Showing papers in "European Journal of Mineralogy in 1996"

Extreme 18 O depletion in eclogite from the Su-Lu terrane in East China

Abstract: The unusually low Ö180 values of -10.3 to -6.8%0 are found in mineral separates from an ec10gite pod (containing quartz-schist bands) in the Su-Lu terrane, East China. These are the lightest oxygen isotope compositions so far reported for metarnorphic rocks in the world. Oxygen isotope equilibrium has been approached among the minerals within both ec10gite and quartz-schist. This not only yields the concordant isotope temperatures of 655 to 765°C, but also indicates that the rocks have exchanged oxygen isotopes with a very 180-depleted fluid. A plausible interpretation for the anomalously low Ö180 values of the ec10gite pod is that it equilibrated isotopically with ancient meteoric water prior to ec1ogite-facies metamorphism and thus preserved the premetamorphic isotopic signature. The hydrothermally altered basaltic rocks kept this isotopic signature through all stages of progressive UHP metamorphism as weIl as subsequent exhumation and cooling. This provides insight into limited crust-mantle interactions under ultrahigh pressure metamorphic conditions in the suture zone of a continent-continent collision. Key-words: oxygen isotopes, water-rock interaction, isotope geothermometry, ec1ogite, UHP metamorphism, continental collision, Dabie Mountains.

Crystallinity, crystallite size and lattice strain of illite-muscovite and chlorite: Comparison of XRD and TEM data for diagenetic to epizonal pelites

Abstract: Crystallinity indices, apparent mean crystallite sizes, lattice strain values and crystallite thickness dis­ tributions of illite-muscovite and chlorite from metapelitic rocks have been determined by powder XRD and/or by HRTEM. The studied slates and phyllite derive from the NE-Hungarian Paleozoic and Mesozoic formations affected by Alpine (Cretaceous) regional metamorphism, and cover a range from late diagenesis to the epizone (greenschist facies chlorite zone). The average crystallite size for both illite-muscovite and chlorite increases with increasing metamorphie grade, although illite-muscovite crystallites are thicker than those of chlorite. XRD­ measured % lattice strain generally decreases with grade, with chlorite showing greater % strain than illite-mus­ covite, consistent with TEM evidence of strain-related textures in chlorite but rarely in illite-muscovite. There is reasonably good correlation between TEM-and XRD-determined crystallite sizes, especially for mean thickness as determined by the Scherrer method. However, there are significant differences in results for four different XRD data-based methods (Scherrer, Voigt, variance and Warren-Averbach), inferred to be caused by approximations in each method, and between results for different standards. Therefore, ace urate, absolute values of mean thickness cannot be predicted on the basis of illite and chlorite crystallinity or XRD line-profiles, and it is not possible to eorrelate precise values with boundaries of the zone of diagenesis, anchizone and epizone. Nevertheless, illite or chlorite erystallinity can be correlated with reasonably narrow ranges of mean crystallite size as a well-behaved function of metamorphic grade. Key-words: crystallinity, crystallite size, lattiee strain, illite-muscovite, chlorite, very low-grade metamorphism, anehizone, epizone. *e-mail : arkai@sparc.core.hu 001:10.1127/ejm/8/5/1119 0935-1221/96/00081 119 $ 4.75 © 1996 E . Schweizcrbart'sche Verlagsbuchhandlung. D-70176 Swtlgart

Nafertisite, a layer titanosilicate member of a polysomatic series including mica

Abstract: Nafertisite, (Na,K)3(Fe +,Fe +,D)IO[Tiz(Si,Fe +,AI)12037](OH,O)6, is an alkali ne titanosilicate from the Khibina massif (Kola Peninsula, Russia). It is monoclinic A2/m with a = 5.353(4), b = 16.176(12), C = 21.95(2) A, ß = 94.6(2r, Z = 2. The comparison with bafertisite, Ba2(Fe,MnMTi2Si4017](OH,Oh, and astrophyllite, (K,Na)3(Fe,Mn)7 [TizSiR027](OH,O)4, allowed to obtain a structural model for nafertisite which has been tested against X-ray diffraction data obtained from a very poor crystal. On the basis of a bafertisite-like B module (A'oh(MD)4 [X2T4017](OHh and a mica-like M module (A,D)(MDh[T401O](OH)2, a polysomatic series BMn can be defined where bafertisite, astrophyllite, nafertisite and mica are the members with n = 0, 1, 2, 00, respectively. It is called heterophyllosilicate series because the crystal structures of the members consist of 2: 1 HOH layers where 0 is an octahedral sheet and H is a tetrahedral-like sheet which differs from the T sheet of the phyllosilicates for the insertion of Ti octahedra; these play a role similar to that of Si tetrahedra. A group of titanosilicates is shown to be derivatives of the bafertisite structure with different interlayer contents. Key-words: nafertisite, crystal structure, polysomatic series, titanosilicates, phyllosilicates.

