Coesite

Abstract: Mixing properties for muscovite–celadonite–ferroceladonite solid solutions are derived from combining available experimental phase equilibrium data with tabulated thermodynamic data for mineral end-members. When a partially ordered solution model is assumed, the enthalpy of mixing among the end-members muscovite–celadonite–ferroceladonite is nearly ideal, although the Gibbs energies of muscovite–celadonite and muscovite–ferroceladonite solutions are asymmetric due to an asymmetry in the entropy of mixing. Thermodynamic consistency is achieved for data on phengite compositions inassemblages with (a) pyrope+kyanite+quartz/coesite (b) almandine+kyanite+quartz/coesite (c)talc+kyanite+quartz/coesite and (d) garnet–phengite pairs equilibrated both experimentally at high temperatures and natural pairs from low-grade schists. The muscovite–paragonite solvus has been reanalysed using the asymmetric van Laar model, and the effects of the phengite substitution into muscovite have been quantitatively addressed in order to complete the simple thermodynamic mixing model for the solid solution among the mica end-members. Results are applied to a natural pyrope–coesite–phengite–talc rock from the Western Alps, and to investigate the conditions under which biotite-bearing mica schists transform to whiteschist-like biotite-absent assemblages for average pelite bulk compositions.

Abstract: [1] Geochemical tracers demonstrate that elements are cycled from subducted sediments into the arc melting regime at subduction zones, although the transfer mechanism is poorly understood. Are key elements (Th, Be, Rb) lost during sediment dehydration or is sediment melting required? To investigate this question, we conducted phase equilibria and trace element partitioning experiments on a pelagic red clay for conditions appropriate to the slab beneath arc volcanoes (2–4 GPa, 600°–1000°C). Using both piston cylinders and multianvils, we determined the solidus, phase stabilities, and major element compositions of coexisting phases. The solidus (H2O + Cl fluid-saturated) was located at 775 ± 25°C at 2 GPa, 810 ± 15°C at 3 GPa, and 1025 ± 25°C at 4 GPa with noevidence for complete miscibility between melt and fluid. This sediment composition produces a profusion of phases both above and below the solidus: garnet, jadeitic pyroxene, alkali-rich amphibole, phengite, biotite, magnetite, coesite, kyanite, apatite, zircon, Cl-rich fluids, and peraluminous to peralkaline granitic melts. At 2 GPa the phengite dehydration solidus is at 800°–825°C, while biotite breaks down between 850° and 900°C. To explore trace element partitioning across the solidus at 2 GPa, we used diamonds to trap fluids and melts. Both the bulk sediment residues and diamond traps were analyzed postexperiment by inductively coupled plasma–mass spectrometry (ICP-MS) and inductively coupled plasma–atomic emission spectrometry (ICP-AES) for 40 elements for which we calculated bulk partition coefficients (D = Csolid/Cfluid). Below the solidus, Rb, Sr, Ba, and Pb showed the greatest mobility (D ∼ 0.5–1.0), while at the solidus, Th and Be became notably partitioned into the melt (D values changing from >2.0 to <1.0). K and Rb D values fall below 1.0 when the micas breakdown. Only at the solidus do Th and Rb attain similar partition coefficients, a condition required by arc data. Taken together, the experimental results indicate that critical elements (Th and Be) require sediment melting to be efficiently transferred to the arc. This conclusion is at odds with most thermal models for subduction zones, which predict slab temperatures more than 100°C lower than sediment solidi. Thus the condition of sediment melting (with oceanic crust dehydration) may provide new constraints on the next generation of thermal/geodynamical models of subduction zones.

Abstract: Coesite is reported for the first time from the Caledonide orogen. It occurs as inclusions in clinopyroxene in the dolomite-eclogite at Grytting, Norway, and provides valuable new evidence to support the hypothesis of extremely high pressure (≥30 kbar) in certain Norwegian eclogites. This finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure, and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport.

Abstract: Determining the depth to which continental lithosphere can be subducted into the mantle at convergent plate boundaries is of importance for understanding the long-term growth of supercontinents as well as the dynamic processes that shape such margins. Recent discoveries of coesite and diamond in regional ultrahigh-pressure (UHP) metamorphic rocks has demonstrated that continental material can be subducted to depths of at least 120 km (ref. 1), and subduction to depths of 150-300 km has been inferred from garnet peridotites in orogenic UHP belts based on several indirect observations. But continental subduction to such depths is difficult to trace directly in natural UHP metamorphic crustal rocks by conventional mineralogical and petrological methods because of extensive late-stage recrystallization and the lack of a suitable pressure indicator. It has been predicted from experimental work, however, that solid-state dissolution of pyroxene should occur in garnet at depths greater than 150 km (refs 6-8). Here we report the observation of high concentrations of clinopyroxene, rutile and apatite exsolutions in garnet within eclogites from Yangkou in the Sulu UHP metamorphic belt, China. We interpret these data as resulting from the high-pressure formation of pyroxene solid solutions in subducted continental material. Appropriate conditions for the Na2O concentrations and octahedral silicon observed in these samples are met at depths greater than 200 km.