The Hielo Patagónico (Patagonia Icefield), South America, is the largest temperate ice body in the Southern Hemisphere with more than 70 major outlet glaciers and a total area of ca. 17,200 km². For the Hielo Patagónico, two major schemes for the Holocene glaciations (Neoglaciations) have been proposed. Mercer first proposed three Neoglaciations, I at 4500-4000 years before the present (yr BP), II at 2700-2000 yr BP and III at the 17-19th centuries. Aniya later postulated a scheme of four Neoglaciations, I at ca. 3600 yr BP, II at 2400-2200 yr BP, III at 1600-900 yr BP, and IV at the 17-19th centuries, which is congruent with other regions of the Andes. The main differences are the age of Neoglaciation I and the existence of a Neoglaciation at 1600-900 yr BP. After examining the dates given by other studies, although scanty, I propose a new scheme of five Neoglaciations, I at 4500-4000 yr BP, II at 3600-3300 yr BP, III at 2700-2000 yr BP, IV at 1600-900 yr BP and V at 17-19th centuries, that is, the Little Ice Ace (LIA). In addition, there were two earlier Holocene glaciations, probably at 5700-5000 yr BP and 8100-6800 (or 7500) yr BP. Two older ages, 8800-8500 yr BP and 9700-9100 yr BP, are uncertain.

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To investigate the melting history of the Patagonian Ice Sheet (PIS) during the last deglaciation, we reconstructed Al₂O₃ flux in sediments from the Magellanes fjord (53°S) and southern Patagonia (55°S: Drake Passage) over the past 12 and 31 kiloyears (kyr), respectively. We confirm that the melting history of the PIS is recorded as changes in sedimentological signals and find that the melting ceased between ~10 and 8 Calendar kyr before present (cal kyr BP). The timing of major melting events of the PIS was similar to that of the Northern Hemisphere ice sheets.

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We report on ²³⁰Th-normalized fluxes of biogenic components—total organic carbon (TOC), total nitrogen (TN), calcium carbonate (CaCO₃), and biogenic opal (Si_OPAL)—from two sediment cores collected at 36°S off central Chile and covering the past 22 thousand years (kyr) (site PC-1), and at 52°S near the Pacific entrance of the Strait of Magellan and encompassing the past 13 kyr (site PC-3). During 13-8 calendar kyr before present (cal kyr BP), the ²³⁰Th-normalized TOC flux at the PC-1 site was relatively high, pointing to increased productivity, whereas a marked decrease in the flux characterized the periods around 22-14 and 8-5 cal kyr BP. In contrast, at the PC-3 site, the ²³⁰Th-normalized TOC flux was low during the last deglaciation until ~6 cal kyr BP, and then abruptly increased in the late Holocene. The ²³⁰Th-normalized fluxes suggest that, compared to other periods, the biological pump functioned less effectively during 22-14 cal kyr BP and the middle Holocene off central Chile, and during 13-6 cal kyr BP off southernmost Patagonia. The changes from glacial to interglacial in the ²³⁰Th-normalized biogenic components, which were controlled by changes in upwelling intensity at the PC-1 site and by the inflow of nutrients from the Pacific at the PC-3 site, can be explained by changes in wind direction and intensity associated with the latitudinal displacement of the southern westerly belt. In contrast, we found no obvious relationship during the deglacial and Holocene periods between El Niño activity and biogenic component fluxes at the PC-1 site, even though at present El Niño events are negatively correlated with primary productivity in the upwelling area off Chile.