Vein antigorites from Elba Island, Italy

Abstract: Antigorite in veins from Elba Island, Italy, appears as light-green splintery fibers, discontinuously sUITounding the massive dark serpentinite, extending along fractures and deformed by compressive shearing. The chemical compositions cluster around Mgl.66Feo.12Alo.Q2Sh.oo05(OHh,60 with the exact value depending upon the polysomatic structural modulations. Electron diffraction shows superpe­ riodicities variable from 33 to 49 A and changing from vein to vein, with no relation with the metamorphic grade of the sUITounding rocks. Common polysomatic faulting, ( 001) twinning and parallel association with polygonal serpentine as weil as with anomalous (thin wall tubes) chrysotile are the most evident microstructural features. A complete set of X-ray diffraction, IR, NMR and thermal analyses results is presented for a selected number of specimens. With respect to massive antigorite serpentinites, antigorite veins show periodicities shorter than expected; this feature is interpreted as due to shearing stress and/or favourable kynetics able to accelerate the sluggish chrysotile­ antigorite transformations. Key-words: serpentine, antigorite, Elba, spectroscopy, diffraction, polysomatism.

Standardized clay mineral crystallinity data from the very low-grade metamorphic facies rocks of southern New Zealand

Abstract: X-ray diffraction determined clay mineral crystallinity data, standardized to the Crystallinity Index Standard (CIS) scale, is presented for illite (white mica)and chlorite-bearing pelitic rocks of the zeolite, prehnite-pumpellyite, pumpellyite-actinolite and the greenschist facies. Good correlations exist between both illite and chlorite crystallinity and crystallite size data, with respect to the mineral (metabasite) facies conditions, using both the Full-Width-at Half-Maximum and a Siemens Single-Line Fourier method, while calculations employing the Warren-Averbach method yielded unacceptably high degrees of error, and hence poor correlations. these preliminary data conform with the commonly held view that the diagenetic-, anchi-, and epi-zones of Kublers illite crystallinity boundary limits, adopted by the CIS, correspond with the zeolite, prehnite-pumpellyite-actinolite, and greenschist facies, respectively. The CIS scale is therefore considered a reliable indicator of metamorphic grade, when adequate attention is given to the calibration of the experimental data and the selection of suitable peak reflections. The air-dried Sr-or Ca-saturated illite 002 and chlorite 003 peak combination are recommended as two useful reflections for accurate and rapid XRD grade determinations in very low-grade metamorphic rocks.

Microstructural controls on sites of metamorphic reaction; a case study of the inter-relationship between deformation and metamorphism

Abstract: Compositional zonation in garnet porphyroblasts in two samples from the same outcrop within the Meran-Mauls Basement of the Austro-Alpine of South Tyrol show at least four growth stages with three of them characterized by repetitive grossular zoning. This occurred due to repeated cycles of growth commencing early during a deformation and then ceasing and resulted from reaction overstepping and termination of micrometaso- matic access of material to the porphyroblast margins as the matrix foliation intensified. The composition of these garnets varies from smooth to abrupt reductions in grossular and spessartine content (and a rise in X\ig) in different microstructural sites. The transitions are smooth where the deformation event accompanying each phase of garnet growth was partitioned such that it was only weakly developed against the porphyroblast. However, they are abrupt where the deformation was more intensely developed against the earlier- formed garnet core due to dissolution of the rim of this portion. Both features indicate a strong microstructural control on the sites of porphyroblast growth and associated chemical reaction within syntectonically metamorphosed rocks. This control has resulted from the effects of partitioning of the deformation, and consequent variation in the intensity of foliation development, on both the access of material to the growth site in zones of progressive shortening plus dissolution of portions of the porphy­ roblast rims along zones of considerable progressive shearing. In one of the samples this microstructural control on sites of growth and associated chemical reaction has resulted in cessation of growth throughout one whole period of deformation.

