A thermodynamic model for silicate melt in CaO–MgO–Al2O3–SiO2 to 50 kbar and 1800 °C

A fully thermodynamic model for mafic melt in CaO–MgO–Al₂O₃–SiO₂ (CMAS) has been calibrated, for calculation of melting equilibria in the pressure range 0–50 kbar. It is intended as a preliminary step towards a large‐system melt model, suitable for exploring melting, melt loss and crystallization processes in a wide range of natural rock compositions. Calibration was performed with attention to the model's behaviour in its compositional subsystems, as a rigorous test of model structure and parameterization. The model is consistent with the latest Holland & Powell thermodynamic data set, and can therefore be used to calculate phase relations in conjunction with the many solid‐phase activity–composition models written for the data set. Model calculations successfully reproduce experimental melting reactions in CMAS spinel lherzolite and garnet lherzolite assemblages, as well as sapphirine‐ and kyanite‐bearing assemblages, at moderate to high pressure. Thermodynamically sensitive features, such as thermal divides are also recovered. However, some changes to the model structure will be required before the model can describe the full range of mafic and ultramafic melt compositions known from experiment at low pressures.     

Garnet and spinel lherzolite assemblages in MgO–Al2O3–SiO2 and CaO–MgO–Al2O3–SiO2: thermodynamic models and an experimental conflict

The recent publication of an updated thermodynamic dataset for petrological calculations provides an opportunity to illustrate the relationship between experimental data and the dataset, in the context of a new set of activity–composition models for several key minerals. These models represent orthopyroxene, clinopyroxene and garnet in the system CaO–MgO–Al₂O₃–SiO₂ (CMAS), and are valid up to 50 kbar and at least 1800 °C; they are the first high‐temperature models for these phases to be developed for the Holland & Powell dataset. The models are calibrated with reference to phase‐relation data in the subsystems CaO–MgO–SiO₂ (CMS) and MgO–Al₂O₃–SiO₂ (MAS), and will themselves form the basis of models in larger systems, suitable for calculating phase equilibria in the crust and mantle. In the course of calibrating the models, it was necessary to consider the reaction orthopyroxene + clinopyroxene + spinel = garnet + forsterite in CMAS, representing a univariant transition between simple spinel and garnet lherzolite assemblages. The high‐temperature segment of this reaction has been much disputed. We offer a powerful thermodynamic argument relating this reaction to the equivalent reaction in MAS, that forces us to choose between good model fits to the data in MAS or to the more recent data in CMAS. We favour the fit to the MAS data, preserving conformity with a large body of experimental and thermodynamic data that are incorporated as constraints on the activity–composition modelling via the internally consistent thermodynamic dataset.     

Two-pyroxene thermobarometry with new experimental data in the system CaO-MgO-Al2O3-SiO2

In the system CaO-MgO-Al₂O₃-SiO₂ (CMAS), equilibrium alumina contents of orthopyroxene and clinopyroxene, both coexisting with spinel + forsterite or spinel + anorthite, have been reversed in 16 runs at 1,300–1,400°C and 10.2–20.8 kbar, using PbO flux. The present data and the data of Perkins and Newton (1980) have been modeled using the Redlich-Kister equation. The resulting model satisfies most of the reversed data in the CMAS system, agrees very well with thermochemical measurements, and is consistent with the model for the enstatite-diopside join of Lindsley et al. (1981) and with the system MgO-Al₂O₃-SiO₂ of Gasparik and Newton (1984). The present data, however, do not confirm the negative slopes of Al-isopleths in the spinel lherzolite field suggested by Dixon and Presnall (1980). The new model has been used to calculate a graphical two-pyroxene thermobarometer applicable to natural two-pyroxene assemblages closely approaching in composition the CMAS system.     

Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgO-Al2O3-SiO2-CO2 from 3 to 7 GPa

We have experimentally determined the solidus position of model lherzolite in the system CaO-MgO-Al₂O₃-SiO₂-CO₂ (CMAS.CO₂) from 3 to 7 GPa by locating isobaric invariant points where liquid coexists with olivine, orthopyroxene, clinopyroxene, garnet and carbonate. The intersection of two subsolidus reactions at the solidus involving carbonate generates two invariant points, I_1A and I_2A, which mark the transition from CO₂-bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing lherzolite respectively. In CMAS.CO₂, we find I_1A at 2.6 GPa/1230 °C and I_2A at 4.8 GPa/1320 °C. The variation of all phase compositions along the solidus has also been determined. In the pressure range investigated, solidus melts are carbonatitic with SiO₂ contents of <6 wt%, CO₂ contents of ˜45 wt%, and Ca/(Ca+Mg) ratios that range from 0.59 (3 GPa) to 0.45 (7 GPa); compositionally they resemble natural magnesiocarbonatites. Volcanic magnesiocarbonatites may well be an example of the eruption of such melts directly from their mantle source region as evidenced by their diatremic style of activity and lack of associated silicate magmas. Our data in the CMAS.CO₂ system show that in a carbonate-bearing mantle, solidus and near-solidus melts will be CO₂-rich and silica poor. The widespread evidence for the presence of CO₂ in both the oceanic and continental upper mantle implies that such low degree SiO₂-poor carbonatitic melts are common in the mantle, despite the rarity of carbonatites themselves at the Earth's surface. Received: 9 April 1997 / Accepted: 25 November 1997     

雷琼地区晚新生代玄武岩地球化学：EM2成分来源及大陆岩石圈地幔的贡献

对雷琼地区21个晚新生代玄武岩样品的主量、微量元素和Sr、Nd、Pb同位素分别用湿化学法、ICP-MS和MC-ICPMS进行了测定.这些玄武岩主要为石英拉斑玄武岩,其次为橄榄拉斑玄武岩和碱性玄武岩.大多数样品的微量元素和同位素成分与洋岛玄武岩(OIBs)相似,而且随着SiO_2不饱和度增加,不相容元素含量也增加.除R4-1可能受到地壳混染外,其他样品相对均一的Nd同位素(ε_(Nd)=2.5-6.0)以及变化明显但范围有限的Sr同位素(0.703106～0.704481),可能继承了地幔源区的特征.~(87)Sr/~(86)Sr与~(206)Pb/~(204)Pb的正相关和~(143)Nd/~(144)Nd与~(206)Pb/~(204)Pb的负相关特征暗示DM(软流圈地幔)与EM2(岩石圈地幔)的混合.地幔捕虏体的同位素特征暗示EM2成分不可能存在于尖晶石橄榄岩地幔,而La/Yb和Sm/Yb系统表明岩浆由石榴石橄榄岩部分熔融产生,这意味着EM2成分可能存在于石榴石橄榄岩地幔.雷琼地区玄武岩的地球化学变化可以用软流圈地幔为主的熔体加入不同比例石榴石橄榄岩地幔不同程度熔融产生的熔体来解释:碱性玄武岩和橄榄拉斑玄武岩是软流圈熔体与石榴石橄榄岩地幔较低程度(7%～9%)熔融体混合,而石英拉斑玄武岩是软流圈熔体与石榴石橄榄岩地幔较高程度(10%～20%)熔融体的混合.     

