Sediment Melts at Sub-arc Depths: an Experimental Study

The phase and melting relations in subducted pelites have beeninvestigated experimentally at conditions relevant for slabsat sub-arc depths (T = 600–1050°C, P = 2·5–4·5GPa). The fluid-present experiments produced a dominant paragenesisconsisting of garnet–phengite–clinopyroxene–coesite–kyanitethat coexists with a fluid phase at run conditions. Garnet containsdetectable amounts of Na₂O (up to 0·5 wt%), P₂O₅ (upto 0·56 wt%), and TiO₂ (up to 0·9 wt%) in allexperiments. Phengite is stable up to 1000°C at 4·5GPa and is characterized by high TiO₂ contents of up to 2 wt%.The solidus has been determined at 700°C, 2·5 GPaand is situated between 700 and 750°C at 3·5 GPa.At 800°C, 4·5 GPa glass was present in the experiments,indicating that at such conditions a hydrous melt is stable.In contrast, at 700°C, 3·5 and 4·5 GPa, asolute-rich, non-quenchable aqueous fluid was present. Thisindicates that the solidus is steeply sloping in P–T space.Fluid-present (vapour undersaturated) partial melting of thepelites occurs according to a generalized reaction phengite+ omphacite + coesite + fluid = melt + garnet. The H₂O contentof the produced melt decreases with increasing temperature.The K₂O content of the melt is buffered by phengite and increaseswith increasing temperature from 2·5 to 10 wt%, whereasNa₂O decreases from 7 to 2·3 wt%. Hence, the melt compositionschange from trondhjemitic to granitic with increasing temperature.The K₂O/H₂O increases strongly as a function of temperatureand nature of the fluid phase. It is 0·0004–0·002in the aqueous fluid, and then increases gradually from about0·1 at 750–800°C to about 1 at 1000°C inthe hydrous melt. This provides evidence that hydrous meltsare needed for efficient extraction of K and other large ionlithophile elements from subducted sediments. Primitive subduction-relatedmagmas typically have K₂O/H₂O of 0·1–0·4,indicating that hydrous melts rather than aqueous fluids areresponsible for large ion lithophile element transfer in subductionzones and that top-slab temperatures at sub-arc depths are likelyto be 700–900°C. KEY WORDS: experimental petrology; pelite; subduction; UHP metamorphism; fluid; LILE     

Crystallization of Chromite and Chromium Solubility in Basaltic Melts

The equilibrium between chromite and melt has been determinedon four basalts at temperatures of 1200–1400?C over arange of oxygen fugacity (f_o2) and pressures of 1 atm and 10kb. The Cr content of chromite-saturated melts at 1300?C and1 atm ranges from 0?05 wt.% Cr₂O₃ at a log f_o2= –3 to1?4 wt.% at a log f_o2=–12?8. The Cr²⁺/Cr³⁺ of melt increaseswith decreasing f_o2 and is estimated by assuming a constantpartitioning of Cr³⁺ between chromite and melt at constant temperature.The estimated values of Cr²⁺/Cr³⁺ in the melt are at f_o2 valuesof 4–5 orders of magnitude lower than the equivalent Fe²⁺/Fe³⁺values. The Cr/(Cr+Al) of chromite coexisting with melt at constanttemperature changes little with variation of f_o2 below log f_o2=–6.Five experiments at 10 kb indicate that Cr₂O₃ dissolved in themelt is slightly higher and the Cr/(Cr + Al) of coexisting chromiteis slightly lower than experiments at 1 atm pressure. Thus variationin total pressure cannot explain the large variations of Cr/(Cr+ Al) that are common to mid-ocean ridge basalt (MORB) chromite. Experiments on a MORB at 1 atm at f_o2 values close to fayalite-magnetite-quartz(FMQ) buffer showed that the Al₂O₃ content of melt is highlysensitive to the crystallization or melting of plagioclase,and consequently coexisting chromite shows a large change inCr/(Cr + Al). It would appear, therefore, that mixing of a MORBmagma containing plagioclase with a hotter MORB magma undersaturatedin plagioclase may give rise to the large range of Cr/(Cr +Al) observed in some MORB chromite.     

