Geochemistry, Geophysics, Geosystems

Lee, Changyeol; King, Scott D.

doi: 10.1029/2008GC002151pmid: N/A

The heat generated by viscous dissipation is consistently evaluated using a 2‐D compressible subduction model with variations of mantle rheology (constant as well as pressure and temperature dependent viscosity), dip, age, and velocity of the subducting slab. For comparison, we also conduct 2‐D incompressible subduction calculations with the same conditions and parameters used in the compressible formulation. The effect of compressibility on the thermal and flow structures of the subduction zones is relatively small and concentrated along the base of the mantle wedge, with temperature differences <100°C and differences in kinematic energy of the mantle wedge <1% between compressible and incompressible models. Mantle rheology has a stronger effect on thermal and flow structures than mantle compressibility as well as the variations of dip, age, and velocity of the subducting slab. The heat from viscous dissipation in the compressible model increases the slab temperatures over the incompressible model (<70°C), as a result of additional conduction across the slab surface (constant viscosity) and thinning of the thermal boundary layer caused by viscosity reduction (pressure‐ and temperature‐dependent viscosity).

Collins, John A.; Blackman, Donna K.; Harris, Amber; Carlson, Richard L.

doi: 10.1029/2008GC002121pmid: N/A

The seismic structure of the upper ∼1 km of the central dome of Atlantis Massif is investigated in the context of lithologies known from seafloor drilling and physical property measurements obtained within the borehole and on core samples. A new analysis of seafloor refraction data and multichannel reflection data acquired in the immediate vicinity of Integrated Ocean Drilling Program (IODP) Site U1309 was motivated by a discrepancy between initial seismic interpretations, which indicated mantle velocities at shallow depth, and the gabbroic sequence recovered by drilling. A new seismic velocity model is derived that is consistent with the full suite of geological and geophysical data in the central dome area; all of these data show that mafic intrusive rocks dominate the upper portion of the footwall of this oceanic core complex and that laterally extensive zones of ultramafic rocks are not required by the data. The origin of subseafloor reflectivity beneath the central dome was also considered. We find that seafloor scattering complicates the interpretation of multichannel seismic data acquired near Site U1309 but that detectable subsurface impedance contrasts do occur. Downhole variations in alteration may generate reflections observed from the upper kilometer of the central dome.

Tominaga, Masako; Teagle, Damon A. H.; Alt, Jeffrey C.; Umino, Susumu

doi: 10.1029/2008GC002143pmid: N/A

The objective of this study is to construct a representative volcanostratigraphy of Ocean Drilling Program Hole 1256D, the first complete penetration of intact upper oceanic crust formed at a superfast spreading rate. An accurate knowledge of the volcanostratigraphy is vital to understand processes of crustal construction and submarine magmatism and to estimate chemical exchange with seawater, but this is rarely achieved due to very low recovery rates in most basement holes. We used two approaches to determine the rock types that form the wall rocks in the basement sections of Hole 1256D: (1) user guided interpretations of electrofacies acquired by imaging tools combined with other wireline tools; and (2) the use of an artificial neutral network to objectively classify the responses of all available logging information. Great availability of formation microscanner (FMS) images provided superior coverage of the borehole wall compared to previous attempts at core‐log integration. This has resulted in more confident and detailed lithologic classifications, such as with the distinction between pillows and different styles of breciation. Ten lithology types are suggested for a volcanostratigraphy model: massive flows, ponded lava, fractured massive flows, fragmented flows, thin flows or thick pillows, pillows, breccias, dikes in dike complex, isolated dikes, and gabbros. Three major lithology types in the extrusive section are massive flows (both massive and fragmented massive flow, 22%), fragmented flows (32%), and breccias (19%). Pillow lavas make up only 1.9% of the volcanic section and are confined to a 100 m interval. Below the extrusive section, subvertical contacts interpreted to be dike margins are typically observed every 1 to 2 m with brecciated zones along the contacts. The dikes dip steeply to the northeast indicating slight rotation away from the ridge axis. We used an artificial neural network (ANN) approach to determine a quantitative lithostratigraphy. The ANN is most strongly influenced by porosity and alteration degrees and the resulting stratigraphy most closely resembles the above classifications when clustered by FMS texture as opposed to lithologic interpretation. The ANN thus provides a porosity‐based stratigraphy of the basement rather than the traditional lithology‐based stratigraphy.

