The Effects of Small Amounts of H2O, CO2 and Na2O on the Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2 {+/-} H2O {+/-} CO2 {+/-} Na2O at 1{middle dot}1 GPa

The effects of small amounts of H₂O (<4 wt % in the melt)on the multiply saturated partial melting of spinel lherzolitein the system CaO–MgO–Al₂O₃–SiO₂ ±Na₂O ± CO₂ have been determined at 1·1 GPa inthe piston-cylinder apparatus. Electron microprobe analysisand Fourier transform infrared spectroscopy were used to analysethe experimental products. The effects of H₂O are to decreasethe melting temperature by 45°C per wt % H₂O in the melt,to increase the Al₂O₃ of the melts, decrease MgO and CaO, andleave SiO₂ approximately constant, with melts changing fromolivine- to quartz-normative. The effects of CO₂ are insignificantat zero H₂O, but become noticeable as H₂O increases, tendingto counteract the H₂O. The interaction between H₂O and CO₂ causesthe solubility of CO₂ at vapour saturation to increase withincreasing H₂O, for small amounts of H₂O. Neglect of the influenceof CO₂ in some previous studies on the hydrous partial meltingof natural peridotite may explain apparent inconsistencies betweenthe results. The effect of small amounts of H₂O on multiplysaturated melt compositions at 1·1 GPa is similar tothat of K₂O, i.e. increasing H₂O or K₂O leads to quartz-normativecompositions, but increasing Na₂O produces an almost oppositetrend, towards nepheline-normative compositions. KEY WORDS: H₂O; CO₂; FTIR; hydrous partial melting; mantle melting; spinel lherzolite; system CaO–MgO–Al₂O₃–SiO₂ ± H₂O ± CO₂ ± Na₂O     

The Effect of Cr2O3 on the Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2-Cr2O3 at 1{middle dot}1 GPa

Chromium as Cr³⁺ substitutes for octahedrally coordinated Alin upper-mantle minerals, thereby reducing the activity of Al₂O₃in the system and hence the concentration of Al₂O₃ in partialmelts. The effect of Cr₂O₃ on melt compositions multiply saturatedwith the spinel lherzolite phase assemblage has been quantifiedin the system CaO–MgO–Al₂O₃–SiO₂–Cr₂O₃at 1·1 GPa as a function of 100 Cr/(Cr + Al) in the spinel(Cr#_sp). The decrease of Al₂O₃ in the melt with increasing Cr#_spis accompanied by increasing MgO and SiO₂, whereas CaO remainsalmost constant. Consequently, the CaO/Al₂O₃ ratio of the meltincreases with Cr#_sp, and the melt becomes richer in normativediopside, hypersthene and quartz. The effect may explain certainmantle melts with unusually high CaO/Al₂O₃ ratios. The concentrationof Cr₂O₃ in the melt remains low even at high Cr#_sp, which meansthat the strong effect of Cr₂O₃ on partial melting equilibriais not readily apparent from its concentration in the melt itself.The existence of a highly refractory major component such asCr₂O₃ nullifies simplified conclusions from the ‘inverseapproach’ in the experimental study of basalt petrogenesis,as there is insufficient information in the composition of thepartial melt to reconstruct the conditions of melting. KEY WORDS: basalt petrogenesis; partial melting; reversal experiment; spinel lherzolite; system CMAS–Cr₂O₃; CaO/Al₂O₃ of melt; effect of Cr₂O₃     

Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2 {+/-} K2O at 1{middle dot}1 GPa

The compositions of multiply saturated partial melts are valuablefor the thermodynamic information that they contain, but aredifficult to determine experimentally because they exist onlyover a narrow temperature range at a given pressure. Here wetry a new approach for determining the composition of the partialmelt in equilibrium with olivine, orthopyroxene, clinopyroxeneand spinel (Ol + Opx + Cpx + Sp + Melt) in the system CaO–MgO–Al₂O₃–SiO₂(CMAS) at 1·1 GPa: various amounts of K₂O are added tothe system, and the resulting melt compositions and temperatureare extrapolated to zero K₂O. The ‘sandwich’ experimentalmethod was used to minimize problems caused by quench modification,and Opx and Cpx were previously synthesized at conditions nearthose of the melting experiments to ensure they had appropriatecompositions. Results were then checked by reversal crystallizationexperiments. The results are in good agreement with previouswork, and establish the anhydrous solidus in CMAS to be at 1320± 10°C at 1·1 GPa. The effect of K₂O is todepress the solidus by 5·8°C/wt %, while the meltcomposition becomes increasingly enriched in SiO₂, being quartz-normativeabove 4 wt % K₂O. Compared with Na₂O, K₂O has a stronger effectin depressing the solidus and modifying melt compositions. Theisobaric invariant point in the system CMAS–K₂O at whichOl + Opx + Cpx + Sp + Melt is joined by sanidine (San) is at1240 ± 10°C. During the course of the study severalother isobaric invariant points were identified and their crystaland melt compositions determined in unreversed experiments:Opx + Cpx + Sp + An + Melt in the system CMAS at 1315 ±10°C; in CMAS–K₂O, Opx + Cpx + Sp + An + San + Meltat 1230 ± 10°C and Opx + Sp + An + San + Sapph +Melt at 1230 ± 10°C, where An is anorthite and Sapphis sapphirine. Coexisting San plus An in three experiments helpdefine the An–San solvus at 1230–1250°C. KEY WORDS: feldspar solvus; igneous sapphirine; mantle solidus; partial melting; systems CMAS and CMAS–K₂O     

Geobarometry for Peridotites: Experiments in Simple and Natural Systems from 6 to 10 GPa

