Feeder Dikes Research Articles

The Feeder System of the Deccan Traps (India): Insights from Dike Geochemistry

Three large dike systems are exposed in the DeccanTraps flood basalt province of India: the dominantly north^south-trending west coast swarm, the east^west-trending Narmada^Tapi swarm in the north^ central Deccan, and the Nasik^Pune swarm in the central western Deccan. Combined major and trace element and Sr^Nd^Pb isotope data reveal that probable feeder dikes for the three main lava formations in the upper part of the lava pile (Poladpur, Ambenali and Mahabaleshwar Formations) are abundantly represented in the Nasik^Pune and coastal swarms. As a group, these dikes have no clear preferred trend. Among the highly oriented dikes of the Narmada^Tapi and west coastal areas, some have affinities with the lower part of the lava pile (Jawhar, Igatpuri, Thakurvadi and Bushe Formations) and these appear to have been intruded under the influence of regional north^south and east^west extension, respectively. Other dikes in the Narmada^Tapi swarm have the high- 206 Pb/ 204 Pb characteristic of flows in the far northeastern Deccan. These data suggest that Deccan lava flows could have reached as much as 700 km in length. Directed lithospheric extension appears to have been an important control on the emplacement of feeder dikes for the lower and middle formations. In contrast, emplacement of the voluminous upper formations, which span the Cretaceous^Tertiary boundary and 29R^N magnetic reversal and are estimated to make up � 50% of Deccan lava volume, was not controlled by directed regional extension. This conclusion contradicts predictions of rifting-based models for Deccan volcanism. Finally, isotopically distinct, north^south-trending dikes cut upper-formation flows and dikes along the coast; these dikes represent minor magmatism linked to early Paleocene east^west extension following the main phase of volcanism, in association with rifting of the Seychelles Bank from India.

Anatomy of an unstable volcano from InSAR: Multiple processes affecting flank instability at Mt. Etna, 1994–2008

Volcano deformation may occur under different conditions. To understand how a volcano deforms, as well as relations with magmatic activity, we studied Mt. Etna in detail using interferometric synthetic aperture radar (InSAR) data from 1994 to 2008. From 1994 to 2000, the volcano inflated with a linear behavior. The inflation was accompanied by eastward and westward slip on the eastern and western flanks, respectively. The portions proximal to the summit showed higher inflation rates, whereas the distal portions showed several sectors bounded by faults, in some cases behaving as rigid blocks. From 2000 to 2003, the deformation became nonlinear, especially on the proximal eastern and western flanks, showing marked eastward and westward displacements, respectively. This behavior resulted from the deformation induced by the emplacement of feeder dikes during the 2001 and 2002–2003 eruptions. From 2003 to 2008, the deformation approached linearity again, even though the overall pattern continued to be influenced by the emplacement of the dikes from 2001 to 2002. The eastward velocity on the eastern flank showed a marked asymmetry between the faster sectors to the north and those (largely inactive) to the south. In addition, from 1994 to 2008 part of the volcano base (south, west, and north lower slopes) experienced a consistent trend of uplift on the order of ∼0.5 cm/yr. This study reveals that the flanks of Etna have undergone a complex instability resulting from three main processes. In the long term (103–104 years), the load of the volcano is responsible for the development of a peripheral bulge. In the intermediate term (≤101 years, observed from 1994 to 2000), inflation due to the accumulation of magma induces a moderate and linear uplift and outward slip of the flanks. In the short term (≤1 year, observed from 2001 to 2002), the emplacement of feeder dikes along the NE and south rifts results in a nonlinear, focused, and asymmetric deformation on the eastern and western flanks. Deformation due to flank instability is widespread at Mt. Etna, regardless of volcanic activity, and remains by far the predominant type of deformation on the volcano.

Explosion dynamics from strainmeter and microbarometer observations, Soufrière Hills Volcano, Montserrat: 2008–2009

Vulcanian explosions with plumes to 12 km occurred at Soufrière Hills volcano (SHV) between July 2008 and January 2009. We report strainmeter and barometric data, featuring quasi‐linear strain changes that correlate with explosive evacuation of the conduit at rates of ∼0.9−2 × 107 kg s−1. July and January explosion‐generated strains were similar, ∼20 nanostrain at ∼5 km, and interpreted as contractions of a quasi‐cylindrical conduit, with release of magmastatic pressure, and exsolution‐generated overpressure of order 10 MPa. The 3 December 2008 event was distinctive with larger signals (∼140–200 nanostrain at 5–6 km) indicating that a rapid pressurization preceded and triggered the explosion. Modeling suggests a dike with ENE trend, implying that feeder dikes at SHV had diverse attitudes at different times during the eruption. All explosions were associated with acoustic pulses and remarkable atmospheric gravity waves.

Geometric difference between non-feeder and feeder dikes

Feeder dikes bring magma to the surface; non-feeder dikes become arrested and never reach the surface. The differences, if any, between these dike types remain largely unexplored because in the field it is normally unknown if a particular dike is a feeder or non-feeder. Here we present measurements of feeder and non-feeder dikes exposed from depths of >200 m to the surface in the walls of the A.D. 2000 caldera collapse of the Miyakejima Volcano, Japan. A typical feeder thickness reaches a maximum of 2–4 m at the surface, decreases rapidly to ∼1 m at a depth of 20–40 m, and then remains constant to the bottom of the exposure. By contrast, a typical non-feeder thickness reaches a maximum of 1.5–2 m at 15–45 m below the tip, and then decreases slowly with depth to 0.5–1 m at the bottom of the exposure. We propose that free-surface effects and magmatic overpressure (driving pressure) changes during the eruption cause the overall shape of a feeder to differ from that of a non-feeder.

Neoproterozoic subaqueous extrusive–intrusive rocks in the Playa Hermosa Formation in Uruguay: Regional and stratigraphic significance

Volcanic rocks in southern Uruguay are linked to an extensional tectonic regime related to the post-collisional Brasiliano–Pan African orogenic cycle. The ca 580 Ma magmatic event is recorded in the Playa Hermosa Formation. In the upper section of this formation several units were defined. The basal unit is a quartz–syenite brecciated deposit, interpreted as the result of explosive episodes of shallow plutonic quartz–syenite intrusion with signs of hydrothermal alteration. Sedimentary and basaltic lithoclasts are occasionally present in the breccia, implying the emplacement of basalts before of that magmatic event. The middle unit is composed of trachytic hyaloclastites. Local distribution and textural evidence indicate that trachytic volcanic pulses have been followed by an explosive event that produced brecciated deposits. The upper unit of the studied sequence is composed of basalts with thin sedimentary intercalations. Most of the outcropping basalts display the typical micro- and macro-features of hyaloclastites; distinctive of extrusions occurred in wet sediments previous to lithification. Basaltic feeder dikes are common in the lower unit. The study of flow structures in the quartz–syenite brecciated deposits led to the recognition of two basaltic inputs. Such differences are related to the intensity of chloritic alteration, which would be related to water contamination. Basaltic peperites are one of the most impressive features of the Playa Hermosa Formation. Hyaloclastites and peperites largely represented among the studied volcanic rocks, and devitrification textures are all evidences of magma/seawater interactions. In the basaltic fragments of the peperites well preserved orange palagonite appears, while the sandstone fractions have lithic clasts of felsic volcanic rocks, such as rhyolitic lavas with perlitic cracks and lithophysae-like structures. Palagonite chemical analyses suggest marine environment and, at the same time, it proves that the studied section lacks of regional metamorphism. The felsic volcanic lithoclasts contained in sandstones could be related to the felsic volcanic lobes intercalations. The studied magmatic event is important in order to formulate correlations with other Neoproterozoic units distributed around the Dom Feliciano belt, like Camaquã and Campo Alegre basins (Brazil), and in the Kaoko belt (NW Namibia) in Africa. Also, Playa Hermosa Formation presents glacigenic features that are well documented, and reinforce the volcanism around 580 Ma in the Rio de la Plata Craton.

A lunar cryptomare dome near Mee H and Drebbel F

In this study, we examine the lunar mare dome Mee 1 situated near the craters Mee H and Drebbel F in a region showing evidence of ancient (pre-Orientale) mare volcanism and cryptomare deposits. Regional stratigraphic relations indicate that Mee 1 was formed prior to the Orientale impact at the beginning of the Imbrian period. Based on a combined photoclinometry and shape from shading technique applied to telescopic CCD images of the dome acquired under oblique illumination, we determined a diameter of Mee 1 of 25 km, a height of 250 m, a flank slope of 1 . 15 ∘ , and a volume of 44 km 3 . Based on rheologic modelling of the dome and a viscoelastic model of the feeder dike, we obtained a magma viscosity of 5.1 × 10 5 Pa s , an effusion rate of 869 m 3 s - 1 , a duration of the effusion process of 1.6 years, a magma rise speed of 1.9 × 10 - 4 ms - 1 , a width of the feeder dike of 32 m, and a horizontal dike length of 144 km. A comparison of Mee 1 with domes with similar morphometric properties, which are located near Milichius and inside the crater Petavius, reveals strong similarities with respect to the viscosity of the dome-forming magma and the feeder dike geometry, while the effusion rate and magma rise speed of Mee 1 are somewhat higher. The pronounced morphometric differences between Mee 1 and a smaller dome situated close to the crater Doppelmayer and characterised by a similar magma viscosity suggest that the growth of that dome was limited by exhaustion of the magma reservoir, while Mee 1 and the other larger domes display morphometric properties presumably coming closer to the cooling limit. The comparison of the ancient dome Mee 1 with the younger (Eratosthenian) edifices near Milichius and Doppelmayer suggests that the conditions in the upper mantle and the crust favoured high eruption volumes, effusion rates, and magma rise speeds, implying the occurrence of large magma reservoirs preventing the limitation of dome growth by magma exhaustion. On the other hand, we observe similar general morphometric, rheologic, and feeder dike characteristics and, thus, conclude that the formation conditions of lunar mare domes did not change fundamentally during the Imbrian period.

Mantle plume: The invisible serial killer — Application to the Permian–Triassic boundary mass extinction

The Earth experienced a severe mass extinction at the Permian–Triassic boundary (PTB) about 252 million years ago. This biological catastrophe was accompanied by major changes in geochemical composition of the atmosphere and ocean and the appearance of sedimentary features which had not occurred since the Precambrian time. The eruption of the largest continental flood basalt, the Siberian Traps, overlapped this mass killing. Many hypotheses have been proposed but no definitive conclusion currently exits. Here we present characteristics of three sections from Iran and China and propose that an active mantle plume initiated a series of processes which led to the mass mortality and produced major sedimentological, mineralogical, and geochemical changes observed in the transition from the Paleozoic to the Mesozoic. The injection of mantle plume-related igneous dike swarms into the continental margin facilitated the release of massive amounts of CH 4 primarily from the dissociation of marine gas hydrates and secondarily from the maturation of organic-rich sediments and fracturing of hydrocarbon reservoirs. The bulk of the CH 4 was aerobically oxidized in the water column producing dissolved CO 2 with low δ 13C values. This CO 2-saturated seawater became acidic to the point of dissolution of shelf carbonates promoting precipitation of siliciclastic-rich strata in the transition from the Permian to the Triassic. Methane-derived CO 2 also lowered carbon isotopic composition of seawater leading to the observed decline in δ 13C composition of organic and inorganic marine carbon at the PTB. Gas-charged oceans released large volumes of CO 2 and CH 4 into the atmosphere which created a severe global warming (the end-Permian inferno) causing the release of additional CH 4 from the dissociation of polar gas hydrates. These events lowered δ 13C compositions of terrestrial carbon. Simultaneously, feeder dikes from the mantle plume formed the Siberian Traps flood basalt. Marine mass extinction was the result of a change in seawater composition due to the injection and oxidation of CH 4 in the water column causing low pH, high concentrations of CO 2, Ca 2+ and HCO 3 −, and low CO 3 2− values. Combined with a hot seawater temperature, these changes made calcification of marine organisms difficult and produced major physiological crisis including reduced metabolic rates, high sensitivity to environmental stress, and hampered growth and reproduction. Terrestrial mass extinction can be attributed to severe global warming and soil acidification produced by increased atmospheric CO 2, acid rain that was generated by SO 2 derived from the Siberian trap eruption, and loss of habitat. Cessation of the plume activity during Early Triassic stopped the release of CH 4 into the ocean and terminated continental flood basalt eruption ending the environment of death on land and in sea. The cut off of CO 2 production in the ocean instantly increased carbonate saturation of seawater resulting in extensive seafloor cementation. It also resulted in the deposition of marine carbonates by microbial activities in the hostile post-extinction environment. From the trigger to recovery, the perturbation which included the end-Permian mass mortality could have lasted for at least 2 Myr. Several major mass extinctions of the Phanerozoic are temporally accompanied by flood basalt eruptions. So far, these two events have been interpreted in a cause-and-effect relation: flood basalt eruption causes mass extinction. This study proposes that flood basalts and their time correlative biological crises are themselves the consequence of a complex perturbation caused by mantle plume activities. If so, major disturbances in the near surface of the Earth are ultimately controlled by changes in the mantle. The physical sedimentological observations presented here combined with previous paleontological evidence cast doubt on other interpretations which use geochemical variables, numerical modeling, biomarkers, and Ce anomaly to suggest that Late Permian ocean was anoxic all the way to the photic zone.

Geochemistry of Cretaceous to Eocene Ophiolitic Rocks of the Central Cordillera: Implications for Mesozoic-Early Cenozoic Evolution of the Northern Philippines

The Central Cordillera in northern Philippines is underlain mainly by ophiolitic basement of Cretaceous to Eocene age. In the central portion of the range, these complexes consist mainly of pillow basalt and basaltic feeder dikes that are unconformably overlain by epiclastic rocks, including turbidites. Field and geochemical evidence reveals that these rocks have a supra-subduction signature, and were generated in a back-arc setting. These relationships are similar to those exhibited by the Cretaceous to Eocene Lepanto Metavolcanics and the Pugo Formation, comprising the ophiolitic basement in the southern portions of the range. This result suggests that the Central Cordillera is floored by a common volcanic basement, possibly forming part of the Philippine Sea plate, proto-Philippine Sea plate, or proto-Molucca Sea plate.

Dike-induced reverse faulting in a graben

Normal-fault slip of the boundary faults of a graben is commonly attributed to dike-induced stresses. Here, however, we report for the first time clear field evidence of a large dike-induced reverse-fault slip on a fault associated with a volcano-tectonic graben. The measured reverse faulting of at least 5 m occurs in the Holocene rift zone of northern Iceland. The reverse slip has apparently occurred on an existing normal fault as a result of over pressure of a nearby 6–13-m-thick 8 ka feeder dike. A numerical model of the dike-fault interaction supports our interpretation. The results indicate that dikes, particularly potential feeder dikes, may cause large reverse slips on nearby normal faults. This conclusion should improve the general understanding of geodetic deformation during volcanic unrest periods and may help forecast dike-fed eruptions.

Methane Release from Igneous Intrusion of Coal during Late Permian Extinction Events

Unusually large and locally variable carbon isotope excursions coincident with mass extinctions at the end of the Permian Period (253 Ma) and Guadalupian Epoch (260 Ma) can be attributed to methane outbursts to the atmosphere. Methane has isotopic values (δ13C) low enough to reduce to feasible amounts the carbon required for isotopic mass balance. The duration of the carbon isotopic excursions and inferred methane releases are here constrained to <10,000 yr by counting annual varves in lake deposits and by estimating peat accumulation rates. On paleogeographic maps, the most marked carbon isotope excursions form linear arrays back to plausible methane sources: end‐Permian Siberian Traps and Longwood‐Bluff intrusions of New Zealand and end‐Guadalupian Emeishan Traps of China. Intrusion of coal seams by feeder dikes to flood basalts could create successive thermogenic methane outbursts of the observed timing and magnitude, but these are unreasonably short times for replenishment of marine or permafrost sources of methane. Methane released by fracturing and heating of coal during intrusion of large igneous provinces may have been a planetary hazard comparable with bolide impact.

Probing Stromboli volcano from the mantle to paroxysmal eruptions

Abstract We investigated the plumbing system of Stromboli volcano from the upper mantle to the surface. Thermobarometric estimates indicate that the deeper detected part of the plumbing system is located in the upper mantle, at approximately 34–24 km depth where, during their ascent, primitive Stromboli basalts (HKCA to shoshonitic) interact with peridotitic materials. In this region magma flow is probably channelled along fracture zones that may converge into a feeder dyke that crosscuts the Moho at about 17 km depth. During their ascent, basaltic magmas will interact with lower crust materials represented by cumulates of earlier Stromboli-type basalts at 13–10 km depth. This zone is also the section of the plumbing system where the feeder dyke is entering the chamber. Thermobarometric estimates, obtained by constructing a grid of selected reactions, indicate that current primitive Stromboli basalts equilibrate at 0.3–0.15 GPa and temperatures approaching 1200 °C, and progressively crystallize and degas before being erupted. Crystal size distributions on lavas and juvenile tephra erupted in 2002–2003 give very variable residence times. Based on average bubble distances, the estimated times for the exsolution of the gaseous phases range from 2–7 days to 45 min for the lavas and scorias, down to about 15 h to 12 min for the pumices erupted during paroxysmal explosions. Estimated syneruptive viscosities range from 10 2 Pa s for the anhydrous basaltic pumices at 1200 °C, to 10 3 –10 4 Pa s for lavas approaching their effusion temperatures (1100–1150 °C). In turn, viscosities for the hydrous basaltic melt that led to the formation of the basaltic pumices may be around 10 Pa s or lower. In the light of the above, we discuss the possible shapes and volumes of Stromboli magma chamber by considering a sphere, an ellipsoid (geometrically concordant with the regional stress distribution) and a feeder dyke, the last two being more likely. In the light of volcanological, structural and geophysical data on conduit thickness, we propose an alternative model that takes into account the volumes of recently erupted lavas. This model consists of a convective ellipsoidal magma chamber ‘injected’ by an active feeder dike of undegassed magma of higher temperature, lower density and lower viscosity. This dyke will evolve into a magma column inside the chamber and will separate the reservoir into two lateral, nearly symmetric convective regions. Crystallization would occur preferentially in the proximity of the wallrocks, particularly where the chamber is entering the conduit. The onset of paroxysmal explosions during major effusive cycles may be explained by a drastic increase in the intrusion rates at the base of the chamber that will produce a progressive inflation of the magma column dynamically transferred to the chamber walls. The ceasing of ‘anomalous’ intrusion rates at the base of the chamber, coupled with higher discharge rates, will progressively depressurize the chamber to a critical threshold, until the stress transferred to the walls is dynamically released: at this point the walls themselves will undergo a nearly instantaneous elastic rebound and contract in the attempt to recover their original pre-eruptive geometry. These dynamics will squeeze up portions of the undegassed magma column, triggering a paroxysmal explosion with the ejection of ‘golden pumices’.

Lunar domes in Mare Undarum: Spectral and morphometric properties, eruption conditions, and mode of emplacement

In this study we examine five lunar domes in Mare Undarum. Four domes termed Condorcet 1–4 are located between the craters Condorcet P and Dubiago, immediately east of Dubiago V and W. The fifth dome, termed Dubiago 3, is located about 35 km further south. The region under study is situated in a major trough concentric to the Crisium basin. The domes Condorcet 1–3 are aligned radially with respect to the Crisium basin. Similar dome configurations aligned radial to major impact basins are known from other lunar mare dome fields. The spectral signature of the domes derived from Clementine UV/VIS imagery reveals that they consist of basaltic lava with a low TiO 2 content below 2 wt% and with a FeO content around 10 wt%. Three examined domes exhibit highland components in their soils, which we attribute to lateral mixing between the material in the mare ponds and the surrounding highland material due to random impacts. All five domes have moderate diameters between 10 and 12 km. Condorcet 1–3 are similar to effusive domes of intermediate flank slope between 1 ∘ and 2 ∘ like those situated in the Hortensius/Milichius/T. Mayer region, while Condorcet 4 has an exceptionally steep flank slope of 2 . 8 ∘ and a large volume. With its low flank slope of 0 . 9 ∘ , the dome Dubiago 3 is morphometrically very similar to a known intrusive dome in the west of Mare Serenitatis. Hence, this structure is possibly of intrusive origin, but with the available data an effusive origin cannot be ruled out. Based on a rheologic model, we infer the physical conditions under which the domes were formed (lava viscosity, effusion rate, magma rise speed) as well as the geometries of the feeder dikes.

Physical volcanology of a large crater-complex formed during the initial stages of Karoo flood basalt volcanism, Sterkspruit, Eastern Cape, South Africa

The Sterkspruit Complex, exposed at the base of the Karoo flood basalts in South Africa, is a complex of mostly massive volcaniclastic rocks, dikes and lava that fills a crater > 45 km 2 cut into country rock and is surrounded by a blanket of more-distal bedded volcaniclastic rocks. The moderately to steeply inward-dipping margins of the crater are delineated by country-rock breccia that grades into rocks within the complex. Intra-complex rocks are dominated by massive, polymict lapilli tuff and tuff breccia > 150 m thick, with intercalated lavas, and minor bedded volcaniclastic rocks. Pillow lava > 50 m thick and thinly bedded, inferred lacustrine deposits cap the complex succession and are in turn covered by subaerial Karoo flood basalts. Beyond the crater a succession of bedded, finer-grained volcaniclastic rocks (100–60 m thick) rests on country rock and thins distally over at least 5 km. Magma emplaced over a broad area through a network of feeder dikes and sheets created many small vents that grew into their neighbours through lateral quarrying and vent migration, and whose deposits overlap both within and outside the complex. An abrupt end to eruptions that formed volcaniclastic deposits is recorded by flooding of the lowest part of the crater by first water then lava, then subsequently by inundation of the complex and the surrounding landscape by voluminous effusion of flood basalt. Formation of the Sterkspruit Complex, and many others like it in South Africa, confirms that the opening phases of Karoo flood basalt volcanism were partly explosive. The abundant volcaniclastic products at Sterkspruit and in other flood basalt provinces result from eruptions that disperse ash and volcanic aerosols differently than effusive eruptions of lava, and thus may have important implications for climate change and landscape development associated with the initial stages of flood basalt provinces.

Influence of pre-salt alignments in post-Aptian magmatism in the Cabo Frio High and its surroundings, Santos and Campos basins, SE Brazil: An example of non-plume-related magmatism

This study, based on interpretations of 2D and 3D seismic data, onshore satellite data, and offshore seismic mapping and well data, addresses the geological evolution of Late Cretaceous and Early Tertiary magmatism in the offshore Cabo Frio area, at the border between the Santos and Campos basins, SE Brazil. The analysis of the available data set suggests an alternative genesis for this magmatism, different from the classic mantle plume model. Seismic mapping of the main feeder dikes reveals a predominant SE–NW orientation, coincident with the alignment of the Cruzeiro do Sul deformation zone, in which the Cabo Frio High is included. This observation is coherent with the characteristics of the major onshore Precambrian strike-slip faults, which were reactivated during the Cenozoic. The largest offshore volumes of magmatic rocks occur intercalated in the sedimentary section and at the intersections of two fault alignments: a SE–NW, strike-slip fault trend and a SW–NE, normal fault trend, similar to the onshore alkaline bodies of the Poços de Caldas–Cabo Frio alignment. The possibly reactivated faults (mainly SE–NW transfer faults) could have cut through the whole lithosphere, reached the asthenosphere, and caused partial melting simply by pressure release.

Shallow plumbing systems for small-volume basaltic volcanoes

Eruptive dynamics in basaltic volcanoes are controlled, in part, by the conduit geometry. However, uncertainties in conduit shape and dike-to-conduit transition geometry have limited our predictive capability for hazards assessments. We characterize the subvolcanic geometry of small-volume basaltic volcanoes (magmatic volatile-driven eruptions, 0.1 to 0.5 km3) based on a synthesis of field studies of five basaltic volcanoes exposed to varying degrees by erosion and exhibiting feeder dikes, conduits, and vent areas ≤250 m depth. Study areas include East Grants Ridge (New Mexico, USA), Basalt Ridge, East Basalt Ridge, Paiute Ridge, and Southeast Crater Flat (Nevada, USA). Basaltic feeder dikes 250 to 100 m deep have typical widths of 4–12 m, with smooth host-rock contacts (rhyolite tuff). At depths less than 100 m, heterogeneities in the host rock form preferential pathways for small dike splays and sills, resulting in a 30-m effective width at 50 m depth. The development of a complex conduit at depths less than 70 m is reflected in bifurcating dikes and brecciation and incorporation of the country rock. The overall zone of effect at depths less than 50 m is ≤110 m wide (220 m elongated along the feeder dike). Based on comparisons with theoretical conduit flow models, the width of the feeder dike at depths from 250 to 500 m is expected to range from 1 to 10 m and is expected to decrease to about 1–2 m at depths greater than 500 m. The flaring shape of the observed feeder systems is similar to results of theoretical modeling using lithostatic pressure-balanced flow conditions. Sizes of observed conduits differ from modeled dimensions by up to a factor of 10 in the shallow subsurface (<50 m depth), but at depths greater than 100 m the difference is a factor of 2 to 4. This difference is primarily due to the fact that observed eroded conduits record the superimposed effects of multiple eruptive events, while theoretical model results define dimensions necessary for a single, steady eruption phase. The complex details of magma-host rock interactions observed at the study areas (contact welding, brecciation, bifurcating dikes and sills, and stoping) represent the mechanisms by which the lithostatic pressure-balanced geometry is attained. The similarity in the normalized shapes of theoretical and observed conduits demonstrates the appropriateness of the pressure-balanced modeling approach, consistent with the conclusions of Wilson and Head (J Geophys Res 86:2971–3001, 1981) for this type of volcano.

Carbonatites in a subduction system: The Pleistocene alvikites from Mt. Vulture (southern Italy)

We report here, for the first time, on the new finding of extrusive calciocarbonatite (alvikite) rocks from the Pleistocene Mt. Vulture volcano (southern Italy). These volcanic rocks, which represent an outstanding occurrence in the wider scenario of the Italian potassic magmatism, form lavas, pyroclastic deposits, and feeder dikes exposed on the northern slope of the volcano. The petrography, mineralogy and whole-rock chemistry attest the genuine carbonatitic nature of these rocks, that are characterized by high to very high contents of Sr, Ba, U, LREE, Nb, P, F, Th, high Nb/Ta and LREE/HREE ratios, and low contents of Ti, Zr, K, Rb, Na and Cs. The O–C isotope compositions are close to the “primary igneous carbonatite” field and, thus, are compatible with an ultimate mantle origin for these rocks. The Sr–Nd–Pb–B isotope compositions, measured both in the alvikites and in the silicate volcanic rocks, indicate a close genetic relationship between the alvikites and the associated melilitite/nephelinite rocks. Furthermore, these latter products are geochemically distinct from the main foiditic-phonolitic association of Mt. Vulture. We propose a petrogenetic/geodynamic interpretation which has important implications for understanding the relationships between carbonatites and orogenic activity. In particular, we propose that the studied alvikites are generated through liquid unmixing at crustal levels, starting from nephelinitic or melilititic parent liquids. These latter were produced in a hybrid mantle resulting from the interaction through a vertical slab window, between a metasomatized mantle wedge, moving eastward from the Tyrrhenian/Campanian region, and the local Adriatic mantle. The occurrence of carbonatite rocks at Mt. Vulture, that lies on the leading edge of the Southern Apennines accretionary prism, is taken as an evidence for the carbonatation of the mantle sources of this volcano. We speculate that mantle carbonatation is related to the introduction of sedimentary carbon from the Adriatic lithosphere during Tertiary subduction.

Vent geometry and eruption conditions of the mixed rhyolite–basalt Námshraun lava flow, Iceland

We describe the morphology and circumstances of eruption of the mixed rhyolite–basalt lava flow Námshraun in the Torfajökull–Vei∂ivötn area of central Iceland. The unusual location and exposure of the elongate fissure vent permits its length along strike (a total of ∼ 275 m) and the width of the dyke feeding it (up to ∼ 10 m) to be estimated in the field. Using analyses of the heat losses during the rise of the mixed magma through the shallow part of its conduit system, we are able to refine the absolute minimum dyke width estimate to ∼ 1.5 m. The lengths of the two main lava flow lobes, assuming that their advance was cooling-limited, imply that the volume effusion rate of the lava varied between ∼ 2.7 and ∼ 1.5 m 3 s − 1 as different parts of the fissure became active. Prior to its emergence at the surface the magma had at most a small yield strength (probably significantly less than ∼ 3000–4000 Pa) and a near-Newtonian viscosity in the range ∼ 1 × 10 4 to ∼ 5 × 10 6 Pa s. After its eruption, the lava formed flows with marginal levées whose sizes imply a yield strength just less than 30 kPa. The lava in the central channels between the levées can be modeled either as a Newtonian fluid with a viscosity of between 3 × 10 7 and 6 × 10 7 Pa s or as a Bingham plastic. Estimates of the plastic viscosity from the two main flow lobes (< 10 4 to ∼ 6 × 10 5 and 1.2 × 10 7 to 1.8 × 10 7 Pa s) differ by a very large factor (at least 30) and are regarded as unreliable; however, they lead to a much smaller range of apparent viscosities, from ∼ 1.5 × 10 7 to ∼ 5.5 × 10 7 Pa s, values very similar to the viscosities found when the rheology is assumed to be Newtonian. If the field estimate of the dyke width is reliable, these results imply that the viscosity (and yield strength) of the magma averaged over the path from its source to the surface had increased by a factor close to 10 by the time that it emerged from the vent; alternatively the feeder dike may have been almost twice as wide during the eruption and relaxed to the presently exposed width as the eruption ended. The typical advance speeds of the two main flow lobes were less than 4 mm s − 1 and their emplacement times were ∼ 2.5 and ∼ 5 days. The implications for the sizes of the conduits feeding other rhyolitic and mixed lavas in central Iceland are discussed.

Formation of lunar mare domes along crustal fractures: Rheologic conditions, dimensions of feeder dikes, and the role of magma evolution

In this study we examine a set of lunar mare domes located in the Hortensius/Milichius/T. Mayer region and in northern Mare Tranquillitatis with respect to their formation along crustal fractures, their rheologic properties, the dimensions of their feeder dikes, and the importance of magma evolution processes during dome formation. Many of these domes display elongated summit vents oriented radially with respect to major impact basins, and several dome locations are also aligned in these preferential directions. Analysis of Clementine UV/VIS and Lunar Prospector gamma ray spectrometer data reveals that the examined mare domes formed from low-Si basaltic lavas of high FeO and low to moderate TiO 2 content. Based on their morphometric properties (diameter, height, volume) obtained by photoclinometric and shape from shading analysis of telescopic CCD images, we derive rheologic quantities (lava viscosity during eruption, effusion rate, duration of the effusion process, magma rise speed) and the dimensions of the feeder dikes. We establish three rheologic groups characterised by specific combinations of rheologic properties and dike dimensions, where the most relevant discriminative parameter is the lava viscosity η. The first group is characterised by 10 4 Pa s < η < 10 6 Pa s and contains the domes with elongated vents in the Milichius/T. Mayer region and two similar domes in northern Mare Tranquillitatis. The second group with 10 2 Pa s < η < 10 4 Pa s comprises the very low aligned domes in northern Mare Tranquillitatis, and the third group with 10 6 Pa s < η < 10 8 Pa s the relatively steep domes near Hortensius and in the T. Mayer region. The inferred dike dimensions in comparison to lunar crustal thickness data indicate that the source regions of the feeder dikes are situated within the upper crust for six of the domes in northern Mare Tranquillitatis, while they are likely to be located in the lower crust and in the upper mantle for the other examined domes. By comparing the time scale of magma ascent with the time scale on which heat is conducted from the magma into the host rock, we find evidence that the importance of magma evolution processes during ascent such as cooling and crystallisation increases with lava viscosity. We conclude that different degrees of evolution of initially fluid basaltic magma are able to explain the broad range of lava viscosities inferred for the examined mare domes. The spectral data reveal that differences in TiO 2 content may additionally account for the systematic difference in lava viscosity between the two examined lunar regions. We show that the described mechanisms are likely to be valid also for other lunar mare domes situated near Cauchy and Arago, regarded for comparison. On the other hand, we find for the Gruithuisen and Mairan highland domes that despite their inferred high lava viscosities of 10 8 – 10 9 Pa s , no significant magma cooling in the dike occurred during ascent, supporting previous findings that the highland domes were formed during a specific phase of non-mare volcanism by highly silicic viscous lavas.

Lunar domes in the Doppelmayer region: Spectrophotometry, morphometry, rheology, and eruption conditions

In this study we examine a lunar volcanic region near Doppelmayer, composed of two domical structures previously not studied in detail and a well-known lunar pyroclastic deposit. Dome 1 is situated at selenographic coordinates 41 . 92 ∘ W and 30 . 08 ∘ S , dome 2 at 43 . 42 ∘ W and 30 . 66 ∘ S . We perform a spectrophotometric study of representative locations of the volcanic region based on Clementine UVVIS data. Relying on ground-based high-resolution CCD imagery, we furthermore examine the morphometric characteristics of the two domes, making use of a combined photoclinometry and shape from shading technique. Based on a rheologic model, we examine the physical conditions under which the domes were formed. Dome 1 is spectrally atypically red for mare domes and shows a very weak mafic absorption, implying a low TiO 2 and FeO content. The overall spectral signature corresponds to that of a mixture between mare and highland soils. Dome 1 was formed of lava with a relatively high viscosity value of ∼ 10 7 Pa s , situated between the ranges of values typically observed for mare domes and for the Gruithuisen and Mairan highland domes, respectively, erupting at moderate rates over a long period of time. Dome 1 is larger, steeper, and more voluminous than typical mare domes but has a much lower flank slope than the Gruithuisen and Mairan highland domes. It is an exemplar of a rare type of unusually steep and voluminous mare domes, similar to the well-known mare domes Hortensius 5 and 6 and Herodotus ω . We discuss the relevance of vertical (assimilation of crustal material) vs. lateral (distribution of material across mare-highland boundaries by random impacts) mixing mechanisms as being responsible for the observed spectral appearance of dome 1. The thermal conditions in the lunar interior did not favour the assimilation of crustal wallrock into the ascending magma. Due to the fact that dome 1 is located right on the boundary between hummocky terrain and a mare pond, lateral mixing of mare and highland soils is a much more natural explanation for the observed spectral signature. For dome 2, we find that it is a typical effusive mare dome, given its spectral and morphometric properties and inferred rheologic parameters. An estimation of the dimensions of the feeder dikes of the two domes reveals that the dike which formed dome 1 was five times as broad as the one which formed dome 2, while the dike lengths only differ by about one third. The dike dimensions suggest that their source regions were located below the lunar crust.

Middle-Late Permian mass extinction on land

The end-Permian mass extinction has been envisaged as the nadir of biodiversity decline due to increasing volcanic gas emissions over some 9 million years. We propose a different tempo and mechanism of extinction because we recognize two separate but geologically abrupt mass extinctions on land, one terminating the Middle Permian (Guadalupian) at 260.4 Ma and a later one ending the Permian Period at 251 Ma. Our evidence comes from new paleobotanical, paleopedological, and carbon isotopic studies of Portal Mountain, Antarctica, and comparable studies in the Karoo Basin, South Africa. Extinctions have long been apparent among marine invertebrates at both the end of the Guadalupian and end of the Permian, which were also times of warm-wet greenhouse climatic transients, marked soil erosion, transition from high- to low-sinuosity and braided streams, soil stagnation in wetlands, and profound negative carbon isotope anomalies. Both mass extinctions may have resulted from catastrophic methane outbursts to the atmosphere from coal intruded by feeder dikes to flood basalts, such as the end-Guadalupian Emeishan Basalt and end-Permian Siberian Traps.