM-H05-b
Temp. (oC) 1140 1120 1000 1000
Avg. of 2 14 3 2
SiO2 0.16(0.09) 0.05(0.04) 0.03(0.02) 0.11(0.00)
TiO2 31.44(0.03) 32.70(0.36) 45.51(1.02) 9.94(2.36)
Al2O3 1.82(0.01) 1.64(0.06) 0.65(0.11) 5.15(0.52)
Cr2O3 0.03(0.04) 0.02(0.02) 0.01(0.01) 0.18(0.07)
Fe2O3 40.48(0.12) 38.56(0.71) 16.24(1.72) 44.81(4.74)
FeO 22.73(0.03) 23.67(0.39) 32.72(0.75) 35.91(0.87)
MnO 0.04(0.06) 0.10(0.03) 0.42(0.07) 0.37(0.01)
70
3-2-2-2 Pyroxene
The compositions of synthesized pyroxenes at 1 GPa are listed in Table 3-11. The wollastonite (Wo) component of the synthesized pyroxenes ranges from 37.77 mole % to 46.48 mole %; Enstatite (En) component from 33.83 mole % to 38.62 mole %; and the ferrosilite (Fs) component from 18.92 mole % to 25.3 mole %. They are classified as diopside and augite (Fig. 3-19). The synthesized pyroxenes of this study were divided into two types at lower temperature (1120 oC). The TiO2 component ranges from 0.23 wt % to 1.78 wt %. The tFeO component ranges from 11.53 wt % to 14.02 wt %.
3-2-2-3 Plagioclase
The compositions of synthesized plagioclases at 1 GPa are listed in Table 3-12.
Most of their An components are from 20.8 mole % to 35.9 mole % and are consistent with andesine and oligoclase (Fig. 3-20). The Al2O3 component ranges from 22.8 wt % to 24.77 wt %. K2O component ranges from 0.51 wt % to 0.91 wt %.
71
Table 3-11. The average compositions of the synthesized pyroxene at 1 GPa of this study.
Run No. BM-H16 BM-H09-a BM-H09-b BM-H05-a BM-H05-b
Temp. (oC) 1140 1120 1120 1000 1000
Avg. of 4 6 3 1 8
SiO2 48.38(0.85) 47.51(0.61) 52.22(0.24) 49.31 52.47(0.34)
TiO2 1.78(0.17) 1.22(0.16) 0.23(0.10) 0.88 0.24(0.11)
Al2O3 6.19(0.66) 7.74(0.96) 1.03(0.18) 6.36 0.94(0.53)
Cr2O3 0.06(0.07) 0.01(0.02) 0.00 0.00 0.00
tFeO 11.66(0.26) 11.82(0.20) 11.53(0.36) 14.02 11.90(0.44)
MnO 0.22(0.04) 0.20(0.05) 0.35(0.10) 0.34 0.32(0.05)
72
(a) 1120
oC
(b) 1000
oC
73
(c) All
Fig. 3-19. (a) The pyroxene is classified as diopside and augite at 1120 oC. (b) The lowest temperature (1000 oC) of experiment run has diopside and augite. (c) Variations of synthesized pyroxene composition at 1 GPa and compared with those of natural pyroxene in BIC. The natural data from Shellnutt and Pang (2012). The boundary lines are based on Morimoto (1988).
74
Table 3-12. The average compositions of the synthesized plagioclase at 1 GPa of this study.
Run No. BM-H09 BM-H05
75
Fig. 3-20. Variations of synthesized plagioclase composition at 1 GPa are compared with natural plagioclases in BIC. The natural plagioclase data are from Shellnutt and Pang (2012). The boundary lines are drawn based on Deer (1963).
76
3-2-2-4 Glass
The composition of the glasses in the run products at temperatures between 1240
oC to 1000 oC were analyzed and are listed in Table 3-13. Figure 3-21 shows that the
variation of glass composition versus temperature. As the temperature decreased, the liquid line of descend demonstrated Al2O3-enriched and K2O-enriched, whereas CaO-depleted and MgO-depleted by the crystallization of pyroxene. The TiO2 and total FeO of the residual liquids became enriched accompanying the presence of
iron-titanium oxide from the initial stage to the middle stage between 1240 oC and 1120
oC, and then depleted from the middle stage to the final stage at lower temperatures. The
SiO2 and Na2O content of the residual liquids became slightly enriched in the beginning then became depleted a little in the middle stage and then enriched in the final stage as the temperature decreased.
77
Table 3-13. The average compositions of the glass at 1 GPa of this study.
Run No. BM-H20 BM-H16 BM-H09 BM-H05
Temp. (oC) 1240 1140 1120 1000
Avg. of 9 9 2 6
SiO2 55.17(0.74) 57.18(0.56) 50.61(0.51) 57.22(0.91)
TiO2 2.94(0.25) 3.26(0.16) 3.75(0.03) 1.46(0.38)
Al2O3 12.75(0.46) 11.28(0.36) 14.75(0.28) 19.25(0.45)
Cr2O3 0.01(0.02) 0.00 0.00 0.01(0.01)
tFeO 10.83(0.89) 11.80(0.27) 13.32(0.43) 8.43(0.30)
MnO 0.13(0.09) 0.15(0.08) 0.18(0.03) 0.14(0.03)
78
Fig. 3-21. Variation of glass compositions versus temperatures at 1 GPa.
79