Experiments with peridotite minerals in simple (MgO–Al₂O₃–SiO₂,CaO–MgO–SiO₂ and CaO–MgO–Al₂O₃–SiO₂)and natural systems were conducted at 1300–1500°Cand 6–10 GPa using a multi-anvil apparatus. The experimentsin simple systems demonstrated consistency with previous lowerpressure experiments in belt and piston–cylinder set-ups.The analysis of spatial variations in pyroxene compositionswithin experimental samples was used to demonstrate that pressureand temperature variations within the samples were less than0·4 GPa and 50°C. Olivine capsules were used in natural-systemexperiments with two mineral mixtures: SC1 (olivine + high-Alorthopyroxene + high-Al clinopyroxene + spinel) and J4 (olivine+ low-Al orthopyroxene + low-Al clinopyroxene + garnet). Theexperiments produced olivine + orthopyroxene + garnet ±clinopyroxene assemblages, occasionally with magnesite and carbonate-richmelt. Equilibrium compositions were derived by the analysisof grain rims and evaluation of mineral zoning. They were comparedwith our previous experiments with the same starting mixturesat 2·8–6·0 GPa and the results from simplesystems. The compositions of minerals from experiments withnatural mixtures show smooth pressure and temperature dependencesup to a pressure of 8 GPa. The experiments at 9 and 10 GPa producedandradite-rich garnets and pyroxene compositions deviating fromthe trends defined by the lower pressure experiments (e.g. higherAl in orthopyroxene and Ca in clinopyroxene). This discrepancyis attributed to a higher degree of oxidation in the high-pressureexperiments and an orthopyroxene–high-P clinopyroxenephase transition at 9 GPa. Based on new and previous resultsin simple and natural systems, a new version of the Al-in-orthopyroxenebarometer is presented. The new barometer adequately reproducesexperimental pressures up to 8 GPa. KEY WORDS: garnet; mineral equilibrium; multi-anvil apparatus; orthopyroxene; geobarometry     

Continuous Gradations among Primary Carbonatitic, Kimberlitic, Melilititic, Basaltic, Picritic, and Komatiitic Melts in Equilibrium with Garnet Lherzolite at 3-8 GPa

Multianvil melting experiments in the system CaO–MgO–Al₂O₃–SiO₂–CO₂(CMAS–CO₂) at 3–8 GPa, 1340–1800°C, involvingthe garnet lherzolite phase assemblage in equilibrium with CO₂-bearingmelts, yield continuous gradations in melt composition betweencarbonatite, kimberlite, melilitite, komatiite, picrite, andbasalt melts. The phase relations encompass a divariant surfacein P–T space. Comparison of the carbonatitic melts producedat the low-temperature side of this surface with naturally occurringcarbonatites indicates that natural magnesiocarbonatites couldbe generated over a wide range of pressures >2·5 GPa.Melts analogous to kimberlites form at higher temperatures alongthe divariant surface, which suggests that kimberlite genesisrequires more elevated geotherms. However, the amount of waterfound in some kimberlites has the potential to lower temperaturesfor the generation of kimberlitic melts by up to 150°C,provided no hydrous phases are present. Compositions resemblinggroup IB and IA kimberlites are produced at pressures around5–6 GPa and 10 GPa, respectively, whereas the compositionsof some other kimberlites suggest generation at higher pressuresstill. At pressures <4 GPa, an elevated geotherm producesmelilitite-like melt in the CMAS–CO₂ system rather thankimberlite. Even when a relatively CO₂-rich mantle compositioncontaining 0·15 wt % CO₂ is assumed, kimberlites andmelilitites are produced by <1% melting and carbonatitesare generated by even smaller degrees of melting of <0·5%. KEY WORDS: carbonatite; CO₂; kimberlite; melilitite; melt generation     

Experimentally Determined Phase Relations in Hydrous Peridotites to 6{middle dot}5 GPa and their Consequences on the Dynamics of Subduction Zones

Fluid-saturated subsolidus experiments from 2·0 to 6·5GPa, and from 680 to 800°C have been performed on threemodel peridotites in the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O(NCFMASH). Amphibole and chlorite coexist up to 2·4 GPa,700°C. Chlorite persists to 4·2 GPa at 680°C.Starting from 4·8 GPa, 680°C a 10 Å phase structurereplaces chlorite in all compositions. The 10 Å phasestructure contains significant Al₂O₃ (up to 10·53 wt%) deviating from the MgO–SiO₂–H₂O 10 Å phase(MSH 10 Å phase). A mixed layered structure (chlorite–MSH10 Å phase) is proposed to account for aluminium observed.In the Tinaquillo lherzolite amphibole breakdown occurs viathe reaction Thermal stabilityof chlorite (chlorite + orthopyroxene = forsterite + garnet+ H₂O) is shifted towards lower temperatures, compared withthe system MASH. Furthermore, the chlorite thermal breakdownis also related to the degenerate reaction Chlorite and the Al-10 Å phase structurecontribute significantly to the water budget in subduction zonesin the depth range relevant for arc magmatism, whereas amphibole-relatedfluid release is restricted to the forearc region. Chloriteand Al-10 Å phase breakdowns might explain the occurrenceof a double seismic zone by dehydration embrittlement. KEY WORDS: amphibole; chlorite; high pressure; peridotites; subduction zones     

Monazite-Xenotime Thermochronometry and Al2SiO5 Reaction Textures in the Picuris Range, Northern New Mexico, USA: New Evidence for a 1450-1400 Ma Orogenic Event

