Understanding magma evolution at Campi Flegrei (Campania, Italy) volcanic complex using melt inclusions and phase equilibria

The magmatic evolution of two eruptive episodes at Campi Flegrei (Italy) has been investigated using phase equilibria modeling (MELTS) and data from melt inclusions (MIs) in phenocrysts from the Fondo Riccio and Minopoli 1 eruptions. Assuming that isobaric fractional crystallization of a mantle-derived parental magma is the dominant petrogenetic process, major element evolution and corresponding changes in the physical and thermodynamic properties of the magma bodies from which Fondo Riccio and Minopoli1 magmas were erupted can be tracked. Fondo Riccio parental magma was trachyandesitic, approximated by the composition of FR-C1-O2-M1, which evolved mainly through fractional crystallization at low pressure (P?≈?0.15?GPa, ≈ 7?km depth), along the QFM, QFM?+?1 oxygen buffer with an initial dissolved H₂O content of ～3?wt%. Minopoli1 parental magma was trachyandesitic, approximated by the chemistry of Mi1-C1-O5-M1, evolved through fractional crystallization at P?≈?0.3?GPa (≈ 12?km depth), with oxygen fugacity along QFM?+?1buffer and initial H₂O content of?～?2 wt%. The relationship between melt fraction and T reveals for Fondo Riccio the presence of a pseudo-invariant temperature at which the physical properties of melt change abruptly. The net effect of these changes is to drive the system towards dynamic instability, which it is suggested to be the trigger mechanism for the eruptions.     

http://dx.doi.org/10.1016/j.gsf.2016.11.007

Lunar anorthosite is a major rock of the lunar highlands,which formed as a result of plagioclasefloatation in the lunar magma ocean(LMO).Constraints on the sufficient conditions that resulted in the formation of a thick pure anorthosite(mode of plagioclase 95 vol.%) is a key to reveal the early magmatic evolution of the terrestrial planets.To form the pure lunar anorthosite,plagioclase should have separated from the magma ocean with low crystal fraction.Crystal networks of plagioclase and mafic minerals develop when the crystal fraction in the magma(φ) is higher than ca.40-60 vol.%,which inhibit the formation of pure anorthosite.In contrast,when φ is small,the magma ocean is highly turbulent,and plagioclase is likely to become entrained in the turbulent magma rather than separated from the melt.To determine the necessary conditions in which anorthosite forms from the LMO,this study adopted the energy criterion formulated by Solomatov.The composition of melt,temperature,and pressure when plagioclase crystallizes are constrained by using MELTS/pMELTS to calculate the density and viscosity of the melt.When plagioclase starts to crystallize,the Mg~# of melt becomes 0.59 at 1291 C.The density of the melt is smaller than that of plagioclase for P 2.1 kbar(ca.50 km deep),and the critical diameter of plagioclase to separate from the melt becomes larger than the typical crystal diameter of plagioclase(1.8-3 cm).This suggests that plagioclase is likely entrained in the LMO just after the plagioclase starts to crystallize.When the Mg~# of melt becomes 0.54 at 1263 C,the density of melt becomes larger than that of plagioclase even for 0 kbar.When the Mg~# of melt decreases down to 0.46 at 1218 C,the critical diameter of plagioclase to separate from the melt becomes 1.5-2.5 cm,which is nearly equal to the typical plagioclase of the lunar anorthosite.This suggests that plagioclase could separate from the melt.One of the differences between the Earth and the Moon is the presence of water.If the terrestrial magma ocean was saturated with H_2O,plagioclase could not crystallize,and anorthosite could not form.     

