TY - JOUR

T1 - Liquid immiscibility and phase relations in the system KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 at 6 GPa

T2 - Implications for diamond-forming melts

AU - Shatskiy, Anton

AU - Arefiev, Anton V.

AU - Podborodnikov, Ivan V.

AU - Litasov, Konstantin D.

N1 - Publisher Copyright: © 2020 Copyright: Copyright 2020 Elsevier B.V., All rights reserved.

PY - 2020/9/20

Y1 - 2020/9/20

N2 - To evaluate the effect of Na on the carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments along some specific joins of the system KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 at 6 GPa. Melting in all studied joins begins at 1000–1050 °C. The melting in the Kfs + Dol system is controlled by the reaction 6 KAlSi3O8 (K-feldspar) + 6 CaMg(CO3)2 (dolomite) = 2 (Can,Mg1-n)3Al2Si3O12 (garnet) + Al2SiO5 (kyanite) + 11 SiO2 (coesite) + 3 K2(Ca1-n,Mgn)2(CO3)3 (carbonatitic melt) + 3 CO2 (fluid), where n ~ 0.3–0.4. A temperature increasing to 1300 °C yields an appearance of the silicic immiscible melt in addition to carbonatitic melt via the reaction K2CO3 (carbonatitic melt) + Al2SiO5 (kyanite) + 5 SiO2 (coesite) = 2 KAlSi3O8 (silicic melt) + CO2 (fluid or solute in melts). The silicic melt composition is close to KAlSi3O8 with dissolved CaMg(CO3)2 and molecular CO2. An addition of NaAlSi2O6 or Na2CO3 to the system results in partial decomposition of K-feldspar and formation of K-bearing carbonates, (K, Na)2Mg(CO3)2 and (K, Na)2Ca3(CO3)4. Their melting produces carbonatite melt with the approximate composition of 4(K, Na)2CO3·6Ca0.6Mg0.4CO3 and magnesite. Besides, the presence of NaAlSi2O6 in the studied system shifts the lower-temperature limit of immiscibility to 1500°С, while the presence of Na2CO3 eliminates the appearance of silicic melt by the following reaction: 2 KAlSi3O8 (in the silicic melt) + Na2CO3 = 2 NaAlSi2O6 (in clinopyroxene) + K2CO3 (in the carbonatitic melt) + SiO2 (coesite). Thus, an increase of the Na2O content in the system Na2O-K2O-CaO-MgO-Al2O3-SiO2-CO2 consumes Al2O3 and SiO2 from silicic melt to form clinopyroxene. We found that grossular-pyrope and diopside-jadeite solid solutions can coexist with CO2 fluid at 900–1500 °C and 6 GPa. Thus, CO2 fluid is stable in the eclogitic suite in the diamond stability field under temperature conditions of the continental lithosphere and subducting slabs. Variations in the Na2O content observed in carbonatitic melts trapped by natural in diamonds exceed those derived by the pelite melting. The present experiments show that an addition of NaAlSi2O6 to the Kfs + Dol system does not cause an increase of the Na2O content in the carbonatitic melt, whereas the addition of Na2CO3 at Na2O/Al2O3 > 1 yields the formation of the melts with the Na2O contents covering the entire range of natural compositions. Thus, only the presence of additional salt components can explain the elevated Na2O content in the melts trapped in lithospheric diamonds. In addition to carbonates, sodium can be hosted by chlorides, sulfates, etc.

AB - To evaluate the effect of Na on the carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments along some specific joins of the system KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 at 6 GPa. Melting in all studied joins begins at 1000–1050 °C. The melting in the Kfs + Dol system is controlled by the reaction 6 KAlSi3O8 (K-feldspar) + 6 CaMg(CO3)2 (dolomite) = 2 (Can,Mg1-n)3Al2Si3O12 (garnet) + Al2SiO5 (kyanite) + 11 SiO2 (coesite) + 3 K2(Ca1-n,Mgn)2(CO3)3 (carbonatitic melt) + 3 CO2 (fluid), where n ~ 0.3–0.4. A temperature increasing to 1300 °C yields an appearance of the silicic immiscible melt in addition to carbonatitic melt via the reaction K2CO3 (carbonatitic melt) + Al2SiO5 (kyanite) + 5 SiO2 (coesite) = 2 KAlSi3O8 (silicic melt) + CO2 (fluid or solute in melts). The silicic melt composition is close to KAlSi3O8 with dissolved CaMg(CO3)2 and molecular CO2. An addition of NaAlSi2O6 or Na2CO3 to the system results in partial decomposition of K-feldspar and formation of K-bearing carbonates, (K, Na)2Mg(CO3)2 and (K, Na)2Ca3(CO3)4. Their melting produces carbonatite melt with the approximate composition of 4(K, Na)2CO3·6Ca0.6Mg0.4CO3 and magnesite. Besides, the presence of NaAlSi2O6 in the studied system shifts the lower-temperature limit of immiscibility to 1500°С, while the presence of Na2CO3 eliminates the appearance of silicic melt by the following reaction: 2 KAlSi3O8 (in the silicic melt) + Na2CO3 = 2 NaAlSi2O6 (in clinopyroxene) + K2CO3 (in the carbonatitic melt) + SiO2 (coesite). Thus, an increase of the Na2O content in the system Na2O-K2O-CaO-MgO-Al2O3-SiO2-CO2 consumes Al2O3 and SiO2 from silicic melt to form clinopyroxene. We found that grossular-pyrope and diopside-jadeite solid solutions can coexist with CO2 fluid at 900–1500 °C and 6 GPa. Thus, CO2 fluid is stable in the eclogitic suite in the diamond stability field under temperature conditions of the continental lithosphere and subducting slabs. Variations in the Na2O content observed in carbonatitic melts trapped by natural in diamonds exceed those derived by the pelite melting. The present experiments show that an addition of NaAlSi2O6 to the Kfs + Dol system does not cause an increase of the Na2O content in the carbonatitic melt, whereas the addition of Na2CO3 at Na2O/Al2O3 > 1 yields the formation of the melts with the Na2O contents covering the entire range of natural compositions. Thus, only the presence of additional salt components can explain the elevated Na2O content in the melts trapped in lithospheric diamonds. In addition to carbonates, sodium can be hosted by chlorides, sulfates, etc.

KW - Carbonatite

KW - Diamond formation

KW - Earth's mantle

KW - High-density fluids

KW - High-pressure experiment

KW - KAlSiO-CaMg(CO)

KW - Liquid immiscibility

KW - HIGH-PRESSURE TRANSITIONS

KW - CRYSTALLIZATION

KW - COESITE

KW - INCLUSIONS

KW - MANTLE FLUIDS

KW - RAMAN

KW - CARBONATED PELITES

KW - KAlSi3O8-CaMg(CO3)(2)

KW - FIBROUS DIAMONDS

KW - SILICATE

KW - HEAT-CAPACITY

UR - http://www.scopus.com/inward/record.url?scp=85086078745&partnerID=8YFLogxK

U2 - 10.1016/j.chemgeo.2020.119701

DO - 10.1016/j.chemgeo.2020.119701

M3 - Article

AN - SCOPUS:85086078745

VL - 550

JO - Chemical Geology

JF - Chemical Geology

SN - 0009-2541

M1 - 119701

ER -