Cooling Rate Data Research Articles

Cation ordering in orthopyroxenes from two stony-iron meteorites: implications for cooling rates and metal-silicate mixing

We have determined the cooling rates of orthopyroxene crystals from two group IVA stony iron meteorites—Steinbach (ST) and São João Nepomuceno (SJN)—on the basis of their Fe–Mg ordering states. The rate constant was calibrated as a function of temperature by controlled cooling experiments using orthopyroxene crystals separated from ST. These data were used along with earlier calibrations of the equilibrium intracrystalline fractionation of Fe and Mg as a function of temperature for crystals separated from both meteorites to calculate their cooling rates. The site occupancies of the orthopyroxene crystals were determined by single-crystal X-ray diffraction subject to the bulk compositional constraints. The closure temperatures (Tc) of cation ordering for the untreated crystals from SJN are ∼400°C, whereas those from ST vary between ∼430 and 470°C. Reconciliation of the metallographic and orthopyroxene cooling rate data, within the framework of the metal–silicate mixing model of Haack et al. (1995), suggests that these two stony irons had cooled at a similar rate of ∼400°C/Ma through the closure temperatures for cation ordering in the orthopyroxenes. This was followed by slow cooling for ST at ∼50°C/Ma at T < 425°C. Similar slow cooling was not recorded by the metals in SJN, which implies that if this stony iron were subjected to slow cooling, it must have been below 350°C. The similar cooling rates above 425°C for both ST and SJN, as required to reconcile the metal and orthopyroxene cooling rate data, is at variance with the earlier notion (Rasmussen et al., 1995) of distinctly different cooling rates for the high and low Ni IVA irons and stony irons. The cation ordering and metallographic cooling rate data are also amenable to an alternative interpretation, which requires two different parent bodies for the two stony irons, and mixing of the metal and silicate components of ST after the metals had cooled below the closure temperature of Fe–Ni interdiffusion. However, the available textural data for ST seems to argue against such metal-silicate mixing model.

Determination of the critical cooling rate of heavy metal fluoride glasses containing nucleating agents

Application of the Christiansen–Shelyubskii method optimized the melting technology for sample preparation. Homogeneity with respect to glass composition, fining, and bubbling were improved. The critical cooling rate, a homogeneity sensitive property, was determined on samples obtained from this optimized standard process. These results were compared with critical cooling rate data from samples containing nucleating agents (AgF, PtCl 2) in three concentration steps. A well-defined variation of homogeneity of the undoped base glass was observed.

Revised model calculations for the thermal histories of ordinary chondrite parent bodies

Abstract— Recent measurements of ordinary chondrite physical and thermal properties along with new geothermometry studies have provided the necessary parameters for updating a previously proposed model (Miyamoto et al., 1981) for the thermal evolution and internal structure of ordinary chondrite parent bodies. Model calculations assumed a heat source term derived from the decay of 26Al (justification is provided). Differences from the previous model include: varying the thermal diffusivity parameter with increasing temperature (and decreasing porosity), using variable physical and thermal parameters to provide end member models, and incorporating a shortened thermal history of 60 Ma (obtained from new Pb‐Pb chronology of phosphates) rather than 100 Ma. Times of isotopic closure in chondrite phosphates overlap the thermal model estimates, and postmetamorphic cooling rates from the model approximately coincide, in both trend and magnitude, with metallographic and fission track cooling rate data. Model calculations attempt to match peak metamorphic conditions in the central portions of these bodies and yield accretion ages between 1.4 to 3.1 Ma after calcium‐aluminum inclusion (CAI) formation. Model calculations also predict that both the H and the L chondrite parent asteroids consisted of ∼80% equilibrated and 20% unequilibrated chondritic material and that their original radii ranged from 80 to 95 km.

Contrasting Thermal Evolution within the Ross Orogen, Antarctica: Evidence from Mineral $$^{40}Ar/^{39}Ar$$ Ages

