David H. Green

An experimental study of the effect of Ca upon garnet-clinopyroxene Fe-Mg exchange equilibria

AbstractA series of basaltic compositions and compositions within the simple system CaO-MgO-FeO-Al2O3-SiO2 have been crystallized to garnetclinopyroxene bearing mineral assemblages in the range 24–30 kb pressure, 750°–1,300° C temperature. Microprobe analyses of coexisting garnet and clinopyroxene show that KD(Fe2+/MgG+/Fe2+/MgCpx) for the Fe-Mg exchange reaction between coexisting garnet and clinopyroxene is obviously dependent upon the Ca-content and apparently independent of the Mg/(Mg+Fe) content of the clinopyroxene and garnet. The Ca-effect is believed to be due to a combination of non-ideal Ca-Mg substitutions in the garnet and clinopyroxene.Our data and interpretation reconciles previous inconsistencies in the temperature dependence of KD− values determined in experimental studies of simple systems, complex basalt, grospydite and garnet peridotite compositions. Previous differences between the effect of pressure upon KDas predicted from simple system theory (Banno, 1970), and that observed in experiments on multicomponent natural rock compositions (Råheim and Green, 1974a) can now be resolved.We have determined KDas a function of P, T, and XGtCa(grossular) and derived the empirical relation n

Evidence for carbonatite metasomatism in spinel peridotite xenoliths from western Victoria, Australia

Tleft( {^circ {text{K}}} right) = frac{{3104X_{{text{Ca}}}^{{text{Gt}}} + 3030 + 10.86Pleft( {{text{kb}}} right)}}{{ln K_{text{D}} + 1.9034}}

Empirical geothermobarometry for garnet peridotites and implications for the nature of the lithosphere, kimberlites and diamonds

n.This empirical relationship has been applied to garnet-clinopyroxene bearing rocks from a wide range of geological environments. The geothermometer yields similar estimates for garnet-clinopyroxene equilibration for neighbouring rocks of different composition and different KDvalues. In addition, temperature estimates using the above relationship are more consistent with independent temperature estimates based on other geothermometers than previous estimates which did not correct for the Ca-effect.An alternative approach to the above empirical geothermometer was attempted using regular solution models to derive Margules parameters for various solid solutions in garnets and clinopyroxenes. The derived Margules parameters are broadly consistent with those determined from binary solution studies, but caution must be exercised in interpreting them in terms of actual thermodynamic properties of the relevant crystalline solid solutions because of the assumptions which necessarily have to be made in this approach.

Internally consistent gahnitic spinel-cordierite-garnet equilibria in the FMASHZn system: geothermobarometry and applications

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.

Experimental demonstration of refractory carbonate-bearing eclogite and siliceous melt in the subduction regime

The experimental discovery that sodic dolomitic carbonatite melt can be generated in equilibrium with pargasitic lherzolite [1] at pressures corresponding to depths of 90 km or more, has led to the development of models for mantle metasomatism by ephemeral carbonatite melts [2]. A suite of mantle xenoliths (magnesian wehrlites to lherzolites) from Mt Leura and Mt Shadwell (western Victoria, Australia) display the predicted petrographic and geochemical signatures of such metasomatism, namely replacement of primary orthopyroxene by jadeitic clinopyroxene and forsteritic olivine, the presence of significant accessory apatite, unusually high whole-rock CaO/Al2O3 and Na2O/Al2O3 values, and extreme LILE enrichment without concomitant TiO2 enrichment. Isotopic data suggest a petrogenetic association of the carbonatite metasomatic event with the host alkali olivine basalts. We propose a model whereby carbonated, undersaturated silicate melts crystallizing pargasitic amphibole at depths of ⩾ 90 km, produce a mobile carbonatite melt which can segregate and ascend at very low melt fractions, and undergo decarbonation reactions with refractory lithosphere at 55–65 km depth, producing highly magnesian, apatite-bearing wehrlites and lherzolites.

