Plateau ages and excess argon in phengites: an 40Ar_39Ar laser probe study of Alpine micas (Sesia Zone, Western Alps, northern Italy) (original) (raw)
Related papers
Earth and Planetary Science Letters, 2018
The preservation of 40 Ar/ 39 Ar ages of high pressure (HP) metamorphic white mica reflects an interplay of processes that mobilise 40 Ar, either through mica recrystallisation or by diffusive 40 Ar loss. The applicability of resulting ages for dating tectonic processes is critically dependent on whether either of these processes can be proven to be efficient and exclusively active in removing 40 Ar from mica. If not, preservation of an inherited or mixed age signal in a sample must be considered for interpretation. The Cycladic Blueschist Unit on Syros has become a new focal area in the discussion of the geological significance of argon age results from multi-grain step heating experiments. While some argue that age results can directly be linked to deformation or metamorphic growth events, others interpret age results to reflect the interplay of protracted recrystallisation and partial resetting, preserving a mixed age signal. Here, we demonstrate the potential of a new approach of multiple single grain fusion dating. Using the distribution of ages at the sample, section and regional scale, we show that in Northern Syros mica ages display systematic trends that can be understood as the result of three competing processes: 1) crystallisation along the prograde to peak metamorphic path, 2) a southward trend of increasing 40 Ar loss by diffusion and 3) localised and rock type dependent deformation or metamorphic reactions leading to an observed age spread typically limited to ∼10 Myr at the section scale. None of the sections yielded the anomalously old age results that would be diagnostic for significant excess 40 Ar. The recorded trends in ages for each of the studied sections reflect a range of P-T conditions and duration of metamorphism. Diffusion modelling shows that in a typical subduction metamorphic loop, subtle variations in P-T-t history can explain that age contrasts occur on a regional scale but are limited on the outcrop scale. Our new approach provides a comprehensive inventory of the range of ages present in different rocks and at different scales, which results in a more refined understanding of argon retention and isotopic closure of phengite and the geological significance of the ages. We verify the added value of our new approach by comparison with multi-grain step heating experiments on selected samples from the same sections.
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.
Interpreting high-pressure phengite 40Ar/39Ar laserprobe ages: an example from Saih Hatat, NE Oman
Contributions to Mineralogy and Petrology, 2011
New single grain fusion and core-rim 40 Ar/ 39 Ar laserprobe phengite data from the Saih Hatat high-pressure terrane in NE Oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane. The majority of the determined ages are older than the previously reported U-Pb zircon peak metamorphic age. Core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores, or, rarely, older rims; some samples manifest all three patterns. Numerical diffusion modelling shows that due to the peak temperature of ca. 550°C, the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system. The age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm-cm scale coupled with pervasive and heterogeneously distributed excess argon. Anomalously old eclogite phengite 40 Ar/ 39 Ar ages are due either to internally derived 40 Ar inherited from a K-bearing precursor, or externally derived 40 Ar distributed by grain boundary fluids. Micarich schists within the eclogite boudins yield younger phengite ages, suggesting excess argon was absent or diluted. Pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist-facies overprint and yield very variable ages commonly with apparently older rims on younger cores. The grain-and sample-scale age variations measured in Saih Hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport, whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle.
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.
Earth and Planetary Science Letters
This study reports the results of the first 4°Ar/39mr 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 4°Ar 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 D 2.
