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The article focuses on gradual accumulation of rhyolitic magma in the upper crust beneath Laguna del Maule, Chile. It states that the rate of growth of a rhyolite-producing magma reservoir is determined through the measurement of a deformed paleoshore surface, isotopic dating of volcanism and surface exposure. It refers to the consistency of the long-term rate of magma input with reservoir freezing and pluton formation.

We have applied the unspiked K-Ar and the 40 Ar/ 39 Ar methods to samples precisely collected and localised, on both Central Indian Ridge flanks, to test their effectiveness and reliability when applied to the dating of recent (i.e. less than 1 Ma) MORBs. Twenty six samples) from the sixty five samples collected every ∼500 m up to the Brunhes-Matuyama boundary on both ridge flanks, were selected based on their distance from the ridge axis. Therefore, we can evaluate whether the isotopic ages are a good indicator of the crystallisation age by considering their geographic position with respect to the ridge axis (zero age) and the B/M magnetic boundary. Direct comparison of the isotopic and model ages shows that only 9 out of 26 samples were successfully dated. The GIMNAUT – MORB's test case amply demonstrates that the unspiked K-Ar technique, when applied to submerged volcanic samples, is subject to potentially defective assumptions of trapped atmospheric argon, excess/fractionated argon and extremely sensitive to alteration. Although the unspiked K-Ar technique is theoretically capable to produce high precision ages, the comparison with the 40 Ar/ 39 Ar techniques reveals that only 15% (i.e. 4 samples out of 26) of the ages obtained here are geologically meaningful. Five of the seven 40 Ar/ 39 Ar incremental heating experiments provide meaningful ages. Because potential sources of systematic errors such as excess 40 Ar*, recoil of 39 Ar K and 37 Ar Ca can be identified and because effects of alteration are significantly reduced by the pre-heating of the samples up to 500–600°c, the 40 Ar/ 39 Ar incremental heating method appears to be the method of choice to date MORBs. [ABSTRACT FROM AUTHOR]

Accurate and precise dating of Quaternary lavas and pyroclastic flow or fall deposits is essential for understanding the evolution of active volcanoes and providing context for future eruptions and hazard assessment. The 40Ar/39Ar method is commonly employed to date these volcanic materials, however, dating young (<150 ka) K 2 O-poor materials can be challenging owing to low radiogenic 40Ar* contents that can be difficult to distinguish from trapped atmospheric argon. To address this challenge, a collaborative intercalibration exercise involving the University of Wisconsin-Madison WiscAr Laboratory and the 40Ar/39Ar Laboratory of the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile was conducted on a common set of samples with the aim of refining our methods and optimizing precision and accuracy of age determinations. Groundmass and plagioclase samples were analyzed on a 5-collector Noblesse ion counting mass spectrometer in the WiscAr lab, whereas measurements in the SERNAGEOMIN lab were performed using an ARGUS VI spectrometer equipped with faraday detectors and one compact discrete dynode electron multiplier. Samples for the intercalibration were collected jointly from three Andean Southern Volcanic Zone volcanoes to evaluate the capability of each laboratory to date different materials. Samples from lava flows with 1.0–3.2 wt % K 2 O from Planchon-Peteroa volcanic complex and with <1.0 wt % K 2 O from Calbuco Volcano that are the focus of ongoing geological studies were measured in both laboratories. Single crystals of plagioclase (0.6–1.0 wt% K 2 O) were measured from the voluminous Diamante (Pudahuel) ignimbrite sourced from the Diamante Caldera. Multiple rounds of experiments were conducted including co-irradiation of samples at Oregon State University, as well as irradiations using the CCHEN reactor in Chile to investigate differences in neutron fluence parameters. As a result, SERNAGEOMIN has modified long-used protocols for the CCHEN reactor so that Quaternary samples may be irradiated for periods of time most appropriate for their age. Although less precise than plateau ages, the isochron ages generated in the two laboratories agree at 2σ for each sample. Six of six co-irradiated samples from Planchon-Peteroa yield plateau ages that also show inter-lab agreement at 2σ. The low K 2 O lavas from Calbuco proved more challenging with only three out of five plateau ages in agreement between labs. SERNAGEOMIN blanks were higher and more variable in Calbuco experiments, thus, differences in the variability of the measured 36Ar blanks between the two laboratories may explain the discrepancy in plateau ages. Analysis of single plagioclase crystals from the Diamante Ignimbrite show excellent agreement between labs for both weighted mean apparent ages and isochron ages. We favor an isochron age for the ignimbrite of 132.4 ± 2.2 ka, however, discrepancies in results between samples from three different outcrops present an interesting geochronologic problem that warrants further study. Overall, the consistency of the results between labs is promising. These new precise age determinations significantly improve our understanding of the temporal evolution of these active volcanoes. • Broad agreement of ages from tephras and lavas measured in two labs. • 40Ar/39Ar dating of low K 2 O lavas from Calbuco is challenging due to limited 40Ar*. • We favor an isochron age of ∼132 ± 2 ka ka for the 270 km3 Diamante Ignimbrite. • Care in sample preparation and irradiation, and control of blanks are essential. [ABSTRACT FROM AUTHOR]

