Quaternary Lavas Research Articles

Two-million-year eruptive history of Laguna del Maule volcanic field

The Laguna del Maule (LdM) volcanic field, which surrounds the 54-km2 lake of that name, covers ∼500 km2 of mountainous glaciated terrain with Quaternary lavas and tuffs that extend 40 km westward from the Argentine frontier and 30 km north-south from the Río Campanario to Laguna Fea. Complementing recent investigations of postglacial volcanism and the ongoing geophysical unrest around the lake, we here review the longer eruptive history that spanned the entire Quaternary.The distributed rear-arc LdM volcanic field is contiguous with the Tatara-San Pedro stratovolcano complex on the volcanic front of the Quaternary Andean arc. The LdM field has had only a few large edifices, but we identified at least 140 separate vents, from which >350 km3 of products have erupted since 1.5 Ma. Eruptive products of 14 (early and middle Pleistocene) stratocones and shields, and of ∼125 monogenetic cones, domes, and lava flows, were mapped on foot, studied petrographically, and chemically analyzed. More than 80 40Ar/39Ar and K-Ar ages have been determined to calibrate the Pleistocene eruptive sequence. An extensive welded ignimbrite erupted at 1.5 Ma and was followed by another at ∼950 ka, producing a 12 x 8 km-wide caldera that underlies the north part of the lake basin and the ruggedly eroded highlands north of it. Outside the caldera, the southern two-thirds of the lake basin is a drainage network cut on Tertiary andesites and dacites.A ring of ∼29 postglacial rhyolite and rhyodacite coulees and domes plus associated pyroclastic deposits that erupted from >30 separate vents (and together cover ∼100 km2) encircles the lake. The large number of postglacial silicic vents around the lake basin, several comagmatic multi-vent compositional arrays, and scarcity of mafic enclaves in the rhyolites are features that suggest growth of a latest Pleistocene to Holocene magma reservoir beneath the LdM Basin. The Barrancas center on the divide southeast of the Basin has an additional 21 lavas from 15 vents and represents a second independent postglacial rhyolitic reservoir. About 21 postglacial mafic and intermediate eruptive units accompany the rhyolites around the margins of the LdM Basin. Ongoing work by Fierstein et al. (this volume) has brought the total number of postglacial vents recognized to more than 73 and has determined ∼70 radiocarbon dates that bracket the abundant tephra deposits, thus providing a 17,000-year-long calibration of the postglacial eruptive sequence.In addition to the many postglacial silicic units, glacially eroded silicic lavas yield ages of 3.7, 2.5, 2.4, 2.0, 1.6, and 1.35 Ma, and 924, 880, 712, 695, 680, 460, 335, 240, 203, 114, 97, 83, and 25 ka, providing evidence of a prolonged history of explosive silicic eruptions from vents scattered throughout the volcanic field. Production of widely distributed rhyolites throughout the long history of the volcanic field demonstrates intensive crustal processing as well as the enduring potential for explosive eruptions.For the Quaternary LdM volcanic field, chemical analyses define an array continuous from 49% to 77.6% SiO2, medium-K toward its mafic end (1.5% K2O @ 55% SiO2) but high-K at its silicic end (4.5% K2O @ 75% SiO2). Quaternary eruptive units include 5 basalts, ∼30 mafic andesites (52–57% SiO2), 33 andesites, 11 dacites (63–68% SiO2), 25 rhyodacites, ∼27 rhyolites (>72% SiO2), and 6 ignimbrites (andesitic to rhyolitic). None of the basalts is primitive, and most of the mafic rocks display petrographic and/or chemical evidence for diverse crustal contributions (Hildreth et al., 2010).

Deciphering mantle source heterogeneity in space and time in the back-arc of a contemporary subduction system: A regional study of the Sredinny Range, Kamchatka

We present here the first regional set of Sr-Nd-Pb isotopic compositions, major and trace element compositions and KAr ages for a representative suite of back-arc samples of the Sredinny Range (SR) of Kamchatka. Together with previously published analyses, this unique dataset allowed us to trace the source heterogeneity and invoke new mechanisms to explain the variably enriched geochemical signatures. Our results indicate that the Sr isotopic ratios and the LILE content of the studied rocks were mainly influenced by the subduction fluid. Neodymium isotopes, as well as the HFSE and REE distributions in the rocks, require a more complex explanation, depending on the geographical location of the studied samples and the age of their eruption. Melts representing Miocene rocks of the northern part of the SR and most of the rocks from the eastern and central flanks of the SR (from Miocene until present) were produced from a depleted MORB-like metasomatized mantle, and were then exposed to varying degrees of crustal assimilation and fractional crystallization. Higher HFSE contents and lower 143Nd/144Nd ratios in the western flank lavas, as well as high HFSE contents in the Quaternary lavas of the northern part of the SR and some of the lavas from the eastern flank of the southern part of the SR, require an alternative source for their enrichment. Delamination of the lithosphere explains the unusual, enriched signature of these rocks, whereas their variably enriched neodymium isotope ratios identify the various ages of the separation of the lithosphere from the mantle. The Nd isotopic composition of the rocks together with their HFSE content, therefore, serves as an unusual tracer for the enriched mantle domain, showing the presence of the older lithospheric blocks and indicating the timeframes for this source involvement in magma generation.

