Experimental studying of alcalic-carbonaceous
metasomatism. Genesis of alkaline and carbonatites magmas.
Kostyuk
A.V.,
Institute
of Experimental Mineralogy RAS,
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
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 â 5ºC for temperature and â 0.1 GPa for pressure measurements. Duration of
experiments were from
Association of graphite-garnet-clinopyroxene-phlogopite with accessory chromite
were formed at near-solidus (×=850áÂ, Á=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. ×=850ºÂ, Á=3.5 GPa.
|
Ga |
Cpx |
Flog |
m |
Cht |
SiO2 |
39.6 |
50.7 |
36.2 |
52.54 |
0.2 |
TiO2 |
0.4 |
0.1 |
0.3 |
0.75 |
0.4 |
Al2O3 |
19.1 |
7.0 |
18.0 |
13.44 |
31.3 |
Cr2O3 |
1.0 |
- |
0.3 |
0.79 |
33.6 |
FeO |
11.6 |
8.9 |
23.6 |
8.65 |
16.6 |
MgO |
1.2 |
13.8 |
10.8 |
5.01 |
16.2 |
CaO |
25.9 |
13.7 |
0.3 |
10.47 |
0.1 |
Na2O |
0.6 |
2.1 |
0.6 |
7.6 |
0.3 |
K2O |
<0.1 |
3.0 |
9.5 |
0.64 |
|
Total |
99.5 |
99.3 |
99.6 |
99.89 |
98.7 |
With increases of temperature
up to 1300ºÂ 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. ×=1300ºÂ, Á=3.9 GPa.
|
Cpx |
Flog |
m |
SiO2 |
46.72 |
41.66 |
46.10 |
TiO2 |
3.60 |
1.55 |
0.37 |
Al2O3 |
14.44 |
14.61 |
19.87 |
Cr2O3 |
0.11 |
0.37 |
0.12 |
FeO |
2.53 |
1.69 |
1.38 |
MnO |
0.2 |
0.2 |
0.1 |
MgO |
11.27 |
25.22 |
0.2 |
CaO |
20.36 |
0.15 |
0.32 |
Na2O |
2.08 |
0.56 |
8.60 |
K2O |
<0.1 |
9.01 |
3.61 |
Total |
98.24 |
95.13 |
81.50 |
|
Fig. 1.
Backscattered electron photograph of run products. Melting of eclogite in C-saturated system at T=1300ºÂ, Á=3.9GPa. |
Composition of liquidus associations in high-temperature (up to 1450áÂ)
experiments differs from phase composition of near-solidus associations in low-temperature
(850áÂ) experiments. At high-temperature experiments we observedš 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 Âà2. Balance Â-Âà-Âà2
is displaced aside Âà in the presence of graphite at ×-Á 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 ×=1200áÂ, Á=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. ×=1200ºÂ Á=3.8 GPa.
|
Cpx |
Flog |
m |
ÿÒ |
Cht |
SiO2 |
52.1 |
40.1 |
49.0 |
2.4 |
0.65 |
TiO2 |
0.5 |
1.4 |
0.5 |
0.1 |
0.7 |
Al2O3 |
7.8 |
12.9 |
14.9 |
1.0 |
21.75 |
Cr2O3 |
0.8 |
0.2 |
0.2 |
0.1 |
37.93 |
FeO |
8.4 |
18.0 |
5.0 |
12.0 |
28.79 |
MgO |
10.0 |
7.9 |
1.2 |
1.2 |
7.05 |
CaO |
14.3 |
1.7 |
2.4 |
20.6 |
0.26 |
Na2O |
4.3 |
1.8 |
6.3 |
12.6 |
0.23 |
K2O |
0.1 |
6.1 |
6.1 |
1.1 |
0.08 |
Total |
98.4 |
92.2 |
83.6 |
52.0 |
97.44 |
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 Âà2) by Ca (to 40 wt.% of CaO), Na (to 25 wt.% Na2O),
contain SiO2 (to 5 wt.%), and šcharacterized by high solubility of sulphur (to 3 wt.% SO3) (fig. 2).
š
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.