Absarokites from the western Mexican Volcanic Belt: constraints on mantle wedge conditions

We have investigated the near liquidus phase relations of a primitive absarokite from the Mascota region in western Mexico. Sample M.102 contains ~11.6 wt% MgO, Mg#=0.73 and the lava contains Fo₉₀ olivine phenocrysts, indicating near equilibrium with the mantle. High-pressure experiments on a synthetic analogue of the absarokite composition containing low and high H₂O abundances of (~2 and ~5 wt%, respectively) were performed in a piston cylinder apparatus over the pressure range of 1.2 to 2.0 GPa. The composition containing ~2 wt% H₂O is multiply saturated with olivine and orthopyroxene at 1.6 GPa and 1,400 °C. At the same pressure, clinopyroxene appears 30 °C below the liquidus. At an H₂O content of ~5 wt% the multiple saturation with olivine and orthopyroxene occurs at 1.7 GPa and 1,300 °C. Assuming a batch-melting process, we suggest that the primitive absarokite was segregated from a depleted lherzolite or harzburgite residue at ~50 km, placing the depth of origin well within the mantle wedge beneath the Jalisco Block. A low degree (<5 %wt%) batch-melt of an original metasomatized depleted lherzolite or harzburgite source would contain the observed trace element abundances found in M.102. The liquidus phase relations are not consistent with the presence of non-peridotitic veins at the depth of last equilibration. Therefore, we propose that the Mascota absarokites segregated at an apparent melt fraction of less than 5% from a depleted peridotitic source. Melting first began at a greater depth as a small degree H₂O- and trace element- rich melt of a metasomatized peridotite that ascended into the overlying wedge and re-equilibrated with shallower, hotter mantle.Editorial responsibility: J. Hoefs     

Melting relations of basalt-andesile-rhyolite-H2O and a pelagic red clay at 30 kb

An olivine basalt, a tonalite (andesite), a granite (rhyolite), and a red clay (pelagic sediment) were reacted, with known quantities of water in sealed noble metal capsules, in a piston-cylinder apparatus at 30 kb pressure. For the pelagic sediment, with H₂O+=7.8% and no additional water, the liquidus temperature is 1240°C, the primary phases are garnet and kyanite. The subsolidus phase assemblage is phengite mica+garnet+clinopyroxene+coesite+kyanite. With 5 wt.% water added, the liquidus temperatures and primary phases for the calc-alkaline rocks are 1280°-1180°-1080°, garnet+clinopyroxene, garnet, and quartz respectively. Garnet and clinopyroxene occur throughout the melting interval of the olivine tholeiite for all water contents. Garnet is joined by clinopyroxene 80° below the andesite plus 5% H₂O liquidus, quartz is joined by clinopyroxene 180° below the rhyolite plus 5% H₂O liquidus. The subsolidus phase assemblage is garnet+clinopyroxene+coesite+minor kyanite for all the calc-alkaline compositions. We conclude that calc-alkaline andesites and rhyolites are not equilibrium partial melting pruducts of subducted oceanic crust consisting of olivine tholeiite basalt and siliceous sediments. Partial melting in subduction zones produces broadly acid and intermediate liquids, but these liquids lie off the calc-alkaline basalt-andesite-rhyolite join and must undergo modification at lower pressures to produce calcalkaline magmas erupted in overlying island arcs.     

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.     

The melting relationships of a madupite from the Leucite Hills,Wyoming, to 30 Kb

The water-undersaturated melting relationships of a mafic, peralkaline, potassic madupite (with about 3% H₂O as shown by chemical analysis) from the Leucite Hills, Wyoming, have been studied at pressures up to 30 kb. At low pressures (<5 kb) leucite is the dominant liquidus phase, but it is replaced at higher pressures by clinopyroxene plus olivine (<5–7 kb), clinopyroxene (7–12.5 kb), clinopyroxene plus minor spinel (12.5–17.5 kb), and clinopyroxene alone (17.5–> 30 kb). At all pressures there is a reaction relationship with falling temperature between melt, olivine and probably clinopyroxene to yield phlogopite. Apatite is stable within the melting interval to pressures above 25 kb. Electron microprobe analyses demonstrate that the clinopyroxene is diopsidic, with low aluminium and titanium contents. Pressure has relatively little effect on the composition of the pyroxene. Phlogopite is also aluminium-poor and has only a moderate titanium content. The experimental results indicate that madupite is not the partial melting product of hydrous lherzolite or garnet lherzolite in the upper mantle and it seems improbable that it is derived by melting of mantle peridotite with a mixed H₂O-CO₂ volatile component. Madupite could, however, be the partial melting product of mica-pyroxenite or mica-olivine-pyroxenite in the upper mantle. It is pointed out that the chemistry of some potassium-rich volcanics may have been affected by volatile transfer and other such processes during eruption and that experimental studies of material affected in this way have little bearing upon the genesis of potassic magmas. Finally, the experimental results enable constraints to be placed upon the P-T conditions of the formation of richterite-bearing mica nodules found in kimberlites and associated rocks. Maximum conditions are 25 kb and 1,100 ° C.     

