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We have estimated the geochemical equilibration depth and temperature of the widespread Neogene alkaline basalts in the Cordillera of Alaska, Northwest Canada, and in Mexico using geobarometry on bulk compositions that have been minimally differentiated in upward transit. The method has uncertainties of about [+ or -]10 km and Key words: mantle, lithosphere, asthenosphere, alkaline, lava, depth, Cordillera, Introduction In this study, we report estimates of the depth and temperature of geochemical equilibration of Neogene and younger mafic lavas in Alaska and Northwest Canada using geochemical thermobarometers and [...]

A large dyke of quartz tholeiitic gabbronorite has been mapped for 59 km in southern New Brunswick, Canada, between Lepreau River in the northeast and Indian Island in the southwest. Scattered outcrops occur along a positive aeromagnetic lineament, providing a dyke strike of N42[degrees]E overall (segments N30[degrees]E to N72[degrees]E), dips of 80[degrees] to 90[degrees]NNW, and widths of 4 to 30 m. A [new.sup.40]Ar/[.sup.39]Ar plagioclase age of 201.67 6 0.35 Ma for the Lepreau River Dyke is similar to dates for the massive North Mountain Basalt in the Fundy Basin to the east. The dyke is associated with the Ministers Island and Christmas Cove dykes, which are indistinguishable in chemistry, petrology, and probable age, and we regard them as segments of the same co-magmatic dyke system. In addition, their petrology is similar to that of the basalts of the adjacent Early Mesozoic Fundy and Grand Manan basins. We propose that the Lepreau River and associated dykes were sources for the regional basin basalts, which in turn are part of the Central Atlantic Magmatic Province (CAMP) that overlaps the Triassic-Jurassic boundary and associated mass extinction event. Key words: Mesozoic dykes, Central Atlantic Magmatic Province (CAMP), flood basalt, Fundy Basin. Un grand dyke de gabbronorite tholeitique quartzifere a ete cartographie sur une distance de 59 km dans le sud du Nouveau-Brunswick (Canada) entre la riviere Lepreau, au nord-est, et l'ile Indian, au sud-ouest. Des affleurements disperses sont presents le long d'un lineament aeromagnetique positif, qui donnent une orientation globale de N[42.sup.o]E pour le dyke (les differents troncons allant de N30[degrees]E a N72[degrees]E), des pendages de 80[degrees] a 90[degrees] vers le NNO et des largeurs de 4 m a 30 m. Un nouvel [age.sup.40]Ar/[.sup.39]Ar sur plagioclase de 201,67 6 0,35 Ma pour le dyke de la riviere Lepreau est semblable a des ages obtenus pour le basalte massif de North Mountain, dans le bassin de Fundy plus a l'est. Le dyke est associe aux dykes de l'ile Minister et de Christmas Cove, dont la chimie, la petrologie et l'age probable sont essentiellement identiques, et nous estimons qu'ils constituent des troncons d'un meme systeme de dykes comagmatiques. Leur petrologie est en outre semblable a celle des basaltes des bassins de Fundy et de Gand Manan attenants, d'age mesozoique precoce. Nous proposons que le dyke de la riviere Lepreau et les dykes associes constituaient des sources pour les basaltes de bassin regionaux qui, eux, font partie de la province magmatique centre atlantique (PMCA) qui chevauche la limite Trias-Jurassique et l'episode d'extinction massive associe. [Traduit par la Redaction] Mots-cles : dykes mesozoiques, province magmatique centre atlantique (PMCA), basalte de plateau, bassin de Fundy., Introduction Flood basalts in large igneous provinces are produced by fissure eruptions, which can be exceptionally large (tens to hundreds of kilometres long), and with huge eruption volumes of tens [...]

