[SIT27-P06] Phase relations and liquid immiscibility in KAlSi3O8- and CaMg(CO3)2-bearing systems at 6 GPa: Implications for origin of fibrous diamonds
キーワード：Phase relations, Liquid immiscibility, KAlSi3O8, CaMg(CO3)2, fibrous diamonds
The reaction between potassium feldspar (Kfs) and dolomite (Dol) could define the solidus of carbonated pelite below 7 GPa under nominally anhydrous conditions (Grassi and Schmidt, 2011). To investigate this reaction subsolidus and melting phase relations in the systems KAlSi3O8 + CaMg(CO3)2 (Kfs + Dol), KAlSi3O8 + CaMgSi2O6 + NaAlSi2O6 + CaMg(CO3)2 (Kfs + Di + Jd + Dol), and KAlSi3O8 + Na2CO3 + CaMg(CO3)2 (Kfs + Na2 + Dol) have been examined at 6 GPa and compared with those established in dry carbonated pelite (DG2).
In the system Kfs+Dol at 1000 °C, the subsolidus assemblage consists of Kfs and Dol. Besides, Kfs contains numerous inclusions of K2Si4O9 wadeite (K-wad), kyanite (Ky), coesite (Coe). At lower temperature 900 °C, Kfs and Dol are not stable and the assemblage includes K-wad, Ky, Coe, aragonite (Arg) and magnesite (Mgs). The system begins to melt at 1100 °C via reaction 6KAlSi3O8 (Kfs) + 6CaMg(CO3)2 (Dol) = 2(Can,Mg1-n)3Al2Si3O12 (Grt) + Al2SiO5 (Ky) + 11SiO2 (Coe) + 3K2(Ca1-n,Mgn)2(CO3)3 (L) + 3CO2 (F and/or L), where n ~ 0.3-0.4, leading to essentially carbonate liquid, L(C), with composition of 32K2CO3·68(Ca0.63Mg0.37)CO3 + 1-2 mol% SiO2 + 0.5-1 mol% Al2O3. Kfs and Dol completely disappear at 1150 °C. Starting from 1300 °C, immiscible silicate melt, L(S), coexists with L(C). L(S) contains (in mol%): 64 SiO2, 9 Al2O3, 2 MgO, 2 CaO, 11 K2O, and 12 CO2. The Grt + Coe + Ky + L(C) + L(S) phase assemblage remains stable up to 1500 °C.
Adding Di-Jd clinopyroxene in the Kfs + Dol system does not affect phase relations. At 1000 °C, the subsolidus assemblage is Kfs + CPx + Dol. Kfs is partially replaced by K-wad + Ky + Coe assemblage. Above the solidus at 1150 °C, Cpx, Grt, Ky, Coe coexist with L(C) [34(K0.92Na0.08)2CO3·66(Ca0.62Mg0.38)CO3 + 3 mol% SiO2 + 1 mol% Al2O3]. L(S) appears at 1250 °C and contains (in mol%): 67 SiO2, 7 Al2O3, 4 MgO, 4 CaO, 1 Na2O, 7 K2O, and 10 CO2. The Cpx + Grt + Ky + Coe + L(C) + L(S) phase assemblage was established at 1250 and 1350 °C, whereas at 1500 °C only CPx remains in coexistence with L(C) and L(S).
At 900 °C in the Kfs + Na2 + Dol system, the subsolidus assemblage consists of Cpx (Jd87-90Di10-13), (Na0.55K0.45)2Mg0.90Ca0.10(CO3)2 (NaKMg), (Na0.63K0.37)2(Ca0.93Mg0.07)3(CO3)4 (NaKCa3), Coe, and Mgs. As temperature increases to 1000 °C, NaKMg and NaKCa3 disappear, while minor amount of Mgs and Coe remains. The sample consists of Cpx (Jd78Di22) and L(C) [44(Na0.51K0.49)2CO3·56Ca0.57Mg0.43CO3]. Thus, an addition of Na2CO3 into the Kfs+Dol system yields formation of Cpx, NaKMg and NaKCa3 at the expense of K-wad, Ky, Mgs, and Arg. Molar abundances of phases and their compositions change slightly with further temperature increase up to 1500 °C. Thus, Na2CO3 causes redistribution of K2O into carbonate phases, whereas SiO2 and Al2O3 are consumed on jadeite formation: Na2CO3 + KAlSi3O8 (Kfs or L(S)) = NaAlSi2O6 + SiO2 + NaKCO3 (L(C)). As a result, immiscible L(S) does not appear in this system.
In dry carbonated pelite (DG2), subsolidus assemblage established at 1000 °C is represented by Cpx, Grt, Ky, Coe, Dol, and K2Ti1-nSi3+nO9 wadeite. Two immissible liquids were established at 1350 and 1500 °C in coexistence with Cpx, Coe, Grt, Ky and CO2 fluid. At 1350 °C, L(S) contains (in mol%): 56.6 SiO2, 1.8 TiO2, 8.3 Al2O3, 2.9 FeO, 1.3 MgO, 3.2 CaO, 1.4 Na2O, 9.7 K2O, and 15.0 CO2, whereas L(C) composition is (K0.74Na0.26)2(Ca0.58Mg0.18Fe0.24)4CO3 + 4.2 mol% SiO2 + 1.7 mol% TiO2 + 1.4 mol% Al2O3.