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The Chile Triple Junction (CTJ), an RTT-type triple junction located at 46°13′ S, 75°48′ W off the western coast of Chile, is characterized by the subducting Chile Ridge, which is the constructive plate boundary that generates both the Nazca Plate and the Antarctic Plate. The ridge subduction mechanism and the regional tectonics around the CTJ were investigated primarily using marine geophysical data (topography, gravity, geomagnetic field and single-channel seismics) collected during the SORA2009 cruise (Cruise ID = MR08-06) by R/V MIRAI together with other cruise data from the National Geophysical Data Center. The segment of the ridge axis just before the subduction around the CTJ is associated with an axial deep covered with thick sediment unlike that seen in typical ridge crests. The profiles of both topography and the free air anomaly around the CTJ show quite different patterns from those of ordinary subduction zones. However, topographic features typical of a slow-spreading type ridge, including a median valley and both flanks, remain in the seaward side of the trench. Even after the subduction of the eastern flank, the topographic features of the western flank remain. A slight Outer Rise and an Outer Gravity High, which are common in the western Pacific area, were observed in an area far away from the CTJ on both Nazca and Antarctic plate sides. The geomagnetic anomaly pattern around the Chile Ridge near the CTJ shows that the estimated spreading rate decreases gradually towards the ridge crest. This suggests that volcanic activity diminishes gradually towards the subducting ridge axis. The lithosphere under the Chile Ridge might have amalgamated with the surrounding oceanic lithosphere due to heat loss after the cessation of volcanic activity. The oceanic lithosphere towards the trench also thickens rapidly due to heat loss. Consequently, shallow-angle subduction of the youngest and most immature oceanic plate occurs smoothly via slab-pull force without any resistance along the interface between the South American continental plates.

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To understand the processes of melt eduction in a ridge subduction zone, we performed U-Pb dating on zircons separated from igneous and sedimentary rocks that were newly dredged from the Chile Triple Junction area and from volcanic rocks collected from the Taitao peninsula, southern Chile. The youngest fraction of the U-Pb age population was used to estimate the age of magmatism or sedimentation. Our new results indicate that the fore-arc region became volcanically active over a period of ~0.4 m.y., after obduction of the Taitao ophiolite (~5.7 to 5.2 Ma) from the west and after granite intrusions related to ridge subduction at ~6 Ma. Fore-arc volcanism produced ejecta of basaltic to dacitic compositions and migrated from offshore (~5.3 Ma) to inland (~4.6 Ma) along the Chile Margin Unit that trends northeast-southwest. The volcanism further extended east to produce the dacitic volcanic plug of Pan de Azucar (~4.3 Ma) and lavas in Fjord San Pedro (~2.9 Ma). The migration took place at a rate of ~2.3 cm/y to ~5.3 cm/y. Another intrusion of a granite pluton, widely distributed offshore of the Taitao ophiolite, took place at ~4.0 Ma. Distributions of old detrital zircon and zircon xenocryst ages were used to evaluate, respectively, the influence of subducted sediments and igneous crustal material. Our results indicate that crustal material influenced only Pan de Azucar and Fjord San Pedro dacites; other acidic magmatism shows moderate evidence for incorporation of subducted sediments. Therefore, melts were formed at shallow depths near the triple junction, due mainly to partial melting of the subducted slab and sediments, and then ejected instantaneously. Depleted oceanic materials became anhydrous, and a volcanic gap was formed along the Andean arc.

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Geochemical compositions are reported for Late Miocene to Pliocene granitoids from the Taitao Peninsula near the Chile ridge subduction zone. Major element compositions of Taitao granitoids show a resemblance with those of TTG suites. However, trace element compositions are characterized by low Sr (50-300 ppm), moderately high Y (10-45 ppm) and Yb concentrations (1-5 ppm), and low Sr/Y and La/Yb ratios compared with those of typical adakites, which are presumably produced by melting of young and hot oceanic crust under eclogite to garnet amphibolite conditions. Instead, trace element composition of the Taitao granitoids resembles that of typical calc-alkaline arc magmas. Based on trace element compositions, together with tectonic constraints, we infer that the Taitao granitoids were generated by partial melting of the subducted oceanic crust in garnet-free amphibolite conditions at depths shallower than 30 km. Our results indicate that slab-melting-related magmas do not necessarily show a HREE-depleted signature, which was used as evidence for slab-melting for granitic rocks of the TTG suites.