Derivation of potassic (shoshonitic) magmas by decompression melting of phlogopite+pargasite lherzolite

A model metasomatized lherzolite composition contains phlogopite and pargasite, together with olivine, orthopyroxene, clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at ≥1.5 GPa, phlogopite is stable above the dehydration solidus, determined by the melting behaviour of pargasite and coexisting phases. At 2.8 GPa, melts with residual phlogopite+garnet lherzolite mineralogy at 1195 °C and with garnet lherzolite mineralogy at 1250 °C are both olivine nephelinite with K/Na (atomic)=0.51 and K/Na=0.65, respectively. Recent work shows that melting along the dehydration (fluid-absent) solidus of the phlogopite+pargasite lherzolite at pressures <1.5 GPa is very different with the presence of phlogopite, decreasing the solidus below that of pargasite lherzolite. At 1.0 GPa, both phlogopite and pargasite disappear at temperatures at or slightly above the solidus. The compositions of two melts at 1.0 GPa, 1075 °C (with different water contents), in equilibrium with residual spinel lherzolite mineralogy are silica-saturated trachyandesite (5% melt fraction, 3% H₂O) to silica-oversaturated basaltic andesite (8% melt fraction, 4.5% H₂O). Both compositions may be classified as ‘shoshonites’ on the basis of normative compositions, silica-saturation, and K/Na ratio. Decompression melting of metasomatized lithospheric lherzolite with minor phlogopite and pargasite may produce primary ‘shoshonitic’ magmas by dehydration melting at 1 GPa, 1050–1150 °C. Such magmas may be parental to Proterozoic batholithic syenites occurring in Brazil.     

Phase Relations of an Armalcolite-Phlogopite Lamproite from Smoky Butte, Montana: Applications to Lamproite Genesis

Suprasolidus phase relations at pressures from 8 to 30 kb andtemperatures from 950 to 1380C have been determined experimentallyfor a glassy armalcolite–phlogopite lamproite from thechilled margin of a medium–grained lamproite from SmokyButte, Montana: The armalcolite-phlogopite lamproite has microphenocrystsof olivine in a groundmass of phlogopite, sanidine, armalcolite,clinopyroxene, chromite, priderite, apatite, and abundant glass.The lamproite is SiO₂-rich and has high F/H₂O relative to lamproitesthat have been investigated in previous experimental studies.Our data show that with decreasing temperature from the liquidusat pressures above 12 kb, melt coexists successively with:olivine; orthopyroxene + clinopyroxene; orthopyroxene + clinopyroxene+ phlogopite; clinopyroxene +phlogopite; and clinopyroxene +orthopyroxene + K-richterite. Below 12 kb, the assemblage successionis: olivine; olivine + clinopyroxene; olivine + clinopyroxene+ phlogopite; and olivine +clinopyroxene + phlogopite + armalcolite.The main difference from the natural paragenesis is that therock does not contain any orthopyroxene—a feature thatis rather remarkable inasmuch as it has 16% normative hypersthene—andthe rock differs also in that it contains sanidine and priderite.In the experiments, sanidine is observed only as ghostlike domainsin some of the glass and appears to have formed during quenching. The solid phases crystallized experimentally are generally compositionallysimilar to the minerals in the rock. These similarities andthe experimental phase relations support the concept of a rapidinitial magma ascent with only a small temperature drop andcrystallization of olivine, but not of orthopyroxene. At lowerpressures, less than 12 kb, it appears that the magma ascendedmore slowly with a larger temperature drop suggested by thesimilarity of the experimentally determined sequence of assemblagesto the paragenesis of the rock. No quasi-invariant multiphase-saturation point was found suchas might be indicative of pressure and temperature conditionsfor formation of the lamproite magma by eutectic-type partialmelting of a mantle source. The occurrence of olivine, orthopyroxene,and clinopyroxene near the liquidus, and the high proportionof normative hypersthene in the melt suggest that lherzoliteor harzburgite was probable in the magma source rock. The highSiO₂ and MgO contents of the Smoky Butte lamproites may indicatethat orthopyroxene was a source mineral even though it did notcrystallize under near-surface conditions. The curve definingthe appearance of phlogopite appears at progressively lowertemperatures from the liquidus as pressure increases, so itwould appear that either phlogopite was not the mantle K-reservoir,or it was entirely consumed during the partial melting process.The composition of the near-liquidus glass in the experimentsis likely to be the composition of the bulk rock less the verysmall amounts of olivine + clinopyroxene + orthopyroxene crystallizedwithin a few degrees below the liquidus. From the inferred compositionof this glass, anhydrous phlogopite is a potential mineral.The principal variable that determines whether phlogopite crystallizesas a near-liquidus mineral is F/H₂O; low values of this ratiopromote the presence of phlogopite as a near-liquidus mineralwhereas high values deter its crystallization. The common practiceof adding H₂O but not F in experiments to compensate for degassingmay obscure the role of phlogopite in the evolution of lamproitemagmas.     