Possible role of amphibole in the origin of andesite: some experimental and natural evidence

Spontaneous pattern formation in reactive media has been thoroughly investigated in homogeneous oscillatory systems, mainly in the BelousovZhabotinsky reaction1. Spatiotemporal patterns have been observed in some studies of catalytic surfaces2,3 and of travelling electrochemical waves4. Here we report on oscilla-tory wave patterns observed during electrochemical dissolution of a nickel wire in acidic media. We show that space-averaged poten-tial or current oscillations are associated with the creation of an inhomogeneous current distribution, and that the selection of a specific spatial current pattern depends on the current control mode of the electrochemical cell. In the almost potentiostatic (fixed potential) mode of operation, a train of travelling pulses prevails, whereas antiphase oscillations occur in the galvanostatic (constant average current) mode. The latter, as far as we know, have never before been reported in any reactive system.

The genesis of refractory melts in the formation of oceanic crust

Abstract As a result of a study on the influence of Cr on geothermometry and geobarometry empirical garnet-orthopyroxene geobarometry has been refined. The applicability to simple and complex systems is demonstrated and the modelling procedure briefly outlined. The suitability of some geothermometers for application to garnet lherzolite xenoliths and their limits of precision are discussed. Fields of pressure-temperature estimates for garnet lherzolites from various provinces are shown. Low-temperature xenoliths ( 1050°C) give near-isobaric estimates for a range of temperatures. The depth of this “temperature discontinuity” is shallower for off-craton than for on-craton suites. Similar patterns are indicated for suites from U.S.A. and the U.S.S.R. The temperature discontinuity is interpreted as a geologically short-lived transient state, where the deepest parts of the lithosphere are in the process of adjustment to higher heat flux from below. At the highest temperatures recorded by those suites “wet” peridotitic solidi are intersected and thus the xenoliths record events, were P,T conditions may have allowed the formation of kimberlitic or strongly undersaturated magmas at about the depths of this discontinuity. This process is inferred to lead to thermal erosion of the base of the lithosphere. The generation and subsequent crystallization of magmas at the base of the lithosphere is inferred to be the source for diamond formation. The distinctive pattern of high- and low-temperature xenoliths in relation to the diamond stability field shows the existence of a rather small “diamond window” between about 900 and 1300°C at 40–55 kb. This window exists only underneath old, stable continents with a thick lithosphere. If recent age determinations on diamonds are correct, this implies the existence of thick continental lithosphere with similar characteristics to present-day lithosphere in the Archean. A model is presented arguing for hot, upwelling convection streams as a possible source of heat. The model implies differences in lithospheric development according to the speed of plates, carrying a continent: Continuous lithospheric thinning by thermal erosion for steady or very slow moving plates with the break-up of a continent and the formation of an ocean as one extreme, the continuous repetition of thermal erosion and growth on cooling (underplating) of the continental lithosphere as the other extreme for fast moving plates. The latter may be the cause of a number of complexities and heterogeneities both within the deep continental lithosphere as well as in the convecting mantle.

The effect of bulk rock composition on the stability of amphibole in the upper mantle: Implications for solidus positions and mantle metasomatism

The equilibrium (Mg, Fe, Zn)3Al2Si3O12+2Al2SiO5=3(Mg, Fe, Zn)Al2O4+5SiO2 garnet + sillimanite/kyanitc = spinel + quartz was calibrated in the piston-cylinder apparatus between 11 and 30 kbar, and over the temperature range of 950 to 1200°C. Three experimental mixes of Mg no. [100*MgO/(MgO+FeO)] 40, 47 and 60, in the FeO −MgO−Al2O3−SiO2−ZnO (FMASZn) system were used under low oxygen fugacities and anhydrous conditions. We derive a ternary Fe−Mg−Zn symmetric mixing model for aluminous spinels in equilibrium with garnet, to quantify the increase in gahnitic end-member of spinel with increasing pressure and descreasing temperature. Further experiments in the spinel-cordieritequartz-sillimanite field were combined with garnet-cordierite data from the literature to produce a consistent set of equations describing the exchange reactions in FMASHZn relevant to quartz-sillimanite bearing rocks at granulite facies conditions. As spinel is an important mineral participating in many rocks of aluminous composition at granulite-facies conditions, and as zinc contributes to an enlargement of spinels stability field towards higher pressures and lower temperatures, the thermobarometric calibrations presented here will be most significant in delineating the prograde and retrograde trajectory of P-T paths.