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     

A modified iterative sandwich method for determination of near-solidus partial melt compositions. II. Application to determination of near-solidus melt compositions of carbonated peridotite

We performed modified iterative sandwich experiments (MISE) to determine the composition of carbonatitic melt generated near the solidus of natural, fertile peridotite + CO₂ at 1,200–1,245°C and 6.6 GPa. Six iterations were performed with natural peridotite (MixKLB-1: Mg# = 89.7) and ∼10 wt% added carbonate to achieve the equilibrium carbonatite composition. Compositions of melts and coexisting minerals converged to a constant composition after the fourth iteration, with the silicate mineral compositions matching those expected at the solidus of carbonated peridotite at 6.6 GPa and 1,230°C, as determined from a sub-solidus experiment with MixKLB-1 peridotite. Partial melts expected from a carbonated lherzolite at a melt fraction of 0.01–0.05% at 6.6 GPa have the composition of sodic iron-bearing dolomitic carbonatite, with molar Ca/(Ca + Mg) of 0.413 ± 0.001, Ca# [100 × molar Ca/(Ca + Mg + Fe*)] of 37.1 ± 0.1, and Mg# of 83.7 ± 0.6. SiO₂, TiO₂ and Al₂O₃ concentrations are 4.1 ± 0.1, 1.0 ± 0.1, and 0.30 ± 0.02 wt%, whereas the Na₂O concentration is 4.0 ± 0.2 wt%. Comparison of our results with other iterative sandwich experiments at lower pressures indicate that near-solidus carbonatite derived from mantle lherzolite become less calcic with increasing pressure. Thus carbonatitic melt percolating through the deep mantle must dissolve cpx from surrounding peridotite and precipitate opx. Significant FeO* and Na₂O concentrations in near solidus carbonatitic partial melt likely account for the ∼150°C lower solidus temperature of natural carbonated peridotite compared to the solidus of synthetic peridotite in the system CMAS + CO₂. The experiments demonstrate that the MISE method can determine the composition of partial melts at very low melt fraction after a small number of iterations.     

The system CaO-MgO-SiO2-CO2 at 1 GPa, metasomatic wehrlites, and primary carbonatite magmas

New experimental data in CaO-MgO-SiO₂-CO₂ at 1 GPa define the vapor-saturated silicate-carbonate liquidus field boundary involving primary minerals calcite, forsterite and diopside. The eutectic reaction for melting of model calcite (1% MC)-wehrlite at 1 GPa is at 1100 °C, with liquid composition (by weight) 72% CaCO₃ (CC), 9% MgCO₃ (MC), and 18% CaMgSi₂O₆ (Di). These data combined with previous results permit construction of the isotherm-contoured vapor-saturated liquidus surface for the calcite/dolomite field, and part of the adjacent forsterite and diopside fields. Nearly pure calcite crystals in mantle xenoliths cannot represent equilibrium liquids. We recently determined the complete vapor-saturated liquidus surface between carbonates and model peridotites at 2.7 GPa; the peritectic reaction for dolomite (25% MC)-wehrlite at 2.7 GPa occurs at 1300 °C, with liquid composition 60% CC, 29% MC, and 11% Di. The liquidus field boundaries on these two surfaces provide the road-map for interpretation of magmatic processes in various peridotite-CO₂ systems at depths between the Moho and about 100 km. Relationships among kimberlites, melilitites, carbonatites and the liquidus phase boundaries are discussed. Experimental data for carbonatite liquid protected by metasomatic wehrlite have been reported. The liquid trends directly from dolomitic towards CaCO₃ with decreasing pressure. The 1.5 GPa liquid contains 87% CC and 4% Di, much lower in silicate components than our phase boundary. However, the liquids contain approximately the same CaCO₃ (90 ± 1 wt%) in terms of only carbonate components. For CO₂-bearing mantle, all magmas at depth must pass through initial dolomitic compositions. Rising dolomitic carbonatite melt will vesiculate and may erupt as primary magmas through cracks from about ˜70 km. If it percolates through metasomatic wehrlite from 70 km toward the Moho at 35–40 km, primary calcic siliceous carbonatite magma can be generated with silicate content at least 11–18% (70–40 km) on the silicate-carbonate boundary. Received: 22 June 1998 / Accepted: 7 July 1999     

Experimental reconstruction of sodic dolomitic carbonatite melts from metasomatised lithosphere