Mafic rocks containing eclogite relics are fairly widespread in the crystalline nappe Stack of lhe Swiss Central Alps. This study addresses the spatial distribution of eclogite relics in the Central Alps, their field relations. structural and petrological characteristics. and their PTt-history. Implications for the assembly of the nappe Stack are explored. The majority of eclogite-facies relics is confined lo a single super-unit of tectonic melange. interpreted as a tec¬ tonic accretion Channel (TAC). Numerous mafic high-pressure (HP) lenses have been discovered through systematic fieldwork in the TAC units of the Central Alps, an up-lo-date inventory of which is presented. Systematic documentation of select samples with HP imprint yields clockwise PT-paths. Prograde phase relations are seldom preserved. except in the chemical zoning of garnet porphyroblasts. However. when present. relic assemblages indicate HP-LT conditions indicative of a subduction setting. Maximum recorded pressures are substantially different from one loca¬ tion to the next (1.9 to 3.3 GPa). Depending on the degree of rehydration. reaction sequences are derived from observed relics. local replacement relations and assemblages. Quantitative constraints on the detailed PT-path are extracted by combining isochemical phase diagrams and 7Wß-thermobarometry with petrographie information. HP lenses from different locations display substantially different paths. both within and between different melange zones of the TAC. PT-conditions reflecting the late-Alpine Barrovian overprint of mafic HP lenses are in agreement with the coherent regional pattern derived from metasediments. i.e.. maximum temperatures (-600°C in the central Lepontine belt. 700-750°C in the southern parts) were reached at pressures between 0.75 and 0.55 GPa Four samples have been dated by Lu-Hf isotopic analysis of garnet. clinopyroxene. malrix phases and wholerock powders.The age span Covers a ränge from >70 to-.36 Ma.much larger than previously documented for Alpine HP rocks from the Central Alps. Petrological data of the samples and their Lu-Hf isotopic system indicate a protracted HP historv for at least some of the sub-units of the TAC. with garnet growth under eclogite-facies conditions starting before 70 Ma in some parts of the TAC. and continuing as late as 36 Ma in others. Iliese data have implications for the dynamics of melange formation within the TAC with internal fragmentation and mixing. and pronounced mobility of lhe tectonic zones, probably during the early. subduclional stages and again during the post-collisional extrusion along the plate boundary. After 32 Ma. when the Barrovian overprint reached its maximum temperature. the TAC appears to have been exhumed as part of the then-coherenl crystalline nappe Stack.
Geochimica et Cosmochimica Acta, 1992
High-pressure white micas from a restricted area in the Dora-Maira nappe of the western Alps (Italy) were analyzed by the conventional 40Ar/3gAr bulk-sample step-heating method and by the stepheating and the spot-fusion 40Ar/3gAr continuous laser-probe techniques on single grains. High-Si, Ferich phengites from fine-grained gneisses display homogeneous 35-40 Ma plateau and spot-fusion ages, consistent with the upper Eocene 40Ar/3gAr phengite ages documented over the whole internal western Alps. In contrast, phengite separates and single-grains of highSi and Mg-rich to intermediate composition selected from metapelitic and Mg-rich mica schist samples yield older discordant apparent ages, even when sampled in the same outcrop as the fine-grained gneisses. These local age variations are interpreted to reflect primarily an increasing Ar retentivity with increasing Mg content in phengites. In metapelitederived white micas, the isotopic discordance is marked by (1) a characteristic bulk-sample upwardconvex spectrum shape, (2) single-grain disturbed age spectra, (3) a systematic radially outward decrease in spot ages across individual mica cleavage planes, and (4) strong grain-to-grain spot-age variations in rock sections. In Mg-rich mica schists, the isotopic discordance is observed as (1) a reproducible upwardconvex bulk-sample age spectrum with high (140 Ma) intermediate apparent ages, (2) upward-convex single-grain age spectra, and (3) irregular intra-and intergrain age gradients. On geochemical grounds, it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the [ (Mg,Fe) Si-Ah] celadonitic re-equilibration of the high-pressure phengites during greenschist overprinting. In Mg-rich phengites, this chemical t-e-equilibration may induce Ar loss below their presumed higher blocking temperature. A model of Ar behaviour in high-pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the Mg/Fe compositional control on Ar retentivity and the [ (Mg,Fe) Si-Al21 modifications occurring in phengites during progressive overprinting. INTRODUCIION IN RECENT YEARS, the use of metamorphic white micas in 40Ar/3gAr geochronology has found increasing application in thermotectonic reconstructions of P-T-Z (pressure-temperature-time) paths of crustal sections involved in collision zones and HP/LT (high pressure-low temperature) to greenschistfacies transformations. Although many unaltered high-pressure assemblages have been shown to provide undisturbed age spectra (e.g.