Volcanic Crust Research Articles

Widespread seawater circulation in 18–22 Ma oceanic crust: Impact on heat flow and sediment geochemistry

On the basis of heat-flow measurements, seismic mapping, and sediment pore-water analysis, we demonstrate widespread and efficient ventilation of the 18–22 Ma oceanic crust of the northeast equatorial Pacific Ocean. Recharge and discharge appear to be associated with basement outcrops, including seamounts and north-south–trending faults, along which sediment cover thins out and volcanic rocks are exposed. Low-temperature hydrothermal circulation through the volcanic crust leads to the reduction of heat flow through overlying sediments, with measured heat-flow values that are well below those expected from conductive cooling curves for lithosphere of this age. Typically, dissolved pore-water oxygen decreases from the sediment surface downward, reaching minimum values at mid-depth and rising again in the lower part of the cores investigated, clearly indicating oxygen-rich seawater circulation through the oceanic crust underneath the sediments. If the residence time of the circulating fluids in the upper crust is short or the fluid flux is large, oxic conditions may be preserved, and oxygen can diffuse upwards into the sediments. This process, leading to widespread oxic conditions in the near-basement sediments, may cause the oxidation of residual reduced material stored in the deeper sediments, resulting in downward fluxes of the reaction products into the basement and from there back into the oceans. Considering the widespread existence of this type of off-axis ventilation, the net effect of the resulting return flow of reaction products on biogeochemical cycles and element fluxes (e.g., carbon and nitrogen) may be very large.

Seismic structure and segmentation of the axial valley of the Mid‐Cayman Spreading Center

AbstractWe report the results of a two‐dimensional tomographic inversion of marine seismic refraction data from an array of ocean‐bottom seismographs (OBSs), which produced an image of the crustal structure along the axial valley of the ultraslow spreading Mid‐Cayman Spreading Center (MCSC). The seismic velocity model shows variations in the thickness and properties of the young oceanic crust that are consistent with the existence of two magmatic‐tectonic segments along the 110 km long spreading center. Seismic wave speeds are consistent with exhumed mantle at the boundary between these two segments, but changes in the vertical gradient of seismic velocity suggest that volcanic crust occupies most of the axial valley seafloor along the seismic transect. The two spreading segments both have a low‐velocity zone (LVZ) several kilometers beneath the seafloor, which may indicate the presence of shallow melt. However, the northern segment also has low seismic velocities (3 km/s) in a thick upper crustal layer (1.5–2.0 km), which we interpret as an extrusive volcanic section with high porosity and permeability. This segment hosts the Beebe vent field, the deepest known high‐temperature black smoker hydrothermal vent system. In contrast, the southern spreading segment has seismic velocities as high as 4.0 km/s near the seafloor. We suggest that the porosity and permeability of the volcanic crust in the southern segment are much lower, thus limiting deep seawater penetration and hydrothermal recharge. This may explain why no hydrothermal vent system has been found in the southern half of the MCSC.

Silicate mineralogy at the surface of Mercury

NASA’s MESSENGER spacecraft has revealed geochemical diversity across Mercury’s volcanic crust. Near-infrared to ultraviolet spectra and images have provided evidence for the Fe2+-poor nature of silicate minerals, magnesium sulfide minerals in hollows and a darkening component attributed to graphite, but existing spectral data is insufficient to build a mineralogical map for the planet. Here we investigate the mineralogical variability of silicates in Mercury’s crust using crystallization experiments on magmas with compositions and under reducing conditions expected for Mercury. We find a common crystallization sequence consisting of olivine, plagioclase, pyroxenes and tridymite for all magmas tested. Depending on the cooling rate, we suggest that lavas on Mercury are either fully crystallized or made of a glassy matrix with phenocrysts. Combining the experimental results with geochemical mapping, we can identify several mineralogical provinces: the Northern Volcanic Plains and Smooth Plains, dominated by plagioclase, the High-Mg province, strongly dominated by forsterite, and the Intermediate Plains, comprised of forsterite, plagioclase and enstatite. This implies a temporal evolution of the mineralogy from the oldest lavas, dominated by mafic minerals, to the youngest lavas, dominated by plagioclase, consistent with progressive shallowing and decreasing degree of mantle melting over time. The MESSENGER spacecraft has revealed geochemical diversity across Mercury’s surface. Magma crystallization experiments suggest a crustal mineralogy consistent with a transition towards shallower and cooler mantle melting conditions.