Dissolution of Quartz, Albite, and Orthoclase in H2O-Saturated Haplogranitic Melt at 800{degrees}C and 200 MPa: Diffusive Transport Properties of Granitic Melts at Crustal Anatectic Conditions

We have conducted experiments on dissolution of quartz, albite,orthoclase, and corundum into H₂O-saturated haplogranite meltat 800°C and 200 MPa over a duration of 120–1488 hwith the aim of ascertaining the diffusive transport propertiesof granitic melts at crustal anatectic temperatures. Cylindersof anhydrous starting glass and a single mineral phase (quartzor feldspar) were juxtaposed along flat and polished surfacesinside gold or platinum capsules with 10 wt % added H₂O. Concentrationprofiles in glass (quenched melt) perpendicular to the mineral–glassinterfaces and comparison with relevant phase diagrams suggestthat melts at the interface are saturated in the dissolvingphases after 384 h, and with longer durations the concentrationprofiles are controlled only by diffusion of components in themelt. The evolution of the concentration profiles with timeindicates that uncoupled diffusion in the melt takes place alongthe following four linearly independent directions in oxidecomposition space: SiO₂, Na₂O, and K₂O axes (Si-, Na-, and K-eigenvectors,respectively), and a direction between the Al₂O₃, Na₂O, andK₂O axes (Al-eigenvector), such that the Al/Na molar ratio isequal to that of the bulk melt and the Al/(Na + K) molar ratiois equal to the equilibrium ASI (= mol. Al₂O₃/[Na₂O + K₂O])of the melt. Experiments in which a glass cylinder was sandwichedbetween two mineral cylinders—quartz and albite, quartzand K-feldspar, or albite and corundum—tested the validityof the inferred directions of uncoupled diffusion and exploredlong-range chemical communication in the melt via chemical potentialgradients. The application of available solutions to the diffusionequations for the experimental quartz and feldspar dissolutiondata provides diffusivities along the directions of the Si-eigenvectorand Al-eigenvector of (2·0–2·8) x 10^–15m²/s and (0·6–2·4) x 10^–14 m²/s, respectively.Minimum diffusivities of alkalis [(3–9) x 10^–11m²/s] are orders of magnitude greater than the tetrahedral componentsof the melt. The information provided here determines the rateat which crustal anatexis can occur when sufficient heat issupplied and diffusion is the only mass transport (mixing) processin the melt. The calculated diffusivities imply that a quartzo-feldspathicsource rock with initial grain size of 2–3 mm undergoinghydrostatic, H₂O-saturated melting at 800°C (infinite heatsupply) could produce 20–30 vol. % of homogeneous meltin less than 1–10 years. Slower diffusion in H₂O-undersaturatedmelts will increase this time frame. KEY WORDS: chemical diffusion; haplogranite; mineral dissolution experiments; crustal anatexis     

Peralkalinity, Al{rightleftharpoons}Si Substitution, and Solubility Mechanisms of H2O in Aluminosilicate Melts

The solubility mechanisms of H₂O in peralkaline sodium aluminosilicatequenched melts (anhydrous NBO/T = 0.5) have been studied withRaman spectroscopy as a function of Al/(Al + Si) (0–0–3)and H²O content (0–7.5 wt.%). The coexisting structuralunits in the anhydrous quenched melts are TO2 (Q4), T2O5(Q3),and TO3 (Q2). In Al-free Na₂Si₄O₉ (NS4) melt, H₂O forms complexes with Na⁺(Na–OH bonds) and with Si⁴⁺ (Si–OH bonds). MolecularH₂O is also detected. TO3 structural units are not detectedin this composition. In the H₂O concentration range between0 and 4 wt.%, there is an approximately 20% increase in NBO/Tresulting from the increased abundance ratio, T₂O₅/TO2. Withfurther increments in water activity, the NBO/T of hydrous NS4melt is reduced. The depolymerization results from hydroxylationof the silica tetrahedra, whereas polymerization is due to formationof complexes with Na–OH bonding. In Al-bearing compositions on the Na₂Si₄O₉–Na₂(NaAl)₄O₉–join, there is evidence for Al–OH bonding in additionto Na–OH and Si–OH bonds. Among these complexes,the relative abundance of those with Si–OH bonds diminisheswith increasing Al/(A1 + Si), whereas complexes with Al–OHand Na–OH bonds become more important. Complexes withNa–OH bonds dominate for H₂O4 wt.%, whereas complexeswith Al–OH dominate at higher water content. The threestructural units, TO₃, T₂O₅, and TO², were observed in bothanhydrous and hydrous peralkaline sodium aluminosilicate melts.Their abundance varies, however, with the H₂O concentrationin the melts. The NBO/T decreases to a minimum (a 30–50%lowering of NBO/T relative to anhydrous materials) for low H₂Ocontents (3–4 wt.% H₂O), and increases as the H₂O contentis increased further.     