Experimental constraints on ultrapotassic magmatism from the Bohemian Massif (durbachite series, Czech Republic)

The equilibrium phase relations of a mafic durbachite (53 wt.% SiO₂) from the Třebíč pluton, representative of the Variscan ultrapotassic magmatism of the Bohemian Massif (338–335 Ma), have been determined as a function of temperature (900–1,100°C), pressure (100–200 MPa), and H₂O activity (1.1–6.1 wt.% H₂O in the melt). Two oxygen fugacity ranges were investigated: close to the Ni–NiO (NNO) buffer and 2.6 log unit above NNO buffer (∆NNO + 2.6). At 1,100°C, olivine is the liquidus phase and co-crystallized with phlogopite and augite at 1,000°C for the whole range of investigated pressure and water content in the melt. At 900°C, the mineral assemblage consists of augite and phlogopite, whereas olivine is not stable. The stability field of both alkali feldspar and plagioclase is restricted to low pressure (100 MPa) at nearly water-saturated conditions (<3–4 wt.% H₂O) and T < 900°C. A comparison between experimental products and natural minerals indicates that mafic durbachites have a near-liquidus assemblage of olivine, augite, Ti-rich phlogopite, apatite and zircon, followed by alkali feldspar and plagioclase, similar to the mineral assemblage of minette magma. Natural amphibole, diopside and orthopyroxene were not reproduced experimentally and probably result from sub-solidus reactions, whereas biotite re-equilibrated at low temperature. The crystallization sequence olivine followed by phlogopite and augite reproduces the sequence inferred in many mica-lamprophyre rocks. The similar fractionation trends observed for durbachites and minettes indicate that mafic durbachites are probably the plutonic equivalents of minettes and that K- and Mg-rich magmas in the Bohemian Massif may have been generated from partial melting of a phlogopite–clinopyroxene-bearing metasomatized peridotite. Experimental melt compositions also suggest that felsic durbachites can be generated by simple fractionation of a more mafic parent and mixing with mantle-derived components at mid crustal pressures.     

An experimental study of a minette from the Milk River area, southern Alberta, Canada

Buhlmann et al. (Can J Earth Sci 37: 1629–1650, 2000) studied the minettes and xenoliths from the Milk River area of southern Alberta, Canada. Based on previous work, they hypothesized that the minettes were derived from a source containing phlogopite?+?clinopyroxene?±?olivine, at pressures ≥1.7?GPa. To test this hypothesis, liquidus experiments were performed on a primitive minette between 1.33 and 2.21?GPa and between 1,300 and 1,400?°C to constrain the mineralogy of its source region. We found a multiple saturation point along the liquidus at 1.77 GPa and 1,350?°C, where the liquid coexists with orthopyroxene and olivine. Neither phlogopite nor clinopyroxene were found to be liquidus phases, which is inconsistent with Buhlmann et al.’s hypothesis. We suggest that our minette is not primary, but had re-equilibrated with harzburgitic mantle subsequent to formation. In such a scenario, partial melting of a veined source containing mica and clinopyroxene occurred at or near the base of the Wyoming craton (~200?km). Minimal heating or the introduction of hydrous fluids into the source would be required to induce partial melting. Rapid ascent rates, coupled with slow cooling rates, of the “primary minette magma” would preserve the high temperature observed in our experiments. At ~58?km, our “primary minette magma” likely stalled and re-equilibrated with the harzburgite surroundings.     

