Evidence for the alkaline nature of parental carbonatite melts at Oka complex in Canada

Abstract

The Earth's sole active carbonatite volcano, Oldoinyo Lengai (Tanzania), is presently erupting unique natrocarbonatite lavas that are characterized by Na-and K-bearing magmatic carbonates of nyerereite [Na 2 Ca(CO 3 ) 2 ] and gregoryite [(Na 2 ,K 2 ,Ca)CO 3 ]. Contrarily, the vast majority of older, plutonic carbonatite occurrences worldwide are dominated by Ca-(calcite) or Mg-(dolomite)-rich magmatic carbonates. Consequently, this leads to the conundrum as to the composition of primary, mantle-derived carbonatite liquids. Here we report a detailed chemical investigation of melt inclusions associated with intrusive (plutonic) calcite-rich carbonatites from the B120 Ma carbonatite complex of Oka (Canada). Melt inclusions are hosted by magnetite (Fe 3 O 4 ), which crystallizes through a significant period of carbonatite melt solidification. Our results indicate mineral assemblages within the melt inclusions that are consistent with those documented in natrocarbonatite lavas. We propose therefore that derivation of alkali-enriched parental carbonatite melts has been more prevalent than that preserved in the geological record.

Chlorine in mantle-derived carbonatite melts revealed by halite in the St.-Honoré intrusion (Québec, Canada)

“…2) that are proposed to represent an early crystallization stage (Fournier, 1993) indicates Figure DR4 Although other carbonatite complexes are known to have halitebearing melt inclusions in a variety of phenocrysts (apatite, carbonates, fluorite, magnetite, perovskite; e.g., Andreeva et al, 2006;Chen et al, 2013;Panina, 2005;Sharygin et al, 2011;Zaitsev et al, 2002), none are reported to contain macroscopic segregations and/or veins with significant modal halite such as those in the St.-Honoré carbonatite. The only known exception is the natrocarbonatite lavas of the Oldoinyo Lengai volcano, where groundmass chlorides (halite and sodian sylvite), fluorite, and alkali carbonate phenocrysts (gregoryite and nyerereite) are primary magmatic minerals (Keller and Krafft, 1990;Mitchell, 1997Mitchell, , 2006.…”

“…The occurrence of primary melt inclusions with chloride-and alkali carbonate-rich compositions in several intrusive carbonatites ( Fig. DR5; also see Andreeva et al, 2006;Chen et al, 2013;Panina, 2005;Sharygin et al, 2011;Zaitsev et al, 2002) suggests that the current dolomitic-calcitic mineralogy of these carbonatites is not representative of their parental melts. This is supported by experiments that indicate that primary, mantle-derived carbonatitic melts have sodic dolomite compositions (Litasov et al, 2013;Wallace and Green, 1988) and generate alkali carbonate-chloride liquids by immiscibility at mantle conditions (Safonov et al, 2007).…”

“…Low-temperature (<600 °C) natrocarbonatitic melts at Oldoinyo Lengai are strongly enriched in Na 2 O and K 2 O (total of 38-40 wt%) and halogens (to 4.5 wt% F and 5.7 wt% Cl) and have low Ca (15 wt%) and insignificant Mg-Fe contents (<1 wt%) (Keller and Krafft, 1990;Keller and Zaitsev, 2012), unlike the Ca-Mg-dominated compositions typical of intrusive carbonatites. Such unusual compositions at the only active carbonatite volcano, coupled with petrographic and geochemical evidence of alkali-rich compositions in prehistoric extrusive carbonatites (Clarke and Roberts, 1986;Deans and Roberts, 1984;Guzmics et al, 2011;Hay, 1983;Zaitsev, 2010;Zaitsev et al, 2013) and some intrusive carbonatites (Andreeva et al, 2006;Chen et al, 2013;Kogarko et al, 1991;Sharygin et al, 2011;Zaitsev et al, 2002), suggest that the much more common Ca-Mg-dominated compositions may not represent primary liquid compositions. Calcitic and dolomitic carbonatites may instead represent cumulates and/or residues and alteration products of originally alkali-rich magmatic liquids.…”

The K₂CO₃ fusion curve revisited: New experiments at pressures up to 12 GPa

“…the exception of alkali-rich natrocarbonatite lavas erupted at Oldoinyo Lengai in Tanzania (Dawson, 1962). However, many studies (Dawson, 1993;Zaitsev and Keller, 2006;Le Bas, 2008;Chen et al, 2013) argue that the parental carbonatite liquid is alkali-rich. Such a carbonate component could be easily lost within a few days through replacement by secondary calcite at atmospheric conditions, or the loss of alkalis can occur during melt evolution and crystallization such as that during fenitization of country rocks.…”

Raman spectra of nyerereite, gregoryite, and synthetic pure Na2Ca(CO3)2: diversity and application for the study micro inclusions

“…Micro‐sized and nano‐sized inclusions of Na–Ca carbonates are, however, quite common in minerals from various carbonatite and ultramafic alkaline rocks, e.g. the Vulture, Kerimasi, Tinderet, and Catanda volcanoes, Guli pluton, Afrikanda, Gardiner, and Kovdor ultramafic‐alkaline complexes, and the Oka carbonatite complex . In addition, (Na,K) 2 Ca(CO 3 ) 2 and/or (Na,K) 2 Ca(CO 3 ,SO 4 ) 2 magmatic carbonates occur occasionally in kimberlite groundmass and chloride‐carbonate segregations and have been more often identified among daughter phases in melt inclusions in various minerals from kimberlites and their mantle xenoliths from different parts of the world .…”