Experimental studying of alcalic-carbonaceous metasomatism. Genesis of alkaline and carbonatites magmas.

Kostyuk A.V., Gorbachev N.S.

Institute of Experimental Mineralogy RAS, Chernogolovka, Russia

Eclogites along with peridotites are widespread in nodules mantle xenolites from kimberlites and alkalic basalts. They have their origin in eclogitization processes of basaltic rocks (basalt, gabbro) during the oceanic slab recycling. Evidences of partial melting are widely spread in xenolites. A number of xenolites contains silicate glasses which are enriched in alkalis (up to 16 wt.%), phlogopites, carbonates, sulphates, sanidine. Formation of such exotic structures is resulted from partial melting of eclogites caused by the influence of deep alkaline fluids. We studied interactions of eclogites with alcalic-carbonaceous at temperatures from 850 to 1450бC and at pressure 3.8-4 GPa experimentally.

Experiments were carried out in an anvil-with-hole apparatus by a quenching technique. The temperature is measured by a Pt30Rh/Pt6/Rh thermocouple. At high temperature, pressure is calibrated using a curve of balance quartz - coesite. Uncertainties are в 5ºC for temperature and в 0.1 GPa for pressure measurements. Duration of experiments were from 6 to 8 hours. Products of experiments were studied by PC-controlled scanning electron microscope Tescan VEGA TS 5130MM with detector of secondary and backscattered electron on the YAG-crystals and energy dispersive X-ray microanalyzer with semi-conductor Si(Li) detector INCA Energy 350.

Association of graphite-garnet-clinopyroxene-phlogopite with accessory chromite were formed at near-solidus (Ч=850бВ, Б=3.5GPa) alcalic-carbonaceous metasomatism of eclogite in C-saturated system. This association coexist with intergranular alkali silicate melt (not more than 5%). Carbonate phase has not been found out. Clinopyroxenes contain 11-13 wt.% of CaO and up to 3.0 wt.% K₂O (tab.1).

Table 1. Representative phase composition of run products in C-saturated system. Ч=850ºВ, Б=3.5 GPa.

With increases of temperature up to 1300ºВ at the same compositions degree of melting of eclogite increases (up to 30 % and more)). The sample is presented by clinopyroxene, phlogopite, accessory chromite which cementing by alkaline Na-K silicate melt. However, the carbonate phase is not found out too. (Fig. 1, tab. 2)

Table 2. Representative phase composition of run products in C-saturated system. Ч=1300ºВ, Б=3.9 GPa.

Composition of liquidus associations in high-temperature (up to 1450бВ) experiments differs from phase composition of near-solidus associations in low-temperature (850бВ) experiments. At high-temperature experiments we observed? only Ca-clinopiroxene (with <0.1 % K₂O), garnete and carbonate are not present. Possible explanation of carbonate phases absence in these experiments it is low activity Ва₂. Balance В-Ва-Ва₂ is displaced aside Ва in the presence of graphite at Ч-Б parametres of experiment. Absence of a garnet on the liquidus of alkaline silicate melts we also can explain as temperature effect.

Carbonatization of silicate melts were observed at alcalic-carbonaceous metasomatism and partial melting of eclogite at Ч=1200бВ, Б=3.8GPa. Carbonatization of silicate melts occurred in C-unsaturated system with formation of immiscibility alkaline silicate and carbonate liquids. Alkaline melts of phonolite composition coexist with carbonate melts, Na-clinopyroxenes, phlogopites and chromites (tabl.3, fig.2).

Table 3. Composition of coexisting phase at alcalic-carbonaceous metasomatism and melting of eclogite. Ч=1200ºВ Б=3.8 GPa.

Experimental samples are presented by large (tens micron) the tabular form silicate minerals, which are cementing by silicate glass (quench silicate melt) with the oval form inclusions of carbonate phases (quench carbonate melt). The size of carbonate phase is about 5-10 mm. Carbonates are enriched (in recalculation on 100% oxide, without Ва₂) by Ca (to 40 wt.% of CaO), Na (to 25 wt.% Na₂O), contain SiO₂ (to 5 wt.%), and ?characterized by high solubility of sulphur (to 3 wt.% SO₃) (fig. 2).

?

Fig. 2. Melting of metasomatic eclogite, liquid silicate-carbonate immiscibility. At the left - structures of co-existing phases: m - silicate melt, Ka - carbonate melt, Cpx - clinopyroxen, Flog - phlogopite. On the right - backscattered electron photograph of run products.

During researches has been finding out that alcalic-carbonaceous metasomatism and melting of eclogite leads to formation of alkaline and carbonate melts. Features of melt composition and associations of minerals coexisting with them show us efficiency of alcaline-carbonate melts in metasomatic transformation of the upper mantle and the important role of these processes in mantle magma-formation, including formation alkaline and carbonatite magmas.