Emplacement conditions of komatiite magmas from the 3.49 Ga Komati Formation, Barberton Greenstone Belt, South Africa

This paper provides new constraints on the crystallization conditions of the 3.49 Ga Barberton komatiites. The compositional evidence from igneous pyroxene in the olivine spinifex komatiite units indicates that the magma contained significant quantities of dissolved H₂O. Estimates are made from comparisons of the compositions of pyroxene preserved in Barberton komatiites with pyroxene produced in laboratory experiments at 0.1 MPa (1 bar) under anhydrous conditions and at 100 and 200 MPa (1 and 2 kbar) under H₂O-saturated conditions on an analog Barberton composition. Pyroxene thermobarometry on high-Ca clinopyroxene compositions from ten samples requires a range of minimum magmatic water contents of 6 wt.% or greater at the time of pyroxene crystallization and minimum emplacement pressures of 190 MPa (6 km depth). Since high-Ca pyroxene appears after 30% crystallization of olivine and spinel, the liquidus H₂O contents could be 4 to 6 wt.% H₂O. The liquidus temperature of the Barberton komatiite composition studied is between 1370 and 1400°C at 200 MPa under H₂O-saturated conditions. When compared to the temperature-depth regime of modern melt generation environments, the komatiite mantle source temperatures are 200°C higher than the hydrous mantle melting temperatures inferred in modern subduction zone environments and 100°C higher than mean mantle melting temperatures estimated at mid-ocean ridges. When compared to previous estimates of komatiite liquidus temperatures, melting under hydrous conditions occurs at temperatures that are 250°C lower than previous estimates for anhydrous komatiite. Mantle melting by near-fractional, adiabatic decompression takes place in a melting column that spans 38 km depth range under hydrous conditions. This depth interval for melting is only slightly greater than that observed in modern mid-ocean ridge environments. In contrast, anhydrous fractional melting models of komatiite occur over a larger depth range ( 130 km) and place the base of the melting column into the transition zone.     

Pyroxene-garnet solid-solution equilibria in the systems Mg4Si4O12Mg3Al2Si3O12 and Fe4Si4O12Fe3Al2Si3O12 at high pressures and temperatures

Pyroxene-garnet solid-solution equilibria have been studied in the pressure range 41–200 kbar and over the temperature range 850–1,450°C for the system Mg₄Si₄O₁₂Mg₃Al₂Si₃O₁₂, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂. At 1,000°C, the solid solubility of enstatite (MgSiO₃) in pyrope (Mg₃Al₂Si₃O₁₂) increases gradually to 140 kbar and then increases suddenly in the pressure range 140–175 kbar, resulting in the formation of a homogeneous garnet with composition Mg₃(Al_0.8Mg_0.6Si_0.6)Si₃O₁₂. In the MgSiO₃-rich field, the three-phase assemblage of β- or γ-Mg₂SiO₄, stishovite and a garnet solid solution is stable at pressures above 175 kbar at 1,000°C. The system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂ shows a similar trend of high-pressure transformations: the maximum solubility of ferrosilite (FeSiO₃) in almandine (Fe₃Al₂Si₃O₁₂) forming a homogeneous garnet solid solution is 40 mol% at 93 kbar and 1,000°C.If a pyrolite mantle is assumed, from the present results, the following transformation scheme is suggested for the pyroxene-garnet assemblage in the mantle. Pyroxenes begin to react with the already present pyrope-rich garnet at depths around 150 km. Although the pyroxene-garnet transformation is spread over more than 400 km in depth, the most effective transition to a complex garnet solid solution takes place at depths between 450 and 540 km. The complex garnet solid solution is expected to be stable at depths between 540 and 590 km. At greater depths, it will decompose to a mixture of modified spinel or spinel, stishovite and garnet solid solutions with smaller amounts of a pyroxene component in solution.     

