and [(Mg,Fe)Si-Al2] compositional control on argon behaviour in high-pressure white micas: A continuous laser-probe study from the Dora-Maira nappe of the internal western Alps, Italy (original) (raw)
Related papers
Chemical Geology, 1995
Bulk samples and single grains of high-pressure white micas and a biotite from two restricted areas (Mucrone and Marine) 10 km apart in the Sesia Zone (Western Alps, Italy) were analysed by the conventional (step-heating) and continuous laser probe (step-heating and spot fusion) 4°Ar/39Ar techniques, respectively. The analysed minerals crystallized during the Eoalpine, eclogitic facies metamorphism. The phengites from the Mucrone area display ages which scatter between ~ 104 and ~ 180 Ma. In the Marine area (a 50-m-wide outcrop) the phengites from four samples with different bulk-rock compositions display precise plateau ages, which scatter between 69.4___ 0.7 and 76.9 + 0.6 Ma, and homogeneous age maps, whereas two minerals (phengite and biotite) from the same rock sample provide two very "discordant" plateau ages, 69.4 + 0.7 and 140.5 __+ 0.6 Ma, respectively. In the light of the present knowledge of the history of the Alps, ages as old as 180 Ma are unrealistic and are related to excess argon, whereas ages in the range 70-80 Ma, perhaps even 140 Ma, could be significant. We suggest that excess argon, probably conveyed by a fluid phase and trapped in defects within the structure of the phengite grains at the time of their isotopic closure, is the best explanation for the observed spread in apparent ages on a regional to single grain scale, in spite of a nearly universal existence of plateau ages and homogeneous age maps. We propose that the age of closure of the phengites to argon loss is ~<69.4+ 0.7 Ma.
Contributions to Mineralogy and Petrology, 1986
Overprinting of white micas from high pressure, low to medium temperature (M0 metamorphic assemblages in pelitic schists on Naxos during subsequent thermal dome (M2) metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome. The original (M0 minerals are phengites (Si 4+ =6.7-7.0) and the overprinting minerals are muscovites (Si 4+ =6.0-6.45). 4~ step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra, characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between. These upwardconvex age spectra are shown to be caused by mixing of two generations of micas, each of which has a different age spectrum and argon release pattern. Seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages. Further, the upward-convex age spectra have been used to trace the isotopic signature of phengites toward increasing 3/2 metamorphic grade, and suggest that as long as phengites can be observed in the rocks upward-convex age spectra occur. On Naxos, crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas. However, there is also good evidence for argon loss by volume diffusion from phengites. Simple diffusion calculations suggest that the 3/2 metamorphism was caused by a shortlived heat source.
Journal of Metamorphic Geology, 1996
A B S TR A CT Two samples from the Eclogite Micaschist Complex (EMC) and the Seconda Zona Diorito-Kinzigitica (IIDK) of the Sesia Zone have been studied using a high-spatial resolution laser probe 40Ar/39Ar technique with the aim of investigating the complexities of argon behaviour in metamorphic rocks and comparing their thermal histories. Data from a single large phengite grain from the EMC show a range of ages from mid-Jurassic to Upper Cretaceous. These 'apparent age' variations are spatially related to both location within the grain and to intragrain microstructure. Modelling of the data shows that the profile formed by the diffusion of an excess argon component into the grain, parallel to the mica cleavage. Profile asymmetry is explained by temporal variations in microstructural development enabling excess argon to enter the grain at different times in different places. The temperatures of the initiation of deformation and the possible time-scales for the deformation can be calculated as a function of cooling rate. All estimates suggest deformation at greenschist facies, in accord with the observed retrograde mineral assemblage. Absolute temperature estimates for deformation vary by less than 22°C for different cooling rates of 10 and 30°C Ma−1 but vary by 80°C with different estimates of diffusion parameters. The duration of deformation was for at least 2 Ma at 10°C Ma−1 or 0.7 Ma at 30°C Ma−1. Biotites from the IIDK sample record a Permian to Upper Cretaceous age range that correlates with grain size, the smallest grain sizes yielding the youngest ages. This relationship is best explained by a partial resetting of biotites during an Alpine thermal event initiated not more than 70 Ma ago. Modelling of these data suggest that the sample never exceeded 300°C during the Alpine. The profoundly different thermal histories of the two units-the EMC recrystallized at 550°C whilst the IIDK remained below 300°C -suggests that they may not have been juxtaposed until much later than the eclogite facies metamorphism.