Steady-state Mantle-Melt Interactions in One Dimension: II. Thermal Interactions and Irreversible Terms

Progress in development of thermodynamically based models ofsilicate equilibria with explicit entropy budgets has motivateda reexamination of the conclusion of McKenzie (Journal of Petrology25, 713–765, 1984) that isentropic upwelling sufficesas a model of mantle melting. An entropy budget equation forfractional melting with melt migration in an upwelling two-phasecontinuum is presented. The energetically self-consistent meltproduction model predicted by MELTS is used to evaluate numericallythe magnitudes of differences between fractional melting (withmelt migration) and equilibrium melting (without relative movement)that can be bounded in one dimension: chemical advection byout-of-equilibrium melt; thermal disequilibrium between migratingliquid and residue; frictional dissipation of gravitationalpotential; dissipation as a result of solid compaction. Likethe familiar isobaric case in which fractional melting is significantlyless productive than equilibrium melting, chemical isolationof the escaping melts from the residue reduces the oceanic crustalthickness by     

Dehydration melting of tonalites. Part II. Composition of melts and solids

 Dehydration melting of tonalitic compositions (phlogopite or biotite-plagioclase-quartz assemblages) is investigated within a temperature range of 700–1000°C and pressure range of 2–15 kbar. The solid reaction products in the case of the phlogopite-plagioclase(An₄₅)-quartz starting material are enstatite, clinopyroxene and potassium feldspar, with amphiboles occurring occasionally. At 12 kbar, zoisite is observed below 800°C, and garnet at 900°C. The reaction products of dehydration melting of the biotite (Ann₅₀)-plagioclase (An₄₅)-quartz assemblage are melt, orthopyroxene, clinopyroxene, amphibole and potassium feldspar. At pressures > 8 kbar and temperatures below 800°C, epidote is also formed. Almandine-rich garnet appears above 10 kbar at temperatures ≥ 750°C. The composition of melts is granitic to granodioritic, hence showing the importance of dehydration melting of tonalites for the formation of granitic melts and granulitic restites at pressure-temperature conditions within the continental crust. The melt compositions plot close to the cotectic line dividing the liquidus surfaces between quartz and potassium feldspar in the haplogranite system at 5 kbar and a _{H 2O} = 1. The composition of the melts changes with the composition of the starting material, temperature and pressure. With increasing temperature, the melt becomes enriched in Al₂O₃ and FeO+MgO. Potash in the melt is highest just when biotite disappears. The amount of CaO decreases up to 900°C at 5 kbar whereas at higher temperatures it increases as amphibole, clinopyroxene and more An-component dissolve in the melt. The Na₂O content of the melt increases slightly with increase in temperature. The composition of the melt at temperatures > 900°C approaches that of the starting assemblage. The melt fraction varies with composition and proportion of hydrous phases in the starting composition as well as temperature and pressure. With increasing modal biotite from 20 to 30 wt%, the melt proportion increases from 19.8 to 22.3 vol.% (850°C and 5 kbar). With increasing temperature from 800 to 950°C (at 5 kbar), the increase in melt fraction is from 11 to 25.8 vol.%. The effect of pressure on the melt fraction is observed to be relatively small and the melt proportion in the same assemblage decreases at 850°C from 19.8 vol.% at 5 kbar to 15.3 vol.% at 15 kbar. Selected experiments were reversed at 2 and 5 kbar to demonstrate that near equilibrium compositions were obtained in runs of longer duration. Received: 27 December 1995 / Accepted: 7 May 1996     

Modeling diffusive dissolution in silicate melts

Here empirical models for calculating self-diffusion coefficients and diffusion matrices are combined with MELTS, a thermodynamic model for silicate minerals and melts, to estimate diffusive dissolution rates, interface melt compositions and melt diffusivities. Simulations of olivine dissolution experiments in basalt show that the overall model is capable of accurately reproducing diffusive dissolution rates, and the resulting diffusion profiles, over a range of pressures and temperatures. However, the overall model is less successful at reproducing olivine dissolution in andesite, diopside dissolution in either basalt or andesite, or anorthite dissolution in picrite. Yet, even for these systems the predicted dissolution rates are generally within about a factor of two of the measured ones. Comparisons between simulations and experiments suggest that errors in the self-diffusion and thermodynamic models are responsible for the differences, and show that dissolution experiments could be a powerful way of testing and calibrating these and similar models. The overall model will also be a useful tool for designing future experiments, and for identifying the parameters that control diffusive dissolution (and crystallization) in silicate melts under a wide range of conditions.     

