Minerals In Metamorphic Rocks Research Articles

Al and H incorporation and Al-diffusion in natural rutile and its high-pressure polymorph TiO 2 (II)

Abstract Rutile is an important accessory mineral in metamorphic rocks and is used as a geothermobarometer or geochronometer. This study aims to bridge the gap between diffusion studies in simplified and complex natural systems by investigating the incorporation and mobility of Al in natural rutile and its high-pressure polymorph TiO 2 (II). Experiments were performed at 0.1 MPa to 7 GPa, 1223–1373 K, at buffered μ(Al 2 O 3 ) and with f O 2 constrained to ≤CCO, which is the equilibrium between graphite and a CO-CO 2 gas phase. Based on electron probe microanalysis, secondary ion mass spectrometry and Fourier transform infrared analyses, we suggest a complex combination of mechanisms to explain the incorporation of Al and H in natural rutile and TiO 2 (II). This includes: (1) the incorporation of Al 3+ on octahedral Ti-sites charge balanced by the formation of oxygen vacancies; and (2) the incorporation of oxygen in interstitial positions charge balanced by hydrogen interstitials. Determined Al-diffusivities in natural TiO 2 are approximately eight to nine orders of magnitude faster compared to previously published data. A possible explanation includes a significantly enhanced rate of ionic diffusion through the combined effect of hydrolytic weakening, enhanced Al-diffusion through extended defects and to a minor extent oxygen fugacity variations. Consequently, results of this study question that the inferred high closure temperatures for the Al-in-rutile geothermobarometer can be applied to all natural systems.

Ti-Bearing Minerals: from the Ocean Floor to Subduction and Back

Abstract Rutile, titanite, and ilmenite are the most common Ti-bearing minerals in metamorphic rocks. Experimental constraints have shown that titanite is stable at low-grade metamorphic conditions, rutile at high pressure (HP), and ilmenite at high temperature, low pressure (HT-LP) conditions. Yet, petrological evidence suggests that titanite can also be stable at low temperature, HP (LT-HP). This implies that both titanite and rutile can be used to develop proxies to track HP metamorphism, which can have interesting applications. In this study, we have investigated the natural occurrence and chemistry of Ti-bearing minerals in gabbroic rocks and basalts that record different degrees of metamorphism, including LP amphibole-bearing gabbros from the ocean floor (Mid-Atlantic and Indian ridge IODP LEGs) and from an obducted ophiolite (Chenaillet) and HP Alpine metagabbros and metabasalts, including blueschist and eclogite facies rocks from the Western Alps and Corsica. We have performed detailed petrography, Raman spectroscopy and analyzed major and trace elements mineral chemistry using EPMA and LA-ICPMS. We found that rutile is stable at low pressure (< 2 kbar) in ocean-floor amphibole-bearing gabbros, lower than experimental constraints had previously suggested. Rutile is also found in eclogitic metagabbros from the Western Alps and can be chemically distinguished from LP rutile. Blueschist metagabbros from the Western Alps and eclogitic metabasalts from Corsica have titanite stable instead of rutile. While the titanite to rutile transition is pressure- and temperature-dependent, we demonstrate how small variations in bulk-rock Ti/Ca and Ca/Al values within the NCKFMASHTO chemical system may shift their stabilities. High-pressure titanite from these metamafic rocks exhibits La depletion and low La/SmN values in comparison to titanite from amphibolite-facies mafic rocks. La/SmN or Nb together with Yb and V can be used to distinguish HP titanite from titanite formed under other P–T settings. These new systematics can be useful in studies using detrital Ti-bearing minerals to probe the HP metamorphic record through time.

