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 1460^OC, chloride-carbonate melt (L_CC) 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 L_CC: En + {2/3CaCO₃ + 1/6Na₂CO₃} = 1/6KNCM + 1/3Fo + {5/6CO₂}. Silica undersaturated Cl-bearing strongly carbonate-normative alkalic melt (L_S) (~32 wt. % of SiO₂, ~28 wt. % of K₂O+Na₂O, and 3.5 wt. % of Cl) immiscible with L_CC, appears at 1460^OC, being presumably produced via a complex peritectic reaction KNCM + Grt + L_CC = Mrw + Cpx + L_S. Cpx and Mrw disappear at higher temperatures, and L_S coexists with forsterite and L_CC. With increasing temperature, the SiO₂, MgO, CaO contents of L_S slightly increases being accompanied by depletion in Cl and alkalis. At 1560^OC, 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 1200-1300^OC 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 (1050^OC) assemblage at 1 GPa includes melilite, monticelite, and forsterite coexisting with L_CC. Again, this assemblage is produced via reaction of enstatite and pyrope constituents of peridotite with L_CC: Prp + 3En + {5CaCO₃} = Mel (Ak + Geh) + Mtc + 2Fo + {5CO₂}. At 1100^OC, the assemblage Mel + Mtc reacts incongruently in the CO₂-saturated L_CC 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 L_CC 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 1200^OC. 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.

References:

Edgar A.D., Lloyd F.E., Vukadinovic D. The role of fluorine in evolution of ultrapotassic magmas // Mineralogy and Petrology. 1994. Vol. 51. P. 173-193.

Foley S.R. Vein-plus-wall-rock melting mechanisms in the lithosphere and the origin of potassic alkaline magmas // Lithos. 1992. Vol. 28. P. 435-453.

Kamenetsky V.S., Metrich N., Cioni R. Potassic primary melts of Vulsini (Roman Province): evidence from mineralogy and melt inclusions // Contribution to Mineralogy and Petrology. Vol. 120. P. 186-196.

Panina L.I., Motorina I.V. Liquid immiscibility in deep-seated magmas and the generation of carbonatite melts // Geochemistry International. 2008. Vol. 46. P. 448-464.

Panina L.I., Stoppa F., Usol’tseva L.M. Genesis of melilite rocks of Pian di Celle volcano, Umbrian kamafugite province, Italy: evidence from melt inclusions in minerals // Petrology. 2003. Vol. 11. P. 365-382.

Rosenthal A., Foley S.F., Pearson D.G., Nowell G.M., Tappe S. Petrogenesis of strongly alkaline primitive volcanic rocks at the propagating tip of the western branch of the East African Rift // Earth and Planetary Science Letters. 2009. Vol. 284. P. 236-248.

Safonov O.G., Perchuk L.L., Litvin Yu.A. Melting relations in the chloride-carbonate-silicate systems at high-pressure and the model for formation of alkalic diamond-forming liquids in the upper mantle // Earth and Planetary Science Letters. 2007. Vol. 253, P. 112-128.

Safonov О.G., Chertkova N.V., Perchuk L.L., Litvin Yu.А. Experimental model for alkalic chloride-rich liquids in the upper mantle // Lithos. 2009. Vol. 112S, P. 260-273.

Stoppa F., Lloyd F., Rosatelli G. CO₂ as the virtual propellant of carbonatite-kamafugite conjugate pairs and the eruption of diatremic tuffisite // Periodico di Mineralogia. 2003. Vol. 72, P. 1-18.

Stoppa F., Sharygin V.V., Cundari A. New mineral data from the kamafugitecarbonatite association: The melilitolite from Pian di Celle, Italy // Mineralogy and Petrology. 1997. Vol. 61. P. 27-45.