Marco Brenna

Modern analogues for Miocene to Pleistocene alkali basaltic phreatomagmatic fields in the Pannonian Basin: “soft-substrate” to “combined” aquifer controlled phreatomagmatism in intraplate volcanic fields Research Article

The Pannonian Basin (Central Europe) hosts numerous alkali basaltic volcanic fields in an area similar to 200 000 km2. These volcanic fields were formed in an approximate time span of 8 million years producing smallvolume volcanoes typically considered to be monogenetic. Polycyclic monogenetic volcanic complexes are also common in each field however. The original morphology of volcanic landforms, especially phreatomagmatic volcanoes, is commonly modified. by erosion, commonly aided by tectonic uplift. The phreatomagmatic volcanoes eroded to the level of their sub-surface architecture expose crater to conduit filling as well as diatreme facies of pyroclastic rock assemblages. Uncertainties due to the strong erosion influenced by tectonic uplifts, fast and broad climatic changes, vegetation cover variations, and rapidly changing fluvio-lacustrine events in the past 8 million years in the Pannonian Basin have created a need to reconstruct and visualise the paleoenvironment into which the monogenetic volcanoes erupted. Here phreatomagmatic volcanic fields of the Miocene to Pleistocene western Hungarian alkali basaltic province have been selected and compared with modern phreatomagmatic fields. It has been concluded that the Auckland Volcanic Field (AVF) in New Zealand could be viewed as a prime modern analogue for the western Hungarian phreatomagmatic fields by sharing similarities in their pyroclastic successions textures such as pyroclast morphology, type, juvenile particle ratio to accidental lithics. Beside the AVF two other, morphologically more modified volcanic fields (Pali Aike, Argentina and Jeju, Korea) show similar features to the western Hungarian examples, highlighting issues such as preservation potential of pyroclastic successions of phreatomagmatic volcanoes.

Diffusion-zoned pyroxenes in an isotopically heterogeneous mantle lithosphere beneath the Dunedin Volcanic Group, New Zealand, and their implications for intraplate alkaline magma sources

An important task in assessing the magma source of an intraplate volcanic province is establishing the composition of the underlying lithospheric mantle. Pyroxenes in peridotite xenoliths from the ∼10,000 km2 Dunedin Volcanic Group in New Zealand reveal that the underlying lithospheric mantle is chemically and isotopically heterogeneous and has a complex thermal history. Portions of this mantle have light rare earth element–depleted clinopyroxene trace element concentrations and distinctly radiogenic Nd compositions (eNd(20 Ma) ≥ + 15.5) with model depleted mantle ages that are ≥100 m.y. older than the overlying Jurassic crust (type 1). The Nd isotopic composition of these moderately fertile domains is distinct from any Dunedin Volcanic Group magma, but the domains are embedded within enriched peridotitic mantle (type 2) that has formed through reaction with a light rare earth element–rich fluid that imparted an isotopic composition in strongly metasomatized xenoliths of 87Sr/86Sr(20 Ma) = 0.7028–0.7029, eNd(20 Ma) = + 5.0 to + 5.1, and 206Pb/204Pb = 19.9, 207Pb/204Pb = 15.5, and 208Pb/204Pb = 39.6. These isotope ratios overlap with the isotopically homogeneous high time-integrated U/Pb–like source signature of the host Dunedin Volcanic Group. However, all metasomatic and nonmetasomatic pyroxenes are zoned in temperature-sensitive elements (Al, Cr, Mg ± Ca), with trends indicating element exchange during cooling and the results of diffusion calculations implying that the zoning formed over hundreds of thousands to millions of years prior to late Oligocene–Miocene xenolith entrainment. These data, along with calculated pyroxene rare earth element homogenization diffusion rates, indicate that mantle metasomatism predated entrainment in the host magmas by millions of years. Furthermore, the presence of the cooling trends in all but one sample indicates that this upper lithosphere mantle preserves little or no sign of a rise in the geotherm at the time of magmatism. Zoning patterns in peridotite pyroxenes can therefore provide useful insight into the role of portions of the lithospheric mantle in formation of intraplate alkaline basalts.

Genesis of the world’s largest rare earth element deposit, Bayan Obo, China: Protracted mineralization evolution over ∼1 b.y.

The unique, giant, rare earth element (REE) deposit at Bayan Obo, northern China, is the world’s largest REE deposit. It is geologically complex, and its genesis is still debated. Here, we report in situ Th-Pb dating and Nd isotope ratios for monazite and Sr isotope ratios for dolomite and apatite from fresh drill cores. The measured monazite ages (361–913 Ma) and previously reported whole-rock Sm-Nd data show a linear relationship with the initial Nd isotope ratio, suggesting a single-stage evolution from a Sm-Nd source that was formed before 913 Ma. All monazites show consistent to those of the adjacent 1.3 Ga carbonatite and mafic dikes. The primary dolomite and apatite show lower than the recrystallized dolomite (0.7038–0.7097). The REE ores at Bayan Obo are interpreted to have originally formed as products of ca. 1.3 Ga carbonatitic magmatism and to have undergone subsequent thermal perturbations induced by Sr-rich, but REE-poor, metamorphic fluids derived from nearby sedimentary rocks.eNd(1.3Ga) values (0.3 ± 0.6) close87Sr/86Sr ratios (0.7024–0.7030)

Conceptual Development of a National Volcanic Hazard Model for New Zealand

We provide a synthesis of a workshop held in February 2016 to define the goals, challenges and next steps for developing a national probabilistic volcanic hazard model for New Zealand. The workshop involved volcanologists, statisticians, and hazards scientists from GNS Science, Massey University, University of Otago, Victoria University of Wellington, University of Auckland, and University of Canterbury. We also outline key activities that will develop the model components, define procedures for periodic update of the model, and effectively articulate the model to end-users and stakeholders. The development of a National Volcanic Hazard Model is a formidable task that will require long-term stability in terms of team effort, collaboration and resources. Development of the model in stages or editions that are modular will make the process a manageable one that progressively incorporates additional volcanic hazards over time, and additional functionalities (e.g. short-term forecasting). The first edition is likely to be limited to updating and incorporating existing ashfall hazard models, with the other hazards associated with lahar, pyroclastic density currents, lava flow, ballistics, debris avalanche, and gases/aerosols being considered in subsequent updates.

Mantle heterogeneity controls on small-volume basaltic volcanism: COMMENT

A recent article published in Geology by McGee et al. (2015) investigates the relationship between chemical characteristics and erupted volumes in the Auckland Volcanic Field (AVF), New Zealand. In the study, five eruptions from a possible 53 were selected from the field to provide insight into mantle source properties and processes. Based on this sample suite, a strong correlation is hypothesized between eruption volumes and chemical parameters used to draw a number of conclusions on the timing and nature of mantle melting. We here wish to question the selection of the small sample suite and the robustness of the volumecorrelation conclusions compared to using a more comprehensive AVF dataset available in the literature. Rare earth element (REE) data are available for 14/53 eruptions with major element data available for 41/53 eruptions in the AVF (Heming and Barnet, 1986; Huang et al., 1997; McGee et al., 2013; Needham et al., 2011; Searle, 1960; Smith et al., 2008). From this suite, we show here that P2O5 and REE concentrations correlate extremely well (R = 0.96; Figs. 1A and 1B) and that P2O5 is thus a robust proxy for REE abundances for the eruptions without trace element data. When the most primitive sample for each eruption is plotted (highest MgO content, which is >10wt% for most eruptions), any fractional crystallization effect is minimized. Using this proxy, the conclusions of McGee et al. (2015) can be tested on 41/53 eruptions in the field, rather than five eruptions selected based on unclear criteria.

Dyke-diatreme transition in monogenetic volcanoes: insights from the Hillier Bay volcanic complex, Western Australia

The factors controlling phreatomagmatism and diatreme formation are still poorly constrained and understood. Here, we describe the field relationships between mafic intrusions and volcaniclastic deposits observed at Hillier Bay, Western Australia, and discuss the implications for the formation of monogenetic basaltic volcanoes involving both phreatomagmatic and magmatic eruption phases. The Hillier Bay volcanic complex consists of a series of basaltic sheeted dykelets and larger dykes injected within the metamorphic basement. Volcaniclastic lithologies also occur, usually trapped between basaltic dykes and the basement. These vary between mixtures of juvenile basaltic fragments, metamorphic basement fragments and quartz/feldspar sand in different relative amounts. Based on the textures of the clastic lithologies, we argue that initial phreatomagmatic phases resulted from sequential injections of thin dykelets due to ascending magma struggling to open a path to the surface. The relatively lower magma/water ratio maximises the efficiency of premixing leading to phreatomagmatic explosions. Later, as the main body of magma reaches the shallow conduit through wider dykes, magma overcomes water availability, and the eruption style switches to magmatic/effusive. This implies that magma flux may be a determining factor controlling eruption style in at least some monogenetic volcanoes.

L. Beccaluva, G. Bianchini and M. Wilson (eds): Volcanism and evolution of the African lithosphere

Eastern Africa is the cradle of mankind as well as being the cradle of a new ocean, at the Great Rift Valley, where East Africa is breaking apart along a volcanically active spreading centre. Beyond the rift, the African continent and tectonic plate encompass a great variety of other tectonic settings and volcanic features. This collection of 16 articles does a good job of presenting all different aspects of the geological life of a tectonic plate from its rifting to create new oceans to its destruction through subduction. Each article forms a standalone chapter, but altogether, they are woven so as to present a journey through the whole tectonic plate. The book results from the symposium “Cenozoic volcanism and evolution of the African lithosphere” held at the 33rd International Geological Congress in Oslo, Norway, in August 2008. The first half of the book is dedicated to the rifting of East Africa, with six chapters focusing on the lithosphere’s evolution prior to and during the rifting event. These studies mainly investigate the lithospheric structure through the geochemistry of mantle xenoliths and lava erupted in the rift region and through seismic surveys. Two further chapters focus on magmatic aspects of volcanic activity along the rift system, at Kilimanjaro and Oldoinyo Lengai. Each article in the remainder of the book treats a distinct aspect of the African lithosphere, mainly through the study of the geochemistry and mineralogy of volcanic and xenolith suites from Cameroon to Angola, from Madagascar to Cape Verde, and ending in the Tyrrhenian Sea. On the magmatic aspect, there is something for every taste, from basanite to rhyolite, from tholeiite to kimberlite, from syenite to granite and also a good account of carbonatitic and dolomitic volcanism (the latter being only one among three such cases recorded worldwide). As well as the breadth of themes treated, the information on very rare occurrences, such as the dolomitic volcanism, makes this book an excellent opportunity to learn about poorly understood and researched phenomena. Given the wide coverage of geological aspects, there is no broad introduction to the tectonic and magmatic settings of the African plate, though each article gives a brief but comprehensive introduction and background to the particEditorial responsibility: K. Németh