 Investigations of peridotite xenolith suites have identified a compositional trend from lherzolite to magnesian wehrlite in which clinopyroxene increases at the expense of orthopyroxene and aluminous spinel, and in which apatite may be a minor phase. Previous studies have shown that this trend in mineralogy and chemical composition may result from reaction between sodic dolomitic carbonatite melt and lherzolite at pressures around 1.7 to 2 GPa. This reaction results in decarbonation of the carbonatite melt, releasing CO₂-rich fluid. In this study, we have experimentally reversed the decarbonation reaction by taking two natural wehrlite compositions and reacting them with CO₂ at a pressure of 2.2 GPa and temperatures from 900 to 1150° C. Starting materials were pargasite-bearing wehrlites, one with minor apatite (composition 71001^*) and one without apatite (composition 70965^*). At lower temperatures (900° C) the products were apatite+pargasite+magnesite harzburgite for runs using composition 71001^*, and pargasite+dolomite lherzolite for runs using composition 70965^*. At and above 1000° C, carbonatite melt with harzburgite residue (olivine+orthopyroxene+spinel) and with lherzolite residue (olivine+orthopyroxene+clinopyroxene+ spinel) were produced respectively. Phase compositions in reactants and products are consistent with the documented carbonatite/lherzolite reactions, and also permit estimation of the carbonatite melt compositions. In both cases the melts are sodic dolomitic carbonatites. The study supports the hypothesis of a significant role for ephemeral, sodic dolomitic melts in causing metasomatic changes in the lithosphere at P≤2 GPa. The compositions of wehrlites imply fluxes of CO₂, released by metasomatic reactions, which are locally very large at around 5 wt% CO₂. Received: 15 December 1995/Accepted: 14 February 1996     

Experimental study of the phase and melting relations of homogeneous basalt + peridotite mixtures and implications for the petrogenesis of flood basalts

Flood basalt provinces may constitute some of the most catastrophic volcanic events in the Earth's history. A popular model to explain them involves adiabatic ascent of plumes of anomalously hot peridotite from a thermal boundary layer deep in the mantle, across the peridotite solidus. However, peridotitic plumes probably require unreasonably high potential temperatures to generate sufficient volumes of magma and high enough melting rates to produce flood volcanism. This lead to the suggestion that low melting eclogitic or pyroxenitic heterogeneities may be present in the source regions of the flood basalts. In order to constrain petrogenetic models for flood basalts generated in this way, an experimental investigation of the melting relations of homogeneous peridotite + oceanic basalt mixtures has been performed. Experiments were conducted at 3.5 GPa on a fertile peridotite (MPY90)–oceanic basalt (GA1) compositional join. The hybrid basalt + peridotite compositions crystallised garnet lherzolite at subsolidus temperatures plus quenched ne-normative picritic liquids at temperatures just above the solidus, over the compositional range MPY90 to GA1₅₀MPY90₅₀. The solidus temperature decreased slightly from ∼1500 °C for MPY90 to ∼1450 °C for GA1₅₀MPY90₅₀. Compositions similar to GA1₃₀MPY90₇₀ have 100% melting compressed into a melting interval which is approximately 50–60% smaller than that for pure MPY90, due to a liquidus minimum. During adiabatic ascent of hybrid source material containing a few tens of percent basalt in peridotite, the lower solidus and compressed solidus–liquidus temperature interval may conspire to substantially enhance melt productivity. Mixtures of recycled oceanic crust and peridotite in mantle plumes may therefore provide a viable source for some flood volcanics. Evidence for this would include higher than normal Fe/Mg values in natural primary liquids, consistent with equilibration with more Fe-rich olivine than normal pyrolitic olivine (i.e. <Fo_89–92). Modelling of fractionation trends in West Greenland picrites is presented to demonstrate that melts parental to the Greenland picrites were in equilibrium at mantle P–T conditions with olivine with Fo_84–86, suggesting an Fe-enriched source compared with normal peridotite, and consistent with the presence of a basaltic component in the source. Received: 29 October 1999 / Accepted: 3 February 2000     

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 (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 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 [affecting CH₄:H₂O:CO₂ ratios] and to the amount of H₂O and carbon (CO₂, CH₄) present. At higher 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 >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.     