Al₂SiO₅ reaction textures in aluminous schist and quartziteof the northern Picuris range, north-central New Mexico, recorda paragenetic sequence of kyanite to sillimanite to andalusite,consistent with a clockwise P–T loop, with minor decompressionnear the Al₂SiO₅ triple-point. Peak metamorphic temperaturesare estimated at 510–525°C, at 4·0–4·2kbar. Kyanite and fibrolite are strongly deformed; some prismaticsillimanite, and all andalusite are relatively undeformed. Monaziteoccurs as inclusions within kyanite, mats of sillimanite andcentimetre-scale porphyroblasts of andalusite, and is typicallyaligned subparallel to the dominant regional foliation (S₀/S₁or S₂) and extension lineation (L₁). Back-scatter electron imagesand X-ray maps of monazite reveal distinct core, intermediateand rim compositional domains. Monazite–xenotime thermometryfrom the intermediate and rim domains yields temperatures of405–470°C (±50°C) and 500–520°C(±50°C), respectively, consistent with the progradeto peak metamorphic growth of monazite. In situ, ion microprobeanalyses from five monazites yield an upper intercept age of1417 ± 9 Ma. Near-concordant to concordant analyses yield²⁰⁷Pb–²⁰⁶Pb ages from 1434 ± 12 Ma (core) to 1390± 20 Ma (rim). We find no evidence of older regionalmetamorphism related to the 1650 Ma Mazatzal Orogeny. KEY WORDS: Al₂SiO₅; metamorphism; monazite; thermochronometry; triple-point     

Liquidus Phase Relations in the System CaO-MgO-Al2O3-SiO2 at 2{middle dot}0 GPa: Applications to Basalt Fractionation, Eclogites, and Igneous Sapphirine

To model magmatic crystallization processes for mafic to intermediatecompositions at high pressure, liquidus phase relations in theforsterite–anorthite–diopside–silica (FADS)tetrahedron within the CaO–MgO–Al₂O₃–SiO₂system have been determined at 2·0 GPa. Compositionsof five liquidus invariant points have been determined and theapproximate compositions of five others have been inferred.These involve primary phase volumes for forsterite (fo), enstatite(en), diopside (di), high quartz (qz), spinel (sp), sapphirine(sa), garnet (gt), anorthite (an), and corundum (cor). The determined(with wt % coefficients) and inferred reactions (without coefficients)that define each isobaric invariant point are as follows: 23 en + 68 di + 9 sp = 84 liq + 16 fo 37 di + 63 sa = 47 liq + 40 sp + 13 en 100 gt = 21 liq + 27 sa + 55 en + 18 di 1 di + 59 en + 41 an = 43 liq + 57 gt 18 di + 21 qz + 15 en + 47 an = 100 liq di + an + gt = liq + sa an + gt = liq + sa + en sa + an + di = liq + sp sa + an = liq + cor + sp di + cor = liq + an + sp. These phase relations provide a diverse range of constraintson igneous processes at pressures near 2 GPa. They show thatfractional crystallization of a model basalt gives a residualliquid strongly enriched in SiO₂, strongly depleted in MgO,and mildly enriched in Al₂O₃. Such a trend is consistent withthe calc-alkaline fractionation trend observed at subductionzones, but is in disagreement with suggestions that fractionationof tholeiitic basalt in this pressure range yields an alkalicbasalt. Both trends may occur for natural basalts dependingon the Na₂O content of the parental magma. Also, the data showthat the minimum pressure for the formation of cumulate eclogitesand garnet pyroxenites is about 1·8–1·9GPa. The lower limit of pressure at which sapphirine can crystallizefrom a liquid in the FADS tetrahedron is estimated to be 1·1–1·5GPa and the upper limit is >3 GPa. Sapphirine crystallizesfrom magmas intermediate in composition between basalt and andesite.Probable igneous sapphirine in mafic associations is rare, butit occurs as part of a pyroxenite xenolith from Delegate, Australia,that we suggest is a cumulate assemblage and in a sapphirinenorite at Wilson Lake, Labrador, Canada. KEY WORDS: basalt; eclogite; sapphirine; fractional crystallization     

Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral-Fluid Equilibria and Petrogenetic Grids

The role of clinopyroxene in producing grandite garnet is evaluatedusing data from an ultrahigh-temperature metamorphosed calc-silicategranulite occurrence in the Eastern Ghats Belt, India. ‘Peak’pressure–temperature conditions of metamorphism were previouslyconstrained from associated high Mg–Al granulites as c.0·9 GPa, >950°C, and the rocks were near-isobaricallycooled to c. 750°C. Grandite garnet of variable compositionwas produced by a number of reactions involving phases suchas clinopyroxene, scapolite, plagioclase, wollastonite and calcite,in closely spaced domains. Compositional heterogeneity is preservedeven on a microscale. This precludes pervasive fluid fluxingduring either the peak or the retrograde stage of metamorphism,and is further corroborated by computation of fluid–rockratios. With the help of detailed textural and mineral compositionalstudies leading to formulation of balanced reactions, and usingan internally consistent thermodynamic dataset and relevantactivity–composition relationships, new petrogenetic gridsare developed involving clinopyroxene in the system CaO–Al₂O₃–FeO–SiO₂–CO₂–O₂in T–aCO₂–fO₂ space to demonstrate the importanceof these factors in the formation of grandite garnet. Two singularcompositions in garnet-producing reactions in this system arededuced, which explain apparently anomalous textural relations.The possible role of an esseneite component in clinopyroxenein the production of grandite garnet is evaluated. It is concludedthat temperature and fO₂ are the most crucial variables controllinggarnet composition in calc-silicate granulites. fO₂, however,behaves as a dependent variable of CO₂ in the fluid phase. Externalfluid fluxing of any composition is not necessary to producechemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidation–decarbonation equilibria     