The near-solidus transition from garnet lherzolite to spinel lherzolite

The position of the transition from spinel lherzolite to garnet lherzolite in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) has been determined experimentally at near-solidus temperatures. In reversed experiments, the transition occurs between 18 and 20 kbar at 1200 °C and between 26 and 27 kbar at 1500 °C, corresponding to higher pressures than previously envisaged. A position for the transition deeper within the Earth further complicates the explanation of the so-called garnet signatures in the trace element and isotope patterns of mid-ocean ridge basalts. If melting during adiabatic upwelling beneath a mid-ocean ridge begins at the depth required for the stability of garnet in peridotitic compositions, simple melting models predict that the amount of melt produced should be much greater than the observed thickness of the oceanic crust. A partial solution to the apparent conflict might be that (1) the rather simplistic melting models are in error, (2) that melting begins in garnet pyroxenite veins that are believed to be stable at lower pressures than garnet lherzolite or (3) that melting does not involve garnet at all, but it is clinopyroxene causing the trace element patterns observed in basalts erupted at mid-ocean ridges. A second set of reversal experiments were conducted to investigate the solubility of alumina in both orthopyroxenes and clinopyroxenes at the high temperatures near the solidus in the system CMAS. The results are compatible with most previous studies, and may be used as a starting point to calibrate thermodynamic models for pyroxenes in chemical systems, approximating upper mantle chemistry. Received: 9 August 1999 / Accepted: 29 October 1999     

Garnet and spinel in fertile and depleted mantle: insights from thermodynamic modelling

We performed thermodynamic calculations based on model and natural peridotitic compositions at pressure and temperature conditions relevant to the Earth’s upper mantle, using well-established free energy minimization techniques. The model is consistent with the available experimental data in Cr-bearing peridotitic systems and can therefore be used to predict phase relations and mineral compositions in a wide range of realistic mantle compositions. The generated phase diagrams for six different bulk compositions, representative of fertile, depleted and ultra-depleted peridotitic mantle, shown that the garnet + spinel stability field is always broad at low temperatures and progressively narrows with increasing temperatures. In lithospheric sections with hot geotherms (ca. 60 mW/m²), garnet coexists with spinel across an interval of 10–15 km, at ca. 50–70 km depths. In colder, cratonic, lithospheric sections (e.g. along a 40 mW/m² geotherm), the width of the garnet–spinel transition strongly depends on bulk composition: In fertile mantle, spinel can coexist with garnet to about 120 km depth, while in an ultra-depleted harzburgitic mantle, it can be stable to over 180 km depth. The formation of chromian spinel inclusions in diamonds is restricted to pressures between 4.0 and 6.0 GPa. The modes of spinel decrease rapidly to less than 1 vol % when it coexists with garnet; hence, spinel grains can be easily overlooked during the petrographical characterization of small mantle xenoliths. The very Cr-rich nature of many spinels from xenoliths and diamonds from cratonic settings may be simply a consequence of their low modes in high-pressure assemblages; thus, their composition does not necessarily imply an extremely refractory composition of the source rock. The model also shows that large Ca and Cr variations in lherzolitic garnets in equilibrium with spinel can be explained by variations of pressure and temperature along a continental geotherm and do not necessarily imply variations of bulk composition. The slope of the Cr# [i.e. Cr/(Cr + Al)mol] isopleths in garnet in equilibrium with spinel changes significantly at high temperatures, posing serious limitations to the applicability of empirical geobarometric methods calibrated on cratonic mantle xenoliths in hotter, off-craton, lithospheric mantle sections.     

Melting experiments on the enstatite-diopside join at 70–224 kbar,including the melting of diopside

 Melting relations on the enstatite−diopside (En, Mg₂Si₂O₆−Di, CaMgSi₂O₆) join, including the compositions of crystalline phases and melts coexisting along the solidi, were experimentally determined in the pressure range 70–224 kbar with a split-sphere anvil apparatus (USSA-2000). Melting is peritectic in enstatite-rich compositions at 70–124 kbar (1840–2100° C) and eutectic at higher pressures, while the diopside-rich clinopyroxene melts azeotropically at 70–165 kbar and up to 300° C lower temperatures than the eutectic. Orthopyroxene is replaced with enstatite-rich clinopyroxene at 120 kbar and 2090°C. First garnet with 17 mol% Di forms on the solidus at 158 kbar and 2100° C. Two garnets coexist on the solidus at 165–183 kbar and 2100° C, garnet coexists with CaSiO₃ perovskite at 183–224 kbar (2100–2230° C) and two coexisting perovskites are stable at higher pressures. The melting curve of diopside was determined at 80–170 kbar; the slope becomes negative at 140 kbar and 2155° C. At 170 kbar and 2100° C, diopside with 96% Di breaks down to garnet with 89% Di and CaSiO₃ perovskite. The new data were used to calculate an improved temperature-pressure phase diagram for the CMAS system, which can be useful for estimating the mineralogy of the Earth's upper mantle. Received: 15 October 1994 / Accepted: 15 October 1995     