Gondwana Research, 2004
Anomalously large chemical ranges in muscovite-paragonite and muscovite-celadonite systems are observed in white micas from the Piemonte calcschists in the Chisone valley area, internal western Alps. The petrographical and chemical observations on white mica strongly suggest that most mica crystals with high Na/K ratios in the chlorite zone are of detrital origin, and were derived from the pre-Alpine high-temperature metamorphic sequence such the Caledonian and/or Variscan. Submicroscopic muscovite (Ms)paragonite (Pg) composite aggregates occur in the chlorite zone and their EPMA analyses give an apparent chemical composition range from Ms,,Pgo, to Ms,,Pg,,. In the rutile zone, the paragonite content of the white micas is less than 20%, suggesting that the white micas have been homogenized during the Alpine metamorphism even if detrital white micas existed. Metamorphic mica is also very heterogeneous. The total range in Si content becomes wider with increasing of metamorphic grade: 3.22-3.39 pfu for the chlorite zone, 3.07-3.45 pfu for the chloritoid zone and 3.06-3.59 pfu for the rutile zone. This clearly indicates that the micas have experienced significant retrogressive chemical reactions during cooling and exhumations of the host schists. The detrital white mica in the chlorite zone has not reset well in its K-Ar system during the Alpine subductionrelated metamorphism. The wide range of the white mica K-Ar ages from 115 to 41 Ma must be due to a mixture of various amounts of detrital white mica in the separates. This feature is also observed in the chloritoid zone though the age variation is not so large as that in the chlorite zone. In contrast, the mica in the rutile zone, which was higher than 450"C, has been reset completely during Alpine HP metamorphism.
The basement of Sardinia represents a nearly complete section of a segment of the Variscan belt that experienced a polyphase tectonometamorphic evolution and Barrovian metamorphism. This basement is well suited to investigate the relationship between tectono-metamorphic evolution and argon isotope records in white mica. Micaschists from the garnet zone (maximum T of up to 520---560 C) contain two texturally and chemically resolvable generations of white mica: (1) deformed celadonite-rich flakes, defining a relict S 1 foliation preserved within the main S 2 foliation or enclosed in rotated albite porphyroblasts; (2) celadonite-poor white micas aligned along the main S 2 foliation. The S 1 foliation developed earlier and at a deeper crustal level with respect to that at which the thermal peak was reached. From the staurolite zone ( T of up to 590---625 C) to the sillimanite þ K-feldspar zone, white mica is nearly uniform in composition (muscovite) and is predominantly aligned along the S 2 foliation or is of later crystallization. In situ 40 Ar---39 Ar laser analyses of white mica yielded ages of 340−−−315Mainthegarnetzone,and340---315 Ma in the garnet zone, and 340−−−315Mainthegarnetzone,and320---300 Ma in the staurolite and sillimanite þ K-feldspar zones. Results highlight a close link between argon isotope records in white mica and both textures and structure-forming major elements. The oldest ages were detected in samples where the earlier syn-D 1 white mica generation did not texturally and chemically re-equilibrate at upper-crustal levels. This study suggests that the white micas that escaped recrystallization retain argon isotope records of an earlier metamorphic stage that survived a later event characterized by temperatures higher than 500 C.
