The K2CO3–CaCO3–MgCO3 System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts

Carbonate micro inclusions with abnormally high K2O appear in diamonds worldwide. However, the precise determination of their chemical and phase compositions is complicated due to their sub-micron size. The K2CO3−CaCO<...

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Main Authors:	Anton V. Arefiev, Anton Shatskiy, Ivan V. Podborodnikov, Konstantin D. Litasov
Format:	Article
Language:	English
Published:	MDPI AG 2019-09-01
Series:	Minerals
Subjects:	double potassium carbonates bütschliite ultrapotassic carbonatite melt high-pressure experiment diamond formation continental lithosphere
Online Access:	https://www.mdpi.com/2075-163X/9/9/558

id	doaj-e2797cf0f79e4894880926c2e0ce344c
record_format	Article
collection	DOAJ
language	English
format	Article
sources	DOAJ
author	Anton V. Arefiev Anton Shatskiy Ivan V. Podborodnikov Konstantin D. Litasov
spellingShingle	Anton V. Arefiev Anton Shatskiy Ivan V. Podborodnikov Konstantin D. Litasov The K<sub>2</sub>CO<sub>3</sub>–CaCO<sub>3</sub>–MgCO<sub>3</sub> System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts Minerals double potassium carbonates bütschliite ultrapotassic carbonatite melt high-pressure experiment diamond formation continental lithosphere
author_facet	Anton V. Arefiev Anton Shatskiy Ivan V. Podborodnikov Konstantin D. Litasov
author_sort	Anton V. Arefiev
title	The K<sub>2</sub>CO<sub>3</sub>–CaCO<sub>3</sub>–MgCO<sub>3</sub> System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts
title_short	The K<sub>2</sub>CO<sub>3</sub>–CaCO<sub>3</sub>–MgCO<sub>3</sub> System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts
title_full	The K<sub>2</sub>CO<sub>3</sub>–CaCO<sub>3</sub>–MgCO<sub>3</sub> System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts
title_fullStr	The K<sub>2</sub>CO<sub>3</sub>–CaCO<sub>3</sub>–MgCO<sub>3</sub> System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts
title_full_unstemmed	The K<sub>2</sub>CO<sub>3</sub>–CaCO<sub>3</sub>–MgCO<sub>3</sub> System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts
title_sort	k<sub>2</sub>co<sub>3</sub>–caco<sub>3</sub>–mgco<sub>3</sub> system at 6 gpa: implications for diamond forming carbonatitic melts
publisher	MDPI AG
series	Minerals
issn	2075-163X
publishDate	2019-09-01
description	Carbonate micro inclusions with abnormally high K<sub>2</sub>O appear in diamonds worldwide. However, the precise determination of their chemical and phase compositions is complicated due to their sub-micron size. The K<sub>2</sub>CO<sub>3</sub>−CaCO<sub>3</sub>−MgCO<sub>3</sub> is the simplest system that can be used as a basis for the reconstruction of the phase composition and <i>P−T</i> conditions of the origin of the K-rich carbonatitic inclusions in diamonds. In this regard, this paper is concerned with the subsolidus and melting phase relations in the K<sub>2</sub>CO<sub>3</sub>−CaCO<sub>3</sub>−MgCO<sub>3</sub> system established in Kawai-type multianvil experiments at 6 GPa and 900−1300 °C. At 900 °C, the system has three intermediate compounds K<sub>2</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub> (Ca# ≥ 97), K<sub>2</sub>Ca(CO<sub>3</sub>)<sub>2</sub> (Ca# ≥ 58), and K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub> (Ca# ≤ 10), where Ca# = 100Ca/(Ca + Mg). Miscibility gap between K<sub>2</sub>Ca(CO<sub>3</sub>)<sub>2</sub> and K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub> suggest that their crystal structures differ at 6 GPa. Mg-bearing K<sub>2</sub>Ca(CO<sub>3</sub>)<sub>2</sub> (Ca# ≤ 28) disappear above 1000 °C to produce K<sub>2</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub> + K<sub>8</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>7</sub> + K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub>. The