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Kamafugitic melts resulted from interaction of peridotite with CaCO3-Na2CO3-KCl liquids at pressures 1-7 GPa: an experimental study.

 Oleg Safonov

 Institute of Experimental Mineralogy, Chernogolovka, Russia



Kamafugitic (SiO2-undersaturated potassic) melts are believed to originate from peridotite source mixed with phlogopite-pyroxene-carbonate veins (Foley, 1992) formed via an influx of alkali-rich fluids at 4-6 GPa (e.g. Rosenthal et al., 2009). Pairing of kamafugites with carbonatites (e.g. Stoppa et al., 2003), as well as fluid/melt inclusions in minerals of these rocks (Stoppa et al., 1997; Panina et al., 2003; Panina, Motorina, 2008) suggest that evolution of kamafugitic melts was accompanied by their equilibrium with complex non-silicate liquids composed of Ca-K-Na carbonates, halides, sulfur, phosphorus, etc. These components are able to provoke a multi-stage liquid immiscibility at different depth levels (e.g. Panina, Motorina, 2008). At pressures above 3 GPa, however, only chlorides could be responsible for immiscibility (Safonov et al., 2007, 2009).

In order to demonstrate the role of liquid immiscibility and carbonate-silicate decarbonation reactions in generation of the kamafugite-like melts from the peridotite source at different pressures, experiments on interaction of a model lherzolite Fo63En30Prp5Di2 with the [CaCO3]25[Na2CO3]25[KCl]50 liquid (peridotite/liquid  = 80/20) were performed at 7.0 with the Bridgmen anvil-with-hole apparatus, and 2.0 and 1.0 GPa with the piston-cylinder appratus.

At 7 GPa below 1460OC, chloride-carbonate melt (LCC) coexists with forsterite, grossular-rich garnet, and previously unknown KNCM-silicate phase (Na, K)2Ca4Mg2Si4O15. This phase forms at expense of the enstatite component of the peridotite interacting with the carbonate constituent of LCC: En + {2/3CaCO3 + 1/6Na2CO3} = 1/6KNCM + 1/3Fo + {5/6CO2}. Silica undersaturated Cl-bearing strongly carbonate-normative alkalic melt (LS) (~32 wt. % of SiO2, ~28 wt. % of K2O+Na2O, and 3.5 wt. % of Cl) immiscible with LCC, appears at 1460OC, being presumably produced via a complex peritectic reaction KNCM + Grt + LCC = Mrw + Cpx + LS. Cpx and Mrw disappear at higher temperatures, and LS coexists with forsterite and LCC. With increasing temperature, the SiO2, MgO, CaO contents of LS slightly increases being accompanied by depletion in Cl and alkalis. At 1560OC, these melts are more depleted in normative carbonate and become larnite-normative. Nevertheless, they are still strongly silica-undersaturated and highly alkalic (33-34 wt. % of SiO2, 24-25 wt. % of K2O+Na2O).

In contrast, alkalic silicate melt forming at 2 GPa within the interval 1200-1300OC are more silica-rich and less alkalic (38-39 wt. % of SiO2, 20-21 wt. % of K2O+Na2O) with about 1.5 wt. % of Cl. These melts are both carbonate and larnite-normative. They form via melting of the assemblage forsterite + clinopyroxene + kalsilite, which was generated from reactions of enstatite and pyrope constituents of the starting peridotite with the CaCO3+Na2CO3+KCl liquid: Prp + 2En + {2KCl + Na2CO3 + 1/3CaCO3} = 2Kls + 7/3Fo + 1/3Di + {2NaCl + 4/3CO2}. Melting is presumably accompanied by the formation of CO2-rich fluid.

The low-temperature (1050OC) assemblage at 1 GPa includes melilite, monticelite, and forsterite coexisting with LCC. Again, this assemblage is produced via reaction of enstatite and pyrope constituents of peridotite with LCC: Prp + 3En + {5CaCO3} = Mel (Ak + Geh) + Mtc + 2Fo + {5CO2}. At 1100OC, the assemblage Mel + Mtc reacts incongruently in the CO2-saturated LCC producing Ca-Tschermack-rich clinopyroxene and Ca-rich carbonate-silicate melt: Mel (Ak + Geh) + 3Mtc + {4CO2} = Cpx (2Di+CaTs) + Fo + {4CaCO3}. This melt (28 wt. % of SiO2) contains up to 15 wt. % of Al2O3 and up to 28 wt. % CaO, while its alkali concentration does not exceed 5 wt. %. It coexists with CaO-rich LCC formed due to immiscible separation of both chloride and carbonate components. However, with increasing temperature both melts rapidly loose CO2 approaching in composition to essentially silicate and essentially chloride liquids, respectively, at 1200OC. Chemical characteristics of these strongly larnite-normative silicate melt (up to 45 wt. % of SiO2, about 16 wt. % of K2O+Na2O, and just 1.0-1.2 wt. % of Cl) are very close to those of the kamafugitic magmas.

Thus, interaction of peridotite with the chloride-carbonate liquid within wide pressure interval includes (1) peritectic reactions of silicates with the liquid resulting in formation of CO2-rich fluid; (2) dissolution of silicates in the liquid saturating the melt in silica and alumina and (3) splitting of the liquid into silicate-rich (carbonate-silicate) and chloride-rich (chloride-carbonate) melts. Carbonates are responsible to the strong undersaturation of the melts in SiO2, while immiscibility provoked by chlorides (and carbonates at low pressures) assists to their enrichment in potassium. At high pressure (7 GPa) the melts are enriched in carbonates. A dramatic lost of CO2 with decompression is accompanied by unmixing of Cl and lowering in alkalinity. For example, correlation between CO2 content and alkalinity is well-known for potassic lavas from East African rift, and it is supposed to be a consequence of different depths of melting (Rosenthal et al., 2009). Thus, being formed within mantle peridotite metasomatized by alkaline non-silicate liquids, the melts loose Cl during their ascent (Kamenetsky et al., 1995; Edgar et al., 1994) preserving high potassium content. The experiments clearly support that “at earlier stages of ascent, where a free vapor phase is unlikely, Cl may remain in the melt and participate in melting of mantle veined material” (Edgar et al., 1994, p. 191). In addition, the experiments prove close relations between kamafugitic melts and halogene-enriched carbonatites, which can be both precursors for the silicate melts at low degree of partial melting of the metasomatized peridotite and coexist as immiscible fractions during the ascent of the magmas.


The study is supported by the RFBR (10-05-00040), Russian President Grant (MD-380.2010.5) and Russian Science Support Foundation.



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