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Scientific paper
Mar 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55..761b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 3, pp.761-767
Other
1
Scientific paper
The conventional interpretation of silicate liquid immiscibility in the quartz-fayalite-leucite (Q-F-L) system SiO 2 (Q)-Fe 2 SiO 4 (F)-KAlSi 2 O 6 (L) is that there is only one liquation field but it is revealed twice above the liquidus: the Greig field (G) at high temperatures (>1690°C), and the Roedder (R) field at lower temperatures (1100-1270°C), elsewhere being metastable below the quartz and fayalite liquidus surfaces. Published experimental work has established that there are two very different tie-line patterns for liquation in synthetic systems. If alkalies are absent, liquation in four to seven component alumino-silicate melts has tie-lines that simply radiate from Q, in effect a multicomponent continuation of the G region from the binary and ternary SiO 2 -MO melts (M = Ca, Mg, Mn, Fe). This is in strong contrast to the experimental tie-lines on the R region in the alkali-bearing Q-F-L system which reflect mild partitioning of Al 2 O 3 and K 2 O. The known consolute points for the R region in the Q-F-L system are strongly offset from the anticipated position of the consolute on the binary G solvus in Q-F. If it is assumed that there is only one simple liquation meta/stable solvus in Q-F-L, this consolute mismatch requires a highly deformed tie-line pattern which does not agree with the published experimental tie-lines. There must be two distinct solv in the Q-F-L system, one extending from G and the other from R. The above two very different tie-line patterns show that in Q-F-L the consolute locus for the G region must diverge radically from that for the R region, confirming that the R and G regions belong to two totally different liquation solvi. Furthermore, the most likely interaction of R and G will generate a third liquation solvus (X) situated between Q and L, below the liquidus of Q and orthoclase. A metastable three-liquid field is predicted, with end-points corresponding to approximate compositions of ultramafic, highly silicic and alkalic magmas. About half of the title system is covered by the complex of three liquation solvi, but most of the liquation is metastable below liquidus temperatures, so the complexity will not be reflected in the liquidus surfaces except in an indirect manner: metastable liquation may cause significant flattening of the liquidus surface of a particular mineral, thus influencing its fractionation paths and element partitioning. Fundamentally, the actual geometry of binary to multicomponent silicate liquation must guide the selection of suitable thermodynamic solution models for silicate melts, because only certain models can produce the liquation geometries invoked above; these models have the correct thermodynamic power-law to fit activities across the whole compositional range in the melt. Other solution models, calibrated with the wrong power-law, may extrapolate incorrectly outside their very narrow temperature-composition calibration range.
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