Timm John

Evidence for a Neoproterozoic ocean in south-central Africa from mid-oceanic-ridge-type geochemical signatures and pressure-temperature estimates of Zambian eclogites

Precambrian eclogites, metagabbros, and gabbros occur in an similar to200-km-long by 40-km-wide zone in central Zambia. Pressure-temperature (P-T) estimates of kyanite-bearing eclogites (kyanite eclogites) throughout the zone give temperatures of 590-750 degreesC at minimum pressures of 20 kbar. Phengite-bearing eclogites equilibrated at 720-755 degreesC and 26-28 kbar and show evidence for a clockwise P-T path. These P-T conditions imply a low geothermal gradient of similar to8 degreesC/km and a subduction depth of similar to90 km. The eclogites, metagabbros, and gabbros show incompatible element patterns similar to those of recent mid-oceanic-ridge basalts, and thus are interpreted to represent former oceanic crust. The low geothermal gradient indicates a cold subducted oceanic lithosphere, implying long-lived, fast convergence and a relatively large (>1000 km) associated ocean basin. A Sm-Nd isochron defines an age of 595 +/- 10 Ma for the eclogite facies metamorphism. These results imply that a Neoproterozoic suture zone exists between the Congo and Kalahari cratons. Suturing occurred during the same orogenic cycle that formed the Zambezi belt and is related to the assembly of Gondwana.

Interrelations between intermediate-depth earthquakes and fluid flow within subducting oceanic plates: Constraints from eclogite facies pseudotachylytes

Field evidence preserved in ancient subducted oceanic crust documents that eclogite facies frictional failure with melting (pseudotachylyte formation during intermediate-depth earthquakes) was accompanied and followed, not preceded, by infiltration of external fluids and progressive vein formation in the eclogites. Eclogitization began during seismic failure and fluid passage through the shear zone. Subsequent fluid flow produced hydraulic fracturing and continuous vein formation during ongoing burial. We suggest that this kind of shear zone may allow channellized fluid flow within and out of slabs.

The mechanism of porosity formation during solvent-mediated phase transformations

Solvent-mediated solid–solid phase transformations often result in the formation of a porous medium, which may be stable on long time scales or undergo ripening and consolidation. We have studied replacement processes in the KBr–KCl–H2O system using both in situ and ex situ experiments. The replacement of a KBr crystal by a K(Br,Cl) solid solution in the presence of an aqueous solution is facilitated by the generation of a surprisingly stable, highly anisotropic and connected pore structure that pervades the product phase. This pore structure ensures efficient solute transport from the bulk solution to the reacting KBr and K(Br,Cl) surfaces. The compositional profile of the K(Br,Cl) solid solution exhibits striking discontinuities across disc-like cavities in the product phase. Similar transformation mechanisms are probably important in controlling phase-transformation processes and rates in a variety of natural and man-made systems.

Influence of temperature and Cl on the hydrothermal replacement of calcite by apatite and the development of porous microstructures

Abstract Calcite (CaCO3) powder was replaced by hydroxylapatite [Ca5(PO4)3OH] in hydrothermal experiments at temperatures between 120 and 180 °C, and times between 2 and 12 h, in 1.0 M (NH4)2HPO4 solutions. Additional experiments using concentrations of up to 2.0 M NH4Cl as an additional component were carried out to examine the influence of Cl on the process. X‑ray powder diffraction and Rietveld refinements were used to determine the proportion of parent and product phases after each experiment. The resultant data on the fraction transformed as a function of temperature and time were plotted according to the Avrami equation and an activation energy of 118 kJ/mol was determined for the replacement process without Cl in solution, whereas with 0.5-2.0 M Cl in solution, the activation energy varied from 131 to 145 kJ/mol. No simple correlation between the chlorinity of the solution and the value of the activation energy was observed. However, the data indicate that the mechanism controlling the reaction changes between 140 and 160 °C, correlating with systematic differences in abundance, size, and geometry of the porosity in the apatite formed in the replacement process. At lower temperature, the pores are small and oriented parallel to the reaction interface, whereas at higher temperature, the pores are larger and orientated perpendicular to the interface. Changes in the porosity during the replacement process make it problematic to determine a meaningful value for the activation energy for such replacement processes, since the mechanism controlling the reaction depends on factors other than the temperature, such as the availability and form of the pathways allowing the fluid to reach the unreacted core of the single crystals.

Fluid-induced breakdown of white mica controls nitrogen transfer during fluid–rock interaction in subduction zones

ABSTRACT In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic systematics in high-pressure metamorphic rocks, we investigated three different profiles representing three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan (ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent with a batch devolatilization process. A second profile from the Tianshan represents a retrograde eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to +5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N isotopic compositions (δ15N = + 2.1 to +3.6‰). Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances and isotopic composition of N are controlled by the stability and presence of white mica. Phengite breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid. Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace the extent of N transport during metasomatic processes. The Vendée profile demonstrates that this process occurs over several tens of metres and affects both N concentrations and N isotopic compositions.

Synthesis of trace element bearing single crystals of Chlor-Apatite (Ca5(PO4)3Cl) using the flux growth method

We present a new strategy on how to synthesize trace-element bearing (REE, Sr) chlorapatites Ca5(PO4)3Cl using the flux growth method. Synthetic apatites were up to several mm long, light blue in colour. The apatites were characterized using XRD, electron microprobe and laser ablation ICP-MS (LA-ICPMS) techniques and contained several hundred μg/g La, Ce, Pr, Sm, Gd and Lu and about 1700 μg/g Sr. The analyses indicate that apatites were homogenous (within the uncertainties) for major and trace elements.