The source and tectonic implications of late Carboniferous–early Permian A-type granites and dikes from the eastern Alataw Mountains, Xinjiang: geochemical and Sr–Nd–Hf isotopic constraints

ABSTRACTZircon U–Pb ages, and geochemical, Sr–Nd and zircon Hf isotopic compositions are reported for the A-type granites and dikes in the Alataw Mountains of the northwestern Tianshan Orogenic Belt (NTOB), with the aim of investigating the sources and genesis of A-type granites and dikes. The laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating of A-type granites yielded a concordant weighted mean 206Pb/238U age of 297.4 ± 1.5 and 300.6 ± 0.9 Ma, respectively, defining a late Carboniferous–early Permian magmatic event. Geochemically, the granitic intrusions and dikes are characterized by high SiO2 and total alkalies (K2O + Na2O), high Zr, Nb, Ta content, and Ga/Al ratio with prominent negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate that the granitic intrusions and dikes in the eastern Alataw Mountains are of an A-type affinity. The depleted Nd isotope compositions of the granitic intrusions and dikes are consistent with those of the Carboniferous volcanic rocks in the Alataw Mountains, especially Carboniferous adakites (εNd(t) = +3.6 to +6.6), suggesting that they were likely generated by partial melting of less evolved crustal materials, such as oceanic crust stored in the middle and/or lower crust or Carboniferous volcanic arc crust. The widespread late Carboniferous–early Permian magmatism in the NTOB may have been related to a ridge subduction accompanied by slab roll-back of the subducting plate of the North Tianshan Ocean.

Cross-hole tracer experiment reveals rapid fluid flow and low effective porosity in the upper oceanic crust

Numerous field, laboratory, and modeling studies have explored the flows of fluid, heat, and solutes during seafloor hydrothermal circulation, but it has been challenging to determine transport rates and flow directions within natural systems. Here we present results from the first cross-hole tracer experiment in the upper oceanic crust, using four subseafloor borehole observatories equipped with autonomous samplers to track the transport of a dissolved tracer (sulfur hexafluoride, SF6) injected into a ridge-flank hydrothermal system. During the first three years after tracer injection, SF6 was transported both north and south through the basaltic aquifer. The observed tracer transport rate of ∼2–3 m/day is orders of magnitude greater than bulk rates of flow inferred from thermal and chemical observations and calculated with coupled fluid-heat flow simulations. Taken together, these results suggest that the effective porosity of the upper volcanic crust through which much tracer was transported is <1%, with fluid flowing rapidly along a few well-connected channels. This is consistent with the heterogeneous (layered, faulted, and/or fractured) nature of the volcanic upper oceanic crust.

Xujiaweizi Depression Deep Volcanic Rock Weathering Crust Research

This paper based on the field outcrop ,core data, logging data , physical property data and testing data to research the weathering crust of the deep volcanic in xujiaweizi depression. Take red clay layer that developed on the top of volcanic rock as the identification mark, and take the logging response characteristics of volcanic rock weathering crust as division standard, to research weathering crust of volcanic rock. Results show that recognized 31 wells that developed volcanic rock weathering crust of deep K1yc1 and K1yc3 volcanic of xujiaweizi depression that mainly distributed in Xuzhong and Anda region. Find that the main components of weathering crust are composed of rhyolite, volcanic breccia and tuff, and divide out dissolution and disintegration belt that are in conformity with the actual gas reservoir exploration position.

Weathered volcanic crust and its petroleum geological significance: A case study of the Carboniferous volcanic crust in northern Xinjiang, NW China

Abstract The weathered Carboniferous volcanic crust in the northern Xinjiang was analyzed in this paper, using outcrop and core chemical analysis, oil testing and logging data, in order to clarify the characteristics, distribution and petroleum geological significance of weathered volcanic crusts. A complete weathered volcanic crust consists of a 6 layered structure, comprising of a soil layer, hydrolysis zone, leached zone, disintegration zone I, disintegration zone II and parent rock, all of which have different characteristics and markers. The effective reservoir is developed mainly within the leached zone and disintegration zone I. The duration of weathering controls the thickness of the weathered crust. The crust thickness can reach 550 m deep and dates back to approximately 46.3Ma. The lithology, lithofacies and weathering degree control the properties of the weathered crust reservoir. Faults control the seepage of the weathered crust reservoir: the closer a reservoir is to a fault, the larger is its flow radius and seepage. Weathered volcanic crusts were mostly found at palaeogeomorphic high points, slope zones and fault-developed low-lying points, and oil layers near faults have a larger vertical span. We have established a model to evaluate the degree of weathering and effective reservoir generation of volcanic rock, while the well logging analysis could be used to evaluate weathered crust reservoir effectiveness for wells without core data.