SPINMELT-2.0: Simulation of spinel–melt equilibrium in basaltic systems under pressures up to 15 kbar: I. model formulation,calibration, and tests

The paper presents results of testing currently used models proposed to describe Cr-spinel–melt equilibrium: models of the MELTS family by M.S. Ghiorso with colleagues, the SPINMELT program by A.A. Ariskin and G.S. Nikolaev, and the “MELT–CHROMITE spinel calculator” by А.А. Pustovetov and R.L. Roeder. The new calibration of the SPINMELT model presented in this publication enables calculating a sixcomponent (Mg, Fe²⁺, Cr, Al, Fe³⁺, and Ti) composition of Cr-spinel and the \(T - {f_{{o_2}}}\) parameters of its stability on the liquidus of basaltic melts under pressures up to 15 kbar. The model is based on results of 392 runs from 43 experimental studies, including systems of normal alkalinity at \({f_{{o_2}}}\) ≤ QFM + 2. The experimental dataset (which was extended compared to that used for the previous calibration) allowed us not only to estimate the pressure effect, but also apply the model to aluminous and hydrous systems. Tests of the SPINMELT-2.0 model show that the errors of the calculated temperature of the spinel–melt equilibrium increase with pressure from 16°C at 1 atm to 50°C at 15 kbar. Experimental spinel compositions are reproduced by the model accurate to < 3 at % Al and Cr, and no worse 1 at % for the other cations.     

Viscous Energy Dissipation and Strain Partitioning in Partially Molten Rocks

We develop a steady-state fluid-mechanical analysis describingthe effect of strain partitioning on viscous energy dissipation.As observed in experimental studies of shear deformation ofpartially molten rocks, strain partitions when melt segregatesbecause viscosity is reduced in regions of elevated melt fraction.The equations derived here are based on parameters measuredin experiments, describing the evolution of melt distributionand rheological properties. We find that the dissipation dependsstrongly on the configuration of the melt-rich network of shearzones, including the average angle, volume fraction of meltand amplification of strain rate in the melt-rich bands. Minimain energy dissipation as a function of band angle develop, correspondingto configurations of melt networks that minimize the differencein mean stress between the band and the non-band regions. Wepropose that the organization of band networks occurs by theinterplay between strain localization and viscosity variationsassociated with melt segregation. The band networks maintaina steady-state angle during shear by continuously pumping meltthrough the network. The development of strain partitioningin melt-rich networks will modify the energetics of meltingand melt transport by efficiently extracting melt and reducingeffective viscosity. KEY WORDS: melt transport; rheology; self-organization; strain localization; strain partitioning     

Consequences of crystal shape and fabric on anisotropic permeability in magmatic mush

Crystals that form an interconnected porous network can become preferentially oriented both prior to and during compaction of magmatic mush. This introduces anisotropy in the melt pore-space that can reduce permeability in the direction of compaction and in turn decrease melt flux and compaction rate. Using a number of grain-scale numerical models, the consequences of end-member magmatic fabrics on the directional dependence of permeability are tested over a range in melt fraction from 22 to 77%. As the crystal aspect ratio (i.e. ratio of long to short axis length) increases from 2 to 10, isotropic permeability decreases by a factor of 2 and 5 for randomly oriented prolate and oblate-shaped crystals, respectively, at a melt fraction of 22%. With a flattening fabric, permeability is reduced in the compaction direction no more than approximately a factor of 2 relative to the isotropic permeability at the same melt fraction and crystal shape for both oblate and triaxial prisms. However, permeability is enhanced in directions orthogonal to the compaction direction. For example, permeability is enhanced up to a factor of 11 relative to the isotropic permeability at a melt fraction of 22% for oblate prisms with a ratio of the long to short axis length of 10. Anisotropy in permeability increases as the melt fraction decreases and the crystal aspect ratio increases. Ratios of the principal permeabilities are sufficiently large based on the realistic crystal shapes tested here to warrant including anisotropic permeability into macroscale melt segregation models including those for compaction.     

