Cooling Rate Estimates Research Articles

Carbonate clumped isotopes and blocking temperatures of marbles from the Backbone Range, Taiwan

Clumped isotopic compositions (abundance of 13C16O18O in CO2, expressed by Δ47) in massive calcite marble (CM) and dolomite marble (DM) were reported to refine our understanding of greenschist-facies metamorphism during the still on-going arc-continent collision from the northern, central, and southern localities along the northern-central Tailuko belt of the Backbone Range of Taiwan. Black calcite marble (BCM), white calcite marble (WCM), and DM lenses with cataclastic texture were analyzed. We observed that the carbonate clumped isotope reordering is independent of the grain size of minerals and is less likely affected by lattice defects and active stress. The averaged Δ47 values for CMs and DMs are 0.308 ± 0.004 ‰ (n = 26) and 0.235 ± 0.003 ‰ (n = 13), respectively, giving blocking temperatures of 211 ± 6 °C and 332 ± 8 °C. The blocking temperatures infer mean cooling rates of 614 °C/m.y. and 8.8 × 104 °C/m.y for calcites and dolomites, respectively, estimated based on the available models and clumped reordering mechanisms. We noted that the variations in cooling rates widely differ with different reordering models, especially with different temperature calibrations. Some cooling rate estimates are in agreement with those based on zircon/apatite thermochronological studies. Further refinements on Δ47 - temperature calibration, reordering mechanism and estimation of thermodynamic constants, in addition to improvement in analytical precision, are needed for practical applications of carbonate clumped thermometry as a tectonic speedometer for metamorphic terranes.

Cooling-rate constraints from metapelites across two inverted metamorphic sequences of the Alpine-Himalayan belt; evidence for viscous heating

Despite the occurrence of high-grade metamorphic rocks next to and along crustal-scale shear zones, the temporal character of their formation and evolution is difficult to extract. We utilize the major-element diffusion in the compositional re-adjustment of garnet from metapelites in two crustal-scale shear zones as a complementary method to extract cooling rates from deforming/reacting rocks. The two thrust zones, the Nestos Thrust Zone (NTZ) in Rhodope, Greece, and the Main Central Thrust (MCT) in Sikkim, Himalaya, exhibit inverted metamorphic zonation. We applied phase equilibria modelling and geothermometry to constrain the peak- and the post-peak-temperature conditions relevant for the cooling-rate estimates. Results are 50–80 °C/Myr in the footwalls of both thrust zones, in consistency with published estimates using geochronology methods for MCT. However, results are much less (∼0.5–5°C/Myr) for the base of the MCT hanging wall. The estimated cooling rates are between 300 and 2500 °C/Myr for the NTZ hanging wall. The exceedingly fast cooling rates indicate the operation of transient and proximal thermo-mechanical processes consistent with the contribution of thrust-related viscous heating during metamorphism. The very slow cooling rate of the MCT hanging wall may reflect a complex thermal history or other overlooked processes.

Faster geospeedometry: A Monte Carlo approach to relaxational geospeedometry for determining cooling rates of volcanic glasses

The thermal history of natural glasses is critical to understanding a wide range of geologic processes. Relaxation geospeedometry has been used to infer the cooling rates of naturally formed glasses across a wide range of compositions and geologic settings. However, using the Tool-Narayanaswamy (TN) geospeedometer is time consuming, requiring multiple isobaric heat capacity (CP) measurements to constrain four sample-specific model parameters, before quantifying natural cooling history of a sample. Here we present a Monte Carlo-inspired numerical solver called CoolMonte, which automates the fitting procedure of experimental CP measurements, and is capable of determining an unknown cooling rate using a single CP measurement. We compare quantitative cooling rates of four naturally cooled obsidian lava samples determined using the traditional approach with multiple CP measurements, to cooling rates determined using a single CP measurement without sample-specific model calibration. Cooling rates calculated using a single CP measurement are within 0.1 to 1.3 log10 K s−1 of cooling rates determined using multiple CP measurements. We also assessed CoolMonte using 50 synthetic datasets with known cooling rates (≳0.1 K per year) and 10 previously published natural cooling rates which were reproduced within 0.8 log10 K s−1. CoolMonte reduces the time necessary to determine cooling rates of natural samples, improves the reproducibility of cooling rate estimates, and makes relaxational geospeedometry a more accessible method for the study of thermal histories of geologic systems.

Ferrovolcanism on metal worlds and the origin of pallasites

As differentiated planetesimals cool, their cores can solidify from the outside-in, as evidenced by paleomagnetic measurements and cooling rate estimates of iron meteorites. The details of outside-in solidification and fate of residual core melt are poorly understood. For a core primarily composed of Fe and Ni alloyed with lighter constituent elements, like sulfur, such inward core growth would likely be achieved by growth of solid FeNi dendrites. Growth of FeNi dendrites results in interconnected pockets of residual melt that become progressively enriched in sulfur up to a eutectic composition of 31 wt percent sulfur as FeNi continues to solidify. Here we show that regions of residual sulfur-enriched FeNi melt in the core attain sufficient excess pressures to propagate via dikes into the mantle. Thus, core material will intrude into the overlying rocky mantle or possibly even erupt onto the plantesimals surface. We refer to these processes collectively as ferrovolcanism. Our calculation show that ferrovolcanic surface eruptions are more likely on bodies with mantles less than 50 km thick. We show that intrusive ferromagmatism can produce pallasites, an enigmatic class of meteorites composed of olivine crystals entrained in a matrix of FeNi metal. Ferrovolcanic eruptions may explain the observations that Psyche has a bulk density inconsistent with iron metorites yet shows evidence of a metallic surface composition.

Meteorite cloudy zone formation as a quantitative indicator of paleomagnetic field intensities and cooling rates on planetesimals

Metallic microstructures in slowly-cooled iron-rich meteorites reflect the thermal and magnetic histories of their parent planetesimals. Of particular interest is the cloudy zone, a nanoscale intergrowth of Ni-rich islands within a Ni-poor matrix that forms below ∼350°C by spinodal decomposition. The sizes of the islands have long been recognized as reflecting the low-temperature cooling rates of meteorite parent bodies. However, a model capable of providing quantitative cooling rate estimates from island sizes has been lacking. Moreover, these islands are also capable of preserving a record of the ambient magnetic field as they grew, but some of the key physical parameters required for recovering reliable paleointensity estimates from magnetic measurements of these islands have been poorly constrained. To address both of these issues, we present a numerical model of the structural and compositional evolution of the cloudy zone as a function of cooling rate and local composition. Our model produces island sizes that are consistent with present-day measured sizes. This model enables a substantial improvement in the calibration of paleointensity estimates and associated uncertainties. In particular, we can now accurately quantify the statistical uncertainty associated with the finite number of islands acquiring the magnetization and the uncertainty on their size at the time of the record. We use this new understanding to revisit paleointensities from previous pioneering paleomagnetic studies of cloudy zones. We show that these could have been overestimated by up to one order of magnitude but nevertheless still require substantial magnetic fields to have been present on their parent bodies. Our model also allows us to estimate absolute cooling rates for meteorites that cooled slower than <10,000°CMy−1. We demonstrate how these cooling rate estimates can uniquely constrain the low-temperature thermal history of meteorite parent bodies. Using the main-group pallasites as an example, we show that our results are consistent with the previously-proposed unperturbed, conductive cooling at low temperature of a ∼200-km radius main-group pallasite parent body.

Generating potassium abundance variations in the Solar Nebula

An intriguing aspect of chondritic meteorites is that they are complementary: while their separate components have wildly varying abundances, bulk chondrites have nearly solar composition. This implies that the nearly-solar reservoirs in which chondrites were born were in turn assembled from sub-reservoirs of differing compositions that birthed the different components. We focus on explaining the potassium abundance variations between chondrules even within a single chondrite, while maintaining the observed CI $^{41}$K to $^{39}$K ratios. This requires physically separating potassium and chondrules while the temperature is high enough for K to be in the gas phase. We examine several mechanisms which could drive the dust through gas and show that to do so locally would have required long (sub-orbital to many orbits) time scales; with shortest potassium depletion time scales occurring in a scenario where chondrules formed high above the midplane and settled out of the evaporated potassium. While orbital time scales are at odds with laboratory chondrule cooling rate estimates, any other model for the origin for the potassium abundance variation has to wrestle with the severe logistical difficulty of generating a plethora of correlated reservoirs which varied strongly in their potassium abundances, but not in their potassium isotope ratios.

Petrology, geochemistry, and age of low-Ti mare-basalt meteorite Northeast Africa 003-A: A possible member of the Apollo 15 mare basaltic suite

Northeast Africa 003 (NEA 003) is a lunar meteorite found as a two paired stones (6 and 118 g) in Libya, 2000 and 2001. The main portion (∼75 vol%) of the 118 g meteorite, used for this study, (NEA 003-A) consists of mare-basalt and a smaller adjacent portion (∼25 vol%) is a basaltic breccia (NEA 003-B). NEA 003-A has a coarse-grained magmatic texture consisting mainly of olivine, pyroxene and plagioclase. The late-stage mineral association is composed mainly of elongated plagioclase, ilmenite, troilite, fayalite, Si–K-rich glass, apatite, and a rare SiO 2 phase. Other accessory minerals include ulvöspinel, chromite, and trace Fe–Ni metal. Olivine and pyroxene contain shock-induced fractures, and plagioclase is completely converted into maskelynite. The Fe/Mn values of the whole rock, olivines and pyroxenes, and the bulk-rock oxygen isotopic composition provide evidence for the lunar origin of NEA 003-A meteorite. This is further supported by the presence of Fe–Ni metal and the anhydrous mineral association. NEA 003-A is geochemically and petrographically distinct from previously described mare-basalt meteorites and is not paired with any of them. The petrography and major element composition of NEA 003-A is similar to the composition of low-Ti olivine mare basalts from Apollo 12 and olivine-normative basalts from Apollo 15. The NEA 003-A meteorite shows obvious geochemical similarities in trace elements contents with Apollo 15 olivine-normative basalts and could represent a yet unknown geochemically primitive member of the olivine-normative basalt series. The meteorite is depleted in rare earth elements (REE) and incompatible trace elements indicating a primitive character of the parental magma. The bulk-rock chemical composition demonstrates that the parent melt of NEA 003-A was not contaminated with KREEP components as a result of magma mixing or assimilation processes. Results of crystallization modelling and low minimum cooling rate estimates (∼0.07 °C/h) suggest that the parent melt of NEA 003-A crystallized in the lower part of a lava flow containing cumulate olivine (∼10%) and was probably derived from more primitive picritic magma by fractional crystallization processes. Sm–Nd dating yields an age of 3.09 ± 0.06 Ga which corresponds to the period of lower Eratosthenian lunar volcanic activity, and the near-chondritic ε Nd value of −0.4 ± 0.3 indicates that the meteorite could be derived from a slightly enriched mantle source similar to the Apollo 15 green glasses. Ar–Ar step release results are inconsistent with Sm–Nd ages suggesting that NEA 003-A was exposed to one or more impact events. The most extensive event took place at 1.8 Ga and the shock intensity was likely between 28 and 45 GPa. The absence of solar Ar suggests that NEA 003-A has not been directly exposed at the lunar surface but the cosmic ray exposure age of 209 ± 6 Ma suggests that NEA 003-A resided in the upper regolith for part of its history.

Cooling rate of chondrules in ordinary chondrites revisited by a new geospeedometer based on the compensation rule

For several decades efforts to constrain chondrite cooling rates from diffusion zoning in olivine gave rise to a range of values from 5 to 8400 K/h (Desch, S.J., Connolly Jr., H.C., 2002. A model for the thermal processing of particles in solar nebula shocks: application to cooling rates of chondrules. Meteorit. Planet. Sci. 37, 183–208; Greeney, S., Ruzicka, A., 2004. Relict forsterite in chondrules: implications for cooling rates. Lunar Planet. Sci. XXXV, abstract # 1246.). Such large uncertainties directly reflect the variability of diffusion data. Alternatively, from this variability results a compensation rule, log D 0 = a + bE (diffusion coefficients are written D = D 0 exp(− E/ RT)). We test a new geospeemetry approach, based on this rule, on cooling of chondrules in chondrites, Sahara-97210 LL 3.2 and Wells LL 3.3. Greeney and Ruzicka (2004) matched Fe–Mg diffusion profiles in olivine from these chondrites with cooling rates between 200 and 6000 K/h. In our geospeedometry model, the use of the compensation rule greatly reduces the uncertainties by avoiding the choice of one diffusion coefficient among many. The cooling rates we found are between 700 and 3600 K/h for Sahara and 700–1600 K/h for Wells. Finally, we discuss the influence of our analytical model parameters on our cooling rate estimates.

Reconciling temperatures of metamorphism, fluid fluxes, and heat transport in the upper crust at intermediate to fast spreading mid‐ocean ridges

Current models of the mineralogical and chemical modification of the crust within mid‐ocean ridge hydrothermal systems at intermediate to fast spreading ridges are difficult to reconcile with the temperature distribution predicted by the required fluid fluxes. In particular, the sharp increase in metamorphic grade passing down from the lavas into the sheeted dike complex places important constraints on the flow of fluid through the crust. This distribution of metamorphic temperatures has typically been discussed in terms of a step in permeability in the downwelling limb of a single‐pass convection cell. Simple modeling of this system shows that downwelling fluid cools the crust too efficiently for high‐temperature metamorphism of the upper dikes to occur in the downflow zone. In fact, temperatures throughout most of the sheeted dike complex are too high. Three alternative models are considered. Fluid‐rock reaction during the cooling of each dike from magmatic temperatures is difficult to reconcile with cooling rate estimates for dikes, reaction rates required to grow metamorphic minerals, and the relatively constant composition of many hydrothermal vents. Metamorphism during reheating of the crust after fluid flow slows or stops is difficult to reconcile with the required fluid fluxes during metamorphism, based on the Sr‐isotopic composition of the sheeted dike complex, and cannot produce high enough temperatures in the upper dikes and low temperatures in the lower lavas. In contrast, fluid‐rock reaction in the sheeted dike complex as hot fluids migrate upward after being heated by the underlying magma source appears to be consistent with the data. In this model, downwelling seawater cools the recharge zone, and the fluid only becomes hot enough to significantly alter the crust once it approaches the conductive boundary layer overlying the magma chamber. Metamorphism of the sheeted dike complex then largely occurs in the discharge zone.

Turbulent heating of the interstellar medium: cascade spectrum cut-offs and anisotropy effects

Inclusion of turbulent cascade cut-offs in wavenumber space for the fast- and slow-magnetosonic modes is shown to have a profound effect on estimates of heating rates for the interstellar medium from the two wave types. Using a generalization of the Goldreich–Sridhar cut-off procedure, this paper shows that the effect of the turbulent cut-offs is to lower the previous estimates of interstellar heating rates (which were about 10–11 orders of magnitude higher than cooling rate estimates) to precisely the level of the cooling rate estimates. Thereby a major discordance is removed between heating and cooling rate estimates that had persisted for over a quarter of a century. The electron number density calculated in this paper which follows a roughly k−5/3 rule over at least 8–10 orders of magnitude, is in accord with observed spectra in the interstellar medium. Application of the same sort of turbulent wavenumber cascade cut-off to an anisotropic wave spectrum (proposed by Lerche and Schlickeiser as a possible candidate to achieve balance between the interstellar heating and cooling rates) is shown to modify that spectrum so that it more closely mimics the wave power spectra derived by Goldreich and Sridhar using turbulent MHD arguments. The wavenumber cut-offs are shown to have a power index different than the canonical value of 1/3 suggested by Goldreich and Sridhar and are closer to 1. Numerical turbulent cascade calculations in the framework of a particle kinetic picture rather than an MHD procedure would likely be required to improve the understanding of wave cut-offs from a purely theoretical perspective.

Diffusion‐driven kinetic isotope effect of Fe and Ni during formation of the Widmanstätten pattern

Abstract— Iron meteorites show resolvable Fe and Ni isotopic fractionation between taenite and kamacite. For Toluca (IAB), the isotopic fractionations between the two phases are around +0.1‰/amu for Fe and −0.4‰/amu for Ni. These variations may be due to i) equilibrium fractionation, ii) differences in the diffusivities of the different isotopes, or iii) a combination of both processes. A computer algorithm was developed in order to follow the growth of kamacite out of taenite during the formation of the Widmanstätten pattern as well as calculate the fractionation of Fe and Ni isotopes for a set of cooling rates ranging from 25 to 500 °C/Myr. Using a relative difference in diffusion coefficients of adjacent isotopes of 4‰/amu for Fe and Ni (β = 0.25), the observations made in Toluca can be reproduced for a cooling rate of 50 °C/Myr. This value agrees with earlier cooling rate estimates based on Ni concentration profiles. This supports the idea that the fractionation measured for Fe and Ni in iron meteorites is driven by differences in diffusivities of isotopes. It also supports the validity of the value of 0.25 adopted for β for diffusion of Fe and Ni in Fe‐Ni alloy in the temperature range of 400–700 °C.

Silica and pyroxene in IVA irons; possible formation of the IVA magma by impact melting and reduction of L-LL-chondrite materials followed by crystallization and cooling

Group IVA is a large magmatic group of iron meteorites. The mean Δ 17O (=δ 17O − 0.52·δ 18O) of the silicates is ∼+1.2‰, similar to the highest values in L chondrites and the lowest values in LL chondrites; δ 18O values are also in the L/LL range. This strongly suggests that IVA irons formed by melting L-LL parental material, but the mean Ni content of IVA irons (83 mg/g) is much lower than that of a presumed L-LL parent (∼170 mg/g) and the low-Ca pyroxene present in two IVA meteorites is Fs13, much lower than the Fs20-29 values in L and LL chondrites. Thus, formation from L-LL precursors requires extensive addition of metallic Fe, probably produced by reduction of FeS and FeO. Group IVA also has S/Ni, Ga/Ni, and Ge/Ni ratios that are much lower than those in L-LL chondrites or any chondrite group that preserves nebular compositions, implying loss of these volatile elements during asteroidal processing. We suggest that these reduction and loss processes occurred near the surface of the asteroid during impact heating, and resulted partly from reduction by C, and partly from the thermal dissociation of FeS and FeO with loss of O and S. The hot (∼1770 K) low-viscosity melt quickly moved through channels in the porous asteroid to form a core. Two members of the IVA group, São João Nepomuceno (hereafter, SJN) and Steinbach, contain moderate amounts of orthopyroxene and silica, and minor amounts of low-Ca clinopyroxene. Even though SJN formed after ∼26% crystallization and Steinbach formed after ∼77% crystallization of the IVA core, both could have originated within several tens of meters of the core-mantle interface if 99% of the crystallization occurred from the center outwards. Two other members of the group (Gibeon and Bishop Canyon) contain tabular tridymite, which we infer to have initially formed as veins deposited from a cooling SiO-rich vapor. The silicates were clearly introduced into IVA irons after the initial magma crystallized. Because the γ-iron crystals in SJN are typically about 5 cm across, an order of magnitude smaller than in IVA irons that do not contain massive silicates, we infer that the metal was in the γ-iron field when the silicates were injected. The SJN and Steinbach silicate compositions are near the low-Ca-pyroxene/silica eutectic compositions. We suggest that a tectonic event produced a eutectic-like liquid and injected it together with unmelted pyroxene grains into fissures in the solid metal core. Published estimates of IVA metallographic cooling rates range from 20 to 3000 K/Ma, leading to a hypothesized breakup of the core during a major impact followed by scrambling of the core and mantle debris [Haack, H., Scott, E.R.D., Love, S.G., Brearley, A. 1996. Thermal histories of IVA stony-iron and iron meteorites: evidence for asteroid fragmentation and reaccretion. Geochim. Cosmochim. Acta 60, 3103–3113]. This scrambling model is physically implausible and cannot explain the strong correlation of estimated cooling rates with metal composition. Previous workers concluded that the low-Ca clinopyroxene in SJN and Steinbach formed from protopyroxene by quenching at a cooling rate of 10 12 K/Ma, and suggested that this also supported an impact-scrambling model. This implausible spike in cooling rate by a factor of 10 10 can be avoided if the low-Ca clinopyroxene were formed by a late shock event that converted orthopyroxene to clinopyroxene followed by minimal growth in the clinopyroxene field, probably because melt was also produced. We suggest that metallographic cooling-rate estimates (e.g., based on island taenite) giving similar values throughout the metal compositional range are more plausible, and that the IVA parent asteroid can be modeled by monotonic cooling followed by a high-temperature impact event that introduced silicates into the metal and a low-temperature impact event that partially converted orthopyroxene into low-Ca clinopyroxene.

Ubiquitous low-FeO relict grains in type II chondrules and limited overgrowths on phenocrysts following the final melting event

Type II porphyritic chondrules commonly contain several large (>40 μm) olivine phenocrysts; furnace-based cooling rates based on the assumption that these phenocrysts grew in a single-stage melting-cooling event yield chondrule cooling-rate estimates of 0.01–1 K s −1. Because other evidence indicates much higher cooling rates, we examined type II chondrules in the CO3.0 chondrites that have experienced only minimal parent-body alteration. We discovered three kinds of evidence indicating that only minor (4–10 μm) olivine growth occurred after the final melting event: (1) Nearly all (>90%) type II chondrules in CO3.0 chondrites contain low-FeO relict grains; overgrowths on these relicts are narrow, in the range of 2–12 μm. (2) Most type II chondrules contain some FeO-rich olivine grains with decurved surfaces and acute angles between faces indicating that the grains are fragments from an earlier generation of chondrules; the limited overgrowth thicknesses following the last melting event are too thin to disguise the shard-like nature of these grains. (3) Most type II chondrules contain many small (<20 μm) euhedral or subhedral phenocrysts with central compositions that are much more ferroan than the centers of the large phenocrysts; their small sizes document the small amount of growth that occurred after the final melting event. If overgrowth thicknesses were small (4–10 μm) after the final melting event, it follows that large fractions of coarse (>40 μm) high-FeO phenocrysts are relicts from earlier generations of chondrules, and that cooling rates after the last melting event were much more rapid than indicated by models based on a single melting event. These observations are thus inconsistent with the “classic” igneous model of formation of type II porphyritic chondrules by near-total melting of a precursor mix followed by olivine nucleation on a very limited number of nuclei (say, ≤10) and by growth to produce the large phenocrysts during a period of monotonic (and roughly linear) cooling. Our observations that recycled chondrule materials constitute a large component of the phenocrysts of type II chondrules also imply that this kind of chondrule formed relatively late during the chondrule-forming period.

New analytical and numerical geospeedometers tested on garnet pyroxenites from Bragança Nappe Complex (NE Portugal)

Two new geospeedometric approaches, an analytical one [Phys. Earth Planet. Inter. 110 (1999) 95] and a numerical one [Am. J. Sci. 29 (1998) 30], are tested to evaluate cooling rate on upper mantle-derived pyroxenites from the Bragança massif (NE Portugal). These estimations, coupled with standard geobarometric techniques, allow theoretically the determination of exhumation rates responsible for incorporation of these rocks within the lower crust. In the present paper, we discuss the most significant sources of uncertainties in geospeedometry (whether numerical or analytical): precision of temperature estimates and input diffusion data. None of the available geospeedometrical tools, when used independently, is able to provide definitive results with less than a factor 10 in precision. Cross-calibration is, thus, required in order to provide reliable cooling rate estimates. We obtain for pyroxenites from the Bragança massif, a minimum cooling rate from 1000 to 750 °C of the order of 250 °C/Ma for a depth interval of 10–15 km.

Regolith and Megaregolith Formation of H-Chondrites: Thermal Constraints on the Parent Body

Spectral reflectivity data and its location near an orbital resonance suggest that Asteroid 6 Hebe may be the source body for H-chondrites, the second largest meteorite group. Recent spacecraft images of asteroids and theoretical modeling indicate that, contrary to previous ideas, asteroids can retain thick regoliths. We model the thermal evolution of a Hebe-sized object coated with a thick insulating regolith and heated by26Al and other long-lived radionuclides. The heat conduction equations for spherically symmetric objects were solved using finite-difference approximations. We assumed a three-layer structure with regolith and megaregolith overlying a rocky core. The three layers differed in bulk density, porosity, and thermal conductivity. Interior peak temperatures were set to match metamorphic temperatures of H6 chondrites.The regolith has a major influence on thermal history, and the results are very different from those for a simple rocky body published by various authors. Regolith insulation produces a uniform interior peak temperature of ∼1250 K and moves the petrographic type boundaries close to the surface of the parent body. Petrologic types 3–6 can be produced within 10 km of the asteroid's surface with only moderate (∼1 km) regolith thicknesses. The calculations indicate that H4–H6 formation would be consistent with the cooling rate estimates and Pb–Pb formation ages if the material originated in the near surface regions. We suggest that many if not all H-chondrites could have been formed in a megaregolith and thick regolith. Their observed properties are consistent with this environment, especially the abundance of regolith breccias and H-chondrites of all petrologic types with implanted solar wind gases.

40Ar/39Ar and K–Ar age constraints on shear zone evolution, southern Taltson magmatic zone, northeastern Alberta

New 40Ar/39Ar analyses of hornblende, muscovite, biotite, and K-feldspar constrain the timing of deformation and cooling of the southern Taltson magmatic zone, which underwent lower granulite to upper amphibolite grade deformation, in part synchronous with voluminous 1.99–1.92 Ga magmatism. New data are combined with existing K–Ar dates into a regional cooling framework to provide thermotemporal constraints on the deformational history. 40Ar/39Ar hornblende ages of ca. 1900 Ma are interpreted to record relatively rapid cooling following ductile thrusting on the Andrew Lake shear zone, and younger anatectic magmatism. These data, with published K–Ar and U–Pb data, support relatively rapid cooling of the Taltson magmatic zone from monazite closure temperature of 725 °C at ca. 1930 Ma to 525 °C at ca. 1900 Ma. Cooling rate estimates are about 7 °C/Ma, which suggests moderate exhumation rates during the high-grade part of the deformational history. A muscovite 40Ar/39Ar plateau age of 1803 ± 11 Ma is consistent with the mean muscovite K–Ar age of 1792 Ma, indicating regional cooling through about 350 °C at ca. 1800 Ma. 40Ar/39Ar ages from magmatic biotite of 1856 and 1799 Ma also suggest slow cooling during greenschist grade deformation, which can be no older than ca. 1860 Ma. A K-feldspar 40Ar/39Ar age of 1681 Ma provides a lower limit for the time of greenschist grade deformation. Cooling rate estimates during amphibolite to greenschist grade deformation are 1.75–2.25 °C/Ma.

Very high rates of cooling and uplift in the Alpine belt of the Betic Cordilleras, southern Spain

Cooling-rate estimates of 150-350 °C/m.y., suggesting magnitudes of uplift and exhumation of ∼15-20 km at rates of 5-10 km/m.y., are proposed for the final stage of the orogenic development in a major part of the Betic Cordilleras. The results are based on a combination of radiometric and paleontological dates. Six isotopic chronometers (muscovite and biotite whole-rock Rb-Sr, muscovite and biotite 40 Ar/ 39 Ar, biotite K-Ar, and whole-rock Rb-Sr) yield analytical ages between 23 and 18.5 Ma for rocks from Alpine nappe complexes. Nappe-sealing marine sedimentary rocks contain early Miocene foraminifera and nannoplankton indicating minimum ages of 18-15.5 Ma. The very high estimates of cooling and uplift-exhumation rates suggest tectonic unroofing, which is tentatively connected geodynamically to lithospheric slab detachment and concomitant diapirism in the upper mantle, and extensional tectonics in the crustal section.

The metallic microstructures and thermal histories of severely reheated chondrites

The metallographic structures of eight severely reheated chondrites (Farmington, Ramsdorf, Orvinio, Wickenburg, Lubbock, Rose City, Arapahoe and Tadjera) have been studied by optical, scanning electron microscope and electron microprobe techniques. Unreheated chondrites and experimentally heat treated chondritic material have also been examined. The following metallographic characteristics can be used to estimate the post-shock residual temperature of reheated chondrites: melted appearance of metal-troilite, presence of martensite, P enrichment of the metal, and the averaging of central metal grain compositions. Metallographic characteristics used to estimate the cooling rates of the severely reheated chondrites are the Ni content of troilite, the Ni gradients in metal grain rims, and the presence of secondary kamacite and phosphides. Farmington, Ramsdorf, Orvinio, Lubbock, Rose City and several of the heat treatment specimens have substantial P in solution in the metal grains ( > 0.1 wt%). P enrichment is apparently caused by reduction of phosphates upon severe reheating and partial melting of metal-troilite areas in chondritic meteorites. The eight severely reheated chondrites studied showed evidence of reheating to temperatures ranging from ∼ 950°C to ∼ 1250°C. Ramsdorf has the highest reheating temperature (1200–1250°C) and the fastest cooling rate ∼ 100°C/day. Wickenberg has the lowest reheating temperature (950–1000°C) and the slowest cooling rate, ∼ 1°C/100yrs. Cooling rate estimates correspond to post-reheating burial depths of less than 1 to ∼ 1000m.