Journal Article Potassium-rich Rocks: Phase Analysis and Heteromorphic Relations Get access H. S. YODER, Jr H. S. YODER, Jr Geophysical Laboratory, Carnegie Institution of WashingtonWashington D.C Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 27, Issue 5, October 1986, Pages 1215–1228, https://doi.org/10.1093/petrology/27.5.1215 Published: 01 October 1986 Article history Received: 26 April 1986 Accepted: 12 May 1986 Published: 01 October 1986

The experimental techniques for measuring at high temperatures and pressures the transport properties of magma, a concentrated solution of silicates, are at a primitive stage. The fluxes of energy, mass, and momentum are usually interrelated in nature and are difficult to isolate in the laboratory.

Journal Article Twenty Years of Service to Petrology Get access H. S. YODER, JR. H. S. YODER, JR. Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 21, Issue 1, February 1980, Page 1, https://doi.org/10.1093/petrology/21.1.1 Published: 01 February 1980

Basic magma generation in the mantle at the present stage of earth history probably begins most commonly in metamorphosed garnet peridotite at those points where the four major phases meet; the solidus defines the spatial limits of the region of melting at the site of origin. On the basis of the forsterite-diopside-pyrope system and the melting relations of natural garnet peridotite at high pressures, the melting is invariant-like up to about 30% liquid. If the melt is fractionally removed, melting temporarily ceases after this limit is reached, terminating the production of liquid of invariant-like composition. Because one phase is eventually consumed at the invariant-like point, melting might be resumed at a higher temperature, generating a different liquid of more basic composition at the invariant-like point governing the assemblage of remaining phases. The garnet peridotite becomes permeable to melt almost immediately after the melting process begins, as has been demonstrated by the large increase in measured electrical conductivity. A large volume of relatively homogeneous liquid can, therefore, be extracted as it is produced.

The maximum stability limit of akermanite in the presence of excess CO2 was found to be about 6 kbar. Below that pressure, at relatively lower temperatures, akermanite reacts with CO2 to form diopside + calcite. Akermanite is unstable below 1010°C, 2 kbar; 1180°, 5 kbar; and 1240°, 6.1 kbar. The melting temperature of akermanite is depressed by CO2 owing to the solubility of CO2 in the liquid: 1454°C, 1 kbar; 1405°, 2 kbar; 1365°, 5 kbar; and 1355°, 5.8 kbar. The complete melting of diopside + calcite (1:1 mole) above 6 kbar and the melting of akermanite in the presence of excess CO2 below 6 kbar produce homogeneous liquids; therefore, carbonates are not necessarily immiscible in silicate melts of all ultrabasic compositions. The restriction of akermanite to relatively low pressures in the presence of CO2 and H2O, respectively, indicates that melilite-bearing rocks are not likely sources of kimberlite in the mantle. Melilite-bearing rocks recrystal-lize at high pressures to pyroxenites in the absence of CO2 and H2O. On the other hand, simplified reactions representing the conversion of ugandite or mafurite in the presence of CO2 and H2O to forsterite + phlogopite + calcite, the groundmass assemblage of kimberlite, indicates that a magma having melilite affinities could be transformed into kimberlite with the aid of suitable volatiles. The experimental results are also pertinent to BOWEN'S step 8 in the metamorphism of a siliceous dolomite. Because of the exceptionally high temperatures involved where CO pressure equals the total pressure, the reaction in nature appears to run at very low partial pressures of CO2. Preliminary results on the melting of calcite cleavage rhombs indicate congruent melting at 1355°C, 2 kbar; 1395°, 5 kbar; 1430°, 7.5 kbar; and 1460°, 10 kbar.

Melting relations for the MgSiO 3 composition have been studied at pressures between 5 and 30 kb under hydrous conditions. The beginning of melting of enstatite is about 1360°, 1280°, and 1270° C at 10, 20, and 30 kb, respectively. The incongruent melting behavior of enstatite, which disappears at pressures higher than about 5 kb under anhydrous conditions, persists to at least 30 kb water pressure. The melting interval where forsterite, liquid, and vapor co-exist is 30° to 50° C in the pressure range 10 to 30 kb, compared with about 20° at 1 atm. Below the solidus the assemblage enstatite + forsterite + vapor is obtained from the MgSiO 3 composition, because the vapor probably contained silica in excess of that in stoichiometric MgSiO 3 . The maximum content of H 2 O that dissolves in the initial H 2 O-saturated melt, determined by the phase-assemblage method, is about 11 weight percent at 10 kb. It is suggested that silica-saturated magmas such as a quartz tholeiite magma may be generated at considerable depths (at least up to 100 km) under hydrous conditions, either by direct partial melting of peridotites or by fractional crystallization of olivine tholeiitic magmas. Even if the H 2 O content is small, its effect cannot be disregarded.

Synopsis Experiments were conducted at 30 kilobars on dry synthetic mixtures in the join diopside (CaMgSi 2 O 6 )-pyrope (Mg 3 Al 2 Si 3 O 12 ), on a small number of compositions in adjacent parts of the plane CaSiO 3 -MgSiO 3 -Al 2 O 3 and one composition in the plane diopside-pyrope-forsterite. In addition, experiments were made at the same pressure on the joins chrome diopside-enstatite, chrome diopside-pyrope, and chrome diopside-pyrope-olivine using the separated analysed minerals of a natural garnet-peridotite nodule in kimberlite, and the join garnet-omphacite using the separated analysed minerals of an eclogite nodule in kimberlite. A few experiments were made on the whole-rock sample of some eclogite nodules in kimberlite. The results have an important bearing on the range ofcomposition of clinopyroxene solid solutions, the conditions of equilibration and original formation of eclogite and kimberlite, and the existence of an orthopyroxene-liquid reaction relationship in olivine and hypersthene-normative liquids at 30 kilobars pressure. They lead to the construction of a new fractionation scheme at high pressure in which a hypersthene-normative picrite or olivine tholeiite magma is the primary liquid, formed by partial melting of a garnet-peridotite mantle. Precipitation of garnets and clinopyroxenes (eclogite accumulates) leads to me development of alkaline and silica-poor mafic magmas from this primary liquid.