Interaction of model peridotite with H2O-KCl fluid: Experiment at 1.9 GPa and its implications for upper mantle metasomatism

Mineralogical and geochemical data suggest that chloride components play an important role in the transformation and partial melting of upper mantle peridotites. The effect of KCl on the transformation of hydrous peridotite rich in Al₂O₃, CaO, and Na₂O was examined in experiments aimed at studying interaction between model NCMAS peridotite with H₂O-KCl fluid under a pressure of 1.9 GPa, temperatures of 900–1200°C, and various initial H₂O/KCl ratios. The experimental results indicate that KCl depresses the solidus temperature of the hydrous peridotite: this temperature is <900°C at 1.9 GPa, which is more than 100°C lower than the solidus temperature (1000–1025°C) of hydrous peridotite in equilibrium with KCl-free fluid. The reason for the decrease in the melting temperature is that the interaction of KCl with silicates prevails over the effect of chloride on the water activity in the fluid. Experimental data highlight the key role of Al₂O₃ as a component controlling the whole interaction process between peridotite and H₂O-KCl fluid. Garnet, spinel, and pargasite-edenite amphibole in association with aluminous orthopyroxene are unstable in the presence of H₂O-KCl fluid at a chloride concentration in the fluid as low as approximately 2 wt % and are replaced by Cl-bearing phlogopite (0.4–1.1 wt % Cl). Interaction with H₂O-KCl fluid does not, however, affect clinopyroxene and forsterite, which are the Al poorest phases of the system. Chlorine stabilizes phlogopite at relatively high temperatures in equilibrium with melt at temperatures much higher than the solidus (>1200°C). The compositional evolution of melt generated during the melting of model peridotite in the presence of H₂O-KCl fluid is controlled, on the one hand, by the solubility of the H₂O-KCl fluid in the melt and, on the other hand, by phlogopite stability above the solidus. At temperatures below 1050°C, at which phlogopite does not actively participate in melting reactions, fluid dissolution results in SiO₂-undersaturated (35–40 wt %) and MgO-enriched (up to 45 wt %) melts containing up to 4–5 wt % K₂O and 2–3 wt % Cl. At higher temperatures, active phlogopite dissolution and, perhaps, also the separation of immiscible aqueous chloride liquid give rise to melts containing >10 wt % K₂O and 0.3–0.5 wt % Cl. Our experimental results corroborate literature data on the transformation of upper mantle peridotites into phlogopite-bearing associations and the formation of ultrapotassic and highly magnesian melts.     

Experimental Petrology of a Highly Potassic Magma

The melting behaviour of a highly potassic biotite mafuriteof the Central African olivine leucitite kindred has been studiedexperimentally as a function of pressure (to 30kb) temperature,and water content (0%, 5%, 15%, 25%, and 40% H₂O). Olivine isthe liquidus phase up to 30 kb for all water contents studiedexcept for anhydrous (clinopyroxene on the liquidus) and 15%H₂O (phlogopite on the liquidus) conditions. Analyses of phasescrystallizing from the biotite mafurite show that pressure hasvery little effect on the composition of clinopyroxene whichis extremely calcium-rich, and low in Al₂O₃ and TiO₂ for allconditions investigated. Phlogopite has low TiO₂ content andtitanphlogopite cannot be a refractory phase in the upper mantlecausing Ti-depletion in partial melts in equilibrium with titanphlogopite.There are apparently no conditions where the extremely potassicbiotite mafurite could be a partial melt from pyrolite but derivationfrom an olivine+clinopyroxene+phlogopite+ilmenite assemblageoccurring as ‘enriched’ patches in the upper mantle,is possible. Liquids in equilibrium with phlogopite as a residualphase at 30 kb would be olivine nephelinites with approximately5% K₂O, Na₂O/K₂O 1 and TiO₂ > 5+. Crystal elutriation withtransported residual phlogopite reacting (phlogopite+liquid1 olivine+liquid 2) at lower pressures provides a mechanismfor K-enrichment and generating Na₂O/K₂O < 1.     

