Partial fusion of basic granulites at 5 to 15 kbar: implications for the origin of TTG magmas

Partial fusion experiments with basic granulites (S6, S37) believed to represent the lower crust beneath the Eifel region (Germany) were performed at pressures from 5 to 15 kbar. Water-undersaturated experiments were carried out in the presence of 1 wt% H₂O plus 2.44 or 0.81 wt% CO₂ equivalent to mole fractions of H₂O/(H₂O + CO₂) of 0.5 and 0.75, respectively, of the volatile components added. At temperatures from 850 to 1100 °C the weight proportions of melt range from 7 to 30 %. Melt compositions change from trondhjemitic over tonalitic to dioritic with increasing degree of partial melting. Crystalline residua are plagioclase/pyroxene dominated at 5 kbar to garnet/pyroxene dominated at 15␣kbar. Dehydration melting was studied in granulite S35 similar in composition to S6. The magmatic precursors of the granulite xenoliths used in this study had geochemical characteristics of cumulate gabbro (metagabbro S37) and evolved melts (metabasalts S6, S35), respectively. Melts from granulite S37 match the major element compositions of natural trondhjemites and tonalites. At 5 kbar, their Al₂O₃ is relatively low, similar to tonalites from ophiolites. At 15 kbar, Al₂O₃ in the melts is high due to the near absence of plagioclase in the crystalline residua. The Al₂O₃ concentrations in 15 kbar melts from S6 (˜20 wt%) are higher than in natural tonalites. Depth constraints on the formation of tonalitic magmas in the continental crust are provided by REE (rare earth element) patterns of the synthetic melts calculated from the known REE abundances in metagabbro S37 and metabasalt S6 assuming batch melting and using partition coefficients from the literature. The REE patterns of tonalites from active continental margins and Archean trondhjemite-tonalite-granodiorite␣associations low in REE with La_N (chondrite normalised) from 10 to 30 and Yb_N from 1 to 2 are reproduced at pressures of 10 and 12.5 kbar from metagabbro S37 which displays a slightly L(light)REE enriched pattern with La_N = 8 and Yb_N = 3. Natural tonalites with La_N from 30 to 100 require a source richer in REE than granulite S37. At 15 kbar, H(heavy)REE_N in melts from granulite S37 are depressed below the level observed in natural tonalites due to the high proportion of garnet (>30 wt%) in the residue. Melts from metabasalt S6 (enriched in REE with La_N = 38 and Yb_N = 16) do not match the REE characteristics of natural tonalites under any conditions. Received: 1 July 1994 / Accepted: 11 September 1996     

Metamorphic equilibria in the siliceous dolomite system: 6 kbar experimental data and geologic implications

Hydrothermal experiments with H₂O-CO₂ fluids at P_fluid = 6 kbar yielded the following quilibrium conditions for reactions important in metamorphosed siliceous dolomites (T = °C; X = X_co2): (3) dolomite + 2 quartz = diopside + 2 CO₂T = 620 ± 8X = 0.73 ± 0.03 (5) 5 dolomite + 8 quartz + H₂O = tremolite + 3 calcite + 7 CO₂T = 600 ± 5 550 ±5 540±5 500±5X = 0.66 ± 0.03 0.21 ± 0.03 0.21 ± 0.04 0.06 ± 0.02 (7) 3 dolomite + 4 quartz + H₂O = talc + 3 calcite + 3 CO₂T = 550±5 500±5 450 ±5X = 0.25 ± 0.05 0.07 ± 0.02 0.03 ± 0.02 (8) 2 dolomite + talc + 4 quartz = tremolite + 4 CO₂T = 550 ± 5 540 ±5 500 ± 5X = 0.22 ± 0.03 0.21 ± 0.02 0.06 ± 0.02 A thermodynamically self-consistent 6 kbar T-X_CO2, topology results by extrapolating equilibria from experimental brackets using a modified Redlich-Kwong equation for activities in H₂O-CO₂ mixtures. This topology restricts the assemblage talc + calcite to a narrow stability band in T-X_CO2 space at X_CO2 < 0.55 and T < 590°C. Accordingly, the occurrence of talc + calcite in pure siliceous dolomites metamorphosed at P_fluid = 6 kbar implies correspondingly water-rich fluids.     

