[SIT27-P06] Phase relations and liquid immiscibility in KAlSi3O8- and CaMg(CO3)2-bearing systems at 6 GPa: Implications for origin of fibrous diamonds
Keywords: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).