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New eclogite localities and new 40 Ar/ 39 Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh-pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km 2 ; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 · 50 · 5 km. 40 Ar/ 39 Ar mica and K-feldspar ages show that this outcrop pattern is the result of gentle regional-scale folding younger than 380 Ma, and possibly 335 Ma.The UHP and intervening high-pressure (HP) domains are composed of eclogite-bearing orthogneiss basement overlain by eclogite-bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc-silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15-cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near-isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12-17 kbar and c. 750°C during exhumation from mantle depths, followed by a low-pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750°C. New 40 Ar/ 39 Ar hornblende ages of 402 Ma document that this decompression from eclogite-facies conditions at 410-405 Ma to mid-crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20-30 km. 40 Ar/ 39 Ar muscovite ages of 397-380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30°C Myr )1 ), perhaps because of continued tectonic unroofing.

In understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting, and the rate and duration of that thrusting. Hornblende 4øAr/39Ar ages demonstrate that the igneous oceanic crust in Oman crystallized and cooled to-825 K in 1-2 m.y. Hornblende ages from metamorphic rocks and from cross-cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below-825 K in 1-2 m.y. Motion along the sole thrust accounted for 200 km of displacement at a rate of 100-200 mm/yr. On the basis of age relationships and thermal considerations, we favor a twostage model for the initial stages of Samall ophiolite emplacement: intraoceanic thrusting over < 2-m.y.-old lithosphere at 150 km/m.y. parallel to a spreading ridge for 1-2 m.y., followed by equally rapid and brief thrusting over cold and old lithosphere. Preservation of the Samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure, and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting. Locations of Ar/ Ar samples from this study are indicated by sample number. Ages (in Ma) of U/Pb zircon samples [Tilton et al., 1981] are preceded with a "z". Orientations of sheeted dikes after Nicolas et al. [1988]. spreading and subsequent melt extraction during intraoceanic thrusting [Ernewein et al., 1988; Pfiumio, 1991]. Plutonic rocks comprise an early gabbroic series (60-80 vol% of the crust) and a late wehrlitic series (20-40%) [Browning and Smewing, 1981; Juteau et al., 1988; Lippard et al., 1986]. The early series is a gabbro-dioriteplagiogranite suite that was the source of the sheeted dike complex and the lower extrusive unit [Lippard et al., 1986; Nicolas and Boudier, 1991]. The wehrlitic series is dominantly wehrlite, with subordinate dunite, gabbro, plagiogranite, and granite, which intruded all younger crustal rocks before complete crystallization of the gabbroic sequence. It is genetically related to the middle extrusive sequence, V2, and is inferred to have been emplaced during

Clumped-isotope" thermometry is an emerging tool to probe the temperature history of surface and subsurface environments based on measurements of the proportion of 13 C and 18 O isotopes bound to each other within carbonate minerals in 13 C 18 O 16 O 2 2À groups (heavy isotope "clumps"). Although most clumped isotope geothermometry implicitly presumes carbonate crystals have attained lattice equilibrium (i.e., thermodynamic equilibrium for a mineral, which is independent of solution chemistry), several factors other than temperature, including dissolved inorganic carbon (DIC) speciation may influence mineral isotopic signatures. Therefore we used a combination of approaches to understand the potential influence of different variables on the clumped isotope (and oxygen isotope) composition of minerals.We conducted witherite precipitation experiments at a single temperature and at varied pH to empirically determine 13 C-18 O bond ordering (D 47 ) and d 18 O of CO 3 2À and HCO 3 À molecules at a 25°C equilibrium. Ab initio cluster models based on density functional theory were used to predict equilibrium 13 C- 18 O bond abundances and d 18 O of different DIC species and minerals as a function of temperature. Experiments and theory indicate D 47 and d 18 O compositions of CO 3 2À and HCO 3 À ions are significantly different from each other. Experiments constrain the D 47 -d 18 O slope for a pH effect (0.011 ± 0.001; 12 P pH P 7). Rapidly-growing temperate corals exhibit disequilibrium mineral isotopic signatures with a D 47 -d 18 O slope of 0.011 ± 0.003, consistent with a pH effect. Our theoretical calculations for carbonate minerals indicate equilibrium lattice calcite values for D 47 and d 18 O are intermediate between HCO 3 À and CO 3 2À. We analyzed synthetic calcites grown at temperatures ranging from 0.5 to 50°C with and without the enzyme carbonic anhydrase present. This enzyme catalyzes oxygen isotopic exchange between DIC species and is present in many natural systems. The two types of experiments yielded statistically indistinguishable results, and these measurements yield a calibration that overlaps with our theoretical predictions for calcite at equilibrium. The slow-growing Devils Hole calcite exhibits D 47 and d 18 O values consistent with lattice equilibrium.Factors influencing DIC speciation (pH, salinity) and the timescale for DIC equilibration, as well as reactions at the mineral-solution interface, have the potential to influence clumped-isotope signatures and the d 18 O of carbonate minerals. In fast-growing carbonate minerals, solution chemistry may be an important factor, particularly over extremes of pH and salinity. If a crystal grows too rapidly to reach an internal equilibrium (i.e., achieve the value for the temperature-dependent mineral lattice equilibrium), it may record the clumped-isotope signature of a DIC species (e.g., the temperature-dependent equilibrium of HCO 3 À ) or a mixture of DIC species, and hence record a disequilibrium mineral composition. For extremely slow-growing cry...

[1] We present new equation-of-state (EoS) data acquired by shock loading to pressures up to 245 GPa on both low-density samples (MgSiO 3 glass) and high-density, polycrystalline aggregates (MgSiO 3 perovskite + majorite). The latter samples were synthesized using a large-volume press. Modeling indicates that these materials transform to perovskite, postperovskite, and/or melt with increasing pressure on their Hugoniots. We fit our results together with existing P-V-T data from dynamic and static compression experiments to constrain the thermal EoS for the three phases, all of which are of fundamental importance to the dynamics of the lower mantle. The EoS for perovskite and postperovskite are well described with third-order Birch-Murnaghan isentropes, offset with a Mie-Grüneisen-Debye formulation for thermal pressure. The addition of shock data helps to distinguish among discrepant static studies of perovskite, and for postperovskite, constrain a value of K 0 significantly larger than 4. For the melt, we define for the first time a single EoS that fits experimental data from ambient pressure to 230 GPa; the best fit requires a fourth-order isentrope. We also provide a new EoS for Mg 2 SiO 4 liquid, calculated in a similar manner. The Grüneisen parameters of the solid phases decrease with pressure, whereas those of the melts increase, consistent with previous shock wave experiments as well as molecular dynamics simulations. We discuss implications of our modeling for thermal expansion in the lower mantle, stabilization of ultra-low-velocity zones associated with melting at the core-mantle boundary, and crystallization of a terrestrial magma ocean.