Experimental study of quartz inclusions in garnet at pressures up to 3.0 GPa: evaluating validity of the quartz-in-garnet inclusion elastic thermobarometer

Garnet crystals with quartz inclusions were hydrothermally crystallized from oxide starting materials in piston–cylinder apparatuses at pressures from 0.5 to 3 GPa and temperatures ranging from 700 to 800 °C to study how entrapment conditions affect remnant pressures of quartz inclusions used for quartz-in-garnet (QuiG) elastic thermobarometry. Systematic changes of the 128, 206 and 464 cm^?1 Raman band frequencies of quartz were used to determine pressures of quartz inclusions in garnet using Raman spectroscopy calibrations that describe the P–T dependencies of Raman band shifts for quartz under hydrostatic pressure. Within analytical uncertainties, inclusion pressures calculated for each of the three Raman band frequencies are equivalent, which suggests that non-hydrostatic stress effects caused by elastic anisotropy in quartz are smaller than measurement errors. The experimental quartz inclusions have pressures ranging from ??0.351 to 1.247 GPa that span the range of values observed for quartz inclusions in garnets from natural rocks. Quartz inclusion pressures were used to model P–T conditions at which the inclusions could have been trapped. The accuracy of QuiG thermobarometry was evaluated by considering the differences between pressures measured during experiments and pressures calculated using published equation of state parameters for quartz and garnet. Our experimental results demonstrate that Raman measurements performed at room temperature can be used without corrections to estimate garnet crystallization pressures. Calculated entrapment pressures for quartz inclusions in garnet are less than ~?10% different from pressures measured during the experiments. Because the method is simple to apply with reasonable accuracy, we expect widespread usage of QuiG thermobarometry to estimate crystallization conditions for garnet-bearing silicic rocks.     

Raman spectroscopic study of high-pressure phase transitions in cristobalite

The pressure dependence of the cristobalite Raman spectrum has been investigated to 22 GPa at room temperature, using single-crystal Raman spectroscopy with a diamond-anvil cell. We observe a rapid, first-order phase transition on increasing pressure, consistent with the cristobalite I?II transition revealed in previous x-ray diffraction experiments. The phase transition has been bracketed at 1.2±0.1 GPa on increasing pressure and 0.2±0.1 GPa on decreasing pressure. The pressure shifts II) of 11 Raman bands in the high-pressure phase (cristobalite have been measured. Evidence for an unusual hybridization of modes at 490–500 cm^?1 is found. Changes in the Raman spectra also reveal an additional phase transition to a new phase at P ≈ 11 GPa, which remains to be fully characterized.     

石英高压相变研究进展

文中总结了前人有关石英高温高压相变的实验结果。根据以前的实验,在静水压条件下,石英-柯石英-斯石英-CaCl2结构超斯石英相-α-PbO2结构超斯石英相之间的相变方程分别是:p(GPa)=(2.11±0.03)+(9.8×10-4±1.2×10-4)×T(℃),p(GPa)=(8.0±0.2)+(1.1×10-3±3×10-4)×T(℃),p(GPa)=(51±2)+(0.012±0.005)×T(K),p(GPa)=98+(0.0095±0.0016)×T(K)。文中还初步探讨了非静水压状态对石英相变的影响。实验结果表明,差应力的出现降低了石英相变所需要的围压,即相变边界向低压方向偏移,在周永胜等人实验数据的基础上,笔者尝试将二维的相图扩展到三维相图以考虑差应力的影响。最后讨论了石英相变在地学研究中的作用,对比不同的观点分析了前人对超高压变质作用过程的解释,希望可以为以后解释地质资料提供较为广泛的可能性,促进我们对地球内部动力学过程的了解。     

Determination of the phase boundary between ilmenite and perovskite in MgSiO3 by in situ X-ray diffraction and quench experiments

