Geodynamics of rapid voluminous felsic magmatism through time

Two end member geodynamic settings produce the observed examples of rapid voluminous felsic (rhyolitic) magmatism through time. The first is driven by mantle plume head arrival underneath a continent and has operated in an identifiable and regular manner since at least 2.45 Ga. This style produces high temperature (≤ 1100 °C), low aspect ratio rheoignimbrites and lavas that exhibit high SiO₂/Al₂O₃ ratios, high K₂O/Na₂O ratios, and where available data exists, high Ga/Al₂O₃ ratios (> 1.5) with high F (in thousands of parts per million) and low water content. F concentration is significant as it depolymerizes the silicate melt, influencing the magmas' physical behavior during development and emplacement. These rhyolites are erupted as part of rapidly emplaced (10–15 Myr) mafic LIPs and are formed primarily by efficient assimilation-fractional crystallization processes from a mafic mantle parent. The second is driven by lithospheric extension during continental rifting or back arc evolution and is exclusive to the Phanerozoic. SLIPs (silicic large igneous provinces) develop over periods < 40 Myr and manifest in elongate zones of magmatism that extend up to 2500 km, contrasting with the mafic LIP style. Some of the voluminous felsic magmas within SLIPs appear to have a very similar geochemistry and petrogenesis to that of the rhyolites within mafic LIPs. Other voluminous felsic magmas within SLIPs are sourced from hydrous lower crust, and contrast with those sourced from the mantle. They exhibit lower temperatures (< 900 °C), explosive ignimbrites with lower SiO₂/Al₂O₃ ratios, and lower K₂O/Na₂O ratios. Rapid voluminous felsic magmatism represents both extreme examples of continental growth since the Archean, and also dramatic periods of crustal recycling and maturation during the Phanerozoic.     

Expert assessment of vulnerability of permafrost carbon to climate change

Approximately 1700 Pg of soil carbon (C) are stored in the northern circumpolar permafrost zone, more than twice as much C than in the atmosphere. The overall amount, rate, and form of C released to the atmosphere in a warmer world will influence the strength of the permafrost C feedback to climate change. We used a survey to quantify variability in the perception of the vulnerability of permafrost C to climate change. Experts were asked to provide quantitative estimates of permafrost change in response to four scenarios of warming. For the highest warming scenario (RCP 8.5), experts hypothesized that C release from permafrost zone soils could be 19–45 Pg C by 2040, 162–288 Pg C by 2100, and 381–616 Pg C by 2300 in CO₂ equivalent using 100-year CH₄ global warming potential (GWP). These values become 50 % larger using 20-year CH₄ GWP, with a third to a half of expected climate forcing coming from CH₄ even though CH₄ was only 2.3 % of the expected C release. Experts projected that two-thirds of this release could be avoided under the lowest warming scenario (RCP 2.6). These results highlight the potential risk from permafrost thaw and serve to frame a hypothesis about the magnitude of this feedback to climate change. However, the level of emissions proposed here are unlikely to overshadow the impact of fossil fuel burning, which will continue to be the main source of C emissions and climate forcing.     

1600–1500 Ma hotspot track in eastern Australia: implications for Mesoproterozoic continental reconstructions

Mesoproterozoic A‐type magmatic rocks in the Gawler Craton, Curnamona Province and eastern Mount Isa Inlier, form a palaeo‐curvilinear belt for reconstructed plate orientations. The oldest igneous rocks in the Gawler Craton are the Hiltaba Granite Suite: c. 1600–1575 Ma. The youngest in the Mount Isa Inlier are the Williams‐Naraku Batholiths: c. 1545–1500 Ma. The belt is interpreted as a segment of a hotspot track that evolved between c. 1600 and 1500 Ma. This hotspot track may define a quasilinear part of Australia’s motion between 1636 and 1500 Ma, and suggests that Australia drifted to high latitudes. An implication of this interpretation is that Australia and Laurentia may not have been fellow travellers leading to the formation of Rodinia. A hotspot model for A‐type magmatism in Australia differs from geodynamic models for this style of magmatism on other continents. This suggests that multiple geologic processes may be responsible for the genesis of Proterozoic A‐type magmas.     

A thermodynamic model of high temperature lava vaporization on Io

We modified the MAGMA chemical equilibrium code developed by Fegley and Cameron (1987, Earth Planet. Sci. Lett. 82, 207-222) and used it to model vaporization of high temperature silicate lavas on Io. The MAGMA code computes chemical equilibria in a melt, between melt and its equilibrium vapor, and in the gas phase. The good agreement of MAGMA code results with experimental data and with other computer codes is demonstrated. The temperature-dependent pressure and composition of vapor in equilibrium with lava is calculated from 1700 to 2400 K for 109 different silicate lavas in the ONaKFeSiMgCaAlTi system. Results for five lavas (tholeiitic basalt, alkali basalt, Barberton komatiite, dunite, and a molten type B1 Ca, Al-rich inclusion) are discussed in detail. The effects of continuous fractional vaporization on chemistry of these lavas and their equilibrium vapor are presented. The predicted abundances (relative to Na) of K, Fe, Si, Al, Ca, and Ti in the vapor equilibrated with lavas at 1900 K are lower than published upper limits for Io's atmosphere (which do not include Mg). We predict evaporative loss of alkalis, Fe, and Si during volcanic eruptions. Sodium is more volatile than K, and the Na/K ratio in the gas is decreased by fractional vaporization. This process can match Io's atmospheric Na/K ratio of 10±3 reported by Brown (2001, Icarus 151, 190-195). Silicon monoxide is an abundant species in the vapor above lavas. Spectroscopic searches are recommended for SiO at IR and mm wavelengths. Reactions of metallic vapors with S- and Cl-bearing volcanic gases may form other unusual gases including MgCl₂, MgS, MgCl, FeCl₂, FeS, FeCl, and SiS.     

Age and metasomatic alteration of the Mt Neill Granite at Nooldoonooldoona Waterhole,Mt Painter Inlier,South Australia

Quartz feldspar augen gneisses, quartz augen schists and trondhjemites outcrop at Nooldoonooldoona Waterhole in the southwestern corner of the Proterozoic Mt Painter Inlier, northern Flinders Ranges, South Australia. These rocks were previously interpreted as having different origins and ages. However, we argue that all rock types were the result of deformation and strong metasomatic alteration of one common precursor: the Mt Neill Granite. Our conclusion is based on field observations that show that the different lithologies grade into each other and that intrusive contacts are lacking. Whole rock major and trace element analyses also point to a common protolith. Finally, Pb/Pb dating of magmatic zircons gave the same ca 1576 Ma age for the different rock types. Our findings necessitate a re evaluation of the published regional geology and lithostratigraphy of the Mt Painter Inlier. They also indicate that extreme care should be taken in the classification and genetic interpretation of rocks that have experienced extensive metasomatic alteration, which is common in many high grade terrains in Australia.     

Determinability of NAPL mass by measuring concentrations downstream of a contaminant source

Organic contaminants in aquifers are often present as non-aqueous phase liquids (NAPL), which are long-lasting sources for groundwater contamination. The existing NAPL mass is an important parameter for the persistence of the source, but its determination is difficult. One possible detection method is based on the ideal multicomponent dissolution theory, using aqueous concentrations downstream of a fully mixed NAPL source to calculate its mass. In this publication, the applicability of this method is tested for a source size of about 5 m, using numerical methods. In contrast to fully mixed source zones, on this scale the NAPL sources are not in contact with each other, do not mix and develop independently over time. Highly soluble NAPL components can be depleted or the NAPL phase can be completely exhausted locally, while in other portions of the source zone NAPL is still present with all its components. Hence, the interpretation of the resulting aqueous concentrations downstream using the ideal dissolution theory leads to erroneous NAPL masses of several orders of magnitude in the investigated scenarios.     

Paleogeography of the Upper Rhine Graben (URG) and the Swiss Molasse Basin (SMB) from Eocene to Pliocene

Twenty paleogeographic maps are presented for Middle Eocene (Lutetian) to Late Pliocene times according to the stratigraphical data given in the companion paper by Berger et al. this volume. Following a first lacustrine-continental sedimentation during the Middle Eocene, two and locally three Rupelian transgressive events were identified with the first corresponding with the Early Rupelian Middle Pechelbronn beds and the second and third with the Late Rupelian Serie Grise (Fischschiefer and equivalents). During the Early Rupelian (Middle Pechelbronn beds), a connection between North Sea and URG is clearly demonstrated, but a general connection between North Sea, URG and Paratethys, via the Alpine sea, is proposed, but not proved, during the late Rupelian. Whereas in the southern URG, a major hiatus spans Early Aquitanian to Pliocene times, Early and Middle Miocene marine, brackish and freshwater facies occur in the northern URG and in the Molasse Basin (OMM, OSM); however, no marine connections between these basins could be demonstrated during this time. After the deposition of the molasse series, a very complex drainage pattern developed during the Late Miocene and Pliocene, with a clear connection to the Bresse Graben during the Piacenzian (Sundgau gravels). During the Late Miocene, Pliocene and Quaternary sedimentation persisted in the northern URG with hardly any interruptions. The present drainage pattern of the Rhine river (from Alpine area to the lower Rhine Embayment) was not established before the Early Pleistocene.     

Application of an equilibrium vaporization model to the ablation of chondritic and achondritic meteoroids

We modeled equilibrium vaporization of chondritic and achondritic materials using the MAGMA code. We calculated both instantaneous and integrated element abundances of Na, Mg, Ca, Al, Fe, Si, Ti, and K in chondritic and achondritic meteors. Our results are qualitatively consistent with observations of meteor spectra.     

Structures of Mesocirculations Producing Tornadoes Associated with Tropical Cyclone Frances (1998)

Radar structures of one mesocyclone and one mesocirculation (the term mesocirculation refers to a class of rotating updrafts, which may or may not be as spatially and temporally large as a typical mesocyclone) that developed a total of four tornadoes in association with Tropical Cyclone (TC) Frances 1998 are presented. One tornado developed within an inner rainband near the time of landfall while three of the other tornadoes developed within an outer rainband nearly 24 hours after the landfall. Radar reflectivities of the tornadic circulations averaged upwards of 40 dBZ while Doppler radar wind components directed toward the radar averaged 11 m s−1. It is realized that although TC Frances was a minimal hurricane it spawned several tornadoes (four of which were studied) causing damage exceeding $2 million. These tornadoes were not all located close to the TC center, serving as a caution to forecasters and emergency personnel that the immediate landfalling area is not the only place to watch.While it is difficult to accurately predict the TC tornado location and time of occurrence, the degree of low-level baroclinicity seems to play an important role in tornadogenesis. Another significant finding is that the tornadoes were produced on the inward side of an inner rainband, as well as the inward side of an outer rainband. Consistent with climatology, the forward right quadrant of the TC developed the four tornadoes studied here.The lead author, Professor G. V. Rao died 31 July 2004 at the age of 70. He fell victime to the waves while swimming in Mazatlan, Mexico. This is the last paper he publilshed as lead author.     

The 1999 eruption of Shishaldin Volcano, Alaska: monitoring a distant eruption

Shishaldin Volcano, in the central Aleutian volcanic arc, became seismically restless during the summer of 1998. Increasing unrest was monitored using a newly installed seismic network, weather satellites, and rare local visual observations. The unrest culminated in large eruptions on 19 April and 22-23 April 1999. The opening phase of the 19 April eruption produced a sub-Plinian column that rose to 16 km before rapidly dissipating. About 80 min into the 19 April event we infer that the eruption style transitioned to vigorous Strombolian fountaining. Exceptionally vigorous seismic tremor heralded the 23 April eruption, which produced a large thermal anomaly observable by satellite, but only a modest, 6-km-high plume. There are no ground-based visual observations of this eruption; however we infer that there was renewed, vigorous Strombolian fountaining. Smaller low-level ash-rich plumes were produced through the end of May 1999. The lava that erupted was evolved basalt with about 49% SiO₂. Subsequent field investigations have been unable to find a distinction between deposits from each of the two major eruptive episodes.