Spinel-garnet lherzolite transition in the system CaO-MgO-Al2O3-SiO2 revisited: an in situ X-ray study

Large discrepancies are reported for the near-solidus, pressure-temperature location of the spinel to garnet lherzolite univariant curve in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS). Experimental data obtained previously from the piston-cylinder apparatus indicate interlaboratory pressure differences of up to 30% relative. To investigate this disparity—and because this reaction is pivotal for understanding upper mantle petrology—the phase boundary was located by means of an independent method. The reaction was studied via in situ X-ray diffraction techniques in a 6-8 type multianvil press. Pressure is determined by using MgO as an internal standard and is calculated from measured unit cell volume by using a newly developed high-temperature equation of state for MgO. Combinations of real-time and quenched-sample observations are used to bracket the phase transition. The transition between 1350 and 1500°C was reversed, and the reaction was further constrained from 1207 to 1545°C. Within this temperature range, the transition has an average dT/dP slope of ∼40 ± 10°C/kbar, consistent with several previous piston-cylinder studies. Extrapolation of our curve to 1575°C, an established temperature of the P-T invariant point, yields a pressure of 25.1 ± 1.2 kbar. We also obtained a real-time reversal of the quartz-coesite transition at 30.5 ± 2.3 kbar at 1357°C, which is about 2 to 4 kbar lower in pressure than previously determined in the piston-cylinder apparatus.     

Parameters of hotspots and thermochemical plumes during their ascent and eruption

Thermochemical plumes develop at the core-mantle boundary in the presence of a heat flow from the outer core and at local chemical doping that decreases the melting temperature near the bottom of the lower mantle (this dope triggers the melting of the mantle material and the ascent of the plume). The paper presents evaluations for the heat power of the Hawaiian and Iceland plumes and the results of the experimental modeling of a thermochemical plume. The diameter of a plume conduit was determined to remain virtually unchanging in the course of plume ascent. When the top of a plume reaches a “refractory” layer, whose melting temperature is higher than the melt temperature in the plume conduit, a mushroom-shaped head of the plume develops beneath the bottom of this layer. The analysis of geological and geophysical data and the results of experimental modeling are used to develop a thermal physical model for a thermochemical plume. The balance relations for the mass and thermal energy and systematic tendencies in the heat and mass transfer during free convection were utilized to derive a system of equations for the heat and mass transfer of a thermochemical plume. Parameters were determined for a thermochemical plume ascending from the core-mantle boundary. Geodynamic processes are considered that occur during the ascent of a plume before it reaches the surface. The effect of the P-T conditions on the shape and size of a plume roof is analyzed, and a model is proposed for mass transfer between a thermochemical plume and the lithosphere, when the plume reaches the bottom of a “refractory” layer in the lithosphere.     

Garnet-bearing Xenoliths from Salt Lake Crater, Oahu, Hawaii: High-Pressure Fractional Crystallization in the Oceanic Mantle