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We present a dataset for boron and other trace element contents obtained from samples from 13 volcanoes distributed along the Quaternary volcanic front of the Southern Volcanic Zone (SVZ) of the Chilean Andes. The dataset shows constraints on the nature of slab-derived component to mantle source. Analyzed samples show large negative Nb and Ta anomalies, and enrichment of alkaline earth elements and Pb, which are features of typical island arc volcanic rocks. Boron contents of SVZ volcanic rocks range 2.3-125.5 ppm, exhibiting marked enrichment relative to N-MORB and OIB. Both the boron contents and B/Nb ratios of the volcanic rocks increase from the southern SVZ (SSVZ) to central SVZ (CSVZ). Fluid mobile/immobile element ratios (B/Nb, Ba/Nb, Pb/Nb, and K/Nb) are used to examine slab-derived component to mantle source. Trace element compositions of altered oceanic crust (AOC)-derived fluid, sediment-derived fluid, and sediment melt are modeled. Mantle sources of volcanic rocks in CSVZ with high B/Nb ratios were contaminated by both AOC-derived and sediment-derived fluids. In contrast, mantle sources of volcanic rocks in SSVZ with a low B/Nb ratio were contaminated with ca. 3 wt% melt of subducted sediment, which had suffered from loss of boron during progressive devolatilization before melting.

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The ophiolitic remnants of the Upper Mesozoic Rocas Verdes basin in southernmost South America were studied from the perspectives of petrography, chemistry of minerals, bulk-rock geochemistry, and U-Pb geochronology. The study aimed to unravel the tectonic, magmatic, and metamorphic evolution of a suprasubduction rift zone that underwent a transition to a back-arc basin. The rifting phase and bimodal magmatism within the Rocas Verdes basin started prior to or during the Late Jurassic, as indicated by a gabbro in contact with pillow basalts that dated at 154 Ma. In the Late Jurassic Capitán Aracena and Carlos III complexes, tholeiitic basalts are geochemically comparable to enriched mid-oceanic ridge basalts. Back-arc basin development continued for 35 myr until the Early Cretaceous, as suggested by the ages of detrital zircons in cherty layers within pillow basalts and metamorphic titanite that crystallized during seafloor metamorphism near the spreading/magmatic axis. In the Early Cretaceous Tortuga Complex, tholeiitic basalts are comparable to normal mid-oceanic ridge basalts. Non-deformative metamorphism converted the primary mineralogy of the ophiolites to low- to intermediate-grade metamorphic assemblages formed during ocean-floor type alteration in a suprasubduction setting. Fossilized bacteria, preserved as rounded aggregates of titanite microcrystals, were identified in the pillow basalts up to the Early Cretaceous. The Rocas Verdes basin closed during the Andean orogeny, which started during the Late Cretaceous, and ophiolites were tectonically juxtaposed and thrust over the sedimentary infill of the quasi-oceanic basin in which they developed. The tectonic emplacement of the ophiolitic complexes was complete before the latest Cretaceous, as indicated by crystallization ages of granites intruded into the ophiolitic complexes.

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This paper presents the results of new petrochemical studies carried out on mantle xenoliths hosted in Pleistocene basaltic rocks from the Agua Poca volcano in central-western Argentina. Mantle xenoliths studied are shown to be mainly anhydrous spinel lherzolites with minor amounts of harzburgite and banded pyroxenite, showing highly variable equilibrium temperatures ranging from 820°C to 1030°C at 1.0 to 2.0 GPa. This constitutes evidence that the mantle xenoliths are representative of a large portion of the lithospheric mantle column and that the geothermal gradient is not very elevated as reported in some other Patagonian provinces. Geochemical characteristics of clinopyroxene in the mantle xenoliths allow classification into two groups; Groups 1 and 2. Group 1 contains most of the lherzolites and has light-REE depletion, with slightly positive anomalies of Eu in some samples and extreme Nb and Ta depletion. Group 2 consists of two harzburgitic samples, has flat REE patterns with lower Sm to Lu concentrations, with enriched Sr and negative HFSE anomalies. Based on mineral and residua compositions estimated assuming equilibrium with clinopyroxenes, Group 1 can be considered to be refractory residua after up to 7%, non-modal, near-fractional melting of a spinel-facies Primitive Mantle. Group 2 can be considered to be after ca. 13% of partial melting. It is inferred that partial melting events in the lithospheric mantle beneath the Agua Poca occurred in different ages since the Proterozoic, but compared with Group 1, the metasomatic overprint is dominant in Group 2 mantle xenoliths. The calculated melt compositions from Group 2 are interpreted to be transient liquid compositions developed during melt-peridotite interaction, and are different from the host alkaline basalts. The HFSE-depleted composition estimated for the rising melt suggests the presence of a slab-derived component, although the possibility cannot be disregarded (on the basis of present data) that such a geochemical feature is due to segregation of HFSE-bearing minerals during the interaction with the peridotite. Thus, we attribute the metasomatic agent to a basaltic melt and to a minor amount of slab-derived fluids.

