Abstract 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.

中文翻译：

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系统中的液体不混溶性和相关系 KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 在 6 GPa：对金刚石形成熔体的影响

摘要 为了评估 Na 对金刚石稳定性领域中碳酸盐-硅酸盐液体不混溶性的影响，我们在 6 GPa 下沿 KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 系统的一些特定连接进行了实验。所有研究过的接头的熔化开始于 1000–1050 °C。Kfs + Dol 系统中的熔化由反应控制 6 KAlSi3O8 (钾长石) + 6 CaMg(CO3)2 (白云石) = 2 (Can,Mg1-n)3Al2Si3O12 (石榴石) + Al2SiO5 (蓝晶石) + 11 SiO2（柯石英）+ 3 K2(Ca1-n,Mgn)2(CO3)3（碳酸岩熔体）+ 3 CO2（流体），其中 n ~ 0.3–0.4。通过反应 K2CO3（碳酸熔体）+ Al2SiO5（蓝晶石）+ 5 SiO2（柯石英）= 2 KAlSi3O8（硅熔体）+ CO2（流体或熔体中的溶质）。硅熔体组成接近 KAlSi3O8，并含有溶解的 CaMg(CO3)2 和分子 CO2。向系统中添加 NaAlSi2O6 或 Na2CO3 会导致钾长石部分分解并形成含钾碳酸盐 (K, Na)2Mg(CO3)2 和 (K, Na)2Ca3(CO3)4。它们的熔融产生碳酸岩熔体，其成分近似为 4(K, Na)2CO3·6Ca0.6Mg0.4CO3 和菱镁矿。此外，所研究体系中 NaAlSi2O6 的存在将不混溶的温度下限移至 1500°С，而 Na2CO3 的存在通过以下反应消除了硅熔体的出现：2 KAlSi3O8（硅熔体中）+ Na2CO3 = 2 NaAlSi2O6（在单斜辉石中）+ K2CO3（在碳酸岩熔体中）+ SiO2（柯石英）。因此，Na2O-K2O-CaO-MgO-Al2O3-SiO2-CO2体系中Na2O含量的增加消耗了硅熔体中的Al2O3和SiO2形成单斜辉石。我们发现，在 900-1500 °C 和 6 GPa 下，粗火石和透辉石-硬玉固溶体可以与 CO2 流体共存。因此，在大陆岩石圈和俯冲板块的温度条件下，CO2 流体在金刚石稳定场中的榴辉岩套件中是稳定的。在钻石中天然捕获的碳酸岩熔体中观察到的 Na2O 含量的变化超过了泥质岩熔体产生的变化。本实验表明，在 Kfs + Dol 系统中添加 NaAlSi2O6 不会导致碳酸岩熔体中 Na2O 含量的增加，而在 Na2O/Al2O3 中添加 Na2CO3 > 图 1 产生了熔体的形成，其中 Na2O 含量覆盖了整个天然成分范围。因此，只有额外盐成分的存在才能解释岩石圈金刚石中捕获的熔体中 Na2O 含量升高的原因。除了碳酸盐外，钠还可以包含在氯化物、硫酸盐等中。