A vast portion of the plumbing system of the Siberian Traps Large Igneous Province (STLIP) is emplaced in the Tunguska Basin, where borehole data reveal ubiquitous and abundant sills with great lateral extension. These intrusions intersect Cambrian-Ordovician evaporite, carbonate and siliciclastic series, and locally coal-bearing Permian host rocks, with a high potential for thermogenic gas generation. Here we present new geochemical data from 71 magmatic and 4 sedimentary rock samples from the Tunguska Basin center and periphery, recovered from 15 deep sills intercepted by boreholes. The studied samples are all low-Ti basalt and basaltic andesites, confirming absence of high-Ti and alkaline STLIP magmatism in the Tunguska Basin. The sills derive from picritic parental melts produced by extensive melting of a mantle source with recycled crustal components below a thinned lithosphere (50-60 km), within the spinel stability field. The mantle source was dominantly peridotitic, with enriched pyroxenitic domains formed by recycled lower crust, in agreement with previous models for the main tholeiitic STLIP phase. Limited amounts (up to 5%) of highly radiogenic granitoids or moderately radiogenic metapelites were assimilated in upper crustal magma reservoirs. After emplacement, sills intruded in Cambrian evaporites assimilated marlstones and interacted with the evaporitic host rocks, probably via fluids and brines. This is the first time that such process is described in subvolcanic rocks from all across the volcanic basin. The sills are correlated geochemically with the established chemostratigraphy for the on-craton STLIP lava piles and intrusions (Norilsk region). Sills correlated with the Morongovsky-Mokulaevsky Fm. and the Norilsk-type intrusions are the most voluminous, present all across the central Tunguska Basin, and bear the strongest evidence of interaction with evaporites. Massive discharge of thermogenic volatiles is suggested by explosive pipes and hydrothermal vent structures throughout the Tunguska Basin. We propose that this voluminous pulse of magmatism is a good candidate for the hitherto unidentified early intrusive phase of the STLIP, and may link the deep Tunguska basin sills to the end-Permian environmental crisis., Author(s): S. Callegaro [sup.1], H. H. Svensen [sup.1], E. R. Neumann [sup.1], A. G. Polozov [sup.2], D. A. Jerram [sup.1] [sup.3], F. M. Deegan [sup.4], S. Planke [sup.1] [sup.5], O. [...]

Both spilitic and non-spilitic metabasaltic sills are hosted by greenschist-facies metasediments in the SW Scottish Highlands. Spilitization is mainly characterized by enrichment in [Na.sub.2]O, elevated modal plagioclase and epidote segregations. Mapping of the spatial distribution of spilitic metabasalts reveals an ancient sub-sea-floor fluid cell centred on the extrusive Tayvallich Volcanics. Fluid circulation was most extensive at shallow levels where most sills were spilitized. We attribute this to pervasive flow of saline fluid, which was thermally driven by the cooling suite of lava flows and sills. Spilitization below this lithostratigraphic depth was restricted to only a few sills. Their spilitization is largely unrelated to specific properties of these sills (e.g. width, chemistry or host lithology). We conclude that fluid channelling was an intrinsic property of sub-sea-floor fluid flow either at deeper levels or earlier during fluid circulation. By profiling of the size distributions of relic phenocrysts in a partly spilitized sill, we conclude that spilitization proceeds with the symmetric propagation of a spilitization front from the sill margins towards the sill interior. Based on chemical profiling across the margin of an epidote segregation, we conclude that spilitization is associated with chemical transport on scales ranging from 0.1 to 10m. doi: 10.1144/0016-76492009-130.

North of the Lebanese restraining bend, the northern Dead Sea fault in Syria cuts Late MiocenePliocene intraplate alkali basalts that have [sup.40]Ar/[sup.39]Ar ages ranging between 6.4 [+ or -] 0.1 and 3.7 [+ or -] 0.1 Ma. Despite a wide (40 m) gouge zone in places, only between 5.3 and 16.8 km of Pliocene Recent left-lateral offset occurs along the segment south of Mesyef, where up to 1100 m of down-to-the-east throw has been measured using the base Neogene volcanic rocks as a datum horizon. Although theoretically possible, there is no geological evidence for pre-Pliocene movement along the fault in NW Syria. The fault splays into two transtensional faults bounding the Pliocene A1-Ghab depression and cuts basalts dated at 4.0-3.7 Ma. The minor lateral geological offsets and matching of Mesozoic and Tertiary stratigraphy across the fault show that the northern Dead Sea fault is not a plate boundary, but merely an intraplate fault. The kinematics of NW Syria can be related to a dynamically evolving stress field with time from Miocene NW--SE compression to Pliocene--Recent north--south strike-slip faulting, coupled with minor anti-clockwise rotation, initiation of the Dead Sea left-lateral transtensional strike-slip fault and the transtensional Al-Ghab basin. Supplementary material: Analytical procedures, data tables and concordia diagrams are available at http:// www.geolsoc.org.uk/SUP18423. doi: 10.1144/0016-76492010-009.

