Petrogenetic grid for siliceous dolomites extended to mantle peridotite compositions and to conditions for magma generation
- Univ. of Chicago
Decarbonation reactions in the system CaO--MgO-SiO/sub 2/--CO/sub 2/ involve calcite, dolomite, magnesite, and quartz, and the products enstatite, forsterite, diopside, and wollastonite, among others. Each decarbonation reaction terminates at an invariant point involving a liquid, CO/sub 2/ vapor, carbonate minerals, and one or more of the silicate minerals. Fusion curves for mantle mineral assemblages involving forsterite, orthopyroxene, and clinopyroxene in the presence of CO/sub 2/, extending from higher temperature regions, terminate at these same invariant points. The points are connected by a series of liquidus reactions involving the carbonates and mantle silicates, at temperatures generally lower than the silicate-CO/sub 2/ melting reactions. Experimental data and theoretical analysis permit construction of a series of partly schematic phase diagrams. Petrological and geophysical conclusions include the following: (1) Free CO/sub 2/ cannot exist in the mantle; it is stored as carbonate. (2) CO/sub 2/ appears to be as effective as H/sub 2/O in causing incipient melting of mantle peridotite, and this remains our preferred explanation for the seismic low-velocity zone. (3) At depths greater than about 80 km, mantle peridotite with CO/sub 2/ (as carbonate) yields carbonatitic magmas with about 40 percent CO/sub 2/ and 10 percent silicates in solution; with progressive fusion the liquid becomes kimberlitic. (4) Primary carbonatite or kimberlite magmas rising from the asthenosphere must evolve CO/sub 2/ near 80 km depth, which would contribute to their explosive eruption. (5) Through a wide pressure range, SiO/sub 2/-undersaturated basic magmas with CO/sub 2/ in solution can yield residual kimberlitic or carbonatitic magmas. (6) Deep mantle magmas may include the carbonated alkali ultrabasic magmas that have been proposed as the parents from which continental associations of highly alkalic rocks are derived.
- OSTI ID:
- 6866944
- Journal Information:
- Am. Mineral.; (United States), Vol. 61:7/8
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
DOLOMITE
CHEMICAL REACTIONS
EARTH MANTLE
GEOTHERMOMETRY
MAGMA
ORIGIN
MAGNESIUM CARBONATES
PERIDOTITES
QUARTZ
CALCITE
CALCIUM OXIDES
CARBON DIOXIDE
CARBONATES
DIOPSIDE
ENSTATITE
MAGNESIUM OXIDES
MELTING
MINERALS
PETROLOGY
PYROXENES
REMOVAL
SILICA
SYNTHESIS
VERY HIGH TEMPERATURE
ALKALINE EARTH METAL COMPOUNDS
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CARBON COMPOUNDS
CARBON OXIDES
CARBONATE ROCKS
CHALCOGENIDES
GEOLOGY
IGNEOUS ROCKS
MAGNESIUM COMPOUNDS
MAGNESIUM SILICATES
OXIDES
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
RESERVOIR ROCK
ROCKS
SILICATES
SILICON COMPOUNDS
SILICON OXIDES
152004* - Geothermal Data & Theory- Isotope & Trace Element Studies