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The rear-arc Laguna del Maule volcanic field (LdM) in the Andean Southern Volcanic Zone, 36°S, is among the most active latest Pleistocene-Holocene rhyolitic centers globally and has been inflating at a rate of >20 cm a-1 since 2007. At least 50 eruptions during the last 26 kyr allow for a thorough interrogation of changes in the physical and chemical state of this large, ~20 km diameter, silicic system. Trace element concentrations and Sr, Pb and Th isotope ratios indicate that the mafic precursors to the LdM rhyolites result from mixing between partial melts of garnet-bearing mantle and crust in Th-excess and partial melts of garnet-free crust in U-excess. The 238U/230Th ratios of the LdM lavas are decoupled from the slab fluid signature, similar to several recently studied frontal arc volcanic centers in the Southern Volcanic Zone. A narrow range of radiogenic isotope compositions and increasing isotopic homogeneity with differentiation indicate that silicic magma is generated by magma hybridization and crystallization in the upper crust with limited involvement of older, radiogenic material. New 40Ar/39Ar and 36Cl ages reveal a wide footprint of silicic volcanism during the early post-glacial (25-19 ka) and Holocene (c. 8-2 ka) periods, but focused within a single eruptive center during the interim period. Subtle temporal variations in trace element compositions and two-oxide temperatures indicate that these eruptions, issued from vents distributed within a similar area, tapped at least two physically discrete rhyolite reservoirs. This compositional distinction favors punctuated extraction and ephemeral storage of the erupted magma batches. Frequent mafic recharge incubates this long-lived, growing shallow silicic magma reservoir above the granite eutectic, which favors magma interactions over rejuvenation of near- to sub-solidus silicic cumulates. A long-term rate of mass addition--extrapolated from surface deformation accumulated over the past decade--is comparable with those that have produced moderate- to large-volume caldera-forming eruptions elsewhere. [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]

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: 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]

Calbuco (41.3°S) is a highly active composite frontal arc volcano in the Andean Central Southern Volcanic Zone (CSVZ). It is currently ranked by Chile's Geological Survey (SERNAGEOMIN) as among the most dangerous volcanic centers in the Southern Andes, with sub-Plinian explosive activity occurring as recently as 2015. Compositional and geochronologic data from 65 samples constrain the Pleistocene-Holocene eruptive evolution of Calbuco. New 40Ar/39Ar ages from lava flows, revised mapping, and existing 14C dates for Holocene tephras reveal that much of the current edifice is <20 ka in age. Thus, Calbuco has produced a greater Holocene eruptive flux, 3.4 km3/ka averaged over the last 10 ka and 4.8 km3/ka over the last 4 ka, than other arc volcanoes both in the CSVZ and globally. Morphologic and stratigraphic observations along with new whole-rock geochemical data suggest that this remarkable magma flux is expressed as repeated eruptions of lavas and tephra with a narrow SiO 2 range (54–58 wt%). This compositional monotony, conspicuous resorbed amphibole, and elevated fluid-mobile trace element abundances differ from the post-glacial compositions and mineralogy observed at other CSVZ arc front volcanoes. Mid-crustal processing at or just below the depth where amphibole is stable plays a key role in the genesis of andesite at Calbuco. The precision of 40Ar/39Ar dates from Latest Pleistocene to Holocene low K 2 O mafic lavas remains a challenge to constructing eruptive chronologies for comparison to records of glacier expansion and retreat. Notwithstanding, our findings from Calbuco imply that petrologic factors, including mantle melt flux, depth of magma storage, and melt H 2 O content, may promote distinctive eruptive responses at adjacent CSVZ volcanoes following rapid retreat of the Patagonian Ice Sheet at 18 ka. • New 40Ar/39Ar ages from lava flows reveal that Calbuco Volcano grew at a fast rate (3.4 km3/ka) during the Holocene. • The precision of 40Ar/39Ar dates for young, low K 2 O mafic lavas remains a challenge to constructing eruptive chronologies. • Factors like magma storage depth may promote unique eruptive responses at adjacent volcanoes following fast deglaciation. [ABSTRACT FROM AUTHOR]