Hydrogeochemistry of Fluorine in Groundwater in Humid Mountainous Areas: A Case Study at Xingguo County, Southern China

The understanding of F− concentration in groundwater in humid areas is limited although there are lots of research on high-fluoride groundwater in arid areas. In this paper, with controlling factors of F− concentrations in humid areas as the focus, 130 groundwater samples, obtained from four subsystems in Northwest Xingguo County, Jiangxi Province, China, were investigated to demonstrate the controlling factors of F− concentrations in humid areas. According to analytical results, the following hydrogeochemical characteristics of the fluorine in humid mountainous areas were determined: (1) F− concentration is positively correlated with total dissolved solids (TDS), Ca2+, HCO 3 − , and pH; (2) the groundwater features a high flow rate and low TDS; (3) the equilibrium constant of CaF2 is less than its solubility product constant, and the fluorine-bearing minerals in rocks are in a dissolved state; and (4) the dissolved fluoride-bearing minerals constitute the main sources of F− in the groundwater. Fluorine mainly comes from groundwater fluorine-bearing minerals in metamorphic rocks. Moreover, the low F− concentration in the groundwater mainly results from the fast flow rate of groundwater. Fluoride in groundwater has great potential hazards in humid areas.

Stability conditions and compositional variations of deerite in high-pressure meta-ironstone during subduction–exhumation processes (SW Tianshan, China)

Index minerals in metamorphic rocks, such as coesite, glaucophane, or lawsonite, are of great importance to understand high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism, the thermal gradient, and the geodynamics of subduction zones. Deerite is thought to be diagnostic for blueschist-facies metamorphism in cold subduction environments and a sensitive indicator of the oxygen fugacity (fO2) in Fe-rich rocks. However, little is known concerning its stability and compositional variation during subduction and exhumation cycles. In this study, we provide the first detailed investigation of deerite in eclogite-facies meta‑ironstones (deerite–garnet riebeckitite) from the Southwestern Tianshan (U)HP metamorphic belt. Petrological observation and textural evidence suggest that the meta‑ironstones –enclosing eclogite blocks– form part of an initially coherent volcano-sedimentary sequence that underwent a common tectono-metamorphic evolution, including HP metamorphism and subsequent exhumation. Two types of deerite, which display a different texture (inclusion vs. matrix) and composition (TiO2 content), occur in the deerite–garnet riebeckitite. Dee_1 and aegirine–augite appear exclusively as fine-grained inclusions in garnet, and together with garnet, riebeckite, quartz and magnetite form the peak metamorphic mineral assemblages at P–T conditions of ~510 °C and 2.6 GPa. Matrix deerite (Dee_2), on the other hand, mainly occurs as coarse-grained rods and acicular needle-like grains in quartz, and rarely as inclusions in riebeckite and garnet intimately in the riebeckitite, representing the blueschist-facies retrogression of the meta‑ironstones during exhumation. The garnet–clinopyroxene oxybarometer and thermodynamic calculations show that the fO2 of the deerite–garnet riebeckitite increases from ΔFMQ–0.64 at the peak eclogite-facies stage to ΔFMQ+1.5 at the blueschist-facies retrogression. This fO2 variation corresponds to the chemical variation of deerite, whereby the TiO2 content increases gradually from <0.1 wt% in the Dee_1 in garnet to about 5 wt% in the matrix Dee_2. Our study reveals the detailed metamorphic P–T–fO2 evolution of the prograde eclogite- and retrograde blueschist-facies deerite-bearing meta‑ironstones during subduction–exhumation processes. This study further emphasizes the potential of deerite to reveal the P–T–fO2 evolution of subduction-related high-pressure rocks.

Iterative thermodynamic modelling—Part 2: Tracing equilibrium relationships between minerals in metamorphic rocks