Carbonatite metasomatized peridotite xenoliths from southern Patagonia: implications for lithospheric processes and Neogene plateau magmatism

The mineral chemistry, major and trace element, and Sr–Nd isotopic composition of Cr-diopside, spinel peridotite xenoliths from the Estancia Lote 17 locality in southern Patagonia document a strong carbonatitic metasomatism of the backarc continental lithosphere. The Lote 17 peridotite xenolith suite consists of hydrous spinel lherzolite, wehrlite, and olivine websterite, and anhydrous harzburgite and lherzolite. Two-pyroxene thermometry indicates equilibration temperatures ranging from 870 to 1015 °C and the lack of plagioclase or garnet suggests the xenoliths originated from between ˜40 and 60 km depth. All of the xenoliths are LILE- and LREE-enriched, but have relatively low ⁸⁷Sr/⁸⁶Sr (0.70294 to 0.70342) and high ɛ_Nd (+3.0 to +6.6), indicating recent trace element enrichment (∼25 Ma, based on the low ⁸⁷Sr/⁸⁶Sr and high Rb concentrations of phlogopite separates) in the long-term, melt-depleted Patagonian lithosphere. Lote 17 peridotite xenoliths are divided into two basic groups. Group 1 xenoliths consist of fertile peridotites that contain hydrous phases (amphibole ± phlogopite ± apatite). Group 1 xenoliths are further subdivided into three groups (a, b, and c) based on distinctive textures and whole-rock chemistry. Group 1 xenolith mineralogy and chemistry are consistent with a complex metasomatic history involving variable extents of recent carbonatite metasomatism (high Ca/Al, Nb/La, Zr/Hf, low Ti/Eu) that has overprinted earlier metasomatic events. Group 2 xenoliths consist of infertile, anhydrous harzburgites and record cryptic metasomatism that is attributed to CO₂-rich fluids liberated from Group 1 carbonatite metasomatic reactions. Extremely variable incompatible trace element ratios and depleted Sr–Nd isotopic compositions of Lote 17 peridotite xenoliths indicate that the continental lithosphere was neither the primary source nor an enriched lithospheric contaminant for Neogene Patagonian plateau lavas. Neogene plateau magmatism associated with formation of asthenospheric slab windows may have triggered this occurrence of “intraplate-type” carbonatite metasomatism in an active continental backarc setting. Received: 26 January 2000 / Accepted: 1 March 2000     

Liquid immiscibility between nephelinite and carbonatite from 1.0 to 2.5 GPa compared with mantle melt compositions

In order to define the conditions for the formation of immiscible carbonatite magmas in the lithosphere and in the crust, we have conducted phase equilibrium experiments to determine the effect of pressure and temperature on the silicate-carbonate liquid miscibility gap in bulk compositions appropriate for magmas in the upper mantle. A primitive (magnesian) nephelinite (NEPH) was used as a starting material, mixed with carbonates. Experiments were made with mixtures in the joins NEPH-dolomite-Na₂CO₃ (NEPH-Dol-NC) at 1.0 to 2.5 GPa, and NEPH-calcite (NEPH-CC) at 1.0 GPa. The miscibility gap was intersected by the join NEPH-Dol-NC (liquids with olivine), but not by NEPH-CC. Together with previous results for the Mg-free system (Na₂O-CaO-Al₂O₃-SiO₂-CO₂), it was established that the size of the miscibility gap for magnesian compositions increases with decreasing pressures from depths of ˜100 km to ˜ 35 km; it increases further as compositions are changed by decreasing Mg/Ca. The maximum CaCO₃ in liquids associated with the miscibility gap is 50 wt % for Mg-bearing liquids, and 80 wt % for Mg-free liquids. There is no experimental evidence for nearly pure-CaCO₃ immiscible liquids, but abundant evidence for the precipitation of rounded calcite crystals from carbonate-rich liquids. The join NEPH-CC locates a piercing point on the liquidus field boundary for coprecipitation of olivine and calcite at NEPH₅₀CC₅₀ (wt %), part of the silicate-carbonate liquidus field boundary which defines the locus of liquids formed from carbonate-peridotites. The miscibility gap results are compared with magmas formed during partial fusion of CO₂-bearing mantle peridotites, and during fractional crystallization of mantle-derived magmas. None of the probable magma paths in mantle processes intersects the miscibility gap. CO₂-bearing mantle-derived alkalic magmas such as nephelinites and melilitites may fractionate during uprise through the mantle and crystallization within the crust. The compositions of these evolved nephelinites and phonolites approach the silicate side of the miscibility gap, confirming the probable generation of immiscible, alkalic carbonate-rich liquids at crustal pressures. Received: 29 January 1996 / Accepted: August 14, 1996     