Determination of the phase boundary between ilmenite and perovskite structures in MgSiO₃ has been made at pressures between 18 and 24 GPa and temperatures up to 2000 °C by in situ X-ray diffraction measurements using synchrotron radiation and quench experiments. It was difficult to precisely define the phase boundary by the present in situ X-ray observations, because the grain growth of ilmenite hindered the estimation of relative abundances of these phases. Moreover, the slow reaction kinetics between these two phases made it difficult to determine the phase boundary by changing pressure and temperature conditions during in situ X-ray diffraction measurements. Nevertheless, the phase boundary was well constrained by quench method with a pressure calibration based on the spinel-postspinel boundary of Mg₂SiO₄ determined by in situ X-ray experiments. This yielded the ilmenite-perovskite phase boundary of P (GPa) = 25.0 (±0.2) – 0.003 T (°C) for a temperature range of 1200–1800 °C, which is generally consistent with the results of the present in situ X-ray diffraction measurements within the uncertainty of ∼±0.5 GPa. The phase boundary thus determined between ilmenite and perovskite phases in MgSiO₃ is slightly (∼0.5 GPa) lower than that of the spinel-postspinel transformation in Mg₂SiO₄. Received: 19 May 1999 / Accepted: 21 March 2000     

Phase transitions in the system CaCO<Subscript>3</Subscript> at high P and T determined by in situ vibrational spectroscopy in diamond anvil cells and first-principles simulations

The stability of the high-pressure CaCO₃ calcite (cc)-related polymorphs was studied in experiments that were performed in conventional diamond anvil cells (DAC) at room temperature as a function of pressure up to 30 GPa as well as in internally heated diamond anvil cells (DAC-HT) at pressures and temperatures up to 20 GPa and 800 K. To probe structural changes, we used Raman and FTIR spectroscopy. For the latter, we applied conventional and synchrotron mid-infrared as well as synchrotron far-infrared radiation. Within the cc-III stability field (2.2–15 GPa at room temperature, e.g., Catalli and Williams in Phys Chem Miner 32(5–6):412–417, 2005), we observed in the Raman spectra consistently three different spectral patterns: Two patterns at pressures below and above 3.3 GPa were already described in Pippinger et al. (Phys Chem Miner 42(1):29–43, 2015) and assigned to the phase transition of cc-IIIb to cc-III at 3.3 GPa. In addition, we observed a clear change between 5 and 6 GPa that is independent of the starting material and the pressure path and time path of the experiments. This apparent change in the spectral pattern is only visible in the low-frequency range of the Raman spectra—not in the infrared spectra. Complementary electronic structure calculations confirm the existence of three distinct stability regions of cc-III-type phases at pressures up to about 15 GPa. By combining experimental and simulation data, we interpret the transition at 5–6 GPa as a re-appearance of the cc-IIIb phase. In all types of experiments, we confirmed the transition from cc-IIIb to cc-VI at about 15 GPa at room temperature. We found that temperature stabilizes cc-VI to lower pressure. The reaction cc-IIIb to cc-VI has a negative slope of ?7.0 × 10^?3 GPa K^?1. Finally, we discuss the possibility of the dense cc-VI phase being more stable than aragonite at certain pressure and temperature conditions relevant to the Earth’s mantle.     

Thermal and redox state of the subcontinental lithospheric mantle of NE Spain from thermobarometric data on mantle xenoliths

Mantle xenoliths in within-plate Cenozoic alkaline mafic lavas from NE Spain are used to assess the local subcontinental lithospheric mantle geotherm and the influence of melting and metasomatism on its oxidation state. The xenoliths are mainly anhydrous spinel lherzolites and harzburgites and gradations between, with minor pyroxenites. Most types show protogranular textures, but transitional protogranular–porphyroclastic and equigranular lherzolites also exist. Different thermometers used in the estimates provide higher subsolidus equilibrium temperatures for harzburgites (1,062 ± 29°C) than for lherzolites (972 ± 89°C), although there is overlap; the lowest temperatures correspond to porphyroclastic lherzolites, whereas pyroxenites give the highest temperatures (up to 1,257°C). Maximum pressures for subsolidus equilibrium of peridotites are at 2.0–1.8 GPa. Later they followed adiabatic decompression and harzburgites registered lower pressures (1.02 ± 0.19 GPa) than lherzolites (1.41 ± 0.27 GPa). One pyroxenite gives values consistent with the spinel lherzolite field (1.08 GPa). The shallowest barometric data are in agreement with the highest local conductive geotherms, which implies that the lithosphere–asthenosphere boundary is at 70–60 km minimum depth. Higher equilibrium temperatures for the harzburgites could be explained by the existence of mafic magma bodies or dykes at the lower crust–mantle boundary. Paleo-fO₂ conditions during partial melting as inferred from the covariation between V and MgO concentrations are mainly between QFM−1 and QFM−2 in log units. However, most thermobarometric fO₂ estimates are between QFM−1 and QFM+1, suggesting oxidation caused by later metasomatism during uplift and cooling.     