AbstractUnderstanding equilibrium relationships between minerals is a fundamental goal of metamorphic petrology, especially for the quantification of metamorphic conditions recorded by crustal rocks. Information on pressure (P) and temperature (T) can be derived from mineral compositions based on the a priori assumption that frozen‐in assemblages reflect chemical equilibrium at the time of formation. While the stable coexistence of minerals is commonly assessed from a combination of microstructural criteria and interpretation of phase diagrams, there is no robust modelling strategy available so far for the unambiguous recognition of minerals that grew at different conditions. This study uses iterative thermodynamic models and the computer program Bingo‐Antidote to propose a general strategy for assessing the stable coexistence of minerals and therefore for tracing multiple partial equilibrium relationships preserved in metapelites to reconstruct their P–T evolutions. First the principles of this approach are elaborated in a theoretical example. Application to a metapelite from the Western Central Alps reveals a detailed, clockwise P–T path with at least four different equilibration stages including a poorly defined peak pressure at 1–1.1 GPa, ~560°C, followed by peak temperatures of ~620°C at 0.9 GPa and a late amphibolite stage at ~580°C, 0.65 GPa. The investigation of the natural metapelite via iterative thermodynamic models (ITM) allows the observed mineral formation sequence to be combined with a higher accuracy with variable mineral compositions and results in a detailed P–T path. The metastable perseverance of one or more mineral phases can be statistically inferred via ITM. In the investigated metapelite, only ~50 vol.% of the rock volume equilibrated at peak temperature conditions of 620°C. In addition, ITM enables a more detailed investigation of partial re‐equilibration by diffusion during subsequent metamorphic stages, showing that prograde garnet growth could have been modified during subsequent 10 Ma of amphibolite facies metamorphism in the Central Alps.

A kinetic model of oriented nucleation under nonhydrostatic stress: Implications for the preferred orientation of columnar and platy minerals in metamorphic rocks

We propose a kinetic model for the nucleation rate of columnar and platy minerals in nonhydrostatically stressed metamorphic rocks. Nucleation rate I in our model is expressed as IT,σn=Aexp−ΔU∗−TΔS∗+σnΔV∗kBT, Where ΔU∗, ΔS∗ and ΔV∗ are the internal energy, entropy and volume of activation, respectively, kB is the Boltzmann constant, T is absolute temperature, σn is the magnitude of the normal stress applied to the site of heterogeneous nucleation, and A is a constant. As σn varies with respect to the spatial orientation of the nucleation site in nonhydrostatically stressed rocks, the nucleation rate I also varies accordingly. As the number of nucleated grains is proportional to I, anisotropic nucleation can produce a preferred orientation of mineral grains. The model predicts the von Mises distributions of the long axes of columnar mineral grains and poles to the basal planes of platy mineral grains on any plane cut perpendicular to the principal-stress axes. The model also uses the exponential form of the Lamé’s stress ellipsoid to plot contoured equal-area stereographic fabric diagrams. As the generated patterns are similar to those observed in metamorphic rocks, the model is likely useful for analyses of natural deformation.

Metamorphic pressure variation in a coherent Alpine nappe challenges lithostatic pressure paradigm

Pressure–temperature–time paths obtained from minerals in metamorphic rocks allow the reconstruction of the geodynamic evolution of mountain ranges under the assumption that rock pressure is lithostatic. This lithostatic pressure paradigm enables converting the metamorphic pressure directly into the rock’s burial depth and, hence, quantifying the rock’s burial and exhumation history. In the coherent Monte Rosa tectonic unit, Western Alps, considerably different metamorphic pressures are determined in adjacent rocks. Here we show with field and microstructural observations, phase petrology and geochemistry that these pressure differences cannot be explained by tectonic mixing, retrogression of high-pressure minerals, or lack of equilibration of mineral assemblages. We propose that the determined pressure difference of 0.8 ± 0.3 GPa is due to deviation from lithostatic pressure. We show with two analytical solutions for compression- and reaction-induced stress in mechanically heterogeneous rock that such pressure differences are mechanically feasible, supporting our interpretation of significant outcrop-scale pressure gradients.

Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps

AbstractGarnet is a prototypical mineral in metamorphic rocks because it commonly preserves chemical and textural features that can be used for untangling its metamorphic development. Large garnet porphyroblasts may show extremely complex internal structures as a result of a polycyclic growth history, deformation, and modification of growth structures by intra‐ and intercrystalline diffusion. The complex internal structure of garnet porphyroblasts from garnet–phengite schists (GPS) of the Zermatt area (Western Alps) has been successfully decoded. The centimetre‐sized garnet porphyroblasts are composed of granulite facies garnet fragments overgrown by a younger generation of grossular‐rich eclogite facies garnet. The early granulite facies garnet (G‐Grt) formed from low‐P, high‐T metamorphism during a pre‐Alpine orogenic event. The late garnet (E‐Grt) is typical of high‐pressure, low‐temperature (HPLT) metamorphism and can be related to Alpine subduction of the schists. Thus, the garnet of the GPS are polycyclic (polymetamorphic). G‐Grt formation occurred at ~670 MPa and 780°C, E‐Grt formed at ~1.7 GPa and 530°C. The G‐Grt is relatively rich in Prp and poor in Grs, while E‐Grt is rich in Grs and poor in Prp. The Alm content (mol.%) of G‐Grt is 68 of E‐Grt 55. After formation of E‐Grt between and around fragmented G‐Grt at 530°C, the GPS have been further subducted and reached a maximum temperature of 580°C before exhumation started. Garnet composition profiles indicate that the initially very sharp contacts between the granulite facies fragments of G‐Grt and fracture seals of HPLT garnet (E‐Grt) have been modified by cation diffusion. The profiles suggest that Ca did not exchange at the scale of 1 µm, whereas Fe and Mg did efficiently diffuse at the derived maximum temperature of 580°C for the GPS at the scale of 7–8 µm. The Grt–Grt diffusion profiles resulted from spending c. 10 Ma at 530–580°C along the P–T–t path. The measured Grt composition profiles are consistent with diffusivities of log DMgFe = −25.8 m2/s from modelled diffusion profiles. Mg loss by diffusion from G‐Grt is compensated by Fe gain by diffusion from E‐Grt to maintain charge balance. This leads to a distinctive Fe concentration profile typical of uphill diffusion.

Identification of metamorphic rocks in Wuliangshan Mountains (Southwest China) using ASTER data

Metamorphic rocks have scientific significance for parsing the evolutionary process of an orogenic belt. The Wuliangshan area is a very complex region of metamorphism, where it is nearly impossible to conduct conventional geological investigation because of the extremely rugged topography and difficult travel conditions. Previous research on the area’s metamorphic rocks is limited. There are two metamorphic belts, the Yunling-Jinghong and Wuliangshan metamorphic belts, identified by means of traditional field survey. To better identify the metamorphic rocks of the Wuliangshan area, this study adopted the principal component analysis (PCA) and spectrum-area (S-A) methods, using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data, to extract the anomalies of diagnostic metamorphic minerals in metamorphic rocks, such as actinolite, chlorite, epidote, biotite, muscovite, hornblende, andalusite, staurolite, cordierite, and sillimanite. The methods are based on the principle that the metamorphic degree is reflected by the diagnostic metamorphic minerals. Profile investigation and petrographic analysis show that the identification accuracy of the diagnostic metamorphic minerals reached 85.65%. Seven concentrations of metamorphic rocks were delineated based on the extracted metamorphic mineral anomalies. Compared with previous study results, this study showed that there are metamorphic rocks in defined Regions 1, 2, 3, 4 and 6, which are consistent with existing data; whereas in Regions 5 and 7, there is new information regarding the existence of metamorphic rocks, which may offer new clues about the formation of this area through additional studies. The results of this study not only reveal detailed information for geological survey of the Wuliangshan area but also provide useful methods for future surveys.

