The anhydrous melting behaviour of two synthetic peridotite compositions has been studied experimentally at temperatures ranging from near the solidus to about 200° C above the solidus within the pressure range 0–15 kb. The peridotite compositions studied are equivalent to ‘Hawaiian’ pyrolite and a more depleted spinel lherzolite (Tinaquillo peridotite) and in both cases the experimental studies used peridotite −40% olivine compositions. Equilibrium melting results in progressive elimination of phases with increasing temperature. Four main melting fields are recognized; from the solidus these are: olivine (ol)+orthopyroxene (opx)+clinopyroxene (cpx)+Al-rich phase (plagioclase at low pressure, spinel at moderate pressure, garnet at high pressure)+liquid (L); ol+opx+cpx+Cr-spinel+L; ol+opx+Cr-spinel +L: ol±Cr-spinel+L. Microprobe analyses of the residual phases show progressive changes to more refractory compositions with increasing proportion of coexisting melt i.e. increasing Mg/(Mg+Fe) and Cr/(Cr+Al) ratios, decreasing Al2O3, CaO in pyroxene. The degree of melting, established by modal analysis, increases rapidly immediately above the solidus (up to 10% melting occurs within 25°–30° C of the solidus), and then increases in roughly linear form with increasing temperature. Equilibrium melt compositions have been calculated by mass balance using the compositions and proportions of residual phases to overcome the problems of iron loss and quench modification of the glass. Compositions from the melting of pyrolite within the spinel peridotite field (i.e. ∼ 15 kb) range from alkali olivine basalt (<15% melting) through olivine tholeiite (20–30% melting) and picrite to komatiite (40–60% melting). Melting in the plagioclase peridotite field produces magnesian quartz tholeiite and olivine-poor tholeiite and, at higher degrees of melting (30–40%), basaltic or pyroxenitic komatiite. Melts from Tinaquillo lherzolite are more silica saturated than those from pyrolite for similar degrees of partial melting, and range from olivine tholeiite through tholeiitic picrite to komatiite for melting in the spinel peridotite field. The equilibrium melts are compared with inferred primary magma compositions and integrated with previous melting studies on basalts. The data obtained here and complementary basalt melting studies do not support models of formation of oceanic crust in which the parental magmas of common mid-ocean ridge basalts (MORB) are attributed to segregation from source peridotite at shallow depths (≦ 25 km) to leave residual harzburgite. Liquids segregating from peridotite at these depths are more silica-rich than common MORB.

Anhydrous melting of peridotite at 0–15 Kb pressure and the genesis of tholeiitic basalts

The question of whether basaltic rocks in continental flood basalt provinces are primary magmas or whether they are descended in general from picritic parent magmas is reviewed. It is suggested that the latter is more likely to be correct on the evidence of phase relations and the relative rareness of mantle materials with appropriate Fe/Mg ratios. Major element variations in the residual liquids of fractional and equilibrium crystallization of basaltic magmas are modelled for a variety of crystallizing assemblages. It is concluded that crystallization of olivine, clinopyroxene, and plagioclase has a marked effect on buffering chemical change in many important elements. It is this effect which accounts for the apparent uniformity of large volumes of flood basalts, not, as has sometimes been supposed, a series of implausible coincidences in the amount of material fractionated from each magma batch. It is further argued that much of the variation seen in basalts may be imposed by polybaric fractionation operating throughout crustal depths, that is at pressures up to at least 12 kb. Parental picritic magmas rising from the mantle reach the surface in exceptional areas of crustal thinning. More usually, however, it is suggested that they intrude the base of the crust as a series of sills which differentiate into upper gabbroic and lower ultramafic portions. Much of the 'low pressure' fractionation of basaltic magmas may take place in this deep crustal sill complex and evolved liquids are transmitted to the surface as their density becomes sufficiently low. This implies that in areas of flood vulcanism a potentially large new contribution to the crust is made by underplating, the volumes of concealed cumulates being at least as large as the amount of erupted surface lava.

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A Model for Flood Basalt Vulcanism

Geochemistry and mineralogy of two spinel peridotite suites from Dreiser Weiher, West Germany

Flood Basalt to Rhyolite Suites in the Southern Paranà Plateau (Brazil): Palaeomagnetism, Petrogenesis and Geodynamic Implications

Three arguments based on geological evidence are put forward to support the importance of magmatic underplating processes during continental flood basalt vulcanism. (1) Petrological evidence of gabbro fractionation in erupted basaltic sequences allows estimates to be made of the minimum total mass of concealed cumulate material, which is retained in deep crustal magma chambers, possibly along the Moho, and is comparable in amount to the erupted material. (2) In the Karoo province (southern Africa) large volumes of rhyolite along the S.E. continental margin were generated from basaltic precursors, either as partial melts of alreadyemplaced solid basic material or as crystal fractionation products of large volumes of basic magma. In either case very substantial volumes of concealed basic rocks are at least locally implied. (3) Studies of geomorphology suggest that the area of the Karoo province experienced at least 1 km of permanent uplift associated with the vulcanism. This appears to be the consequence of the emplacement of an underplated gabbroic layer ca . 5km thick.

Continental magmatic underplating

A relatively Fe-rich lherzolite xenolith (mg=79) in analcimite from Spring Mount, New South Wales, belongs to the Cr-diopside inclusion suite and accordingly may be assigned an accidental, upper mantle origin. More Mg-rich variants, transitional to the most common Cr-diopside xenoliths (mg=88–92), are also upper mantle components. The relative abundance of Mg-rich xenoliths may reflect factors controlling xenolith preservation in higher temperature lavas. Significant inhomogeneity with respect to Mg/Fe is indicated for the upper mantle, thereby greatly expanding the spectrum of possible primary melts. The abundance of ‘evolved’ volcanics in continental flood ‘basalt’ provinces in considered in terms of this proposal. Least squares mixing calculations suggest that anorogenic tholeiitic andesites may represent primary magmas in their own right, formed by partial melting of relatively Fe-rich lherzolite.

R. A. Binns

Papers

Relatively iron-rich lherzolite xenoliths of the Cr-diopside suite: A guide to the primary nature of anorogenic tholeiitic andesite magmas