Liquidus Equilibria in the System K2O-Na2O-Al2O3-SiO2-F2O-1-H2O to 100 MPa: I. Silicate-Fluoride Liquid Immiscibility in Anhydrous Systems

Liquidus relations in the four-component system Na₂O–Al₂O₃–SiO₂–F₂O_–1were studied at 0· 1 and 100 MPa to define the locationof fluoride–silicate liquid immiscibility and outlinedifferentiation paths of fluorine-bearing silicic magmas. Thefluoride–silicate liquid immiscibility spans the silica–albite–cryoliteand silica–topaz–cryolite ternaries and the haplogranite-cryolitebinary at greater than 960°C and 0· 1–100 MPa.With increasing Al₂O₃ in the system and increasing aluminum/alkalication ratio, the two-liquid gap contracts and migrates fromthe silica liquidus to the cryolite liquidus. The gap does notextend to subaluminous and peraluminous melt compositions. Forall alkali feldspar–quartz-bearing systems, the miscibilitygap remains located on the cryolite liquidus and is thus inaccessibleto differentiating granitic and rhyolitic melts. In peralkalinesystems, the magmatic differentiation is terminated at the albite–quartz–cryoliteeutectic at 770°C, 100 MPa, 5 wt % F and cation Al/Na =0· 75. The addition of topaz, however, significantlylowers melting temperatures and allows strong fluorine enrichmentin subaluminous compositions. At 100 MPa, the binary topaz–cryoliteeutectic is located at 770°C, 39 wt % F, cation Al/Na 0·95, and the ternary quartz–topaz–cryolite eutecticis found at 740°C, 32 wt % F, 30 wt % SiO₂ and cation Al/Na 0· 95. Such location of both eutectics enables fractionationpaths of subaluminous quartz-saturated systems to produce fluorine-rich,SiO₂-depleted and nepheline-normative residual liquids. KEY WORDS: silicate melt; granite; rhyolite; fluorine; liquid immiscibility     

Exhumation History of a Garnet Pyroxenite-bearing Mantle Section from a Continent-Ocean Transition (Northern Apennine Ophiolites, Italy)

Garnet clinopyroxenite and garnet websterite layers occur locallywithin mantle peridotite bodies from the External Liguride Jurassicophiolites (Northern Apennines, Italy). These ophiolites werederived from an ocean–continent transition similar tothe present-day western Iberian margin. The garnet clinopyroxenitesare mafic rocks with a primary mineral assemblage of pyrope-richgarnet + sodic Al-augite (Na₂O 2·5 wt %, Al₂O₃ 12·5wt %), with accessory graphite, Fe–Ni sulphides and rutile.Decompression caused Na-rich plagioclase (An_50–45) exsolutionin clinopyroxene porphyroclasts and extensive development ofsymplectites composed of secondary orthopyroxene + plagioclase(An_85–72) + Al-spinel ± clinopyroxene ±ilmenite at the interface between garnet and primary clinopyroxene.Further decompression is recorded by the development of an olivine+ plagioclase-bearing assemblage, locally under syn-kinematicconditions, at the expense of two-pyroxenes + Al-spinel. Mg-richgarnet has been also found in the websterite layers, which arecommonly characterized by the occurrence of symplectites madeof orthopyroxene + Al-spinel ± clinopyroxene. The enclosingperidotites are Ti-amphibole-bearing lherzolites with a fertilegeochemical signature and a widespread plagioclase-facies myloniticfoliation, which preserve in places a spinel tectonite fabric.Lu–Hf and Sm–Nd mineral isochrons (220 ±13 Ma and 186.0 ± 1·8 Ma, respectively) have beenobtained from a garnet clinopyroxenite layer and interpretedas cooling ages. Geothermobarometric estimates for the high-pressureequilibration have yielded T 1100°C and P 2·8 GPa.The early decompression was associated with moderate cooling,corresponding to T 950°, and development of a spinel tectonitefabric in the lherzolites. Further decompression associatedwith plagioclase–olivine growth in both peridotites andpyroxenites was nearly isothermal. The shallow evolution occurredunder a brittle regime and led to the superposition of hornblendeto serpentine veining stages. The garnet pyroxenite-bearingmantle from the External Liguride ophiolites represents a raretectonic sampling of deep levels of subcontinental lithosphereexhumed in an oceanic setting. The exhumation was probably accomplishedthrough a two-step process that started during Late Palaeozoiccontinental extension. The low-pressure portion of the exhumationpath, probably including also the plagioclase mylonitic shearzones, was related to the Mesozoic (Triassic to Jurassic) riftingthat led to continental break-up. In Jurassic times, the studiedmantle sequence became involved in an extensional detachmentprocess that resulted in sea-floor denudation. KEY WORDS: garnet pyroxenite; ophiolite; non-volcanic margin; mantle exhumation; Sm–Nd and Lu–Hf geochronology     

Cr-Saturation Arrays in Concentrate Garnet Compositions from Kimberlite and their Use in Mantle Barometry