High-pressure behavior of intermediate scapolite: compressibility, structure deformation and phase transition

Scapolites are common volatile-bearing minerals in metamorphic rocks. In this study, the high-pressure behavior of an intermediate member of the scapolite solid solution series (Me47), chemical formula (Na1.86Ca1.86K0.23Fe0.01)(Al4.36Si7.64)O24[Cl0.48(CO3)0.48(SO4)0.01], has been investigated up to 17.79 GPa, by means of in situ single-crystal synchrotron X-ray diffraction. The isothermal elastic behavior of the studied scapolite has been described by a III-order Birch–Murnaghan equation of state, which provided the following refined parameters: V0 = 1110.6(7) A3, $${K_{{V_0}}}$$ = 70(2) GPa ( $${\beta _{{V_0}}}$$ = 0.0143(4) GPa−1) and $${K_{{V}}^\prime}$$ = 4.8(7). The refined bulk modulus is intermediate between those previously reported for Me17 and Me68 scapolite samples, confirming that the bulk compressibility among the solid solution increases with the Na content. A discussion on the P-induced structure deformation mechanisms of tetragonal scapolite at the atomic scale is provided, along with the implications of the reported results for the modeling of scapolite stability. In addition, a single-crystal to single-crystal phase transition, which is displacive in character, has been observed toward a triclinic polymorph at 9.87 GPa. The high-pressure triclinic polymorph was found to be stable up to the highest pressure investigated.

Natural End Member Samples of Pyrope and Grossular: A Cathodoluminescence-Microscopy and -Spectra Case Study

Garnet is one of the most significant minerals in metamorphic rocks, that provides key information on prograde, peak-metamorphic and retrograde parts of the pressure-temperature (PT) path. Such results require a detailed knowledge of its different growth domains. For iron-poor compositions, the cathodoluminescence (CL) microscopy is an important and often overlooked method and allows to identify the internal structures of all garnet grains in one thin section within only a few seconds. The advantage of the CL-microscope is to deliver low magnification images in true color, not only of garnet but also, for instance, of other rock forming silicates, carbonates, sulfates, etc., of metamorphic, but also of sedimentary and magmatic origin, using polished thin sections. Internal structures of grossular from Mexico and pyrope from the Italian Alps were characterized and visualized by CL-microscopy. The different growth domains were additionally studied using CL-spectra and electron microprobe (EMP) analysis. Grossular shows a patchy zonation in its core while in mantle and rim zones oscillatory zoning is observed. It contains zones of anomalous birefringence, zones of orange and bluish luminescence and zones lacking luminescence. Different but low amounts of the activator elements Mn2+ and Eu2+ are responsible for the orange and bluish luminescent domains. Pyrope is also characterized by oscillatory growth zones, shows a dull luminescent core with a change of crystal morphology during growth, and displays an increase of brightness from core towards rim—the outermost rim, however, is lacking luminescence. The different luminescent zones are characterized by different amounts of Dy3+, Tb3+, Sm3+ and Sm2+ as activator elements. Because of slow diffusion rates of activators such as the REEs Sm, Dy and Tb, it can be still possible to visualize possible prograde and/or peak pressure stage growth domains of garnet, even if later high temperature events may have homogenized the major element profiles. Such domains may help to identify respective assemblages of mineral inclusions, and hence these results can represent an integral part of a detailed PT path. Thus the CL-information can be used as an important pathfinder prior to supplementary investigations, as for instance EMP, ion probe, mineral or fluid inclusion studies.

Zr-Ti bearing minerals in metamorphic rocks from the Koyubi-Ridge, Sør Rondane Mountains, East Antarctica

Zr-Ti bearing minerals in metamorphic rocks from the Koyubi-Ridge, Sør Rondane Mountains, East Antarctica

XMapTools: A MATLAB©-based program for electron microprobe X-ray image processing and geothermobarometry

