TEM investigation of the crystal microstructures in a quartz-coesite assemblage of the western alps

Atransmission electron microscope (TEM) study of quartz-coesite inclusions in garnet in crustal rocks from the Western Alps is presented. Coesite shows a low dislocation density (<10⁷ cm^?2), and quartz a higher density of defects, Brasil twins (10⁴ cm^?1) and dislocations (10⁸ cm^?2). It is concluded that coesite has been not or only slightly plastically deformed and that the yield strength of coesite is higher than that of quartz. The large scale deformation implications are briefly discussed. TEM observations show no systematic topotactic relationship between the two polymorphs and their boundaries have a scalloped morphology which suggests that growth of quartz from coesite was controlled by a diffusion process.     

Raman spectroscopy of natural silica in Chicxulub impactite,MexicoSpectroscopie Raman de silices naturelles dans l'impactite de Chicxulub,Mexique

A series of natural silica impactite samples from Chicxulub (Mexico) was investigated by Raman microprobe (RMP) analysis. The data yield evidence for high-pressure shock metamorphism in the rock. The impactite contains three polymorphs of silica: the original α-quartz, and two high-pressure varieties – coesite and disordered quartz representing various degrees of crystallinity. We found systematic changes in frequencies and half-widths of the Raman bands, caused by increasing irregularities of bond-lengths and bond-angles and a general breaking-up of the structure as a result of impact events. Therefore, RMP is an adequate tool for measuring the crystallinity of disordered quartz. The half-width Γ and the frequency ω of the symmetric SiOSi stretching vibrational band (A₁ mode) of the SiO₄ tetrahedra are the most amenable parameters for estimating the degree of crystallinity. In well-crystallized quartz, Γ=5 cm^?1 and ω=464 cm^?1, while in highly disordered quartz this line shifts up to ω=455 cm^?1 and broadens up to Γ=30 cm^?1. The Raman lineshapes appear to depend strongly on the degree of lattice disorder subsequent to impact events. To cite this article: M. Ostroumov et al., C. R. Geoscience 334 (2002) 21–26     

Factors affecting preservation of coesite in ultrahigh‐pressure metamorphic rocks: Insights from TEM observations of dislocations within kyanite

To understand the preservation of coesite inclusions in ultrahigh‐pressure (UHP) metamorphic rocks, an integrated petrological, Raman spectroscopic and focussed ion beam (FIB) system–transmission electron microscope (TEM) study was performed on a UHP kyanite eclogite from the Sulu belt in eastern China. Coesite grains have been observed only as rare inclusions in kyanite from the outer segment of garnet and in the matrix. Raman mapping analysis shows that a coesite inclusion in kyanite from the garnet rim records an anisotropic residual stress and retains a maximum residual pressure of ~0.35 GPa. TEM observations show quartz is absent from the coesite inclusion–host kyanite grain boundaries. Numerous dislocations and sub‐grain boundaries are present in the kyanite, but dislocations are not confirmed in the coesite. In particular, dislocations concentrate in the kyanite adjacent to the boundary with the coesite inclusion, and they form a dislocation concentration zone with a dislocation density of ~10⁹ cm^?2. A high‐resolution TEM image and a fast Fourier transform‐filtered image reveal that a tiny dislocation in the dislocation concentration zone is composed of multiple edge dislocations. The estimated dislocation density in most of the kyanite away from the coesite inclusion–host kyanite grain boundaries is ~10⁸ cm^?2, being lower than that in kyanite adjacent to the coesite. In the case of a coesite inclusion in a matrix kyanite, using Raman and TEM analyses, we could not identify any quartz at the grain boundaries. Dislocations are not observed in the coesite, but numerous dislocations and stacking faults are developed in the kyanite. The estimated overall dislocation density in the coesite‐bearing matrix kyanite is ~10⁸ cm^?2, but a high dislocation density region of ~10⁹ cm^?2 is also present near the coesite inclusion–host kyanite grain boundaries. Inclusion and matrix kyanite grains with no coesite have dislocation densities of ≤10⁸ cm^?2. Dislocation density is generally reduced during an annealing process, but our results show that not all dislocations in the kyanite have recovered uniformly during exhumation of the UHP rocks. Hence, one of the key factors acting as a buffer to inhibit the coesite to quartz transformation is the mechanical interaction between the host and the inclusion that lead to the formation of dislocations in the kyanite. The kyanite acts as an excellent pressure container that can preserve coesite during the decompression of rocks from UHP conditions. The search for and study of inclusions in kyanite may be a more suitable approach for tracing the spatial distribution of UHP metamorphic rocks.     