The role of sphene as an accessory phase in the high-pressure partial melting of hydrous mafic compositions

In a high-pressure experimental study of reactions and possible melt products occurring in the deep continental crust or in subducted oceanic crust, sphene has been identified over a pressure range of 10–18 kbar and to temperatures of 1020°C. Sphene may be a refractory phase with up to 60% partial melting for hydrous mafic compositions. Sphene breaks down at lower pressure than the maximum pressure stability of amphibole in hydrous mafic compositions, and rutile rather than sphene is the important Ti-bearing accessory phase at pressures greater than 16–18 kbar. Sphene and rutile coexist to pressures as low as 14 kbar. This implies that amphibole eclogites containing primary sphene and no rutile have most likely formed at depths less than 45 km.The presence of minor sphene as a residual phase in equilibrium with low-Ti silicic liquids may have a marked effect on the REE distribution in derivative liquids. Thus melts in equilibrium with a garnet and sphene-bearing residuum may have less light-REE-enriched patterns than those predicted when garnet is a residual phase without coexisting sphene. This effect is modelled using REE patterns for sphenes from high-grade metamorphic terrains of western Norway.Both the new REE data and the experimental study have important implications for the genesis of low-Ti magmas formed in continental margins and island arcs.     

Geochemistry of Archaean spinifex-textured peridotites and magnesian and low-magnesian tholeiites

Major and trace element (Rb, Sr, Ba, Zr, Y, Nb, Ni, Co, V, Cr) data are presented for 11 spinifex-textured peridotites (STP) and a number of high-magnesian and low-magnesian tholeiitic basalts. The STP, representing high-magnesian liquids, come from the Yilgarn Block of Western Australia, Munro Township in the Abitibi Belt of Canada and one sample from the Barberton area of South Africa. All of the basaltic samples come from the Yilgarn Block.The STP and high-magnesian rocks are considered to belong to the komatiite suite (1, 2) despite their low CaO/Al₂O₃ ratios. It is argued that the high values (about 1.5) reported for this ratio from the Barberton area can be explained by a combination of factors, viz. garnet separation, Al loss or Ca addition during metamorphism. The processes can be evaluated using CaO/TiO₂, Al₂O₃/TiO₂ ratios, the REE group and trace elements (e.g. Y, Sc). It would appear that most STP from other Archaean belts do not have abnormal CaO/Al₂O₃ ratios.The STP display close to chondritic ratios for Ti/Zr, Zr/Nb, Zr/Y, and TiO₂/Al₂O₃ and are considered to represent liquids produced by large amounts of partial melting of the Archaean mantle. The data suggest that virtually all phases other than olivine were removed by melting during the production of STP liquids. In the STP, Ti/V, Ti/P ratios are non-chondritic, suggesting original depletion and/or incorporation into the core.For lower levels of partial melting, including mid-ocean ridge basalts (MORB) non-chondritic ratios are exhibited by Zr/Y, TiO₂/Al₂O₃, TiO₂/CaO, suggesting controlling phases in the residue for Y, Ca, Al. It is apparent that for STP, Cr is not being controlled, indicating the absence of chromite in the residual. However, at about 15% MgO the data suggest that chromite becomes a residual phase.The transition metals, with the exception of Mn, have higher abundances in Archaean basaltic rocks than in MORB. This is interpreted as being mainly due to more extensive partial melting of the mantle in the Archaean, as a result of higher temperatures.It is suggested that the generation of STP liquids with about 32% MgO is due to upwelling mantle diapirs which probably originated at depths greater than 400 km and at temperatures in excess of 1900°C.Modern equivalents to Archaean greenstone sequences are lacking. The closest tectonic analogue would be the development of oceanic crust within a rifted continental block.     

High-pressure phase equilibria in a garnet lherzolite,with special reference to Mg2+Fe2+ partitioning among constituent minerals

Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg²⁺Fe²⁺, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg₂SiO₄Fe₂SiO₄. At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe²⁺/(Mg + Fe²⁺) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe²⁺/(Mg + Fe²⁺) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe²⁺ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg²⁺Fe²⁺ partition relation among coexisting minerals, an increase of about 1% in the Fe₂SiO₄ component in (Mg,Fe)₂SiO₄ modified spinel and spinel was observed compared with olivine.These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km.     

