Martin Svojtka

3 – Diamond and Other Possible Ultradeep Evidence Discovered in the Orogenic Spinel-Garnet Peridotite from the Moldanubian Zone of the Bohemian Massif, Czech Republic

Publisher Summary In the Plesovice peridotite, several lines of evidence suggesting an ultradeep condition are found. All of these facts suggest that the Plesovice peridotite once resided in the diamond-stability field, which could not be identified by the previous geothermobarometric studies. This suggests that diamond was stable in the rock before Stage I (high-temperature spinel6garnet peridotite stage), which is the oldest generation of mineral assemblage previously recognized in the Plesovice peridotite. Furthermore, the presence of pyroxene exsolution lamellae in the earlier generation of spinel (Stage I) implies the preexisting high-pressure stage before Stage I. If graphite in MS inclusions were originally crystallized as diamond, parental fluids responsible for MS inclusion was most likely entrapped in the host mineral before Stage I, and they should have crystallized daughter minerals during the cooling/exhumation trajectory from the diamond-stability field, through Stage I, to the chlorite peridotite stage. For MS inclusions within Spinel-I, there is a good possibility original fluid/melt was entrapped in the diamond-stability field, if host Spinel-I was originally its high-pressure polymorph, Ca-ferrite, or Ca-titanite. On the other hand, one could not find the typical pyroxene exsolution texture in garnet. This probably suggests that most garnet was secondary formed at the expense of Spinel-I at relatively low-pressure condition. Therefore, if graphite in garnet was originally diamond, it is most likely that the diamond was graphitized before the entrapment within garnet crystal.

Trace element composition of quartz from different types of pegmatites: A case study from the Moldanubian Zone of the Bohemian Massif (Czech Republic)

Abstract The evolution of the trace-element patterns of quartz during crystallization of pegmatite melt was investigated using laser ablation inductively coupled plasma mass spectrometry. The contents of Al, B, Ba, Be, Cr, Fe, Ge, Li, Mn, P, Rb, Sn, Sr and Ti were analysed in quartz from the border, intermediate and core zones of four granitic pegmatites differing in degree of fractionation and origin. The material investigated originates from the pegmatite district of the Strážek Unit, Moldanubian Zone, Bohemian Massif, Czech Republic and includes: lepidolite LCT (Li-Cs-Ta) pegmatite from Rožná; berylcolumbite LCT pegmatite from Věžná; anatectic pegmatite from Znětínek; and intragranitic NYF (Nb- Y-F) pegmatite Vladislav from the Třebíč Pluton. The abundances of the elements analysed varied over wide intervals: <1 to 32 ppm Li, 0.5 to 6 ppm B, <1 to 10 ppm Ge, 1 to 10 ppm P, 10 to 450 ppm Al, 1 to 45 ppm Ti and <1 to 40 ppm Fe (average sample contents). Concentrations of Be, Rb, Sr, Sn, Ba, Cr and Mn are usually <1 ppm. Quartz from LCT pegmatites exhibits a distinct evolutionary trend with a decrease in Ti and an increase in Al, Li and Ge from the pegmatite border to the core. In comparison with the most fractionated rare-metal granites, pegmatitic quartz is relatively depleted in Al and Li, but strongly enriched in Ge. Quartz from simple anatectic and NYF pegmatites is poor in all trace elements with their evolution marked by a decrease in Ti and a small increase in Ge. There is little Al or Li and neither shows any systematic change with pegmatite evolution. Using the Ti-in-quartz thermobarometer, the outer zones of the Znětínek and Vladislav pegmatites crystallized at ~670°C, whereas the border zone in the Rožná pegmatite yields a temperature near 610°C.

Evaluation of Molar Volume Effect for Calculation of Diffusion in Binary Systems

This article relates the theoretical evaluation of the effect of a molar volume change in the diffusion profile of a binary system. This study is based on the regression fit of a diffusion profile using a selected function in an inverse form. For the calculation of concentration-dependent diffusivity according to the Sauer and Freise (and Wagner) treatment of the Boltzmann-Matano relation, this inverse regression function is further modified by the inclusion of molar volume, which is dependent on the molar composition of a diffusing two-component mixture. Following the conversion of the initial relation to the form that enables the direct integration of the used inverse function, which fits the diffusion profile, the formulas for the analytical calculation of diffusivity are obtained. These formulas are derived both for the linear change in molar volume in the diffusion profile corresponding to Vegard’s rule and for the case of the positive or negative deviations from the linearity representing the swell or shrink course Vm. The analysis results show that the drop and swell shapes of the course of molar volume in a diffusion profile of the binary system increase the resulting diffusivity. Conversely, the increase of Vm and its shrink course are factors that lower diffusivity. The regression function that was used in its inverse form was suitable for fitting the diffusion profile of any asymmetrical shape. The derived relations are therefore practically applicable for the direct analytical calculation of concentration-dependent diffusivity in binary systems.