40Ar/39Ar geochronology of basaltic to rhyolitic lavas, domes, and pyroclastic deposits from Ascension Island indicates that the maximum age of subaerially exposed samples is 1094 ka. Thirty-eight 40Ar/39Ar ages, coupled with new and existing geochemical data, constrain the eruptive histories of the four distinct mafic magma types (high Zr/Nb, low Zr/Nb, intermediate Zr/Nb, and Dark Slope Crater) and document temporal variations in magma sources. Lavas from the eastern felsic complex, previously assumed to be as old as or slightly younger than the 602–1094 ka Middleton Ridge complex, are as young as 52 ka. Basaltic to benmoreitic scoria cones and associated flows of the intermediate Zr/Nb magma type have been inferred to be the most recent eruptive products, yet their eruptive histories extend back to 705 ka. These intermediate Zr/Nb magmas are likely to be parental to the abundant trachytic to rhyolitic lavas and domes, which contradicts previous interpretations that call upon a high Zr/Nb parental basalt. Two distinct fractionation trends are observed in the trace element variations of Ascension trachytes and rhyolites. 40Ar/39Ar ages of the samples defining the two trends suggest that ilmenite fractionation dominated the Nb budget and thus controlled Zr/Nb ratios in early (>931 ka) Ascension evolved magmas, whereas zircon–titanite fractionation was predominant in younger felsic magmas. The eruptive sequence and compositions of the subaerial lavas and domes at Ascension Island are unique in comparison with other ocean island volcanoes because of its on-axis location and eruptions of high-SiO2 trachyte and rhyolite. [ABSTRACT FROM PUBLISHER]

Abstract: Eighteen lavas erupted from cinder cones behind the volcanic front in southeastern Guatemala and western El Salvador have been dated using the 40Ar/39Ar method on groundmass separates. Plateau ages range between 1069.5±22.1and 37±12.0ka. The majority of dated lavas are younger than 600ka. Importantly, cinder cone lavas become progressively younger toward the volcanic front/trench. In addition, the lavas become systematically depleted in Cs, Ba, rare earth elements, Cs/Zr, La/Zr, and δ 18O with time. The baseline values of these same geochemical parameters increase with distance from the volcanic front. The cinder cone lavas are closely associated with rifting south of the North American–Caribbean plate boundary. The trenchward migration of behind-the-front volcanism may be linked to a westward migration of this rifting. Existing evidence suggests there was an important change in the face of volcanism behind the volcanic front in southeastern Guatemala: from a small number of large polygenetic volcanoes to abundant smaller monogenetic cones. The temporal/spatial changes in the geochemistry of cinder cone lavas is attributed to decreasing contamination by Paleozoic basement rocks, both with time and with diminishing distances to the volcanic front. [Copyright &y& Elsevier]