Miocene Volcanism of the Baikal Rift Across the Boundary of the Siberian Craton: Evidence for Lithospheric Mantle Melting

Abstract Continental rifting is usually viewed in terms of two contrasting models of active and passive extension. The origin of the Baikal Rift, adjacent to the southern part of the Siberian Craton, has been described by both models in the past. It is expected that basaltic magmatism in an active model scenario should be primarily sourced from a mantle plume or plume-fed asthenosphere, whereas melting of the lithospheric mantle is expected to be a predominant source for magmatism in the passive model. In this paper, we focus on the Miocene volcanic rocks sampled along two 60-km-long profiles that cross the boundary between the Neoproterozoic Tuva-Mongolian massif and the Archean-Paleoproterozoic Siberian Craton. Most of the samples studied are trachybasalts. In terms of trace element concentrations normalised to primitive mantle, the lavas mimic oceanic island basalt-like patterns with troughs at Rb, Th–U, Pb, and Y, and peaks at Ba, Nb, Ta, K, and Sr. Moreover, similar trace element patterns to the studied samples are also observed for Miocene and Quaternary lavas located in the southwestern of the Baikal Rift, and adjacent regions of non-rifted Mongolia. According to the ratio of CaO to MgO, and TiO2/Al2O3 to SiO2, the compositions of the studied lavas coincide with experimental melts derived from mafic lithologies. Trace element data of samples suggest that garnet was a residual phase during partial melting. The Sr-Nd isotopic characteristics of the studied lavas are 87Sr/86Sr 0.70427–0.70469 and 143Nd/144Nd 0.51267–0.51284. They are identical to the coeval Miocene lavas of neighbouring volcanic fields, but they differ from the Quaternary lavas that extend to lower 87Sr/86Sr (0.7038–0.7044) with near identical 143Nd/144Nd. Isotopes of Hf for studied samples show values εHf = 6.0–7.7, except for the two samples taken within the boundary between two lithospheric blocks with εHf 4.6 and 4.8. The δ18O of olivine from lava samples is everywhere higher than that of the asthenospheric mantle and ranges from 5.5 to 6.4‰. Variations of δ18O versus Mg#, 87Sr/86Sr and εHf in the studied samples do not correlate, but do unequivocally rule out crustal assimilation. The isotopic variations are consistent with recycling of mafic crustal lithologies at mantle depths. Lavas from the Tuva-Mongolian massif and the Siberian Craton differ in lead isotopes by lower values of 206Pb/204Pb (< 17.785) and higher values of Δ8/4Pb (61–75) for on-cratonic samples and the reverse relationship for off-cratonic lava (> 17.785 and 55–61), respectively. The equation for Δ8/4Pb = [208Pb/204Pb-(1.209*(206Pb/204Pb) +15.627)] *100 is from Hart (Nature, 309, 753–757, 1984). The correlation of lead isotopes with the mafic recycled component, the sharp change of lead isotopic values at the cratonic boundary and decoupling of lead isotope ratios from other isotopic ratios lead us to suggest that the values of 206Pb/204Pb and Δ8/4Pb are associated with an ancient accessory mineral phase such as sulphide confined within the lithospheric mantle. The predominant role of the lithospheric sources in the formation of the Miocene volcanic rocks indicate that the volcanism of the Baikal Rift was caused by a passive tectonic process, rather than active rifting.

Sediment recycling by continental subduction indicated by B-Hf-Pb-Nd isotopes from Miocene–Quaternary lavas in the northern margin of Tibet

