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An interlaboratory comparison (ILC) was organised to characterise 87Sr/86Sr isotope ratios in geological and industrial reference materials by applying the so‐called conventional method for determining 87Sr/86Sr isotope ratios. Four cements (VDZ 100a, VDZ 200a, VDZ 300a, IAG OPC‐1), one limestone (IAG CGL ML‐3) and one slate (IAG OU‐6) reference materials were selected, covering a wide range of naturally occurring Sr isotopic signatures. Thirteen laboratories received aliquots of these six reference materials together with a detailed technical protocol. The consensus values for the six reference materials and their associated measurement uncertainties were obtained by applying a Gaussian, linear mixed effects model fitted to all the measurement results. By combining the consensus values and their uncertainties with an uncertainty contribution for potential heterogeneity, reference values ranging from 0.708134 mol mol‐1 to 0.729778 mol mol‐1 were obtained with relative expanded uncertainties of ≤ 0.007 %. This study represents an ILC on conventional 87Sr/86Sr isotope ratios, within which metrological principles were considered and the compatibility of measurement results obtained by MC‐ICP‐MS and by MC‐TIMS is demonstrated. The materials characterised in this study can be used as reference materials for validation and quality control purposes and to estimate measurement uncertainties in conventional 87Sr/86Sr isotope ratio measurement. [ABSTRACT FROM AUTHOR]

The [sup.40]Ar/[sup.39]Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra--marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between [sup.14]C and longer-lived isotopic systems that are too imprecise for dating young samples. However, [sup.40]Ar/[sup.39]Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, [sup.238]U-[sup.230]Th disequilibrium, to demonstrate that [sup.40]Ar/[sup.39]Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous [sup.40]Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in [sup.40]Ar/[sup.39]Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of [sup.40]Ar due to rapid loss to an 'infinite reservoir' such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced. doi: 10.1130/G31064.1

The 163 k.y. history as well as the chemical and 46 km3 volumetric evolution of Volcan Parinacota are described in detail by new mapping, stratigraphy, and 57 [sup.40]Ar/[sup.39]Ar ages determined from groundmass or sanidine crystals in basaltic andesitic to rhyolitic lavas. A more precise chronology of eruptions and associated eruptive volumes of this central Andean volcano, which was built upon 70-km-thick crust, provides a more complete view of how quickly volcanic edifices are built in this setting and how their magmatic systems evolve during their lifetime. Development of the complex involved initial eruption of andesitic lava flows (163-117 ka) followed by a rhyodacite dome plateau (47-40 ka) synchronous with the onset of the building of a stratocone (52-20 ka), which was later destroyed by a debris avalanche ~ 3 times larger than that at Mount St. Helens in 1980. Dome plateau emplacement occurred faster and later than has previously been published, implying a compressed duration of cone building and introducing a preceding 65 k.y. hiatus. Debris avalanche timing is refined here to be older than 10 but younger than 20 ka. Rapid postcollapse rebuilding of the volcanic edifice is delineated by 16 groundmass and whole-rock [sup.40]Ar/[sup.39]Ar ages, which include some of the youngest lava flows dated by this method. Increase in cone-building rate and a continued trend toward more mafic compositions following collapse imply an inter-relationship between the presence of the edifice and flux of magma from the feeding reservoir. Cone-building rates at Parinacota are similar to those at other well-dated volcanoes on thinner crust; however, the distributed basaltic volcanism prevalent in those other arcs is virtually absent both at Parinacota and elsewhere in the Central Volcanic Zone. This suggests that while the hydrous, calc-alkaline magmas that make up the central volcanoes are not significantly retarded by thick crust, primitive, dry basalts might be. Keywords: Andes, Central Volcanic Zone, Parinacota, [sup.40Ar]/[sup.39]Ar geochronology, eruptive rates.

The determination of accurate and precise isochron ages for igneous rocks requires that the initial isotope ratios of the analyzed minerals are identical at the time of eruption or emplacement. Studies of young volcanic rocks at the mineral scale have shown this assumption to be invalid in many instances. Variations in initial isotope ratios can result in erroneous or imprecise ages. Nevertheless, it is possible for initial isotope ratio variation to be obscured in a statistically acceptable isochron. Independent age determinations and critical appraisal of petrography are needed to evaluate isotope data. If initial isotope ratio variability can be demonstrated, however, it can be used to constrain petrogenetic pathways. Keywords: geochronology, igneous rocks, isotope geochemistry, isochrons.

The 40Ar/39Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra-marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between 14C and longer-lived isotopic systems that are too imprecise for dating young samples. However, 40Ar/39Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, 238U-230Th disequilibrium, to demonstrate that 40Ar/39Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous 40Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in 40Ar/39Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of 40Ar due to rapid loss to an "infinite reservoir" such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced. [ABSTRACT FROM AUTHOR]

The Miocene Kaiserstuhl volcanic complex in the Rhine graben rift is known for simultaneously exposing both intrusive and erupted (pyroclastic) calciocarbonatites. This makes Kaiserstuhl a promising candidate for studying the field and genetic relations between intrusive calciocarbonatite and its eruptive equivalent, and the processes enabling eruption of the calciocarbonatite at the surface in particular. Eruptive calciocarbonatites in Kaiserstuhl are represented by carbonatite tuff and lapillistone beds covering a debrite fan on the western flank of the volcano. The debrites are interpreted as lahar (debris flow) and possibly also debris-avalanche deposits. Based on the observed textures, the debris flows were most likely derived by water dilution from debris avalanches resulting from edifice failure, which occurred in the central part of the Kaiserstuhl volcanic complex. The edifice failure ultimately exposed the intrusive system, and the carbonatite pyroclasts (lapilli and ash) were ejected from narrow vents represented by open-framework tuff-breccias aligned along the detachment scarp. Since the Ca-carbonates break down rapidly at high temperatures and low pressures, calciocarbonatites are unlikely to form surface lavas. On the other hand, the presence of the calciocarbonatite pyroclastic deposits suggests that some geological process faster than the high-temperature breakdown of Ca-carbonate may facilitate calciocarbonatite eruption. We suggest that the sudden exposure and decompression of a suprasolidus high-level carbonatite intrusion by edifice collapse may be a suitable scenario enabling calciocarbonatite eruption. The absence of edifice failures on alkaline volcanoes, where carbonatite intrusion is either supposed or exposed, may explain the overall scarcity of erupted calciocarbonatites. [ABSTRACT FROM AUTHOR]