Forty-three 40Ar/39Ar age determinations of lava flows, domes, ignimbrites, and dikes, plus 14C dates from seven distal tephra layers, combined with stratigraphy, geochemistry, and Sr and Th isotope data, establish an eruptive chronology for the Puyehue-Cordón Caulle volcanic complex at 40.5° S in the Andean southern volcanic zone (SVZ). The complex preserves ~131 km³ of lava and tephra that erupted from numerous vents widely separated in time and space. Approximately 80% of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka. The modern Puyehue stratovolcano was built on the southerly shield during the past 69 k.y. following a hiatus of 25 k.y. Puyehue has erupted ~15 km³ of basaltic to rhyolitic magma that spans the entire compositional range found in the southern SVZ and evolved via at least six phases including: (1) basaltic andesitic to dacitic lavas between 69 and 32 ka, (2) a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of MgO-rich basaltic andesite at 34 ka, (3) dacitic to rhyolitic flows and domes from 19 to 12 ka, (4) basaltic to basaltic andesitic flows between 15 and 12 ka, (5) subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka, followed by (6) a powerful series of phreatomagmatic and sub-Plinian eruptions at ca. 1.1 ka that obliterated the preceding rhyolite domes and formed the present 2.5-km-diameter, 280-m-deep summit crater. Along the Cordón Caulle fissure zone, ~5 km³ of rhyodacitic to rhyolitic lavas, domes, and cones have formed during the past ~16.5 k.y., including explosive and effusive eruptions in 1921-1922 and 1960. Eruptive rates were nonuniform over time, with background growth at 0.04 km³/k.y. or less, punctuated by spurts at up to 0.90 km³/ k.y. The time-averaged rate, 0.42 km³/k.y., is nearly double that at the Tatara-San Pedro complex 500 km to the north during the past 300 k.y. These findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable. The last three stratocone-building events on Puyehue began during periods of deglaciation, suggesting a relationship between unloading of ice and ease of magma ascent. Puyehue basalt exhibits subtle changes in 238U-230Th, 87Sr/86Sr, and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge, or the extent to which basalt was modified by assimilation of heterogeneous crustal melts. The complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt. Despite the high flux of basalt during the past 300 k.y., no large silicic magma reservoir formed in the upper crust. Instead, 238U-230Th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust, promoted perhaps by conduits that reflect strike-slip faulting beneath the complex. [ABSTRACT FROM AUTHOR]

Abstract: Eight basaltic lavas from the Auckland volcanic field, New Zealand, record three distinct sets of excursional geomagnetic field directions and low paleointensities, however the timing and therefore paleomagnetic significance of these records have been poorly understood. Radiocarbon, K–Ar, and thermoluminescence dating constrain these lavas to have erupted during the last 75 ka, a period during which as many as three excursions have been recorded differentially at several northern and fewer southern hemisphere sites. Forty 40Ar/39Ar incremental heating experiments conducted on groundmass from seven of these excursional lavas indicate that they erupted during at least two periods, at 39.1±4.1 and 31.6±1.8 ka, coincident with 40Ar/39Ar, astrochronologic, and 14C ages determined for the Laschamp and Mono Lake excursions, respectively. Samples from a lava flow associated with a third cluster of virtual geomagnetic poles (VGPs) are complicated by low concentrations of radiogenic argon in the presence of excess argon, and thus yield discordant age spectra and an imprecise age of 26.6±8.1 ka. Our findings indicate that the Laschamp and Mono Lake excursions, until recently identified unequivocally and isotopically dated only in the Northern Hemisphere, were globally synchronized at 40±1 and 32±1 ka. However, the VGPs of lavas that record the Laschamp excursion in New Zealand and France are inconsistent with a simple clockwise looping geometry inferred from VGP paths obtained in eight marine sediment cores spanning the Laschamp excursion. We suggest that the differences in VGPs recorded at the various sites are significant and may point to non-axial dipole components and lower mantle control on transitional fields during short-lived excursions. [Copyright &y& Elsevier]