A New Zealand-Specific Pseudospectral Acceleration Ground-Motion Prediction Equation for Active Shallow Crustal Earthquakes Based on Foreign Models

Abstract The applicability of foreign ground‐motion prediction equations (GMPEs) for predicting geometric‐mean pseudospectral acceleration amplitudes from active shallow crustal earthquakes in New Zealand (NZ) is examined. Four different foreign GMPEs were considered, as well as the NZ‐based McVerry et al. (2006) (McV06) model. It was found that the McV06 model exhibited the lowest applicability with a database of 2437 recorded ground motions, and that the Chiou et al. (2010) (C10) modification of the Chiou and Youngs (2008b) (CY08) model was the most applicable. Discrepancies between the C10 model and the NZ database, which were empirically identified and theoretically justified, were used to modify the C10 model for: (1) small magnitude scaling; (2) scaling of short period ground motion from normal faulting events in volcanic crust; (3) scaling of ground motions on very hard rock sites; (4) anelastic attenuation in the NZ crust; and (5) consideration of the increased anelastic attenuation in the Taupo Volcanic Zone. The developed NZ‐specific model therefore contains features as evident from recorded ground motions in NZ and consistent scaling for parameters not well constrained by NZ data. Comparisons with ground motions from the 4 September 2010 Darfield and 22 February 2011 Christchurch earthquakes, which occurred following completion of the NZ‐specific model, illustrate that it provides an empirical prediction with sufficient accuracy and precision.

Mantle composition controls the development of an Oceanic Core Complex

AbstractThe thickness and continuity of oceanic crust is variable. Slow‐spreading ridge segments often contain areas of ‘amagmatic’ or tectonic extension, exposing areas of lower‐crust and upper‐mantle, and having little or no recent volcanism. These are interspersed with areas of ‘normal’ volcanic crust generated by ‘robust’ magmatic accretion. Tectonic spreading is accommodated by displacement on low‐angle extensional detachment faults, forming Oceanic Core Complexes. Although ‘amagmatic’ extension appears to be common at slow spreading rates, little is known about the mechanisms that drive the transition from magmatic spreading. Here, we report results from a detailed study of the Mid‐Atlantic Ridge (13°N–14°N) and show, paradoxically, that despite the presence of several Core Complexes, melt production remains similar along the present‐day spreading axis, which erupts homogeneous ‘normal’ mid‐ocean ridge basalt. However, melt production during formation of the older crust off‐axis was derived from substantially lower degrees of melting of a heterogeneous mantle. During this magmatically restricted phase, melt production was limited by source composition. Small volumes of an enriched basalt (M1) were produced, derived from low‐fraction melts of enriched compositional heterogeneities embedded in an otherwise compositionally depleted upper‐mantle, which, in turn, erupted low‐fraction incompatible‐element‐poor basalts (M2). As a consequence of low magma flux, the crust was thin and insufficient to fully accommodate seafloor spreading. Faulting of this thin crust resulted in the development of detachment faults and the formation of OCCs. Thus, we propose that periods of low melt production, resulting directly from depleted, heterogeneous mantle drives the transition from magmatic to amagmatic spreading.