The focus of this study is a suite of garnet-bearing mantlexenoliths from Oahu, Hawaii. Clinopyroxene, olivine, and garnetconstitute the bulk of the xenoliths, and orthopyroxene is presentin small amounts. Clinopyroxene has exsolved orthopyroxene,spinel, and garnet. Many xenoliths also contain spinel-coredgarnets. Olivine, clinopyroxene, and garnet are in major elementchemical equilibrium with each other; large, discrete orthopyroxenedoes not appear to be in major-element chemical equilibriumwith the other minerals. Multiple compositions of orthopyroxeneoccur in individual xenoliths. The new data do not support theexisting hypothesis that all the xenoliths formed at 1 6–22GPa, and that the spinel-cored garnets formed as a consequenceof almost isobaric subsolidus cooling of a spinel-bearing assemblage.The lack of olivine or pyroxenes in the spinel–garnetreaction zones and the embayed outline of spinel grains insidegarnet suggest that the spinel-cored garnets grew in the presenceof a melt. The origin of these xenoliths is interpreted on thebasis of liquidus phase relations in the tholeiitic and slightlysilica-poor portion of the CaO–MgO–Al₂O₃–SiO₂(CMAS) system at pressures from 30 to 50 GPa. The phase relationssuggest crystallization from slightly silica-poor melts (ortransitional basaltic melts) in the depth range 110–150km beneath Oahu. This depth estimate puts the formation of thesexenoliths in the asthenosphere. On the basis of this study itis proposed that the pyroxenite xenoliths are high-pressurecumulates related to polybaric magma fractionation in the asthenosphere,thus making Oahu the only locality among the oceanic regionswhere such deep magmatic fractional crystallization processeshave been recognized. KEY WORDS: xenolith; asthenosphere; basalt; CMAS; cumulate; oceanic lithosphere; experimental petrology; mantle; geothermobarometry; magma chamber     

Melts of garnet lherzolite: experiments, models and comparison to melts of pyroxenite and carbonated lherzolite

Phase equilibrium experiments on a compositionally modified olivine leucitite from the Tibetan plateau have been carried out from 2.2 to 2.8 GPa and 1,380–1,480 °C. The experiments-produced liquids multiply saturated with spinel and garnet lherzolite phase assemblages (olivine, orthopyroxene, clinopyroxene and spinel ± garnet) under nominally anhydrous conditions. These SiO₂-undersaturated liquids and published experimental data are utilized to develop a predictive model for garnet lherzolite melting of compositionally variable mantle under anhydrous conditions over the pressure range of 1.9–6 GPa. The model estimates the major element compositions of garnet-saturated melts for a range of mantle lherzolite compositions and predicts the conditions of the spinel to garnet lherzolite phase transition for natural peridotite compositions at above-solidus temperatures and pressures. We compare our predicted garnet lherzolite melts to those of pyroxenite and carbonated lherzolite and develop criteria for distinguishing among melts of these different source types. We also use the model in conjunction with a published predictive model for plagioclase and spinel lherzolite to characterize the differences in major element composition for melts in the plagioclase, spinel and garnet facies and develop tests to distinguish between melts of these three lherzolite facies based on major elements. The model is applied to understand the source materials and conditions of melting for high-K lavas erupted in the Tibetan plateau, basanite–nephelinite lavas erupted early in the evolution of Kilauea volcano, Hawaii, as well as younger tholeiitic to alkali lavas from Kilauea.     

High-pressure phase transitions of anorthosite crust in the Earth's deep mantle

We investigated phase relations, mineral chemistry, and density of lunar highland anorthosite at conditions up to 125 GPa and 2000 K. We used a multi-anvil apparatus and a laser-heated diamond-anvil cell for this purpose. In-situ X-ray diffraction measurements at high pressures and composition analysis of recovered samples using an analytical transmission electron microscope showed that anorthosite consists of garnet, CaAl₄Si₂O₁₁-rich phase (CAS phase), and SiO₂ phases in the upper mantle and the mantle transition zone. Under lower mantle conditions, these minerals transform to the assemblage of bridgmanite, Ca-perovskite, corundum, stishovite, and calcium ferrite-type aluminous phase through the decomposition of garnet and CAS phase at around 700 km depth. Anorthosite has a higher density than PREM and pyrolite in the upper mantle, while its density becomes comparable or lower under lower mantle conditions. Our results suggest that ancient anorthosite crust subducted down to the deep mantle was likely to have accumulated at 660–720 km in depth without coming back to the Earth's surface. Some portions of the anorthosite crust might have circulated continuously in the Earth's deep interior by mantle convection and potentially subducted to the bottom of the lower mantle when carried within layers of dense basaltic rocks.     