Trace element partitioning between mica- and amphibole-bearing garnet lherzolite and hydrous basanitic melt: 1. Experimental results and the investigation of controls on partitioning behaviour

Thirty five minor and trace elements (Li, Be, B, Sc, Cu, Zn, Ga, Ge, As, Rb, Nb, Mo, Ag, Cd, In, Sn, Sb, Cs, Ba, La, Ce, Nd, Sm, Tb, Ho, Tm, Lu, Hf, Ta, W, Tl, Pb, Bi, Th and U) in experimentally produced near-liquidus phases, from a primitive nelpheline basanite from Bow Hill in Tasmania (Australia), were analysed by LAM ICP-MS. A number of halogens (F, Cl and I) were also analysed by electron microprobe. The analyses were used to determine mineral/melt partition coefficients for mica, amphibole, garnet, clinopyroxene, orthopyroxene and olivine for conditions close to multiple saturation of the basanite liquidus with garnet lherzolite (approximately 2.6 GPa and 1,200°C with 7.5 wt% of added H₂O). A broader range of conditions was also investigated from 1.0 GPa and 1,025°C to 3.5 GPa and 1,190°C with 5–10 wt% of added H₂O. The scope and comprehensiveness of the data allow them to be used for two purposes, these include the following: an investigation of some of the controlling influences on partition coefficients; and the compilation of a set partition coefficients that are directly relevant to the formation of the Bow Hill basanite magma by partial melting of mantle peridotite. Considering clinopyroxene, the mineral phase for which the most data were obtained, systematic correlations were found between pressure and temperature, mineral composition, cation radius and valence, and ΔG ^coulb (the coulombic potential energy produced by substituting a cation of mismatched valence into a crystallographic site). ΔG ^coulb is distinctly different for different crystallographic sites, including the M2 and M1 sites in clinopyroxene. These differences can be modelled as a function of variations in optimum valence (expressed as 1 sigma standard deviations) within individual M1 and M2 site populations.     

Partial melting of secondary pyroxenite at 1 and 1.5 GPa,and its role in upwelling heterogeneous mantle

We performed partial melting experiments at 1 and 1.5 GPa, and 1180–1400 °C, to investigate the melting under mantle conditions of an olivine-websterite (GV10), which represents a natural proxy of secondary (or stage 2) pyroxenite. Its subsolidus mineralogy consists of clinopyroxene, orthopyroxene, olivine and spinel (+garnet at 1.5 GPa). Solidus temperature is located between 1180 and 1200 °C at 1 GPa, and between 1230 and 1250 °C at 1.5 GPa. Orthopyroxene (±garnet), spinel and clinopyroxene are progressively consumed by melting reactions to produce olivine and melt. High coefficient of orthopyroxene in the melting reaction results in relatively high SiO₂ content of low melt fractions. After orthopyroxene exhaustion, melt composition is controlled by the composition of coexisting clinopyroxene. At increasing melt fraction, CaO content of melt increases, whereas Na₂O, Al₂O₃ and TiO₂ behave as incompatible elements. Low Na₂O contents reflect high partition coefficient of Na between clinopyroxene and melt (\(D_{{{\text{Na}}_{ 2} {\text{O}}}}^{{{\text{cpx}}/{\text{liquid}}}}\)). Melting of GV10 produces Quartz- to Hyperstene-normative basaltic melts that differ from peridotitic melts only in terms of lower Na₂O and higher CaO contents. We model the partial melting of mantle sources made of different mixing of secondary pyroxenite and fertile lherzolite in the context of adiabatic oceanic mantle upwelling. At low potential temperatures (T _P < 1310 °C), low-degree melt fractions from secondary pyroxenite react with surrounding peridotite producing orthopyroxene-rich reaction zones (or refertilized peridotite) and refractory clinopyroxene-rich residues. At higher T _P (1310–1430 °C), simultaneous melting of pyroxenite and peridotite produces mixed melts with major element compositions matching those of primitive MORBs. This reinforces the notion that secondary pyroxenite may be potential hidden components in MORB mantle source.     

