Experimental melting and crystallisation of glassy olivine melilitites |
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| Authors: | F E Lloyd |
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| Institution: | (1) Department of Geology, University of Reading, Reading, England |
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| Abstract: | A remarkable variety of unusual melilite and feldspathoid-bearing lavas characterises the volcanism of Katwe-Kikorongo and
Bunyaruguru, central South West Uganda. The magmas were silica undersaturated, mafic, highly potassic and volatile-rich. The
most peralkaline compositions are found in rapidly quenched small bombs and lapilli, which are invariably glass enclosing
olivine and melilite phenocrysts. Slowly cooled lava flows and ejected blocks are non-glassy, feldspathoid-bearing, clinopyroxene-rich
assemblages with mol. Na2O+K2O∶Al2O3 close to unity or less.
One atmosphere melting experiments were carried out between 1,330 and 1,050° C for two glassy olivine melilitites. Both have
identical mineralogy and similar chemistry, except that one is more hydrated, carbonated and oxidised and has lost alkalis,
principally sodium. The fresher material crystallised, in the following order, olivine-melilite-leucite-clinopyroxene-kalsilite.
The other sample crystallised olivine-leucite-clinopyroxene-kalsilite and melilite was absent at all temperatures. The separate addition to this sample (BN20,A) of (i) Na2CO3 (ii) K2CO3 (iii) CaCO3, in each case induced melilite to crystallise. Adition of ammonium carbonate, however, did not, showing that melilite appearance
is not linked solely with the presence of CO2. The separate and combined roles of Na, K and Ca, in the crystallisation of melilite, were examined by comparing the chemistry
of melilite-bearing with related melilite-free, highly alkaline lavas, that had been melted and recrystallised at one atmosphere
(this study and published material). The data indicate that high Na∶Si + Al favours melilite separation, while K and Ca, and
by inference any element capable of reducing the influence of Si and Al, serve effectively to increase this ratio. Loss of
Na and K from BN20,A led to direct and indirect reduction of the Na∶Si+Al ratio and consequent inhibition of melilite crystallisation.
Thus loss of alkalis, in particular Na, can relate olivine melilitite to leucite clinopyroxenite and leucite kalsilite clinopyroxenite.
This implies that mode of eruption, cooling history and extent of degassing, with concomitant loss of alkalis, is capable
of explaining much of the diversity of mineralogy that typifies the volcanic products of the South West Ugandan fields. |
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