Hans-Joachim Massonne

Microdiamond daughter crystals precipitated from supercritical COH + silicate fluids included in garnet, Erzgebirge, Germany

Microdiamonds associated with phlogopite, quartz, paragonite, phengite, apatite, and rutile are found as regular constituents of minute polyphase inclusions in garnet of ultrahigh-pressure ( P ) metamorphic gneiss lenses within migmatites of the gneiss-eclogite unit, Erzgebirge, Germany. These aggregates are interpreted to represent original inclusions of a supercritical dense COH fluid rich in K, Na, and SiO 2 . From this fluid, diamond was precipitated as a daughter crystal due to cooling at ultrahigh- P conditions. Brittle failure of the garnet host due to overpressure during release of confining pressure is demonstrated by healed radial cracks. During further cooling, the silicate phase assemblage of the inclusions crystallized at reduced internal pressures outside the stability field of diamond, as indicated by the presence of quartz, paragonite, and plagioclase. It is proposed that the mica-dominated mineral assemblage of the inclusions formed by reaction between the fluid and the garnet host, the previously formed diamond daughter crystals being preserved metastably. These diamond-bearing inclusions provide an unequivocal record of dense supercritical COH fluids rich in alkalies and silica within subducted continental crust during ultrahigh- P metamorphism.

A comparison of the evolution of diamondiferous quartz-rich rocks from the Saxonian Erzgebirge and the Kokchetav Massif: are so-called diamondiferous gneisses magmatic rocks?

Abstract The petrography and chemical composition of minerals of quartz-rich diamondiferous rocks from the Kokchetav Massif, especially the zonation of garnet, were studied and compared with diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge. Many compositional and textural features were found to be similar. For instance, microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks. On the basis of experimental data, a magmatic scenario was constructed to check if the quartz-rich diamondiferous rocks are of magmatic origin. By this, the P–T paths, derived here for the Kokchetav rocks, and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts. These melts originated by anatexis of deeply submerged metasediments (Erzgebirge: at T as high as 1200°C, Kokchetav Massif: at 50–100°C lower T) and ascended from at least 200 km depth. Relics of the pre-anatectic evolution are still present, for instance, as garnet cores. After ascent and emplacement of the magma in deep portions of thickened continental crust (Kokchetav Massif: 45–50 km close to 800°C, Erzgebirge: 55–60 km at 30–50°C lower T) considerable quantities of (white and/or dark) micas formed by peritectic reactions from melt. For instance, garnets could be resorbed at this stage and biotite grew instead. After the magmatic stage, retrogression took place much stronger in the Kokchetav Massif. This was accompanied by deformation transforming broadly the magmatic texture of quartz-rich diamondiferous rocks from the Kokchetav Massif to a gneissic texture.

Microdiamonds from the Saxonian Erzgebirge, Germany in situ micro-Raman characterisation

The identity of diamond inclusions in quartz-muscovite gneisses occurring in the Saxonian Erzgebirge near the Saidenbach reservoir was proven by laser-Raman micro-spectroscopy. The identification was additionally backed-up by graphitization of diamond inclusions as a result of irradiation with a high-energy laser beam. Enclosed microdiamonds are mostly idiomorphic and are up to 30 μm in size. They occur preferentially in garnet but were also found in zircon and kyanite. The microdiamond inclusions give evidence for ultra-high pressure metamorphism experienced by the host rock.

First find of coesite in the ultrahigh-pressure metamorphic area of the central Erzgebirge, Germany

In the ultrahigh-pressure metamorphic area of the Saidenbach reservoir, Erzgebirge, where microdiamond- bearing gneiss occurs, coesite was found as inclusion in garnet and omphacite in an unusually well preserved eclog- ite. Most of the eclogite bodies of this area are thoroughly altered, with all omphacite transformed to plagioclase + amphibole symplectites. The coesite-bearing eclogite contains abundant dolomite in the matrix and as surprisingly large inclusions in garnet, which is another unusual feature compared to the many eclogites of the Saxonian Erzgebirge. Coesite is at least partly replaced by quartz showing a palisade texture. This pattern is accompanied by abundant radial cracks in the host minerals. Polycrystalline inclusions of equigranular quartz are also related to former coesite because some of them preserve minor coesite relics. In such cases, cracks in the host mineral are not more pronounced than around moderately transformed coesite. No K-feldspar was observed in inclusions consisting of partly replaced coesite. Thus, inclusions in omphacite of K-feldspar-quartz symplectites with abundant cracks around, which occur as well in eclogites near the Saidenbach reservoir, cannot be coesite pseudomorphs.