An experimental study of partitioning of fluorine between K-richterite,apatite, phlogopite,and melt at 20 kbar

Phase relations have been determined at 20 kbar and primarily under suprasolidus conditions in the Fe−Ti-free F-bearing K-richterite—phlogopite and K-richterite—apatite systems in order to assess the partitioning of F among phlogopite, K-richterite, apatite, and melt under upper-mantle conditions. Both systems are pseudoternary because they contain forsterite, enstatite and a diopside-rich clinopyroxene from the breakdown of the mica and K-richterite. The F-bearing K-richterite systems have lower minimum melting temperatures than the F-bearing phlogopite —apatite system at the same pressure. However in the systems studied, F in phlogopite appears the most effective component in altering minimum liquid compositions whereas comparison between the present study and previous systems suggests that the presence of P₂O₅ during melting may result in more K-enriched melts. Variations in the compositions of the F-bearing phases are primarily controlled by the bulk compositions of the end-member minerals and by temperature, although buffering by non-F bearing minerals (e.g. clinopyroxene) may be effective. Distribution coefficients (as wt% ratios) between F-bearing minerals and coexisting liquids have been determined as functions of bulk composition and temperature for products of experiments. Distribution coefficients between K-richterite—liquid, apatite—liquid, and phlogopite—liquid are ≥1 to slightly <1 for most bulk compositions, indicating thatF is generally a compatible element. This conclusion is in agreement with the sequence ofF distribution for similar phases in ultrapotassic rocks. These results preclude F-bearing mineral reservoirs in the mantle, at depths corresponding to 20 kbar, being capable of producing F-enrichment in ultrapotassic magmas, or being effective in redox melting processes. Editorial responsibility: K. Hodges     

Alkali exchange equilibria between a silicate melt and coexisting magmatic volatile phase: an experimental study at 800°C and 100 MPa

Many experimental studies have been performed to evaluate the composition of coexisting silicate melts and magmatic volatile phases (MVP). However, few studies have attempted to define the relationship between melt chemistry and the acidity of a chloride-bearing fluid. Here we report data on melt composition as a function of the HCl concentration of coexisting brines. We performed 35 experimental runs with a NaCl-KCl-HCl-H₂O brine (70 wt% NaCl [equivalent])-silicate melt (starting composition of Qtz_0.38Ab_0.33Or_0.29, anhydrous) assemblage at 800°C and 100 MPa. We determined an apparent equilibrium constant

     

Predicting phosphate saturation in silicate magmas: An experimental study of the effects of melt composition and temperature

A series of 1 atm experiments has been performed to test the influence of iron content and oxidation state on the saturation of phosphate minerals in magmatic systems. Four bulk compositions of different iron content have been studied. The experiments cover a range of temperature from 1030 to 1070 °C and oxygen fugacity from 1.5 log units below to 1.5 log units above the Fayalite-Magnetite-Quartz buffer. The results demonstrate that neither iron content of the liquid nor oxidation state play a significant role on phosphate saturation. On the other hand, SiO₂ and CaO contents of the liquid strongly influence the appearance of a crystalline phosphate. Our results are combined with data from the literature to define an equation which predicts the P₂O₅ content of silicate liquids saturated in either whitlockite or fluorapatite:

     

Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites

Peraluminous granitoid magmas are a characteristic product of ultrametamorphism leading to anatexis of aluminous metasedimentary rocks in the continental crust. The mechanisms and characteristic length-scales over which these magmas can be mobilized depend strongly on their melt fraction, because of their high viscosities. Thus, it is of fundamental importance to understand the controls exerted by pressure, temperature and bulk composition of the source material on melt productivity. We have studied experimentally the vapour-absent melting behaviour of a natural metapelitic rock and our results differ greatly from those of previous experimental and theoretical investigations of melt productivity from metamorphic rocks. Under H₂O-undersaturated conditions, bulk composition of the source material is the overriding factor controlling melt fraction at temperatures on the order of 850–900° C. Granitoid melts formed in this temperature interval by the peritectic dehydration-melting reaction: $$\begin{gathered} Biotite + plagioclase + aluminosilicate + quartz \hfill \\ = melt + garnet \hfill \\ \end{gathered} $$ have a restricted compositional range. As a consequence, melt fractions will be maximized from protoliths whose modes coincide with the stoichiometry of the melting reaction. This “optimum mode” (approximately 38% biotite, 32% quartz, 22% plagioclase and 8% aluminosilicate) reflects the fact that generation of low-temperature granitoid liquids requires both fusible quartzo-feldspathic components and H₂O (from hydrous minerals). Metapelitic rocks rich in mica and aluminosilicate and poor in plagioclase contain an excess of refractory material (Al₂O₃, FeO, MgO) with low solubility in low-temperature silicic melts, and will therefore be poor magma sources. Melt fraction varies inversely with pressure in the range 7–13 kbar, but the effect is not strong: the decrease (at constant temperature) over this pressure range is of at most 15 vol% (absolute). The liquids produced in our experiments are silicarich (68–73 wt% SiO₂), strongly peraluminous (2–5 wt% normative corundum) and very felsic (MgO+FeO^* +TiO₂ less than 3 wt%, even at temperatures above 1000° C). The last observation suggests that peraluminous granitoids with more than 10% mafic minerals (biotite, cordierite, garnet) contain some entrained restite. Furthermore, because liquids are also remarkably constant in composition, we believe that restite separation is more important than fractional crystallization in controlling the variability within and among peraluminous granitoids. We present liquidus phase diagrams that allow us to follow the phase relationships of melting of silica-and alumina-saturated rocks at pressures corresponding to the mid- to deep-continental crust. Garnet, aluminosilicate, quartz and ilmenite are the predominant restitic phases at temperatures of about 900° C, but Ti-rich biotite or calcic plagioclase can also be present, depending on the bulk composition of the protolith. At temperatures above 950–1050° C (depending on the pressure) the restitic assemblage is: hercynitic spinel+ilmenite+quartz±aluminosilicate. Our results therefore support the concept that aluminous granulites (garnet-spinel-plagioclase-aluminosilicate-quartz) can be the refractory residuum of anatectic events.     

Dehydration-melting of amphibolite at 10 kbar: the effects of temperature and time

We have simulated the dehydration-melting of a natural, low-K, calcic amphibolite (67.4% hornblende, 32.5% anorthite) in piston-cylinder experiments at 10 kbar and 750–1000°C, for 1–9 days. The solidus temperature is lower than 750°C; garnet appears at 850°C. The overall reaction is: Hb+PL+Cpx+Al-Hb+Ca-Hb+Ga+Opx. Three stages of reaction are: (1) melting dominated by the growth of clinopyroxene and garnet, with little change in composition of liquid or garnet, (2) a reversal of this reaction between 875°C and 900°C, with decreases in the amounts of liquid and garnet, and (3) a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase. Garnet is enriched in pyrope and zoned from Fe-cores to Mg-edges (range 3 mol % pyrope); liquid composition is enriched first in An (to 950°C) and then in Ab. The liquids are more calcic and aluminous than natural tonalites, which is attributed to the plagioclase composition (An₉₀). The formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite — not H₂O-saturated conditions. The results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet-absent field (900–1000°C), but with a solidus backbend at 7–9 kbar, coincident with the garnet-in boundary. Hornblende breakdown due to garnet formation in a closed system must make H₂O available for H₂O-undersaturated melting right down to the H₂O-saturated solidus, below 700°C, which defines a large low-temperature PT area where hydrous granitoid melts can be generated with residual garnet and hornblende.     

An analytical and experimental study of zoning in plagioclase

A detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars. Discontinuous, sector, and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends. The relative accuracy of 3 element (Na, Ca, K) microprobe traverses was found statistically to be 2 mole percent. Comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio-chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals.     

Dissolution kinetics of plagioclase in the melt of the system diopside-albite-anorthite,and origin of dusty plagioclase in andesites

The textures and kinetics of reaction between plagioclase and melts have been investigated experimentally, and origin of dusty plagioclase in andesites has been discussed. In the experiments plagioclase of different compositions (An₉₆, An₆₁, An₅₄, An₂₃, and An₂₂) surrounded by glasses of six different compositions in the system diopside-albite-anorthite was heated at temperatures ranging from 1,200 to 1,410° C for 30 min to 88 h. Textures were closely related to temperature and chemical compositions. A crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt (glass) and remained its original euhedral shape below the liquidus. Whatever the temperature, the crystal-melt interface became rough and often more complicated (sieve-like texture composed of plagioclase-melt mixture in the scale of a few m was developed from the surface of the crystal inward; formation of mantled plagioclase) if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt, and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase. From these results the following two types of dissolution have been recognized; (1) a crystal simply dissolves in the melt which is undersaturated with respect to the phase (simple dissolution), and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt (partial dissolution). The amount of a crystal dissolved and reacted increased proportional to the square root of time. This suggests that these processes are controlled by diffusion, probably in the crystal.Mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so-called resorbed plagioclase in igneous rocks. Chemical compositions and textures of plagioclase phenocrysts in island-arc andesites of magma mixing origin have been examined. Cores of clear and dusty plagioclase were clacic (about An₉₀) and sodic (about An₅₀), respectively. This result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing.     