Degrees of melting in mantle diapirs and the origin of ultrabasic liquids

The temperature and degree of melting in an upwelling diapir in the mantle may be considerably less than that anticipated from an adiabatic cooling curve. Several geological and thermodynamic parameters may be incorporated to produce a more realistic melting model in diapirs. The latent heat of fusion of mantle material is the greatest buffer on degrees of melting. Models are presented which suggest that an uprising diapir intersecting the anhydrous solidus of mantle material at 50 kbars may be only 29% melted on reaching the surface. A diapir initiated at 100 kbars may be 69% melted. These are maximum values. These calculations imply that the generation of komatiitic liquids by diapiric uprise alone demands that the diapir originate at depths in excess of 300 km. Melting of mantle with an irregular geotherm is preferred for the origin of these liquids.     

A double partial melt zone in the mantle beneath mid-ocean ridges

For a lherzolite mantle with about 0.1 wt.-percent CO₂ or less, and a CO₂/H₂O mole ratio greater than about one, the mantle solidus curve in P-T space will have two important low-temperature regions, one centered at about 9 kbar (30 km depth) and another beginning at about 28 kbar (90 km depth). It is argued that the depth of generation of primary tholeiitic magmas beneath ridge crests is about 9 kbar, and that the geotherm changes from an adiabatic gradient at greater pressures to a strongly superadiabatic gradient at lesser pressures. Such a ridge geotherm would intersect the solidus at two separate depth intervals corresponding to the two low-temperature regions on the solidus. With increasing age and cooling of the lithosphere, the shallow partial melt zone would pinch out and the thickness of the deep partial melt zone would decrease. With increasing depth in a mature oceanic lithosphere, the rock types would consist of depleted harzburgite from directly beneath the crust to about 30 km depth, fertile spinel lherzolite from about 30 km to 50–60 km, and fertile garnet lherzolite from about 50–60 km to the top of the deep partial melt zone at about 90 km.     

Komatiites and the structure of the Archaean mantle

The existence of Archaean komatiites with eruption temperatures greater than 1650°C requires that the mantle be vertically differentiated by the time of komatiite eruption. If in the unlikely event that undifferentiated mantle had survived primordial planetary differentiation and had been hot enough to deliver 1650°C komatiite, it would have been extensively molten to depths of ～250 km, resulting in rapid, profound, vertical differentiation anyway. During primordial differentiation (or Archaean komatiite petrogenesis) the high density and compressibility of ultrabasic melt allowed storage of a global melt layer beneath a buoyant residue of dunite and/or harzburgite. This refractory cap segregated by extraction of melt both upwards and downwards from the depth at which the density contrast between crystals and liquid vanishes. Eruption of komatiite from the melt layer by corrosion of the cap was the Archaean earth's principal means of dissipating excess heat. This subterranean magma ocean precluded vertical homogenization of the Archaean mantle by convection but effectively absorbed lateral mantle heterogeneities and imposed the relative uniformity of maximum eruption temperature and MgO contents (～32%) seen in primitive Archaean komatiites on all continents.Verification of the postulated density relations of liquids and crystals to 100 kbar becomes a pressing concern in view of the expected consequences these relations may have had.     

The mineralogy of an eclogitic earth mantle

Pyroxene (omphacitic) and garnet (pyrope-rich) are the two major mineral components of an eclogite. No high-pressure phase transformation has been observed in omphacite and pyrope in the pressure range between 30 and 200 kbar and at 1000°C. The phase behaviour of the DSDP3-18 glass (basaltic and eclogitic composition) has been investigated in the pressure range between 100 and 280 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. Both omphacite and garnet were observed in the range 100 to 150 kbar and garnet is the only phase observed in the 180-kbar run. However, it was inferred from other evidence that garnet also coexists with diopside (II) in the 180-kbar run. Diopside (II) is an unquenchable phase which is impossible to preserve on release of pressure. Glasses were the only products quenched from runs carried out at pressures greater than 210 kbar. These glasses were also interpreted as diopside(II). The phase behaviour of this complex eclogite composition at pressures below 200 kbar generally resembles that of a simple enstatite-pyrope system; pyroxene progressively dissolves in garnet with increasing pressure. The P-T conditions for the pyroxene ? garnet transition and the accompanying density (or velocity) change in the eclogitic composition are not consistent with those of the 400-km discontinuity in the Earth's mantle. Thus, an eclogitic mantle composition would not undergo a phase transformation which would be capable of accounting for the major seismic discontinuity observed in the vicinity of 400 km.     