Pressure-induced incipient amorphization of α-quartz and transition to coesite in an eclogite from Antarctica: a first record and some consequences

Monocrystalline quartz inclusions in garnet and omphacite from various eclogite samples from the Lanterman Range (Northern Victoria Land, Antarctica) have been investigated by cathodoluminescence (CL), Raman spectroscopy and imaging, and in situ X‐ray (XR) microdiffraction using the synchrotron. A few inclusions, with a clear‐to‐opalescent lustre, show ‘anomalous’ Raman spectra characterized by weak α‐quartz modes, the broadening of the main α‐quartz peak at 465 cm^?1, and additional vibrations at 480–485, 520–523 and 608 cm^?1. CL and Raman imaging indicate that this ‘anomalous’α‐quartz occurs as relicts within ordinary α‐quartz, and that it was preserved in the internal parts of small quartz inclusions. XR diffraction circular patterns display irregular and broad α‐quartz spots, some of which show an anomalous d‐spacing tightening of ～2%. They also show some very weak, hazy clouds that have d‐spacing compatible with coesite but not with α‐quartz. Raman spectrometry and XR microdiffraction characterize the anomalies with respect to α‐quartz as (i) a pressure‐induced disordering and incipient amorphization, mainly revealed by the 480–485 and 608‐cm^?1 Raman bands, together with (ii) a lattice densification, evidenced by d‐spacing tightening; (iii) the cryptic development of coesite, 520–523 cm^?1 being the main Raman peak of coesite and (iv) Brazil micro‐twinning. This ‘anomalous’α‐quartz represents the first example of pressure‐induced incipient amorphization of a metastable phase in a crustal rock. This issue is really surprising because pressure‐induced amorphization of metastable α‐quartz, observed in impactites and known to occur between 15 and 32 GPa during ultrahigh‐pressure (UHP) experiments at room temperature, is in principle irrelevant under normal geological P–T conditions. A shock (due to a seism?) or a local overpressure at the inclusion scale (due to expansion mismatch between quartz and its host mineral) seem the only geological mechanisms that can produce such incipient amorphization in crustal rocks. This discovery throws new light on the modality of the quartz‐coesite transition and on the pressure regimes (non‐lithostatic v. lithostatic) during high‐pressure/UHP metamorphism. In particular, incipient amorphization of quartz could favour the quartz‐coesite transition, or allow the growth of metastable coesite, as already experimentally observed.     

The crystal chemistry of birefringent natural uvarovites. Part III. Application of the superposition model of crystal fields with a characterization of synthetic cubic uvarovite

