1-atm experiments with
alkaline melts: problems, solutions and some results
Borisov A.A.
Russian Academy of Sciences,
Moscow,
A vertical tube
furnace with controlled oxygen fugacities in
combination with a loop technique is a standard experimental design for one atm studies in geochemistry, cosmochemistry
and petrology. However, the investigation of alkalis-containing system,
especially at low fO2 values, is very limited. The main reason is
the loss of Na and K in open systems where the oxygen fugacity is imposed by a
gas mixture (CO/CO2, H2/CO2 etc). Therefore, a
new experimental device CSLT (crucible supported loop technique) was designed
to prevent Na losses of experimental charges primarily for experiments at low
oxygen fugacities (Borisov
et al., 2006). Later CSLT was modified to allow not only preventing alkalis losses,
but also saturating of initially alkalis-free melts with Na, K and Rb.
In brief, a single
or a few sample loops place above alkali-rich melts in a quartz crucible. A lid
loosely covers the crucible, so that exchange with the surrounding gas is
possible. Exchange with the furnace atmosphere is necessary for controlling fO2
inside the crucible. The alkali-rich atmosphere inside the crucible allows
maintenance of alkali concentrations in the loop samples in the weight-percent
level for hours or even days. In our practice we have used two types of
crucibles. Small crucibles (
CSLT has some limitations, however a few experimental studies in different
areas of earth science were conducted:
1.) We have
experimentally studied the effect of Na2O on the solubility of Fe,
Co, and Pd (Borisov et al., 2006). All solubilities clearly decrease (correspondent activity
coefficients increase) with increasing Na2O-contents. The increase
of FeO activity coefficients with increasing sodium
found in this study may provide an explanation for decreasing FeO with decreasing degrees of partial melting in
experimentally produced mantle melts (Hirschmann et
al. 1998), as melts at low melting degrees are sodium-rich.
2.) We have
reported experimentally determined 1 atm olivine/melt
DNa partitioning data for low fO2,
a variety of melt compositions and a temperature range of 1325-1522ºC (Borisov et al., 2008). We demonstrated that high-current
electron microprobe analyses (EPMA, I = 500 nA, 600 s on the peak) allow quantitative
determination of Na2O in olivine down to about 10 ppm. The mean olivine/melt DNa from 12 experimental runs is
0.0031╠0.0007 (1σ). This is the recommended value for low pressures and a
wide range of natural compositions.
3.) We have found
relative activity coefficients of alkalis in multicomponent
silicate melts (Borisov, 2008; Borisov,
submitted). Model melts (Di-An eutectic modified with
variable amounts of silica and Ca2Si3O8
modified with variable amounts of alumina) were experimentally saturated with
Na, K and Rb at temperatures of 1300√1470° C. Either
binary Na2O-SiO2 and K2O-SiO2 or
complex Na2O-K2O-SiO2 and Na2O-K2O-Rb2O-SiO2
melts were used as alkalis sources. It was shown that alumina content in many
cases has little influence on the degree of silicate melt saturation with
alkalis. In contrast, silica content strongly affects the solubility of alkalis
in silicate melts. Model calculations showed that the perfectly mobile behavior
of alkalis leads to a potassium specifics in basalts
contaminated by crustal materials and, correspondingly, a sodic
specifics in silicic melts from crustal xenoliths contaminated
by the enclosing basaltic melt.
References:
Borisov A.A. Experimental
investigation of K and Na partitioning between miscible liquids. // Petrology.
2008. Vol. 16. P. 552-564.
Borisov A.A. Dependence of alkalis activity coefficients on
silica and alumina contents in silicate melts: an experimental investigation.
// Petrology. Submitted.
Borisov A., Lahaye Y. and Palme H. The effect of sodium on the solubilities of metals in silicate melts. // American
Mineralogist. 2006. Vol. 91. P. 762-771.
Borisov A., Pack A., Kropf A. and
Palme H. Partitioning of Na between olivine and melt: An experimental study
with application to the formation of meteoritic Na2O-rich chondrule glass and refractory forsterite
grains. // Geochimica Cosmochimica
Acta. 2008. Vol. 72. P. 5558-5573.
Hirschmann
M.M., Baker M.B. and Stolper E.M. The effect of
alkalis on the silica content of mantle-derived melts. // Geochimica
et Cosmochimica Acta. 1998. Vol. 62. P. 883-902.