Geochimica et Cosmochimica Acta, 1999
Condensation in Dust-enriched Systems, by D.S. Ebel and L. Grossman,
Geochimica et Cosmochimica Acta, 1999
------------------ Figure 10: Compositions of condensate liquids at (a), (b) 10-3 bar and a dust enrichment of 100x; (c), (d) 10-3 bar and a dust enrichment of 1000x; and (e), (f)10-6 bar and a dust enrichment of 1000x. In all cases, the vertical line marks the condensation temperature of olivine, where the transition between CMAS and MELTS liquid models is made. In (e) and (f), Na2O (> or =) 0.01 wt%. Inflection points are due to the onset of crystallization or disappearance of a coexisting phase, and are labelled as follows: a, spinel in; d, metal in; e, orthopyroxene in; f, Cr-spinel in; g, feldspar in; h, clinopyroxene in; k, rhombohedral oxide in; p, perovskite in; q, clinopyroxene in and liquid out. (enlarge a) (enlarge b) (enlarge c) (enlarge d) (enlarge e) (enlarge f) |
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The temperature variation of the composition of the silicate melt is shown for the cases of 10-3 bar
and a dust enrichment of 100x in Figs. 10a and b, 10-3 bar and a dust enrichment of 1000x in Figs. 10c and d, and
for 10-6 bar and a dust enrichment of 1000x in Figs.10e and f. The evolution of the liquid composition is similar
in all cases, but some exceptions are noteworthy. Because Al is more refractory than Ca, the Al2O3 content
of the initial liquid is very high but falls with decreasing temperature due to dilution by CaO which condenses more gradually with falling temperature.
Similarly, at lower temperatures, incipient condensation of more volatile Si and Mg into the liquid causes the concentrations of
SiO2 and MgO to increase, diluting both Al2O3 and CaO.
Comparing Figs. 10a and c, it is seen that, for liquids that form at higher temperatures than olivine, similar liquid compositions are stable
at temperatures 300K higher when the dust enrichment is increased by a factor of 10 at 10-3 bar.
Similarly, comparing Figs. 10c and e reveals that liquids of similar composition form about 400K higher when Ptot is
increased by a factor of 1000 at a dust enrichment of 1000x. Note, however, that, at the condensation temperatures of olivine, liquid compositions are quite
different from one another at different combinations of Ptot and dust enrichment.
For example, the concentrations of MgO and SiO2 in the liquid at a dust enrichment of 1000x are almost the same where olivine
condenses at 10-3 bar (Fig. 10c) but the SiO2 content is more than double that of MgO
at 10-6 bar (Fig. 10e). One way of understanding this is by considering the fact that the solubility of olivine in a melt
of a given composition is quite different at temperatures hundreds of degrees apart.
Below the condensation temperature of olivine, the main difference in major element trends of the liquid at different combinations
of Ptot and dust enrichment, aside from those of FeO and alkalis, is the failure of the SiO2
concentration to level off with falling temperature at a dust enrichment of 1000x at 10-3 bar (Fig. 10c) as it does at a dust
enrichment of 100x at 10-3 bar (Fig. 10a) and 1000x at 10-6 bar (Fig. 10e).
This is entirely due to the absence of orthopyroxene from the crystalline assemblage in equilibrium with the liquid at a dust enrichment of 1000x
at 10-3 bar. It is the condensation of this phase which triggers the flattening of the SiO2 curve
under the other sets of conditions. Of the three cases shown, it is at a dust enrichment of 1000x and 10-3 bar where
the Cr2O3 content of the liquid is highest, climbing to 1.2 wt% with falling temperature and then
declining after Cr-spinel becomes stable at 1710K (Fig. 10d). At a dust enrichment of 100x at 10-3 bar, this phase becomes
stable at 1600K, before the Cr2O3 content of the liquid reaches 0.36 wt% (Fig. 10b). At a dust enrichment
of 1000x at 10-6 bar, Cr-spinel coexists with the MELTS liquid over its entire stability range, preventing its
Cr2O3 content from exceeding 0.28 wt% (Fig. 10f). As the amount of liquid becomes vanishingly small
during near-solidus crystallization of clinopyroxene and plagioclase, concentrations of TiO2 are seen to build up in the
last dregs of liquid. Only at a dust enrichment of 1000x at 10-3 bar does this trend reverse itself. This is due to
stabilization of a pyrophanite-rich solid solution at a temperature above that for the disappearance of liquid. As seen by comparing Figs. 10b, d and f,
alkali contents of the liquid increase both with increasing Ptot and with increasing dust enrichment because the partial
pressures of sodium and potassium increase with both parameters. As a result, Na2O and K2O
concentrations in the liquid are negligible at 10-6 bar, even at a dust enrichment of 1000x. In the other cases shown,
Na2O and K2O concentrations rise above negligible levels only within 100-200K of the
temperature of disappearance of liquid, reaching maxima of 10.1 wt% and 1.3 wt%, respectively, at 10-3 bar and a dust
enrichment of 1000x. At a dust enrichment of 1000x, the FeO content of the liquid at 10-3 bar is higher than
at 10-6 bar at most temperatures (Figs. 10c and 10e), considerably so at some temperatures.
Because fO2 is only weakly dependent on Ptot at 1500-1600K, the higher FeO
content of the liquid is due simply to the higher PFe at higher Ptot, which causes a greater proportion
of the iron to be condensed at any given temperature.
In most cases, the liquid disappears in the temperature interval 1370 to 1400K, the approximate location of the peridotite solidus at 1 bar (see Table 5).
An exception to this general rule is found in Table 8 for the case of a dust enrichment of 1000x at 10-3 bar, where the liquid
persists to 1310K. At the same Ptot and a dust enrichment of 500x, the liquid disappears at a significantly higher temperature, 1400K.
Similarly, at the same dust enrichment, 1000x, and lower Ptot, 10-6 bar, the liquid also disappears
at a much higher temperature, 1370K. At 10-3 bar, the reason for the different solidification temperatures at the different dust
enrichments is evident from a comparison of the liquid compositions in the two cases at 1410K, the last temperature step before the liquid disappears at a dust
enrichment of 500x. At this temperature, the liquid at the lower dust enrichment contains slightly less Na2O, 2.67 wt%, and much
less FeO, 3.26%, than the liquid at the higher dust enrichment, 3.81 and 9.46%, respectively, and high concentrations of both of these oxides are known to depress
solidus temperatures. At a dust enrichment of 1000x, the reason for the different solidification temperatures at the different total pressures is found in the
different liquid compositions at 1380K, the last temperature step before the liquid disappears at 10-6 bar. Although the FeO
content of the liquid is slightly lower at 10-3 than at 10-6 bar, 7.54 vs 9.51 wt%,
the Na2O concentration is much higher at 10-3 than at 10-6 bar,
5.89 vs <0.01 wt%. This is because the partial pressure of Na is more than a factor of 200 higher at 1380K at 10-3 bar
than at 10-6 bar. Furthermore, because Na continues to condense into the liquid in this temperature range, the lower the temperature
to which the liquid persists, the higher its Na2O content becomes, and this further lowers the ultimate temperature of its disappearance.
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