From the simplest chemical system to the natural one: garnet peridotite barometry

The available experimental data on garnet-bearing-assemblages for synthetic chemical systems (MAS, FMAS, CMAS) have been used to calibrate consistent models for the Al-solubility in orthopyroxene coexisting with garnet, on the basis of equilibrium reaction Py(opx) ? Py(gt). The alternative reaction En(opx)+MgTs(opx) ? Py(gt) is discarded as it yields larger a-posteriori uncertainties. To provide a reliable equation, directly applicable to natural garnet lherzolites, each successive synthetic-system calibration is tested against Mori and Green's (1978) natural-system reequilibration data. For the MAS system, an ideal solution model with constant ΔH°, ΔV° and ΔS° based on 12-oxygen structural formulae for aluminous pyroxenes yields the best fit (GPa, K), $${\text{25,134 + 9,941 }}P - 23.177{\text{ }}T{\text{ + }}RT{\text{ ln (}}X_{{\text{Al}}}^{TB'} {\text{) = 0}}$$ . The MAS synthetic-system calibration can be directly applied to the FMAS system by adding an empirical correction term (20,835 [X _Fe ^gt ]²) independent of either pressure and temperature. However, this correction term is not important because of the limited Fe content of mantle peridotites. When calcium is added to the MAS system, the equilibrium constant is calculated as: $$K_{{\text{CMAS}}} = {{[(1 - X_{{\text{Ca}}}^{M2} )^2 (X_{{\text{Al}}}^{TB'} )]} \mathord{\left/ {\vphantom {{[(1 - X_{{\text{Ca}}}^{M2} )^2 (X_{{\text{Al}}}^{TB'} )]} {[(1 - X_{{\text{Ca}}}^X )^3 (X_{{\text{Al}}}^Y )^2 ]}}} \right. \kern-\nulldelimiterspace} {[(1 - X_{{\text{Ca}}}^X )^3 (X_{{\text{Al}}}^Y )^2 ]}}$$ where M2 and TB′ are pyroxene sites and X and Y are garnet sites. Up to 5 GPa, X _Ca ^X ～ and the CMAS experimental data agree well with the MAS model, but for Yamada and Takahashi's (1983) higher pressure experiments (up to 10 GPa), this no longer holds. Indeed, the garnet solid solution does not behave ideally and an asymmetric regular solution model is needed for application to the deepest natural samples available (>7GPa). Calibration based on new high pressure data yields, $$\begin{gathered} \Delta G_{{\text{CMAS}}}^{XS} = (X_{{\text{Ca}}}^X )(1 - X_{{\text{Ca}}}^X )(0.147 - X_{{\text{Ca}}}^X ) \hfill \\ {\text{ }} \cdot {\text{(6,440,535 - 1,490,654 }}P{\text{)}} \hfill \\ \end{gathered}$$ . According to tests of the inferred solution model, the CFMAS system is a good analogue of natural systems in the pressure, temperature and composition ranges covered by the natural-system reequilibration data (up to 1,500° C and 4 GPa). Simultaneous application of this thermobarometer and of the two-pyroxene mutual solubility thermometer (Bertrand and Mercier 1985) to the phases of the garnet-peridotite xenoliths from Thaba Putsoa, Lesotho, yields a refined paleogeotherm for southern Africa strongly contrasting with previous results. The “granular” nodules yield a thermal gradient of about 8 K/km characteristic of a lithospheric-type environment, whereas the “sheared” ones show a lower gradient of about 1 K/km. This is a typical geotherm expected for a steady thermal state with an inflexion point at the depth of about 160 km corresponding to the lithosphere/asthenosphere boundary.     