Experimental study on the tremolite-pargasite join at variable temperatures under 10 kbar

The join tremolite (Tr)-pargasite (Pa) was studied at temperatures between 800 and 1150°C under water vapour pressure of 10 kbar. The results show a continuous solid solution of amphibole between the composition Tr₈₀Pa₂₀ and Pa₁₀₀ at 800°C and 10kb. Pargasite melts incongruently and breaks down at high temperature to clinopyroxene+forsterite+spinel+L+V. A single phase amphibole with composition lying between Tr₈₀Pa₂₀ and nearly pure Pa, breaks down to amphibole of different composition plus other phases. The stability fields of amphibole spread toward higher temperature side with increasing pargasite content, and pargasite itself has the widest stability field. At subliquidus, the composition of amphibole coexisting with other phases becomes more pargasitic with increasing temperature. The compositions of liquid, which are formed by partial melting of amphibole of Tr₄₀Pa₆₀ composition (Fo-normative) under water vapour pressure of 10 kbar, are alumina-rich and Qz-normative.     

An experimental study of Ostwald ripening of olivine and plagioclase in silicate melts: implications for the growth and size of crystals in magmas

We carried out an experimental study to characterize the kinetics of Ostwald ripening in the forsterite-basalt system and in the plagioclase (An₆₅)-andesite system. Eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution (CSD) with time t; the experiments were performed in a 1-atmosphere quench furnace, at 1,250°C for plagioclase and 1,300°C for olivine. Very contrasted coarsening kinetics were observed in the two series. In the plagioclase series, the mean grain size increased as log(t), from ≈3 μm to only 8.7 μm in 336 h. The kinetic law in log(t) means that Ostwald ripening was rate-limited by surface nucleation at plagioclase-liquid interfaces. In the olivine series, the mean grain size increased as t ^1/3, from ≈3 μm to 23.2 μm in 496 h. A kinetic law in t ^1/3 is expected when Ostwald ripening is rate-limited either by diffusion in the liquid or by grain growth/dissolution controlled by a screw dislocation mechanism. The shape of olivine CSDs, in particular their positive skewness, indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism, not by diffusion. As the degrees of undercooling ΔT (or supersaturation) involved in Ostwald ripening are essentially <1°C, the mechanisms of crystal growth identified in our experiments are expected to be those prevailing during the slow crystallisation of large magma chambers. We extrapolated our experimental data to geological time scales to estimate the effect of Ostwald ripening on the size of crystals in magmas. In the case of plagioclase, Ostwald ripening is only efficient for mean grain sizes of a few microns to 20 μm, even for a time scale of 10⁵ years. It can, however, result in a significant decrease of the number of small crystals per unit volume, and contribute to the development of convex upwards CSDs. For olivine, the mean grain size increases from 2–3 μm to ≈70 μm in 1 year and 700 μm in 10³ years; a mean grain size of 3 mm is reached in 10⁵ years. Accordingly, the rate of grain size-dependent processes, such as compaction of olivine-rich cumulates or melt extraction from partially molten peridotites, may significantly be enhanced by textural coarsening.     

Trace element partitioning and melt structure: An experimental study at 1 atm pressure