Synthetic, flux-grown uvarovite, Ca₃Cr₂ [SiO₄]₃, was investigated by optical methods, electron microprobe analysis, UV-VIS-IR microspectrometry, and luminescence spectroscopy. The crystal structure was refined using single-crystal X-ray CCD diffraction data. Synthetic uvarovite is optically isotropic and crystallizes in the “usual” cubic garnet space group Ia3¯d [a=11.9973 Å, Z=8; 21524 reflections, R1=2.31% for 454 unique data and 18 variables; Cr–O=1.9942(6), Si–O=1.6447(6), Ca–O_a=2.3504(6), Ca–O_b= 2.4971(6) Å]. The structure of Ca₃Cr₂[SiO₄]₃ complies with crystal-chemical expectations for ugrandite group garnets in general as well as with predictions drawn from “cubically averaged” data of non-cubic uvarovite–grossular solid solutions (Wildner and Andrut 2001). The electronic absorption spectra of Cr³⁺ in trigonally distorted octahedra of synthetic uvarovite were analyzed in terms of the superposition model (SM) of crystal fields. The resulting SM and interelectronic repulsion parameters are =9532 cm^?1, =4650 cm^?1, power law exponent t ₄=6.7, Racah B₃₅=703 cm^?1 at 290 K (reference distance R ₀=1.995 Å; fixed power law exponent t ₂=3 and spin-orbit parameter ζ=135 cm^?1). The interelectronic repulsion parameters Racah B ₅₅=714 cm^?1 and C=3165 cm^?1 were extracted from spin-forbidden transitions. This set of SM parameters was subsequently applied to previously well-characterized natural uvarovite–grossular solid solutions (Andrut and Wildner 2001a; Wildner and Andrut 2001) using their extrapolated Cr–O bond lengths to calculate the energies of the spin-allowed bands. These results are in very good agreement with the experimentally determined band positions and indicate the applicability of the superposition model to natural 3d ^N prevailing systems in geosciences. Single-crystal IR absorption spectra of synthetic uvarovite in the region of the OH-stretching vibration exhibit one isotropic absorption band at 3508 cm^?1 at ambient conditions, which shifts to 3510 cm^?1 at 77 K. This band is caused by structurally incorporated hydroxyl groups via the (O₄H₄)-hydrogarnet substitution. The water content, calculated using an integral extinction coefficient ?=60417 cm^?2 l mol^?1, is c _H2O=33 ppm.     

Measurement of the polarized absorption spectra of synthetic transition metal-bearing silicate microcrystals in the spectral range 44,000-4,000 cm−1

A new single beam microtechnique has been developed for measuring the polarized absorption spectra in the region 44,000-4,000 cm^?1. Spectra of a natural garnet (Spess₇₀Alm₃₀), measured by the microtechnique and by conventional macrotechniques, are consistent and thus prove the applicability of the microtechnique described. It is possible to obtain well resolved spectra down to about 13,000 cm^?1 with crystals as small as about 10 μm. Thus spectra of crystals obtained in routine high-pressure high-temperature silicate syntheses can be measured. The polarized spectra of Mn³⁺, Fe³⁺, Fe²⁺, and Cr³⁺ in the following synthetic silicate minerals are presented: piemontite (I), acmite (II), orthoferrosilite (III), and kyanite (IV) or uvarovite (V), respectively. O-Cr³⁺, O-Mn³⁺, and O-Fe²⁺ charge transfer band maxima in the UV region are identified at 38,700 cm^?1, in V; at 33,200, 35,300, and 39,000 cm^?1, in I; and at 32,800, 35,200, and 37,300 cm^?1, in III, respectively. Bands in the region ≦25,000 cm^?1 are assigned to spin-allowed and spin-forbidden dd transitions as predicted from crystal field theoretical considerations for the foregoing ions in the respective structures.     

First-principles study of OH defects in zircon

The infrared spectroscopic properties of selected OH defects in zircon are investigated by first-principles calculations. The explicit treatment of the coupled nature of OH motions in the stretching modes, together with the calculation of the intensity and polarization of absorption bands, makes it possible to directly compare theoretical and experimental data. The bands observed at 3,420 cm^?1 (polarization parallel to c axis) and 3,385 cm^?1 (polarization perpendicular to c axis) in natural and synthetic samples correspond to the IR-active vibrational modes of the hydrozircon defect, that is, fully protonated Si vacancy. The broad band observed at 3,515 cm^?1 in the spectrum of zircon crystals grown in F-rich environments is consistent with the occurrence of composite (OH,F) tetrahedral defects. Calculations also show that the band observed at 3,200 cm^?1 in the spectrum of synthetic undoped samples can be ascribed to fully protonated Zr vacancies. The theoretical values of integrated absorption coefficients indicate that general correlations can be reasonably used to determine the concentration of OH groups in zircon.     

