Diffusion vs. fluid alteration in alkali feldspar ⁴⁰Ar/³⁹Ar thermochronology: does cross-correlation of log(r/r₀) and age spectra validate thermal histories?

For six decades geoscientists have been trying to quantitatively understand the nature of radiogenic Ar loss from alkali feldspar. Some researchers suggest that volume diffusion is the dominant mechanism, and they use conventional step-heating ⁴⁰Ar/³⁹Ar data from alkali feldspar to recover the thermal histories of rocks. They argue that a high degree of correlation between log(r/r₀) and ⁴⁰Ar/³⁹Ar age spectra, which is observed in a number of natural examples, justifies this hypothesis. In contrast, other investigators suggest that fluid-mediated recrystallisation and alteration control the radiogenic Ar redistribution, hence rendering alkali feldspar useless as a thermochronometer. By means of numerical modelling, we found that the latter mechanism as well is able to produce samples with highly correlated log(r/r₀) and ⁴⁰Ar/³⁹Ar age spectra. In addition, we show that apparent thermal histories recovered for altered alkali feldspar crystals by interpreting step-heating ⁴⁰Ar/³⁹Ar data may be grossly inaccurate, and yet seemingly fit the prevailing understanding of regional geology. Such inaccurate apparent thermal histories can be obtained even from alkali feldspar crystals that underwent volumetrically low degrees of alteration. Therefore, we conclude that conventional step-heating ⁴⁰Ar/³⁹Ar data are insufficient to support the assumption that radiogenic Ar loss from alkali feldspar occurred solely by volume diffusion and validate the constrained thermal histories, even if upheld by a priori knowledge of regional tectonics. We further suggest that all thermochronological constraints obtained using such data should be supported by detailed petrological characterisation of alkali feldspar

Rock uplift and exhumation of continental margins by the collision, accretion, and subduction of buoyant and topographically prominent oceanic crust

Understanding the causes of rock and surface uplift is important because they control the location of mountain building, depocenters, and drainage characteristics and can influence climate. Here we combine previous thermochronological data with field observations to determine the amount of exhumation, rock, and surface uplift that occurs in the upper plate of Central and South American subduction zones during the collision, accretion, and subduction of oceanic plateaus and aseismic ridges. The collision of buoyant and topographically prominent oceanic plateaus and ridges can drive at least 5 km of rock uplift within 2 Ma. Uplift appears to be an immediate response to collision and is generally independent of the slab dip. The amount of rock uplift is controlled mainly by excess topography associated with the ridge (ultimately linked to buoyancy) and erosion, while it is also influenced by the strength of the subduction interface related to the presence of volcanic asperities and overpressured sediments in the subduction channel. The quantity of exhumation is strongly dependant on climate-induced erosion and the lifespan over which the topography is uplifted and supported. Sediment draining into the trench may leave the elevated ridge axis sediment starved, increasing the shear stresses at the ridge subduction interface, leading to positive feedback between ridge subduction, rock uplift, and exhumation. Trench-parallel variations in exhumation have a direct impact on exploration paradigms for porphyry-related metalliferous deposits, and it is likely that porphyry systems are completely eroded by the impingement of plateaus and aseismic ridges within temperate and tropical climates

(U–Th)/He thermochronometric constraints on the late Miocene–Pliocene tectonic development of the northern Cordillera Real and the Interandean Depression, Ecuador

The low sensitivity of apatite fission track (AFT) thermochronometry at temperatures less than ∼60 °C suggests that AFT data sets from the Andean Cordilleras may have frequently failed to identify specific periods after 9 Ma when cooling rates were high. Forward modeling of (U–Th)/He apatite age data obtained from the juxtaposed Paleozoic–Mesozoic Alao, Loja, and Salado terranes in the northern Cordillera Real, Ecuador, has improved the resolution of previous AFT thermal histories for the past 9 My. The Alao and Loja terranes form a coherent, structural block that resided at temperatures greater than 70–80 °C until ∼3.3–2.8 Ma and then cooled rapidly to less than 40 °C at rates of >15 °C/My. Intraterrane variations in the cooling and exhumation histories in the Salado terrane suggest that nonterrane-bounding faults played a significant role during its Pliocene–Recent evolution. The Salado terrane preserves an older history that reveals elevated cooling rates during 22–19 and 18–15 Ma. Subsequently, the terrane cooled rapidly from greater than 90 °C to less than 40 °C during 11–8 and 5.5–3.5 Ma at rates of >8 °C/My. Vertical reactivation of the Llanganates fault, which separates the Salado and Loja terranes, during the Pliocene–Recent coincides with the main stages of formation of the juxtaposed Interandean Depression, which provides further constraints on the growth phases of the depression and the Cordillera