Phase transformations in a harzburgite composition to 26 GPa: implications for dynamical behaviour of the subducting slab

The mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5–26 GPa at temperatures of 1300–1400°C. The pyroxene-garnet component of the harzburgite composition (harzburgite minus 82 wt.% olivine) transforms to majorite garnet by 18–19 GPa, and further disproportionates to the assemblage of garnet + stishovite + Mg₂SiO₄ spinel above 20 GPa. At still higher pressures, first ilmenite (22–24 GPa) and then perovskite MgSiO₃ (24–26 GPa) are found to coexist with garnet. Garnet disappears at 26 GPa and almost complete transition to perovskite is achieved at this pressure. The mineral proportions and density profiles in the subducting oceanic lithosphere, modelled by a combination of 80% harzburgite + 20% primitive MORB compositions are calculated as a function of depth under conditions isothermal with surrounding pyrolite mantle, and also for a temperature distribution in which the slab is substantially cooler than surrounding mantle to below 700 km. Under isothermal conditions, the slab has a density similar to surrounding mantle to a depth of 600 km. However, between 600 and 700 km, the slab is up to 0.08 g/cm³ denser than surrounding mantle. This is caused primarily by the higher alumina content in pyrolite as compared to harzburgite, which causes the transition to perovskite in pyrolite to occur at substantially higher pressures than in harzburgite. The presence of alumina also smears out the garnet-perovskite transition in pyrolite over a depth interval of 50 km, whereas this transformation is much sharper in the harzburgite composition. Calculations based on the observed phase equilibria also show that a subducted cool slab remains much denser (by 0.1–0.3 g/cm³) than surrounding mantle to a depth of 700 km but possesses a density similar to surrounding mantle below this depth. These results have important implications for the dynamical behaviour of slabs possessing different thermal regimes when they encounter the 670 km discontinuity and also for the nature of this discontinuity.     

Composition,temperature, and thickness of the lithosphere of the Archean Kaapvaal craton

A new model is proposed for the structure of the Kaapvaal craton lithosphere. Based on chemical thermodynamics methods, profiles of the chemical composition, temperature, density, and S wave velocities are constructed for depths of 100–300 km. A solid-state zone of lower velocities is discovered on the S velocity profile in the depth interval 150–260 km. The temperature profiles are obtained from absolute values of P and S velocities, taking into account phase transformations, anharmonicity, and anelastic effects. The examination of the sensitivity of seismic models to the chemical composition showed that relatively small variations in the composition of South African xenoliths result in lateral temperature variations of ～200°C. Inversion of some seismic profiles (including IASP91) with a fixed bulk composition of garnet peridotites (the primitive mantle material) leads to a temperature inversion at depths of 200–250 km, which is physically meaningless. It is supposed that the temperature inversion can be removed by gradual fertilization of the mantle with depth. In this case, the craton lithosphere should be stratified in chemical composition. The depleted lithosphere composed by garnet peridotites exists to depths of 175–200 km. The lithospheric material at depths of 200–250 km is enriched in basaltoid components (FeO, Al₂O₃, and CaO) as compared with the material of garnet peridotites but is depleted in the same components as compared with the fertile substance of the underlying primitive mantle. The material composing the craton root at a depth of ～275 km does not differ in its physical and chemical characteristics from the composition of the normal mantle, and this allows one to estimate the thickness of the lithosphere at 275 km. The results of this work are compared with data of seismology, thermal investigations, and thermobarometry.     

Is upper-mantle phosphorus contained in sodic garnet?