Abstract: Basaltic to rhyolitic lavas and tephras erupted over the last 70 kyr at the Puyehue–Cordón Caulle volcanic complex in the Andean Southern Volcanic Zone (SVZ) were analyzed for major and trace element, Sr isotope, and U–Th isotope compositions to constrain the timescales of magmatic processes and identify the subducted and crustal components involved in magma genesis. Internal U–Th mineral isochrons from five lavas and three tephra fall deposits are indistinguishable from their eruption ages, indicating a short period (<1000 yr) of crystal residence in the magma prior to eruption. The (230Th/232Th) ratios define a narrow range (0.80–0.83) compared to that of all SVZ lavas (0.72–0.97), suggesting that Puyehue basalt was derived from a relatively uniform mantle source. Dacites and rhyolites have the largest U excesses and likely evolved via fractional crystallization of a plagioclase-dominated mineral assemblage. In contrast, basalts have 1 to 6% 230Th excesses, a characteristic not previously observed in frontal arc stratovolcanoes of the Andean SVZ. The 230Th excesses are interpreted to reflect a relatively small degree of fluid flux melting coupled with assimilation and melting of the lower crust. Lower crustal processes, therefore, have dampened the 238U excesses that were generated during fluid addition to the mantle wedge. Although prior 238U–230Th–226Ra studies of lavas from other southern SVZ stratovolcanoes (36 to 41° S) have inferred that slab additions and the extent of mantle melting were nearly constant along strike of the arc, our results suggest that MASH processes envisioned by Hildreth and Moorbath [W. Hildreth, S. Moorbath, Crustal contributions to arc magmatism in the Andes of central Chile, Contrib. Mineral. Petrol. 98 (1988) 455-489] in the northern SVZ also occur in the southern SVZ, where the crust is relatively thin. [Copyright &y& Elsevier]

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]

New 40Ar/39Ar dating coupled with detailed field mapping, stratigraphy, and chemical analyses have established an eruptive chronology that reveals and constrains the compositional and volumetric evolution of Seguam Island in the Aleutian Island arc, Alaska. Sixty new 40Ar/39Ar ages from lavas, domes, and pyroclastic deposits were obtained using furnace incremental-heating techniques on replicate samples of whole-rock and ground-mass separates, and they constrain the duration of Pleistocene to Holocene subaerial volcanism to 318 k.y. The 40Ar/39Ar plateau ages indicate that over 85% of the complex, 68 km³ of material, was erupted almost continuously between 318 ka and 9 ka. At ca. 9 ka, a stratocone on the eastern half of the island partially collapsed producing a 4-km-wide crater. Rhyolitic dome-forming eruptions followed from vents in the newly created crater, and were likely contemporaneous with 8.0 km³ of basaltic and basaltic andesitic effusions from Pyre Peak, and a 1.4 km³ basaltic eruption from a monogenetic cone on the far eastern end of the island. Geochemical changes over the last 318 k.y. are subtle. Most notably, the earliest eruptions from 318 to 142 ka produced no andesite, and basalt from this period has larger ranges in Zr/Rb and La/Yb than younger basalts. Small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal-liquid fractionation process that varied only slightly in successive eruptive phases over 318 k.y. We identified minor geochemical changes in magma composition during each of the three main stages of volcanism, but overall the monotonic variations in major- and trace-element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed-system fractional crystallization of basalt. Using the 40Ar/39Ar geochronology and estimates of individual flow volumes, we calculated a time-averaged eruptive rate at Seguam that is similar to growth rates of other well-dated arc volcanoes in the Cascades and Chilean Southern volcanic zone but less than that of Mount Katmai and Mount Mageik, which are located on the Alaska Peninsula. The eruptive flux at Seguam has been highly variable, fluctuating more than an order of magnitude, from 0.07 km³/k.y. during the early history of bimodal volcanism to 1.18 km³/k.y. over the past 9 k.y. [ABSTRACT FROM AUTHOR]

Radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the Aleutian Island Arc provides insight into the timing of arc formation in the middle Eocene. Twenty-eight 40Ar/39Ar ages constrain the duration of arc magmatism to the last 46 m.y. Basaltic lavas from the Finger Bay volcanics, the oldest exposed rocks in the arc, gave an isochron age of 37.4 ± 0.6 Ma, which is 12–17 m.y. younger than a widely cited age of 55–50 Ma. Three main pulses of arc-wide magmatism occurred at 38–29, 16–11, and 6–0 Ma, which coincide with periods of intense magmatism in other western Pacific island arcs. Using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m.y., we calculate a time-averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude. [ABSTRACT FROM AUTHOR]

