• Volume 99, Issue 1

March 1990,   pages  1-165

• Foreword - Experimental petrology: A perspective

• Experimental petrology: Quantitative boundaries for petrogenesis

The paper is an overview of the current status on experimental petrology — its objectives and its major role in solving problems related to various earth processes. It describes how investigations related to solid-solid transitions, dehydration or decarbonation reactions, and melting studies have helped to formulate petrological models for the earth’s internal structure. It also describes how measurements of physical properties of minerals, melts and vapour under extreme conditions have provided vital information of fluid dynamics of magmatic systems. The paper narrates the role of experimental petrology in calibrating geophysical processes with petrological consequences. Model P-T-X (SiO2)-fluid systems are considered to emphasize the role of various gas species in shifting the solidus in a P-T space, in degrees of melting and composition of the melt. Synthetic models and study of whole rock systems are considered to discuss the zonation and metasomatic processes in the mantle of the earth. The paper is also concerned with mantle convection and the uprise of thermal plumes, particularly, in the oceanic environment. It discusses the petrological structures associated with the plume and shows how static petrological maps are modified by the dynamics of the plumes.

• Patent mantle-metasomatism: Inferences based on experimental studies

K, Na and Ca are the most common elements transported during mantle metasomatism and result in formation of phlogopite (K), amphibole (Na) and clinopyroxene (Ca) by various reactions. This review presents models for this type of metasomatism based on experiments on the pyrolite-K2CO3-H2O, pyrolite-Na2 CO3-H2O systems and on the pyrolite-CaCO3 system. The addition of K2CO3 and Na2CO3 lowers the liquidus of pyrolite providing a low temperature, alkali-rich hydrous melt which may ascend and metasomatize overlying mantle regions. Several reactions are proposed for the formation of phlogopite and amphibole (pargasite-edenite) in these systems. The compositions of amphiboles correspond to those found in metasomatized mantle xenoliths. In contrast, Ca-metasomatism is considered to be mainly an anhydrous process in which orthopyroxene and carbonate react to produce clinopyroxene, olivine and CO2. High pressure liquids in this model system are of carbonatitic composition and this low viscosity melt can ascend converting harzburgite mantle assemblages to olivine-rich wehrlite.

Based on an inverse experimental approach, moderately high degrees of partial melting of a model metasomatized alkali clinopyroxenite xenolith yield liquids at 30kb which are very comparable in composition to the lavas enclosing such types of xenoliths.

Experimental modelling of mantle metasomatism produces assemblages which are in good agreement with the mineral assemblages and textural relationships found in metasomatized mantle xenoliths from areas such as West Eifel and South-West Uganda.

• Clinopyroxene on the join CaMgSi2O6-CaFe3+AlSiO6-CaTiAl2O6 at low oxygen fugacity

Subsolidus phase relations on the join CaMgSi2O6-CaFe3+ AlSiO6-CaTiAl2O6 were studied by the ordinary quenching method at$$f_{O_2 } = 10^{ - 11}$$ atm and 1,100°C. Crystalline phases encountered are clinopyroxeness (ss:solid solution) (Cpxss), melilite (Mel), perovskite (Pv), spinelss (Spss), magnetitess (Mtss) and anorthite (An). There is no Cpxss single phase field, and the following assemblages were found; Cpxss+Mel, Cpxss+Mel+Spss, Cpxss+Mel+Pv, Cpxss+Mel+Spss+Pv, Cpxss+Pv+Spss+An, Spss+Pv+Mel+An+Cpxss, Mel+Mtss+An+Spss+Cpxss+liquid and Mel+Mtss+An+Spss+Cpxss+Pv. Mössbauer spectral study revealed that Cpxss contains both Fe2+ and Fe3+ in the octahedral site, and it was confirmed that the CaFe3+ AlSiO6 content in the Cpxss at low$$f_{O_2 }$$ is considerably less than that in the Cpxss crystallized in air, whereas the CaFe2+Si2O6 component increases. The maximum solubility of CaTlAl2O6 component in the Cpxss at low$$f_{O_2 }$$ is higher than that in air. The decrease of CaFe3+ AlSiO6 in the Cpxss at low$$f_{O_2 }$$ may cause increase of CaTial2O6 in the Cpxss.