Although it has been argued that sediment recycling plays an important role in the differentiation of the continental crust, boron (B) isotopic data does not support a direct input of subducted sediments into arc magmas. This raises questions about the viability of sediment recycling as a process in the differentiation of continental crust. Here, we report B isotopic data from Miocene–Quaternary lavas derived from different sources in the northern margin of Tibet. These lavas have high B contents and negative δ11B values close to those of continental sediments. Strongly peraluminous rhyolites have the highest B (93 to 1559 ppm) contents with negative δ11B (−9.7 to −17.9) values. Adakitic dacites and trachyandesites exhibit the lowest B (18 to 29 ppm) contents with markedly negative δ11B (−12.0 to −35.7) values whereas olivine leucitites have B (37.2 to 59.3 ppm) contents with negative δ11B (−8.3 to −15.6) values. These lavas also have enriched Hf-Pb-Nd isotopic compositions similar to those of sediments. This data, combined with numerical modelling and geophysical and tectonic data for Cenozoic continental subduction in the northern margin of Tibet, indicates that: (1) the strongly peraluminous rhyolites were generated by partial melting of mica-bearing continental sedimentary rocks subducted to the depth of mid-to-lower crust; (2) adakitic lavas were derived by partial melting of sediment-bearing thickened lower crust underwent dehydration and eclogites-facies metamorphism; and (3) olivine leucitites were generated by partial melting of enriched mantle metasomatized by sediment-bearing eclogite-facies crust-derived melts. Thus, at continental convergent margins, continental subduction is an important mechanism for sediment recycling and the evolution of continental crust.

Quaternary lava tubes in northern Hainan Island: Morphology and significance of comparative planetology

Quaternary lava tubes in northern Hainan Island: Morphology and significance of comparative planetology

Intercalibration of the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile and WiscAr 40Ar/39Ar laboratories for Quaternary dating

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 40 Ar/ 39 Ar 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 40 Ar* 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 40 Ar/ 39 Ar 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 36 Ar 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. • 40 Ar/ 39 Ar dating of low K 2 O lavas from Calbuco is challenging due to limited 40 Ar*. • We favor an isochron age of ∼132 ± 2 ka ka for the 270 km 3 Diamante Ignimbrite. • Care in sample preparation and irradiation, and control of blanks are essential.

First study of the bat fossil record of the mid-Atlantic volcanic islands

ABSTRACTBats are one of the most abundant and important mammals in ecosystems. However, their fossil record is scarce and fragile, making them difficult to find. Accordingly, there is no record of this group in the volcanic islands of the mid-Atlantic Ocean apart from the Canary Islands. This paper studies the first bat fossil record of the Canary Islands (Spain). The material studied is found within two Quaternary lava tubes, Cueva de los Verdes on Lanzarote and Cueva Roja on the island of El Hierro. The dental and humeral morphology and biometry are analysed and compared with current specimens. Among our results we highlight the first fossil data of two species endemic to the islands of the mid-Atlantic Ocean,Plecotus teneriffaeandPipistrellus maderensis, the former from the Canary Islands and the latter from the Azores, Madeira and the Canary Islands. We also confirm the presence ofPipistrellus kuhliiin the fossil record of the island of Lanzarote. No differences are observed between the dental morphology of the current and the fossil populations ofP. maderensisandPl. teneriffae. In the case ofP. kuhlii, the populations of the Canary Islands and the Iberian Peninsula show differences in the paraconule with respect to the populations from central Europe. Palaeoecological studies of these taxa suggest that these islands presented a similar habitat when the sites were formed to the present-day habitat.

Late Quaternary lahars and lava dams: Fluvial responses of the Upper Tana River (Kenya)

Geomorphological and sedimentary records near the confluences of the Tana River and major tributaries draining the eastern slopes of Mt. Kenya and the Nyambeni Range, indicate impacts of Late Quaternary volcanic activity in their fluvial records. The main reconstructed event was triggered by a 366.9 ka basalt flow (40Ar/39Ar dated) which flowed along Kazita River from the Nyambeni Range blocking both Kazita River and Tana River near Kibuka Grand Falls, causing a temporary lake. Consequently, Tana River and Kazita River started to build volcanoclastic Gilbert type deltas. The preserved pro-delta sediments rich in trachytic pumice fragments display a mineralogical and age match with known Ithanguni trachytic tuffs, indicating delta build up right after a contemporary Ithanguni eruption. This trachytic eruption caused the deposition of lahars and fluvial volcaniclastic sediments in all river records draining the Eastern side of Mt. Kenya. The multiple lahars seem to be triggered by eruptions under glacial conditions (basalt age indicates MIS 10). The lava dammed lake was only short lived (estimated to have lasted only a few years to decades) and breached before a complete lake infill could occur. The current Kibuka Grand Falls can be viewed as the delayed incisional response of this lava dam breach, indicating that after >366.9 ka, Tana River is still responding and adjusting to this short-lived disruptive phase. The current Kazita River has re-incised adjacent to a MIS 4 basalt flow down into the crystalline Basement System rocks. The MIS 10 pre-volcanic sedimentary record indicates that more sediments were in the fluvial system during glacial conditions than during the interglacial conditions. An implication of our reconstruction is that the Late Quaternary fluvial record of Tana River is of only limited use to reconstruct uplift rates because reconstructed Quaternary incision rates are reflecting both volcanic disruptions as climate change trends of aridification and decreasing glaciation extents.

