N. L. BOWEN AND THE ORIGIN OF ULTRAMAFIC ROCKS

In the first half of the twentieth century, petrologists commonly accepted the concept of ultramafic magmas. Harker and Vogt suggested that ultramafic liquids formed by the re-melting of accumulated olivine and pyroxene. Harker and Hess proposed that the liquidus temperatures of ultramafic melts were significantly lowered by dissolved volatiles. Vogt maintained that the temperatures of olivine-rich melts were lowered substantially by addition of iron. Bowen, however, consistently rejected the concept of ultramafic magma. Although Bowen appealed to the lack of ultramafic lavas and the general paucity of contact metamorphism around ultramafic plutons as evidence against ultramafic magmas, experimental data clinched the matter for him. In 1914, he found that forsterite melts at 1890oC, a temperature far too high for dunite magma to exist in the crust. In the 1930s, Bowen and Schairer determined a significant portion of the olivine solid solution loop, and, contrary to the opinion of Vogt, they showed that olivine with a substantial amount of iron still required unreasonably high temperatures to exist as melt in the crust. As a result, Bowen maintained that ultramafic plutons represented accumulations of olivine and pyroxene that were intruded as mushes of solid crystals lubricated by thin films of fluid. In his final study on ultramafic rocks with Tuttle in 1949, Bowen again showed that no liquid exists in the system MgO-SiO2-H2O at crustal temperatures. He also argued that serpentinites could not crystallize directly from a magma of serpentine composition but formed by absorption of water from wall rocks and alteration of olivine.