A close look at dihedral angles and melt geometry in olivine-basalt aggregates: a TEM study

Olivine-basalt aggregates sintered at high P/T have been used as a simplest approximation of partially molten upper mantle peridotite. In the past, geometry of partial melt in polycrystalline olivine (and other materials) has been characterised by dihedral (wetting) angles which depend upon surface free energy. However, since olivine (like most other crystalline materials) is distinctively anisotropic, the simple surface energy balance defining the dihedral angles cos(Θ/2)=_gb/2_sl is not valid and melt geometry is more complicated than can be expressed by a single dihedral angle value. We examine in detail melt geometry in aggregates held at high temperature and pressure for very long times (240–612 h). We show the simple dihedral angle concept to be invalid via transmission electron microscope images. Olivine-basalt interfaces are frequently planar crystal faces (F-faces) which are controlled by the crystal structure rather than the surface area minimisation used in the simple dihedral angle concept. Nevertheless, the dihedral angles may provide useful insights in some situations. They may give a rough estimation of the wetting behaviour of a system, or be used to approximate the melt distribution if F-faces are not present (possibly at large grain size and very low melt fraction). Our measurements, excluding F-faces, give a range of dihedral angle values from 0 to 10° which is significantly lower than reported previously (20–50°). The nature of 0° angles (films and layers up to 1 μm in thickness) is unclear but their frequency compared to dry grain boundaries depends on grain size and melt fraction (e.g. 70% for grain size 43 μm and melt fraction 2%). Received: 13 April 1997 / Accepted: 2 October 1997     

熔体的形态与分布特征对岩石流变的影响

熔体的形态与分布研究表明,在静态条件下,熔融程度比较低时,熔体主要分布于三个矿物颗粒之间,形成三角形状熔体结构,熔体二面角在0°~60°;熔融程度比较高时,熔体沿多个颗粒边界形成孤立的三角形或四边形结构,熔体三联点的二面角接近60°或大于60°。在动态条件下,在部分或全部矿物颗粒边界出现熔体薄膜,把熔体三角形连通,形成局部熔体网络,熔体三联点的二面角接近0°。如果熔体呈孤立的三角形或四边形结构时,熔体对岩石流变的影响比较小:当熔体含量小于2%~3%,熔体对岩石流变基本没有影响;只有熔体含量接近或超过3%~5%,熔体对流变强度的弱化作用才出现,当熔体含量达到10%时,流变强度弱化增加3倍左右。如果矿物颗粒边界出现熔体薄膜,微量熔体(小于1%)就对岩石流变强度有显著的弱化作用。流变实验表明,在颗粒边界含有小于1%的熔体时,熔体对流变强度的弱化达到4倍,当颗粒边界含有3%的熔体时,这种弱化作用达到10倍。     

Constraints from melt inclusions and their host olivines on the petrogenesis of Oligocene-Early Miocene Xindian basalts, Chifeng area, North China Craton

The geochemical characteristics of melt inclusions and their host olivines provide important information on the processes that create magmas and the nature of their mantle and crustal source regions. We report chemical compositions of melt inclusions, their host olivines and bulk rocks of Xindian basalts in Chifeng area, North China Craton. Compositions of both bulk rocks and melt inclusions are tholeiitic. Based on petrographic observations and compositional variation of melt inclusions, the crystallizing sequence of Xindian basalts is as follows: olivine (at MgO > ~5.5 wt%), plagioclase (beginning at MgO = ~5.5 wt%), clinopyroxene and ilmenite (at MgO < 5.0 wt%). High Ni contents and Fe/Mn ratios, and low Ca and Mn contents in olivine phenocrysts, combining with low CaO contents of relatively high MgO melt inclusions (MgO > 6 wt%), indicate that Xindian basalts are possibly derived from a pyroxenite source rather than a peridotite source. In the CS-MS-A diagram, all the high MgO melt inclusions (MgO > 6.0 wt%) project in the field between garnet + clinopyroxene + liquid and garnet + clinopyroxene + orthopyroxene + liquid near 3.0 GPa, further suggesting that residual minerals are mainly garnet and clinopyroxene, with possible presence of orthopyroxene, but without olivine. Modeling calculations using MELTS show that the water content of Xindian basalts is 0.3–0.7 wt% at MgO = 8.13 wt%. Using 20–25 % of partial melting estimated by moderately incompatible element ratios, the water content in the source of Xindian basalts is inferred to be ≥450 ppm, much higher than 6–85 ppm in dry lithospheric mantle. The melting depth is inferred to be ~3.0 GPa, much deeper than that of tholeiitic lavas (<2.0 GPa), assuming a peridotite source with a normal mantle potential temperature. Such melting depth is virtually equal to the thickness of lithosphere beneath Chifeng area (~100 km), suggesting that Xindian basalts are derived from the asthenospheric mantle, if the lithospheric lid effect model is assumed.     