Garnets crystallized experimentally from within the anhydrous melting ranges of an olivine tholeiite, a tholeiitic andesite and an augite leucitite at pressures between 18 and 45 kbars contain up to 0.4% Na₂O and 0.6% P₂O₅. The Na and P are thought to form a substitution couple, replacing Ca and Si in the garnet structure; representing limited solid solution between grossular (Ca₃Al₂Si₃O₁₂) and the phosphate Na₃Al₂P₃O₁₂. This substitution is enhanced by increasing pressure and by falling temperature (increasing degree of crystallization) at constant pressure.Current knowledge of the crystalline site of P in the upper mantle is hampered by lack of data on the stability of apatite and other phosphates at appropriate pressures and temperatures. If all samples of garnetiferous upper mantle brought to the surface by magmatic processes have been depleted to some extent by previous escape of a partial-melt fraction, P₂O₅ concentrations below 0.1% in their garnets could nevertheless signify that this phase was the sole predepletion host for P in the upper mantle, at the depths from which such inclusions are derived. If garnet and apatite are the principal minerals containing P in the upper mantle, it may be possible to use covariances between P and rare-earth elements in mafic liquids to detect which of these phases was the dominant host for P at the site of magma genesis. This approach confirms the widely-held opinion that strongly alkalic mafic magmas are products of upper-mantle partial fusion in the presence of residual garnet. It also leads to a contrasting proposal that mid-ocean ridge basalts may be generated by upper-mantle partial fusion at comparatively small depths, in the presence of residual apatite.     

Apatite and phosphorus in mantle source regions: An experimental study of apatite/melt equilibria at pressures to 25 kbar

The solubility of fluorapatite in a wide variety of basic magmatic liquids was experimentally determined over a range of upper mantle P-T conditions (8–25 kbar, 1275–1350°C). Fluorapatite is stable over the entire range of conditions investigated, but its solubility in melts is variable, depending negatively on SiO₂ content of the melt and positively upon temperature, with relatively little sensitivity to pressure above 8 kbar. At upper mantle pressures and a temperature of 1250°C, molten basalt (50% SiO₂) will dissolve 3–4 wt.% P₂O₅ before saturation in apatite is reached. For a magma 100°C cooler or containing 10 wt.% more SiO₂, apatite saturation occurs at less than 2 wt.% dissolved P₂O₅. The observed high solubility of apatite in basic magmas at their normal near-liquidus temperatures virtually precludes the occurrence of residual apatite in mantle source regions. If relatively low-temperature melting conditions prevail (e.g., 1100°C), as might be possible in H₂O-bearing regions of the upper mantle, apatite could remain in the residue, but only in amounts too small to have significant effects on the rare earth patterns of the liquids.Because of the high solubility of apatite in basic magmas, phosphorus can be confidently treated as an incompatible element in peridotite melting models. Such models, in combination with observed characteristics of basic lavas, indicate that the upper mantle contains ～200 ppm of phosphorus, much less than the chondritic abundance of ～900 ppm.     

Anhydrous and water-saturated melting experiments of an olivine andesite from Mt Yakushi-Yama, northeastern Shikoku, Japan

Abstract Melting experiments have been carried out on an olivine andesite of Mt Yakushi-Yama from the Miocene Setouchi volcanic belt in northeastern Shikoku, Japan. This andesite has been characterized by a low ratio of FeO*/Mg° (= 0.78). Phase relations have been determined within the pressure range of 2.8 to 19.3 kbar at 1000-1300°C under anhydrous and water-saturated conditions. At pressures less than 8.8 kbar, olivine is a liquidus phase. Orthopyroxene appears on the liquidus at 9.3 kbar under the anhydrous conditions. The multiple saturation point rises up to 17.5 kbar for water-saturated experiments. The andesite melt coexists with olivine and orthopyroxene just below the liquidus at 8.8–9.3 kbar and 1230°C for dry conditions, and at 17.5 kbar and 1060°C under water-saturated conditions. These experimental results indicate that the Yakushi-Yama olivine andesite magma could coexist with a harzburgitic mantle at depths between about 30 and 60 km, and at temperatures between 1060 and 1230°C. Experimental data also suggest a possibility that a high magnesian andesite magma would be generated by a direct partial melting of the uppermost harzburgitic mantle under anhydrous conditions.     