Geochronology and Thermochronology Using Apatite: Time and Temperature, Lower Crust to Surface

Apatite can provide geologists with an exceptionally wide range of ages and temperatures to investigate processes that operate from Earth's surface right down to the lower crust. Apatite is a widespread accessory mineral in igneous, metamorphic, and clastic sedimentary rocks and can be dated using four radioactive decay schemes, each with a different temperature window for isotopic closure: Lu–Hf (675–750 °C); U–Pb (350–550 °C); apatite fission track (60–110 °C); (U–Th)/He (40–80 °C). The fission-track and (U–Th)/He methods are popular for studying upper-crustal and near-surface processes, whereas the U–Pb and Lu–Hf systems are used to investigate the thermal, tectonic, and magmatic histories of the deeper crust

Geochronology and stable isotope signature of alteration related to hydrothermal magnetite ores in Central Anatolia, Turkey

Hydrothermal iron ores at Divriği, east Central Anatolia, are contained in two orebodies, the magnetite-rich A-kafa and the limonitic B-kafa (resources of 133.8 Mt with 56% Fe and 0.5% Cu). The magnetite ores are hosted in serpentinites of the Divriği ophiolite at the contact with plutons of the Murmano complex. Hydrothermal biotite from the Divriği A-kafa yield identical weighted mean plateau ages of 73.75 ± 0.62 and 74.34 ± 0.83 Ma (2σ). This biotite represents a late alteration phase, and its age is a minimum age for the magnetite ore. Similar magnetite ores occur at Hasançelebi and Karakuz, south of Divriği. There, the iron ores are hosted in volcanic or subvolcanic rocks, respectively, and are associated with a voluminous scapolite ± amphibole ± biotite alteration. At Hasançelebi, biotite is intergrown with parts of the magnetite, and both minerals formed coevally. The weighted mean plateau ages of hydrothermal biotite of 73.43 ± 0.41 and 74.92 ± 0.39 Ma (2σ), therefore, represent mineralization ages. Hydrothermal biotite from a vein cutting the scapolitized host rocks south of the Hasançelebi prospect has a weighted mean plateau age of 73.12 ± 0.75 Ma (2σ). This age, together with the two biotite ages from the Hasançelebi ores, constrains the minimum age of the volcanic host rocks, syenitic porphyry dikes therein, and the scapolite alteration affecting both rock types. Pyrite and calcite also represent late hydrothermal stages in all of these magnetite deposits. The sulfur isotope composition of pyrite between 11.5 and 17.4‰ δ34S(VCDT) points towards a non-magmatic sulfur source of probably evaporitic origin. Calcite from the Divriği deposit has δ18O(VSMOV) values between +15.1 and +26.5‰ and δ13C(VPDB) values between −2.5 and +2.0‰, which are compatible with an involvement of modified marine evaporitic fluids during the late hydrothermal stages, assuming calcite formation temperatures of about 300°C. The presence of evaporite-derived brines also during the early stages is corroborated by the pre-magnetite scapolite alteration at Divriği, and Hasançelebi-Karakuz, and with paleogeographic and paleoclimatic reconstructions. The data are compatible with a previously proposed genetic model for the Divriği deposit in which hydrothermal fluids leach and redistribute iron from ophiolitic rocks concomitant with the cooling of the nearby plutons

Time scales of mineral systems - Advances in understanding over the past decade

The establishment of accurate time scales of mineral systems is essential to construct reliable genetic models about their formation. Time scales of fossil mineral systems are directly determined through radiometric dating of different stages of development of the mineral system. In theory, porphyry systems are, among mineral sys- tems, those whose duration can be bracketed with most accuracy and precision, because of the universal occur- rence of ore and gangue minerals that can be dated with the high precision U-Pb zircon, Re-Os molybdenite, and 40Ar/39Ar dating techniques. Time scales of fossil porphyry systems reported in the literature range between 4 Ma. The long dura- tions (>1 Ma) of magmatic-hydrothermal activity measured in several porphyry systems are likely the result of multiple magmatic pulses in agreement with field observations indicating that porphyry systems are associated with several intrusive events. Nonetheless, estimated long durations could also be affected by methodological problems. One methodological problem is the accuracy of the intercalibration among the three different meth- ods. It has become evident during the last 15 years that 40Ar/39Ar dates are systematically younger compared to U-Pb dates. This has been attributed to incorrect values of the secondary standard (Fish Canyon Tuff sanidine), most commonly used to calculate 40Ar/39Ar ages, and/or of the 40K decay constant. Systematic cross calibrations to check the consistency between Re-Os and U-Pb dates are lacking and should also be carried out. Another possible cause of erroneous long durations of porphyry systems concerns the way to determine the emplacement age of the causative intrusion. The current high precision (≤0.1%) of single zircon U-Pb dating by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) shows that zircon grains extracted from a single sample of intermediate/felsic magmatic rocks do not overlap in age. This is so because zircon grains record a protracted evolution of magmas within the crust lasting several hundreds of thousands of years. Under these conditions, the emplacement age of a magmatic intrusion is best approximated by the youngest ID-TIMS age measured from a population of zircon grains. In contrast, spot ages measured with in situ techniques, due to their lower precisions (1−3%), are not able to discriminate such protracted magmatic evolution recorded by different zircon grains. This allows pooling together spot ages of different zircons, resulting in a statistically significant mean age with a low uncertainty. In reality this is a mixed age that is characteristically older (by up to a few hundreds of thousands of years) than the age of the youngest single zircon grain measured by ID-TIMS. A further problem in estimating the duration of magmatic-hydrothermal activity in porphyry systems derives from the widespread use of 40Ar/39Ar dating. Because this method does not date the crystallization of a mineral but rather its cooling below its closure temperature, 40Ar/39Ar dates may be affected by (hydro-)thermal activity that postdates the mineralization