Significance of Zr-in-Rutile Thermometry for Deducing the Decompression P–T Path of a Garnet–Clinopyroxene Granulite in the Moldanubian Zone of the Bohemian Massif

This work aims to show the importance of Zr-in-rutile thermometry for evaluating the P–T history of granulite-facies rocks, where higher diffusion rates in the main constituent minerals impede the use of geothermometers based on element distributions. We apply Zr-in-rutile thermometry to a garnet–clinopyroxene (Grt–Cpx) granulite from the Moldanubian Zone of the Bohemian Massif. Three major metamorphic evolutionary stages are identified from the Grt–Cpx granulite. The early high-pressure (HP) stage is represented by an inclusion assemblage in garnet: a high-Ca garnet core (32–38% grossular, 30–32% pyrope and 32–35% almandine)þomphacite (36–39% jadeite and 3–5% Ca-Tschermak)þplagioclase (18% anorthite)þpargasitic amphiboleþ rutileþ zirconþquartz. The subsequent medium-pressure (MP) stage is represented by matrix minerals composed of augitic clinopyroxene (2–6% jadeite and 2–6% Ca-Tschermak)þorthopyroxeneþ ternary feldspar (17–23% anorthite, 41–44% albite, 33–43% orthoclase; reintegrated compositions from antiperthite grains in the matrix)þ rutileþ ilmeniteþquartz. The final low-pressure (LP) stage is represented by a symplectic corona composed of calcic plagioclase ( 90% anorthite)þorthopyroxeneþmagnetite. Application of Grt–Cpx and/or jadeite–quartz–albite geobarometers gives pressures of 1 8 GPa for the early HP stage and 1 3–1 4 GPa for the MP stage. The final LP stage is constrained to lower than 0 7 GPa using conventional geothermobarometers. Rutile inclusions in high-grossular garnet have a rather low and limited range of Zr contents (mostly 1100–1500 ppm), regardless of inclusion size. This suggests that rutile inclusions preserved the initial Zr compositions without much modification by later re-equilibration. Application of Zr-in-rutile thermometry yields a temperature of 830 C at 1 8 GPa for the early HP stage of granulite evolution. Rutile grains in undeformed clinopyroxene-rich domains of the matrix generally occur as small euhedral crystals and have higher Zr contents (mostly 8000–10000 ppm), corresponding to 980–1066 C at 1 35 GPa using Zr-in-rutile thermometry. In contrast, those in strongly deformed quartz-rich domains of the matrix occur as coarser and more elongated grains with lower Zr contents (3000– 5000 ppm), yielding slightly lower temperatures owing to retrogressive re-equilibration. Based on these results, we reveal that the studied Grt–Cpx granulite underwent a significant heating by about 200 C during the early stage of decompression from the peak pressure. Sensitive high-resolution ion microprobe U–Pb dating for the zircon inclusions in high-grossular garnet indicates that the HP stage of the studied granulite occurred at c. 340 Ma, which is indistinguishable from reported LP VC The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 1173 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 6, 1173–1198 doi: 10.1093/petrology/egx050 Advance Access Publication Date: 3 August 2017

Fossilized Melts in Mantle Wedge Peridotites

The shallow oxidized asthenosphere may contain a small fraction of potassic silicate melts that are enriched in incompatible trace elements and volatiles. Here, to determine the chemical composition of such melt, we analysed fossilized melt inclusions, preserved as multiphase solid inclusions, from an orogenic garnet peridotite in the Bohemian Massif. Garnet-poor (2 vol.%) peridotite preserves inclusions of carbonated potassic silicate melt within Zn-poor chromite (<400 ppm) in the clinopyroxene-free harzburgite assemblage that equilibrated within the hot mantle wedge (Stage 1, > 1180 °C at 3 GPa). The carbonated potassic silicate melt, which has a major element oxide chemical composition of K2O = 5.2 wt.%, CaO = 17 wt.%, MgO = 18 wt.%, CO2 = 22 wt.%, and SiO2 = 20 wt.%, contains extremely high concentrations of large ion lithophile elements, similar to kimberlite melts. Peridotites cooled down to ≅800 °C during Stage 2, resulted in the growth of garnet relatively poor in pyrope content, molar Mg/(Mg + Fe + Ca + Mn), (ca. 67 mol.%). This garnet displays a sinusoidal REE pattern that formed in equilibrium with carbonatitic fluid. Subsequently, subduction of the peridotite resulted in the formation of garnet with a slightly higher pyrope content (70 mol.%) during the Variscan subduction Stage 3 (950 °C, 2.9 GPa). These data suggest the following scenario for the generation of melt in the mantle wedge. Primarily, infiltration of sediment-derived potassic carbonatite melt into the deep mantle wedge resulted in the growth of phlogopite and carbonate/diamond. Formation of volatile-bearing minerals lowered the density and strength of the peridotite. Finally, phlogopite-bearing carbonated peridotite rose as diapirs in the mantle wedge to form carbonated potassic silicate melts at the base of the overriding lithosphere.