The spinel–garnet transition in Cr/Al-enriched peridotiticbulk compositions is known from experimental investigationsto occur at 20–70 kbar, within the pressure range sampledby kimberlites. We show that the Cr₂O₃–CaO compositionsof concentrate garnets from kimberlite have maximum Cr/Ca arrayscharacterized by Cr₂O₃/CaO 0·96–0·81, andinterpret the arrays as primary evidence of chromite–garnetcoexistence in Cr-rich harzburgitic or lherzolitic bulk compositionsderived from depth within the lithosphere. Under Cr-saturatedconditions on a known geotherm, each Cr/Ca array implicitlydelineates an isobar inside a garnet Cr₂O₃–CaO diagram.This simplification invites a graphical approach to calibratean empirical Cr/Ca-in-pyrope barometer. Carbonaceous chromite–garnetharzburgite xenoliths from the Roberts Victor kimberlite tightlybracket a graphite–diamond constraint (GDC) located atCr₂O₃ = 0·94CaO + 5·0 (wt %), representing a pivotalcalibration corresponding to 43 kbar on a 38 mW/m² conductivegeotherm. Additional calibration points are established at 14,17·4 and 59·1 kbar by judiciously projecting garnetcompositions from simple-system experiments onto the same geotherm.The garnet Cr/Ca barometer is then simply formulated as follows(in wt %):

if Cr₂O₃ 0·94CaO + 5, then P₃₈ (kbar) = 26·9+ 3·22Cr₂O₃ – 3·03CaO, or

if Cr₂O₃ <0·94CaO + 5, then P₃₈ (kbar) = 9·2+ 36[(Cr₂O₃+ 1·6)/(CaO + 7·02)].

A small correction to P₃₈ values, applicable for 35–48mW/m² conductive geotherms, is derived empirically by requiringconventional thermobarometry results and garnet concentratecompositions to be consistent with the presence of diamondsin the Kyle Lake kimberlite and their absence in the Zero kimberlite.We discuss application of the P₃₈ barometer to estimate (1)real pressures in the special case where chromite–garnetcoexistence is known, (2) minimum pressures in the general casewhere Cr saturation is unknown, and (3) the maximum depth ofdepleted lithospheres, particularly those underlying Archaeancratons. A comparison with the P_Cr barometer of Ryan et al.(1996, Journal of Geophysical Research 101, 5611–5625)shows agreement with P₃₈ at 55 ± 2 kbar, and 6–12%higher P_Cr values at lower P₃₈. Because the P_Cr formulationsystematically overestimates the 43 kbar value of the GDC by2–6 kbar, we conclude that the empirical Cr/Ca-in-garnetbarometer is preferred for all situations where conductive geothermsintersect the graphite–diamond equilibrium. KEY WORDS: Cr-pyrope; chromite; P₃₈ barometer; mantle petrology; lithosphere thickness     

Shear Zone-hosted Migmatites (Eastern India): the Role of Dynamic Melting in the Generation of REE-depleted Felsic Melts, and Implications for Disequilibrium Melting

In the Ranmal migmatite complex, non-anatectic foliated graniteprotoliths can be traced to polyphase migmatites. Structural–microtexturalrelations and thermobarometry indicate that syn-deformationalsegregation–crystallization of in situ stromatic and diatexiteleucosomes occurred at 800°C and 8 kbar. The protolith,the neosome, and the mesosome comprise quartz, K-feldspar, plagioclase,hornblende, biotite, sphene, apatite, zircon, and ilmenite,but the modal mineralogy differs widely. The protolith compositionis straddled by element abundances in the leucosome and themesosome. The leucosomes are characterized by lower CaO, FeO+MgO,mg-number, TiO₂ , P₂O₅ , Rb, Zr and total rare earth elements(REE), and higher SiO₂ , K₂O, Ba and Sr than the protolith andthe mesosome, whereas Na₂O and Al₂O₃ abundances are similar.The protolith and the mesosome have negative Eu anomalies, butprotolith-normalized abundances of REE-depleted leucosomes showpositive Eu anomalies. The congruent melting reaction for leucosomeproduction is inferred to be 0·325 quartz+0·288K-feldspar+0·32 plagioclase+0·05 biotite+0·014hornblende+0·001 apatite+0·001 zircon+0·002sphene=melt. Based on the reaction, large ion lithophile element,REE and Zr abundances in model melts computed using dynamicmelting approached the measured element abundances in leucosomesfor >0·5 mass fraction of unsegregated melts withinthe mesosome. Disequilibrium-accommodated dynamic melting andequilibrium crystallization of melts led to uniform plagioclasecomposition in migmatites and REE depletion in leucosome. KEY WORDS: migmatite; REE; trace element; partial melting; P–T conditions     

A Metamorphosed Early Cambrian Crust-Mantle Transition in the Eastern Alps, Austria

In the Speik Complex (Eastern Alps, Austria), highly melt-depleted,metamorphosed harzburgites with abundant pods and layers ofchromitite are interlayered with a suite of metamorphosed orthopyroxenites,clinopyroxenites and gabbros. Coarse-grained orthopyroxenitesoccur as centimetre- to metre-wide veinlets and pods, but alsoas intrusive plugs several tens of metres wide. Intimately associatedmetaclinopyroxenite and metagabbro are present as bodies upto several metres thick at a distinct stratigraphic level withinthe complex. In the ultramafic rocks, relict magmatic olivine,orthopyroxene, clinopyroxene and spinel have been overprintedby a metamorphic assemblage of forsterite, diopside, tremolite,anthophyllite, chlorite, serpentine, talc and Cr–Fe-richspinel. Hornblende, epidote, zoisite and chlorite dominate themetamorphic paragenesis in metagabbros, in addition to rarerelicts of clinopyroxene and two phases of Ca-rich garnet. Thepolymetamorphic evolution of the Speik Complex includes rarelypreserved pre-Variscan (400 Ma) eclogite-facies conditions,Variscan (330 Ma) amphibolite-facies conditions (600–700°C,>5 kbar) and Eoalpine (100 Ma) greenschist- to amphibolite-faciesconditions reaching 550°C and 7–10 kbar. Orthopyroxenitesare characterized by high concentrations of SiO₂, MgO and Cr,and by U-shaped chondrite-normalized rare earth element (REE)patterns similar to those of their harzburgite hosts. The REEpatterns of the clinopyroxenites are flat to slightly enrichedin light REE. Metagabbro compositions are variable, but generallycharacterized by low SiO₂ and high mg-numbers (61–78).Their REE patterns all have Gd_N/Yb_N > 1; some samples havelarge positive Eu anomalies implying the original presence ofcumulus plagioclase. In the orthopyroxenites, clinopyroxenitesand some peridotites, Pt, Pd and Re are distinctly enrichedcompared with Os, Ir and Ru, whereas most harzburgites haveunfractionated to slightly fractionated platinum-group element(PGE) patterns with respect to average upper mantle. The Re–Osisotope compositions of the pyroxenites define an errorchronat 550 ± 17 Ma and a supra-chondritic ¹⁸⁷Os/¹⁸⁸Os of0·179 ± 0·003. An isochron age of 554 ±37 Ma with _Nd(i) +0·7 is indicated by the Sm–Ndisotope compositions of whole-rock pyroxenite and gabbro samples,whereas the harzburgites plot on an errorchron of 745 ±45 Ma and _Nd(i) +6. The pyroxenites and gabbros probably representa cogenetic suite of magmatic dykes intruded into uppermost,highly depleted, suboceanic mantle below the crust–mantletransition zone in an oceanic basin close to the northwesternmargin of Gondwana. KEY WORDS: pyroxenite; metagabbro; geochemistry; Re–Os isotopes; Sm–Nd isotopes     