Experimental study of the CaMgSi2O6–CO2 system at 3–8 GPa

The melting relationships in the system CaMgSi₂O₆ (Di)–CO₂ have been studied in the 3–8 GPa pressure range to determine if there is an abrupt decrease in the temperature of the solidus accompanying the stabilization of carbonate as a subsolidus phase. Such a decrease has been observed previously in peridotitic and some eclogitic systems. In contrast, the solidus in the Di–CO₂ system was found to decrease in a gradual fashion from 3 to 8 GPa. This decrease accompanies an evolution in the composition of the melt at the solidus from silicate-rich with minor CO₂ at 3 GPa to carbonatitic at 5.5 GPa, where the carbonation reaction Diopside + CO₂ = Dolomite (Dol) + Coesite (Cst) intersects the solidus. The near-solidus melt remains carbonatitic at higher pressure, consistent with carbonate being the dominant contributor to the melt. Based on previous studies in both eclogitic and peridotitic systems, this conclusion can be extended to more complicated systems: once carbonate is a stable subsolidus phase, it plays a major role in controlling both the temperature of melting and the composition of the melt produced.     

Carbonation and melting reactions in the system CaO–MgO–SiO2–CO2 at mantle pressures with geophysical and petrological applications

Bowen's petrogenetic grid was based initially on a series of decarbonation reactions in the system CaO-MgO-SiO₂-CO₂ with starting assemblages including calcite, dolomite, magnesite and quartz, and products including enstatite, forsterite, diopside and wollastonite. We review the positions of 14 decarbonation reactions, experimentally determined or estimated, extending the grid to mantle pressures to evaluate the effect of CO₂ on model mantle peridotite composed of forsterite(Fo)+orthopyroxene(Opx)+clinopyroxene(Cpx). Each reaction terminates at an invariant point involving a liquid, CO₂, carbonates, and silicates. The fusion curves for the mantle mineral assemblages in the presence of excess CO₂ also terminate at these invariant points. The points are connected by a series of reactions involving liquidus relationships among the carbonates and mantle silicates, at temperatures lower (1,100–1,300° C) than the silicate-CO₂ melting reactions (1,400–1,600° C). Review of experimental data in the bounding ternary systems together with preliminary data for the system CaO-MgO-SiO₂-CO₂ permits construction of a partly schematic framework for decarbonation and melting reactions at upper mantle pressures. The key to several problems in the peridotite-CO₂ subsystem is the intersection of a subsolidus carbonation reaction with a melting reaction at an invariant point near 24 kb and 1,200°C. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO₂-saturated liquidus surfaces. Conclusions include the following: (1) Peridotite Fo+Opx+Cpx can be carbonated with increasing pressure, or decreasing temperature, to yield Fo+Opx+Cpx+Cd (Cd=calcic dolomite), Fo+Opx+Cd, Fo+Opx+Cm (Cm=calcic magnesite), and finally Qz+Cm. (2) Free CO₂ cannot exist in subsolidus mantle peridotite with normal temperature distributions; it is stored as carbonate, Cd. (3) The CO₂ bubbles in peridotite nodules do not represent free CO₂ in mantle peridotite along normal geotherms. (4) CO₂ is as effective as H₂O in causing incipient melting, our preferred explanation for the low-velocity zone. (5) Fusion of peridotite with CO₂ at depths shallower than 80 km produces basic magmas, becoming more SiO₂-undersaturated with depth. (6) The solubility of CO₂ in mantle magmas is less than about 5 wt% at depths to 80 km, increasing abruptly to about 40 wt% at 80 km and deeper. (7) Deeper than 80 km, the first liquids produced are carbonatitic, changing towards kimberlitic and eventually, at considerably higher temperatures, to basic magmas. (8) Kimberlite and carbonatite magmas rising from the asthenosphere must evolve CO₂ at depths 100-80 km, which contributes to their explosive emplacement. (9) Fractional crystallization of CO₂-bearing SiO₂-undersaturated basic magmas at most pressures can yield residual kimberlite and carbonatite magmas.     