Brzak, Keith; Gu, Yu Jeffrey; Ökeler, Ahmet; Steckler, Michael; Lerner‐Lam, Arthur

doi: 10.1029/2008GC002234pmid: N/A

This study combines migration and forward source modeling techniques to examine the existence and location of persistent seismic noise near southern Italy. Our results demonstrate that noise source modeling is both feasible and recommended in validating the “ambient source” assumption prior to noise‐based velocity analyses. Persistent noise sources near the Gargano promontory and the Tyrrhenian Sea coast are strongly suggested by the observed cross‐correlations. The presence of a single point source or a cluster of point sources could both produce coherent Rayleigh wave energy in southern Italy. While the nature of the noise sources is still uncertain, baroclinic estimates and dynamic topography models favor an explanation that encompasses atmosphere‐ocean coupling and heightened wave interaction off the Adriatic coast. Our records indicate that these noise sources can maintain their average location for up to 7 months despite seasonal and, possibly, daily variations.

O'Driscoll, Brian; Petronis, Michael S.

doi: 10.1029/2008GC002274pmid: N/A

Serpentinization of ultramafic rocks results in the growth of serpentine and magnetite, following hydration of magmatic olivine. Cr‐spinel seams in ultramafic intrusions that also contain olivine might produce magnetite during the serpentinization process. However, hysteresis and temperature dependence of susceptibility experiments on serpentinized chromitite (>80% Cr‐spinel) from the Dawros peridotite (western Ireland) reveal the presence of small quantities of a Fe‐Ti oxide phase, likely titanomaghemite. Reflected light petrography reveals that the oxides occur as 0.1 μm to 20 μm grains that are typically intergrown with serpentine. It is suggested that the formation of the minute quantities of low‐susceptibility‐high Curie point titanomaghemite occurred by low‐temperature alteration of the magnetite produced during serpentinization. The presence of such low quantities of the oxide phase may be either a function of the very high Mg content of the magmatic olivine or the decoupling of oxide growth from the process of serpentinization, where magnetite formed later than the reaction of olivine with the infiltrating fluids. Our data highlight the usefulness of rock magnetic techniques to the investigation of oxide mineral phases in serpentinized ultramafic rocks.

DiCaprio, Lydia; Müller, R. Dietmar; Gurnis, Michael; Goncharov, Alexey

doi: 10.1029/2008GC002222pmid: N/A

The Tonga‐Kermadec subduction system in the southwest Pacific preserves a series of crustal elements and sediments which have recorded subduction initiation, rift, and back‐arc basin formation. The Norfolk Basin is the farthest landward of all back‐arc basins formed in the Tonga‐Kermadec region and may preserve the earliest record of subduction initiation regionally. For the Norfolk Basin, we use a set of multibeam bathymetry, magnetic, and seismic reflection and refraction data to constrain basin structure and the mode and timing of formation. A structural interpretation reveals a two‐stage tectonic evolution: (1) a convergent tectonic regime until 38–34 Ma, alternatively related to island arc collision or subduction initiation, and (2) lithospheric extension after 34 Ma. These observations may help to constrain mechanical models that predict rapid extension following convergence of the overriding plate during subduction initiation or arc reversals.

Chang, Liao; Roberts, Andrew P.; Rowan, Christopher J.; Tang, Yan; Pruner, Petr; Chen, Qianwang; Horng, Chorng‐Shern

doi: 10.1029/2008GC002276pmid: N/A

We provide comprehensive low‐temperature magnetic results for greigite (Fe3S4) across the spectrum from superparamagnetic (SP) to multidomain (MD) behavior. It is well known that greigite has no low‐temperature magnetic transitions, but we also document that it has strong domain‐state dependence of magnetic properties at low temperatures. Blocking of SP grains and increasing thermal stability with decreasing temperature is apparent in many magnetic measurements. Thermally stable single‐domain greigite undergoes little change in magnetic properties below room temperature. For pseudo‐single‐domain (PSD)/MD greigite, hysteresis properties and first‐order reversal curve diagrams exhibit minor changes at low temperatures, while remanence continuously demagnetizes because of progressive domain wall unpinning. The low‐temperature demagnetization is grain size dependent for PSD/MD greigite, with coarser grains undergoing larger remanence loss. AC susceptibility measurements indicate consistent blocking temperatures (TB) for all synthetic and natural greigite samples, which are probably associated with surficial oxidation. Low‐temperature magnetic analysis provides much more information about magnetic mineralogy and domain state than room temperature measurements and enables discrimination of individual components within mixed magnetic mineral assemblages. Low‐temperature rock magnetometry is therefore a useful tool for studying magnetic mineralogy and granulometry of greigite‐bearing sediments.