Phase Relations and Stability of Magnetoplumbite- and Crichtonite-Series Phases under Upper-Mantle P-T Conditions: an Experimental Study to 15 GPa with Implications for LILE Metasomatism in the Lithospheric Mantle

High-pressure–high-temperature experiments were performedin the range 7–15 GPa and 1300–1600°C to investigatethe stability and phase relations of the K- and Ba-dominantmembers of the crichtonite and magnetoplumbite series of phasesin simplified bulk compositions in the systems TiO₂–ZrO₂–Cr₂O₃–Fe₂O₃–BaO–K₂Oand TiO₂–Cr₂O₃–Fe₂O₃–BaO–K₂O. Both seriesof phases occur as inclusions in diamond and/or as constituentsof metasomatized peridotite mantle xenoliths sampled by kimberlitesor alkaline lamprophyres. They can accommodate large ion lithophileelements (LILE) and high field strength elements (HFSE) on awt % level and, hence, can critically influence the LILE andHFSE budget of a metasomatized peridotite even if present onlyin trace amounts. The Ba and K end-members of the crichtoniteseries, lindsleyite and mathiasite, are stable to 11 GPa and1500–1600°C. Between 11 and 12 GPa, lindsleyite breaksdown to form two Ba–Cr-titanates of unknown structurethat persist to at least 13 GPa. The high-pressure breakdownproduct of mathiasite is a K–Cr-titanate with an idealizedformula KM₇O₁₂, where M = Ti, Cr, Mg, Fe. This phase possessesspace group P6₃/m with a = 9·175(2) Å, c = 2·879(1)Å, V = 209·9(1) ų. Towards high temperatures,lindsleyite persists to 1600°C, whereas mathiasite breaksdown between 1500 and 1600°C to form a number of complexTi–Cr-oxides. Ba and K end-members of the magnetoplumbiteseries, hawthorneite and yimengite, are stable in runs at 7,10 and 15 GPa between 1300 and 1400°C coexisting with anumber of Ti–Cr-oxides. Molar mixtures (1:1) of lindsleyite–mathiasiteand hawthorneite–yimengite were studied at 7–10GPa and 1300–1400°C, and 9–15 GPa and 1150–1400°C,respectively. In the system lindsleyite–mathiasite, onehomogeneous Ba–K phase is stable, which shows a systematicincrease in the K/(K + Ba) ratio with increasing pressure. Inthe system hawthorneite–yimengite, two coexisting Ba–Kphases appear, which are Ba rich and Ba poor, respectively.The data obtained from this study suggest that Ba- and K-dominantmembers of the crichtonite and magnetoplumbite series of phasesare potentially stable not only throughout the entire subcontinentallithosphere but also under conditions of an average present-daymantle adiabat in the underlying asthenosphere to a depth ofup to 450 km. At still higher pressures, both K and Ba may remainstored in alkali titanates that would also be eminently suitablefor the transport of other ions with large ionic radii. KEY WORDS: crichtonite; magnetoplumbite; high-P–T experiments; phase relations; upper mantle     

Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from Petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS Dating

Metapelitic rock samples from the NE Shackleton Range, Antarctica,include garnet with contrasting zonation patterns and two agespectra. Garnet porphyroblasts in K-rich kyanite–sillimanite–staurolite–garnet–muscovite–biotite schistsfrom Lord Nunatak show prograde growth zonation, and give Sm–Ndgarnet, U–Pb monazite and Rb–Sr muscovite ages of518 ± 5, 514 ± 1 and 499 ± 12 Ma, respectively.Geothermobarometry and P–T pseudo-section calculationsin the model system CaO–Na₂O–K₂O– TiO₂–MnO–FeO–MgO–Al₂O₃–SiO₂–H₂Oare consistent with garnet growth during prograde heating from540°C/7 kbar to 650°C/7·5 kbar, and partial resorptionduring a subsequent P–T decrease to <650°C at <6kbar. All data indicate that rocks from Lord Nunatak were affectedby a single orogenic cycle. In contrast, garnet porphyroblastsin K-poor kyanite–sillimanite– staurolite–garnet–cordierite–biotite-schistsfrom Meade Nunatak show two growth stages and diffusion-controlledzonation. Two distinct age groups were obtained. Laser ablationplasma ionization multicollector mass spectrometry in situ analysesof monazite, completely enclosed by a first garnet generation,yield ages of c. 1700 Ma, whereas monazite grains in open garnetfractures and in most matrix domains give c. 500 Ma. Both agegroups are also obtained by U–Pb thermal ionization massspectrometry analyses of matrix monazite and zircon, which fallon a discordia with lower and upper intercepts at 502 ±1 and 1686 ± 2 Ma, respectively. Sm–Nd garnet datingyields an age of 1571 ± 40 Ma and Rb–Sr biotiteanalyses give an age of 504 ± 1 Ma. Integrated geochronologicaland petrological data provide evidence that rocks from MeadeNunatak underwent a polymetamorphic Barrovian-type metamorphism:(1) garnet 1 growth and subsequent diffusive garnet annealingbetween 1700 and 1570 Ma; (2) garnet 2 growth during the RossOrogeny at c. 500 Ma. During the final orogenic event the rocksexperienced peak P–T conditions of about 650°C/7·0kbar and a retrograde stage at c. 575°C/4·0 kbar. KEY WORDS: garnet microtexture; P–T pseudosection; geochronology; polymetamorphism; Shackleton Range; Antarctica     