IODP Expedition 327 and Atlantis Expedition AT 18-07:Observatories and Experiments on the Eastern Flank of the Juan de Fuca Ridge

Abstract. Integrated Ocean Drilling Program (IODP) Expedition 327 (summer 2010) was designed to resolve the nature of fluid-rock interactions in young, upper volcanic crust on the eastern flank of the Juan de Fuca Ridge. Expedition 327 drilled, cased and cored two new basement holes, conducted hydrogeologic experiments, and installed subseafloor borehole observatories (Circulation Obviation Retrofit Kits, CORKs). These CORKs were intended to allow borehole conditions to recover to a more natural state after the dissipation of disturbances caused by drilling, casing, and other operations; provide a long-term monitoring and sampling presence for determining fluid pressure, temperature, composition, and microbiology; and facilitate the completion of active experiments to resolve crustal hydrogeologic conditions and processes. Expedition 327 was followed (summer 2011) by R/V Atlantis Expedition AT18-07, with the remotely-operated vehicle (ROV) Jason, to service these CORKs, collect subseafloor pressure data, recover and deploy autonomous fluid and microbial samplers, collect large volumes of borehole fluids, and initiate a cross-hole hydrogeologic experiment using an electromagnetic flow meter. In addition, Atlantis Expedition AT18-07 refurbished an old CORK that could not be replaced during IODP Expedition 327, completing a critical part of the three-dimensional observation network that is currently being used to monitor a large-scale, directional formation response to long-term fluid flow from the crust. doi: 10.2204/iodp.sd.13.01.2011

The Strangest Terrestrial Planet

NASA's MESSENGER mission previously revealed that Mercury has a volcanic crust; now it finds evidence for an inner “anticrust.”

IODP Expedition 327 and &lt;i&gt;Atlantis&lt;/i&gt; Expedition AT 18-07: Observatories and Experiments on the Eastern Flank of the Juan de Fuca Ridge

Integrated Ocean Drilling Program (IODP) Expedition 327 (summer 2010) was designed to resolve the nature of fluid-rock interactions in young, upper volcanic crust on the eastern flank of the Juan de Fuca Ridge. Expedition 327 drilled, cased and cored two new basement holes, conducted hydrogeologic experiments, and installed subseafloor borehole observatories (&lt;i&gt;Circulation Obviation Retrofit Kits&lt;/i&gt;, CORKs). These CORKs were intended to allow borehole conditions to recover to a more natural state after the dissipation of disturbances caused by drilling, casing, and other operations; provide a long-term monitoring and sampling presence for determining fluid pressure, temperature, composition, and microbiology; and facilitate the completion of active experiments to resolve crustal hydrogeologic conditions and processes. Expedition 327 was followed (summer 2011) by R/V &lt;i&gt;Atlantis&lt;/i&gt; Expedition AT18-07, with the remotely-operated vehicle (ROV) &lt;i&gt;Jason&lt;/i&gt;, to service these CORKs, collect subseafloor pressure data, recover and deploy autonomous fluid and microbial samplers, collect large volumes of borehole fluids, and initiate a cross-hole hydrogeologic experiment using an electromagnetic flow meter. In addition, &lt;i&gt;Atlantis&lt;/i&gt; Expedition AT18-07 refurbished an old CORK that could not be replaced during IODP Expedition 327, completing a critical part of the three-dimensional observation network that is currently being used to monitor a large-scale, directional formation response to long-term fluid flow from the crust. &lt;br&gt;&lt;br&gt; doi:&lt;a href="http://dx.doi.org/10.2204/iodp.sd.13.01.2011" target="_blank"&gt;10.2204/iodp.sd.13.01.2011&lt;/a&gt;

Field evidence of Eros-scale asteroids and impact-forcing of Precambrian geodynamic episodes, Kaapvaal (South Africa) and Pilbara (Western Australia) Cratons

The role of asteroid and comet impacts as triggers of mantle–crust processes poses one of the fundamental questions in Earth science. I present direct field evidence for close associations between impact ejecta/fallout units, major unconformities and lithostratigraphic boundaries in Archaean and early Proterozoic terrains, including abrupt changes in the composition of volcanic and sedimentary assemblages across stratigraphic impact boundaries, with implications for the nature and composition of their provenance terrains. As originally observed by D.R. Lowe and G.R. Byerly, in the Barberton Greenstone Belt, eastern Kaapvaal Craton, South Africa, 3.26–3.24 Ga asteroid mega-impact units are closely associated with the abrupt break between an underlying simatic mafic–ultramafic volcanic crust and an overlying association of turbidites, banded iron formations, felsic tuff and conglomerates of continental affinities. Contemporaneous stratigraphic relationships are identified in the Pilbara Craton, Western Australia. Evidence for enrichment of seawater in ferrous iron in the wake of major asteroid impacts reflects emergence of new source terrains, likely dominated by mafic compositions, attributed to impact-triggered oceanic volcanic activity. Relationships between impact and volcanic activity are supported by the onset of major mafic dyke systems associated with ~ 2.48 Ga and possibly the 2.56 Ga mega-impact events.