Experimental evidence for the exsolution of cratonic peridotite from high-temperature harzburgite

Phase equilibrium experiments were performed on typical ‘oceanic’ and ‘cratonic’ peridotite compositions and a Ca, Al-rich orthopyroxene composition, to test the proposal that garnet lherzolites exsolved from high-temperature harzburgites, and to further our understanding of the origin of ancient cratonic lithospheres. ‘Oceanic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1450–1600°C, but at 5 GPa and temperatures less than 1450°C, crystallize clinopyroxene to become true lherzolites. ‘Cratonic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1300–1600°C. Garnet-free harzburgite crystallizes from both ‘cratonic’ and ‘oceanic’ peridotite at temperatures above 1450°C and pressures below 4.5–5 GPa. Phase relations for the high Ca, Al-rich orthopyroxene composition essentially mirror those for ‘oceanic’ peridotite.The complete solution of garnet and clinopyroxene into orthopyroxene observed in all three starting compositions at temperatures near or above the mantle solidus at pressures less than 6 GPa supports the hypothesis that garnet lherzolite could have exsolved from harzburgite. The inferred cooling path for the original high-temperature harzburgite protoliths of garnet lherzolites differs depending on bulk composition. The precursor harzburgite protoliths of garnet lherzolites and harzburgites with ‘cratonic’ bulk compositions apparently experienced simple isobaric cooling from formation temperatures near the peridotite solidus to those at which most of these peridotites were sampled in the mantle (< 1200°C). The cooling histories for harzburgite protoliths of sheared garnet lherzolites with ‘oceanic’ compositional affinity are speculated to have involved convective circulation of mantle material to depths deeper than those at which it was originally formed.Phase equilibria and compositional relationships for orthopyroxenes produced in phase equilibrium experiments on peridotite and komatiite are consistent with an origin for ‘cratonic’ peridotite as a residue of Archean komatiite extraction, which has since cooled and exsolved clinopyroxene and garnet to become the common low-temperature, coarse-grained peridotite thought to comprise the bulk of the mantle lithosphere beneath the Archean Kaapvaal craton.     

The petrogenesis of komatiites and related rocks as evidence for a layered upper mantle

Although the CaO/Al₂O₃ ratio of komatiites has been regarded as one of the distinguishing features of these rocks, a comparison of various komatiite and oceanic tholeiite analyses suggests that there is a continuum of ratios between the two. The extremely high MgO values of peridotitic komatiites suggest that they are the result of high degrees of partial melting of the mantle, leaving a harzburgitic residuum depleted in CaO and Al₂O₃, and hence preserving in the melt the original CaO/Al₂O₃ ratio of the parental material. Available chemical models of the mantle have CaO/Al₂O₃ ratios too low to explain the origin of komatiite by such a process. Shallow-level melting of a layered mantle in which clinopyroxene content decreases and garnet content increases with depth, may explain the chemistry of komatiites and related ultrabasic lavas.     

The eclogite-garnetite transformation at high pressure and some geophysical implications

Mineral assemblages displayed by MORB and alkali-poor olivine tholeiites have been investigated over the pressure interval 4.6–18 GPa at 1200°C. Both compositions crystallize to form normal eclogites between 4.6 and 10 GPa and there is little change in the relative proportions of garnet and pyroxene over this range. However, the proportion of garnet increases rapidly above 10 GPa as pyroxene dissolves in the garnet structure and pyroxene-free garnetites (±stishovite) are produced by 14–15 GPa, dependent upon composition. The garnetite facies for both compositions possess zero-pressure densities of 3.75 g/cm³, implying that subducted oceanic crust remains appreciably denser than surrounding mantle to depths exceeding 600 km. It is demonstrated that the seismic velocity distributions in the mantle between 400 and 650 km are inconsistent with Anderson's hypothesis that this region is of eclogitic composition.     