Phase relations and dehydration behaviour of psammopelite and mid-ocean ridge basalt at very-low-grade to low-grade metamorphic conditions

P–T pseudosections were calculated in the system Na–Ca–K–Fe–Mg–Al–Si–Ti–H–O with the PERPLE_X software package for the pressure–temperature range 1–25 kbar and 150–450 °C to gain a better understanding of the phase relations and the dehydration behaviour of psammopelite and mid-ocean ridge basalt (MORB) during prograde metamorphism at very-low-grade and low-grade. For this purpose, the thermodynamic data set of Holland & Powell was enlarged by end-member data for Fe2+- and Fe3+-pumpellyite, Fe2+- and Mg-stilpnomelane, actinolite, and magnesioriebeckite. In addition, a three-component solid-solution model for pumpellyite, a two-component model for stilpnomelane, and four-component models for amphibole and sodic pyroxene were created. Studied metamorphosed MORB and psammopelite contain around 6 wt.% structural H2O bound in minerals at 150 °C and pressures up to 5 kbar. Prograde metamorphism causes dehydration patterns, which are, for example, important for an understanding of the formation of accretionary-wedge systems: along a relatively high geotherm of 15 °C/km, significant dehydration (1.5 wt.% H2O release) of metapsammopelite can be noted in the temperature range 220–240 °C. We believe that this process leads to softening of the sedimentary cover of oceanic crust during early subduction so that this material can be scraped off the basic crust, which then would dehydrate at higher T , to form frontal accretionary prisms. Basal accretionary prisms are generated at lower geotherms ( e.g ., 12 °C/km) by dehydration of metapsammopelite between 260 and 300 °C. Again, metabasic material would dehydrate at significantly higher T and is, therefore, only subordinately involved in accretionary wedges. Along geotherms lower than 7 °C/km, almost no water is released up to temperatures of 400 °C and more. Thus, the corresponding material is subducted to mantle depths as in the subduction/exhumation channels of collision zones. We also hypothesize that accretionary-wedge complexes of the hot subduction zones in Precambrian times should mainly have formed by frontal accretion.

Focused ion beam technique and transmission electron microscope studies of microdiamonds from the Saxonian Erzgebirge, Germany

A focused ion beam of Ga ions is a relatively new technique that has been developed for microelectronic industries. Now researchers of the Earth sciences find it to be a promising tool for studying various geological materials. Using the FIB technique and an FEI Strata DB 235 dual beam system, we have successfully prepared several electrontransparent foils, which crossed Wm-sized diamonds included in host minerals such as zircon and garnet from quartzofeldspathic rocks of the Saxonian Erzgebirge, Germany. Scanning and transmission electron microscopy applied to these foils revealed that the diamonds contain crystalline nanometric inclusions. These inclusions consist of minerals of known stoichiometries such as SiO2 and Al2SiO5, whereas others are characterized by different combinations of Si, K, P, Ti, and Fe in the presence of oxygen (stoichiometries are not clear at this stage of research). One suite of inclusions is assumed to be represented by archerite, KH2PO4, which is known to be stable at pressures of 4^22 GPa, and one nanocrystal containing Pb, oxygen and carbon is interpreted to be PbxOy or PbCO3. Along with solid crystalline inclusions, the diamonds contain cavities filled by liquid/gas that escaped during sample preparation. These are associated with dislocations of diamond growth. Our data are consistent with the concept of diamond crystallization from a COH-rich multicomponent supercritical fluid and suggest that the composition of such a fluid is more consistent with a local crustal source rather than that of a mantle origin. @ 2003 Elsevier Science B.V. All rights reserved.

A high precision U–Pb age of metamorphic rutile in coesite-bearing eclogite from the Dabie Mountains in central China: a new constraint on the cooling history

This paper first reports a high precision U–Pb age of 218±1.2 Ma for rutile in coesite-bearing eclogite from Jinheqiao in the Dabie Mounteins, east–central China. This work shows that the U–Pb mineral (rutile+omphacite) isochron age of 218±2.5 Ma and conventional rutile U–Pb concordia age of 218±1.2 Ma obtained by common Pb correction based on the Pb isotopic composition of omphacite in the same eclogite sample are consistent, proving that the omphacite with low U/Pb ratio (μ=2.8) can be used for common Pb correction in U–Pb dating of rutile. Oxygen isotope analysis of rutile aliquots gave the consistent δ18O values of −6.1±0.1%, demonstrating oxygen isotope homogenization in the rutile of different grains as inclusion in garnet and grain in matrix. Oxygen isotope thermometry yields temperatures of 695±35 and 460±15 °C for quartz–garnet and quartz–rutile pairs, respectively. These oxygen isotopic observations suggest that the diffusion of oxygen in rutile as inclusion in garnet is not controlled by garnet. According to field-based thermochronological studies of rutile, an estimate of the Tc of about 460 °C for U–Pb system in rutile under rapid cooling conditions (∼20 °C/Ma) was advised. Based on this U–Pb age as well as the reported chronological data with their corresponding metamorphic and/or closure temperature, an improved T–t path has been constructed. The T–t path confirms that the UHPM rocks in South Dabie experienced a rapid cooling following the peak metamorphism before 220 Ma and a long isothermal stage from 213 to 180 Ma around 425 °C.