Microearthquake streaks and seismicity triggered by slow earthquakes on the mobile south flank of Kilauea Volcano, Hawai'i

We perform waveform cross correlation and high precision relocation of both background seismicity and seismicity triggered by periodic slow earthquakes at Kilauea Volcano's mobile south flank. We demonstrate that the triggered seismicity dominantly occurs on several preexisting fault zones at the Hilina region. Regardless of the velocity model employed, the relocated earthquake epicenters and triggered seismicity localize onto distinct fault zones that form streaks aligned with the slow earthquake surface displacements determined from GPS. Due to the unknown effects of velocity heterogeneity and nonideal station coverage, our relocation analyses cannot distinguish whether some of these fault zones occur within the volcanic crust at shallow depths or whether all occur on the decollement between the volcano and preexisting oceanic crust at depths of ∼8 km. Nonetheless, these Hilina fault zones consistently respond to stress perturbations from nearby slow earthquakes.

Evidence for hydrothermal Archaea within the basaltic flanks of the East Pacific Rise

Little is known about the fluids or the microbial communities present within potentially vast hydrothermal reservoirs contained in still-hot volcanic ocean crust beneath the flanks of the mid-ocean ridge. During Alvin dives in 2002, organic material attached to basalt was collected at low, near-ambient temperatures from an abyssal hill fault scarp in 0.5 Ma lithosphere on the western ridge flank of the East Pacific Rise. Mineral analysis by X-ray diffractometry and scanning electron microscopy revealed high-temperature (> 110 degrees C) phases chalcopyrite (Cu(5)FeS(4)) and 1C pyrrhotite (Fe(1-x)S) within the fault scarp materials. A molecular survey of archaeal genes encoding 16S rRNA identified a diverse hyperthermophilic community, including groups within Crenarchaeota, Euryarchaeota, and Korarchaeota. We propose that the sulfide, metals and archaeal communities originated within a basalt-hosted subseafloor hydrothermal habitat beneath the East Pacific Rise ridge flank and were transported to the seafloor during a recent episode of hydrothermal venting from the abyssal hill fault. Additionally, inferred metabolisms from the fault scarp community suggest that an ecologically unique high-temperature archaeal biosphere may thrive beneath the young East Pacific Rise ridge flank and that abyssal hill fault scarps may present new opportunities for sampling for this largely unexplored microbial habitat.

When did subduction start—And how did it evolve?

Cardiff University, UK ; PearceJA@cf.ac.uk This key question for understanding the Early Earth and the history of subduction recycling is surprisingly difficult to resolve. Much of the evidence for subduction in the first billion years of Earth history is based on the identification of bonintes and adakites, both minor subduction-related rock types at the present day. Moreover, each has other explanations. In the case of boninites, the present-day petrogenetic requirements of mantle depletion by melt extraction during subduction initiation followed by addition of a subduction component can be replicated in the Archaean by melt extraction during komatiite genesis followed by further decompression melting and crustal interaction of the resulting magma. In the case of adakites, as a number of authors have pointed out, the present-day requirement of slab melting during hot subduction can be replicated in the Archaean by stacking of altered lava flows, delamination of thick volcanic crust or shallow underthrusting - to name but three alternatives to subduction. Recognition of subduction in boninites and adakites thus requires more precise fingerprinting. As will be demonstrated, this presently indicates a higher probablitity for non-subduction origins. In the second billion years of Earth history, there are more rocks resembling present-day arc volcanics but, even here, few are unambiguous. The usual criterion of negative-Nb anaomalies can be achieved even in the Phanerozoic in a number of ways that do not require deep subduction, the most important being crustal contamination of hot basic-ultrabasic magma and melting of enriched mantle during delamination or rifting. It is interesting that, if one also applies alteration-insensitive proxies for degree of melting (e.g. Y at 200ppm Cr) or oxygen fugacity (e.g. V-Y covariation), many Archaean ‘arc’ lavas do not satisfy present-day subduction criteria. Early examples best match the Cascades (relatively shallow subduction of hot lithosphere with inflow of hot, enriched mantle) in terms of the range of rock types present. Overall, a pattern of no subduction, followed by shallow subduction and, eventually, deep subduction is an attractive model that can be justified in part, though this does not take into account possible complex recycling when a magma ocean was active. In terms of the session theme, the implication of such a model is that the depleted components of the mantle will initially result from recycling of plume residues; only later will recycled subducted oceanic lithosphere become important. Interestingly, these two types of depleted mantle will have experienced melt extraction under different P,T conditions and may be distinguishable. Similarly, deep subduction recycling of crust, as opposed to burial-delamination recycling, may be more recent than commonly supposed.

