Metasomatic interaction of the eutectic Na- and K-bearing carbonate melts with natural garnet lherzolite at 6 GPa and 1100–1200°C: Toward carbonatite melt composition in SCLM

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The range of carbonatite melts in equilibrium with the subcontinental lithospheric mantle (SCLM) under geothermal conditions is limited by alkali-rich near-eutectic compositions. Therefore, here we employed eutectic Na/K-Ca-Mg-Fe carbonate mixtures to model the interaction of a metasomatic carbonatite melt with natural garnet lherzolite. The experiments were performed at 1100 and 1200 °C and 6 GPa in graphite capsules using a multianvil press. The run duration was 111 and 86 h, respectively. To verify achieving an equilibrium, a synthetic mixture identical to natural lherzolite was also employed.We have found that both Na- and K-bearing carbonatite melts cause wehrlitization accompanying by the elimination of orthopyroxene and an increase of CaO in garnet at a constant Cr2O3. Interaction with the K‑carbonatite melt alters clinopyroxene composition toward lower Na2O (0.2–0.3 wt%), and higher K2O (0.5–1.0 wt%), whereas the Na‑carbonatite melt revealed the opposite effect. The resulting melts have a following approximate composition [40(Na, K)2CO3∙60Ca0.5Mg0.4Fe0.1CO2 + 0.6–1.4 wt% SiO2] displaying a decrease in Ca# at a nearly constant alkali content relative to the initial composition, where Ca# = 100∙Ca/(Ca + Mg + Fe). We have also found that alkali-poor (≤ 20 mol% (Na, K)2CO3) carbonate mixtures do not melt completely but yield magnesite and alkali- and Ca-rich melts like those in the systems with eutectic mixtures.Under SCLM P-T conditions the range of carbonatite melt compositions is restricted by the full melting field of alkali-rich carbonates in the corresponding Na/K-Ca-Mg carbonate systems. Infiltration of less alkaline higher-temperature carbonatite melt in SCLM and its subsequent cooling to the ambient mantle temperature, 1100–1200 °C at 6 GPa, should cause crystallization of magnesite and shift the melt composition to [30(Na, K)2CO3∙70Ca0.6Mg0.3Fe0.1CO3]. Owing to its high Ca#, this melt is not stable in equilibrium with orthopyroxene yielding its disappearance by CaMg exchange reaction producing clinopyroxene, magnesite, and shifting the melt composition toward higher alkali content. The melts containing 40–45 mol% of alkaline carbonates have no limitation in Ca# because the corresponding binary NaMg and KMg carbonate eutectics are located near 1200 °C. Therefore, these melts can achieve Ca# ≤ 30–40 and, be in equilibrium with garnet lherzolites and harzburgites under the geothermal condition of SCLM.Considering the present results and previous experimental data the following ranges of carbonatite melt compositions can be expected in equilibrium with garnet peridotites at the base of SCLM: Ca# < 30 and > 30 mol% (K, Na)2CO3 in equilibrium with harzburgite; Ca# 30–40, >25 mol% (K, Na)2CO3 in equilibrium with lherzolite; and Ca# 40–60 and >20 mol% (K, Na)2CO3 in equilibrium with wehrlite.