Diopside-melt and forsterite-melt rare earth (REE) and Ni partition coefficients have been determined as a function of bulk compositions of the melt. Available Raman spectroscopic data have been used to determine the structures of the melts coexisting with diopside and forsterite. The compositional dependence of the partition coefficients is then related to the structural changes of the melt.The melts in all experiments have a ratio of nonbridging oxygens to tetrahedral cations ($\text{NBO}\text{T}$) between 1 and 0. The quenched melts consist of structural units that have, on the average, 2 (chain), 1 (sheet) and 0 (three-dimensional network) nonbridging oxygens per tetrahedral cation. The proportions of these structural units in the melts, as well as the overall $\text{NBO}\text{T}$, change as a function of the bulk composition of the melt.It has been found that Ce, Sm, Tm and Ni crystal-liquid partition coefficients ($\text{K}{}^{\mathrm{crystal?liq}}{}_{\mathrm{i}}\text{=}\text{C}{}^{\mathrm{crystal}}{}_{\mathrm{i}}\text{C}{}^{\mathrm{liq}}{}_{\mathrm{i}}$) decrease linearly with increasing $\text{NBO}\text{T}$. The values of the individual REE crystal-liquid trace element partition coefficients have different functional relations to $\text{NBO}\text{T}$, so that the degree of light REE enrichment of the melts would depend on their $\text{NBO}\text{T}$.The solution mechanisms of minor oxides such as CO₂, H₂O, TiO₂, P₂O₅ and Fe₂O₃ in silicate melts are known. These data have been recast as changes of $\text{NBO}\text{T}$ of the melts with regard to the type of oxide and its concentration in the melt. From such data the dependence of crystal-liquid partition coefficients on concentration and type of minor oxide in melt solution has been calculated.     

The origin of plagioclase phenocrysts in basalts from continental monogenetic volcanoes of the Kaikohe-Bay of Islands field,New Zealand: implications for magmatic assembly and ascent

Late Quaternary, porphyritic basalts erupted in the Kaikohe-Bay of Islands area, New Zealand, provide an opportunity to explore the crystallization and ascent history of small volume magmas in an intra-continental monogenetic volcano field. The plagioclase phenocrysts represent a diverse crystal cargo. Most of the crystals have a rim growth that is compositionally similar to groundmass plagioclase (~?An₆₅) and is in equilibrium with the host basalt rock. The rims surround a resorbed core that is either less calcic (~?An_20–45) or more calcic (>?An₇₀), having crystallized in more differentiated or more primitive melts, respectively. The relic cores, particularly those that are less calcic (<?~?An₄₅), have ⁸⁷Sr/⁸⁶Sr ratios that are either mantle-like (~?0.7030) or crustal-like (~?0.7040 to 0.7060), indicating some are antecrysts formed in melts fractionated from plutonic basaltic forerunners, while others are true xenocrysts from greywacke basement and/or Miocene arc volcanics. It is envisaged that intrusive basaltic forerunners produced a zone where various degrees of crustal assimilation and fractional crystallization occurred. The erupted basalts represent mafic recharge of this system, as indicated by the final crystal rim growths around the entrained antecrystic and xenocrystic cargo. The recharge also entrained cognate gabbros that occur as inclusions, and produced mingled groundmasses. Multi-stage magmatic ascent and interaction is indicated, and is consistent with the presence of a partial melt body in the lower crust detected by geophysical methods. This crystallization history contrasts with traditional concepts of low-flux basaltic systems where rapid ascent from the mantle is inferred. From a hazards perspective, the magmatic system inferred here increases the likelihood of detecting eruption precursor phenomena such as seismicity, degassing and surface deformation.     

高温高压条件下闪长岩部分熔融产物的反常现象

为了考察岩石熔融过程中熔体成分的变化规律,以细粒闪长岩为样品,进行了不同熔融程度的实验研究。通过对熔体成分的分析,得出以下结论:在初始熔融条件下,熔体的成分为玄武质,随着熔融程度的增高,熔体的成分由基性向中酸性,由富钾向富钠,由强碱性向亚碱性方向演化。     

Systematics of calcium partitioning between olivine and silicate melt: implications for melt structure and calcium content of magmatic olivines