Experimental phase and melting relations of metapelite in the upper mantle: implications for the petrogenesis of intraplate magmas

We performed a series of piston-cylinder experiments on a synthetic pelite starting material over a pressure and temperature range of 3.0–5.0 GPa and 1,100–1,600°C, respectively, to examine the melting behaviour and phase relations of sedimentary rocks at upper mantle conditions. The anhydrous pelite solidus is between 1,150 and 1,200°C at 3.0 GPa and close to 1,250°C at 5.0 GPa, whereas the liquidus is likely to be at 1,600°C or higher at all investigated pressures, giving a large melting interval of over 400°C. The subsolidus paragenesis consists of quartz/coesite, feldspar, garnet, kyanite, rutile, ±clinopyroxene ±apatite. Feldspar, rutile and apatite are rapidly melted out above the solidus, whereas garnet and kyanite are stable to high melt fractions (>70%). Clinopyroxene stability increases with increasing pressure, and quartz/coesite is the sole liquidus phase at all pressures. Feldspars are relatively Na-rich [K/(K + Na) = 0.4–0.5] at 3.0 GPa, but are nearly pure K-feldspar at 5.0 GPa. Clinopyroxenes are jadeite and Ca-eskolaite rich, with jadeite contents increasing with pressure. All supersolidus experiments produced alkaline dacitic melts with relatively constant SiO₂ and Al₂O₃ contents. At 3.0 GPa, initial melting is controlled almost exclusively by feldspar and quartz, giving melts with K₂O/Na₂O ~1. At 4.0 and 5.0 GPa, low-fraction melting is controlled by jadeite-rich clinopyroxene and K-rich feldspar, which leads to compatible behaviour of Na and melts with K₂O/Na₂O ≫ 1. Our results indicate that sedimentary protoliths entrained in upwelling heterogeneous mantle domains may undergo melting at greater depths than mafic lithologies to produce ultrapotassic dacitic melts. Such melts are expected to react with and metasomatise the surrounding peridotite, which may subsequently undergo melting at shallower levels to produce compositionally distinct magma types. This scenario may account for many of the distinctive geochemical characteristics of EM-type ocean island magma suites. Moreover, unmelted or partially melted sedimentary rocks in the mantle may contribute to some seismic discontinuities that have been observed beneath intraplate and island-arc volcanic regions.     

Compressibility of nanocrystalline forsterite

We established an equation of state for nanocrystalline forsterite using multi-anvil press and diamond anvil cell. Comparative high-pressure and high-temperature experiments have been performed up to 9.6 GPa and 1,300°C. We found that nanocrystalline forsterite is more compressible than macro-powder forsterite. The bulk modulus of nanocrystalline forsterite is equal to 123.3 (±3.4) GPa whereas the bulk modulus of macro-powder forsterite is equal to 129.6 (±3.2) GPa. This difference is attributed to a weakening of the elastic properties of grain boundary and triple junction and their significant contribution in nanocrystalline sample compare to the bulk counterpart. The bulk modulus at zero pressure of forsterite grain boundary was determined to be 83.5 GPa.     

The interaction between reaction and deformation: an experimental study using a biotite + plagioclase + quartz gneiss

Metamorphic reactions commonly accompany ductile deformation of crustal rocks. We performed an experimental study to determine: (i) the effect of syn-deformation reaction on strain weakening and localization, and (ii) the effect of crystal plastic deformation on reaction extent and distribution. Experiments were conducted on a fine-grained gneiss (58 vol.% quartz, forming the interconnected matrix, 13 vol.% biotite, 28 vol.% plagioclase and 1 vol.% garnet/Fe-Ti oxides). General shear experiments were performed at 745 and 800 °C, 1.5 GPa, two strain rates, and shear strain ( γ ) from 0.6 to 5, yielding three suites with initial phase strength contrast between the matrix quartz and weak biotite of 45×, 25× and 10×; hydrostatic experiments were performed on cores and powders at 750 and 800 °C and 1.5–2 GPa for the same times. At these conditions, biotite reacts with plagioclase and quartz to form garnet, K-feldspar and water (no melt was observed). Greater reaction extent was observed in deformed samples than in equivalent hydrostatic samples, because of the increased surface area and internal strain energy. In all of the deformed samples, reaction contributes to strain weakening, due principally to a switch to grain boundary sliding in the fine-grained reaction products. The degree to which syn-deformational reaction causes strain weakening and localization in this polyphase aggregate depends on the phase strength contrast and how it evolves. In samples with low-phase strength contrast, strain and reaction are homogeneously distributed; however, in samples with high-phase strength contrast, ductile strain and reaction interact positively to produce a narrow ductile shear zone. Similar concentration of reaction is observed in some natural ductile shear zones.     