Calculation of Phase Relations Involving Haplogranitic Melts Using an Internally Consistent Thermodynamic Dataset

A simple thermodynamic model is developed for silicate meltsin the system CaO–Na₂O–K₂O–Al₂O₃–SiO₂–H₂O(CNKASH). The Holland & Powell (Journal of Metamorphic Geology,16, 289–302, 1998) internally consistent thermodynamicdataset is extended via the incorporation of the experimentallydetermined melting relationships in unary and binary subsystemsof CNKASH. The predictive capability of the model is evaluatedvia the experimental data in ternary and quaternary subsystems.The resulting dataset, with the software THERMOCALC, is thenused to calculate melting relationships for haplogranitic compositions.Predictions of the P–T stabilities of assemblages in water-saturatedand -undersaturated bulk compositions are illustrated. It isnow possible to make useful calculations of the melting behaviourof appropriate composition rocks under crustal conditions. KEY WORDS: thermodynamics; melts; granite; dataset     

The Distribution of H2O between Cordierite and Granitic Melt: H2O Incorporation in Cordierite and its Application to High-grade Metamorphism and Crustal Anatexis

Experiments defining the distribution of H₂O [D_w = wt % H₂O(melt)/wt% H₂O(crd)]) between granitic melt and coexisting cordieriteover a range of melt H₂O contents from saturated (i.e. coexistingcordierite + melt + vapour) to highly undersaturated (cordierite+ melt) have been conducted at 3–7 kbar and 800–1000°C.H₂O contents in cordierites and granitic melts were determinedusing secondary ion mass spectrometry (SIMS). For H₂O vapour-saturatedconditions D_w ranges from 4·3 to 7 and increases withrising temperature. When the system is volatile undersaturatedD_w decreases to minimum values of 2·6–5·0at moderate to low cordierite H₂O contents (0·6–1·1wt %). At very low aH₂O, cordierite contains less than 0·2–0·3wt % H₂O and D_w increases sharply. The D_w results are consistentwith melt H₂O solubility models in which aH₂O is proportionalto X_w² (where X_w is the mole fraction of H₂O in eight-oxygenunit melt) at X_w     

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     

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     

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     

Experimental Petrology of the 1991-1995 Unzen Dacite, Japan. Part II: Cl/OH Partitioning between Hornblende and Melt and its Implications for the Origin of Oscillatory Zoning of Hornblende Phenocrysts

High-temperature–pressure experiments were carried outto determine the chlorine–hydroxyl exchange partitioncoefficient between hornblende and melt in the 1992 Unzen dacite.Cl in hornblende and melt was analyzed by electron microprobe,whereas OH in hornblende and melt was calculated assuming anionstoichiometry of hornblende and utilizing the dissociation reactionconstant for H₂O + O = 2(OH) in water-saturated melt, respectively.The partition coefficient strongly depends on the Mg/(Mg + Fe)ratio of hornblende, and is expressed as ln K₁ = (Cl/OH)_hb/(Cl/OH)_melt= 2·37 – 4·6[Mg/(Mg + Fe)]_hb at 2–3kbar and 800–850°C. The twofold variation in Cl contentin the oscillatory zoned cores of hornblende phenocrysts inthe 1991–1995 dacite cannot be explained by the dependenceof the Cl/OH partition coefficient on the Mg/(Mg + Fe)_hb ratio,and requires c. 80% variation of the Cl/OH ratio of the coexistingmelt. Available experimental data at 200 MPa on Cl/OH fractionationbetween fluid and melt suggest that c. 1·2–1·8wt % degassing of water from the magma can explain the required80% variation in the Cl/OH ratio of the melt. The negative correlationbetween Al content and Mg/(Mg + Fe) ratio in the oscillatoryzoned cores of the hornblende phenocrysts is consistent withrepeated influx and convective degassing of the fluid phasein the magma chamber. KEY WORDS: chlorine; element partitioning; hornblende; oscillatory zoning; Unzen volcano     

Coexisting High- and Low-Calcium Melts Identified by Mineral and Melt Inclusion Studies of a Subduction-Influenced Syn-collisional Magma from South Sulawesi, Indonesia