Paragenetic relations of diamond with silicate and carbonate minerals in the carbonatite-diamond system: Experiments at 8.5 GPa

Phase relations of diamond and syngenetic minerals were experimentally investigated in the multicomponent system natural carbonatite-diamond at a pressure of 8.5 GPa and temperatures of 1300–1800°C (within the thermodynamic stability field of diamond). Under such conditions, the natural carbonatite of the Chagatai complex (Uzbekistan) acquires the mineralogy of Ca-rich eclogites (grospydites). The melting phase diagram of this system (syngenesis diagram) was constructed; an important element of this diagram is the diamond solubility curve in completely miscible carbonate-silicate melts (solubility values are 15–18 wt % C). The diamond solubility curve divides the phase diagram into two fields corresponding to (1) phase relations involving diamond-undersaturated melts-solutions of carbon with garnet as a liquidus phase (region of diamond dissolution) and (2) phase relations with diamond-saturated melts-solutions with diamond as a liquidus phase (region of diamond crystallization). During a temperature decrease in the region of diamond crystallization from carbonate-silicate melts, the crystallization of diamond is accompanied by the sequential formation of the following phase assemblages: diamond + garnet + melt, diamond + garnet + clinopyroxene + melt, and diamond + garnet + clinopyroxene + carbonate + melt, and the subsolidus assemblage diamond + garnet + clinopyroxene + carbonate is eventually formed. This is indicative of the paragenetic nature of silicate and carbonate minerals co-crystallizing with diamond and corresponding primary inclusions trapped by the growing diamond. A physicochemical mechanism was proposed for the formation of diamond in carbonate-silicate melts. The obtained results were used to analyze the physicochemical behavior of a natural diamond-forming magma chamber.     

Determining the temperature of the Earth’s continental upper mantle from geochemical and seismic data

A method is proposed for determining the temperature of the Earth’s upper mantle from geochemical and seismic data. The data are made consistent by physicochemical simulations, which enable one to derive physical characteristics from geochemical compositional models (direct problem) and to convert seismic velocity profiles into model for the temperature distribution (inverse problem). The methods were used to simulate temperature distribution profiles in the “normal” and “cold” mantle on the basis of profiles for the velocities of P and S waves in the IASP91 model and regional models for the Kaapvaal craton. The constraints assumed for the chemical composition included the depleted material of garnet peridotites and the fertile primitive mantle. The conversion of seismic into thermal profiles was conducted by minimizing the Gibbs free energy with the use of equations of state for the mantle material with regard for anharmonicity and the effects of inelasticity. The sensitivity of the model to the chemical composition and its importance in application to the solution of inverse problems is demonstrated. Temperature profiles derived from the IASP91 and some regional models for depths of 200–210 km display an inflection on geotherms toward decreasing temperatures, which is physically senseless. This anomaly cannot be related to either the presence of volatiles or the occurrence of partial melting, because both of them should have resulted in a decrease, but not an increase, in the seismic velocities. Temperature inversion can be ruled out by the gradual fertilization of the mantle with depth. In this situation, the upper mantle material at depths of 200–300 km should be enriched in FeO, Al₂O₃, and CaO relative to garnet peridotites and be simultaneously depleted in these oxides relative to the pyrolite material of the primitive mantle. It can be generally concluded that both the lithosphere and sublithospheric mantle of the Kaapvaal craton, as well as the normal mantle, should be chemically stratified.     

The stability of sapphirine + clinopyroxene: implications for phase relations in the CaO-MgO-Al2O3-SiO2 system under deep-crustal and upper mantle conditions

