Abstract
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–CaCO3–MgCO3 is the simplest system that can be used as a basis for the reconstruction of the phase composition and P–T 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 K2CO3–CaCO3–MgCO3 system established in Kawai-type multianvil experiments at 6 GPa and 900–1300 °C. At 900 °C, the system has three intermediate compounds K2Ca3(CO3)4 (Ca# ≥ 97), K2Ca(CO3)2 (Ca# ≥ 58), and K2Mg(CO3)2 (Ca# ≤ 10), where Ca# = 100Ca/(Ca + Mg). Miscibility gap between K2Ca(CO3)2 and K2Mg(CO3)2 suggest that their crystal structures differ at 6 GPa. Mg-bearing K2Ca(CO3)2 (Ca# ≤ 28) disappear above 1000 °C to produce K2Ca3(CO3)4 + K8Ca3(CO3)7 + K2Mg(CO3)2. The system has two eutectics between 1000 and 1100 °C controlled by the following melting reactions: K2Ca3(CO3)4 + K8Ca3(CO3)7 + K2Mg(CO3)2 → [40K2CO3∙60(Ca0.70Mg0.30)CO3] (1st eutectic melt) and K8Ca3(CO3)7 + K2CO3 + K2Mg(CO3)2 → [62K2CO3∙38(Ca0.73Mg0.27)CO3] (2nd eutectic melt). The projection of the K2CO3–CaCO3–MgCO3 liquidus surface is divided into the eight primary crystallization fields for magnesite, aragonite, dolomite, Ca-dolomite, K2Ca3(CO3)4, K8Ca3(CO3)7, K2Mg(CO3)2, and K2CO3. The temperature increase is accompanied by the sequential disappearance of crystalline phases in the following sequence: K8Ca3(CO3)7 (1220 °C) → K2Mg(CO3)2 (1250 °C) → K2Ca3(CO3)4 (1350 °C) → K2CO3 (1425 °C) → dolomite (1450 °C) → CaCO3 (1660 °C) → magnesite (1780 °C). The high Ca# of about 40 of the K2(Mg, Ca)(CO3)2 compound found as inclusions in diamond suggest (1) its formation and entrapment by diamond under the P–T 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% K2CO3 is stable under P–T 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.
Highlights
IntroductionPresence of crystalline carbonates at different mantle levels follows from the occurrence of magnesite, dolomite, calcite, and/or aragonite in spinel peridotite and eclogite xenoliths [1,2,3], as primaryMinerals 2019, 9, 558; doi:10.3390/min9090558 www.mdpi.com/journal/mineralsMinerals 2019, 9, 558 inclusions in Cr-pyropes derived from 100–130 km depth [4,5], and as inclusions in diamonds derived from the base of the continental lithosphere and deeper levels [6,7,8,9,10,11,12,13]
The temperature increase is accompanied by the sequential disappearance of crystalline phases in the following sequence: K8 Ca3 (CO3 )7 (1220 ◦ C) → K2 Mg(CO3 )2 (1250 ◦ C) → K2 Ca3 (CO3 )4 (1350 ◦ C) → K2 CO3 (1425 ◦ C) → dolomite (1450 ◦ C) → CaCO3 (1660 ◦ C) →
Our results suggest that the partial substitution of Ca with Mg extends its stability to 1000 ◦ C
Summary
Presence of crystalline carbonates at different mantle levels follows from the occurrence of magnesite, dolomite, calcite, and/or aragonite in spinel peridotite and eclogite xenoliths [1,2,3], as primaryMinerals 2019, 9, 558; doi:10.3390/min9090558 www.mdpi.com/journal/mineralsMinerals 2019, 9, 558 inclusions in Cr-pyropes derived from 100–130 km depth [4,5], and as inclusions in diamonds derived from the base of the continental lithosphere and deeper levels [6,7,8,9,10,11,12,13]. The inclusions retain high internal pressure suggesting the mantle origin of the entrapped melt [26] This melt was found as micro inclusions in the central part of a gem-quality diamond crystal [27] and along the twinning plane in ancient diamonds [28]. This suggests that alkali-rich carbonate melts have been introduced into the reduced lithospheric mantle since the Archaean and that these melts could be responsible for the formation of most lithospheric diamonds [28]
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