Distribution of natural radionuclides in the soils and assessment of radiation hazards in the Khrami Late Variscan crystal massif (Georgia)

This study was performed to determine the distributions of radionuclides (238U, 232Th and 40K naturally occurring in Khrami Late Variscan crystal massif and soils overlapping its adjacent territory built by Neogene and Quaternary lava using a gamma-ray spectrometry with an HPGe detector. By identification of artificial radionuclide 137Cs concentration revealed the character of radioactive contamination of the area under investigation. Based on the obtained results, the radiological parameters (outdoor absorbed gamma dose rate, annual effective dose; radium equivalent activity) were estimated to assess radiation hazards caused by use of industrial materials. The difference was established between concentrations of radionuclides having emerged at the expense of Late Variscan crystal substrate and recently erupted lavas. The results were compared with similar studies conducted in different countries and with data and recommendations published by international organizations (UNSCEAR, ICRP).

Olivine-hosted melt inclusions in Pliocene–Quaternary lavas from the Qorveh–Bijar volcanic belt, western Iran: implications for source lithology and cooling history

ABSTRACT Silicate melt inclusions (SMIs) are small droplets of magma that become trapped in minerals during crystal growth. SMIs in olivine crystals can provide critical information on the range of melt compositions and processes that occur during melt generation, evolution, transport, and eruption. The Pliocene–Quaternary volcanic rocks in the Qorveh–Bijar volcanic belt of western Iran show porphyritic and microlithic textures, with olivine and clinopyroxene being the dominant minerals. Magnesian olivines in these volcanic rocks contain primary SMIs. The composition and characteristic of olivine-hosted SMI of these rocks are investigated to constrain the source lithology for mafic volcanism. Bulk compositions of the SMIs overlap those of their host rocks and extend to higher CaO/Al2O3 values. The estimated entrapment pressures and temperatures of the studied SMIs are 9.1–10.3 kbar and 1220–1355°C. The calculated mafic parental melt contains 42.36 wt.% SiO2, low total alkalis (3.22 wt.%), and high MgO (16.1 wt.%). Exploratory calculations using pMELTS show that this parental composition underwent variable degrees of fractional crystallization, as reflected by the variable compositions of the SMIs. Several lines of evidence including pyroxene xenocrysts and high FeO/MnO, FC3MS (FeO/CaO – 3*MgO/SiO2), and Zn/Fe ratios (14–21), suggest that a metasomatized pyroxenitic source contributed to the genesis of the parental melt. Amphibole in the SMIs indicates a high volatile content in the parental melt, which we conclude was generated from a metasomatized lithospheric mantle source. The pyroxenite source also contained garnet. Our geochemical results lead us to propose a new petrogenetic model. Specifically, we infer that a dense and unstable portion of the lithosphere underwent localized laminar detachment and downward flow, i.e. lithospheric drip. This drip underwent volatile-enhanced partial melting during descent through the underlying hot asthenosphere and generated the studied volcanic rocks.

Magnetic assessment of OSL and radiocarbon ages of sediments beneath a lava in Jeju Island, Korea: Implication of possible resetting of OSL signals and age constraint of the late Quaternary lava

Abstract Better chronological constraining of Late Pleistocene-Holocene volcanic eruptions is essential for figuring out spatio-temporal frequency distribution of ancient volcanism and assessing volcanic eruptions probability in a volcanic field. For this chronological frame work, radiocarbon (14C) and luminescence dating methods are being attempted alternatively, instead of K-Ar and 40Ar/39Ar techniques with lower resolutions and age limits of applicability. For the sediments capped by a lava flow in a Quaternary basaltic volcanic field of Jeju Island, Korea, we here introduce an interesting and probably frequently-encountered case of the discrepancy in age estimates between 14C and optically stimulated luminescence (OSL) chronologies. For samples with different positions in the sediments, the 14C ages of organic material extracts have a stratigraphically consistent but widely dispersed values between 2.25 ± 0.09 and 27.2 ± 0.5 calibrated (cal.) kyr BP, whereas the OSL ages of quartz extracts have a narrow range between 13.4 ± 1.3 and 16.1 ± 0.7 ka. Multiple magnetic parameters including Koenigsberger ratio, ratio of natural remanent magnetization (NRM) to saturation isothermal remanent magnetization intensities, and similarity in NRM direction compared with paleomagnetic direction for the upper lava comprehensively infer the paleo-heating of the sediments by the lava. Further, paleo-heating temperatures estimation by two independent magnetic methods, based on progressive thermal demagnetizations of NRM and repeated magnetic susceptibility versus temperature measurements with increasing peak temperatures, undoubtedly reveals that the sample positions where the OSL ages were yielded had paleo-temperatures between 200 and 250 °C and much higher than 550 °C, which are sufficiently high to reset their luminescence signals. Therefore, we interpret that a weighted mean OSL age of 14.8 ± 0.7 ka is the most probable age for the lava. Accordingly, the 14C ages older than ∼15.5 cal kyr BP are interpreted to be the deposition ages for their horizons, whereas the three exceptionally young 14C ages (∼2.3–∼7.7 cal kyr BP) for the upper sediments were contaminated by modern carbon. We suggest that magnetic applications, when combined with 14C and luminescence dating techniques, can help greatly improve the reliance of chronologies for late Quaternary volcanism of the world.