Generation and differentiation of group II kimberlites: constraints from a high-pressure experimental study to 10 GPa

Experiments have been performed in the multicomponent (natural) bulk system to constrain the conditions of generation and differentiation of a K-rich group II kimberlite (now also referred to as orangeite). The group II composition examined was saturated in olivine, orthopyroxene, and garnet at near liquidus conditions in the pressure range 4 to 10 GPa. In the range 2 to 3 GPa, the liquidus phase was olivine only. The potassic nature of the melts in the bulk compositions studied was ensured by the absence of any K-bearing phase in the residual assemblage at P > 4 GPa. Phlogopite is destabilized toward higher pressures by a carbonation reaction of the type phlogopite + CO₂ = enstatite + garnet + K₂CO₃ (liquid) + H₂O leading to alkalic, carbonatitic liquids coexisting with a garnet-peridotite (harzburgite or lherzolite) residue over a wide pressure-temperature space at pressures in excess of 4 GPa. Evidently, CO₂-bearing systems do not favor the stability of phlogopite and/or K-richterite amphibole at pressures in excess of 4 to 5 GPa, and it is suggested that the carbonate-bearing and potassic character of any mantle melt originating from this depth is most likely the product of a two-stage process: either a carbonate-bearing protolith is invaded by a potassic melt or fluid (probably supercritical), or a potassic protolith (after metasomatism) has been invaded by a carbonatite melt.     

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.     

Carbonation and decarbonation of eclogites: the role of garnet

Carbonates are potentially significant hosts for primordial and subducted carbon in the Earth's mantle. In addition, the coexistence of carbonate with silicates and reduced carbon (diamond or graphite), allows constraints to be placed on the oxidation state of the mantle. Carbonate-silicate-vapor reactions control how carbonate + silicate assemblages may form from carbon-bearing vapor + silicate assemblages with increasing pressure. In olivine-bearing rocks such as peridotite, considered the dominant rock type in the upper mantle, the lowest-pressure carbonate-forming reactions involve olivine (±clinopyroxene) reacting with CO₂ (e.g., Wyllie et al. 1983). In eclogitic rocks, the essential mineral assemblage is omphacitic clinopyroxene + garnet, without olivine. Therefore, alternative carbonate-forming reactions must be sought. The carbonation of clinopyroxene via the reaction dolomite + 2 coesite = diopside + 2 CO₂ was studied experimentally by Luth (1995). The alternative possibility that garnet reacts with CO₂ is explored here by determining the location of the reaction 3 magnesite + kyanite + 2 coesite = pyrope + 3 CO₂ between 5 and 11 GPa in multi-anvil apparatus. At the temperatures ≥1200 °C, carbonation of eclogitic rocks with increasing pressure will proceed initially by reaction with clinopyroxene, because the pyrope-carbonation reaction lies at higher pressures for a given temperature than does the diopside-carbonation reaction. Diluting the pyrope component of garnet and the diopside component of clinopyroxene to levels appropriate for mantle eclogites does not change this conclusion. At lower temperatures, appropriate for “cold” slabs, it is possible that the converse situation will hold, with initial carbonation proceeding via reaction with garnet, but this possibility awaits experimental confirmation. Decarbonation of an eclogite under “normal mantle” geothermal conditions by a decrease in pressure, as in an ascending limb of a mantle convection cell, would be governed by the formation of clinopyroxene + CO₂. At higher pressure than this reaction, any CO₂ produced by the breakdown of magnesite reacting with kyanite and coesite would react with clinopyroxene to produce dolomite + coesite. Release of CO₂ from eclogite into mantle peridotite would form carbonate at sub-solidus conditions and produce a dolomitic carbonate melt if temperatures are above the peridotite-CO₂ solidus. Received: 4 May 1998 / Accepted: 23 December 1998     

Low solubility of He and Ar in carbonatitic liquids: Implications for decoupling noble gas and lithophile isotope systems

In order to asses the importance of carbonatitic liquids in transporting noble gases in the mantle, the solubilities of He and Ar in carbonatitic liquids were estimated from analyses of calcium-potassium carbonate glasses that had been synthesized at 1 bar and temperatures between 850 and 950 °C under He or Ar enriched atmospheres. Despite poor reproducibility related to difficulties synthesizing carbonatite glass, we have been able to estimate He and Ar solubilities in carbonatite liquids to be 1 × 10⁻⁸ and 2 × 10⁻⁹ mol g⁻¹ at 1 bar respectively (with ?50% uncertainty). Despite the significant uncertainties on these estimates, it is clear that the noble gases are not massively soluble in carbonatite liquids (within error, these solubilities are identical to their equivalent solubilities in tholeiitic melts). Assuming the results of these low pressure experiments can be applied to mantle conditions, it seems unlikely that carbonatite metasomatism per se transports noble gases within the mantle. It is nevertheless possible that partitioning of lithophile trace elements (including the important radioelements, U, K and Th) and noble gases between a carbonatitic melt and a silicate melt could effectively decouple lithophile and noble gas isotope systematics because the carbonatitic melt expressedly does not transport noble gases, yet is known to efficiently transport incompatible trace elements.     