Mineral and melt inclusions in olivines from the most Mg-richmagma from the southern West Sulawesi Volcanic Province indicatethat two distinct melts contributed to its petrogenesis. Thecontribution that dominates the whole-rock composition comesfrom a liquid with high CaO (up to 16 wt %) and low Al₂O₃ contents(CaO/Al₂O₃ up to 1), in equilibrium with spinel, olivine (Fo_85–91;CaO 0·35–0·5 wt %; NiO 0·2–0·30wt %) and clinopyroxene. The other component is richer in SiO₂(>50 wt %) and Al₂O₃ (19–21 wt %), but contains significantlyless CaO (<4 wt %); it is in equilibrium with Cr-rich spinelwith a low TiO₂ content, olivine with low CaO and high NiO content(Fo_90–94; CaO 0·05–0·20 wt %; NiO0·35–0·5 wt %), and orthopyroxene. Boththe high- and low-CaO melts are potassium-rich (>3 wt % K₂O).The high-CaO melt has a normalized trace element pattern thatis typical for subduction-related volcanic rocks, with negativeTa–Nb and Ti anomalies, positive K, Pb and Sr anomalies,and a relatively flat heavy rare earth element (HREE) pattern.The low-CaO melt shows Y and HREE depletion (Gd_n/Yb_n 41), butits trace element pattern resembles that of the whole-rock andhigh-CaO melt in other respects, suggesting only small distinctionsin source areas between the two components. We propose thatthe depth of melting and the dominance of H₂O- or CO₂-bearingfluids were the main controls on generating these contrastingmagmas in a syn-collisional environment. The composition ofthe low-CaO magma does not have any obvious rock equivalent,and it is possible that this type of magma does not easily reachthe Earth's surface without the assistance of a water-poor carriermagma. KEY WORDS: melt inclusions; mineral chemistry; olivine; syn-collisional magmatism; ankaramites; low-Ca magma     

Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific)

Macquarie Island is an exposure above sea-level of part of thecrest of the Macquarie Ridge. The ridge marks the Australia–Pacificplate boundary south of New Zealand, where the plate boundaryhas evolved progressively since Eocene times from an oceanicspreading system into a system of long transform faults linkedby short spreading segments, and currently into a right-lateralstrike-slip plate boundary. The rocks of Macquarie Island wereformed during spreading at this plate boundary in Miocene times,and include intrusive rocks (mantle and cumulate peridotites,gabbros, sheeted dolerite dyke complexes), volcanic rocks (N-to E-MORB pillow lavas, picrites, breccias, hyaloclastites),and associated sediments. A set of Macquarie Island basalticglasses has been analysed by electron microprobe for major elements,S, Cl and F; by Fourier transform infrared spectroscopy forH₂O; by laser ablation–inductively coupled plasma massspectrometry for trace elements; and by secondary ion mass spectrometryfor Sr, Nd and Pb isotopes. An outstanding compositional featureof the data set (47·4–51·1 wt % SiO₂, 5·65–8·75wt % MgO) is the broad range of K₂O (0·1–1·8wt %) and the strong positive covariation of K₂O with otherincompatible minor and trace elements (e.g. TiO₂ 0·97–2·1%;Na₂O 2·4–4·3%; P₂O₅ 0·08–0·7%;H₂O 0·25–1·5%; La 4·3–46·6ppm). The extent of enrichment in incompatible elements in glassescorrelates positively with isotopic ratios of Sr (⁸⁷Sr/⁸⁶Sr= 0·70255–0·70275) and Pb (²⁰⁶Pb/²⁰⁴Pb =18·951–19·493; ²⁰⁷Pb/²⁰⁴Pb = 15·528–15·589;²⁰⁸Pb/²⁰⁴Pb = 38·523–38·979), and negativelywith Nd (¹⁴³Nd/¹⁴⁴Nd = 0·51310–0·51304).Macquarie Island basaltic glasses are divided into two compositionalgroups according to their mg-number–K₂O relationships.Near-primitive basaltic glasses (Group I) have the highest mg-number(63–69), and high Al₂O₃ and CaO contents at a given K₂Ocontent, and carry microphenocrysts of primitive olivine (Fo_86–89·5).Their bulk compositions are used to calculate primary melt compositionsin equilibrium with the most magnesian Macquarie Island olivines(Fo_90·5). Fractionated, Group II, basaltic glasses aresaturated with olivine + plagioclase ± clinopyroxene,and have lower mg-number (57–67), and relatively low Al₂O₃and CaO contents. Group I glasses define a seriate variationwithin the compositional spectrum of MORB, and extend the compositionalrange from N-MORB compositions to enriched compositions thatrepresent a new primitive enriched MORB end-member. Comparedwith N-MORB, this new end-member is characterized by relativelylow contents of MgO, FeO, SiO₂ and CaO, coupled with high contentsof Al₂O₃, TiO₂, Na₂O, P₂O₅, K₂O and incompatible trace elements,and has the most radiogenic Sr and Pb regional isotope composition.These unusual melt compositions could have been generated bylow-degree partial melting of an enriched mantle peridotitesource, and were erupted without significant mixing with commonN-MORB magmas. The mantle in the Macquarie Island region musthave been enriched and heterogeneous on a very fine scale. Wesuggest that the mantle enrichment implicated in this studyis more likely to be a regional signature that is shared bythe Balleny Islands magmatism than directly related to the hypotheticalBalleny plume itself. KEY WORDS: mid-ocean ridge basalts; Macquarie Island; glass; petrology; geochemistry     