High-pressure polymorphism and structural transitions of norsethite,BaMg(CO3)2

In situ high-pressure investigations on norsethite, BaMg(CO₃)₂, have been performed in sequence of diamond-anvil cell experiments by means of single-crystal X-ray and synchrotron diffraction and Raman spectroscopy. Isothermal hydrostatic compression at room temperature yields a high-pressure phase transition at P _c ≈ 2.32 ± 0.04 GPa, which is weakly first order in character and reveals significant elastic softening of the high-pressure form of norsethite. X-ray structure determination reveals C2/c symmetry (Z = 4; a = 8.6522(14) Å, b = 4.9774(13) Å, c = 11.1542(9) Å, β = 104.928(8)°, V = 464.20(12) Å³ at 3.00 GPa), and the structure refinement (R ₁ = 0.0763) confirms a distorted, but topologically similar crystal structure of the so-called γ-norsethite, with Ba in 12-fold and Mg in octahedral coordination. The CO₃ groups were found to get tilted off the ab-plane direction by ~16.5°. Positional shifts, in particular of the Ba atoms and the three crystallographically independent oxygen sites, give a higher flexibility for atomic displacements, from which both the relatively higher compressibility and the remarkable softening originate. The corresponding bulk moduli are K ₀ = 66.2 ± 2.3 GPa and dK/dP = 2.0 ± 1.8 for α-norsethite and K ₀ = 41.9 ± 0.4 GPa and dK/dP = 6.1 ± 0.3 for γ-norsethite, displaying a pronounced directional anisotropy (α: β _a ^?1  = 444(53) GPa, β _c ^?1  = 76(2) GPa; γ: β _a ^?1  = 5.1(1.3) × 10³ GPa, β _b ^?1  = 193(6) GPa β _c ^?1  = 53.4(0.4) GPa). High-pressure Raman spectra show a significant splitting of several modes, which were used to identify the transformation in high-pressure high-temperature experiments in the range up to 4 GPa and 542 K. Based on the experimental series of data points determined by XRD and Raman measurements, the phase boundary of the α-to-γ-transition was determined with a Clausius–Clapeyron slope of 9.8(7) × 10^?3 GPa K^?1. An in situ measurement of the X-ray intensities was taken at 1.5 GPa and 411 K in order to identify the nature of the structural variation on increased temperatures corresponding to the previously reported transformation from α- to β-norsethite at 343 K and 1 bar. The investigations revealed, in contrast to all X-ray diffraction data recorded at 298 K, the disappearance of the superstructure reflections and the observed reflection conditions confirm the anticipated \(R\bar{3}m\) space-group symmetry. The same superstructure reflections, which disappear as temperature increases, were found to gain in intensity due to the positional shift of the Ba atoms in the γ-phase.     

Dehydration and partial melting of tremolitic amphibole coexisting with zoisite, quartz, anorthite, diopside, and water in the system H2O-CaO-MgO-Al2O3-SiO2