Electron paramagnetic resonance,optical absorption,and magnetic circular dichroism studies of the CO 3 − molecular-ion in irradiated natural beryl

Room temperature X-irradiation of some natural beryls produced several new absorption lines in the electron paramagnetic resonance (EPR) spectrum, a known series of optical absorption lines in the 500–700 nm range, and a shift of the absorption edge to lower energies. Several of the new EPR lines and part of the irradiation-induced shift of the absorption edge disappeared after a few days at room temperature, and were not examined in detail. However, three of the paramagnetic centres responsible for the new EPR lines were stable at room temperature and two of these have previously been identified as atomic hydrogen and the methyl radical, CH₃. These species were stable to ～150 and ～450°C respectively. The third stable species, hitherto unreported, showed a single-line EPR spectrum of axial symmetry, with g∥=2.0051 and g⊥=2.0152. This spectrum was found to be intensity-correlated with the series of optical bands in the 500–700 nm range, after thermal bleaching at 175°C. The EPR and optical spectra are therefore assigned to the same species. It is argued that this species is the CO ₃ ^? molecular ion, located in the widest part of the structural channel and aligned with the plane of the molecule perpendicular to the c axis. The EPR spectrum is consistent with a ² A′₂ ground state of a CO ₃ ^? molecule with trigonal symmetry, and this requires that the optical transition has a ² A′₂ → ² E′ character. Most of the features in the optical spectrum can be assigned to coupling of a totally symmetric mode of frequency ～1020 cm^?1 onto a zero-phonon line at 14,490 cm^?1 and a second weaker line at 16,020 cm^?1. However, both of these two fundamental lines are structured, and the two components show strong temperature-dependent derivative-shaped magnetic circular dichroism (MCD). Furthermore, the overall sign of the MCD for the line at 16,020 cm^?1 is opposite to that at 14,490 cm^?1. The separation (～120 cm^?1) of the two components of the 14,490 cm^?1 line is much larger than that expected from spin-orbit interaction, and the origin of this splitting is not yet understood.     

Polarized electronic absorption spectra of 3d3-configurated tetravalent manganese in octahedral fields of Mn(SeO3)2

Polarized optical absorption spectra of Mn(IV) in octahedral crystal fields of Mn(SeO₃)₂ have been studied by means of microscope-spectrometry in the range 40000-4000 cm^?1 and at temperatures between 113 K and 293 K. Intense charge-transfer absorptions (linear absorption coefficient α ? 30000 cm^?1) completely mask the d-d transitions in the UV and VIS region above ≈23000 cm^?1. The optical electronegativity χ _opt of Mn(IV) in Mn(SeO₃)₂ is estimated to be 2.7. In accordance with the d ₃ configuration of tetravalent manganese three d-d bands observed at ambient temperatures at 13250, 14137 (α≈50 cm^?1) and ≈18500 cm^?1 (α≈500–800 cm^?1) are assigned to the spin forbidden ⁴ A _2g →² E _g and ⁴ A _2g →² T _1g transitions as well as to the first spin allowed ⁴ A _2g →⁴ T ^2g transition, respectively. These assignments allow the calculation of the following ligand field parameters: Dq ≈ 1850 cm^?1, B ₅₅ = 869 cm^?1 (β ₅₅ = 0.82), and C = 2346 cm^?1 (293 K).     

Single-crystal polarized FTIR spectroscopy and neutron diffraction refinement of cancrinite

We relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H₂O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with Ow···O donor–acceptor distances >2.7 Å. The FTIR spectra show two main absorptions. The first at 3,602 cm^?1 is polarized for E ⊥ c and is assigned to the ν₃ mode. The second, at 3,531 cm^?1, is also polarized for E ⊥ c and is assigned to ν₁ mode. A weak component at 4,108 cm^?1 could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1,300–1,500 cm^?1 range are strongly polarized for E ⊥ c, and are assigned to the vibrations of the CO₃ group.     