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This paper newly reports results of LA-ICPMS U-Pb dating for 282 zircon crystals separated from a Middle Miocene adakite in Cerro Pampa, southern Argentine Patagonia. With the exception of one spot age, 174 of the U-Pb concordia ages are markedly older (>94 Ma) than the cooling ages of the adakite magma (ca. 12 Ma). The presence of numerous exotic zircon crystals indicates that the adakitic magma carries up information related to the crustal components during its ascent through the entire crust underneath Cerro Pampa. The obtained concordia ages of exotic zircons, 94-1335 Ma, are divisible into five groups having distinctive peaks on a population diagram. The first (94-125 Ma) and second age groups (125-145 Ma) correspond to the age of plutonic activities that formed the main body of the South Patagonian Batholith. The third to fifth groups respectively correspond to activities of the El Qumado-Ibañez volcanic complex (145-170 Ma), plutonic rocks scarcely exposed in Central Patagonia (170-200 Ma), and the Eastern Andean metamorphic complex of Late Paleozoic to Early Mesozoic ages (200-380 Ma). Our data suggest that the crust underneath Cerro Pampa was formed mostly after 380 Ma, the majority forming during the Early Cretaceous to Middle Jurassic. The processes of crustal development ceased for ca. 80 m.y. until the activity of the Cerro Pampa adakite in ca. 12 Ma. In contrast to the existence of numerous Archaean-Palaeoproterozoic exotic zircons in Mesozoic plutonic rocks distributed in Andean Cordillera at around 46°S, no evidence was found for Archaean-Paleoproterozoic crust on the Cerro Pampa region at 48°S. This evidence suggests that two crusts must have aggregated along a boundary between 46°S and 48°S with the continental margin of Gondwana during Late Paleozoic times, as part of the amalgamation of Pangea.

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In 2009, petit-spot submarine volcanoes were discovered off the oceanward slope of the central Chile trench, offshore from Valparaiso, Chile, at around 33°S. Ar-Ar dating of mugearite and alkali-basalt from the volcanoes yields ages of 10.11 ± 0.22 Ma and 6.69 ± 0.88 Ma, respectively. Back-calculations of plate motion along the present absolute movement direction of the Nazca Plate, conducted using the Ar-Ar age data, indicate that eruption occurred above a zone of plate flexure. The back-calculation results suggest that the mugearite was erupted at a flexural arch prior to arrival at the active site of the Juan Fernández hotspot. In contrast, the alkali-basalt was erupted on a plate flexure at a site of interaction between a flexural moat and an outer-rise area, where the source material was probably influenced by the Juan Fernández hotspot. The geochemistry of the lavas supports this interpretation because the concentration ratios of various rare earth and other trace elements in the mugearite are different from those of the alkali-basalt and Juan Fernández hotspot lavas. Consequently, petit-spot melts could reflect the composition of source materials below the plates at sites of plate flexure.

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In this study, we review recent seismological and geochemical studies of the hotspots in the South Pacific superswell. Extensive studies on global-scale seismic tomography have revealed the presence of slow seismic-velocity anomalies in the lower mantle beneath the superswell region although the size and vertical extent of these anomalies are not well constrained. In the last decade, regional seismic networks deployed on islands and the seafloor on the superswell have enabled detailed imaging of the mantle structure. Results show that the low-velocity superplume extended from the core-mantle boundary to a depth of 1000 km, and the low-velocity narrow plumes extend from the top of the superplume toward the South Pacific hotspots. Geochemical studies have suggested that dehydrated subducted oceanic crust is involved in the formation of the HIMU lavas, but the trace element and isotopic composition of HIMU lavas cannot be explained by the inclusion of this crust. Recent studies have revealed that the oceanic crust should sink into the lowermost mantle and melt, which would metasomatize the ambient mantle to form the HIMU reservoir. We present a model incorporating a thermochemical superplume and secondary narrow plumes generated from the superplume; our model can explain seismic structure, geochemical heterogeneities of ocean island basalts in the superswell region, age progression of the South Pacific hotspots, and massive eruption and formation of large oceanic plateaus in the Cretaceous period.

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