• Preliminary phase equilibria of the nepheline-diopside system under variable pressures (up to 28 kb) and temperatures

The nepheline-diopside join defines the ultra-alkaline portion of the basalt tetrahedron and the bulk composition of nephelinitic rocks lie in this join. Schairer and others established that under atmospheric pressure, the join cuts through the primary phase volumes of oliviness, carnegieitess and nepheliness. Melilite coexists with nepheliness, oliviness and diopsidess below 1160±10°C and olivine reacts out at low temperature.

Experimental studies on seven compositions show the presence of a pseudoeutectic at Ne70Di30 and 1420°C, where diopsidess, nepheliness and liquid are in equilibrium. Olivine and melilite do not appear in the system and the assemblage below 1225±20°C is diopsidess+nepheliness.

Four compositions studied at 1000°C show the appearance of the assemblage diopsidess+nepheliness+melilite at 15kb, whereas diopsidess and nepheliness are the stable phases at 20 and 25kb. The appearance of melilite is therefore restricted to a pressure of 18±3kb. Diopsidess and nepheliness coexist without olivine and melilite in albite-nepheline-diopside and sanidine-nepheline-diopside system. However, this study shows that feldspar-free nephelinitic rocks, which are devoid of melilite, may have crystallized under mantle conditions, whereas their melilite-bearing counterparts equilibrated at a shallower depth within the crust.

• A review and assessment of experiments on Kimberlites, Lamproites and Lamprophyres as a guide to their Origin

Liquidus experimental studies on kimberlite, lamproite and lamprophyre compositions are reviewed with respect to the information they carry on the mantle origin of these rock types. This information is coupled with melting experiments on peridotite in the presence of H2O and mixed H2O+CO2 volatile species.

The origin of most lamproites is explained by the melting of mica-harzburgite assemblages at depths ranging from 40km for leucite lamproites to more than 150km for olivine lamproites. Clinopyroxene-rich, silica-poor lamproites remain enigmatic, but are possibly derived by the melting of a mica-bearing ultramafic source richer in clinopyroxene and under more oxidized, CO2-bearing conditions. There are insufficient experimental studies on kimberlite to reasonably constrain their origin, and what remain are only general indications of the compositions of partial melts of mantle under volatile-bearing conditions. Melt compositions are not sufficiently well known to prevent very conceptual use of melt ‘names’ such as ‘kimberlitic’ or ‘carbonatitic’, and melts similar to alkaline and ultramafic lamprophyre may be hidden under this shroud.

Clearer definition of the origins of alkaline melt compositions such as kimberlites and various lamprophyre types badly needs more exact bracketing of melt compositions of a variety of possible mantle mineral assemblages. The recently-developed sandwich reversal technique is ideally suited to study small degrees of partial melting, and could usefully be applied to lherzolitic and non-lherzolitic materials with hydrous and/or carbonate minerals.

• Experimental study on the tremolite-pargasite join at variable temperatures under 10 kbar

The join tremolite (Tr)-pargasite (Pa) was studied at temperatures between 800 and 1150°C under water vapour pressure of 10 kbar. The results show a continuous solid solution of amphibole between the composition Tr80Pa20 and Pa100 at 800°C and 10kb. Pargasite melts incongruently and breaks down at high temperature to clinopyroxene+forsterite+spinel+L+V. A single phase amphibole with composition lying between Tr80Pa20 and nearly pure Pa, breaks down to amphibole of different composition plus other phases. The stability fields of amphibole spread toward higher temperature side with increasing pargasite content, and pargasite itself has the widest stability field. At subliquidus, the composition of amphibole coexisting with other phases becomes more pargasitic with increasing temperature.

The compositions of liquid, which are formed by partial melting of amphibole of Tr40Pa60 composition (Fo-normative) under water vapour pressure of 10 kbar, are alumina-rich and Qz-normative.