XMapTools is a MATLAB©-based graphical user interface program for electron microprobe X-ray image processing, which can be used to estimate the pressure–temperature conditions of crystallization of minerals in metamorphic rocks. This program (available online at http://www.xmaptools.com) provides a method to standardize raw electron microprobe data and includes functions to calculate the oxide weight percent compositions for various minerals. A set of external functions is provided to calculate structural formulae from the standardized analyses as well as to estimate pressure–temperature conditions of crystallization, using empirical and semi-empirical thermobarometers from the literature. Two graphical user interface modules, Chem2D and Triplot3D, are used to plot mineral compositions into binary and ternary diagrams. As an example, the software is used to study a high-pressure Himalayan eclogite sample from the Stak massif in Pakistan. The high-pressure paragenesis consisting of omphacite and garnet has been retrogressed to a symplectitic assemblage of amphibole, plagioclase and clinopyroxene. Mineral compositions corresponding to ~165,000 analyses yield estimates for the eclogitic pressure–temperature retrograde path from 25kbar to 9kbar. Corresponding pressure–temperature maps were plotted and used to interpret the link between the equilibrium conditions of crystallization and the symplectitic microstructures. This example illustrates the usefulness of XMapTools for studying variations of the chemical composition of minerals and for retrieving information on metamorphic conditions on a microscale, towards computation of continuous pressure–temperature-and relative time path in zoned metamorphic minerals not affected by post-crystallization diffusion.

東南極Howard Hillsの変成岩中の鉱物および山鹿地域はんれい岩体に産する珪長質変成岩中のジルコンのU-Pb年代 ～表面電離型質量分析計によるU-Pb分析の例～

東南極Howard Hillsの変成岩中の鉱物および山鹿地域はんれい岩体に産する珪長質変成岩中のジルコンのU-Pb年代 ～表面電離型質量分析計によるU-Pb分析の例～

First find of silver mineral in metamorphic rocks of the Stanovoi Complex

First find of silver mineral in metamorphic rocks of the Stanovoi Complex

Crystal-size distribution and composition of garnets in eclogites from the Dabie orogen, central China

Crystal-size and spatial distributions of minerals in metamorphic rocks provide insight into their nucleation, growth environment, and the metamorphic evolution of their host rocks. Episodic nucleation and growth histories for garnets in eclogites from the Dabie orogen are revealed by variations in crystal size, geochemical zoning, and mineralogy of inclusions in garnets. The studied garnets show pseudo-lognormal crystal-size distributions (CSDs), prograde chemical zoning patterns indexed by mineral inclusions, and are mainly distributed at random. Constant growth rate for each episode is proposed based on the mineralogy of inclusions in garnets and the chemical zoning patterns. The CSD shapes were evaluated in terms of a size-dependent proportionate growth model and a thermally accelerated, diffusion-controlled model but neither is consistent with the geochemical data and the concordant occurrence of mineral inclusions in garnet. Initial increasing nucleation rate followed by a subsequent, medial stage of nearly constant growth and finally declining nucleation rate is inferred from the CSD data. This study suggests that linking chemical analysis with textural analysis is crucial to avoid misleading interpretation solely through CSD shapes of minerals in metamorphic rocks.

Patterns of mineral occurrence in metamorphic rocks

Patterns in the occurrence of minerals in metamorphic rocks suggest additional opportunities for investigating chemical and physical processes during metamorphism. Three such patterns are reviewed. First, trace minerals in metamorphic rocks commonly occur with regular distributions indicating their participation in prograde reactions that can be mapped as isograds. Examples include the distribution of allanite and monazite in pelitic rocks and of zircon and baddeleyite in siliceous dolomites. Recognition of these isograds points to the potential for developing a chronology of specific chemical reactions during metamorphism and for defining the P - T conditions of those reactions. Second, the mineralogical products of retrograde metamorphism in many cases occur in distinctive associations that are consistent with partial mineral-fluid equilibrium. Examples include the distribution of retrograde calcite, quartz, and tremolite in siliceous limestones and of retrograde tremolite, dolomite, brucite, and serpentine in siliceous dolomites from contact aureoles. Among other things, application of partial equilibrium to retrograde metamorphic rocks leads to constraints on the amount and direction of fluid flow in contact aureoles as they cool. Third, pseudomorphs are typically absent from prograde metamorphic rocks but are common in retrograde metamorphic rocks. The distribution may be explained by the effect of “force of crystallization.” The pattern of occurrence of pseudomorphs thus suggests novel phenomena during metamorphism that develop from an interplay between chemical and mechanical processes.

