Picrite basalt

Abstract: This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.

Abstract: 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.

Abstract: This paper presents results of modelling reaction between peridotite and fractionating tholeiitic basalt in simple and complex silicate systems. Reactions are outlined in appropriate binary and ternary silicate systems. In these simple systems, the result of reactions between 'basalt' and 'peridotite' may be treated as a combination of Fe-Mg exchange and mass transfer reactions at constant Fe/Mg. Fe-Mg exchange in ternary and higher-order systems is nearly isenthalpic, and involves a slight decrease in magma mass at constant temperature. Mass transfer reactions, typically involving dissolution of orthopyroxene and consequent crystallization of olivine, are also nearly isenthalpic in ternary and higher-order silicate systems, and produce a slight increase in the magma mass at constant temperature. The combined reactions are essentially isenthalpic and produce a slight increase in magma mass under conditions of constant temperature or constant enthalpy. Initial liquids saturated in plagioclase-I-olivine will become saturated only in olivine as a result of near-constant-temperature reaction with peridotite, and crystal products of such reactions will be dunite. Liquids saturated in clinopyroxene + olivine will remain on the cpx-ol cotectic during reaction with peridotite, but will crystallize much more olivine than clinopyroxene as a result of reaction, i.e., crystal products will be clinopyroxene-bearing dunite and wehrlite rather than olivine clinopyroxenite, which would be produced by cotectic crystallization. The Mg/Fe ratio of crystal products is 'buffered' by reaction with magnesian peridotite, and dunites so produced will have high, nearly constant Mg/Fe. Production of voluminous magnesian dunite in this manner does not require crystal fractionation of a highly magnesian olivine tholeiite or picrite liquid. Combined reaction with ultramafic wall rock and crystal fractionation due to falling temperature produces a calc-alkaline liquid line of descent from tholeiitic parental liquids under conditions of temperature, pressure, and initial liquid composition which would produce tholeiitic derivative liquids in a closed system. Specifically, closed-system differentiates show iron enrichment at near-constant silica concentration with decreasing temperature, whereas the same initial liquid reacting with peridotite produces silica-enriched derivatives at virtually constant Mg/Fe. Reaction between fractionating basalt and mafic to ultramafic rock is likely to be important in subduction-related magmatic arcs, where tholeiitic primary liquids pass slowly upward through hightemperature wall rock in the lower crust and upper mantle. Although other explanations can account for chemical variation in individual calc-alkaline series, none can account as well for the characteristics snared by all calc-alkaline series. This process, if it is volumetrically important on Earth, has important implications for (Phanerozoic) crustal evolution: sub-arc mantle should be enriched in iron, and depleted in silica and alumina, relative to sub-oceanic mantle, acting as a source for sialic crust It is probable that inter-occanic magmatic arcs have basement similar to alpine peridotite, in which suboceanic mantle has been modified by interaction with slowly ascending basaltic liquids at nearly

Abstract: The 1989 IUGS classification of the igneous rocks for the high-Mg and picritic volcanic rocks has been revised. Instead of an 18 wt % MgO minimum limit being applied for all high-Mg and picritic volcanic rocks, that is now applicable only to the high-Mg rocks such as komatiite and meimechite. The minimum MgO requirement for picrite is reduced to 12 wt %. The SiO2 former boundary figure between boninite and komatiite–meimechite–picrite, which was 53 wt %, is reduced to 52 wt %, and the total alkali content for komatiite and meimechite is increased to 2% and for picrite to 3%.