A systematic characterization of the chemical factors that control calcium partitioning between olivine and melt in a magmatic environment was undertaken using experiments performed on compositionally simple systems (CaO-MgO-SiO₂, CaO-MgO-Al₂O₃-SiO₂, CaO-MgO-Al₂O₃-SiO₂-Cr₂O₃, CaO-MgO-Al₂O₃-SiO₂-TiO₂, CaO-MgO-Al₂O₃-SiO₂-Na₂O, CaO-MgO-Al₂O₃-SiO₂-FeO, CaO-MgO-Al₂O₃-SiO₂-FeO-Na₂O) over a wide range of temperature (1050–1530 °C) at one bar pressure. The calcium concentration of olivines is shown to be dependent not only on the forsterite content of the olivine but to a large extent on melt composition. For a fixed CaO content of the melt, these results show that the CaO concentration of olivine is strongly sensitive to the amount of alumina, alkali and ferrous iron present in the coexisting melt. Oxygen fugacity and temperature are not found directly to affect Ca partitioning. It is thus proposed that the systematic variations of the calcium content of olivine may be used as an “in-situ chemical potentiometer” of the lime activity of the melt. Based upon these data in synthetic systems, an empirical model describing Ca partitioning between olivine and melt is developed. When applied to natural olivines this model reproduces their Ca content, where melt composition is known, to within ±10% relative. The model may therefore be used to predict changes in melt composition during olivine crystallization and/or to assess whether an olivine is in equilibrium with its host magma. Finally, the wide range of Ca partitioning observed at fixed crystal composition confirms that minor element partitioning between crystal and melt cannot be predicted from the physical characteristics of the crystal alone, and that the non-ideality of the melt has to be taken into account. Received: 12 June 1998 / Accepted: 1 February 1999     

Anatexis of interlayered amphibolite and pelite at 10 kbar: effect of diffusion of major components on phase relations and melt fraction

 We have conducted H₂O-undersaturated melting experiments on charges consisting of a layer of a sillimanite-bearing metapelite and a layer of garnet-bearing amphibolite, with the goal of studying how layered metamorphic rocks interact during anatexis. Experiments with the layered charges were performed at 10 kbar (1 GPa) at 900 °C (334 h), 925 °C (341 h), and 950 °C (50, 161, 332 h). Dehydration-melting of the amphibolite alone and of the pelite alone were studied at the same P-T conditions. When run alone at 900 °C, the pelite and the amphibolite yielded ca. 40 vol.% and 15 vol.% melt, respectively. Corresponding melt fractions at 950 °C were ca. 50 vol.% and 30 vol.%. When run side by side, melt abundance in the pelite is fairly uniform and ca. 20 vol.% higher than in the solo runs at all temperatures studied. In contrast, complex zoning patterns parallel to the interface develop in the amphibolite layer. Far from the interface the melt fraction is 5–10 vol.% lower than in the amphibolite-solo runs. Closer to the interface the melt fraction in the amphibolite layers are comparable to, or somewhat higher than, in the amphibolite-solo runs, and both clinopyroxene and plagioclase react-out and are replaced by orthopyroxene and quartz. Melt fraction closest to the interface reaches 65–70 vol.%, and the only crystalline phase present is garnet with hollow cores. The increase in melt fraction in the pelite layers of the layered runs is caused by transfer of Na from the amphibolite to the pelite, which forces biotite to be eliminated at lower temperature along the biotite-garnet reaction boundary. K is transferred from the pelite to the amphibolite, and the lower melt fraction in the distal end of the amphibolite layers is caused by faster diffusion of Na compared to K, which results in a net depletion of alkalies. Ca is transferred from the amphibolite to the pelite, and the resulting changes in CaO concentrations affect the stabilities of plagioclase, pyroxenes, garnet and quartz and cause the observed zoning in the amphibolite layer. Ca entering the pelite layer induces crystallization of large plagioclase neoblasts radiating from the interface into the pelite layer. These results show that diffusion of components between contiguous lithologies undergoing anatexis can cause profound changes in phase relations, melt fractions and restite compositions. Received: 21 December 1994 / Accepted: 26 June 1995     

Anhydrous partial melting of MORB pyrolite and other peridotite compositions at 10 kbar: Implications for the origin of primitive MORB glasses

Summary Anhydrous partial melting experiments on four peridotite compositions have been conducted at 10 kbar providing a relatively internally consistent set of data on the character of primary melts expected from the oceanic upper mantle in the mid-ocean ridge setting. The four peridotite compositions are: MORB pyrolite (considered to be suitable for the production of primitive (Mg#0.68) MORB glasses at 10 kbar), Hawaiian pyrolite (representative of enriched upper mantle), Tinaquillo lherzolite (representative of more depleted upper mantle), and the spinel lherzolite KLB-1 which is a suitable composition for the production of primitive MORB glasses. The equilibrium liquids were determined by sandwich experiments. The primitive MORB glass DSDP 3-18-7-1 was used in experiments using MORB pyrolite and KLB-1, while a calculated 10 kbar liquid composition fromJaques andGreen (1980) was used in experiments with Hawaiian pyrolite and Tinaquillo lherzolite. The results of the experiments are used to test a 10 kbar melt model for the generation of primitive MORB glasses, which are parental magmas to typical MORB compositions. The melt compositions from the four peridotites studied are significantly different from primitive MORB glasses in major element chemistry and plot away from the field of primitive MORB glasses in the CIPW molecular normative Basalt tetrahedron. The results indicate that primitive MORB glasses are derivative compositions lying on olivine fractionation lines from picritic parents, which themselves are primary magmas at pressures greater than 10 kbar. The results of this study are integrated with previous 10 kbar experimental studies.