• High pressure-temperature studies on an olivine tholeiite and a tholeiitic picrite from the pavagarh region, Gujarat, India

Experimental studies have been performed on an olivine tholeiite and tholeiitic picrite at pressure and temperature ranges of 20–40 kb and 1200–1300°C. The lower and upper limits of basalt-eclogite transition zone for tholeiitic picrite are 23 kb and 31·67 kb at 1200°C, and 24·67 kb and 33·67 kb at 1300°C, whereas for olivine tholeiite, these are 27 kb and 32·33 kb at 1200°C, and 28·70 kb and 33·70 kb at 1300°C. While the assemblages for both samples below the transition region are Pl+Px+Mt, they are Pl+Gt+Px+Mt within it. The eclogite field has Gt+Px+Mt. The ratio of garnet to plagioclase increases from the transition zone to the eclogite field and with the disappearance of plagioclase, the percentage of garnet increases to 30 in the eclogite field.

Comparison of our results with previous studies on basalt-eclogite transition shows that the transition zone found by us occurs at higher pressure-temperature conditions. Seismic studies of the region below the Deccan Traps show an increase in velocity (1–4%) at depth. It is suggested that after partial melting, during ascent of the basaltic liquid, a significant portion of it crystallizes within the upper mantle as pockets of eclogite. As eclogite is more dense than peridotite, their presence should cause a similar increase in the seismic velocity below the Deccan area.

• Application of infrared spectroscopy to studies of silicate glass structure: Examples from the melilite glasses and the systems Na2O-SiO2 and Na2O-Al2O3-SiO2

Infrared (IR) and Raman spectroscopic methods are important complementary techniques in structural studies of aluminosilicate glasses. Both techniques are sensitive to small-scale (&lt;15 Å) structural features that amount to units of several SiO4 tetrahedra. Application of IR spectroscopy has, however, been limited by the more complex nature of the IR spectrum compared with the Raman spectrum, particularly at higher frequencies (1200–800 cm−1) where strong antisymmetric Si-O and Si-O-Si absorptions predominate in the former. At lower frequencies, IR spectra contain bands that have substantial contributions from ‘cage-like’ motions of cations in their oxygen co-ordination polyhedra. In aluminosilicates these bands can provide information on the structural environment of Al that is not obtainable directly from Raman studies. A middle frequency envelope centred near 700 cm−1 is indicative of network-substituted AlO4 polyhedra in glasses with Al/(Al+Si)&gt;0·25 and a band at 520–620cm−1 is shown to be associated with AlO6 polyhedra in both crystals and glasses. The IR spectra of melilite and melilite-analogue glasses and crystals show various degrees of band localization that correlate with the extent of Al, Si tetrahedral site ordering. An important conclusion is that differences in Al, Si ordering may lead to very different vibrational spectra in crystals and glasses of otherwise gross chemical similarity.

• Problems of pressure estimation in high temperature experiments using solid media apparatus with particular reference to piston-cylinder and anvil-with-hole apparatus (Part I)

The piston-cylinder apparatus has been widely used in geochemical experiments for more than 25 years, but no accurate technique to measure pressures in conjunction with high temperatures has so far been developed. One should however, take into account many possible sources of errors in pressure measurements in case of a piston-cylinder apparatus.

The anvil-with-hole type (AH) high-pressure apparatus is less popular, although it might be successfully employed to carry out experiments at pressures up to 100 kb. In-situ technique for the AH-23M type of apparatus (anvil-with-hole, 23 mm in cell diameter) deals with the measurement of pressure in each run at room temperature by using Bi-Tl pressure gauges. There is uniform pressure distribution within the working part of the cell at room and high temperatures. The temperature distribution is also uniform (within 5°C) at high pressure. The equipment operates with two-stage compression. The use of these methodical approaches makes it possible to accurately perform experiments at pressures from 25 to 50 kb and high temperatures (to 1600°C) with accuracy.

• Problems of pressure estimation in high temperature experiments using solid media apparatus; pressure calibration with reference to breakdown of albite and quartz-coesite transformation (Part II)

The high albite (Ab)⇄jadeite (Jd)+quartz(Q) reaction (1) and the quartz(Q)⇄coesite (Cs) transformation (2) have been determined within the temperature range of 1000–1100°C and 1000–1400°C respectively under variable pressures using an anvil-with-hole apparatus. The equilibrium curves for the two reactions as a function ofP andT are as follows:

P=−1·33+0·0296T (reaction 1);P=18·949+0·0111T(reaction 2). These two lines intersect at 31·1±0·5kb and 1096°C. The possibility of using an anvil-with-hole apparatus for conducting current investigations is discussed in this paper.