The greenschist to amphibolite transition as modeled by the reaction zoisite+tremolite + quartz= anorthite+diopside+water has been experimentally investigated in the chemical system H₂O−CaO− MgO−Al₂O₃−SiO₂ over the range of 0.4–0.8 GPa. This reaction is observed to lie within the stability fields of anorthite + water and of zoisite + quartz, in accord with phase equilibrium principles, and its position is in excellent agreement with the boundary calculated from current internally-consistent data bases. The small dP/dT slope of 0.00216 GPa/K (21.6 bars/K) observed for this reaction supports the pressure-dependency of this transition in this chemical system. Experimental reversals of the Al content in tremolitic amphibole coexisting with zoisite, diopside, quartz, and water were obtained at 600, 650, and 700^°C and indicated Al total cations (atoms per formula unit, apfu) of only up to 0.5±0.08 at the highest temperature. Thermodynamic analysis of these and previous compositional reversal data for tremolitic amphibole indicated that, of the activity/composition relationships considered, a two-site-coupled cation substitution model yielded the best fit to the data and a S ⁰ (1 bar, 298 K) of 575.4±1.6 J/K · mol for magnesio-hornblende. The calculated isopleths of constant Al content in the amphibole are relatively temperature sensitive with Al content increasing with increasing temperature and pressure. Finally, several experiments in the range of 1.0–1.3 GPa were conducted to define the onset of melting, and thus the upper-thermal limit, for this mineral assemblage, which must involve an invariant point located at approximately 1.05 GPa and 770^°C. Received: 24 January 1997 / Accepted: 2 October 1997     

Absorption spectra of Cr3+ in Al2O3 under shock compression

Unpolarized absorption spectra of single crystals of Cr³⁺ doped Al₂O₃ (synthetic ruby) have measured using a new, time-resolving, dispersive, streak photographic system over the range ～350 to ～700 nm during a series of shock loading experiments. The crystal field absorptions assigned to the transition ⁴ A _2g→⁴ T _2g were observed to shift in a series of experiments from 555±1 nm at atmospheric pressure to 503±5 nm at 46 GPa. In a single experiment at 32 GPa the ⁴ A _2g→⁴ T _1g transition was observed to shift from 405±1 to 386±5 nm. The present data extrapolate downwards in compression toward the 10 GPa data of Stephens and Drickamer (1961) although both crystal field absorption energies increase considerably less with compression than predicted by the simple ionic point charge model. The single datum observed for the Racah parameter B, 588±38 cm^?1 at 32 GPa, is consistant with previous results to 10 GPa and the trend of decreasing B, with compression expected from the divergence of the data from the point charge model due to increasing covalancy.     

Coesite inclusions in diamonds of Yakutia

The results of the study of diamonds with inclusions of high-pressure modification of SiO₂ (coesite) by Raman spectroscopy are reported. It is established that the octahedral crystal from the Zapolyarnaya pipe is characterized by the highest residual pressure (2.7 ± 0.07 GPa). An intermediate value of this parameter (2.1 ± 0.07 GPa) was obtained for a crystal of transitional habit from the Maiskaya pipe. The minimal Raman shift was registered for coesite in diamond from the Komsomol’skaya–Magnitnaya pipe and provided a calculated residual pressure of 1.8 ± 0.03 GPa. The residual pressures for crystals from the placer deposits of the Kuoika and Bol’shaya Kuonamka rivers are 2.7 ± 0.07 and 3.1 ± 0.1 GPa, respectively. Octahedral crystals were formed in the mantle at a higher pressure than rhombododecahedral diamonds.     

Shock metamorphism and shock barometry at a complex impact structure: Slate Islands, Canada

The Slate Islands archipelago is believed to represent the central uplifted portion of a complex impact structure. Planar microstructures in quartz and feldspars and shock vitrification of rocks are the most common shock metamorphic features encountered. No diaplectic quartz was identified in the exposed rocks, but minor maskelynite is present. Shatter cones occur on all islands of the archipelago suggesting minimum pressures of 4 ± 2 GPa. The relative frequency of low index planar microstructures of specific, optically determined crystallographic orientations in quartz are correlated with results from shock barometric experiments to estimate peak shock pressures experienced by the exposed target rocks. In general, there is a decrease in shock pressure recorded in the target rocks from about 20–25 GPa in east-central Patterson Island to about 5–10 GPa at the western shore of this island and on Mortimer Island. The shock attenuation gradient is ∼4.5 GPa/km across this section of the island group. However, the shock attenuation has a roughly concentric plan only over the western part of the archipelago. There is no distinct shock center and there are other deviations from circularity. This is probably the result of: (1) the shock wave not having expanded from a point or spherical source because of the ∼1. 0 to 1.5 km size of the impactor; (2) differential movement of large target rock blocks during the central uplift and crater modification phases of the impact process. The orientation of planar deformation features in quartz appears to be independent of the shock wave direction suggesting that crystal structure exerts the primary control on microstructure development. Based on the results of XRD analyses, residual, post-impact temperatures were high enough to cause annealing of submicroscopic damage in shocked quartz. Received: 15 July 1997 / Accepted: 6 October 1997     