Paleosecular variation recorded by Quaternary lava flows from Guadeloupe Island

Paleomagnetic directional data were obtained from fourteen 0 to 2 Ma old lava flows at Basse-Terre Island (Guadeloupe, French West Indies). Five reversed polarity flows are consistent with their Matuyama age between 1.6–1.5 Ma and 875–790 ka while the ages of the other nine normal polarity units tie them to the Olduvai subchron and the Brunhes Chron. These directions have been combined with previous results obtained from Basse-Terre Island. The overall mean direction (D = −1.2°, I = 31.4°, α95 = 3.3°) obtained from the 39 non-transitional flows from Basse-Terre Island is indistinguishable from the expected geocentric axial dipole value (D = 0°, I = 29.8°). The dispersion measured from the angular standard deviation of the Virtual Geomagnetic Poles (VGPs) was found to be close to, but smaller than the predictions of geomagnetic models. Together with further directions from the nearby Martinique Island, the 45 directions obtained within the Brunhes chron provide the most robust estimate of the statistical distribution of paleosecular variation (PSV) at this latitude. The sequence of directions shows episodes of high amplitude secular variation that are coeval with several geomagnetic events including the last reversal documented by five transitional directions. Finally, three lava flows have recorded a transitional behavior which could be link to two excursions, the Laguna del Sello (at ~340 ka) and the Pringle Falls (at ~210 ka) events.

K-Ar Ages of Andesites from Two Volcanic Arrays in Western Chugoku, Southwest Japan

The volcanic front in the Southwest Japan Arc formed relative to the subducting Philippine Sea Plate. In the west Chugoku district, the volcanic front changes direction from E-W to NNE-SSW. Twenty-two Quaternary lava domes in this area can be topographically classified into two arrays (East and West arrays) based on the degree of dissection. K-Ar age results show that domes of the East array formed earlier (0.6-0.3 Ma) than those of the West array (0.2 Ma-recent). The two arrays in different ages may result from the migration of volcanic front caused by westward migration of the PSP slab.

Geochemical and Sr-Nd-Pb Isotopic Constraints on the Origin and Magmatic Evolution of Quaternary Lavas of Sakurajima Volcano, Southern Kyushu Island, Japan(＜Special Section＞Sakurajima Special Issue)

Geochemical and Sr-Nd-Pb Isotopic Constraints on the Origin and Magmatic Evolution of Quaternary Lavas of Sakurajima Volcano, Southern Kyushu Island, Japan(＜Special Section＞Sakurajima Special Issue)

Update on the Volcanic Geoheritage Values of the Pliocene to Quaternary Arxan–Chaihe Volcanic Field, Inner Mongolia, China

The Pliocene to Quaternary small-volume monogenetic volcanic field of the Arxan–Chaihe Volcanic Field (ACVF) hosts some typical volcanic landforms associated with continental basaltic volcanism. The volcanic field volcanic geoheritage values were used to establish two geoparks in 2004 and 2010 based on the volcanic geoheritage values of this region. The geoeducational program was quickly designed but shows signs of some lack of full understanding of the volcanic architecture of the region. This manifests in some misinterpretation of volcanic features that are relatively well-established and known elsewhere, and hence it hinders the effectiveness of the correct scientific information transition via geoeducational programs and geotourism. The geoparks established an excellent logistical network to promote volcanic geoscience, funded new educational centres and the local area economic growth created a situation that geotourism can be the main sector in economic growth in this part of Inner Mongolia. On the basis of these positive signs, we revisited some of the key volcanic geotopes of the region and cross-checked the already identified and established geosites mostly located in volcanologically important areas. There have been four major geotope groups identified such as (1) the scoria and spatter cones, (2) the lava flows and their surface morphological features, (3) the maar volcanism and its pyroclastic deposits and (4) the landscape and sedimentary evidence to demonstrate landscape evolution in a volcanically influenced continental region. Here, we provide new field observation-based evidence to demonstrate the volcanic geoheritage values of these geotopes. This information can be transmitted easily to the general public on the significance and formation of (1) scoria and spatter cones; (2) tumuli, pressure ridges, hornitos and various pahoehoe and transitional lava flow surface textures that are well-preserved on Quaternary lava flow surfaces; (3) base surge dominated tephra ring successions and large maars; and (4) landscape that evolved through the interplay of terrestrial sedimentation and monogenetic volcanic eruption influenced volcanic activity. The complex processes that can be seen at the ACVF make this field a high volcanic geodiversity one that can be used for a near-future application for UNESCO Global Geopark status.