Basic magmatism in the geological history of the Elbrus Neovolcanic area, greater Caucasus: Evidence from K-Ar and Sr-Nd isotope data

The age and genesis of basic rocks in the magmatic evolutionary series are of principal significance for regions with predominant acid, mainly crustal volcanism. The appearance of basic volcanic rocks in the rhyolite‐dacite volcanic sequences is usually a sensitive indicator of a change in the conditions of magma generation, the involvement of different mantle and/or crustal reservoirs, or a change in the geotectonic and geodynamic settings of the volcanic activity. This is evident from the abundance of Neogene‐Quaternary acid and moderately acid volcanism in the Elbrus neovolcanic area, Greater Caucasus. The Neogene‐Quaternary magmatic centers of the Greater Caucasus are localized within three (Kazbek, Central Georgian, and Elbrus) neovolcanic areas. The Elbrus area located in the central part of the northern slope of the Greater Caucasus is bounded by the upper reaches of the Kuban River in the west, the Kuma River in the north, the Cherek River in the east, and the Main Caucasus Range in the south (Fig. 1). Evolution of the young magmatism in the Greater Caucasus is divided into three stages [1] with age boundaries determined by isotope‐geochronological investigations [2‐8]. The first (Late Miocene) stage (9.5‐8.3 Ma) is represented by subalkaline acid intrusive rocks of the Caucasian Mineral’nye Vody area of the Elbrus volcanic area [2, 3]. Products of the second (Pliocene) stage are found in the central sector (Chegem and Tyrnyauz regions). In the Chegem region, the products are represented by acid and moderately acid volcanic rocks of the Upper Chegem caldera and Lower Chegem highland (2.9‐2.7 Ma) [2, 5, 6], andesites of the postcaldera Kumtyube and

Fine‐scale seismic structure of the shallow volcanic crust on the East Pacific Rise at 9°50′N

We use a combination of body wave and interface wave observations from an on‐bottom seismic refraction survey to constrain the fine‐scale seismic structure of the upper crust in a ∼3 × 3 km field area centered on the East Pacific Rise at 9°50′N. We detonated 18 explosive shots (18 sources) in a circular pattern (1.5 km radius) on the rise axis and recorded seismic arrivals with eight ocean bottom seismometers (eight receivers). We observed 30–40 Hz compressional body waves from all shots (144 P waves) and 1–3 Hz Stoneley (interface) waves on a subset of source‐receiver pairs (58 interface waves). Using a station correction inversion, we find that roughly half of the variance in the P wave first‐arrival times results from lateral variations in the thickness of the surface low‐velocity layer (SLVL), a layer of extremely porous lava and basalt breccia with an average P wave velocity of 2.2 km s−1. The SLVL thickness increases from &lt;20 m along the axial summit trough (AST) to ∼120 m at near‐axis lava depocenters, which are not symmetric about the rise axis. Depocenters are located ∼0.5 km to the west and ∼1.5 km to the east of the rise axis. Tomographic inversion of the Stoneley wave first arrivals reveals that shear velocities in the SLVL covary with the layer thickness, exhibiting a similar asymmetric pattern, with shear velocities increasing from ∼320 m s−1 near the AST to ∼520 m s−1 at the near‐axis depocenters. Our analysis demonstrates that the seismic characteristics of the extrusive layer near the rise axis are related primarily to volcanic features and processes. The thickness and velocity of the SLVL are low on the axis and within channel networks that deliver lava flows away from the axis and then increase rapidly at the distal ends of the channels where the lavas are deposited. We find that azimuthal anisotropy exerts only a weak influence on our P wave first‐arrival times, which we model as weak (4%) seismic azimuthal anisotropy in the upper dikes with a fast axis oriented N23°–32°W. We find no evidence for seismic azimuthal anisotropy in the extrusive layer.