Low-temperature ordered phase of CaU(PO 4 ); synthesis and crystal structure

Abstract: The crystal structure of the new compound CaU(P04)2 was determined from X-ray single-crysta! data and refined to R(F) = 0.025 and Rw(F) = 0.023 for 993 independent observed reflections. The unit cell is ortho­ rhombic, a = 13.926(4) A, b = 6.958(3) A and c = 6.136(2) A, space group Pnma. The structure is Ca, U ordered and in some aspects similar to the xenotime and anhydrite structure. The coordination polyhedra for the large meta! ions are described as semi regular trigonododecahedra. Key-words : crysta! structure, synthesis, CaU(P04)2, xenotime, monazite-type.

Fluid inclusion and stable isotope constraints on the genesis of the Cligga Head Sn-W deposit, S.W. England

Abstract: Samples of veins from the Cligga Head granite-hosted Sn-W deposit, S.W. England, have been studied by microthermometry, and analysed for the oxygen-isotope composition of quartz, cassiterite and wolframite and the hydrogen-isotope composition of fluid inclusions. Samples of vein quartz were also analysed for fluid-inclusion chemistry using a crush-leach technique. Oxygen-isotope geothermometry using quartz-cassiterite and quartz-wolframite pairs indicates temperatures of 350 to 400°C, although disequilibrium may be common. Coupled with fluid-inclusion homogenisation temperatures of between 300 to 350°C the data suggests hydrothermal activity at 350 to 400°C, and 0.7 to 1.7 kbar fluid pressure. High Th in some fluid inclusions suggests that pressure may have dropped below 0.7 kbar during periods of vein dilation. The calculated oxygen- and measured hydrogen-isotopic composition of vein fluids (δ18O from 6.0 to 10.8‰; δD from -52 to -22‰) is consistent with an origin as fluid that had equilibrated with granite at temperatures from approximately 500 to 300°C. Crush-leach analyses show the inclusion fluids to be Na-K-Fe-Ca chloride brines with significant levels of B, F and SO4. The K/Na ratios are consistent with equilibration with granite at progressively lower temperatures. Log Br/Cl in the inclusion fluids ranges from -3.40 to -2.90, comparable to the ratios of modern fumarole gases, and consistent with the degassing of magmatic volatiles as the source of salinity.

Methane inclusions in shocked quartz from the Gardnos impact breccia, South Norway

Abstract: A meteorite impact structure of Late-Proterozoic age has recently been recognized at Gardnos in the Hallingdal area, South Norway. The structure is situated in pre-llOOMa gneis ses and metasediments, and consists of a zone of autochtonous breccia, a suevite (impact melt) breccia and a series of unbrecciated crater fill sediments. In the late Silurian, the impact structure was overrun by Caledonian nappes, leading to low-grade metamorphic recrystallization (T rnax :s; 400oC, P rnax � 2.5 kbar). Shocked quartzite from the autochtonous breccia is impregnated by fine-grained carbonaceous material, giving it a nearly black colour in hand specimens. Micro-Raman spectroscopy shows this material to be poorly crystalline. Planar fractures, typical of shocked quartz, are outlined by graphite inclusions and by trails of secondary fluid inclusions. The fluid inclusions comprise methane inclusions, with minor carbon dioxide and no visible water (XCH4 � 96 mol%) and water inclusions with moderate salinity. The methane inclusions show H1, H2 and S2 types of microthermometric behaviour; H I inclusions show a peak of homogenization temperatures to the liquid at 84 to -112 oe. At T :s; 400oC, this corresponds to isochore pressures of 1 to 2.5 kbar, which is compatible with a Caledonian cooling and uplift path, but not with the extreme pressure at the moment of the impact, nor the low pressures (P :s; 0.2 kbar) encountered in the shallow parts of the impact structure during the final stages of post-impact cooling. Methane was formed in-situ by reaction between solid carbonaceous material and aqueous metamorphic fluid, and was trapped as the partly open planar fractures healed. The methane-rich fluid inclusions in shocked quartz from the Gardnos impact structure are thus only indirectly related to the meteorite impact and not at all to hydrocarbons of a deep (mantle) origin. Key-words: methane, fluid inclusions, impact breccia, metamorphism, Caledonides, Gardnos, Norway.