Grain boundary diffusion of Si, Mg, and O in enstatite reaction rims: a SIMS study using isotopically doped reactants

Diffusion-controlled growth rates of polycrystalline enstatite reaction rims between forsterite and quartz were determined at 1,000 °C and 1 GPa in presence of traces of water. Iron-free, pure synthetic forsterite with normal oxygen and silicon isotopic compositions and quartz extremely enriched in ¹⁸O and ²⁹Si were used as reactants. The relative mobility of ¹⁸O and ²⁹Si in reactants and rims were determined by SIMS step scanning. The morphology of the rim shows that enstatite grows by a direct replacement of forsterite. Rim growth is modelled within a mass-conserving reference frame that implies advancement of reaction fronts from the initial forsterite-quartz interface in both directions. The isotopic compositions at the two reaction interfaces are controlled by the partial reactions Mg₂SiO₄=0.5 Mg₂Si₂O₆+MgO at the forsterite-enstatite, and MgO+SiO₂=0.5 Mg₂Si₂O₆ at the enstatite-quartz interface, implying that grain boundary diffusion of MgO is rate-controlling. Isotopic profiles show no silicon exchange across the propagating reaction interfaces. This propagation, controlled by MgO diffusion, is faster than the homogenisation of Si by self-diffusion behind the advancing fronts. From this, and using % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiramaaDa % aaleaacaWGtbGaamyAaiaacYcacaWGfbGaamOBaaqaaiaadAfacaWG % VbGaamiBaaaaaaa!3DD2! D_Si,En^VolD_{Si,En}^{Vol} at dry conditions from the literature, results a % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmirayaafa % Waa0baaSqaaiaadofacaWGPbGaaiilaiaadweacaWGUbaabaaaaOGa % eqiTdqgaaa!3CCD! D￠_Si,En dD'_{Si,En}^{} \delta value of 3᎒^-24 m³ s^-1 at 1,000 °C. The isotopic profiles for oxygen are more complex. They are interpreted as an interplay between the propagation of the interfaces, the homogenisation of the isotope concentrations by grain boundary self-diffusion of O within the rim, and the isotope exchange across the enstatite-quartz interface, which was open to ¹⁸O influx from quartz. Because of overlapping diffusion processes, boundary conditions are unstable and D´_Ox,En' cannot be quantified. Using measured rim growth rates, the grain boundary diffusivity D´_MgO' of MgO in iron-free enstatite is 8᎒^-22 m³ s^-1 at 1,000 °C and 1 GPa. Experiments with San Carlos olivine (fo₉₂) as reactant reveal lower rates by a factor of about 4. Our results show that isotope tracers in rim growth experiments allow identification of the actual interface reactions, recognition of the rate-controlling component and further calculation of D´' values for specific components.     

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge,SW Japan

To understand tectono‐metamorphic processes within or close to the brittle–ductile transition of quartz‐rich crustal rocks in an accretionary wedge, an integrated field, petrological, geochronological and Raman spectroscopic study was conducted on the Mikabu‐Northern Chichibu belt in SW Japan. Field mapping in central Shikoku reveals that the Northern Chichibu belt is comprised of a pile of four tectono‐stratigraphic units, referred to as A, B, C and D units. The A unit (dominated by pelagic sedimentary rocks) represents the structurally lowest and youngest accretionary complex that forms a composite unit with the Mikabu ophiolitic suite. The B unit (consisting of chert‐clastic rock sequences) overlies the A unit and is overlain by the C and D units (mudstone‐matrix mélange units). Raman spectroscopy of carbonaceous material constrains the peak temperature of each unit to be ~290°C for the A unit, 270–290°C for the B unit, 230–250°C for the C unit and ~220°C for the D unit. Ductile deformation and pervasive metamorphism are limited to rocks in the Mikabu, A and B units. Alkali pyroxene and sodic amphibole occur in metabasite from the Mikabu, A and B units, and the widespread occurrence of prograde veins containing lawsonite+quartz pseudomorphs after laumontite was newly recognized from the C unit. Phase petrological data constrain the peak pressure of each unit to be ~0.65 GPa for the Mikabu‐A unit (aragonite stable), ~0.45–0.6 GPa for the B unit (jadeite+albite stable in the structurally lower part), and ~0.35 GPa for the C unit (prehnite+lawsonite stable). The peak metamorphic pressure increases towards structurally lower and younger accretionary complexes, but the thickness of the preserved strata is insufficient to account for the inferred pressure range. The structural–metamorphic relations imply thickening of the accretionary wedge by underplating was followed by a significant phase of thinning by both ductile and brittle processes.     