Melting Behaviour of Model Lherzolite in the System CaO-MgO-Al2O3-SiO2-FeO at 0{middle dot}7-2{middle dot}8 GPa

Fe–Mg exchange is the most important solid solution involvedin partial melting of spinel lherzolite, and the system CaO–MgO–Al₂O₃–SiO₂–FeO(CMASF) is ideally suited to explore this type of exchange duringmantle melting. Also, if primary mid-ocean ridge basalts arelargely generated in the spinel lherzolite stability field bynear-fractional fusion, then Na and other highly incompatibleelements will early on become depleted in the source, and themelting behaviour of mantle lherzolite should resemble the meltingbehaviour of simplified lherzolite in the CMASF system. We havedetermined the isobarically univariant melting relations ofthe lherzolite phase assemblage in the CMASF system in the 0·7–2·8GPa pressure range. Isobarically, for every 1 wt % increasein the FeO content of the melt in equilibrium with the lherzolitephase assemblage, the equilibrium temperature is lower by about3–5°C. Relative to the solidus of model lherzolitein the CaO–MgO–Al₂O₃–SiO₂ system, melt compositionsin the CMASF system are displaced slightly towards the alkalicside of the basalt tetrahedron. The transition on the solidusfrom spinel to plagioclase lherzolite has a positive Clapeyronslope with the spinel lherzolite assemblage on the high-temperatureside, and has an almost identical position in P–T spaceto the comparable transition in the CaO–MgO–Al₂O₃–SiO₂–Na₂O(CMASN) system. When the compositions of all phases are describedmathematically and used to model the generation of primary basalts,temperature and melt composition changes are small as percentmelting increases. More specifically, 10% melting takes placeover 1·5–2°C, melt compositions are relativelyinsensitive to the degree of melting and bulk composition, andequilibrium and near-fractional melting yield similar melt compositions.FeO and MgO are the oxides that exhibit the greatest changein the melt with degree of melting and bulk composition. Theamount of FeO decreases with increasing degree of melting, whereasthe amount of MgO increases. The coefficients for Fe–Mgexchange between the coexisting crystalline phases and melt,K_dFe–Mg^xl–liq, show a relatively simple and predictablebehaviour with pressure and temperature: the coefficients forolivine and spinel do not show significant dependence on temperature,whereas the coefficients for orthopyroxene and clinopyroxeneincrease with pressure and temperature. When melting of lherzoliteis modeled in the CMASF system, a strong linear correlationis observed between the mg-number of the lherzolite and themg-number of the near-solidus melts. Comparison with meltingin the CMASN system indicates that Na₂O has a strong effecton lherzolite melting behaviour only at small degrees of melting. KEY WORDS: CMASF; lherzolite solidus; mantle melting     

Partial Melting Experiments of Peridotite + CO2 at 3 GPa and Genesis of Alkalic Ocean Island Basalts

We document compositions of minerals and melts from 3 GPa partialmelting experiments on two carbonate-bearing natural lherzolitebulk compositions (PERC: MixKLB-1 + 2·5 wt% CO₂; PERC3:MixKLB-1 + 1 wt% CO₂) and discuss the compositions of partialmelts in relation to the genesis of alkalic to highly alkalicocean island basalts (OIB). Near-solidus (PERC: 1075–1105°C;PERC3: 1050°C) carbonatitic partial melts with <10 wt%SiO₂ and 40 wt% CO₂ evolve continuously to carbonated silicatemelts with >25 wt% SiO₂ and <25 wt% CO₂ between 1325 and1350°C in the presence of residual olivine, orthopyroxene,clinopyroxene, and garnet. The first appearance of CO₂-bearingsilicate melt at 3 GPa is 150°C cooler than the solidusof CO₂-free peridotite. The compositions of carbonated silicatepartial melts between 1350 and 1600°C vary in the rangeof 28–46 wt% SiO₂, 1·6–0·5 wt% TiO₂,12–10 wt% FeO*, and 19–29 wt% MgO for PERC, and42–48 wt% SiO₂, 1·9–0·5 wt% TiO₂,10·5–8·4 wt% FeO*, and 15–26 wt% MgOfor PERC3. The CaO/Al₂O₃ weight ratio of silicate melts rangesfrom 2·7 to 1·1 for PERC and from 1·7 to1·0 for PERC3. The SiO₂ contents of carbonated silicatemelts in equilibrium with residual peridotite diminish significantlywith increasing dissolved CO₂ in the melt, whereas the CaO contentsincrease markedly. Equilibrium constants for Fe*–Mg exchangebetween carbonated silicate liquid and olivine span a rangesimilar to those for CO₂-free liquids at 3 GPa, but diminishslightly with increasing dissolved CO₂ in the melt. The carbonatedsilicate partial melts of PERC3 at <20% melting and partialmelts of PERC at 15–33% melting have SiO₂ and Al₂O₃ contents,and CaO/Al₂O₃ values, similar to those of melilititic to basaniticalkali OIB, but compared with the natural lavas they are moreenriched in CaO and they lack the strong enrichments in TiO₂characteristic of highly alkalic OIB. If a primitive mantlesource is assumed, the TiO₂ contents of alkalic OIB, combinedwith bulk peridotite/melt partition coefficients of TiO₂ determinedin this study and in volatile-free studies of peridotite partialmelting, can be used to estimate that melilitites, nephelinites,and basanites from oceanic islands are produced from 0–6%partial melting. The SiO₂ and CaO contents of such small-degreepartial melts of peridotite with small amounts of total CO₂can be estimated from the SiO₂–CO₂ and CaO–CO₂ correlationsobserved in our higher-degree partial melting experiments. Thesesuggest that many compositional features of highly alkalic OIBmay be produced by 1–5% partial melting of a fertile peridotitesource with 0·1–0·25 wt% CO₂. Owing to verydeep solidi of carbonated mantle lithologies, generation ofcarbonated silicate melts in OIB source regions probably happensby reaction between peridotite and/or eclogite and migratingcarbonatitic melts produced at greater depths. KEY WORDS: alkali basalts; carbonated peridotite; experimental petrology; ocean island basalts; partial melting     