The Nimrod Group in the central Transantarctic Mountains and the Lanterman Metamorphic Complex in northern Victoria Land of Antarctica constitute internal metamorphic basement terrains of the Ross orogen that share many first-order similarities in lithology, structure, and metamorphism. However, new cooling ages determined for hornblende and muscovite from tectonites in the Geologists (Nimrod Group) and Lanterman ranges (Lanterman complex) indicate that these terrains experienced different post-kinematic cooling histories. Nimrod ductile L-S tectonites derived from igneous and sedimentary protoliths yield hornblende cooling ages of 524-495 Ma and muscovite cooling ages of 499-496 Ma. These ages are interpreted to date the cooling following syn-metamorphic ductile deformation. Combined and existing U-Pb age data yield an average post-kinematic cooling rate for the Nimrod tectonites of ~10°C/m.y. Metasedimentary L-S tectonites of the Lanterman complex yield cooling ages of ca. 486 Ma (hornblende) and ca. 482 Ma (muscovite). These cooling ages indicate an average post-kinematic Lanterman cooling rate of ~30°C/m.y. We interpret the contrasting thermal histories of these terrains to be the result of different modes of accretion along the orogen, due in part to oblique subduction of paleo-Pacific lithosphere beneath the East Antarctic craton. In the central Transantarctic Mountains, Nimrod cooling rates indicate relatively modest post-tectonic denudation rates (~0.4 mm/a), resulting from crustal thickening in an upper plate-margin setting. Contraction of adjacent continental-margin supracrustal sequences did not involve significant under-thrusting, possibly as a result of a high component of margin-parallel translation. In contrast, markedly faster Lanterman cooling rates indicate more rapid denudation (~1.2 mm/a) of thickened crust in northern Victoria Land. Kinematic and cooling-rate data from the Lanterman complex indicate that accretion of outboard lower Paleozoic volcanic arc and continental-rise assemblages involved near-orthogonal underthrusting beneath crystalline basement, leading ultimately to rapid tectonic denudation.

Rapidly Solidified Microstructures in a Ti-22 Wt% Fe Alloy

ABSTRACTRapidly-solidified microstructures were produced in aTi-22 wt% Fe alloy by laser surface melting, melt extraction, melt spinning and splat cooling. Increased cooling rates during solidification promoted increasingly finer beta grain sizes ranging from approximately 75 microns in a laser melt to 0.5 to 2.0 microns for splats. Beta grain morphologiesw ere generally equiaxed, although epitaxially-nucleated columnar grains were observed at the pool-substrate interface for laser melts. Fitting of the equiaxed beta grain size data to a λ0 = Boε−n0 power relationship for homogenous nucleation and isotropic growth resulted in Bo = 3 × 104 and no = 0.62. The refinement of dendrite structure with increasing cooling rate was also documented for laser melts. Dendrite spacing versus cooling rate data were fitted to a similar general power relationship and resulted in B1 = 45 and n1 = 0.34.

On the Use of Pyrgeometers in Cloud

Laboratory and airborne observations show that the protective hemispheres of aircraft pyrgeometers are partly covered by water when used in cloud. This cover can reduce the incident longwave flux by as much as 60%. Improved agreement between observations and theory is obtained when a parameterization of water cover in terms of cloud liquid water content is used to correct flux divergence and cooling rate data. It is suggested that all previous in-cloud pyrgeometer measurements may suffer cloud-water contamination.

The thermodynamics and kinetics of the β → αm transformation in three Ti-X systems

Continuous cooling experiments were used to study the β → α m transformation in three titanium base eutectoid systems: Ti-Ag, Ti-Au and Ti-Si. Using thermal arrest temperature vs cooling rate data, the T 0 temperature and the enthalpy of transformation. ΔH T β → α m , were determined for eight alloys in these sytems. Subsequent microstructural analysis yielded growth rates as a function of reaction temperature from which an estimation of the activation enthalpy for trans-interphase boundary diffusion, ΔH D b , was made. The phase boundaries of the eutectoid region for each system were re-evaluated and found to agree well with some previous investigations and to differ markedly with others. Values of ΔH T β → α m range from −2470 ± 500 to −3270 ± 460 J/ mol (−590 ± 120 to −790 ± 110 cal/ mol). Most T 0 temperatures agree well with that of the bisector of the ( α + β) region. Values of ΔH D b vary from 50 to 93 kJ/mol (11.7–22.2 kcal/mol). The growth distance of α m in one second is calculated to be ca. 6 × 10 −5 m at 7 K undercooling as compared to the equilibrium precipitate growth distance (when volume diffusion-controlled) of ca. 1 × 10 −7 m at the same time and temperature. These results further confirm the identification of the massive transformation in the alloys investigated.

Thermal Models of Inhomogeneously Accreted Meteorite Parent Bodies

THERMAL calculations on planetary models, together with cooling rate data from studies of Ni diffusion gradients in iron meteorites, have been used to establish constraints on the possible dimensions of meteorite parent bodies1–3. Previously these models assumed spherical chondritic bodies with uniform distribution of radioactive heat sources, although redistribution of heat sources upon melting has been considered. Core formation has usually been regarded as a subsequent metamorphic event. But recent studies suggest that iron was fractionated early in the planetary formation process and that the terrestrial planets were inhomogeneously accreted4. Therefore, considering the differences in thermal properties and distribution of radioactive heat sources that result from initial core formation followed by silicate accretion, we present here thermal calculations on a set of models of meteorite parent bodies in which initial formation of a Ni-Fe core is assumed.