Lithos, 2011
In order to date low-temperature deformation, intensely strained muscovite porphyroclasts and neocrystallized shear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Ar method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300–315 Ma) during the Alpine orogenic event. Mineralogical and textural observations indicate that phengites and chlorites developed from biotite and plagioclase in fluid system during deformation following dissolution–transport–precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite + albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengite developed at the expense of clays following substitution reaction in pelite mylonite. Based on conventional thermobarometry on phengite and chlorite and Pressure–Temperature-aqueous fluid (P–T-MH2O) pseudosections calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at 375 ± 30 °C and 4.8–7 ± 1 kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Ar analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopic resetting, all ages scattering between 296 and 315 Ma. Under conditions of intense ductile deformation and large-scale fluid circulation, muscovite grains formed during the Variscan retain their much older ages. 40Ar/39Ar dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34–20 Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample where phengites crystallized by substitution process while the other mylonitic samples where phengites grow from fluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating of neocrystallized synkinematic white mica allows the determination of precise ages of deformation and fluid activity. Together with precise thermobarometry undertaken on the basis of mineral chemistry and whole-rock composition, 40Ar/39Ar dating of white mica leads to the reconstitution of precise depth-deformation history of low-grade (< 400 °C) metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zone development at 15–21 km depth are dated between 33 and 20 Ma. In the SE part of the massif shear zone ages are well constrained to be either (1) 33.6 ± 0.6 Ma or in the range (2) 26.8 ± 0.7 Ma–26.3 ± 0.7 Ma. In the West of the massif, younger shear zone ages range between (3) 22.2 ± 0.3 Ma and (4) 20.5 ± 0.3 Ma.► Inherited deformed white mica preserve their initial 40Ar/39Ar crystallisation age. ► Deformed minerals cannot provide the time of deformation. ► Neocrystallisation argon age provide a reliable age of deformation-fluid activity. ► Si-phengite can be used as a good geobarometer in the water-saturated domain.
Contributions to Mineralogy and Petrology, 1994
The 4~ 39Ar degassing spectra of white micas and amphiboles from three tectonic units of the central Tauern Window (Pennine basement and cover in the Eastern Alps) have been measured. White micas are classified as (1) pre-Alpine low-Si relic micas with an age value of 292 Ma, variously disturbed by the Alpine metamorphism; (2) Alpine phengitic micas of variable composition with an age between 32 and 36 Ma; (3) Alpine low-Si micas with a maximum age of 27 Ma. We attribute the higher Alpine ages to a blueschist facies event, whereas the lower age reflects the late cooling of the nappe pile. Blueschist facies phengites from the basement (Lower Schieferhfille) and the tectonic cover (Upper Schieferh~lle) crystallized at a temperature below the closure temperature (To) for argon diffusion in white mica and record ages of 32 to 36 Ma. At the same time a thin, eclogite facies unit (Eclogite Zone) was thrust between the Lower and the Upper Schieferhfille and cooled from eclogite facies conditions at about 600~ at 20kbar to blueschist facies conditions at 450~ or even 300~ at >10kbar. Eclogite facies phengites closed for argon diffusion and record cooling ages, coinciding with the crystallization ages in the hanging and the footwall unit. Amphibole age spectra (actinolite, glaucophane, barroisite) are not interpretable in terms of geologically meaningful ages because of excess argon.
The Effects of Retrograde Reactions and of Diffusion on 40Ar-39Ar Ages of Micas
Journal of Petrology, 2011
The effects of metamorphic reactions occurring during decompression were explored to understand their influence on the 40 Ar^3 9 Ar ages of micas. Monometamorphic metasediments from the Lepontine Alps (Switzerland) reached lower amphibolite facies during the Barrovian metamorphism related to the collision between European and African (Adria) continental plates. Mineral assemblages typically composed of garnet, plagioclase, biotite, muscovite and paragonite (or margarite) were screened for petrological equilibrium, to focus on samples that record a minimum degree of retrogression. X-ray diffraction data indicate that some mineral separates prepared for 40 Ar^3 9 Ar stepwise heating analysis are monomineralic, whereas others are composed of two white micas (muscovite with paragonite or margarite), or biotite and chlorite. In monomineralic samples 37 Ar/ 39 Ar and 38 Ar/ 39 Ar (proportional to Ca/K and Cl/K ratios) did not change and the resulting ages can be interpreted unambiguously. In mineral separates containing two white micas, Ca/K and Cl/K ratios were variable, reflecting non-simultaneous laboratory degassing of the two heterochemical Ar reservoirs. These ratios were used to identify each Ar reservoir and to unravel the age. In a chlorite^margarite^biotite calcschist equilibrated near 5608C and 0·65 GPa, biotite, margarite, and muscovite all yield ages around 18 Ma. At slightly higher grade (560^5808C, 0·8^0·9 GPa), the assemblage muscovite^paragonite^plagioclase is in equilibrium and remains stable during retrogression. In this case, muscovite and paragonite yield indistinguishable ages around 16·5 Ma. Above 5908C, paragonite was mostly consumed to form plagioclase 45908C, whereby the relict mica yields an age up to 5·6 Ma younger than muscovite.This partial or total resetting of the Ar clock in paragonite is interpreted to reflect plagioclase growth during decompression. Where biotite is present within this same assemblage, it systematically yields a younger age than muscovite, by 0·5^2 Ma. However, these biotites all show small amounts of retrograde chlorite formation. We conclude that even very minor chloritization of biotite is apparently a more effective process than temperature in resetting the Ar clock, as is the formation of plagioclase from paragonite decomposition. Multi-equilibrium thermobarometry is an excellent means to ensure that equilibrium in investigated samples is preserved, and this helps to obtain geologically meaningful metamorphic ages. However, even samples passing such equilibrium tests may still show retrograde effects that affect the Ar retention of micas. A more robust interpretation of such 40 Ar^3 9 Ar results may require use of a second geochronometer, such as U^Pb on monazite.
Repeated thermal resetting of phengites in the Mulhacen Complex (Betic Zone, southeastern Spain) shown by 40Ar/39Ar stepheating and single grain laser probe dating. , 1992
This study reports the results of the first 40Ar/39Ar combined induction furnace and laser probe dating of phengites from the Mulhacen HP/LT metamorphic complex in the Betic Cordilleras, southern Spain. Laser step heating and spot fusion analyses on different halves of a split single grain were made with a continuous laser probe. Spot fusion analysis resulted in ages of about 30–31 Ma in the core and ages as low as 25–26 Ma in the rim. Laser step heating on the other half of the grain gave a spectrum with apparent ages increasing from about 25 Ma to 29.5 Ma. The age spectrum and the decreasing ages towards the rim of the grain may imply that resetting essentially occurred by volume diffusion of radiogenic 40Ar due to late stage reheating resulting from extensional tectonics. Ages around 30 Ma in the core of the grain are interpreted as minimum estimates of the cooling age of the main tectono-metamorphic phase D2. Induction furnace step heating on phengite separates from mica schists and one gneiss resulted in two types of age spectra. Type I spectra show monotonously rising apparent ages from 14.5 ± 1.9 Ma to 20.7 ± 0.2 Ma, and in a second sample from 16.9 ± 0.7 to 29.7 ± 0.2 Ma. Type II spectra are characterized by plateaus of 14.4 ± 0.1 Ma (the gneiss sample), 17.3 ± 0.1 Ma and 17.6 ± 0.1 Ma. Type II spectra show low temperature apparent ages significantly below the plateau age, implying resetting subsequent to initial cooling. Modelling of the age spectra demonstrated that the plateau ages are possibly the result of strong resetting (75–85% of argon loss) of an older isotope system. Total fusion of a number of phengite single grains from marbles taken close to type II mica schists yielded ages of 15.4 ± 1.2 Ma and 17.0 ± 0.7 Ma. The observed repeated resetting is coeval with major volcanic activity in basins adjacent to the metamorphic ranges, pointing to a resetting by advective fluid transport related to volcanism.