system has two eutectics between 1000 and 1100 °C controlled by the following melting reactions: K<sub>2</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub> + K<sub>8</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>7</sub> + K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub> → [40K<sub>2</sub>CO<sub>3</sub>∙60(Ca<sub>0.70</sub>Mg<sub>0.30</sub>)CO<sub>3</sub>] (1st eutectic melt) and K<sub>8</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>7</sub> + K<sub>2</sub>CO<sub>3</sub> + K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub> → [62K<sub>2</sub>CO<sub>3</sub>∙38(Ca<sub>0.73</sub>Mg<sub>0.27</sub>)CO<sub>3</sub>] (2nd eutectic melt). The projection of the K<sub>2</sub>CO<sub>3</sub>−CaCO<sub>3</sub>−MgCO<sub>3</sub> liquidus surface is divided into the eight primary crystallization fields for magnesite, aragonite, dolomite, Ca-dolomite, K<sub>2</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub>, K<sub>8</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>7</sub>, K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub>, and K<sub>2</sub>CO<sub>3</sub>. The temperature increase is accompanied by the sequential disappearance of crystalline phases in the following sequence: K<sub>8</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>7</sub> (1220 °C) → K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub> (1250 °C) → K<sub>2</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub> (1350 °C) → K<sub>2</sub>CO<sub>3</sub> (1425 °C) → dolomite (1450 °C) → CaCO<sub>3</sub> (1660 °C) → magnesite (1780 °C). The high Ca# of about 40 of the K<sub>2</sub>(Mg, Ca)(CO<sub>3</sub>)<sub>2</sub> compound found as inclusions in diamond suggest (1) its formation and entrapment by diamond under the <i>P−T</i> conditions of 6 GPa and 1100 °C; (2) its remelting during transport by hot kimberlite magma, and (3) repeated crystallization in inclusion that retained mantle pressure during kimberlite magma emplacement. The obtained results indicate that the K−Ca−Mg carbonate melts containing 20−40 mol% K<sub>2</sub>CO<sub>3</sub> is stable under <i>P−T</i> conditions of 6 GPa and 1100−1200 °C corresponding to the base of the continental lithospheric mantle. It must be emphasized that the high alkali content in the carbonate melt is a necessary condition for its existence under geothermal conditions of the continental lithosphere, otherwise, it will simply freeze.
topic	double potassium carbonates bütschliite ultrapotassic carbonatite melt high-pressure experiment diamond formation continental lithosphere
url	https://www.mdpi.com/2075-163X/9/9/558
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spelling	doaj-e2797cf0f79e4894880926c2e0ce344c2020-11-25T01:32:43ZengMDPI AGMinerals2075-163X2019-09-019955810.3390/min9090558min9090558The K<sub>2</sub>CO<sub>3</sub>–CaCO<sub>3</sub>–MgCO<sub>3</sub> System at 6 GPa: Implications for Diamond Forming Carbonatitic MeltsAnton V. Arefiev0Anton Shatskiy1Ivan V. Podborodnikov2Konstantin D. Litasov3Department of Geology and Geophysics, Novosibirsk State University, Novosibirsk 630090, RussiaDepartment of Geology and Geophysics, Novosibirsk State University, Novosibirsk 630090, RussiaDepartment of Geology and Geophysics, Novosibirsk State University, Novosibirsk 630090, RussiaDepartment of Geology and Geophysics, Novosibirsk State University, Novosibirsk 630090, RussiaCarbonate micro inclusions with abnormally high K<sub>2</sub>O appear in diamonds worldwide. However, the precise determination of their chemical and phase compositions is complicated due to their sub-micron size. The K<sub>2</sub>CO<sub>3</sub>−CaCO<sub>3</sub>−MgCO<sub>3</sub> is the simplest system that can be used as a basis for the reconstruction of the phase composition and <i>P−T</i> conditions of the origin of the