The roles of magma and groundwater in the phreatic eurptions at Inyo Craters,Long Valley Caldera,California

The Inyo Craters (North Inyo Crater and South Inyo Crater), and a third crater, Summit Crater, are the largest of more than a dozen 650- to 550-yr-B.p. phreatic craters that lie in a 1-km-square area at the south end of the Inyo Volcanic Chain, on the west side of the Long Valley Caldera in eastern California. The three craters are aligned within a 1-km-long northsouth system of fissures and normal faults, and coincide in age with aligned magmatic vents farther north in the Inyo Volcanic Chain, suggesting that they were all produced by intrusion of one or more dikes. To study the sequence and mechanisms of the eruptions, the deposits were mapped, sampled, and compared with subsurface stratigraphy obtained from the core of a slant hole drilled directly below the center of South Inyo Crater from the southwest. The deposits from the two Inyo Craters are fine-grained (median diameter less than 1 mm), are several meters thick at the crater walls, and cover at most a few km² of ground surface. Stratigraphic relationships between the Inyo Craters and Summit Crater indicate that the eruptions proceeded from north to south, overlapped slightly in time, and produced indistinctly plane-parallel bedded, poorly sorted deposits, containing debris derived primarily from within 450 m of the surface. Debris from the deepest identifiable unit (whose top is at 450 m depth) is present at the very base of both Inyo Craters deposits, suggesting that the eruptive vents were open and tapping debris from at least that depth, probably along preexisting fractures, even at their inception. According to ballistic studies, the greatest velocity of ejected blocks was of the order of 100 m/s. All eruptions, particularly the least powerful, selectively removed debris from the finest-grained, most easily eroded subsurface units. Although juvenile fragments have been previously identified in these deposits, they are confined primarily to the grain-size fraction smaller than 0.25 mm dia. and probably did not constitute more than several percent of the deposit. It is therefore suggested that these juvenile fragments were not the main source of heat for the eruptions, and that the eruptions were caused either by: (1) heating of water by fragmented magma that was not ejected before the eruption shut off; (2) slow heating (over months to years) of groundwater under confined conditions without fragmentation of magma, followed by a second process (pressure buildup, seismic faulting, or intrusions) that breached the confinement; or (3) breach of a pre-existing confined geothermal aquifer.     

Improved prediction and tracking of volcanic ash clouds

During the past 30 years, more than 100 airplanes have inadvertently flown through clouds of volcanic ash from erupting volcanoes. Such encounters have caused millions of dollars in damage to the aircraft and have endangered the lives of tens of thousands of passengers. In a few severe cases, total engine failure resulted when ash was ingested into turbines and coating turbine blades. These incidents have prompted the establishment of cooperative efforts by the International Civil Aviation Organization and the volcanological community to provide rapid notification of eruptive activity, and to monitor and forecast the trajectories of ash clouds so that they can be avoided by air traffic. Ash-cloud properties such as plume height, ash concentration, and three-dimensional ash distribution have been monitored through non-conventional remote sensing techniques that are under active development. Forecasting the trajectories of ash clouds has required the development of volcanic ash transport and dispersion models that can calculate the path of an ash cloud over the scale of a continent or a hemisphere. Volcanological inputs to these models, such as plume height, mass eruption rate, eruption duration, ash distribution with altitude, and grain-size distribution, must be assigned in real time during an event, often with limited observations. Databases and protocols are currently being developed that allow for rapid assignment of such source parameters. In this paper, we summarize how an interdisciplinary working group on eruption source parameters has been instigating research to improve upon the current understanding of volcanic ash cloud characterization and predictions. Improved predictions of ash cloud movement and air fall will aid in making better hazard assessments for aviation and for public health and air quality.     

U–Pb perovskite ages for brazilian kamafugitic rocks: further support for a temporal link to a mantle plume hotspot track

We present the results of a U–Pb perovskite age study of kamafugites from Mata da Corda (MC) and Santo Antônio da Barra (SAB), Minas-Goiás alkaline province, Brazil. Perovskite crystals were separated from MC mafurites, ugandites, and cognate pyroxenites, as well as from SAB melilite mafurite. The range of ages of Brazilian kamafugitic samples is 15 Ma. The ²⁰⁶Pb/²³⁸U perovskite ages generally cluster into three age groupings: 88–90, 80–81, and 75–76 Ma. The two younger periods of kamafugitic magmatism occur in the MC area, whereas the older samples are from the SAB area. These new age results provide the first robust evidence of a progressive eastward younging of mafic alkaline magmatism, most likely related to a mantle plume hotspot track.     

1.1 Ga K-rich alkaline plutonism in the SW Grenville Province

U–Pb zircon and baddeleyite dating of six syenitic stocks establishes that the ultrapotassic, potassic alkaline and shoshonitic magmatism with island-arc affinities in the Central Metasedimentary Belt (CMB) of the southwestern Grenville Province, Canada took place between 1089 and 1076 Ma, along a 400-km-long, northeast-trending plutonic belt. These ages indicate that ultrapotassic rocks with arc affinities are not unique to the Phanerozoic. West to east emplacement ages along a northern and southern cross-section of this belt range from 1083±2 Ma (Kensington), through 1081±2 Ma (Lac Rouge) to 1076 _–1 ⁺³ Ma (Loranger) in the north, and from 1089 _–3 ⁺⁴ Ma (loon Lake) and 1088±2 Ma (Calabogie), to 1076±2 Ma (Westport) in the south. Although closely spaced in time, in detail these ages suggest a slight younging of this magmatic activity to the southeast. Integration of the geochronological data with the spatial extent and potassic character of the plutons shows that the K-rich alkaline suite is distinct from the nepheline-syenite belt of the Bancroft terrane and from the syenite-monzonite suite of the Frontenac terrane of the CMB, and it is considered to be a magmatic episode unique to the Elzevir terrane and its Gatineau segment. The timing and the postmetamorphic emplacement of these plutons indicate that the regional greenschist to granulite-facies metamorphism of the country rock (precise age unknown) is older than 1089 Ma throughout the entire Elzevir terrane. The potassic magmatism is interpreted as the initiation of the 1090–1050 Ma Ottawan Orogeny in the Elzevir terrane; thus, the regional metamorphism in this terrane, previously assigned to the Ottawan Orogeny, is an earlier event. The contemporaneous emplacement of this postmetamorphic plutonic belt with Keweenawan volcanism is at variance with current tectonic models which consider the Keweenawan rift to be formed at the same time as regional metamorphism in the CMB.