Epiphytic nitrogen fixation associated with standing dead shoots of smooth cordgrass, Spartina alterniflora

N₂ fixation associated with the epiphytic community on standing dead Spartina alterniflora shoots was examined in both a natural and transplanted salt marsh in North Carolina. Acetylene reduction (AR) assays were conducted over a 24-mo period to estimate N₂ fixation rates on standing dead stems and leaves. In the natural salt marsh, mean AR rates ranged from 0.5 nmol C₂H₄ cm^?2 h^?1 to 14 nmol C₂H₄ cm^?2 h^?1, while in the transplanted marsh mean AR rates ranged from 1 nmol C₂H₄ cm^?2 h^?1 to 33 nmol C₂H₄ cm^?2 h^?1. Diel AR activity of epiphytic communities in both marshes varied seasonally. Midday incubations yielded higher AR rates than nighttime incubations in the spring, while midday incubations in late summer and fall generally yielded AR rates equal to or lower than nighttime incubations. Desiccation during low tides occasionally repressed AR activity, although AR rates quickly rebounded with wetting. AR activity was localized in the epiphytic community, rather than in the underlying Spartina stem material. Based on the measured AR rates and the density of standing dead stems, the annual input of new N to the natural salt marsh via epiphytic N₂ fixation is estimated to be 2.6 g N m^?2 yr^?1. The estimate of annual input of new N to the transplanted marsh is 3.8 g N m^?2 yr^?1. These estimates should be added to previous estimates of N₂ fixation in marsh sediments to estimate the total contribution of new nitrogen to salt marsh nitrogen budgets.     

Raman spectrum of natural and synthetic stishovite

Raman spectra of natural and synthetic samples of stishovite have been measured with a micro-optical spectrometer system. These spectra have a pattern that is characteristic of rutile-structured oxides. The spectrum of synthetic stishovite is characterized by well-resolved bands at 231, 589, 753, and 967 cm^?1, which are assigned as theB _1g,E _g,A _1g, andB _2g fundamentals, respectively, of the first-order Raman spectrum of the ideal, ordered structure. Natural stishovite obtained from Meteor Crater, Arizona has a first-order Raman spectrum that is fully consistent with that of the synthetic material. The observed spectrum of the natural sample, however, is weaker and has bands in addition to those identified as fundamentals in the spectrum of the synthetic material. A broad band at ～475 cm^?1 may be indicative of glass or contaminants derived from the extraction procedure. Alternatively, this band may arise from multiphonon scattering that is enhanced by poor crystallinity or structural disorder in the natural shocked sample.     

Determination of quantitative cation distribution in orthopyroxenes from electronic absorption spectra

Electronic and Mössbauer absorption spectra and electron microprobe data are correlated for iron-bearing orthopyroxenes. The correlation provides a means of quantitatively determining the distribution of Fe²⁺ between the M(1) and M(2) sites of orthopyroxene crystals from electronic spectra and electron microprobe analysis. The electronic spectra are used to analyze the changes in the Fe²⁺ distribution produced during heating experiments and confirm earlier results from Mössbauer spectra. Two components of the spin-allowed transition of Fe²⁺ in the M(1) site are identified at about 13,000 cm^?1 and 8,500 cm^?1 in γ. Molar absorptivity (?) values for all spin-allowed Fe²⁺ absorption bands in the near-infrared region are determined. The M(2) Fe²⁺ band at ～5,000 cm^?1 in β is the analytically most useful for site occupancy determinations. It remains linear with concentration (?=9.65) over the entire compositional range. The band at ～10,500 cm^?1 in α is the most sensitive to M(2) Fe²⁺ concentration (?=40.8), but deviates from linearity at high iron concentrations. The origins of spin-forbidden transitions in the visible region are examined.     