In countless modern geochemical studies, diverse biological and geologic samples are analyzed for Sr, Nd, and Pb isotopic composition. Such heterogeneity presents challenges for a "one-size-fits-all" approach to sample preparation, necessitating customization of sample preparation and chromatographic separation methods. We present (1) digestion techniques for low-Nd silicates, carbonatites, carbonates, water, plant and wood material, organic soils, aerosols collected via filtration, as well as archaeological samples (alloys, teeth, and bones) (2) a column chromatographic approach for samples with low concentrations (large amounts of a matrix) and (3) method verification via replicate analyses of a wide variety of isotopic standards. [ABSTRACT FROM AUTHOR]

The Trans-Altai Zone in the southern tract of the Central Asian Oceanic Belt is composed of Early Palaeozoic oceanic crust preserved in Ordovician to Devonian ophiolite fragments and Devonian--Carboniferous igneous arcs. The Edren and Baaran subzones at the NW tip of the Trans-Altai Zone were intruded by Late Palaeozoic plutons that have been examined by the combined geochronological and geochemical study. Mississippian subduction-related plutons intruded Devonian and Carboniferous volcano-sedimentary sequences in two magmatic pulses. The older, Tournaisian plutons (dated at 352 ± 1 and 347 ± 4 Ma) occur in both subzones; the younger Visean/Serpukhovian ones (331 ± 1 Ma) are found only at the northern boundary of the Edren Subzone. All Mississippian rocks are high-K calc-alkaline and characterised by a strong enrichment of hydrous fluid mobile lithophile elements over conservative Nb, Ta and Ti relative to normal mid-ocean ridge basalts. Low 87Sr/86Sri (~ 0.7035-0.7038) and highly positive εi Nd values (+ 6.6 to + 5.2) suggest a relatively juvenile parental magma source with a short mean crustal residence. This corresponds well with the age of scarce inherited zircons, none of which is older than 530 Ma. The Early Permian post-tectonic plutons intruded the shallow crust of the Baaran Subzone (Devonian--Carboniferous flysch and Early Carboniferous volcanic arc). The prominent concentric body of the Aaj Bogd Pluton is composed of monzodiorites to monzogabbros (284 ± 1 and 294 ± 3 Ma) in its centre, surrounded by granite with syenite (282 ± 1 Ma) in the main mass of the pluton. Whole-rock Sr--Nd isotopic ratios match those of Carboniferous magmatic rocks, while trace-element patterns point to an intra-plate origin influenced by a fertile asthenospheric mantle component. On the other hand, the slightly older (290 ± 1 Ma) quartz syenites to alkali feldspar granites in the Baaran Subzone have spurious arc-like geochemistry inherited from their arc-related crustal source(s). Regional distribution of the numerous oval-shaped Early Permian alkaline post-orogenic plutons, some with A2-type granite affinity, follows the major Permian strike-slip zones spanning from the Dulate Arc in the west to the Khan Bogd Pluton in the east. These late, transcurrent zones apparently played an important role in late-orogenic magma generation, ascent and emplacement. [ABSTRACT FROM AUTHOR]

The 163 k.y. history as well as the chemical and 46 km³ volumetric evolution of Volcán Parinacota are described in detail by new mapping, stratigraphy, and 57 40Ar/39Ar ages determined from groundmass or sanidine crystals in basaltic andesitic to rhyolitic lavas. A more precise chronology of eruptions and associated eruptive volumes of this central Andean volcano, which was built upon 70-km-thick crust, provides a more complete view of how quickly volcanic edifices are built in this setting and how their magmatic systems evolve during their lifetime. Development of the complex involved initial eruption of andesitic lava flows (163-117 ka) followed by a rhyodacite dome plateau (47-40 ka) synchronous with the onset of the building of a stratocone (52-20 ka), which was later destroyed by a debris avalanche 3 times larger than that at Mount St. Helens in 1980. Dome plateau emplacement occurred faster and later than has previously been published, implying a compressed duration of cone building and introducing a preceding 65 k.y. hiatus. Debris avalanche timing is refined here to be older than 10 but younger than 20 ka. Rapid postcollapse rebuilding of the volcanic edifice is delineated by 16 groundmass and whole-rock 40Ar/39Ar ages, which include some of the youngest lava flows dated by this method. Increase in cone-building rate and a continued trend toward more mafic compositions following collapse imply an inter-relationship between the presence of the edifice and flux of magma from the feeding reservoir. Cone-building rates at Parinacota are similar to those at other well-dated volcanoes on thinner crust; however, the distributed basaltic volcanism prevalent in those other arcs is virtually absent both at Parinacota and elsewhere in the Central Volcanic Zone. This suggests that while the hydrous, calc-alkaline magmas that make up the central volcanoes are not significantly retarded by thick crust, primitive, dry basalts might be. [ABSTRACT FROM AUTHOR]