Oxygen isotope geochemistry of mafic phenocrysts in primitive mafic lavas from the southernmost Cascade Range, California

Previously reported whole-rock δ18O values (5.6–7.8‰) for primitive quaternary mafic lavas from the southernmost Cascades (SMC) are often elevated (up to 1‰) relative to δ18O values expected for mafic magmas in equilibrium with mantle peridotite. Olivine, clinopyroxene, and plagioclase crystals were separated from 29 geochemically well-characterized mafic lavas for δ18O measurements by laser fluorination to assess modification of the mantle sources by ancient and modern subducted components. Oxygen isotope values of olivine phenocrysts in calc-alkaline lavas and contemporaneous high alumina olivine tholeiitic (HAOT) lavas generally exceed depleted mantle olivine values (~4.9–5.3‰). Modern addition of up to 6 wt% slab-derived fluid from Gorda serpentinized peridotite dehydration (~15‰) or chlorite dehydration (~10‰) within the serpentinized peridotite can provide the 18O enrichment detected in olivine phenocrysts (δ18Oolivine = 5.3–6.3‰) in calc-alkaline mafic lavas, and elevate 18O in overlying mantle lithosphere, as well. Specifically, although HAOT δ18Oolivine values (5.5–5.7‰) may reflect partial melting in heterogeneous 18O enriched mantle source domains that developed during multiple subduction events associated with terrane accretion (e.g., <1 wt% of ~15‰ materials), an additional 18O enrichment of up to 2 wt% of 10–15‰ slab-derived hydrous fluids might be accommodated. The calc-alkaline primitive magmas appear to have experienced a continuous range of open system processes, which operate in the mantle and during rapid magma ascent to eruption, and occasionally post quench. Textural relationships and geochemistry of these lava samples are consistent with blends of mafic phenocrysts and degassed melts in varying states of 18O disequilibrium. In lenses of accumulated melt within peridotite near the base of the crust, coexisting olivine and clinopyroxene δ18O values probably are not at isotopic equilibrium because fluids introduced into the system perturbed the δ18Omelt values. A “sudden” melt extraction event interrupts 18O equilibration in phenocrysts and poorly mixed melt(s). Rapid ascent of volatile oversaturated primitive mafic magma through the crust appears to be accompanied by devolatilization and crystallization of anorthite-rich plagioclase with elevated δ18Oplag values. The (Sr/P)N values for the whole rock geochemistry are consistent with a 87Sr/86Sr ~0.7027 slab-derived fluid addition into the infertile peridotite source of magmas, and melt devolatilization is recorded in the mixture of disequilibrium δ18O values for the constituent phases of lavas. Morbidity of the Gorda Plate as it undergoes intense deformation from the spreading ridge to the trench is likely a key factor to developing the carrying capacity of hydrous fluids and mineral phases in the slab subducting into the SMC mantle.

Sistemas de información geográfica y cartografía geomorfológica aplicados al inventario de deslizamientos y cartografía de susceptibilidad en la cuenca del río El Estado, Pico de Orizaba, México