Basanite–nephelinite suite from early Kilauea: carbonated melts of phlogopite–garnet peridotite at Hawaii’s leading magmatic edge

A basanite–nephelinite glass suite from early submarine Kilauea defines a continuous compositional array marked by increasing concentrations of incompatible components with decreasing SiO₂, MgO, and Al₂O₃. Like peripheral and post-shield strongly alkalic Hawaiian localities (Clague et al. in J Volcanol Geotherm Res 151:279–307, 2006; Dixon et al. in J Pet 38:911–939, 1997), the early Kilauea basanite–nephelinite glasses are interpreted as olivine fractionation products from primary magnesian alkalic liquids. For early Kilauea, these were saturated with a garnet–phlogopite–sulfide peridotite assemblage, with elevated dissolved CO₂ contents responsible for the liquids’ distinctly low-SiO₂ concentrations. Reconstructed primitive liquids for early Kilauea and other Hawaiian strongly alkalic localities are similar to experimental 3 GPa low-degree melts of moderately carbonated garnet lherzolite, and estimated parent magma temperatures of 1,350–1,400°C (olivine–liquid geothermometry) match the ambient upper mantle geotherm shortly beneath the base of the lithosphere. The ~3 GPa source regions were too hot for stable crystalline carbonate and may have consisted of ambient upper mantle peridotite containing interstitial carbonate–silicate or carbonatitic liquid, possibly (Dixon et al. in Geochem Geophys Geosyst 9(9):Q09005, 2008), although not necessarily, from the Hawaiian mantle plume. Carbonate-enriched domains were particularly susceptible to further melting upon modest decompression during upward lithospheric flexure beneath the advancing Hawaiian Arch, or by conductive heating or upward drag by the Hawaiian mantle plume. The early Kilauea basanite–nephelinite suite has a HIMU-influenced isotopic character unlike other Hawaiian magmas (Shimizu et al. in EOS Tran Amer Geophys Union 82(47): abstr V12B-0962, 2001; Shimizu et al. in Geochim Cosmochim Acta 66(15A):710, 2002) but consistent with oceanic carbonatite involvement (Hoernle et al. in Contrib Mineral Petrol 142:520–542, 2002). It may represent the melting products of a fertile domain in the ambient upper mantle impinged upon and perturbed by the sustained plume source that feeds later shield-stage magmatism.     

Petrogenesis of the amphibole-rich veins from the Lherz orogenic lherzolite massif (Eastern Pyrenees, France): a case study for the origin of orthopyroxene-bearing amphibole pyroxenites in the lithospheric mantle

The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions. Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic K₂O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase). The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few centimetres by vein–wallrock exchange reactions which continued down to subsolidus temperatures for Mg–Fe. Thermobarometric data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P ≥ 13 kbar and recrystallized at P < 12 kbar. The high Al^VI/Al^IV ratio (>1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar to transitional alkali basalt with about 1–3 wt% H₂O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg=0.6; 4–6 wt% H₂O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process. Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P ≤ 10 kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms of extensive dissolution of olivine and orthopyroxene from wallrock peridotite by alkaline melts produced at the mechanical boundary layer/thermal boundary layer transition (about 45–50 km deep). Continuous fluid ingress allowed remelting of the deeper veined mantle to produce the basanitic, strongly volatiles enriched, melts that precipitated hornblendites. A similar model could be valid for the few orthopyroxene-rich hydrous pyroxenites described in basalt-hosted mantle xenoliths. Received: 15 September 1999 / Accepted: 31 January 2000     

Entrained Macrocryst Minerals as a Key to the Source Region of Olivine Nephelinites: Humberg, Kaiserstuhl, Germany