A modified iterative sandwich method for determination of near-solidus partial melt compositions. II. Application to determination of near-solidus melt compositions of carbonated peridotite

We performed modified iterative sandwich experiments (MISE) to determine the composition of carbonatitic melt generated near the solidus of natural, fertile peridotite + CO₂ at 1,200–1,245°C and 6.6 GPa. Six iterations were performed with natural peridotite (MixKLB-1: Mg# = 89.7) and ∼10 wt% added carbonate to achieve the equilibrium carbonatite composition. Compositions of melts and coexisting minerals converged to a constant composition after the fourth iteration, with the silicate mineral compositions matching those expected at the solidus of carbonated peridotite at 6.6 GPa and 1,230°C, as determined from a sub-solidus experiment with MixKLB-1 peridotite. Partial melts expected from a carbonated lherzolite at a melt fraction of 0.01–0.05% at 6.6 GPa have the composition of sodic iron-bearing dolomitic carbonatite, with molar Ca/(Ca + Mg) of 0.413 ± 0.001, Ca# [100 × molar Ca/(Ca + Mg + Fe*)] of 37.1 ± 0.1, and Mg# of 83.7 ± 0.6. SiO₂, TiO₂ and Al₂O₃ concentrations are 4.1 ± 0.1, 1.0 ± 0.1, and 0.30 ± 0.02 wt%, whereas the Na₂O concentration is 4.0 ± 0.2 wt%. Comparison of our results with other iterative sandwich experiments at lower pressures indicate that near-solidus carbonatite derived from mantle lherzolite become less calcic with increasing pressure. Thus carbonatitic melt percolating through the deep mantle must dissolve cpx from surrounding peridotite and precipitate opx. Significant FeO* and Na₂O concentrations in near solidus carbonatitic partial melt likely account for the ∼150°C lower solidus temperature of natural carbonated peridotite compared to the solidus of synthetic peridotite in the system CMAS + CO₂. The experiments demonstrate that the MISE method can determine the composition of partial melts at very low melt fraction after a small number of iterations.     

Equations of state of magnesium silicates anhydrous B and superhydrous B

High-pressure single crystal X-ray diffraction experiments of phase anhydrous B and superhydrous B have been carried out to 7.3 and 7.7?GPa, respectively, at room temperature. Fitting a third-order Birch-Murnaghan equation of state to the P-V data yields values of V ₀?=?838.86?±?0.04?ų, K_T,0?=?151.5?±?0.9?GPa and K′?=?5.5?±?0.3 for Anhy-B and V ₀?=?624.71?± 0.03?ų, K_T,0?=?142.6?±?0.8?GPa and K′?=?5.8?±?0.2 for Shy-B. A similar analysis of the axial compressibilities in Anhy-B reveals that the c-axis is most compressible (K_c?=?137?±?3?GPa), the b-axis is least compressible (K_b?=?175?±?4?GPa), and the a-axis is intermediate (K_a?=?148?±?1?GPa). In Shy-B, the a-axis is most compressible (K_a?=?135?±?1?GPa), followed by the b- and c-axes which have similar compressibilities (K_b?=?146?±?3?GPa; K_c?=?148?±?3?GPa). The fact that the b-axis of Shy-B is approximately 16% more compressible than Anhy-B is primarily due to differences in the O-T layer in which the H atoms are located and the linkages with the adjacent O layers. The rigid edge-sharing chains of MgO₆ and SiO₆ octahedra in the O layer control compressibility along the a- and c-axes in both structures. The net result is a reduction in the overall anisotropic compression from ～22% in Anhy-B to ～9% in Shy-B.     

