Origin of Basalt Magmas: An Experimental Study of Natural and Synthetic Rock Systems

Natural basalts and eclogites were investigated experimentally at a series of temperatures in the pressure range 1 atm to 40 kb and with water pressures 1 to 10 kb. Some runs were also made on related synthetic systems at 10 and 33 kb. The two principal magma types recognized by field investigators—tholeiite and alkali basalt types-appear to be separated by equilibrium thermal divides at 1 atm. The principal divides were found by experiment at elevated pressures to give way to a new set of equilibrium thermal divides resulting from a new mineralogy. The change of the equilibrium thermal divides with pressure leads to the derivation of the two principal magma trends from the same bulk composition. The melting behavior of basalts and eclogites indicates that both are the partial melting products of a more primitive rock (e.g. garnet peridotite). In the region of magma generation (below 60 km) the parental material, presumed to be garnet peridotite, yields an eclogitic magma and its fractionation depends on the garnet and omphacite of the eclogite, not on plagioclase and clinopyroxene of a basaltic magma. Increase of the garnet constituents in the magma at high pressure by effective removal of omphacite or shift of the garnet-omphacite boundary 'surface' will give rise to a tholeiite-type magma at low pressure. Similarly, increase of the omphacite constituents in the magma at high pressure by physical or physicochemical means will give rise to an alkali basalt-type magma at low pressure. In general, alkali basalt-type magmas are to be expected to be generated at greater depths than tholeiite-type magmas from the same primary source rock. Establishment of the two major basalt series takes place in the region of generation; additional minor diversification of each series may come about after emplacement in or on the crust by crystal settling, oxidation or reduction, gas fluxing, contamination, and other processes. The derivative magmas are greatly restricted by the course of liquid thermal descent imposed at generation. Pressure-temperature limits established experimentally suggest that the basalt-eclogite transformation may be responsible for the Mohoroviĉić discontinuity under the continents, but not under the oceans. The field of stability of basalt is drastically reduced in the presence of water, and amphibolite is produced. The melting of amphibolite takes place over a much greater range of temperature than basalt. At 10 kb water pressure the beginning of melting of amphibolite closely approaches that of granite. Partial melting of amphibolite may yield anorthositic liquids having a relatively low anorthite content at exceptionally low temperatures. Eclogite itself is not stable in the presence of water and gives place to amphibolite or pyroxene hornblendite. Magmas which crystallize to basalt, gabbro, or eclogite must have had a low water-content at the time of crystallization. Fifteen rock and twenty-three mineral analyses as well as numerous partial chemical analyses of experimental products were made by J. H. Scoon in the course of the investigation. These chemical analyses bear on many mineralogical and petrological problems.