The Origin and Composition of Metasomatic Fluids and Amphiboles beneath Malaita, Solomon Islands

A suite of mantle peridotite xenoliths from the Malaitan alnoitedisplay both trace element enrichment and modal metasomatism.Pargasitic amphibole is present in both garnet- and spinelbearingxenoliths, formed by reaction of a metasomatic fluid (representedby H₂O and Na₂O) with the peridotite assemblage. Two pargasite-formingreactions are postulated, whereby spinel is totally consumed: 6MgAl₂O₄ + 8CaMgSi₂O₆ + 7Mg₂Si₂O₆ + 4H₂O + 2Na₂O = 4NaCa₂Mg₄Al₃Si₆O₁₂(OH)₂+ 6Mg₂SiO₄ or spinel is both a reactant (low Cr) and a product (high Cr): 24MgAlCrO₄ + 16CaMgSi₂O₆ + 14Mg₂Si₂O₆ + 8H₂O + 4Na₂O = 8NaCa₂Mg₄Al₃Si₆O₁₂(OH)₂+ 12MgCr₂O₄ + 12Mg₂SiO₄ Seven garnet—spinel-peridotites display cryptic metasomatismas demonstrated by the LREE enrichment in clinopyroxenes. TheLREE enrichment correlates positively with ¹⁴³ND/¹⁴⁴ND (0?512771–0?513093)which defines a mixing line between a mantle MORB source anda metasomatic fluid. Isotopic evidence (Sr and Nd) from garnet,clinopyroxene, and amphibole demonstrate this fluid has notoriginated in the alnoite sensu stricto. Calculated amphiboleequilibrium liquids show a range in La/Yb and Ce/Yb ratios similarto those calculated for the augite and subcalcic diopside megacrysts.Sr and Nd isotope analyses from amphibole are within error ofthe augite (PHN4074) and subcalcic diopside megacrysts (CRN2I6,PHN4069, and PHN4085). It is concluded that fluids emanatedfrom a proto-alnoite magma throughout megacryst fractionation,and the mixing line was generated during the crystallizationof the subcalcic diopsides. This study demonstrates that metasomatismrepresented in these xenoliths is not a prerequisite for alnoitemagmatism, but is a consequence of it.     

The Solubility of Sulphur in Hydrous Rhyolitic Melts

Experiments performed at 2 kbar, in the temperature range 800–1000°C,with fO₂ between NNO–2·3 and NNO+2·9 (whereNNO is the nickel–nickel oxide buffer), and varying amountsof sulphur added to hydrous metaluminous rhyolite bulk compositions,were used to constrain the solubility of sulphur in rhyolitemelts. The results show that fS₂ exerts a dominant control onthe sulphur solubility in hydrous silicate melts and that, dependingon fO₂, a rhyolitic melt can reach sulphur contents close to1000 ppm at high fS₂. At fO₂ below NNO+1, the addition of ironto a sulphur-bearing rhyolite magma produces massive crystallizationof pyrrhotite and does not enhance the sulphur solubility ofthe melt. For a given fO₂, the melt-sulphur-content increaseswith fS₂. For fixed fO₂ and fS₂, temperature exerts a positivecontrol on sulphur solubilities, at least for fO₂ below NNO+1.The mole fraction of dissolved sulphur exhibits essentiallylinear dependence on fH₂S at low fO₂ and, although the experimentalevidence is less clear, on fSO₂ at high fO₂. The minimum insulphur solubility corresponds to the redox range where bothfH₂S and fSO₂ are approximately equal. A thermodynamic modelof sulphur solubility in hydrous rhyolite melts is derived assumingthat total dissolved sulphur results from the additive effectsof H₂S and SO₂ dissolution reactions. The model reproduces wellthe minimum of sulphur solubility at around NNO+1, in additionto the variation of the sulphide to sulphate ratio with fO₂.A simple empirical model of sulphur solubility in rhyoliticmelts is derived, and shows good correspondence between modeland observations for high-silica rhyolites. KEY WORDS: sulphur; solubility; rhyolite; thermodynamics; fO₂; fS₂