High-pressure phase transformations in the system CaSiO3-Al2O3

Phase assemblages for five selected compositions in the system CaSiO₃-Al₂O₃ have been investigated in the pressure range 100–300 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. At pressures below about 250 kbar, the assemblage of grossularite plus corundum is stable for compositions containing more than 25 mole% Al₂O₃. Above about 250 kbar, phase assemblages for the latter compositions are truncated by those in the join CaAl₂O₄-SiO₂. Garnet solid solutions are stable between about 10 and 25 mole% Al₂O₃. Grossularite transforms to a new tetragonal form at pressures greater than about 250 kbar, but the stability field for the garnet solid solutions extends to pressures up to about 300 kbar. The perovskite modification appears to be stable at pressures above about 150 kbar, but is probably limited to nearly pure CaSiO₃ composition. Phase behaviour for calcium-bearing silicates or aluminosilicates in the lower mantle are apparently more complicated than was suggested earlier.     

Basalt-andesite-rhyolite-H2O: Crystallization intervals with excess H2O and H2O-undersaturated liquidus surfaces to 35 kolbras,with implications for magma genesis

Three rocks representing the calc-alkaline rock series gabbro-tonalite-granite or basalt-andesite-rhyolite were reacted with varying percentages of water in sealed capsules between 600 and 1300°C and pressures to 36 kbars, corresponding to depths of more than 120 km within the earth. For each rock we present complete P-T diagrams with excess water, and the water-undersaturated liquids surface projected from P-T-X_H2O space mapped with contours for constant H₂O contents and with the fields for near-liquidus minerals. All changes in liquidus and solidus slopes can be correlated with changes in mineralogy from less dense to more dense, or with expansion of crystallization fields, without appeal to changes in molar volume of H₂O in liquid and vapor phases. The results indicate that tholeiites and andesites of the calc-alkaline series with compositions similar to the rocks studied are not primary magmas from mantle peridotite at depths greater than about 50 km. Primary andesitic magmas from shallower levels would require very high water contents and we do not believe such magmas could normally reach the surface. The liquids results are consistent with the derivation of andesites with little dissolved water as primary magmas from subducted ocean crust (quartz eclogite), but multi-stage models are preferred. Temperatures required for the generation of andesites by fusion of continental crust are higher than considered reasonable. The evidence precludes the generation of primary rhyolites or granites from the mantle of subducted oceanic crust at mantle depths. Primary rhyolite or granite magmas with moderate water contents (saturated or undersaturated) can be generated in the crust at reasonable temperatures, and could reach near-surface levels before vesiculation. Water-undersaturated granite liquid with residual crustal minerals could constitute plutonic magmas of intermediate composition.     

Diapirism of depleted peridotite — a model for the origin of hot spots

A model is proposed for the origin of hot spots that depends on the existence of major-element heterogeneities in the mantle. Generation of basaltic crust at spreading centers produces a layer of residual peridotite ～20–25 km thick directly beneath the crust which is depleted in Fe/Mg, TiO₂, CaO, Al₂O₃, Na₂O and K₂O, and which has a slightly lower density than undepleted peridotite beneath it. Upon recycling of this depleted peridotite back into the deep mantle at subduction zones, it becomes gravitationally unstable, and tends to rise as diapirs through undepleted peridotite. For a density contrast of 0.05 g cm^?3, a diapir 60 km in diameter would rise at roughly 8 cm y^?1, and could transport enough heat to the base of the lithosphere to cause melting and volcanism at the surface. Hot spots are thus viewed as a passive consequence of mantle convection and fractionation at spreading centers rather than a plate-driving force.It is suggested that depleted diapirs exist with varying amounts of depletion, diameters, upward velocities and source volumes. Such variations could explain the occurrence of hot spots with widely varying lifetimes and rates of lava production. For highly depleted diapirs with very low Fe/Mg, the diapir would act as a heat source and the asthenosphere and lower lithosphere drifting across the diapir would serve as the source region of magmas erupted at the surface. For mildly depleted diapirs with Fe/Mg only slightly less than in normal undepleted mantle, the diapir could provide not only the source of heat but also most or all of the source material for the erupted magmas. The model is consistent with isotopic data that require two separate and ancient source regions for mid-ocean ridge and oceanic island basalts. The source for mid-ocean ridge basalts is considered to be material upwelling at spreading centers from the deep mantle. This material forms the oceanic lithosphere. Oceanic island basalts are considered to be derived from varying mixtures of sublithospheric and lower lithospheric material and the rising diapir itself.