Young Lava Research Articles

Movement Patterns of a Native and Non‐native Frugivore in Hawaii and Implications for Seed Dispersal

AbstractAvian frugivores historically played important roles as seed dispersers across the Hawaiian Islands, but presently, the ‘Ōma‘o (Myadestes obscurus) is the only extant native frugivore in the wild on the Island of Hawaii. During recent decades, the introduced generalist Japanese White‐eye (Zosterops japonicus) has become the most common bird in Hawaii. The movements of avian frugivores largely dictate how far seeds get dispersed and into what kinds of microhabitats. This study compares the movement patterns and diet of the ‘Ōma‘o to the Japanese White‐eye to understand how a native differs from a non‐native frugivore in the type and distances of seeds dispersed. Radiotelemetry was conducted on nine ‘Ōma‘o and nine Japanese White‐eyes in a system of natural forest fragments (kīpuka) created by lava flows. Japanese White‐eyes disperse seeds approximately twice as far as ‘Ōma‘o; during the time of gut passage, ‘Ōma‘o move a mean distance of 98.1 m, and Japanese White‐eyes move 170.1–194.8 m. However, the ‘Ōma‘o disperses the seeds of at least seven different native fruit species compared with two dispersed by Japanese White‐eyes. Japanese White‐eyes were found to disperse seeds smaller than 1.5 mm, whereas the ‘Ōma‘o dispersed seeds up to 6 mm in diameter. Despite their ecological differences, both birds distribute certain seeds within and among kīpuka and likely facilitate primary succession of fruiting plants in the young lava matrix. However, this study suggests that if the ‘Ōma‘o were extirpated, a smaller‐bodied generalist cannot entirely substitute for the ecological role played by the native frugivore.

Depósitos de tsunami originados por un deslizamiento gravitacional masivo en Tenerife (islas Canarias)

Mega-landslides that have occurred in the flanks of the volcanic edifices of the Canary Islands caused giant waves (tsunamis) that deposited fossiliferous sediments in nearby coastal platforms. Deposits of this type are located along the northwest coast of Tenerife, the so-called Isla Baja. The stratigraphy, lithology, sedimentology and faunal content of these deposits suggests that they were originated by a tsunami produced by a mega-landslide – and an associated eruptive event – affecting the northern flank and the central zone of the island ca. 160 ka ago. The Caldera de Las Cañadas and a depression called Icod valley, today almost completely filled by younger lavas, were formed in this event, the last large flank collapse reported in Tenerife.

A lesson in defining “extinct”

Toni Feder’s Issues and Events news item about the High-Altitude Water Cherenkov Gamma-Ray Observatory under construction in Mexico ( Physics Today, October 2013, page 22) begins by describing the site as “nestled at 4100 m on the slopes of Sierra Negra, an extinct volcano.” However, as the picture accompanying her piece or a cursory examination in Google Earth makes clear, the site is not on the slopes of Sierra Negra. It sits in the saddle between Sierra Negra and its much larger companion Pico de Orizaba, also known as Citlaltepetl. Sierra Negra, a minor flank cone of that larger volcanic system, may now be extinct, but Pico de Orizaba (http://www.volcano.si.edu/volcano.cfm?vnum=1401-10=) absolutely is not. It is dormant at best, having erupted as recently as 1846. To compound the issue, the photograph shows that the observatory site is built squarely in front of an obviously young lava flow. Now, I’m sure that site selection was done with due attention to natural hazards, but as long as astronomers insist on building expensive observatories on top of volcanoes, there needs to be clear understanding and common vocabulary between astronomers and geologists. “Extinct” is a troublesome word when applied to a volcano.

Andean Adakite-like high-Mg Andesites on the Northern Margin of the Chilean–Pampean Flat-slab (27–28·5°S) Associated with Frontal Arc Migration and Fore-arc Subduction Erosion

Glassy, plagioclase phenocryst-free, ∼7–3 Ma, andesitic lavas erupted at the southern end of the Andean Central Volcanic Zone (CVZ) at 27–28·5°S are argued to contain a component from continental crust that was incorporated into the sub-arc mantle in a major pulse of fore-arc subduction erosion that removed ∼50 km of fore-arc. The 7–3 Ma amphibole-bearing Pircas Negras (54–64% SiO2) and ∼7·7 Ma pyroxene-bearing Dos Hermanos lavas erupted as the frontal arc was displaced some 50 km eastward over a developing bend in the Wadati–Benioff zone at the northern margin of the Chilean–Pampean flat-slab region. Their chemistry differs from that of older Miocene and younger CVZ arc lavas in the region in having more pronounced high field strength element (La/Ta = 40–100; Ba/Ta = 800–2400) and heavy rare earth element (Sm/Yb ∼ 4–9) depletions and an extreme adakitic-like character (∼600–1400 ppm Sr; Sr/Yb ∼ 400–1350). They also differ from older Miocene lavas in having higher 87Sr/86Sr and lower 143Nd/143Nd at the same wt % SiO2, while still showing trends with increasing wt % SiO2 that are best attributed to contamination in the ∼65–70 km thick crust. Overall, the trace element and isotopic features of the Pircas Negras lavas are consistent with modeling that attributes differences from older and younger lavas to enrichment of the arc mantle wedge by partial melts of tectonically eroded fore-arc crust, with the modeled eroded component being an outcrop and geophysically constrained mixture of 80–90% Jurassic mafic magmatic rocks and 10–20% silicic Paleozoic crust. An adakitic-type partial melt of this eroded crust generated at >2 GPa, reacted with mantle peridotite, can explain the high Mg# values (50–61) and Cr (100–350 ppm) and Ni (40–70 ppm) contents in some 5–3 Ma Pircas Negras lavas. Pre-eruption temperatures over 1060°C, calculated from mineral thermometry and the MELTS program, permit these magmas to subsequently melt and react with overlying eclogitic crust. Magma storage in this thick crust then led to amphibole crystallization and suppression of plagioclase fractionation. Quartz and feldspar xenocrysts were acquired from locally pooled mid-crustal magmas and the silicic upper crustal basement during final rapid ascent to the surface. Given a constant arc–trench gap of 300 km over the last 10 Myr, ∼198 km3 Myr–1 km–1 of fore-arc crust needs to be removed by subduction erosion at 8–3 Ma to account for the material balance of the fore-arc crust.

Gypsum, opal, and fluvial channels within a trough of Noctis Labyrinthus, Mars: Implications for aqueous activity during the Late Hesperian to Amazonian

We investigate in detail the morphology, mineralogy, and stratigraphy of light-toned deposits within one trough of Noctis Labyrinthus, centered at −6.8°N, 261.1°E. CRISM spectra taken from light-toned layered deposits in the northern portion of the trough exhibit absorptions around 1.41, 1.92 and 2.21μm, consistent with mixtures of opal and Al-clays that are exposed beneath younger lava flows and between high-standing mesas of chaotic terrain. In the southern portion of the trough, opal occurs as a patchy surficial deposit along the southwestern lower wall. Gypsum appears to be present in the southern portion of the trough where spectra show triplet absorptions at 1.44, 1.48, and 1.54μm, and additional absorptions at 1.20, 1.74, 1.95, 2.22, 2.27, and 2.49μm. The gypsum-bearing materials consist of one to several beds that typically fill low-lying regions, including valleys. A bright mound on the trough floor exhibits spectral features at 1.43, 1.92 and 2.43µm, characteristic of polyhydrated sulfates. The bright mound appears distinct in morphology from chaotic terrain, and along its base are exposures of gypsum-bearing materials.Fluvial channels in the southwestern portion of the trough incise surface slopes at 4–6° and lack obvious sources. The channels display first and second order tributaries arranged in a parallel pattern and may have formed by localized surface discharge from melting snow and/or ice. Both opal and gypsum occur in close proximity in the southwestern region of the trough, but gypsum is found alone in the southeast and opal in association with Al-clays is found to the northwest. We do not believe gypsum and opal formed coevally because they are not always found together, they require different aqueous conditions (i.e., opaline deposits require high silica availability while the gypsum deposits require high Ca availability in solution), and they appear at stratigraphically distinct levels. Although we identified evidence for fluvial landforms within the trough, cross-cutting relations indicate their incision post-dates deposition of the opal and pre-dates deposition of the gypsum. Hence, several periods of aqueous activity and alteration likely occurred within the trough from the Late Hesperian into the Amazonian that reflect favorable localized conditions within the Noctis Labyrinthus region and may be contemporaneous with late aqueous activity occurring elsewhere on Mars.

Renewed melting at the abandoned Húnafloí Rift, northern Iceland, caused by plume pulsing

The abandoned Húnafloí Rift (HR), near Skagi in northern Iceland, provides an opportunity to investigate how relocation of an oceanic spreading ridge influences melt generation. Melting recommenced beneath HR several million years after spreading ceased at 7–4 Ma. Both the older (pre-HR abandonment) and younger (post-abandonment) lavas are tholeiitic basalts with high Fe–Ti and enriched incompatible trace element compositions. The HR lavas are compositionally similar to modern lavas from central Iceland (above the Iceland Plume conduit) and the Eastern Volcanic Zone (a propagating rift). Although older and younger HR lavas are compositionally similar, geological and geophysical observations indicate different mantle melting conditions. For the older lavas, a plume migration reconstruction suggests that the combination of incompatible trace element enrichment and crustal thickness of ∼25 km can be explained by plume-driven upwelling, a scenario similar to central Iceland at present. The smaller volume of the younger HR lavas (<1 km melt thickness) and their greater distance from the plume centre argues against plume-driven upwelling to explain their enrichment. Rare earth element inversion modelling indicates that the younger lavas come from reactivation of the deepest part of the melting region beneath HR. We suggest that the base of the HR melting region was reactivated when a pulse of unusually hot asthenosphere, known to be producing unusually thick crust ∼700 km away from the plume centre today, passed laterally beneath the HR at ∼3 Ma. A thermal and kinematic ridge–plume interaction model indicates that this scenario can explain both the volume and composition of the younger Skagi lavas, provided that a small amount of decompression occurs. The decompression may result either from extension or from buoyancy-driven upwelling of hot mantle beneath the relatively thin lithosphere of the abandoned HR. The hypothesis that a hot plume pulse caused the younger HR lavas is significant not only as an example of interaction between mid-ocean ridge melting and time-dependent mantle convection, but also because this same hot pulse may have subsequently helped promote intensification of the Northern Hemisphere Glaciation when it uplifted the oceanic gateways either side of Iceland.

Evolution of low-18O Icelandic crust

The Krafla central volcano in the neovolcanic zone of Iceland hosts a chemically diverse suite of magmas characterized by anomalously low δ18O values. A rhyolite magma intercepted by the Iceland Deep Drilling Project (IDDP) exploratory well at 2.1km depth provided a unique opportunity to investigate the origins of an unerupted rhyolite melt in the primarily basaltic central volcano at Krafla. Here we compare whole rock hydrogen and oxygen isotopes of this melt to those of lavas within and near the caldera of the Krafla central volcano ranging from recent fissure eruptions to Plio--Pleistocene age (including analyses of 18 new samples, plus previously published values) in order to evaluate the petrogenesis of low-18O magmas within the neovolcanic zone of Iceland.Oxygen isotope values of the IDDP-1 melt (δ18O=+3.2±0.2‰) are within the range of Krafla eruptives that have a bimodal composition of olivine-tholeiite and rhyolite (δ18O=+1.6‰ to +4.5‰). Lavas show significantly more variability in hydrogen isotope values (δD=−161‰ to −92‰) than the IDDP-1 melt (−121±2‰), whose δD is comparable to local hydrothermal epidote (−127 to −108‰), and show significantly lower water contents than IDDP-1 (0.1–1.1wt%, in contrast to ~1.8wt%). Basaltic to dacitic lavas from the proximal Heidarspordur ridge volcanic zone have δ18O between +3.4‰ and +4.2‰ and δD between −105‰ and −99‰.Uniformity of oxygen isotopes in the Heidarspordur ridge lavas suggests that their magmatic compositional variations are a consequence of fractional crystallization. The δD of the glass sampled by IDDP-1 unequivocally identifies the source of silicic low-18O melts like those erupted from within the caldera of the Krafla volcano as anatexis of meteoric-hydrothermally altered basalts resulting from the intrusion of mantle-derived basaltic magma. Finally, mantle-derived basalts in both the Krafla central volcano and Heidarspordur ridge (MgO>5wt%) have δ18O values lower than the typical MORB. There is convincing evidence favoring low-18O compositions in primitive Icelandic basalts as also being the result of assimilating meteoric hydrothermally altered Icelandic crust, particularly in the roots of central volcanic complexes, where high-level intrusions develop. Here hydrothermally altered basalts have a high probability of undergoing partial melting. If this is the case, we hypothesize that there should be a notable secular variation in the δ18O values of upper crustal igneous products of the Iceland mantle plume over the sub-aerial extrusive geologic history of Iceland, in which progressively younger lavas exhibit decreasing δ18O values. Synthesis of published oxygen isotope analyses of unaltered whole rock and mineral separates of lavas from Tertiary age to present lends support to this hypothesis.

The investigation of the geothermal potential at the Silali volcano, Northern Kenya Rift, using electromagnetic data

The Silali volcano is one among a number of volcanoes found along the axis of the Kenya rift system that has experienced immense volcanism and faulting that formed a huge caldera at its summit. In this study, we investigate the resistivity distribution within the Silali volcano region in order to define a magma chamber that was the source of young lava flows covering the region and to better describe the geologic and structural features or faults around this region that may allow fluid movement. The data reveal a magma chamber beneath the caldera and a northeast trending fault west of the volcano in the Kapedo region. Potential fluid circulation zone within the caldera region are also evident. The depth to the magma chamber is estimated from the deeper low resistivity zone to be about 6km from the surface. A low resistivity zone less than 20ohm-m, at 1–2km depth is interpreted to be a low temperature clay (smectite) alteration zone resulting from fluid circulation as heat is transferred to shallower depth. The resistivity then increases abruptly beneath the inferred low temperature alteration zone and this increase in resistivity is interpreted as a change from clay mineralization to a chlorite-dominated system producing the observed decrease in resistivity. The decrease in conductivity may also imply a decrease in permeability due to decreasing connectivity of conductive fluids. The resistivity changes with depth could be temperature dependent and the inferred chlorite zone may suggest a temperature above approximately 240°C. The deeper low resistivity zone is interpreted as molten rocks and can be interpreted to indicate temperatures above 700°C of the magma body at a depth of 6km.

Microtexture development during rapid cooling in three rhyolitic lava flows

The effects of water concentration and degassing history on the development of spherulites and flow banding were examined in three middle Tertiary rhyolitic lava flows from the Atascosa Mountains of southern Arizona. The Hell’s Gate lava flow and the Atascosa lava flow host spherulites of strongly contrasting texture, and neither are flow banded. The Sycamore Canyon lava flow is a flow-banded rhyolite that hosts two populations of spherulites. Spherulites in the Hell’s Gate lava flow consist of two to four generations of bladed radiating alkali feldspar crystals that increase in water concentration along their length. Needle-like radiating feldspar crystals in spherulites in the Atascosa and Sycamore Canyon lava flows are in some cases punctuated by concentric rinds of glass that are reservoirs for water rejected by the feldspar crystals. The differences in spherulite crystal morphology between the Sycamore Canyon and Atasocosa flows (both needle-like) and the Hell’s Gate flow (bladed) may reflect a more rapid cooling rate of the Sycamore Canyon and Atascosa flows. Thick gray flow bands in the Sycamore Canyon lava flow host higher water concentrations than thin orange flow bands, suggesting that flow bands are zones of greater and lesser volatile concentration, deformed by stretching of the flowing magma. Temperature was uniform across the light and dark flow bands of the Sycamore Canyon flow, indicating that water concentration, one of the variables that controls diffusion coefficients, rather than temperature, controlled spherulite size in this case. Phenocrysts in the Hell’s Gate lava flow are strongly resorbed, probably as a result of magma ascent along a near-adiabatic gradient that resulted in exsolution of water from the melt, and subsequent dissolution of existing quartz phenocrysts by the water-rich melt. Resumption of crystallization of anhydrous phases such as quartz and feldspar would have further enriched the melt in water, facilitating the growth of spherulites. Spherulites in two of the lava flows (Sycamore Canyon and Hell’s Gate) increase in water concentration from core to rim, as would be expected in spherulites growing in melt enriched in water rejected by the growing feldspar crystals. Spherulites in the Atascosa rhyolite flow decrease slightly in water concentration from core to rim, possibly because the magma degassed during spherulite growth. Calculation of water concentration profiles in spherulites from all three rhyolite flows on the basis of Rayleigh fractionation of water between sanidine and rhyolitic melt shows that the very high water concentrations in spherulites (typically >0.6 × water concentration in surrounding glass) cannot be accounted for by Rayleigh fractionation. Instead it is likely that sanidine incorporated water as fluid inclusions and/or as water clusters during rapid crystal growth. Water concentration profiles in the glass surrounding spherulites do not preserve the high concentration zone at the spherulite boundary that has been observed in younger lava flows, so that spherulite growth rates cannot be calculated on the basis of mass-balance calculations of distribution of water during spherulite growth. Rather, the water concentration profile in the surrounding glass is a half plateau, the height of which is approximately equivalent to the far-field water concentration in the surrounding glass, indicating that water that accumulated at the spherulite/magma boundary diffused sufficiently rapidly to equilibrate with the surrounding magma as the lava flow cooled.

Petrology of Stewart Mountain basalt field in central Arizona, U.S.A.: A lithospheric source with small-scale trace element and isotopic heterogeneities

The Stewart Mountain basalt field in central Arizona is composed of three horizons of Miocene lavas over ~4 km 2 . The youngest lava is ~15.5 Ma. The field is in the southern Basin and Range at its transition to the Colorado Plateau. It is also at the northwestern margin of the ~8000 km 2 Goldfield-Superstition volcanic province (G-SVP), where basaltic lavas are ~20–19 Ma. Stewart Mountain basalts are alkalic, and most have from 6–8 weight percent (wt%) MgO, but more primitive and evolved lavas (10.7 and 4 wt% MgO, respectively) are also present. Most incompatible element abundances differ widely for basalts within the 6–8 wt% MgO range, and they distinguish the three horizons (e.g., ranges for P 2 O 5 are 0.5–1.4 wt%; Zr 125–250 ppm; La 40–80 ppm). One lava has quartz and plagioclase xenocrysts and even lower incompatible element abundances (e.g., P 2 O 5 0.25 wt%; La 25 ppm). All Stewart Mountain basalts, however, have Nb-Ta negative anomalies, consistent with a lithospheric mantle source that had subduction characteristics. Isotopic compositions differ across the three basalt horizons (e.g., ranges for 87 Sr/ 86 Sr are 0.7049–0.7061; 206 Pb/ 204 Pb 17.7–19.2; e Nd -3.5 to -6.2), where the xenocrystic lava has the lowest Sr and Pb isotopic ratios. Over its life, the Stewart Mountain field radiogenic isotope ratios decreased to reflect source heterogeneities, and its 206 Pb/ 204 Pb range is as wide as that formed by Oligocene–Miocene basalts collectively across the southern Basin and Range and transition zone. Incompatible-element abundances and ratios also reflect source heterogeneities, whereby the greatest differences are observed as abundances decreasing from middle to upper horizon basalts. Several abundance ratios, such as Zr/Nb, Th/Ta, Th/Nb, and Zr/Hf, record some of the source heterogeneities that are manifested over the short geologic time represented by the successive lava horizons. These temporal compositional changes likely reflect partial melts from a variably metasomatized lithospheric mantle. Compared to the compositions of the older, neighboring G-SVP basalts, Stewart Mountain lavas are generally evolved (MgO <8 wt%). The absence of mantle xenoliths in any Stewart Mountain lava and the xenocrystic lava both point to the compositional evolution having occurred in crustal reservoirs; however, based on the lowest isotopic ratios present in the xenocrystic lava, the upper crust was not a reservoir. Comparing Stewart Mountain basalt incompatible-element abundance ratios to those in the neighboring G-SVP shows enough difference to conclude that these two Miocene basalt localities had lithospheric sources with distinct trace element characteristics. The G-SVP source also had higher, distinguishing e Nd (-1 to -2). All characteristics combined, the Stewart Mountain field shows that lithospheric source heterogeneities can be manifested both temporally and spatially over only a small surface area. Stewart Mountain lithospheric source indicates that magmatism in central Arizona did not have asthenospheric sources by 15 Ma.

Geoheritage values of one of the largest maar craters in the Arabian Peninsula: the Al Wahbah Crater and other volcanoes (Harrat Kishb, Saudi Arabia)

AbstractAl Wahbah Crater is one of the largest and deepest Quaternary maar craters in the Arabian Peninsula. It is NW-SE-elongated, ∼2.3 km wide, ∼250 m deep and surrounded by an irregular near-perpendicular crater wall cut deeply into the Proterozoic diorite basement. Very few scientific studies have been conducted on this unique site, especially in respect to understanding the associated volcanic eruption processes. Al Wahbah and adjacent large explosion craters are currently a research subject in an international project, Volcanic Risk in Saudi Arabia (VORiSA). The focus of VORiSA is to characterise the volcanic hazards and eruption mechanisms of the vast volcanic fields in Western Saudi Arabia, while also defining the unique volcanic features of this region for use in future geoconservation, geoeducation and geotourism projects. Al Wahbah is inferred to be a maar crater that formed due to an explosive interaction of magma and water. The crater is surrounded by a tephra ring that consists predominantly of base surge deposits accumulated over a pre-maar scoria cone and underlying multiple lava flow units. The tephra ring acted as an obstacle against younger lava flows that were diverted along the margin of the tephra ring creating unique lava flow surface textures that recorded inflation and deflation processes along the margin of the post-maar lava flow. Al Wahbah is a unique geological feature that is not only a dramatic landform but also a site that can promote our understanding of complex phreatomagmatic monogenetic volcanism. The complex geological features perfectly preserved at Al Wahbah makes this site as an excellent geotope and a potential centre of geoeducation programs that could lead to the establishment of a geopark in the broader area at the Kishb Volcanic Field.

After the lava flow: The importance of external soil sources for plant colonization of recent lava flows in the central Oregon Cascades, USA

Effusive volcanic eruptions repave landscapes rapidly with lava flows, resetting broad areas of the underlying landscape and ecosystem. The unique physical properties of lava pose interesting challenges for ecologic recovery, as lava is dense, sterile, and generally inhospitable towards life. In this study we examine two sites of recent volcanism in the central Oregon Cascades, notable for the juxtaposition of barren exposed lava and mature forests on lava flows of the same or roughly the same age. We use a combination of LiDAR analyses, field observations, and soil characterization to examine soil and vegetation at these two sites, and find that the presence of an external sediment or soil source, particularly flood-borne deposits or syn- or post-eruptive tephra, greatly facilitates plant establishment, growth, and survival. The nature of the external sources of sediment or soil dictates the geographic extent of forests on these young lava flows: flood-borne deposits cover localized regions near river channels, while tephra can cover large regions. In general, our results suggest that external sources of soil provide a substrate for plants to grow in along with key nutrients and sufficient moisture retention. We conclude that external sources of soil source are key for the initial recovery following an effusive volcanic disturbance, in particular in temperate climates. Thus, unrelated geomorphic processes, such as past glaciations that provide local sources of mobile sediments, or concurrent volcanic processes, such as tephra production, dictate the presence or absence of forests on young lava flows.

Cosmogenic 3He and 21Ne dating of young basaltic lavas from southern Mendoza, Argentina

Cosmogenic 3He and 21Ne dating of young basaltic lavas from southern Mendoza, Argentina

Cosmogenic 3He exposure ages of basalt flows in the northwestern Payún Matru volcanic field, Mendoza Province, Argentina

The Payún Matru volcanic field encompasses more than 12,000 km2 in the west-central Argentine province of Mendoza. The northwestern part of the field, called Los Volcanes, includes multiple basaltic lava flows and numerous cinder cones and scoria fields. We sampled primary lava flow features (tumuli, pahoehoe and aa flow surfaces) from two different basaltic lava flows and one fluvial strath for cosmogenic 3He exposure age dating in the Los Volcanes area. Olivines from the sampled basalts have low concentrations of U and Th and a minimal radiogenic 4He correction. We used the recently established R correction factor to correct our measured 3He concentrations for magmatic 3He and 4He and radiogenic 4He. Exposure ages for primary lava flow features covering a large area just east of where Route 40 crosses the Río Grande range from 41 to 43 ka with external uncertainties on the order of 7%. A recently published K→Ar age of 26 ± 5 ka was sampled within a few hundred meters of three of our cosmogenic exposure age samples and appears to be from the same basalt flow. Given the large uncertainties associated with dating young lava flows, the K→Ar age and our cosmogenic ages are in reasonable agreement; however, they do not overlap within uncertainties and suggest possible systematic discrepancies between the two methods of up to 20% in this instance. A sample from a bedrock strath carved by the Río Grande into basalt yielded a surface exposure age of 17 ± 1 ka. The toe of a much younger-looking aa flow about 5 km west of the Rt. 40 Río Grande bridge yielded two ages of 0 and 2 ka, and is presumed to be historical.

Cenozoic Volcanism on the Hangai Dome, Central Mongolia: Geochemical Evidence for Changing Melt Sources and Implications for Mechanisms of Melting

Cenozoic volcanism within Mongolia forms part of a large central Asian province of intra-plate magmatism. Numerous small-volume volcanic cones and alkali basalt lava flows have been formed since c. 30 Ma; from c. 12 Ma activity has been focused on the uplifted Hangai dome. A mechanism for melting beneath the dome has, however, thus far remained enigmatic. Some of the oldest basalts on the Hangai dome erupted at its centre at ∼6 Ma and their geochemistry suggests a garnet lherzolite source region at 90–100 km depth. These lavas have Pb isotope compositions similar to those of depleted Indian mid-ocean ridge basalts (MORB) ( 206 Pb/ 204 Pb = 17·822, 207 Pb/ 204 Pb = 15·482, 208 Pb/ 204 Pb = 37·767), which may be indicative of the involvement of ambient asthenospheric mantle in their petrogenesis. Younger basalts exhibit a gradual shift in isotopic composition towards a source that has less radiogenic Pb and more radiogenic Sr, evidenced by the eruption of lavas with 206 Pb/ 204 Pb = 16·991 and 87 Sr/ 86 Sr = 0·704704. The youngest lavas, dated as younger than ∼8 ka, have the highest K 2 O contents (up to 5·2 wt %) and are characterized by the most enriched trace-element signatures; they are interpreted to represent melting of a metasomatically altered sub-continental lithospheric mantle containing phlogopite. Concurrent with progressive melting of the lithosphere, melting appears to propagate outwards from the centre of the dome to its margins; by 0·7 Ma the marginal magmatism is interpreted to result from melting of a depleted MORB-source mantle component with a smaller contribution from the lithospheric mantle. The spatial and temporal variations in melting beneath the Hangai dome may be explained by either lithospheric delamination or the presence of a small-scale thermal anomaly in the upper mantle. Although it is not possible to distinguish between these models on the basis of geochemistry alone, the lack of a viable mechanism to generate small-scale upwelling lends support to a model involving delamination of the lithospheric mantle beneath the Hangai dome.

Rapid magmatic processes accompany arc–continent collision: the Western Bismarck arc, Papua New Guinea

New U–Th–Ra, major and trace element, and Sr–Nd–Pb isotope data are presented for young lavas from the New Britain and Western Bismarck arcs in Papua New Guinea. New Britain is an oceanic arc, whereas the latter is the site of an arc–continent collision. Building on a recent study of the Manus Basin, contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc–continent collision and possible slab detachment. All three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle. However, the Western Bismarck arc lavas diverge from the Pb isotope mixing array formed by the New Britain and the Manus Basin lavas toward elevated 208Pb/204Pb. We interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision. 238U and 226Ra excesses are preserved in all of the lavas and are greatest in the Western Bismarck arc. High-Mg andesites with high Sr/Y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge. Data for two historical rhyolites are also presented. Although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions, both formed from mafic parents within millennia.

Ballistic blocks around Kīlauea Caldera: Their vent locations and number of eruptions in the late 18th century

Thousands of ballistic blocks occur around Kīlauea Caldera and record part of the latest major period of explosive activity on the volcano, in late 1790 or within a few years thereafter. The sizes of the blocks – the largest of which is more than 2m in nominal diameter – and differences in rock types allow the definition of at least 6 dispersal lobes of mostly undetermined relative age. The orientations of the lobes help approximate the locations of vents or explosion sources on the floor of the caldera, now deeply buried by younger lava flows. The vents may have been distributed northward for about 2km from near the site of the modern Halema'uma'u Crater and were apparently confined to the western half of the caldera. The blocks are entirely lithic except for those in one dispersal lobe, which contains cored bombs and blocks as well as juvenile lapilli. Eruption parameters calculated from EJECT! suggest that the phreatic and phreatomagmatic explosions could have been generated at the water table, about 600m below the high point on the caldera rim.

Paleomagnetism and geochemistry from the Upper Cretaceous Tres Picos Prieto locality (43ºS), Patagonian Plateau Basalts

A paleomagnetic study on nine from lavas from the Upper Cretaceous Tres Picos Prieto locality (43o50'S and 70o3'W), Patagonian Plateau Basaltic Province, revealed that a ca 300 m thick pile of basalts accumulated during three discrete (i.e., temporally separated) volcanic episodes. The chemistry of these lavas shows characteristics compatible with both intraplate and subduction-related magmatism, with the latter having a more important contribution in the younger lavas. Overall, we interpret these rocks to be transitional basalts generated in a suprasubduction environment. The Late Cretaceous to Cenozoic Patagonian Plateau Basaltic Province crops out from eastern Patagonia to the cordillera, and their genesis have been associated to different tectonic processes, even for spatially separated rocks of the same age. Considering that the development of the magmatic province is contemporary with the stage of westward drift of South America, we proposed that magma generation and upwelling due to rapidly shearing asthenosphere inducing circulatory flow associated to asperities in the lithosphere-asthenosphere interface (shear-driven upwelling) can be evaluated as a further potential mechanism to account for many outcrops of these backarc to intraplate lavas.

Volcanism on the flanks of the East Pacific Rise: Quantitative constraints on mantle heterogeneity and melting processes

We present major and trace element and Sr, Nd and triple-spike Pb isotope data for 17 fresh volcanic glasses from Seamount 6, a 10-km diameter seamount located 140km east of the East Pacific Rise at 12°45′N. Geological and geochronological evidence show that magma compositions evolved from tholeiitic basalts to alkalic basalts and basaltic trachyandesites during the 1–2Ma active lifetime of the seamount. Major and trace element compositions in Seamount 6 lavas vary systematically with isotope ratios; the youngest lavas with the highest incompatible trace element concentrations have the highest La/Yb, Nb/Zr, K2O/TiO2, 87Sr/86Sr, 206Pb/204Pb and the lowest 143Nd/144Nd, MgO and CaO. The range in element concentrations, incompatible element ratios, and isotope compositions in Seamount 6 lavas exceeds that observed in lavas erupted at the adjacent ridge axis, and is comparable to the range in lava compositions reported from all near-ridge seamounts studied to date. The observed range in lava compositions is consistent with mixing between enriched and depleted melts at shallow levels in the crust. The inferred difference in composition between these mixing endmembers cannot be explained by variable degrees of melting of a single source composition, and requires that the upper mantle is extremely heterogeneous on the scale of the melting region beneath a single seamount.We can show that the range in composition of EPR seamount lavas cannot be generated by melting of variably heterogeneous mantle in which enriched and depleted materials contribute equally to melting (source mixing). Instead, the trace element and isotope compositions of seamount lavas can be reproduced by melting models in which more enriched, fertile mantle lithologies are preferentially melted during mantle upwelling. At progressively lower degrees of melting, erupted lavas are thus more enriched in incompatible trace elements, have higher La/Yb, K/Ti, 87Sr/86Sr ratios and lower 143Nd/144Nd. If this is a common process, then mantle-derived magmas are unlikely to inherit the average incompatible trace element and isotope composition of their mantle source, which is likely to be significantly more depleted, nor will they display the full range of compositions present in the mantle melting region. These results have implications for the way in which oceanic basalts can be used to infer the composition of the upper mantle.

A unique volcanic field in Tharsis, Mars: Pyroclastic cones as evidence for explosive eruptions

Based on theoretical grounds, explosive basaltic volcanism should be common on Mars, yet the available morphological evidence is sparse. We test this hypothesis by investigating a unique unnamed volcanic field north of the shield volcanoes Biblis Patera and Ulysses Patera on Mars, where we observe several small conical edifices and associated lava flows. Twenty-nine volcanic cones are identified and the morphometry of many of these edifices is determined using established morphometric parameters such as basal width, crater width, height, slope, and their respective ratios. Their morphology, morphometry, and a comparison to terrestrial analogues suggest that they are martian equivalents of terrestrial pyroclastic cones, the most common volcanoes on Earth. The cones are tentatively interpreted as monogenetic volcanoes. According to absolute model age determinations, they formed in the Amazonian period. Our results indicate that these pyroclastic cones were formed by explosive activity. The cone field is superposed on an old, elevated window of fractured crust which survived flooding by younger lava flows. It seems possible that a more explosive eruption style was common in the past, and that wide-spread effusive plain-style volcanism in the Late Amazonian has buried much of its morphological evidence in Tharsis.