The Sulfide Capacity and the Sulfur Content at Sulfide Saturation of Silicate Melts at 1400{degrees}C and 1 bar

The solubility of sulfur as S^2– has been experimentallydetermined for 19 silicate melt compositions in the system CaO–MgO–Al₂O₃–SiO₂(CMAS)± TiO₂ ± FeO, at 1400°C and 1 bar, using CO–CO₂–SO₂gas mixtures to vary oxygen fugacity (fO₂) and sulfur fugacity(fS₂). For all compositions, the S solubility is confirmed tobe proportional to (fS₂/fO₂)^1/2, allowing the definition ofthe sulfide capacity (C_S) of a silicate melt as C_S = [S](fO₂/fS₂)^1/2.Additional experiments covering over 150 melt compositions,including some with Na and K, were then used to determine C_Sas a function of melt composition at 1400°C. The resultswere fitted to the equation     

Aluminum Enrichment in Silicate Melts by Fractional Crystallization: Some Mineralogic and Petrographic Constraints

The degree of aluminum saturation of an igneous rock may bedescribed by its Aluminum Saturation Index (ASI) defined asthe molar ratio Al₂O₃(CaO + K₂O + Na₂O). One suggested originfor mildly peraluminous granites (ASI between 1 and about 1.1)is by fractional crystallization of subaluminous (ASI < 1)magmas; hornblende, having ASI < 0.5, could be a major drivingforce in such a fractionation process. The efficacy of the processdepends not only on precipitation of hornblende and its effectiveremoval from the reacting system, but on the composition andnature of other coprecipitating phases, weighted by their modalabundances in the reactive system. Precipitation of feldspar(ASI = 1), for instance, would retard or even prevent aluminumenrichment in the melt if the ASI of melt is < 1, but wouldenhance such evolution if the ASI of the melt is > 1. Discussionof the efficacy of any mineral must be made in the context ofthe total reacting system. For hornblende to effectively cause a melt to evolve into aperaluminous composition, it must be able to coexist with peraluminousmagmas. Experimental phase equilibrium data show that at pressure> 5 kb hornblende can coexist with strongly peraluminousmelts (ASI {small tilde} 1.5). Scantily phyric volcanic rocksshow that hornblende can coexist with granitic magma havingASI {small tilde} 1.1 –1.2. The aggregate ASI of last-stageminerals of a typical granite is less than this value; therefore,even after hornblende has reacted out, the residual magma maybe expected to continue to evolve toward more aluminous compositions. Potentials and problems of using coarse-grained granitic rocksto probe courses of magmatic evolution are illustrated by asuite of samples from the Grayling Lake pluton of southwesternMontana. Such rocks probably always contain a large cumulatecomponent in their texture and should not be used as primarymeans to test the occurrence or efficacy of a fractionationprocess that might lead to peraluminous melts. The process isunlikely to give rise to peraluminous plutons of batholithicdimensions. A simple differential equation is presented thatallows the direct use of petrographic data (mineral chemistryand modal abundance) to predict the path of incremental evolutionof a given magma.     