Olivine nephelinites commonly contain macrocrysts of olivineand clinopyroxene. Some of these macrocrysts might representfragments of the source region of the host magma transportedto the Earth's surface. If this hypothesis is correct thesefragments can be used to characterize the composition of thesource region and to put constraints on the magma generationprocess. In this study, we investigate the origin of macrocrystsand mineral aggregates from an olivine nephelinite from theKaiserstuhl, Germany. We focus on clinopyroxenes (Cpx), whichcan be divided into three groups. Cpx I is relict Cpx from aggregateswith deformed olivine that is depleted in Ca and characterizedby strong light rare earth element (LREE) fractionation, lowTi/Eu and negative high field strength element (HFSE) anomalies.Its geochemical signature is consistent with formation by carbonatitemetasomatism and with equilibration in the presence of orthopyroxene.Cpx II is Ca-rich Cpx, forming both aggregates with deformedolivine and individual macrocrysts. The LREE, as for Cpx I,are strongly fractionated. Convex REE patterns may be present.The depletion in HFSE is less pronounced. Cpx III is oscillatoryzoned Cpx phenocrysts showing enrichment in Ca, convex REE patternsand no HFSE anomalies. The transition in the trace element abundancesbetween the Cpx of the three groups is gradual. However, CpxI and II did not crystallize from the host magma, as demonstratedby the presence of kink-bands and undulose extinction in theassociated olivine and by the composition of alkali aluminosilicateglass inclusions in Cpx II. Based on the Cpx relationships,we interpret the studied suite of macrocrysts and mineral aggregatesas a mixture of disintegrated fragments of the source regionof the host olivine nephelinite. The process of melt generationwas multi-stage. A primary carbonatite melt ascending from deeperlevels in the mantle, probably from the dolomite–garnetperidotite stability field, reacted with mantle peridotite alongthe solidus ledge in the system lherzolite–CO₂ (< 20–22kbar) and started to crystallize carbonate minerals. Becauseof its low solidus temperature, the resulting carbonate-wehrliteassemblage melted incongruently with the formation of additionalclinopyroxene. The carbonatite melt evolved during crystallizationof carbonate minerals and concomitant incongruent melting ofthe carbonate-wehrlite, accompanied by the segregation of incipientalkali aluminosilicate melts. As a consequence of fast reactionrates in the presence of a carbonatite melt, this process probablytook place under disequilibrium conditions. Further meltingof the assemblage wehrlite + alkali aluminosilicate melt ledto the generation of the olivine nephelinite magma. It entrainedfragments of the wehrlite and brought them to the surface. KEY WORDS: carbonatite; metasomatism; source region; clinopyroxene macrocrysts; wehrlite; olivine nephelinite; Kaiserstuhl     

The effect of bulk composition on the solidus of carbonated eclogite from partial melting experiments at 3 GPa

To explore the effect of bulk composition on the solidus of carbonated eclogite, we determined near-solidus phase relations at 3 GPa for four different nominally anhydrous, carbonated eclogites. Starting materials (SLEC1, SLEC2, SLEC3, and SLEC4) were prepared by adding variable proportions and compositions of carbonate to a natural eclogite xenolith (66039B) from Salt Lake crater, Hawaii. Near-solidus partial melts for all bulk compositions are Fe–Na calcio-dolomitic and coexist with garnet + clinopyroxene + ilmenite ± calcio-dolomitic solid solution. The solidus for SLEC1 (Ca#=100 × molar Ca/(Ca + Mg + Fe_T)=32, 1.63 wt% Na₂O, and 5 wt% CO₂) is bracketed between 1,050°C and 1,075°C (Dasgupta et al. in Earth Planet Sci Lett 227:73–85, 2004), whereas initial melting for SLEC3 (Ca# 41, 1.4 wt% Na₂O, and 4.4 wt% CO₂) is between 1,175°C and 1,200°C. The solidus for SLEC2 (Ca# 33, 1.75 wt% Na₂O, and 15 wt% CO₂) is estimated to be near 1,100°C and the solidus for SLEC3 (Ca# 37, 1.47 wt% Na₂O, and 2.2 wt% CO₂) is between 1,100°C and 1,125°C. Solidus temperatures increase with increasing Ca# of the bulk, owing to the strong influence of the calcite–magnesite binary solidus-minimum on the solidus of carbonate bearing eclogite. Bulk compositions that produce near-solidus crystalline carbonate closer in composition to the minimum along the CaCO₃-MgCO₃ join have lower solidus temperatures. Variations in total CO₂ have significant effect on the solidus if CO₂ is added as CaCO₃, but not if CO₂ is added as a complex mixture that maintains the cationic ratios of the bulk-rock. Thus, as partial melting experiments necessarily have more CO₂ than that likely to be found in natural carbonated eclogites, care must be taken to assure that the compositional shifts associated with excess CO₂ do not unduly influence melting behavior. Near-solidus dolomite and calcite solid solutions have higher Ca/(Ca + Mg) than bulk eclogite compositions, owing to Ca–Mg exchange equilibrium between carbonates and silicates. Carbonates in natural mantle eclogite, which have low bulk CO₂ concentration, will have Ca/Mg buffered by reactions with silicates. Consequently, experiments with high bulk CO₂ may not mimic natural carbonated eclogite phase equilibria unless care is taken to ensure that CO₂ enrichment does not result in inappropriate equilibrium carbonate compositions. Compositions of eclogite-derived carbonate melt span the range of natural carbonatites from oceanic and continental settings. Ca#s of carbonatitic partial melts of eclogite vary significantly and overlap those of partial melts of carbonated lherzolite, however, for a constant Ca-content, Mg# of carbonatites derived from eclogitic sources are likely to be lower than the Mg# of those generated from peridotite.     