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Wasserfreie partielle aufschmelzung von MORB pyrolit und andere peridotit-zusammensetzungen bei 10 kbar: bedeutung für die entstehung primitiver MORB gläser

Zusammenfassung Vier Peridotit-Zusammensetzungen wurden bei 10 kbar unter wasserfreien Bedingungen partiell aufgeschmolzen, und die Ergebnisse mit möglichen primitiven Schmelzen Mittel-Ozeanischer Rücken verglichen.Die folgenden perioditischen Zusammensetzungen wurden untersucht: MORB pyrolite [mögliche Ausgangszusammensetzung für primitive (Mg# > 0.68) MORB-Glaszusammensetzungen bei 10 kbar], Hawaiian pyrolite (representativ für angereicherten Oberen Mantel); Tinaquillo lherzolite (representativ für verarmten' Oberen Mantel) und spinel lherzolite, KLB-1 (im Gleichgewicht mit primitiver MORB-Glaszusammensetzung). Die Schmelzen im Gleichgewicht mit diesen Ausgangszusammensetzungen wurden mittels Sandwich-Experimenten ermittelt.Die primitive MORB-Glaszusammensetzung DSDP 3-18-7-1 wurde mit MORB pyrolite und KLB-1 equilibriert, während eine Modell-Zusammensetzung vonJaques and Green (1980) in Verbindung mit Hawaiian pyrolite und Tinaquillo lherzolite vermischt wurde. Die Resultate der Experimente werden mit einem 10 kbar Aufschmelzungsmodell zur Entstehung primitiver MORB-Gläser verglichen. Die Schmelzen im Gleichgewicht mit den vier Peridotit-Ausgangszusammensetzungen unterscheiden sich wesentlich von primitiven MORB-Gläsern, sowohl hinsichtlich ihrer Hauptelemente als auch ihrer Plot-Parameter im Basalttetraeder. Primitive MORB-Glaszusammensetzungen stellen keine primären Schmelzen dar, sondern sind durch Olivinfraktionierung von primitiven Magmen abzuleiten. Die Resultate dieser Untersuchungen werden mit früheren 10 kbar Experimenten verglichen.

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With 10 Figures     

Interaction of granitic and basic magmas: experimental observations on contamination processes at 10 kbar with H2O

Experiments designed to simulate the interaction of juxtaposed rhyolitic and basic magmas were conducted at 10 kbar with H₂O, using reaction-couples consisting of Westerly granite (WG) against basalt (DW-1) and WG against a synthetic mafic glass (SMG, enriched in MgO and Na₂O relative to DW-1). Each couple was run with 5 and 10 wt% H₂O corresponding respectively to H₂O-undersaturated and H₂O-oversaturated conditions. Experiments were run for 42–44 h at 920° C, above the liquidus of WG and within the melting intervals of DW-1 and SMG. WG was run above the basic material in all but one experiment. The composition of the granitic melt was altered only through material exchange with the adjacent basic melts, whereas that of the basic melts also changed (relative to the bulk basic composition) due to partial crystallization. Some crystallization also occurred within the zone of interaction. For control, the basic compositions were also run alone under the same conditions as the reaction-couple experiments. The crystalline phase assemblages in the basic ends of the coupled experiments differed from those produced from the basic materials alone, demonstrating interaction with the granite melt. Moreover, compositional gradients within the basic ends of coupled experiments are indicated by changes in phase assemblage and compositions with distance from the interface with WG. Microprobe analyses of glass collected along the length of the capsules confirm published observations that alkali diffusion is very fast: K₂O and Na₂O homogenized throughout the capsules in less than the two-day run times. This, coupled with the fact that introduction of K₂O into SMG stabilized biotite, produced the result that after interaction the bulk basic material (melt+crystals) contained more K₂O than the coexisting felsic melt. Only very gentle gradients for CaO, FeO, and MgO are preserved in our experiments, in contrast with published anhydrous results, suggesting that the difference in activity coefficients for these components between basic and felsic melts is reduced by the introduction of H₂O. Gradients for SiO₂ and Al₂O₃ are of comparable length to those of the divalent cations, confirming earlier results that the diffusivities of the network-formers limit the rate of diffusion of Ca, Fe, and Mg.     