Melting of Biotite + Plagioclase + Quartz Gneisses: the Role of H2O in the Stability of Amphibole

Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·38–0·40),42 vol. % plagioclase (An_26–29), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 2–4 wt % H₂O. This reflectsthe fact that conditions of low aH₂O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH₂O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt     

Immiscible Transition from Carbonate-rich to Silicate-rich Melts in the 3 GPa Melting Interval of Eclogite + CO2 and Genesis of Silica-undersaturated Ocean Island Lavas

We explore the partial melting behavior of a carbonated silica-deficienteclogite (SLEC1; 5 wt % CO₂) from experiments at 3 GPa and comparethe compositions of partial melts with those of alkalic andhighly alkalic oceanic island basalts (OIBs). The solidus islocated at 1050–1075 °C and the liquidus at 1415 °C.The sub-solidus assemblage consists of clinopyroxene, garnet,ilmenite, and calcio-dolomitic solid solution and the near solidusmelt is carbonatitic (<2 wt % SiO₂, <1 wt % Al₂O₃, and<0·1 wt % TiO₂). Beginning at 1225 °C, a stronglysilica-undersaturated silicate melt (34–43 wt % SiO₂)with high TiO₂ (up to 19 wt %) coexists with carbonate-richmelt (<5 wt % SiO₂). The first appearance of carbonated silicatemelt is 100 °C cooler than the expected solidus of CO₂-freeeclogite. In contrast to the continuous transition from carbonateto silicate melts observed experimentally in peridotite + CO₂systems, carbonate and silicate melt coexist over a wide temperatureinterval for partial melting of SLEC1 carbonated eclogite at3 GPa. Silicate melts generated from SLEC1, especially at highmelt fraction (>20 wt %), may be plausible sources or contributingcomponents to melilitites and melilititic nephelinites fromoceanic provinces, as they have strong compositional similaritiesincluding their SiO₂, FeO^*, MgO, CaO, TiO₂ and Na₂O contents,and CaO/Al₂O₃ ratios. Carbonated silicate partial melts fromeclogite may also contribute to less extreme alkalic OIB, asthese lavas have a number of compositional attributes, suchas high TiO₂ and FeO^* and low Al₂O₃, that have not been observedfrom partial melting of peridotite ± CO₂. In upwellingmantle, formation of carbonatite and silicate melts from eclogiteand peridotite source lithologies occurs over a wide range ofdepths, producing significant opportunities for metasomatictransfer and implantation of melts. KEY WORDS: carbonated eclogite; experimental phase equilibria; partial melting; liquid immiscibility; ocean island basalts     

Phase Relations of Peralkaline Silicic Magmas and Petrogenetic Implications

The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO₂relative to the Ni–NiO solid buffer), between 800 and660°C and for melt H₂O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckite–arfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H₂O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H₂O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 50–60°C,and that of amphibole by 30°C. Riebeckite–arfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40–100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO₂ lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H₂O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H₂O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO₂ at or below the fayalite–quartz–magnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H₂O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria     

The Composition of Near-solidus Partial Melts of Fertile Peridotite at 1 and 1{middle dot}5 GPa: Implications for the Petrogenesis of MORB

We have determined the near-solidus melt compositions for peridotiteMM-3, a suitable composition for the production of mid-oceanridge basalt (MORB) by decompression partial melting, at 1 and1·5 GPa. At 1 GPa the MM-3 composition has a subsolidusplagioclase-bearing spinel lherzolite assemblage, and a solidusat 1270°C. At only 5°C above the solidus, 4% meltis present as a result of almost complete melting of plagioclase.This melting behaviour in plagioclase lherzolite is predictedfrom simple systems and previous experimental work. The persistenceof plagioclase to > 0·8 GPa is strongly dependenton bulk-rock CaO/Na₂O and normative plagioclase content in theperidotite. At 1·5 GPa the MM-3 composition has a subsolidusspinel lherzolite assemblage, and a solidus at 1350°C.We have determined a near-solidus melt composition at 2% meltingwithin 10°C of the solidus. Near-solidus melts at both 1and 1·5 GPa are nepheline normative, and have low normativediopside contents; also they have the highest TiO₂, Al₂O₃ andNa₂O, and the lowest FeO and Cr₂O₃ contents compared with higherdegree partial melts. Comparison of these near-solidus meltswith primitive MORB glasses, which lie in the olivine-only fieldof crystallization at low pressure, indicate that petrogeneticmodels involving aggregation of near-fractional melts formedduring melting at pressures of 1·5 GPa or less are unlikelyto be correct. In this study we use an experimental approachthat utilizes sintered oxide mix starting materials and peridotitereaction experiments. We also examine some recent studies usingan alternative approach of melt migration into, and entrapmentwithin ‘melt traps’ (olivine, diamond, vitreouscarbon) and discuss optimal procedures for this method. KEY WORDS: experimental petrology; mantle melting; near-solidus; fertile peridotite; MORB     