Diabasic intrusion and lavas,segregation veins,and magma differentiation at Kahoolawe volcano,Hawaii

A mafic sill-like intrusion, ~5?×?30 m, exposed along the eastern shoreline of Kahoolawe Island, Hawaii, represents tholeiitic magma emplaced as diabase among caldera-filling lavas. It differentiated from ~7.8 wt.% MgO to yield low-MgO (2.9 wt.%) vesicular segregation veins. We examined the intrusion for whole-rock and mineral compositions for comparison to Kahoolawe caldera-fill lavas (some also diabasic), to the Uwekahuna laccolith (Kilauea), and to gabbros, diabases, and segregations and oozes of other tholeiitic shield volcanoes (e.g., Mauna Loa and Kilauea lava lakes). We also evaluate this extreme differentiation in terms of MELTS modeling, using parameters appropriate for Hawaiian crystallization environments. Kahoolawe intrusion diabase samples have major and trace element abundances and plagioclase, pyroxene, and olivine compositions in agreement with those in gabbros and diabases of other volcanoes. However, the intrusion samples are at the low-MgO end of the large MgO range formed by the collective comparative samples, as many of those have between 8 and 20 wt.% MgO. The intrusion’s segregation vein has SiO₂ 53.4 wt.%, TiO₂ 3.2 wt.%, FeO 13.5 wt.%, Zr 350 ppm, and La 16 ppm. It plots in compositional fields formed by other Hawaiian segregations and oozes that have MgO <5 wt.%—fields that show large variances, such as factor of ~2 differences for incompatible element abundances accompanying SiO₂ from ~49 to 59 wt.%. Our MELTS modeling assesses the Kahoolawe intrusion as differentiating from ~8 wt.% MgO parent magma beginning along oxygen buffers equivalent to FMQ and FMQ-2, having magmatic H₂O of 0.15 and 0.7 wt.% (plus traces of CO₂ and S), and under 100 and 500 bars pressure. Within these parameters, MELTS calculates that <3 wt.% MgO occurs at ~1,086 to 1,060 °C after ~48 to 63 % crystallization, whereby the lesser crystallization percentages and lower temperatures equate to higher magmatic H₂O, leading to high SiO₂, ~56–58 wt.%. To contrast, greater crystallization is calculated for lower H₂O, for which it achieves less SiO₂, <55 wt.%. While MELTS reliably predicts SiO₂ approaching 58 wt.% for differentiation beyond <4 wt.% MgO, and shows that Kahoolawe intrusion’s segregations and those of Kilauea and Mauna Loa are all reasonably accommodated by the modeled parameters and SiO₂ differentiation curves, MELTS fails where it predicts that Fe enrichment is more robust under FMQ than FMQ-2 buffers. That failure not withstanding, MELTS differentiation from liquidus temperatures ~1,205–1,185 °C (depending on the various parameters) gradually increases fO₂ (up to ~0.4 log units, as normalized to FMQ) until magnetite crystallizes at ~1,090–1,085 °C, which reduces absolute fO₂ ~1 to 1.5 log units. The modeled Kahoolawe intrusion, then, exemplifies how tholeiitic magma differentiation can produce extreme SiO₂ and incompatible element compositions, and how Hawaiian segregations from shallow intrusions and lava lakes can be generally modeled under compositional and physical parameters appropriate for Hawaiian tholeiitic magmatism.     

