Lherzolite - carbonatite melt interaction in the presence of additive CO - PowerPoint Presentation

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Lherzolite - carbonatite melt interaction in the presence of additive CO2 and H2O: Experimental data at 5.5 GPa and 1200-1450°C

V.S. Sobolev Institute of Geology and Mineralogy. Russian Academy of Sciences

A.N. Kruk. A.G. Sokol

According to the existing models of kimberlite origin, free exsolution CO2 may be an important agent in the evolution of primary kimberlite magma and initiation of crack propagation. We study the reaction of garnet lherzolite with carbonatitic melt rich in molecular CO2 and H2O in experiments at 5.5 GPa and 1200–1450 °C.

1 - ZrO

2 cell 2 –graphite heater3

- PtRh6/PtRh30 thermocouple 4 - MgO sleev 5 – Pt capsule

6 –

CsCl

7 - ZrO2 8 – Mo-leads.

M

ulti-anvil high-pressure apparatus of the split-sphere

type and high-pressure cell for studies at 5.5GPa

mm.

The accuracy in pressure and temperature control was

±0.1 GPa and ±20°C. respectively (Palyanov et al.. 2010).

Experiments were performed using a BARS multi

-

anvil apparatus at

5.5

GPa

using gold capsules or platinum capsules with rhenium

lining.

 GSB10

Lc (UD-05-05)Lc-GSLc-B10SiO20.62

16.0143.2429.1834.25

TiO2

0.02

0.19

0.13

0.090.15

Al2

O3

0.730.3

2.25

1.75

1.61Cr2O3–

0.11

0.680.460.49FeO3.9718.3111.198.8113.54MnO–0.340.110.070.19NiO–0.080.250.170.19MgO4.7722.2139.0227.7233.47CaO15.678.32.726.994.56Na2O0.180.470.230.210.31K2O31.35.80.1910.462.04CO242.7427.88–14.19.2Total100100100100100

Compositions of lherzolite from xenolith UD 05-05 (Lc). model carbonate melts (GS. B10) and bulk compositions of experimental samples

The composition GS (

Grassi

and Schmidt. 2011) was used for modeling the

carbonatitic

melt, which may be potentially generated in the subduction zones.

Asthenospheric

carbonatite

was simulated using the composition B10 (

Brey

et al.. 2011).

Peridotite

of a CLM source of

kimberlite

was simulated by fresh garnet

lherzolite

from xenolith UD-05-05 in the

Udachnaya

-East

kimberlite

. Additional H

2

O and molecular CO

2

 were incorporated into the experimental mixes as distilled water or oxalic

acid

.

Experimental samplesHere you can see typical sample textures after experiments. The quenched melt is concentrated in the hot zone of the capsule, and solid phases - in cold part.

Experimental

resultsIn general, all Lc-B10 and Lc-GS samples with added water only contained olivine and garnet but lacked orthopyroxene and magnesite, whereas olivine was low to absent in the Lc-B10 and Lc-GS mixes with added CO2 and H2O. Most samples of the latter group contained magnesite and orthopyroxene, while clinopyroxene and phlogopite were found in a few Lc-GS samples with added H2O only.

2Mg

2SiO4(Ol)+2CO2(melt)↔Mg2Si2O6(Opx)+2MgCO3(Mgs) (Wyllie et al.. 1983)

Experimental results

The interaction of carbonatite melt with peridotite in the presence of about 3% water and 13 wt% carbon dioxide added to the system gives multiphase saturated carbonate-silicate melts only with a low content of silica (<10

wt

% SiO

2

)

2Mg2SiO4(Ol)+2CO2(melt)↔Mg2Si2O6(Opx)+2MgCO3(Mgs) (Wyllie et al.. 1983)

Experimental

resultsThe generated carbonate-silicate melts show no bubbling even at twice greater contents of added CO2 and H2O. So, the supersaturation of kimberlite magma with molecular CO2 is impossible in the presence of source peridotite or mantle xenolith material since CO2 is buffered by carbonation of olivine with formation of orthopyroxene and magnesite under the CLM conditions.

The solubility of molecular carbon dioxide in melts

The solubility of molecular carbon dioxide in melts buffered by carbonization decreases from 25 wt% at low silica contents, typical of carbonatite, to 7–12 wt% at higher silica contents. In general, essentially carbonatitic melts rich in CO2 and H2O can separate from their harzburgite source at depths 150–200 km at typical CLM temperatures. Soon after its separation from the source, the carbonatitic melt enriched in molecular CO2 and H2O can rapidly transform into a melt containing >25 wt% SiO

2 due to partial dissolution and carbonation of peridotitic xenoliths.

Experimental

resultsInteraction at 1350 °C between lherzolite and carbonatitic melt GS, rich carbon dioxide and water, produces immiscible carbonate-silicate and potassium-rich silicate melts. The silicate melt can dissolve CO2, and its quenching with related large-scale ebullition leads to the formation of foam-like vesicular glass globules. Differentiation of the unmixed melts early during their ascent may lead to separation of a more viscous silicate melt from mobile carbonate-silicate magma rich in volatiles. As a result, kimberlite magmas generated in different CLM sources may acquire similar compositions.

You can find more detailed information about the results of these studies in our article.

Thank you for your attention!