Evolution of the East Philippine Arc: experimental constraints on magmatic phase relations and adakitic melt formation

Piston-cylinder experiments on a Pleistocene adakite from Mindanao in the Philippines have been used to establish near-liquidus and sub-liquidus phase relationships relevant to conditions in the East Philippines subduction zone. The experimental starting material belongs to a consanguineous suite of adakitic andesites. Experiments were conducted at pressures from 0.5 to 2 GPa and temperatures from 950 to 1,150°C. With 5 wt. % of dissolved H₂O in the starting mix, garnet, clinopyroxene and orthopyroxene are liquidus phases at pressures above 1.5 GPa, whereas clinopyroxene and orthopyroxene are liquidus (or near-liquidus) phases at pressures <1.5 GPa. Although amphibole is not a liquidus phase under any of the conditions examined, it is stable under sub-liquidus conditions at temperature ≤1,050°C and pressures up to 1.5 GPa. When combined with petrographic observations and bulk rock chemical data for the Mindanao adakites, these findings are consistent with polybaric fractionation that initially involved garnet (at pressures >1.5 GPa) and subsequently involved the lower pressure fractionation of amphibole, plagioclase and subordinate clinopyroxene. Thus, the distinctive Y and HREE depletions of the andesitic adakites (which distinguish them from associated non-adakitic andesites) must be established relatively early in the fractionation process. Our experiments show that this early fractionation must have occurred at pressures >1.5 GPa and, thus, deeper than the Mindanao Moho. Published thermal models of the Philippine Sea Plate preclude a direct origin by melting of the subducting ocean crust. Thus, our results favour a model whereby basaltic arc melt underwent high-pressure crystal fractionation while stalled beneath immature arc lithosphere. This produced residual magma of adakitic character which underwent further fractionation at relatively low (i.e. crustal) pressures before being erupted.     

An experimental study of rare earth element partitioning between a shergottite melt and pigeonite: implications for the oxygen fugacity of the mart ian interior

A series of experiments on a synthetic, pigeonite-saturated, basaltic shergottite were carried out to constrain the variation of D(Eu/Gd)_{pigeonite/melt} and D(Eu/Sm)_{pigeonite/melt} with oxygen fugacity (f_O2). The experiments have been run under both dry and hydrous conditions. The shergottite was doped with 0.1, 0.5, and 1.0 wt.% Eu, Gd, and Sm oxides in different experiments and was equilibrated at liquidus conditions for 24 hours. D(Eu/Gd)_{pigeonite/melt} in dry melts ranges from 0.156 ± 0.014 (f_O2 = IW − 1) to 0.630 ± 0.102 (IW + 3.5). D(Eu/Sm)_{pigeonite/melt} in dry melts ranges from 0.279 ± 0.021 (IW − 1) to 1.114 ± 0.072 (IW + 3.5). Due to difficulties with low-f_O2 experiments, hydrous distribution coefficients were measured, but were not used in the calibration of the Eu-oxybarometers. These two Eu-oxybarometers provide an accurate way to measure f_O2 recorded during pigeonite crystallization, thereby yielding a record of f_O2 during the earliest period of Martian meteorite parent magma crystallization history.Using this new calibration, Martian meteorite pigeonite cores record f_O2 values of IW − 0.6 (±0.3) (QUE94201) to IW + 1.9 (±0.6) (Shergotty). These new values differ in magnitude, but not trend, from previously published data. The pigeonite Eu-oxybarometer yields an f_O2 range in the basaltic shergottites of 2 to 3 orders of magnitude. Several processes have been proposed to explain the origin of this f_O2 range, the majority of which rely on assimilation of an oxidized source. A potential correlation between this new pigeonite data and recent Fe-Ti oxide data, however, is consistent with intrinsic f_O2 differences in the magma source region being responsible for the measured f_O2 variations. This implies that the Martian meteorite source region, the mantle or lithosphere, may be heterogeneous in nature. However, the process of assimilation cannot be completely ruled out in that an assimilation event that took place before crystallization commenced would result in the overprinting of the source region f_O2 signature.