The distribution of sulfur between haplogranitic melts and aqueous fluids

The distribution of sulfur between haplogranitic melt and aqueous fluid has been measured as a function of oxygen fugacity (Co-CoO-buffer to hematite-magnetite buffer), pressure (0.5-3 kbar), and temperature (750-850 °C). Sulfur always strongly partitions into the fluid. At a given oxygen fugacity, pressure and temperature, the distribution of sulfur between melt and fluid can be described by one constant partition coefficient over a wide range of sulfur concentrations. Oxygen fugacity is the most important parameter controlling sulfur partitioning. While the fluid/melt partition coefficient of sulfur is 468 ± 32 under Co-CoO buffer conditions at 2 kbar and 850 °C, it decreases to 47 ± 4 at an oxygen fugacity 0.5-1 log unit above Ni-NiO at the same pressure and temperature. A further increase in oxygen fugacity to the hematite-magnetite buffer has virtually no effect on the partition coefficient (D^fluid/melt = 49 ± 2). The dependence of D^fluid/melt on temperature and pressure was systematically explored at an oxygen fugacity 0.5-1 log units above Ni-NiO. At 850 °C, the effect of pressure on the partition coefficient is small (D^fluid/melt = 58 ± 3 at 0.5 kbar; 94 ± 9 at 1 kbar; 47 ± 4 at 2 kbar and 68 ± 5 at 3 kbar) and temperature also has only a minor effect on partitioning.The data show the “sulfur excess” observed in many explosive volcanic eruptions can easily be explained by the presence of a small fraction of hydrous fluid in the magma chamber before the eruption. The sulfur excess can be calculated as the product of the fluid/melt partition coefficient of sulfur and the mass ratio of fluid over melt in the erupted material. For a plausible fluid/melt partition coefficient of 47 under oxidizing conditions, a 10-fold sulfur excess corresponds to a 17.6 wt.% of fluid in the erupted material. Large sulfur excesses (10-fold or higher) are only to be expected if only a small fraction of the magma residing in the magma chamber is erupted.The behavior of sulfur, which seems to be largely independent of pressure and temperature under oxidizing conditions is very different from chlorine, where the fluid/melt partition coefficient strongly increases with pressure. Variations in the SO₂/HCl ratio of volcanic gases, if they reflect primary processes in the magma chamber, therefore provide an indicator of pressure variations in a magma. In particular, major increases in the S/Cl ratio of an aqueous fluid coexisting with a felsic magma suggest a pressure reduction in the magma chamber and/or magma rising to the surface.     

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.     

贺兰山北段孔兹岩系脱水熔融实验研究——临界熔体比例的确定及意义

 在充分研究贺兰山北段区域地质的基础上,选定该区孔兹岩系中最有代表性的两类岩石:变粒岩类及富铝片麻岩为实验对象,进行天然块状样品的脱水熔融实验研究。根据成分变异的特点,确定岩浆的临界熔体体积比为30%,其不仅影响变质作用p-T-t轨迹的演化路径,而且与原地、半原地及异地型花岗岩的形成有着密不可分的关系。     

Insights into chondrule formation process and shock-thermal history of the Dergaon chondrite (H4-5)

The Dergaon fall represents a shock-melted H4-5(S5) ordinary chondrite which includes at least ten textural varieties of chondrules and belongs to the high chondrule-matrix ratio type.Our study reveals that the chondrules are of diverse mineralogy with variable olivine-pyroxene ratios(Type Ⅱ),igneous melt textures developed under variable cooling rates and formed through melt fractionations from two different melt reservoirs.Based on the experimental analogues,mineralogical associations and phase compositions,it is suggested that the Dergaon chondrules reflect two contrasting environments:a hot,dust-enriched and highly oxidized nebular environment through melting,without significant evaporation,and an arrested reducing environment concomitant with major evaporation loss of alkali and highly volatile trace elements.Coexistence of chlorapatite and merrillite suggests formation of the Dergaon matrix in an acidic accretionary environment.Textural integration and chemical homogenization occurred at ~ 1 atmospheric pressure and a mean temperature of 765 C mark the radiogenic thermal event.Equilibrated shock features(olivine mosaicism,diaplectic plagioclase,polycrystalline troilite) due to an impact-induced thermal event reflect a shock pressure 45 GPa and temperature of 600 C.By contrast,the local disequilibrium shock features(silicate melt veins comprising of olivine crystallites,troilite melt veins and metal droplets) correspond to a shock pressure up to 75 GPa and temperature950 ℃.     