Late‐stage orogenic loading revealed by contact metamorphism in the northern Appalachians,New York

Pelitic schists from contact aureoles surrounding mafic–ultramafic plutons in Westchester County, NY record a high‐P (~0.8 GPa) high‐T (~790 °C) contact overprint on a Taconic regional metamorphic assemblage (~0.5 GPa). The contact metamorphic assemblage of a pelitic sample in the innermost aureole of the Croton Falls pluton, a small (<10 km²) gabbroic body, consists of quartz–plagioclase–biotite–garnet–sillimanite–ilmenite–graphite–Zn‐rich Al‐spinel. Both K‐feldspar and muscovite are absent, and abundant biotite, plagioclase, sillimanite, quartz and ilmenite inclusions are found within subhedral garnet crystals. Unusually low bulk‐rock Na and K contents imply depletion of alkalic components and silica through anatexis and melt extraction during contact heating relative to typical metapelites outside the aureole. Thermobarometry on nearby samples lacking a contact overprint yields 620–640 °C and 0.5–0.6 GPa. In the aureole sample, WDS X‐ray chemical maps show distinct Ca‐enriched rims on both garnet and matrix plagioclase. Furthermore, biotite inclusions within garnet have significantly higher Mg concentration than matrix biotite. Thermobarometry using GASP and garnet–biotite Mg–Fe exchange equilibria on inclusions and adjacent garnet host interior to the high‐Ca rim zone yield ~0.5 ± 0.1 GPa and ~620 ± 50 °C. Pairs in the modified garnet rim zone yield ~0.9 ± 0.1 GPa and ~790 ± 50 °C. Thermocalc average P–T calculations yield similar results for core (~0.5 ± ~0.1 GPa, ~640 ± ~80 °C) and rim (~0.9 ± ~0.1 GPa, ~800 ± ~90 °C) equilibria. The core assemblages are interpreted to record the P–T conditions of peak metamorphism during the Taconic regional event whereas the rim compositions and matrix assemblages are interpreted to record the P–T conditions during the contact event. The high pressures deduced for this later event are interpreted to reflect loading due to the emplacement of Taconic allochthons in the northern Appalachians during the waning stages of regional metamorphism (after c. 465 Ma) and before contact metamorphism (c. 435 Ma). In the absence of contact metamorphism‐induced recrystallization, it is likely that this regional‐scale loading would remain cryptic or unrecorded.     

Single-crystal elastic constants of fluorite (CaF2) to 9.3 GPa

 The second-order elastic constants of CaF₂ (fluorite) have been determined by Brillouin scattering to 9.3 GPa at 300 K. Acoustic velocities have been measured in the (111) plane and inverted to simultaneously obtain the elastic constants and the orientation of the crystal. A notable feature of the present inversion is that only the density at ambient condition was used in the inversion. We obtain high-pressure densities directly from Brillouin data by conversion to isothermal conditions and iterative integration of the compression curve. The pressure derivative of the isentropic bulk modulus and of the shear modulus determined in this study are 4.78 ± 0.13 and 1.08 ± 0.07, which differ from previous low-pressure ultrasonic elasticity measurements. The pressure derivative of the isothermal bulk modulus is 4.83 ± 0.13, 8% lower than the value from static compression, and its uncertainty is lower by a factor of 3. The elastic constants of fluorite increase almost linearly with pressure over the whole investigated pressure range. However, at P ≥ 9 GPa, C ₁₁ and C ₁₂ show a subtle structure in their pressure dependence while C ₄₄ does not. The behavior of the elastic constants of fluorite in the 9–9.3 GPa pressure range is probably affected by the onset of a high-pressure structural transition to a lower symmetry phase (α-PbCl₂ type). A single-crystal Raman scattering experiment performed in parallel to the Brillouin measurements shows the appearance of new features at 8.7 GPa. The new features are continuously observed to 49.2 GPa, confirming that the orthorhombic high-pressure phase is stable along the whole investigated pressure range, in agreement with a previous X-ray diffraction study of CaF₂ to 45 GPa. The high-pressure elasticity data in combination with room-pressure values from previous studies allowed us to determine an independent room-temperature compression curve of fluorite. The new compression curve yields a maximum discrepancy of 0.05 GPa at 9.5 GPa with respect to that derived from static compression by Angel (1993). This comparison suggests that the accuracy of the fluorite pressure scale is better than 1% over the 0–9 GPa pressure range. Received: 10 July 2001 / Accepted: 7 March 2002