Phlogopite-biotite parageneses from the K-mafic-carbonatite effusive magmatic association of Katwe-Kikorongo, SW Uganda

Summary Ti-bearing phlogopite-biotite is dominant in Ugandan kamafugite-carbonatite effusives and their entrained alkali clinopyroxenite xenoliths. It occurs as xeno/phenocrysts, microphenocrysts and groundmass minerals and also as a major xenolith mineral. Xenocrystic micas in kamafugites and carbonatites are aluminous (> 12 wt% Al₂O₃), typically contain significant levels of Cr (up to 1.1 wt% Cr₂O₃), and are Ba-poor. Microphenocryst and groundmass micas in feldspathoidal rocks extend to Al-poor compositions, are depleted in Cr, and are generally enriched in Ba. In general, xenocrystic micas occupy the Al₂O₃ and TiO₂ compositional field of the xenolith mica, and on the basis of Mg#, and high P, T experimental evidence they probably crystallised at mantle pressures. Mica xenocryst Cr contents range from those in Cr-poor megacryst and MARID phlogopite to higher values found in primary and metasomatic phlogopites in kimberlite-hosted peridotite xenoliths. Such Cr contents in Ugandan mica xenocrysts are considered consistent with derivation from carbonate-bearing phlogopite wehrlite and phlogopite-clinopyroxenite mantle. Olivine melilitite xenocryst micas are distinguished by higher Mg# and Cr content than mica in clinopyroxenite xenoliths and mica in Katwe-Kikorongo mixed melilitite-carbonatite tephra. Higher Al₂O₃ distinguishes Fort Portal carbonatite xenocrysts and some contain high Cr. It is suggested that the genesis of Katwe-Kikorongo olivine melilitite and Fort Portal carbonatite involves a carbonate-bearing phlogopite wehrlite source while the source of the mixed carbonatite-melilitite rocks may be carbonate-bearing phlogopite clinopyroxenite. Received January 24, 2000; revised version accepted September 27, 2001     

Ancient melt depletion overprinted by young carbonatitic metasomatism in the New Zealand lithospheric mantle

Spinel facies dunite, harzburgite, lherzolite and wehrlite mantle xenoliths from a cluster of Miocene volcanoes in southern New Zealand preserve evidence of the complex evolution of the underlying continental mantle lithosphere. Spinel Cr# records melt extraction with some values indicative of near complete removal of clinopyroxene. LREE-enriched, low Ti/Eu and low Al₂O₃ clinopyroxene and rare F-, LREE-rich apatite indicates subsequent interaction between peridotite and a metasomatising carbonatitic melt. The clearest metasomatic signature occurs in the formerly highly depleted samples because there was little or no pre-existing clinopyroxene to dilute the carbonatite signature. For the same reason, the isotopic character of the metasomatising agent is best observed in the formerly highly depleted peridotites (⁸⁷Sr/⁸⁶Sr = 0.7028–0.7031; ¹⁴³Nd/¹⁴⁴Nd = 0.5129; ²⁰⁶Pb/²⁰⁴Pb = 20.2–20.3). These isotope ratios are very close to, but slightly less radiogenic than, the HIMU end-member mantle reservoir. Nd isotope data imply carbonatite metasomatism occurred within the last several hundred million years, with ubiquitous pyroxene core-to-rim Al diffusion zoning indicating that it must pre-date cooling of the lithospheric mantle following Late Cretaceous–Eocene rifting of Zealandia from Gondwana. Metasomatism was significantly younger than ancient Re-depletion ages of ~2 Ga and shows that decoupling of peridotite isotope systems has occurred.