Physical properties of upper oceanic crust: Ocean Drilling Program Hole 801C and the waning of hydrothermal circulation

The hydrologic evolution of oceanic crust, from vigorous hydrothermal circulation in young, permeable volcanic crust to reduced circulation in old, cooler crust, causes a corresponding evolution of geophysical properties. Ocean Drilling Program (ODP) Hole 801C, which obtained the world's oldest section of in situ, normal oceanic crust, provides the opportunity to examine relationships among hydrologic properties (porosity, permeability, fluid flow), crustal alteration, and geophysical properties, at both core plug and downhole log scales. Within these upper crustal basalts, fluid flux in zones with high porosity and associated high permeability fosters alteration, particularly hydration. Consequently, porosity is correlated with both permeability and a variety of hydration indicators. Porosity‐dependent alteration is also seen at the log scale: potassium enrichment is strongly proportional to porosity. We extend the crustal alteration patterns observed at Hole 801C to a global examination of how physical properties of upper oceanic crust change as a function of age based on global data sets of Deep Sea Drilling Project and ODP core physical properties and downhole logs. Increasing crustal age entails macroporosity reduction and large‐scale velocity increase, despite intergranular velocity decrease with little microporosity change. The changes in macroporosity and velocity are significant for pillows but minor for flows. Matrix densities provide the strongest demonstration of systematic age‐dependent alteration. On the basis of observed decreases in matrix density that are proportional to the logarithm of age, approximately half of all intergranular‐scale crustal alteration occurs after the first 10–15 Myr. Apparently, crustal alteration continues, at a decreasing rate, throughout the lifetime of oceanic crust.

The role of granitic intrusions in the evolution of the Abitibi belt, Canada

The Abitibi greenstone belt, Canada, constitutes a segment of a broad volcanic arc that experienced three distinct periods of volcanic activity peaking at 2730–2725, 2718–2714 and 2705–2700 Ma. During all three volcanic episodes, which were generated by subduction-related processes, mantle plume-derived komatiitic volcanic rocks were also added to the composite Abitibi arc. Plume activity resulted in local regional extension, and restricted later volcanism within the belt. North–south shortening produced thrusts and folds, followed by SE-trending dextral strike-slip faults between 2700 and 2692 Ma, the principal belt-wide deformation. Subsequent E–W dextral strike-slip faulting, limited to the major faults, continued to 2690–2680 Ma (Destor–Porcupine fault) and even to 2680–2670 Ma (Cadillac–Larder fault). Extensional structures resulting from the thickened crust developed around 2660 Ma. Granitic intrusions are associated with all the volcanic episodes and phases of deformation, and have been used to constrain the age of many of these events. Their role in stabilizing, and adding to the volcanic crust is perhaps less well appreciated. Large synvolcanic tonalite-dominated intrusions of the tonalite–trondhjemite–granodiorite (TTG) suite are associated with the three major periods of volcanic activity of the Abitibi greenstone belt, and are genetically related to the volcanism. These synvolcanic intrusions occur both as high-level, flat, cauldron subsidence complexes and as batholithic masses at the base of the oceanic crust. Tonalite gneiss, dated from 2714 to 2703 Ma, occurring in structural culminations, is considered to be arc-underplated material. The gneiss crystallized at depth, and, in contrast to the other tonalites, does not appear to have intruded volcanic rocks. The gneisses also record the earliest compressional deformation. Numerous intrusions of island arc affinity, both the TTG and monzodiorite (MZD) suites form stocks and small plutons emplaced during the late phases of major deformation. These are more numerous than voluminous, and most of the tonalite end members of the suites probably intruded deeper within the sequence, retarded by the now-thickened crust. Large tonalite intrusions dominate the uplifted deeper sections adjacent to the Abitibi belt in the Kapuskasing uplift and Grenville Parautochthon, and presumably make up much of the deep crust of the Abitibi belt imaged in Lithoprobe seismic profiles. Small stocks and dyke complexes characterize the late, localized strike-slip deformational phase. These intrusions are porphyritic syenites and related rocks, including some alkaline compositions. Garnetiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2660 Ma. Thus, plutonic rocks are a volumetrically major overall component of the Abitibi belt and were implicated in every phase of its development, from early volcanic arc magmatic activity, through the various stages of deformation, to final stabilization as continental crust.