With the aim of raising awareness on the prevention of landslide disasters, this work develops a methodology that incorporates geomorphological mapping into the mapping of landslide susceptibility using Geographic Information Systems (gis) and Multiple Logistic Regression (mlr). In Mexico, some studies have evaluated the stability of hillsides using gis. However, these studies set a general framework and guidance (that includes basic concepts and explanations of landslide classification, triggering mechanisms, criteria, considerations, and analysis for landslide hazard reconnaissance, etc.) for preparing a landslide atlas at state and city levels. So far, these have not developed a practical and standardized approach incorporating geomorphological maps into the landslide inventory using gis. This paper describes the analysis conducted to develop an analytical technique and morphometric analysis for a multi-temporal landslide inventory. Three data management levels are used to create gis thematic layers. For the first level, analogue topographic, geological, land-use, and climate paper are converted to raster format, georeferenced, and incorporated as gis thematic layers. For the second level, five layers are derived from topographic elevation data: slope angles, slope curvature, contributing area, flow direction, and saturation. For the third level, thematic maps are derived from the previous two levels of data: a hypsometric map (heuristically classified to highlight altimetric levels), a reclassified slope map (allowing to highlight differences in relief), and a morphographic map (derived from a heuristic reclassification of the slope map to highlight volcanic landforms). The theoretical aspects of geomorphological mapping contribute to set the conceptual basis to support landslide mapping. The gis thematic layers provide context and establish an overall characterization of landslide processes within the watershed. Through the retrieval and on-off switching of layers in the gis system, a base map is created to assist in the digitizing of landslides and the modeling of susceptibility. A landslide inventory is created from aerial photographs, field investigations, and all the above gis thematic layers. El Estado river watershed on the southwestern flank of Pico de Orizaba volcano has been selected as study area. The watershed is located in the southwestern slope of Citlaltepetl or Pico de Orizaba volcano. Geological (the stream channel of El Estado river erodes Tertiary and Quaternary lavas, disjointed volcanoclastic materials such as pyroclastic flows, fall deposits, lahars deposits, and alluvium) and geomorphological factors (steep slopes, energy relief, and vertical erosion) in combination with high seasonal rainfall (annual rainfall averages 1000-1100 mm/yr at > 4000 m a.s.l. and 927 mm/yr at <1500 m a.s.l.), and the high degree of weathering, make the study area susceptible to landslides. To assess landslide susceptibility, a landslide inventory map and geomorphometric cartography (altimetry, slope and geomorphography) were reviewed, and field work was conducted. In the study area, more than one hundred landslides were mapped. Shallow landslides (including debris slides and debris flows) are the predominant type. Shallow landslides predominate on hills capped with ash and pyroclastic deposits. The second major landslide process includes rock falls (which occur where the stream erodes lava flows and lahars) and deep-seated landslides (which occur in ash and pyroclastic deposits where lava flows act as a slip plane). In parallel, the spatial geodatabase of landslides was constructed from standardized gis datasets. Pertinent attributes are recorded on a geo-dataset. These include 1) mass wasting process, 2) level of certainty of the observation, 3) photo identification date, 4) landslide size, 5) landslide activity, 6) landslide parts (head, evacuation zone, deposit), 7) slope shape, 8) field slope gradient, 9) map gradient measured from the 10 m digital elevation model (dem), 10) landslide delivery, 11) land use, 12) elevation at which the landslide started, 13) aerial photograph identification number, 14) landslide area, and 15) researcher comments. Each attribute is standardized by the geo-dataset domains in the gis system. With this information the landslide susceptibility is modeled using mlr within a gis platform. mlr is used to examine the relation between land sliding and several independent variables (elevation, slope, contributing area, land use, geology, and terrain curvature) to create the susceptibility map. With six independent variables, the multiple logistic model susceptibility map tends to overpredict landslides at a 10 m pixel resolution. However, the model is statistically valid and able to predict 79.81% of the existing landslides. The implementation of a landslide inventory and susceptibility mapping techniques demonstrate the feasibility of the method for use in other volcanic areas of Mexico.

The Role of Cultural and Indigenous Values in Geosite Evaluations on a Quaternary Monogenetic Volcanic Landscape at Ihumātao, Auckland Volcanic Field, New Zealand

The Ihumātao Peninsula is located within the wider Mangere region in South Auckland, New Zealand. Significant sites of geological, cultural and ecological value are a recorded feature of this area. One of the most significant sites in the region is the Otuataua Stonefields Historic Reserve (OSHR). Geologically, this area is a Quaternary lava flow field with tuff rings, scoria and spatter cones. It sits within the Auckland Volcanic Field (AVF), and the landscape of the OSHR and the wider Ihumātao Peninsula may be seen as the physical expression of a unique convergence of ecological, geological and cultural values. Geosite evaluation methods applied to the Ihumātao Peninsula, following two distinct methods, shed light on potential high geoheritage values the region holds. These values may be looked at as a good base to develop effective geoeducational, geoconservational and geotouristic programs. The study also showed that implementing management strategies to add and conserve geosite values in the region could provide positive outcomes; however, reduction of its main geosite values would be inevitable and irreversible should proposed urban development take place on a block of land immediately bordering the OSHR. The Ihumātao Peninsula is one of several areas of South Auckland where urbanization has left significant areas relatively untouched until the present, whereby they are now threatened by intense housing and industrial development. The geological features of these areas unquestionably hold geological heritage values, allowing understanding of the interplay between low coastal land and rising basaltic magma. This interplay has resulted in a landscape potentially featuring the greatest volcanic geodiversity of the entire AVF. In addition, this relatively undeveloped land provides an unbroken physical and cultural record dating to arrival of the first humans in New Zealand. Of particular note is the physical artefacts and archaeological sites telling a story of settlers attracted to, utilizing and shaping this discrete region of the AVF. Geosite evaluations demonstrate that high geoheritage values of regions like the Ihumātao Peninsula are influenced by the strong cultural link between the community (in particular the indigenous population) and the volcanic landscape. These cultural factors could be given more weight in currently used geosite evaluation methods, enabling such geoheritage values to be demonstrated in a more explicit and meaningful way and providing a basis for further community education and protection of specific sites within the geographical context of the Ihumātao Peninsula.

