Dry, hydrous, and CO2-bearing liquidus phase relationships in the CMAS system at 28 KB, and their bearing on the origin of alkali basalts

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Abstract

Dry, hydrous, and CO2-bearing liquidus phase relationships were studied within the CMAS system (CaO-MgO-Al2O3-SiO2) at 28 kb. In the dry system garnet, diopside, enstatite, and forsterite were found to coexist with a liquid of the approximate composition 50% SiO2, 16% Al2O3, 25% MgO, 9% CaO. With 20 wt % H2O added, this liquid is relatively more CaAl2O4-rich and less magnesian than it is under dry conditions. Addition of CO2 results in a large expansion of the garnet and enstatite liquidus fields. This causes CO2-rich liquids in equilibrium with garnet lherzolite to be SiO2-poor and have high CaO/Al2O3. The simple-system liquid trends can be compared with the compositional trends of primitive alkali basalts from southeastern Australia, central Europe, Hawaii, and the Canary Islands. Variation in pressure during partial melting of garnet lherzolite also becomes a possible mechanism for producing the variations found in primitive alkali basalt composition. Both variation in pressure and variation in CO2 concentrations during partial melting of garnet lherzolite appear to be capable of producing the compositional range of the natural alkali basalts. The data presently available do not enable a clear choice between these two possibilities. -from Author

Original languageEnglish
Pages (from-to)709-719
Number of pages11
JournalJournal of Geology
Volume96
Issue number6
Publication statusPublished - 1988
Externally publishedYes

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