Helium measurements of pore fluids obtained from the San Andreas Fault Observatory at Depth (SAFOD, USA) drill cores

⁴He accumulated in fluids is a well established geochemical tracer used to study crustal fluid dynamics. Direct fluid samples are not always collectable; therefore, a method to extract rare gases from matrix fluids of whole rocks by diffusion has been adapted. Helium was measured on matrix fluids extracted from sandstones and mudstones recovered during the San Andreas Fault Observatory at Depth (SAFOD) drilling in California, USA. Samples were typically collected as subcores or from drillcore fragments. Helium concentration and isotope ratios were measured 4?C6 times on each sample, and indicate a bulk ⁴He diffusion coefficient of 3.5?±?1.3?×?10^?C8 cm²?s^?C1 at 21°C, compared to previously published diffusion coefficients of 1.2?×?10^?C18 cm²?s^?C1 (21°C) to 3.0?×?10^?C15 cm²?s^?C1 (150°C) in the sands and clays. Correcting the diffusion coefficient of ⁴He_water for matrix porosity (??3%) and tortuosity (??6?C13) produces effective diffusion coefficients of 1?×?10^?C8 cm^2?s^?C1 (21°C) and 1?×?10^?C7 (120°C), effectively isolating pore fluid ⁴He from the ⁴He contained in the rock matrix. Model calculations indicate that <6% of helium initially dissolved in pore fluids was lost during the sampling process. Complete and quantitative extraction of the pore fluids provide minimum in situ porosity values for sandstones 2.8?±?0.4% (SD, n?=?4) and mudstones 3.1?±?0.8% (SD, n?=?4).     

Low-temperature evolution of OH bands in synthetic forsterite, implication for the nature of H defects at high pressure

We performed in situ infrared spectroscopic measurements of OH bands in a forsterite single crystal between ?194 and 200 °C. The crystal was synthesized at 2 GPa from a cooling experiment performed between 1,400 and 1,275 °C at a rate of 1 °C per hour under high silica-activity conditions. Twenty-four individual bands were identified at low temperature. Three different groups can be distinguished: (1) Most of the OH bands between 3,300 and 3,650 cm^?1 display a small frequency lowering (<4 cm^?1) and a moderate broadening (<10 cm^?1) as temperature is increased from ?194 to 200 °C. The behaviour of these bands is compatible with weakly H-bonded OH groups associated with hydrogen substitution into silicon tetrahedra; (2) In the same frequency range, two bands at 3,617 and 3,566 cm^?1 display a significantly anharmonic behaviour with stronger frequency lowering (42 and 27 cm^?1 respectively) and broadening (~30 cm^?1) with increasing temperature. It is tentatively proposed that the defects responsible for these OH bands correspond to H atoms in interstitial position; (3) In the frequency region between 3,300 and 3,000 cm^?1, three broad bands are identified at 3,151, 3,178 and 3,217 cm^?1, at ?194 °C. They exhibit significant frequency increase (~20 cm^?1) and broadening (~70 cm^?1) with increasing temperature, indicating moderate H bonding. These bands are compatible with (2H)_Mg defects. A survey of published spectra of forsterite samples synthesized above 5 GPa shows that about 75 % of the incorporated hydrogen belongs to type (1) OH bands associated with Si substitution and 25 % to the broad band at 3,566 cm^?1 (type (2); 3,550 cm^?1 at room temperature). The contribution of OH bands of type (3), associated to (2H)_Mg defects, is negligible. Therefore, solubility of hydrogen in forsterite (and natural olivine compositions) cannot be described by a single solubility law, but by the combination of at least two laws, with different activation volumes and water fugacity exponents.     