Application of tritium in precipitation and baseflow in Japan: a case study of groundwater transit times and storage in Hokkaido watersheds

Abstract. In this study, we demonstrate the application of tritium in precipitation and baseflow to estimate groundwater transit times and storage volumes in Hokkaido, Japan. To establish the long-term history of tritium concentration in Japanese precipitation, we used tritium data from the global network of isotopes in precipitation and from local studies in Japan. The record developed for Tokyo area precipitation was scaled for Hokkaido using tritium values for precipitation based on wine grown at Hokkaido. Then, tritium concentrations measured with high accuracy in river water from Hokkaido, Japan, were compared to this scaled precipitation record and used to estimate groundwater mean transit times (MTTs). A total of 16 river water samples in Hokkaido were collected in June, July, and October 2014 at 12 locations with altitudes between 22 and 831 m above sea level and catchment areas between 14 and 377 km2. Measured tritium concentrations were between 4.07 (± 0.07) TU and 5.29 (± 0.09) TU in June, 5.06 (± 0.09) TU in July, and between 3.75 (± 0.07) TU and 4.85 (± 0.07) TU in October. We utilised TracerLPM (Jurgens et al., 2012) for MTT estimation and introduced a Visual Basic module to automatically simulate tritium concentrations and relative errors for selected ranges of MTTs, exponential–piston ratios, and scaling factors of tritium input. Using the exponential (70 %) piston flow (30 %) model (E70 %PM), we simulated unique MTTs for seven river samples collected in six Hokkaido headwater catchments because their low tritium concentrations were no longer ambiguous. These river catchments are clustered in similar hydrogeological settings of Quaternary lava as well as Tertiary propylite formations near Sapporo city. However, nine river samples from six other catchments produced up to three possible MTT values with E70 % PM due to the interference by the tritium from the atmospheric hydrogen bomb testing 5–6 decades ago. For these catchments, we show that tritium in Japanese groundwater will reach natural levels in a decade, when one tritium measurement will be sufficient to estimate a unique MTT. Using a series of tritium measurements over the next few years with 3-year intervals will enable us to estimate the correct MTT without ambiguity in this period. These unique MTTs will allow estimation of groundwater storage volumes for water resources management during droughts and improvement of numerical model simulations. For example, the groundwater storage ranges between 0.013 and 5.07 km3 with saturated water thickness from 0.2 and 24 m. In summary, we emphasise three important points from our findings: (1) one tritium measurement is already sufficient to estimate MTTs for some Japanese catchments, (2) the hydrogeological settings control the tritium transit times of subsurface groundwater storage during baseflow, and (3) in the future, one tritium measurement will be sufficient to estimate MTTs in most Japanese watersheds.

The Quaternary volcanic rocks of the northern Afar Depression (northern Ethiopia): Perspectives on petrology, geochemistry, and tectonics

The northern Afar Depression is one of the most volcano-tectonically active parts of the East African Rift system, a place where oceanic rifting may be beginning to form an incipient oceanic crust. In its center, over an area that is ∼80 km long and ∼50 km wide, there are seven major NNW-SSE-aligned shield volcanoes/volcanic edifices surrounded by compositionally distinct fissure-fed basalts. The Quaternary lavas in this area range from transitional to tholeiitic basalts, with significant across-axis variation both in mineralogy and chemistry. The variation in the contents of the major elements (TiO2, Al2O3, and Fe2O3), incompatible trace elements (Nd, Hf, Th, Ta), and the contents and ratios of the rare earth elements (REE) (e.g., (La/Yb)n = 5.3–8.9) indicate some variation in the petrogenetic processes responsible for the formation of these basalts. However, the variation in isotopic compositions of the mafic lavas is minimal (87Sr/86Sr = 0.7036–0.7041, 143Nd/144Nd = 0.51286–0.51289), which suggests only one source for all the Danakil Depression basalts. These basalts have isotope and incompatible trace element ratios that overlap with those of the Oligocene High-Ti2 flood basalts from the Ethiopian Plateau, interpreted as being derived from the last phase/tail of the Afar mantle plume source. Moreover, the Ce/Pb, Ba/U ratios indicate that the involvement of continental crust in the petrogenesis of the basaltic rocks is minimal; instead, both depth and degree of melting of the source reservoir underneath the northern Afar Depression played a major role for the production of incompatible element-enriched basalts (e.g., AleBagu Shield basalts) and the incompatible element-depleted tholeiitic basalts (e.g., Erta’Ale and Alu Shield basalts).