Liquidus phase relations on the join diopside-forsterite-anorthite from 1 atm to 20 kbar: Their bearing on the generation and crystallization of basaltic magma |
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| Authors: | D C Presnall Selena A Dixon James R Dixon T H O'Donnell N L Brenner R L Schrock D W Dycus |
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| Institution: | (1) Department of Geosciences, University of Texas at Dallas, P.O. Box 688, 75080 Richardson, Texas, USA;(2) Present address: Hawaii Institute of Geophysics, University of Hawaii, 96822 Honolulu, HI, USA;(3) Present address: U.S. Geological Survey, 307 Federal Building, 33901 Ft. Myers, FL, USA;(4) Present address: Information Systems, Indiana University, 47401 Bloomington, IN, USA;(5) Present address: Geochemical Testing Inc., 149 Fuller St., 15501 Somerset, PA, USA;(6) Present address: University of Texas Health Science Center at Dallas, 75235, TX, USA |
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| Abstract: | In the system CaO-MgO-Al2O3-SiO2, the tetrahedron CaMgSi2O6(di)-Mg2SiO4(fo)-SiO2-CaAl2 SiO6(CaTs) forms a simplified basalt tetrahedron, and within this tetrahedron, the plane di-fo-CaAl2Si2O8(an) separates simplified tholeiitic from alkalic basalts. Liquidus phase relations on this join have been studied at 1 atm and at 7, 10, 15, and 20 kbar. The temperature maximum on the 1 atm isobaric quaternary univariant line along which forsterite, diopside, anorthite, and liquid are in equilibrium lies to the SiO2-rich side of the join di-fo-an. The isobaric quaternary invariant point at which forsterite, diopside, anorthite, spinel, and liquid are in equilibrium passes, with increasing pressure, from the silica-poor to the silica-rich side of the join di-fo-an, which causes the piercing points on this join to change from forsterite+diopside+anorthite+liquid and forsterite +spinel+anorthite+liquid below 5 kbar to forsterite +diopside+spinel+liquid and diopside +spinel+anorthite+liquid above 5 kbar. As pressure increases, the forsterite and anorthite fields contract and the diopside and corundum fields expand. The anorthite primary phase field disappears entirely from the join di-fo-an between 15 and 20 kbar. Below about 4 kbar, the join di-fo-an represents, in simplified form, a thermal divide between alkalic and tholeiitic basalts. From about 4 to at least 12 kbar, alkalic basalts can produce tholeiitic basalts by fractional crystallization, and at pressures above about 12 kbar, it is possible for alkalic basalt to be produced from oceanite by crystallization of both olivine and orthopyroxene. If alkalic basalts are primary melts from a lherzolite mantle, they must be produced at high pressures, probably greater than about 12 kbar.Department of Geosciences, University of Texas at Dallas Contribution No. 327. Hawaii Institute of Geophysics Contribution No. 814. |
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