Abstract: Primitive basaltic single eruptions in the Big Pine Volcanic Field (BPVF) of Owens Valley, California show systematic temporal–compositional variation that cannot be described by simple models of fractional crystallization, partial melting of a single source, or crustal contamination. We targeted five monogenetic eruption sequences in the BPVF for detailed chemical and isotopic measurements and 40Ar/39Ar dating, focusing primarily on the Papoose Canyon sequence. The vent of the primitive (Mg#=69) Papoose Canyon sequence (760.8±22.8 ka) produced magmas with systematically decreasing (up to a factor of two) incompatible element concentrations, at roughly constant MgO (9.8±0.3 (1σ) wt.%) and Na2O. SiO2 and compatible elements (Cr and Ni) show systematic increases, while 87Sr/86Sr systematically decreases (0.7063–0.7055) and εNd increases (−3.4 to −1.1). 187Os/188Os is highly radiogenic (0.20–0.31), but variations among four samples do not correlate with other chemical or isotopic indices, are not systematic with respect to eruption order, and thus the Os system appears to be decoupled from the dominant trends. The single eruption trends likely result from coupled melting and mixing of two isotopically distinct sources, either through melt-rock interaction or melting of a lithologically heterogeneous source. The other four sequences, Jalopy Cone (469.4±9.2 ka), Quarry Cone (90.5±17.6 ka), Volcanic Bomb Cone (61.6±23.4 ka), and Goodale Bee Cone (31.8±12.1 ka) show similar systematic temporal decreases in incompatible elements. Monogenetic volcanic fields are often used to decipher tectonic changes on the order of 105–106 yr through long-term changes in lava chemistry. However, the systematic variation found in Papoose Canyon (100–102 yr) nearly spans that of the entire volcanic field, and straddles cutoffs for models of changing tectonic regime over much longer time-scales. Moreover, ten new 40Ar/39Ar ages combined with chemistry from all BPVF single eruption sequences show the long-term trend of BPVF evolution comprises the overlapping, temporal–compositional trends of the monogenetic vents. This suggests that the single eruption sequences contain the bulk of the systematic chemical variation, whereas their aggregate compositions define the long-term trend of volcanic field evolution. [Copyright &y& Elsevier]

Abstract: New paleomagnetic measurements, coupled with Argon–Argon (40Ar/39Ar) radioisotopic dating, are revolutionizing our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the paleomagnetic field. As part of an investigation of the Koolau Volcano, Oahu, and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the volcano''s buttressed flank within Halawa Valley. Prior paleomagnetic and Potassium–Argon (K–Ar) investigations of the Koolau Volcanic Series revealed excursional directions The alkaline composition of lava flows, easy access, and close geographical proximity to K–Ar dated lava flows made this newly studied 120-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 28 successive flow sites, were stepwise demagnetized by both alternating field (5 mT to 100 mT) and thermal (from 28 °C to 575–650 °C) methods. Mean directions were obtained by principal component analysis. All samples yielded a strong and stable ChRM trending towards the origin of vector demagnetization diagrams based on seven or more demagnetization steps, with thermal and AF results differing insignificantly. Low-field susceptibility vs. temperature (k–T) analysis conducted on individual lava flows indicated approximately half with reversible curves. Curie point determinations from these analyses revealed a temperature close to or equal to 580 °C, indicative of almost pure magnetite ranging from single domain (SD) to pseudosingle domain (PSD) grain sizes for most of the flows. The mean directions of magnetization of the entire section sampled indicate a reversed polarity, with ∼10 m of the section characterized by excursional directions (5 lava flows). The corresponding VGPs are located off the southeastern part of Africa, close to Madagascar. 40Ar/39Ar incremental heating experiments on groundmass from nine flow sites at different stratigraphic levels yields isochrons between 2.64±0.23 to 2.37±0.17 Ma with a weighted mean age of 2.514±0.039 Ma, which, combined with the overall reversed polarity and the absence of polarity reversals, strongly suggests that the excursion corresponds to Cryptochron C2r.2r-1 [S. Candle and D.V. Kent, A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, Jour. Geophys Res., 100(1995), 6093–6095.]. Current dating is insufficiently precise to unambiguously define this excursion as a Microchron (<10 kyr). This is potentially the first terrestrial record of Cryptochron C2r.2r-1 and the age is 2–3% older than in the reported timescale [S. Candle and D.V. Kent, A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, Jour. Geophys Res., 100(1995), 6093–6095.]. Our finding places important constraints on the evolution of the entire Koolau shield edifice, since most of the lavas recorded a reversed polarity. [Copyright &y& Elsevier]