• Redox state of the upper mantle

The oxygen fugacity condition of equilibration has been carefully determined from a spinel lherzolite from Mongolia, olivine xenocrysts from chrome pyrope-bearing peridotite nodules from kimberlites of Yakutia, and basaltic samples from ocean floor, iron arcs and the continental areas. These indicate that the spinel lherzolites occurring within alkali basalts from Mongolia, equilibrated under an$$f_{O_2 }$$ condition similar to that of WM buffer. The diamond and chrome pyrope-bearing peridotites from the kimberlite pipes equilibrated between IW and WM buffers. Some of the ilmenite-bearing peridotite crystallized under$$f_{O_2 }$$ conditions similar to that between WM and QFM buffers and chondrites equilibrated below the QFI buffer.

It is concluded that during geochemical processes in the upper mantle the$$f_{O_2 }$$ conditions vary broadly, and are similar to that between FMQ and IW buffers.

There is a dramatic change in the composition of the kimberlitic fluid, which is CH4-bearing at an early stage, but is in equilibrium with H2O and CO2 at a later stage. This is related to mass transfer of fluids from the lower part of the mantle with a low oxidation state to the upper part having a higher$$f_{O_2 }$$ condition.

• The role of oxidation-reduction and C-H-O fluids in determining melting conditions and magma compositions in the upper mantle

High pressure experimental studies of the melting of lherzolitic upper mantle in the absence of carbon and hydrogen have shown that the lherzolite solidus has a positive dP/dT and that the percentage melting increases quite rapidly above the solidus. In contrast, the presence of carbon and hydrogen in the mantle results in a region of ‘incipient’ melting at temperatures below the C,H-free solidus. In this region the presence or absence of melt and the composition of the melt are dependent on the amount and nature of volatiles, particularly the CO2, H2O, and CH4 contents of the potential C-H-O fluid. Under conditions of low$$f_{O_2 }$$ (IW to IW + 1 log unit atP ∼ 20–35kb), fluids such as CH4+H2O and CH4+H2 inhibit melting, having a low solubility in silicate melts. Under these conditions, carbon and hydrogen are mobile elements in the upper mantle. At slightly higher oxygen fugacity (IW+2 log units,P∼20–35 kb) fluids in equilibrium with graphite or diamond in peridotite C-H-O are extremely water-rich. Carbon is thus not mobile in the mantle in this$$f_{O_2 }$$ range and the melting and phase relations for the upper mantle lherzolite approximate closely to the peridotite-H2O system. Pargasitic amphibole is stable to solidus temperatures in fertile lherzolite compositions and causes a distinctive peridotite solidus, the ‘dehydration solidus’, with a marked change in slope (a ‘back bend’) at 29–30kb due to instability of pargasite at high pressure. Intersections of geothermal gradients with the peridotite-H2O solidi define the boundary between lithosphere (subsolidus) and asthenosphere (incipient melt region). This boundary is thus sensitive to changes in$$f_{O_2 }$$ [affecting CH4:H2O:CO2 ratios] and to the amount of H2O and carbon (CO2, CH4) present. At higher$$f_{O_2 }$$ conditions (IW + 3 log units), CO2-rich fluids occur at low pressures but there is a marked depression of the solidus at 20–21 kb due to intersection with the carbonation reaction, producing the low temperature solidus for dolomite amphibole lherzolite (T∼925°C, 21 to &gt;31kb). Melting of dolomite (or magnesite) amphibole lherzolite yields primary sodic dolomitic carbonatite melt with low H2O content, in equilibrium with amphibole garnet lherzolite.

The complexity of melting in peridotite-C-H-O provides possible explanations for a wide range of observations on lithosphere/asthenosphere relations, on mantle melt and fluid compositions, and on processes of mantle metasomatism and magma genesis in the upper mantle.

• # Journal of Earth System Science

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