Reaction textures and chemographic relations in sapphirine-bearing basic granulites at Finero, Italy, suggest that sapphirine and aluminous diopside were formed in mutual equilibrium from an inferred early spinel-bearing assemblage. Finero appears to be the only known locality where this association has been found in situ, although it is also known from kimberlite and breccia pipe nodules elsewhere. The reactions deduced to have occurred in these rocks suggest the existence of stable invariant points involving the phases sapphirine-spinel-orthopyroxene-clinopyroxene-garnet-anorthite and sapphirine-two pyroxenes-garnetanorthite-kyanite (or sillimanite) in the CMAS end-member system. P-T estimates for the relevant rocks, and the available experimental data, suggest that these points lie at around 800°–900° C, 9–11 kbar. A semi-quantitative petrogenetic grid, incorporating these invariant points with previously determined univariant reactions, is proposed. It is inferred that sapphirine+diopside are stable relative to spinel-bearing assemblages below 900° C. The relatively low temperature explains why sapphirine has not to date been reported from experimental work on the CMAS system. It also suggests that sapphirine may be an important aluminous phase in Mg-rich metagabbros under conditions corresponding to the base of the continental crust, despite the observed rarity of such rocks at the surface.     

中国东部及西秦岭地区新生代岩石圈地幔中的相转变带及其温压条件

五相(橄榄石 斜方辉石 单斜辉石 石榴石 尖晶石)共存的地幔橄榄岩捕虏体是来自岩石圈地幔相转变带的直接样品。中国东部及西秦岭地区晚第三至第四纪碱性火山岩携带的少量五相共存的地幔橄榄岩捕虏体为探讨这些地区新生代岩石圈地幔中相转变带提供了宝贵的样品。本文根据地幔橄榄岩捕虏体中石榴石和尖晶石的产出状况,将这些橄榄岩捕虏体分为三类:第一类橄榄岩中尖晶石为粒状残核,尖晶石外缘被石榴石的反应边包围。这种橄榄岩捕虏体代表尖晶石一石榴石相转变带的上限,故称为尖晶石带橄榄岩;第二类橄榄岩中尖晶石和石榴石以单颗粒零散分布为特征,二者共存但未见明显的相转变关系。这类橄榄岩多位于相转变带中部,拟称为尖晶石-石榴石过渡带橄榄岩;第三类橄榄岩中以石榴石为主,尖晶石和辉石等微晶构成石榴石反应边。这类橄榄岩代表尖晶石-石榴石相转变带的下限,故称为石榴石带橄榄岩。因此,根据不同类型橄榄岩捕虏体中矿物的组成,结合温度压力估算即可确定岩石圈地幔中相转变带的深度和厚度。本文通过对中国东部及西秦岭地区晚第三至第四纪碱性火山岩携带的尖晶石-石榴石二辉橄榄岩捕虏体的温度压力估算来进一步厘定中国东部新生代岩石圈地幔中的相转变带深度和厚度。     

山东临朐山旺新生代玄武岩中超镁铁岩包体的研究

山旺新生代玄武岩中的超镁铁质包体分为五类:尖晶石纯橄榄岩、尖晶石二辉橄榄岩、尖晶石方辉橄榄岩、尖晶石石榴石二辉岩和石榴石二辉橄榄岩。对它们的地质学,岩相学、岩石化学,造岩矿物的化学成分,稀土配分模式及热力学计算的研究表明,前三种岩石属原始地幔岩,后二种是地幔中岩浆作用的产物。     

Molecular dynamics study of the structures and bulk moduli of crystals in the system CaO-MgO-Al2O3-SiO2

Molecular dynamics (MD) simulations have been used to calculate the structures and bulk moduli of crystals in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) using an interatomic potential model (CMAS94), which is composed of pairwise additive Coulomb, van der Waals, and repulsive interactions. The crystals studied, total of 27, include oxides, Mg meta- and ortho-silicates, Al garnets, and various Ca or Al bearing silicates, with the coordination number of cations ranging 6 to 12 for Ca, 4 to 12 for Mg, 4 to 6 for Al, and 4 and 6 for Si. In spite of the simplicity of the CMAS94 potential and the diversity of the structural types treated, MD simulations are quite satisfactory in reproducing well the observed structural data, including the crystal symmetries, lattice parameters, and average and individual nearest neighbour Ca-O, Mg-O, Al-O, and Si-O distances. In addition MD simulated bulk moduli of crystals in the CMAS system compare well with the observed values.