Primary basalts and magma genesis

Abstract

Three Eocene lavas from Skye, NW Scotland, have been subjected to anhydrous experimental studies within their melting ranges at pressures up to 30 kb. Two of these, an olivine-phyric magnesian alkali basalt and a near-aphyric Mg-poor transitional basalt, appear to show four-phase points on their liquidi at high pressures which are thought to have genetic significance. From experimental and mineralogical evidence, the magnesian basalt is postulated to be a primary magma, erupted without significant compositional change from its genesis by slight partial melting of a relatively Fe-rich spinel lherzolite upper mantle at about 60 km depth. The liquid seems to have had a reaction relationship with Ca-poor pyroxene (pigeonite) in the residual lherzolite. Partial crystallization of batches of this magma, delayed during its ascent at depths of about 40 km, is thought to have given rise to the Mg-poor basaltic liquids. The third lava studied experimentally, a sparsely olivine-phyric hawaiite, does not have olivine on the liquidus in any part of its anhydrous P-T diagram and therefore cannot have been derived under anhydrous conditions from olivine-saturated sources. The mineralogy and chemistry of the lavas are used to support an hypothesis that the hawaiites are products of partial crystallization of pockets of basalt magma at depths approximating to the crust/ mantle boundary beneath Skye, with \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) rising to sufficient values to make the residual liquids comparatively rich in normative feldspar. Finally, the genesis of all other Skye Eocene lavas is reviewed in the light of the new experimental data.