Application of low-temperature thermochronology to hydrothermal ore deposits: Formation, preservation and exhumation of epithermal gold systems from the Eastern Rhodopes, Bulgaria

New low-temperature thermochronological data have been used to quantify the protracted, Eocene–Miocene cooling histories of upper and lower plate rocks of the Kesebir–Kardamos extensional dome, Eastern Rhodopes, Bulgaria. 40Ar/39Ar and apatite fission-track data reveal that the lower plate has experienced continuous cooling and exhumation, since the Late Eocene. Muscovite 40Ar/39Ar plateau ages of 36.90±0.16 Ma and 37.28±0.19 Ma (2σ) from metamorphic rocks of the footwall reveal the approximate time span during which they cooled below ~350 °C during exhumation caused by detachment faulting. The sedimentary rock-hosted gold mineralization, which represents a thermal event at ~250–220 °C, developed during the early stage of basin formation between 34.71±0.16 Ma and 35.36±0.21 Ma (adularia 40Ar/39Ar plateau ages; 2σ). The termination of hydrothermal mineral deposition at Ada Tepe occurred contemporaneously with the earliest phase of calc–alkaline type magmatism at Iran Tepe (33.97±0.36 Ma to 34.62±0.46 Ma, hornblende and biotite 40Ar/39Ar plateau ages, 2σ). Thermal history modelling of apatite fission-track data shows that the lower plate rocks cooled through ~120 °C at ∼18.3±1.9 Ma (1σ). A time–temperature model obtained from zircon and apatite fission-track data fromthe upper plate reveals that it was being buried during the late Eocene. At ∼33–30 Ma, a dramatic change of the time–temperature path was caused by the initiation of horst–graben structures, resulting in rapid exhumation of the upper plate. Our new thermochronological data reveal many aspects of the mechanisms of formation of sedimentary rock-hosted gold deposits. The heat accumulated during sedimentary burial of the upper plate is a plausible heat source to drive hydrothermal fluid circulation and ore formation. The development of large half-graben basins in the hanging walls of detachment faults, accompanied by a favourable climate, may have created a situation that was adequate to capture a primary meteoricwater supply that subsequently underwent hydrothermal convection. Our new 40Ar/39Ar age data indicate that magmatic activity was contemporaneous with gold deposition and must be considered as a source of heat, sulphur and/or a metal for the ore-forming hydrothermal system. The preservation and exhumation of sedimentary rock-hosted ore deposits is largely controlled by the exhumation history of the upper plate, which has been quantitatively constrained by low-temperature thermochronological methods, and commences with rapid exhumation at ∼33–30 Ma

⁴⁰Ar/³⁹Ar age constraints for an early Alpine metamorphism of the Sakar unit, Sakar–Strandzha zone, Bulgaria

We investigated the Sakar unit metamorphic rocks of the Sakar Strandzha zone in Bulgaria,using 40Ar/39Ar dating of amphibole from the polymetamorphic basement and white mica in the overlying upper Permian metasedimentary rocks of the Paleokastro Formation. The amphibole and white mica revealed plateau ages of 140.50 ± 1.75 Ma and 126.19 ± 1.29 Ma, respectively, indicating an Early Cretaceous cooling history of the regional amphibolite-facies metamorphism to greenschist-facies conditions. Similar metamorphic grades and cooling histories of the Sakar unit share evidence with the nearby Rhodope Massif for the northern Aegean region-wide early Alpine tectonometamorphic event