The obtained immissible L(C) and L(S) resemble compositions of melt inclusions in fibrous diamonds worldwide. Thus, the K-aluminosilicate and K-Na-rich carbonatite melts entrapped by fibrous diamonds could be derived by liquid immissibility during partial melting of carbonated pelite subducted down to 180 km depth and heated to 1300 °C or higher temperature.
This work is financially supported by Russian Science Foundation (No 14-17-00609).
In the system Kfs+Dol at 1000 °C, the subsolidus assemblage consists of Kfs and Dol. Besides, Kfs contains numerous inclusions of K2Si4O9 wadeite (K-wad), kyanite (Ky), coesite (Coe). At lower temperature 900 °C, Kfs and Dol are not stable and the assemblage includes K-wad, Ky, Coe, aragonite (Arg) and magnesite (Mgs). The system begins to melt at 1100 °C via reaction 6KAlSi3O8 (Kfs) + 6CaMg(CO3)2 (Dol) = 2(Can,Mg1-n)3Al2Si3O12 (Grt) + Al2SiO5 (Ky) + 11SiO2 (Coe) + 3K2(Ca1-n,Mgn)2(CO3)3 (L) + 3CO2 (F and/or L), where n ~ 0.3-0.4, leading to essentially carbonate liquid, L(C), with composition of 32K2CO3·68(Ca0.63Mg0.37)CO3 + 1-2 mol% SiO2 + 0.5-1 mol% Al2O3. Kfs and Dol completely disappear at 1150 °C. Starting from 1300 °C, immiscible silicate melt, L(S), coexists with L(C). L(S) contains (in mol%): 64 SiO2, 9 Al2O3, 2 MgO, 2 CaO, 11 K2O, and 12 CO2. The Grt + Coe + Ky + L(C) + L(S) phase assemblage remains stable up to 1500 °C.
Adding Di-Jd clinopyroxene in the Kfs + Dol system does not affect phase relations. At 1000 °C, the subsolidus assemblage is Kfs + CPx + Dol. Kfs is partially replaced by K-wad + Ky + Coe assemblage. Above the solidus at 1150 °C, Cpx, Grt, Ky, Coe coexist with L(C) [34(K0.92Na0.08)2CO3·66(Ca0.62Mg0.38)CO3 + 3 mol% SiO2 + 1 mol% Al2O3]. L(S) appears at 1250 °C and contains (in mol%): 67 SiO2, 7 Al2O3, 4 MgO, 4 CaO, 1 Na2O, 7 K2O, and 10 CO2. The Cpx + Grt + Ky + Coe + L(C) + L(S) phase assemblage was established at 1250 and 1350 °C, whereas at 1500 °C only CPx remains in coexistence with L(C) and L(S).
At 900 °C in the Kfs + Na2 + Dol system, the subsolidus assemblage consists of Cpx (Jd87-90Di10-13), (Na0.55K0.45)2Mg0.90Ca0.10(CO3)2 (NaKMg), (Na0.63K0.37)2(Ca0.93Mg0.07)3(CO3)4 (NaKCa3), Coe, and Mgs. As temperature increases to 1000 °C, NaKMg and NaKCa3 disappear, while minor amount of Mgs and Coe remains. The sample consists of Cpx (Jd78Di22) and L(C) [44(Na0.51K0.49)2CO3·56Ca0.57Mg0.43CO3]. Thus, an addition of Na2CO3 into the Kfs+Dol system yields formation of Cpx, NaKMg and NaKCa3 at the expense of K-wad, Ky, Mgs, and Arg. Molar abundances of phases and their compositions change slightly with further temperature increase up to 1500 °C. Thus, Na2CO3 causes redistribution of K2O into carbonate phases, whereas SiO2 and Al2O3 are consumed on jadeite formation: Na2CO3 + KAlSi3O8 (Kfs or L(S)) = NaAlSi2O6 + SiO2 + NaKCO3 (L(C)). As a result, immiscible L(S) does not appear in this system.
In dry carbonated pelite (DG2), subsolidus assemblage established at 1000 °C is represented by Cpx, Grt, Ky, Coe, Dol, and K2Ti1-nSi3+nO9 wadeite. Two immissible liquids were established at 1350 and 1500 °C in coexistence with Cpx, Coe, Grt, Ky and CO2 fluid. At 1350 °C, L(S) contains (in mol%): 56.6 SiO2, 1.8 TiO2, 8.3 Al2O3, 2.9 FeO, 1.3 MgO, 3.2 CaO, 1.4 Na2O, 9.7 K2O, and 15.0 CO2, whereas L(C) composition is (K0.74Na0.26)2(Ca0.58Mg0.18Fe0.24)4CO3 + 4.2 mol% SiO2 + 1.7 mol% TiO2 + 1.4 mol% Al2O3.
The obtained immissible L(C) and L(S) resemble compositions of melt inclusions in fibrous diamonds worldwide. Thus, the K-aluminosilicate and K-Na-rich carbonatite melts entrapped by fibrous diamonds could be derived by liquid immissibility during partial melting of carbonated pelite subducted down to 180 km depth and heated to 1300 °C or higher temperature.
This work is financially supported by Russian Science Foundation (No 14-17-00609).