The Middle Permian (c. 262 Ma) Emeishan Basalt Formation of SW China is a commonly cited example of a large igneous province (LIP) that formed as a result of a deep-mantle plume impinging on the base of the lithosphere and generating large regional-scale up-doming prior to volcanism. Recently, however, this assertion has been challenged on the grounds that some lava flows close to the centre of the LIP were erupted in a submarine setting. Here we analyse all the available biolithostratigraphical, petrological, geochemical and volcanological information and show that this supports the idea that the terrain was generated by a plume that originated in the mantle. Emeishan basalt magma melted from a hot mantle source with trace element contents that are similar to the source of Icelandic and intraplate ocean island basalts. However, the amount and lateral extent of uplift is significantly less than is predicted by conventional deep-mantle plume models. We conclude that large-scale doming is not a diagnostic feature of mantle plumes; surface topography can be greatly influenced by the type of lower mantle plume (thermal or thermochemical), how it passes through the transition zone, and how it interacts with the lithosphere. doi: 10.1144/0016-76492009-129.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.04.005 Byline: David J. Lawrence (a), William C. Feldman (b), John O. Goldsten (a), Timothy J. McCoy (c), David T. Blewett (a), William V. Boynton (d), Larry G. Evans (e), Larry R. Nittler (f), Edgar A. Rhodes (a), Sean C. Solomon (f) Keywords: Mercury, surface Abstract: MESSENGER Neutron Spectrometer (NS) observations of cosmic-ray-generated thermal neutrons provide the first direct measurements of Mercury's surface elemental composition. Specifically, we show that Mercury's surface is enriched in neutron-absorbing elements and has a measured macroscopic neutron-absorption cross section of 45-81x10.sup.-4 cm.sup.2/g, a range similar to the neutron absorption of lunar basalts from Mare Crisium. The expected neutron-absorbing elements are Fe and Ti, with possible trace amounts of Gd and Sm. Fe and Ti, in particular, are important for understanding Mercury's formation and how its surface may have changed over time through magmatic processes. With neutron Doppler filtering - a neutron energy separation technique based on spacecraft velocity - we demonstrate that Mercury's surface composition cannot be matched by prior models, which have characteristically low abundances of Fe, Ti, Gd, and Sm. While neutron spectroscopy alone cannot separate the relative contributions of individual neutron-absorbing elements, these results provide strong new constraints on the nature of Mercury's surface materials. For example, if all the measured neutron absorption were due to the presence of an Fe-Ti oxide and that oxide were ilmenite, then Mercury's surface would have an ilmenite content of 7-18wt.%. This result is in general agreement with the inference from color imaging and visible-near-infrared spectroscopy that Mercury's overall low reflectance is consistent with a surface composition that is enriched in Fe-Ti oxides. The incorporation of substantial Fe and Ti in oxides would imply that the oxygen fugacity of basalts on Mercury is at the upper range of oxygen fugacities inferred for basalts on the Moon. Author Affiliation: (a) Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA (b) Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA (c) National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA (d) University of Arizona, Lunar and Planetary laboratory, 1629 E. University Blvd., Tucson, AZ 85721, USA (e) Computer Sciences Corporation, Lanham-Seabrook, MD 20706, USA (f) Carnegie Institution of Washington, Washington, DC 20015, USA Article History: Received 23 October 2009; Revised 24 March 2010; Accepted 8 April 2010

Detailed mapping of a portion of the North Spirit Lake greenstone belt reveals a basal basaltic volcanic unit (ca. 3 Ga) with a komatiitic component and strong major and trace element geochemical affinity with typical mid-ocean ridge basalts (e.g., N-MORB). The overlying transitional unit includes basaltic tufts, which have close geochemical affinity to the underlying basalts, and to N-MORB. Interbedded with and overlying the tufts is a thin, laterally continuous sulfide-rich unit with chert interbeds, which is in turn over-lain by interbedded carbonate, chert, and iron-rich sedimentary rocks. The chemical sedimentary rocks of the transitional unit display rare-earth element (REE) patterns and positive Eu anomalies consistent with derivation from fluids associated with submarine hydrothermai vent fields. The sulfide unit is interpreted to represent proximal hydrothermal vent deposits derived from black smoker activity near a spreading center. The overlying carbonate and chert mounds and beds are interpreted as hydrothermal deposits formed at lower temperatures in a more distal setting relative to the spreading center. The carbonate deposits may have formed in an environment analogous to that of the Lost City hydrothermai field (Kelley et al., 2001), where abundant chimneys, dendrites, and beds of carbonate are produced ~15 km from the mid-Atlantic rift axis. The basal volcanic unit consequently is interpreted as ocean-floor basalts formed at an Archean spreading center, with the basaltic tufts representing the last volcanic emanations, and with the chemical sediments representing hydrothermalvents deposits both proximal and distal to the spreading center. The capping siliciclastic unit likely is part of an accretionary wedge, possibly representing ocean closure after a protracted seafloor speading episode. The sedimentary rocks were derived in part, from a weathered tonalite-trondhjemite-granodiorite source. The supracrustal rocks of the North Spirit Lake greenstone belt provide an unusually clear window into some tectonic activity on the primitive Earth, including evidence for the existence of ocean-floor basalts, proximal and distal hydrothermal activity, and possibly a protracted cycle of ocean opening and destruction.

The hydraulic behaviour of meltwater during subglacial basaltic eruptions in temperate ice is of paramount importance in understanding the eruptive processes, lithofacies and architecture of the edifices formed. Hydraulics also determines the timing, location and volume of meltwater discharge, which may be sudden and catastrophic and via subglacial and/or supraglacial routes. Increasing our knowledge of meltwater hydraulics is therefore important for understanding, predicting and mitigating the impact of meltwater release on vulnerable human communities. New observations about eruption-related meltwater hydraulics are presented for well-exposed glaciovolcanic lava-fed deltas on James Ross Island, Antarctica, and accounts of historical eruptions are also re-examined to identify the major meltwater discharge routes. The study provides the first conceptual model for how meltwater escapes supraglacially. In the absence of a crevassed layer (which will dominate any meltwater flow), overflowing may be initiated by enhanced rates of seepage, despite the intrinsically low hydraulic conductivities of snow and fun. Once overflowing is established, the rate of spillway incision is a likely overriding control on the evolution of the system and whether the discharge is unstable (fast) or stable (slower). The James Ross Island sequences demonstrate that meltwater discharge is highly dynamic and may have involved both subglacial and supraglacial escape. Subglacial discharge probably occurs throughout basaltic tuya eruptions but some periods may be dominated by concurrent overflowing. It is still unclear if overflowing systems are sufficiently stable to enable the growth of laterally extensive glaciovolcanic lava-fed deltas. Keywords: subglacial; tuya; basalt; hydraulics; glaciovolcanic; meltwater; supraglacial; James Ross Island

Previously published platinum group element (PGE) and rare earth element data (REE) from a sample suite of the Palaeogene flood basalts of the East Greenland rifted margin are used to approximate primary magma compositions by numerical models of mantle melting. Both high-Ti and low-Ti basalts are found intercalated in the coastal section 'the Sortebre Profile' in central East Greenland, and the apparent lack of mixing between the two series indicates coexistence of two geographically separated melting regions and plumbing systems during continental breakup above the Palaeogene Iceland plume. The lavas show little or no sign of crustal contamination and the limited variation in La/Sm and Cu/Pd ratios can be interpreted to reflect mantle source composition and melting processes. Numerical modelling indicate that the low-Ti series formed by F~20% melting in a columnar melting regime from a slightly depleted upper mantle source with a relatively normal S-content (~180 ppm S). In contrast, the high-Ti series formed by much lower degrees of melting (F~6%) in a spreading-related, triangular melting regime from a relatively S-poor (~100 ppm S) source. The low-Ti suite was S-undersaturated at the stage of melt segregation from a shallow mantle source due to the high degree of melting. In contrast, the high-Ti suite probably formed from a S-poor source where some low degree melt batches were S-saturated at the stage of deep segregation in distal parts of the triangular melting regime. This suite shows a geochemical high pressure garnet-signature and adiabatic decompression could therefore have played a role in keeping the mantle-derived S in solution before Fe-enrichment related to fractional crystallisation also increased the S-capacity of these melts.

Many of the Columbia River flood basalts that erupted in the states of Oregon, Washington, and Idaho during the Miocene are remarkably iron rich (12-17 wt% total FeO) and under volatile-free conditions would have been too dense to erupt through continental crust. Their eruption through >20 km of sialic crust sets limits oil their minimum, pre-eruptive total volatile ([H.sub.2]O + C[O.sub.2]) concentrations. These limits are obtained by quantitatively comparing the average density, of the overlying crustal column with the calculated effect of dissolved [H.sub.2]O or exsolved C[O.sub.2] on the density of various Columbia River tholeiitic basalts as a function of depth. Minimum total volatile ([H.sub.2]O + C[O.sub.2]) concentrations range to >4 wt% and are therefore considerably higher than those found in tholeiitic basalts at oceanic islands, like Hawaii. These conclusions are consistent with the strong enrichments in other incompatible elements (e.g., K, P, Ba) observed in the Columbia River flood basalts. Such high concentrations of [H.sub.2]O + C[O.sub.2] would have had a marked effect on the solidus temperature of the source region, which may explain the remarkably large individual flow volumes (~[10.sup.3] [km.sup.3]) that characterize this flood-basalt province. Keywords: Columbia River, flood basalts, volatiles, melt density.

Mineral dissolution is an important factor in many magmatic processes such as melting, assimilation and magma mixing. Since it is not possible to determine dissolution rates or mechanisms from natural samples, experimental measurements are very useful. However, the geometry of the crystal-melt system can have a large effect on the measured rate, depending on whether the contaminated melt formed during dissolution is gravitationally stable or unstable. This study examines the effects of the crystal-melt geometry on the dissolution rate and mechanism. The experiments were performed using basanite melt and cylinders and spheres prepared from a single crystal of natural quartz. All of the experiments were performed in the piston cylinder apparatus at 0.5 GPa and 1350 [degrees] C. Four crystal-melt geometries were used: (1) quartz cylinders on top of a column of melt; (2) quartz cylinders beneath a column of basanite melt; (3) quartz cylinders in the middle of column of melt; (4) quartz spheres on top of a column of basanite melt. These geometries allow an examination of non-convective, convective and mixed non-convective/ convective dissolution. Sphere experiments were included, as this has been the most commonly used geometry in previous experimental studies. In all of the experiments quartz dissolves directly into the basanite without formation of cristobalite or tridymite. Quartz on top of a column of melt dissolves at a rate almost proportional to the square root of time and forms a silica-rich compositional boundary layer that is gravitationally stable. All of the samples show well-defined compositional gradients in the boundary layer; however, the melt at the interface varies in composition with time and plots of concentration as a function of distance normalized to time show that the diffusion rate of Si[O.sub.2] increases with time. These data suggest that the rate-controlling step during quartz dissolution is interface reaction rather than cation diffusion. Quartz on the bottom of a column of basanite dissolves much more quickly than in the quartz-on-top experiments and the dissolution rate is linear, due to the periodic gravitational instability and resultant convection of the boundary layer. Even though interface kinetics are the rate-controlling step in quartz dissolution, convection causes an increase in dissolution rate because it replenishes the boundary layer with new, silica-undersaturated melt, which dissolves the quartz more quickly than the contaminated melt. These data suggest that the interface reaction rate is controlled by the degree of undersaturation of the solvent melt in the dissolving component. Both quartz-in-middle and quartz sphere experiments dissolve at a rate intermediate between the two extremes and both show a power law rate. Both dissolve by a combination of convective and non-convective dissolution but the sphere experiments are affected by an additional factor. During the experiment the sphere can sink through the capsule causing forced convection which adds another complication to the interpretation of the dissolution rate data. The results of this study indicate that the choice of experiment geometry plays a major role in determining the observed dissolution rate. Mineral spheres, which have been widely used in the past, are not ideal for dissolution studies. Instead, dissolution rates and mechanisms are best determined in the absence of convection. These experiments have an additional advantage in that for diffusion-controlled dissolution, they allow determination of cation diffusivity.

Re-Os isotopes have been measured on postorogenic potassic lavas from the Tibetan Plateau, the Betic domain of southeastern Spain, and the Colorado Plateau of the southwestern United States. Previous work has established that these lavas were all derived from parts of the subcontinental lithospheric mantle that had undergone metasomatic enrichment in incompatible elements, following various degrees of melt depletion. Cratonic depleted subcontinental lithospheric mantle peridotites typically have subchondritic (super 187)Os/(super 188)Os; however, the postorogenic lavas are characterized by radiogenic (super 187)Os/(super 188)Os ratios (0.139-0.559). Simple modeling shows that only very large degrees of melt depletion (>25%-30%) can lower source Re/Os ratios sufficiently to permit time-integrated development of subchondritic (super 187)Os/(super l88)Os ratios. Such processes may have been largely restricted to the older Precambrian, and the peridotite component of the postorogenic lavas source was probably depleted by Keywords: Re-Os isotopes, subcontinental lithospheric mantle, postorogenic lavas.

Samples of hydrothermally altered basaltic rocks from the Mid-Atlantic Ridge, south of Kane Fracture Zone (MARK) were analysed in order to study the processes of hydrothermal alteration in a crustal segment formed at a slow- spreading ridge.The samples were reflective of the MARK environment and were classified into two distinct types. It was suggested that the mineral assemblages in these rocks reflect the total effects of several hydrothermal events. Type I metabasalt formed at 180 to 300 degrees Celsius while type II formed at 250 to over 450 degrees Celsius. Geochemical analysis of the rock types indicate that they were formed under conditions of low water to rock ratio.

An addendum to the paper by Robert A. Berner on inclusion of the weathering of volcanic rocks in the geocarbsulf model is presented since the compiled data for different values of the relative weathering rates of basalts versus granites was missed. This paper shows how usage of different values for the intrinsic weatherability ratio of basalt/granite affects recalculation of the concentration of atmospheric carbon-di-oxide and values for the fraction of silicate weathering contributed by volcanics.

The contribution of ligands during weathering on Earth is discussed. The effects of the ligands generally followed trends in cation-ligand, stability constants, but aromatic ligands are less effective in element mobilization than aliphatic ligands.

A well-known second-order mass extinction took place during the Pliensbachian and Toarcian Stages of the Early Jurassic. First recognized as a minor Pliensbachian peak in the global extinction rate, it has alternatively been interpreted as a regional response to the early Toarcian oceanic anoxic event. Detailed studies established it as a global long-term event spanning five successive ammonoid zones. Here we present a revised time scale based on high-precision U-Pb ages resolved to the zone level, which suggests that elevated extinction rates were sustained for about 4 m.y. and peak extinction occurred at 183 Ma. Recent isotopic dating of flood basalts from the southern Gondwanan Karoo and Ferrar provinces documents a culmination in volcanic activity ca. 183 Ma. The onset of volcanism is recorded as an inflection and start of a rapid rise of the seawater [sup.87]Sr/[sup.86]Sr curve. The synchrony of voluminous flood basalt eruptions and biotic crises, as already noted for three of the major mass extinctions, permits a causal relationship, which in this case may be mediated by widespread oceanic anoxia. Keywords: extinction, Toarcian, flood basalt, oceanic anoxic event, time scale.

The theory that the northern Taiwan volcanic zone is an extension of the Ryukyu volcanic front may not be feasible stemming from geochemical data which reveal wide differences in Nb/Zr and Ba/Zr basaltic rock ratios. Moreover, the termination of magmatism of the northern Taiwan volcanic zone was simply coincidental to the opening of the Okinawa Trough. Data suggest that volcanic rocks older than 2 Ma can be found in both eastern and western sides of the zone. Hence, magmatic activity did not cease due to opening of the Trough.

The article gives details of survey carried out to find out more about the abundance and diversity of microbes that live on seafloor basalt. The survey found much greater diversity of organisms and each environment had its own community of organisms.