Characterization of an early metamorphic stage through inclusions in zircon of a diamondiferous quartzofeldspathic rock from the Erzgebirge, Germany

Abstract About 1000 zircon grains from a diamondiferous quartzofeldspathic rock of the Saxonian Erzgebirge were investigated for inclusions using optical microscopy and confocal laser-Raman spectroscopy. Cathodoluminescence imagery was applied to characterize the growth zone of zircon where the inclusions occurred. The most abundant inclusion minerals are microdiamonds. Coesite was not detected. However, garnet and jadeite occur as rare inclusions in zircon cores where diamonds are lacking. Jadeite was detected for the first time in quartzofeldspathic rocks from the crystalline complex of the Erzgebirge. The compositions of the pristine garnets in the zircons are similar to those of core areas of millimeter-sized garnets but the original garnet composition of the early metamorphic stage is only preserved in zircon. Intracrystalline diffusion at temperatures as high as 1000 °C resulted, for instance, in higher Ti concentrations in garnet cores compared with garnet enclosed in zircon. Rutile, quartz, and the compositions of jadeite and garnet inclusions in zircon and of phengite inclusions in cores of large garnets were applied for geothermobarometry. The results, related to an early metamorphic stage, are 650 °C and 18 kbar, which represent conditions at the base of a thickened continental crust before deeper subduction.

Dating of zircon and monazite from diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge – hints at burial and exhumation velocities

Abstract In order to better understand the formation and evolution processes of ultrahigh pressure (UHP) felsic rocks, we determined the ages of various domains of zircon and monazite crystals from the diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge. According to cathodoluminescence imagery and Th/U ratios, three zircon zones were distinguished. Each was dated using several spot analyses from a sensitive high-resolution ion microprobe analysing Pb, U and Th isotopes. The results were: (1) core zone - 21 analyses: Th/U ≤0.023 and 337.0±2.7 Ma (2σ, combined 206Pb/238U-207Pb/235U age); (2) diamond-bearing intermediate zone - 23 analyses: Th/U ≥0.037 and 336.8±2.8 Ma; and (3) rim zone -12 analyses: Th/U = 0.015-0.038 (plus one analysis of 0.164) and 330.2±5.8 Ma. The U-Pb obtained ages are virtually concordant. Furthermore, two oscillatory zoned zircon cores (Th/U ≥0.8) yielded (~concordant) ages of ~400 Ma. Six SHRIMP analyses of monazites gave an age of 332.4±2.1 Ma. In addition, Pb, Th and U contents in monazite were analysed with an electron microprobe (EMP). A mean age of 324.7±8.0 (2σ) Ma was acquired from 113 analyses. By combining the defined ages with previously published P-T conditions, minimum velocities for burial and exhumation were estimated. In addition, we present a likely geodynamic scenario involving age data from the literature as well as this study: beginning 340 million years ago, gneisses at the base of a thickened continentalcrust (~1.8 GPa, 650ºC) were transported to depths of at least 130 km, possibly as deep as 250 km. Here they were heated (>1050ºC) and partially melted and as a result began to rise rapidly. The burial and subsequent ascent back to a depth of 50 km, where zircon rims and monazite formed, took only a few million years and perhaps significantly less.

Geochronological and Petrological Constraints for Tectonic Evolution of the Central Greater Himalayan Sequence in the Kharta Area, Southern Tibet

Within the Kharta area, east of Mount Qomolangma (Everest), garnet sillimanite gneisses and granites including mafic lenses that form the Greater Himalayan Sequence (GHS) were displaced beneath the North Col Formation by the lower South Tibetan Fault (LSTF) and above the Lesser Himalayan Sequence by the Main Central Thrust (MCT). Zircons from mafic lenses, probably former dikes, were dated by thermal ionization mass spectrometry to yield discordant U‐Pb results. One sample gave a discordia line with an upper intercept age of 971 Ma, suggesting a late Proterozoic age for emplacement of the dike. Zircon SHRIMP analyses show that the garnet sillimanite gneisses and granites were derived mainly from early Paleozoic rocks produced by high‐temperature metamorphism and/or partial melting of Neoproterozoic (meta)sedimentary rocks. These crystalline rocks were buried beneath southern Tibet and experienced metamorphic conditions of 750°–800°C and 14 kbar at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape