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Investigates the utilization of [sup 40]Ar/[sup 39]Ar ultraviolet laser microprobes (UVLAMP) in the analysis of detrital white micas. Characteristic of detrital micas evolution; Efficiency of UVLAMP in the analysis of now-eroded mountain belts; Implication of [sup 40]Ar/[sup 39]Ar geochronology for orogenic uplift.

Excess argon is a common problem in high-pressure metamorphic rocks. Here, we present 40Ar/39Ar data from metabasite, metachert and metapelite lithologies and a range of minerals. The results allow us to discuss the origin of the excess argon and processes leading to its incorporation during high-pressure metamorphism. Samples were derived from blueschist and low-temperature eclogite facies rocks from the Tavsanli Zone of NW Turkey. Samples were analysed using the 40Ar/39Ar Ultra-Violet Laser Ablation Microprobe (UVLAMP) and yield ages ranging from 72±3 to 154±7 Ma. Detailed intra-grain profiling of phengites in situ, and analyses of sodic-amphibole, lawsonite, quartz and garnet, present a detailed picture of the spatial distribution of excess argon within different minerals in the same hand samples, revealing that they acted as near closed systems where excess argon was preferentially partitioned into phengite. Whole rock XRF data indicates a scattered relationship between bulk K2O content of the rock and the concentration of excess argon in phengite. We conclude that excess argon was derived in situ in a relatively closed, fluid-absent system, rather than introduced by fluids. [Copyright &y& Elsevier]

Investigates the geochronology of high-pressure metamorphism and exhumation history of the rocks in Tavsanli zone, Turkey. Timing of the high-pressure metamorphism; Intra-grain analysis of minerals to distinguish isotopic heterogeneities.

[Display omitted] • High-grade metapelitic rocks in the Akdağ Massif metamorphosed at 5–6 kbar/650–700 °C. • The cooling of the Akdağ Massif occurred at 75 ± 0.8 Ma. • The replacements of garnet by sillimanite, cordierite and biotite indicates LP-HT metamorphism. • The Akdağ Massif rapidly exhumed at a cooling rate of ∼ 40 °C/Ma between ∼ 55 and 45 Ma. The Central Anatolian Crystalline Complex (CACC) is a Late Cretaceous continental crystalline basement exposed in central Turkey. Here, we report the metamorphic conditions and cooling history of the southern Akdağ Massif, which is the northeastern margin of the CACC. Thermobarometry performed on the garnet-sillimanite-bearing metapelitic rocks indicate ∼ 5–6 kbar/650–700 °C metamorphic conditions. Following HT-MP metamorphism, conditions of ∼ 3–4 kbar and ∼ 720 °C were also recorded by the metapelitic rocks. The replacement of sillimanite, cordierite, and biotite on the rims of garnet indicates LP-HT metamorphism. The HT decompression path described in this study is similar to conditions that have been previously proposed for the CACC. The LP-HT metamorphism resulted from the increased geothermal gradian arising from local granitoid intrusions, which are most likely related to Late Cretaceous subduction below the CACC, at ∼ 82–72 Ma. The Ar/Ar geochronology indicates that cooling of the high-grade rocks occurred at 75 ± 0.8 Ma and the relatively younger Ar/Ar cooling ages of 61 ± 0.5 Ma (amphibole) and 55.0 ± 0.3 Ma (muscovite) were obtained from the medium/low grade metamorphic rocks of the Akdağ Massif. These cooling ages and stratigraphic evidence from the middle Eocene sedimentary cover, suggests a rapid exhumation and cooling rate of the Akdağ Massif is about ∼ 40 °C/Ma. We suggest therefore, that the exhumation of the Akdağ Massif was finalized by the Late Paleocene – Early Eocene and we suggest that this rapid exhumation can be explained by tectonic effects rather than by erosional alone. [ABSTRACT FROM AUTHOR]

The tephra dispersal mechanisms of rhyolitic glaciovolcanic eruptions are little known, but can be investigated through the correlation of eruptive products across multiple depositional settings. Using geochemistry and geochronology, we correlate a regionally important Pleistocene tephra horizon-the rhyolitic component of North Atlantic Ash Zone II (II-RHY-1)-and the Thórsmörk Ignimbrite with rhyolitic tuyas at Torfajökull volcano, Iceland. The eruption breached an ice mass >400 m thick, leading to the widespread dispersal of II-RHY-1 across the North Atlantic and the Greenland ice sheet. Locally, pyroclastic density currents traveled across the ice surface, depositing the variably welded Thórsmörk Ignimbrite beyond the ice margin and ~30 km from source. The widely dispersed products of this eruption represent a valuable isochronous tie line between terrestrial, marine, and ice-core paleoenvironmental records. Using the tephra horizon, estimates of ice thickness and extent derived from the eruption deposits can be directly linked to the regional climate archive, which records the eruption at the onset of Greenland Stadial 15.2. [ABSTRACT FROM AUTHOR]

Abstract: Infrared laser probe 40Ar/39Ar geochronology, instrumental neutron activation analysis (INAA) and analytical electron microscopy have been performed on four 0.5×1.0×0.3cm polished rock tiles of Apollo 16 and 17 granulitic breccias (60035, 77017, 78155, and 79215). Pyroxene thermometry indicates that these samples were re-equilibrated and underwent peak metamorphic sub-solidus recrystallization at 1000–1100°C, which resulted in homogeneous mineral compositions and granoblastic textures. 40Ar/39Ar data from this study reveal that three samples (60035, 77017, and 78155) have peak metamorphic ages of ∼4.1Ga. Sample 79215 has a peak metamorphic age of 3.9Ga, which may be related to Serenitatis basin formation. All four samples contain moderately high concentrations of meteoritic siderophiles. Enhanced siderophile contents in three of the samples provide evidence for projectile contamination of their target lithologies occurring prior to peak metamorphism. Post-peak metamorphism, low-temperature (<300°C) events caused the partial resetting of argon in the two finer-grained granulites (60035 and 77017). These later events did not alter the mineralogy or texture of the rocks, but caused minor brecciation and the partial release of argon from plagioclase. Interpretation of the low-temperature data indicates partial resetting of the argon systematics to as young as 3.2Ga for 60035 and 2.3Ga for 77017. Cosmic ray exposure ages range from 6.4 to ∼339Ma. Our results increase the amount of high-precision data available for the granulitic breccias and lunar highlands crustal samples. The results demonstrate the survival of pre-Nectarian material on the lunar surface and document the effects of contact metamorphic and impact processes during the pre-Nectarian Epoch, as well as the low-temperature partial resetting of ages by smaller impact events after 3.9Ga. The mineralogy and chemical composition of these rocks, as well as exhumation constraints, indicate that the source of heat for metamorphism was within kilometers of the surface via burial beneath impact-melt sheets or hot ejecta blankets. [Copyright &y& Elsevier]