Piochi, Monica; De Astis, Gianfilippo; Petrelli, Maurizio; Ventura, Guido; Sulpizio, Roberto; Zanetti, Alberto

doi: 10.1029/2008GC002176pmid: N/A

The 1739 A.D. Pietre Cotte lava flow is part of a sequence of low‐explosive to weak effusion events occurred at La Fossa Cone, the active vent on Vulcano Island (Aeolian Arc, Italy). This lava is rhyolitic, texturally heterogeneous, and contains lati‐trachytic enclaves. These compositions are recurrent in the La Fossa volcanic products and are representative of the recent Vulcano plumbing system. The host lava is vesicular, relatively phenocryst‐free, and locally contains microlites and millimeter‐sized spherulites. The enclaves are up to 10 cm in size, display angular to spherical shapes, and can form the core of spherulites. Enclaves mostly consist of plagioclase and augitic phenocrysts set in a weakly vesicular groundmass characterized by variable abundance of glass and feldspar microlites. Field, textural, and fractal data allow us to constrain the rheological features of the rhyolitic and lati‐trachytic magmas. In situ major, trace, and volatile element analyses provide evidence for heterogeneities in the glassy matrix and zoning of phenocrysts. Processes of magma evolution have been quantitatively constrained by using the apparent distribution ratios of trace elements measured between mineral phases and glassy matrices. The collected data in combination with petrological and fluid inclusion data from the literature provides evidence for (1) a genetic relationship between the two magmas through assimilation fractional crystallization process; (2) a mingling mechanism between an uprising rhyolitic magma and a shallower partly crystallized lati‐trachytic magma plug; (3) the desegregation (enclaves) at variable scales of the lati‐trachyte within the rhyolite; and (4) the possible eruptive scenarios consequent to a future magmatic unrest.

Famin, Vincent; Welsch, Benoît; Okumura, Satoshi; Bachèlery, Patrick; Nakashima, Satoru

doi: 10.1029/2008GC002015pmid: N/A

We present major element and volatile concentration analyses in melt and gas inclusions from two recent picrite eruptions (February 2005 and December 2005) at Piton de la Fournaise volcano (La Reunion Island, Indian Ocean). Combined with literature data, our new data show that the large variability of major element compositions in Piton de la Fournaise lavas may be explained by three depth‐dependent differentiation stages of a single transitional parental magma (9–11 wt% MgO; 0.5–0.8 wt% K2O, and 10–12 wt% CaO). The deepest differentiation (>7.5 km) is controlled by the fractional crystallization of clinopyroxene + plagioclase and yields gabbros and basalts enriched in K and depleted in Ca relative to the parental magma. In a shallower storage zone (<2.5 km), differentiation of the transitional parental magma is dominated by the fractionation/accumulation of Fo83–85 olivine phenocrysts, yielding Mg‐poor basalts at the top of the reservoir and picrites at the base. In cooling dykes and subsurface pockets (<0.6 km), Mg‐poor basalts may themselves evolve into more differentiated melts by clinopyroxene + plagioclase fractionation. The incorporation of dunitic xenocrysts or other xenoliths is not necessary to explain the major element chemical diversity of the volcano. The same superposition of differentiation mechanisms may occur at similar depths in other shield volcanoes.

Dutay, J.‐C.; Lacan, F.; Roy‐Barman, M.; Bopp, L.

doi: 10.1029/2008GC002291pmid: N/A

The oceanic distributions of 231Pa and 230Th are simulated with the global coupled biogeochemical‐ocean general circulation model NEMO‐PISCES. These natural nonconservative tracers, which are removed from the water column by reversible scavenging processes onto particles, have been used to study modern and past ocean circulation. Our model includes three different types of particles: particulate organic matter (POM), calcium carbonate (CaCO3), and biogenic silica (BSi). It also considers two particle classes: small particles (POM) that sink slowly (3 m/d) and large particles (POM, CaCO3, BSi) that sink much more rapidly (50 m/d to 200 m/d) in the water column. 231Pa and 230Th are simulated with a reversible scavenging model that uses partition coefficients between dissolved and particulate phases that depend on particle type and size. Model results are then compared with 231Pa and 230Th observations in the water column and modern sediments. A preliminary evaluation of the particle fields simulated by the PISCES model has revealed that particle concentrations are reasonable at the surface but largely underestimated in the deep ocean. Largely to compensate for this, we find it necessary to use partition coefficients that vary as a function of particle size by significantly more that observed to obtain relatively realistic results. In the water column, 231Pa and 230Th fluxes are mainly controlled by the slowly sinking particles and partition coefficients need to be parameterized as a function of particle flux, as suggested by observations. Considering discrepancies between the modeling particle fields and those observed, we were forced to use exaggerated values for partition coefficients in order to get realistic tracer distributions. These 231Pa and 230Th simulations have provided an opportunity to propose some future developments of the PISCES model, in order to make progress in the simulation of trace elements. Assigning calcium carbonate, biogenic silica, and aluminosilicates to the small particle pool represents a credible approach to increase its concentration and subsequently simulate realistic tracer distributions in the water column using reasonable values for the partition coefficients, as well as a realistic fractionation in the sediments at all depths.