Formation of ore minerals in metamorphic rocks of the German continental deep drilling site (KTB)

The German Continental Deep Drilling Program (KTB) drilled a 9.1 km deep profile through amphibolite facies metamorphic rocks and reached in situ temperatures of 265°C. Each lithologic unit is characterized by typical ore mineral assemblages related to the regional metamorphic conditions. Paragneisses contain pyrrhotite + rutile + ilmenite ± graphite, metabasic units bear ilmenite + rutile + pyrrhotite ± pyrite, and additionally, the so‐called variegated units yield pyrrhotite + titanite assemblages. In the latter unit, magnetite + ilmenite + rutile + titanite assemblages related to the lower amphibolite facies breakdown of ilmenite‐hematite solid solution also occur locally. Retrograde hydrothermal mineralization which commenced during Upper Carboniferous times is characterized by the following geochemical conditions: (1) low saline Na‐K‐Mg‐Cl fluids with sulfur and oxygen fugacities at the pyrite‐pyrrhotite buffer and temperatures of 400–500°C, (2) fluids with CO2, CH4±N2, andpH, Eh, sulfur, and oxygen fugacity in the stability field of graphite + pyrite at temperatures of 280–350° and (3) moderate to high saline Ca‐Na‐Cl fluids with CH4+ N2; sulfur and oxygen fugacity are in the stability field of pyrrhotite at temperatures &lt;300°C. The latter environment is confirmed by in situ conditions found at the bottom of the deep drilling. Monoclinic, ferrimagnetic pyrrhotite is the main carrier of magnetization which disappears below about 8.6 km, corresponding to in situ temperatures of about 250°C. Below this depth, hexagonal antiferromagnetic pyrrhotite with a Curie temperature of 260°C is the stable phase. Temperature‐dependent transformation of pyrrhotite and the reaching of its Curie isotherm within the Earth crust are one of the striking results of the KTB deep drilling project.

A crustal mineral in a mantle diamond

RECENT studies of diamonds, inclusions in diamonds, and rock fragments from the Earth's mantle have shown the presence of minerals and rocks with a chemical composition that suggests a previous origin in the crust; presumably, this crustal material was transported into the mantle at a subduction zone. The chemical evidence of previous residence of mantle materials in the crust has come principally from determinations of the proportions of the isotopes of oxygen1,2, sulphur3,4, lead4,5 and carbon6, which typically show a much wider range of composition in the crust than in material of purely mantle origin. Here we report a less subtle crustal signature in a mantle-derived diamond: an inclusion of the mineral staurolite. Staurolite has never previously been found in mantle diamonds or rocks, but is a common mineral in metamorphic rocks in the crust. The characteristics of the staurolite suggest not only the probability that it formed in rocks with a crustal bulk composition, but also the possibility that it formed by metamorphism in the crust or in a subduction zone, before being incorporated in the mantle and, eventually, in a growing diamond.

変成鉱物の粒径分布と空間分布

This paper reviews recent progress in analysis of crystal size distributions (CSD) and spatial dispositions of minerals in metamorphic rocks. Mineral growth in metamorphic rocks consists of two stages: the recrytallization stage in which nucleation and growth makes in progress by net transfer reactions and the annealing stage in which Ostwald ripening is operative. Minerals from contact aureoles will preserve the original CSD produced by nucleation and growth because it may suffer little annealing effects. Two distinct CSD patterns, however, has been reported from contact aureoles. One is that by Kretz, which shows nearly log-normal distribution, indicating time-dependent nucleation rate under the assumption of constant growth rate. The other is that by Cashman and Ferry, which shows CSD pattern caused by constant rates of nucleation and growth. Normal or log-normal size distributions of minerals from regional metamorphic terrain has been also an enigma, because such CSD can be produced either by Ostwald ripening, or by diffusion-controlled nucleation and growth. This paper will give an overview for the resolution of these problems.