K-rich carbonatitic inclusions in diamonds. In this regard, this paper is concerned with the subsolidus and melting phase relations in the K<sub>2</sub>CO<sub>3</sub>−CaCO<sub>3</sub>−MgCO<sub>3</sub> system established in Kawai-type multianvil experiments at 6 GPa and 900−1300 °C. At 900 °C, the system has three intermediate compounds K<sub>2</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub> (Ca# ≥ 97), K<sub>2</sub>Ca(CO<sub>3</sub>)<sub>2</sub> (Ca# ≥ 58), and K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub> (Ca# ≤ 10), where Ca# = 100Ca/(Ca + Mg). Miscibility gap between K<sub>2</sub>Ca(CO<sub>3</sub>)<sub>2</sub> and K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub> suggest that their crystal structures differ at 6 GPa. Mg-bearing K<sub>2</sub>Ca(CO<sub>3</sub>)<sub>2</sub> (Ca# ≤ 28) disappear above 1000 °C to produce K<sub>2</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub> + K<sub>8</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>7</sub> + K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub>. The system has two eutectics between 1000 and 1100 °C controlled by the following melting reactions: K<sub>2</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub> + K<sub>8</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>7</sub> + K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub> → [40K<sub>2</sub>CO<sub>3</sub>∙60(Ca<sub>0.70</sub>Mg<sub>0.30</sub>)CO<sub>3</sub>] (1st eutectic melt) and K<sub>8</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>7</sub> + K<sub>2</sub>CO<sub>3</sub> + K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub> → [62K<sub>2</sub>CO<sub>3</sub>∙38(Ca<sub>0.73</sub>Mg<sub>0.27</sub>)CO<sub>3</sub>] (2nd eutectic melt). The projection of the K<sub>2</sub>CO<sub>3</sub>−CaCO<sub>3</sub>−MgCO<sub>3</sub> liquidus surface is divided into the eight primary crystallization fields for magnesite, aragonite, dolomite, Ca-dolomite, K<sub>2</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub>, K<sub>8</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>7</sub>, K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub>, and K<sub>2</sub>CO<sub>3</sub>. The temperature increase is accompanied by the sequential disappearance of crystalline phases in the following sequence: K<sub>8</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>7</sub> (1220 °C) → K<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub> (1250 °C) → K<sub>2</sub>Ca<sub>3</sub>(CO<sub>3</sub>)<sub>4</sub> (1350 °C) → K<sub>2</sub>CO<sub>3</sub> (1425 °C) → dolomite (1450 °C) → CaCO<sub>3</sub> (1660 °C) → magnesite (1780 °C). The high Ca# of about 40 of the K<sub>2</sub>(Mg, Ca)(CO<sub>3</sub>)<sub>2</sub> compound found as inclusions in diamond suggest (1) its formation and entrapment by diamond under the <i>P−T</i> conditions of 6 GPa and 1100 °C; (2) its remelting during transport by hot kimberlite magma, and (3) repeated crystallization in inclusion that retained mantle pressure during kimberlite magma emplacement. The obtained results indicate that the K−Ca−Mg carbonate melts containing 20−40 mol% K<sub>2</sub>CO<sub>3</sub> is stable under <i>P−T</i> conditions of 6 GPa and 1100−1200 °C corresponding to the base of the continental lithospheric mantle. It must be emphasized that the high alkali content in the carbonate melt is a necessary condition for its existence under geothermal conditions of the continental lithosphere, otherwise, it will simply freeze.https://www.mdpi.com/2075-163X/9/9/558double potassium carbonatesbütschliiteultrapotassic carbonatite melthigh-pressure experimentdiamond formationcontinental lithosphere

The K<sub>2</sub>CO<sub>3</sub>–CaCO<sub>3</sub>–MgCO<sub>3</sub> System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts

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