The Formation of Chrysoberyl in Metamorphosed Pegmatites

Mineral assemblages in pegmatite samples from Kolsva, Swedenand Marikov, Czechoslovakia show that chrysoberyl is alwaysaccompanied by quartz, and is a breakdown product of primarypegmatitic beryl. Textures and the mineral-forming process forthe Kolsva pegmatite are explained by the reactions beryl +K-feldspar + H+ = chrysoberyl + quartz + SiO_{2, aq} + K⁺ + H₂Oor alternatively beryl —K—feldspar + H₂O = chrysoberyl+ quartz + melt. Mineral assemblages from mica-rich parts ofthe pegmatite include sillimanite—K—feldspar, muscovite—K—feldspar—sillimanite,and annite—magnetite—spinel—sillimanite—garnet.Details about the composition and the textural relationshipsof these minerals are given; they indicate a post-pegmatiticmetamorphic event at P—T conditions near to the anatecticregime. The samples from Marikov show textures, which are explainedby the reactions beryl + albite + H⁺ = chrysoberyl + quartz+ Na⁺ + H₂O or alternatively beryl + albite + H₂O = chrysoberyl+ quartz + melt. Breakdown of muscovite produces sillimaniteaccording to the reactions beryl + albite + muscovite + H⁺ =chrysoberyl + quartz + sillimanite + Na⁺ + K⁺ + H₂O or alternativelyberyl + albite + muscovite + H₂O = chrysoberyl + quartz + sillimanite+ melt. Similar reaction textures and mineral assemblages were foundin other chrysoberyl-bearing pegmatites (Maroankora, Madagascar;Helsinki, Finland; Haddam, Greenfield, Greenwood, U.S.A.). Hydrothermal experiments located the reaction beryl + alkalifeldspar + H₂O = chrysoberyl + phenakite + melt at P—Tconditions between the K—feldspar—quartz—H₂Osolidus and the K—feldspar—albite—quartz-H₂Osolidus. It is concluded that the formation of Al-rich minerals likechrysoberyl and sillimanite in pegmatites is due to a post-pegmatiticevent at high P—T conditions. The question as to whichof the alternative set of reactions is more likely, the ionicequilibria or the anatectic chrysoberyl formation, must be leftopen. The previous hypothesis of a desilification of a pegmatitewhich intruded into SiO₂-poor country rocks, or of the assimilationof Al₂O₃-rich country rocks, cannot explain the mineral assemblagesof the two pegmatites.     

Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts

Olivine + clinopyroxene ± amphibole cumulates have beenwidely documented in island arc settings and may constitutea significant portion of the lowermost arc crust. Because ofthe low melting temperature of amphibole (1100°C), suchcumulates could melt during intrusion of primary mantle magmas.We have experimentally (piston-cylinder, 0·5–1·0GPa, 1200–1350°C, Pt–graphite capsules) investigatedthe melting behaviour of a model amphibole–olivine–clinopyroxenerock, to assess the possible role of such cumulates in islandarc magma genesis. Initial melts are controlled by pargasiticamphibole breakdown, are strongly nepheline-normative and areAl₂O₃-rich. With increasing melt fraction (T > 1190°Cat 1·0 GPa), the melts become ultra-calcic while remainingstrongly nepheline-normative, and are saturated with olivineand clinopyroxene. The experimental melts have strong compositionalsimilarities to natural nepheline-normative ultra-calcic meltinclusions and lavas exclusively found in arc settings. Theexperimentally derived phase relations show that such naturalmelt compositions originate by melting according to the reactionamphibole + clinopyroxene = melt + olivine in the arc crust.Pargasitic amphibole is the key phase in this process, as itlowers melting temperatures and imposes the nepheline-normativesignature. Ultra-calcic nepheline-normative melt inclusionsare tracers of magma–rock interaction (assimilative recycling)in the arc crust. KEY WORDS: experimental melting; subduction zone; ultra-calcic melts; wehrlite