Мelting
of garnet-bearing carbonatite of Tromsø area, Norvway
N.S.
Gorbachev,
A.N.
Nekrasov,
A.V.
Kostyuk,
D.M.
Sultanov
Institute
of experimental mineralogy RAS, Chernogolvka, Russia
gor@iem.ac.ru.
In connection
with the problem of the genesis of carbonatites at T-P of upper mantle
(950-1450
º C, 3.8-4.0 GPa) studied the melting of garnet-bearing ultra-high
pressure carbonatite UHPC Tromsø
area,
Norway in "dry" conditions and with H2O+CO2 fluid.
Experiments were carried
out in the IEM RAS on the "anvil with hole" in Au and Fe-bearing
platinum capsules using 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 ± 10ºC for temperature and ± 0.1 GPa for pressure measurements.
Duration of experiments were from 6 to 18 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.
Temperature
T "dry"
liquidus UHPC ~ 1270 º C. It
is established full miscibility between silicate and carbonate melts. At
quenching
carbonatite
melt the mix of
microlites of
variable
composition, from the carbonate-silicate to silicate-carbonate
is formed.
In the range of T
1250-1200ºС
with carbonatite melt garnet Grt co-exists. This garnet differing from
initial garnet UHPC by absence of zonality, higher content of CaO, TiO2,
lower Al2O3, FeO.
Temperature "dry" solidus
is ~1150ºС.
In the range Т
from 1150 to 950ºС
near soliduse and subsolidus associations are presented calcite
Сс,
garnet Grt, clinopyroxene Сpx,
flogopite Flog and accessory minerals - apatite Apt, ilmenite Ilm,
rutile Rt.
In experiments with
Н2О+СО2
fluid temperature
liquidus
goes down to 1250ºС.
At Т=1450ºC,
Р=4
GPa stratification carbonatite melt on highly and low-carbonate-silicate
liquids with graphite allocation was observed (fig. 1).
![](GorbachevN2.files/image004.gif)
Fig. 1. Stratification
carbonatite melt on highly and low- carbonate-silicate liquids with
graphite allocation: microphoto an composition of the
quenching
carbonatite melt
sample. 1450ºC, 4 GPa.
In range T=1250-1150ºС
with carbonatite melt the zonality garnet co-exists. Its reactionary
border in comparison with its central part is enriched in
FeO, MgO, MnO and depleted in . CaO, TiO2, SrO (fig. 2).
Zonality of garnet testifies to interaction of carbonatite melt with
garnet at which the reactionary garnet is enriched in CaO, TiO2,
SrO.
![](GorbachevN2.files/image008.gif)
Fig. 2. Microphoto and
composition of the
quenching
sample with zonality
garnet. T 1150ºС,
Р=4
GPa.
Zonality of garnet
testifies to interaction of carbonatite melt with garnet at which the
reactionary garnet is enriched in CaO, TiO2, SrO.
At melting of UHPC with
Н2О+СО2
a fluid
temperature solidus goes down to
Т ~ 950ºС.
At Т=950ºС,
P=3.8 GPa carbonatite melt co-exists with
Сс,
Flog, Grt, Cpx, Apt. At increase
Т up to 1050ºС
the portion of carbonatite melt increases, disappears Cpx, on liqudus
present Сс,
zonality Grt, Flog,
Results of experiments
testify to formation characteristic for UHPC associations a carbonate-zonality
garnet in the range of Т
=1150-1250ºС.
This study was financially supported by RFBR grant
09-05-0113-a,
12-05-00777-a.- |