Ti-content of high-Ca pyroxenes as a petrogenetic indicator: an experimental study of Mafic Alkaline Rocks from the Mt. Erebus volcanic region,Antarctica

We report chemical and mineralogical data for one atmosphere melting experiments conducted on alkalic rocks from the Mt. Erebus volcanic region: DVDP2 basanite, two hawaiites (DVDP2 and a nepheline-bearing variety), and an anorthoclase phonolite. Temperatures between 1,224 and 1,049°C were investigated at f_O2~QFM. DVDP2 basanite appears to be an intermediate pressure liquid or a cumulate, because only olivine coexists with melt from above 1,224–1,160°C. High-Ca pyroxene joins olivine in the crystallization sequence at 1,138°C. These minerals are joined by plagioclase at a temperature between 1,120 and 1,104°C. In contrast, DVDP2 hawaiite appears to be relatively evolved, because it is multiply saturated with olivine, plagioclase, and high-Ca pyroxene near its liquidus (between 1,120 and 1,104°C). Plagioclase crystallizes in the Ne-hawaiite by 1,160°C followed by olivine below 1,120°C. The liquidus of anorthoclase phonolite is between the lowest temperatures investigated, 1,089 and 1,049°C, and plagioclase is the liquidus mineral. Our results indicate that DVDP2 hawaiite can be derived from a DVDP2 basanitic parental magma by crystal fractionation at low pressures, that the nepheline hawaiite is an olivine cumulate, and that the liquids parental to the anorthoclase phonolite represent the end products of crystal fractionation. They also allow us to illustrate how the Ti-content of pyroxene may be used as a petrogenetic indicator of processes and events in the evolution of the Erebus volcanic system.     

Anhydrous partial melting of an iron-rich mantle I: subsolidus phase assemblages and partial melting phase relations at 10 to 30 kbar

Anhydrous partial melting experiments, at 10 to 30 kbar from solidus to near liquidus temperature, have been performed on an iron-rich martian mantle composition, DW. The DW subsolidus assemblage from 5 kbar to at least 24 kbar is a spinel lherzolite. At 25 kbar garnet is stable at the solidus along with spinel. The clinopyroxene stable on the DW solidus at and above 10 kbar is a pigeonitic clinopyroxene. Pigeonitic clinopyroxene is the first phase to melt out of the spinel lherzolite assemblage at less than 20°C above the solidus. Spinel melts out of the assemblage about 50°C above the solidus followed by a 150° to 200°C temperature interval where melts are in equilibrium with orthopyroxene and olivine. The temperature interval over which pigeonitic clinopyroxene melts out of an iron-rich spinel lherzolite assemblage is smaller than the temperature interval over which augite melts out of an iron-poor spinel lherzolite assemblage. The dominant solidus assemblage in the source regions of the Tharsis plateau, and for a large percentage of the martian mantle, is a spinel lherzolite.     

Melting of Refractory Mantle at 1{middle dot}5, 2 and 2{middle dot}5 GPa under Anhydrous and H2O-undersaturated Conditions: Implications for the Petrogenesis of High-Ca Boninites and the Influence of Subduction Components on Mantle Melting

Boninites are an important ‘end-member’ supra-subductionzone magmatic suite as they have the highest H₂O contents andrequire the most refractory of mantle wedge sources. The pressure–temperatureconditions of boninite origins in the mantle wedge are importantto understanding subduction zone initiation and subsequent evolution.Reaction experiments at 1·5 GPa (1350–1530°C),2 GPa (1400–1600°C) and 2·5 GPa (1450–1530°C)between a model primary high-Ca boninite magma composition anda refractory harzburgite under anhydrous and H₂O-undersaturatedconditions (2–3 wt % H₂O in the melt) have been completed.The boninite composition was modelled on melt inclusions occurringin the most magnesian olivine phenocrysts in high-Ca boninitesfrom the Northern Tongan forearc and the Upper Pillow Lavasof the Troodos ophiolite. Direct melting experiments on a modelrefractory lherzolite and a harzburgite composition at 1·5GPa under anhydrous conditions (1400–1600°C) havealso been completed. Experiments establish a P, T ‘meltinggrid’ for refractory harzburgite at 1·5, 2 and2·5 GPa and in the presence of 2–3 wt % H₂O. Theeffect of 2–3 wt % dissolved H₂O produces a liquidus depressionin primary boninite of     