Electronic absorption spectra of Cr3+ ions in natural clinopyroxenes

The polarized (E‖a′, E‖b and E‖c) electronic absorption spectra of five natural chromium-containing clinopyroxenes with compositions close to chromdiopside, omphacite, ureyite-jadeite (12.8% Cr₂O₃), jadeite, and spodumene (hiddenite) were studied. The polarization dependence of the intensities of the Cr³⁺ bands in the clinopyroxene spectra cannot be explained by the selection rules for the point groups C ₂ or C _2v but can be accounted for satisfactorily with the help of the higher order pseudosymmetry model, i.e. with selection rules for the point symmetry group C _3v. The trigonal axis of the pseudosymmetry crystal field forms an angle of 20.5° with the crystallographic direction c in the (010) plane. D _q increases from diopside (1542 cm^?1) through omphacite (1552 cm^?1), jadeite (1574 cm^?1) to spodumene (1592 cm^?1). The parameter B which is a measure of covalency for Cr³⁺-O bonds at M1 sites in clinopyroxene depends on the Cr³⁺ concentration and the cations at M2 sites.     

Vibrational spectra of single-crystal topaz

A single-crystal of topaz was studied by Raman spectroscopy to assign the internal modes of the high-frequency range and to compare with infrared data. All active modes exhibit an important Davydov splitting (150 cm^?1) but we have found a small Bethe splitting (14.5 cm^?1) consistent with a very regular SiO₄ tetrahedron. Because of a high value of v ₁ (～920 cm^?1) the Raman active modes present a mixed v ₁/v ₃ character. Finally the substitution of OH for F splits an A _g internal mode and lead to some proper modes at 3650 cm^?1, 3639 cm^?1 and 1165 cm^?1.     

Timing of Himalayan ultrahigh-pressure metamorphism: sinking rate and subduction angle of the Indian continental crust beneath Asia

Coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from Higher Himalayan Crystalline rocks in the upper Kaghan Valley, north‐west Himalaya. The metamorphic peak conditions of the coesite‐bearing eclogites are estimated to be 27–32 kbar and 700–770 °C, using garnet–pyroxene–phengite geobarometry and garnet–pyroxene geothermometry, respectively. Cathodoluminescence (CL) and backscattered electron (BSE) imaging distinguished three different domains in zircon: inner detrital core, widely spaced euhedral oscillatory zones, and thin, broadly zoned outermost rims. Each zircon domain contains a characteristic suite of micrometre‐sized mineral inclusions which were identified by in situ laser Raman microspectroscopy. Core and mantle domains contain quartz, apatite, plagioclase, muscovite and rutile. In contrast, the rim domains contain coesite and minor muscovite. Quartz inclusions were identified in all coesite‐bearing zircon grains, but not coexisting with coesite in the same growth domain (rim domain). ²⁰⁶Pb/²³⁸U zircon ages reveal that the quartz‐bearing mantle domains and the coesite‐bearing rim were formed at c. 50 Ma and 46.2 ± 0.7 Ma, respectively. These facts demonstrate that the continental materials were buried to 100 km within 7–9 Myr after initiation of the India–Asia collision (palaeomagnetic data from the Indian oceanic floor supports an initial India‐Asia contact at 55–53 Ma). Combination of the sinking rate of 1.1–1.4 cm year^?1 with Indian plate velocity of 4.5 cm year^?1 suggests that the Indian continent subducted to about 100 km depth at an average subduction angle of 14–19°.     

Olivine as an in situ piezometer in high pressure apparatus

The deviatoric stress produced in a large-volume, high-pressure apparatus of the girdle-anvil type has been estimated from the density of free dislocations induced in natural olivine single crystals (initial density of 2×10⁶ cm^?2). Experiments at maximum pressure P=40 kbar and temperature T=1050°C for t=1 h in NaCl cell assemblies and various P-T paths yield specimens whose dislocation densities are unchanged from this initial value, implying that the deviatoric stress was less than 140 bar. In BN cell assemblies, the recovered specimen from high P-T experiments exhibit much higher densities of dislocations (～10⁹ cm^?2) which have been produced by steady-state plastic deformation of the olivine crystals under a deviatoric stress of ～3 kbar. This value of deviatoric stress in BN has been corroborated by observations of the subgrain size and recrystallized grain size in specimens of longer run duration (3 h).