贺兰山北段孔兹岩系脱水熔融实验研究——临界熔体比例的确定及意义

在充分研究贺兰山北段区域地质的基础上,选定该区孔兹岩系中最有代表性的两类岩石:变粒岩类及富铝片麻岩为实验对象,进行天然块状样品的脱水熔融实验研究。根据成分变异的特点,确定岩浆的临界熔体体积比为30%,其不仅影响变质作用p-T-t轨迹的演化路径,而且与原地、半原地及异地型花岗岩的形成有着密不可分的关系。     

Criteria for the recognition of partial melting

Partial melting changes rocks from single phase (solid) to two phase (solid+melt) systems. The bulk viscosity decreases as the melt fraction increases and this effect raises the rate of deformation and heat transfer, as well as causing crustal differentiation. Therefore, it is important to be able to recognise which rocks have partially melted.Macroscopic textures provide the simplest criteria for recognising partial melting. Melting and deformation are generally synchronous, and when the melt fraction retained is low (<20%) metatexite migmatites are formed. Typically, these are morphologically complex because the melt fraction is squeezed out of the deforming matrix and collects in whatever dilatant sites are present. The presence of melanosome layers and patches provides the best evidence of where the melt formed, and the leucosomes where it collected. Diatexite migmatites can be easily recognised by the presence of a flow foliation, schlieren, enclaves and vein like leucosomes, and are evidence of a high melt fraction and pervasive partial melting. For the unusual case of melting without synchronous deformation, rounded neosome patches containing both the melt and solid fractions of the melt-producing reaction develop and, as the degree of melting increases these enlarge, to form diatexite migmatites. In both cases the characteristic feature is an increased grainsize and loss of pre-migmatization structures. Migmatite textures are robust, they survive later deformation well.Microscopic textures such as: (1) thin films of quartz, plagioclase and K-feldspar along brain boundaries that represent crystallized melt and, (2) melt-solid reaction textures, also provide good criteria for identifying partially melted rocks. However, these textures are fragile and easily destroyed by deformation. The identification of mineral assemblages from which melt-forming reactions can be inferred is another reliable critera for recognising partial melting, but post-migmatization rehydration in granulite terranes can destroy this evidence.Whole rock geochemistry can be used to model the partial melting process, but problems in identifying the palaeosome and an unmodified melt compositions can restrict its application. However, whole rock geochemistry coupled with good field based control, can be used to deduce what processes have occurred in a terrane where the rocks have partially melted.Variations in field appearance, texture and composition are, in large part a consequence of whether, or not, and when, the melt-fraction separated from the solid fraction.     

Experimental deformation of partially melted granite revisited: implications for the continental crust

A review and reinterpretation of previous experimental data on the deformation of partially melted crustal rocks reveals that the relationship of aggregate strength to melt fraction is non‐linear, even if plotted on a linear ordinate and abscissa. At melt fractions, Φ < 0.07, the dependence of aggregate strength on Φ is significantly greater than at Φ > 0.07. This melt fraction (Φ = 0.07) marks the transition from a significant increase in the proportion of melt‐bearing grain boundaries up to this point to a minor increase thereafter. Therefore, we suggest that it is the increase of melt‐interconnectivity that causes the dramatic strength drop between the solidus and a melt fraction of 0.07. We term this drop the ‘melt connectivity transition’ (MCT). A second, less‐pronounced strength drop occurs at higher melt fractions and corresponds to the breakdown of the solid (crystal) framework. This is the ‘solid‐to‐liquid transition’ (SLT), corresponding to the well known ‘rheologically critical melt percentage’. Although the strength drop at the SLT is about four orders of magnitude, the absolute value of this drop is small compared with the absolute strength of the unmelted aggregate, rendering the SLT invisible in a linear aggregate strength v. melt‐fraction diagram. On the other hand, the more important MCT has been overlooked in previous work because experimental data usually are plotted in logarithmic strength v. melt‐fraction diagrams, obscuring large strength drops at high absolute strength values. We propose that crustal‐scale localization of deformation effectively coincides with the onset of melting, pre‐empting attainment of the SLT in most geological settings. The SLT may be restricted to controlling flow localization within magmatic bodies, especially where melt accumulates.