Phase relations of phlogopite with magnesite from 4 to 8 GPa

To evaluate the stability of phlogopite in the presence of carbonate in the Earth’s mantle, we conducted a series of experiments in the KMAS–H₂O–CO₂ system. A mixture consisting of synthetic phlogopite (phl) and natural magnesite (mag) was prepared (phl₉₀-mag₁₀; wt%) and run at pressures from 4 to 8 GPa at temperatures ranging from 1,150 to 1,550°C. We bracketed the solidus between 1,200 and 1,250°C at pressures of 4, 5 and 6 GPa and between 1,150 and 1,200°C at a pressure of 7 GPa. Below the solidus, phlogopite coexists with magnesite, pyrope and a fluid. At the solidus, magnesite is the first phase to react out, and enstatite and olivine appear. Phlogopite melts over a temperature range of ~150°C. The amount of garnet increases above solidus from ~10 to ~30 modal% to higher pressures and temperatures. A dramatic change in the composition of quench phlogopite is observed with increasing pressure from similar to primary phlogopite at 4 GPa to hypersilicic at pressures ≥5 GPa. Relative to CO₂-free systems, the solidus is lowered such, that, if carbonation reactions and phlogopite metasomatism take place above a subducting slab in a very hot (Cascadia-type) subduction environment, phlogopite will melt at a pressure of ~7.5 GPa. In a cold (40 mWm⁻²) subcontinental lithospheric mantle, phlogopite is stable to a depth of 200 km in the presence of carbonate and can coexist with a fluid that becomes Si-rich with increasing pressure. Ascending kimberlitic melts that are produced at greater depths could react with peridotite at the base of the subcontinental lithospheric mantle, crystallizing phlogopite and carbonate at a depth of 180–200 km.     

Hydrogen partitioning between melt,clinopyroxene, and garnet at 3 GPa in a hydrous MORB with 6 wt.% H<Subscript>2</Subscript>O

To understand partitioning of hydrogen between hydrous basaltic and andesitic liquids and coexisting clinopyroxene and garnet, experiments using a mid-ocean ridge basalt (MORB) + 6 wt.% H₂O were conducted at 3 GPa and 1,150–1,325°C. These included both isothermal and controlled cooling rate crystallization experiments, as crystals from the former were too small for ion microprobe (SIMS) analyses. Three runs at lower bulk water content are also reported. H₂O was measured in minerals by SIMS and in glasses by SIMS, Fourier Transform infrared spectroscopy (FTIR), and from oxide totals of electron microprobe (EMP) analyses. At 3 GPa, the liquidus for MORB with 6 wt.% H₂O is between 1,300 and 1,325°C. In the temperature interval investigated, the melt proportion varies from 100 to 45% and the modes of garnet and clinopyroxene are nearly equal. Liquid composition varies from basaltic to andesitic. The crystallization experiments starting from above the liquidus failed to nucleate garnets, but those starting from below the liquidus crystallized both garnet and clinopyroxene. SIMS analyses of glasses with >7 wt.% H₂O yield spuriously low concentrations, perhaps owing to hydrogen degassing in the ultra-high vacuum of the ion microprobe sample chamber. FTIR and EMP analyses show that the glasses have 3.4 to 11.9 wt.% water, whilst SIMS analyses indicate that clinopyroxenes have 1,340–2,330 ppm and garnets have 98–209 ppm H₂O. D _H ^cpx−gt is 11 ± 3, D _H ^cpx−melt is 0.023 ± 0.005 and D _H ^gt−melt is 0.0018 ± 0.0006. Most garnet/melt pairs have low values of D _H ^gt−melt, but D _H ^gt−melt increases with TiO₂ in the garnet. As also found by previous studies, values of D _H ^cpx−melt increase with Al₂O₃ of the crystal. For garnet pyroxenite, estimated values of D _H ^{pyroxenite−melt} decrease from 0.015 at 2.5 GPa to 0.0089 at 5 GPa. Hydration will increase the depth interval between pyroxenite and peridotite solidi for mantle upwelling beneath ridges or oceanic islands. This is partly because the greater pyroxene/olivine ratio in pyroxenite will tend to enhance the H₂O concentration of pyroxenite, assuming that neighboring pyroxenite and peridotite bodies have similar H₂O in their pyroxenes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Water-saturated and undersaturated melting relations in a Paricutin andesite and an estimate of water content in the natural magma

Phase relations have been determined for a Paricutin Volcano andesite at pressures to 10 kilobars and for H₂O contents in the melt of 2 to 10 weight percent. Runs were made under H₂O-saturated and undersaturated conditions. In undersaturated runs a H₂O-CO₂ fluid phase was always present. Fugacity of H₂O in melt, which is directly related to H₂O content in the melt, was calculated from thermodynamic data. Plagioclase was found to be the liquidus phase when H₂O contents in melt were less than about two percent. With more H₂O, orthopyroxene, in some cases joined by olivine, assumes the liquidus. Clinopyroxene crystallizes near the liquidus only for H₂O contents greater than five percent. The upper temperature stability limit of hornblende is about 950° C, well below the other silicate liquidi except at H₂O-saturated conditions above 5 kb.The geometry of undersaturated liquidi and experimental phase compositions may be compared to the mode and phase compositions of the natural rock. From this comparison, megaphenocrysts of the natural rock are interpreted to have crystallized from a lava which had a water content of 2.2±0.5 percent and a temperature of 1110±40°C. Mass-balance calculations on experimental and natural phase assemblages show that the Paricutin series could not have formed by fractionation at pressures less than 10 kb; rather, it was probably derived by partial melting of subducted basaltic oceanic floor.     

A high pressure experimental study on a magnesian-rich leucite-lamproite from the West Kimberley area,Australia: petrogenetic implications

Liquidus and subliquidus phase relations of a leucite-lamproite (wolgidite) from the West Kimberley area, Australia have been studied experimentally under the volatile conditions of 3.22 wt.% H₂O ( \(X_{CO_2 }\) =0.11) and 13.0 wt.% H₂O ( \(X_{CO_2 }\) =0.03) between 10 to 40 kbar. Under these conditions, liquids are vapour undersaturated. In experiments with 13.0 wt.% H₂O, olivine is the liquidus phase up to 24 kbar and orthopyroxene above 24 kbar. Phlogopite and rutile occur close to the liquidus above 16 kbar. Crystallization temperatures of clinopyroxenes are 50–120° C below the liquidus. Based on these results, wolgidite magma is unlikely to be a partial melt of a garnet- or spinel-lherzolite mantle but could be derived from phlogopite+rutile±olivine±or-thopyroxene assemblages occurring as metasomatized mantle.     

Phase relations and chemistry of Ti-rich K-richterite-bearing mantle assemblages: an experimental study to 8.0 GPa in a Ti-KNCMASH system

Experiments have been conducted in a peralkaline Ti-KNCMASH system representative of MARID-type bulk compositions to delimit the stability field of K-richterite in a Ti-rich hydrous mantle assemblage, to assess the compositional variation of amphibole and coexisting phases as a function of P and T, and to characterise the composition of partial melts derived from the hydrous assemblage. K-richterite is stable in experiments from 0.5 to 8.0 GPa coexisting with phlogopite, clinopyroxene and a Ti-phase (titanite, rutile or rutile + perovskite). At 8.0 GPa, garnet appears as an additional phase. The upper T stability limit of K-richterite is 1200–1250 °C at 4.0 GPa and 1300–1400 °C at 8.0 GPa. In the presence of phlogopite, K-richterite shows a systematic increase in K with increasing P to 1.03 pfu (per formula unit) at 8.0 GPa/1100 °C. In the absence of phlogopite, K-richterite attains a maximum of 1.14 K pfu at 8.0 GPa/1200 °C. Titanium in both amphibole and mica decreases continuously towards high P with a nearly constant partitioning while Ti in clinopyroxene remains more or less constant. In all experiments below 6.0 GPa ΣSi + Al in K-richterite is less than 8.0 when normalised to 23 oxygens+stoichiometric OH. Rutiles in the Ti-KNCMASH system are characterised by minor Al and Mg contents that show a systematic variation in concentration with P(T) and the coexisting assemblage. Partial melts produced in the Ti-KNCMASH system are extremely peralkaline [(K₂O+Na₂O)/Al₂O₃ = 1.7–3.7], Si-poor (40–45 wt% SiO₂), and Ti-rich (5.6–9.2 wt% TiO₂) and are very similar to certain Ti-rich lamproite glasses. At 4.0 GPa, the solidus is thought to coincide with the K-richterite-out reaction, the first melt is saturated in a phlogopite-rutile-lherzolite assemblage. Both phlogopite and rutile disappear ca. 150 °C above the solidus. At 8.0 GPa, the solidus must be located at T≤1400 ^°C. At this temperature, a melt is in equilibrium with a garnet- rutile-lherzolite assemblage. As opposed to 4.0 GPa, phlogopite does not buffer the melt composition at 8.0 GPa. The experimental results suggest that partial melting of MARID-type assemblages at pressures ≥4.0 GPa can generate Si-poor and partly ultrapotassic melts similar in composition to that of olivine lamproites. Received: 23 December 1996 / Accepted: 20 March 1997