Abstract. We present results from a combined multichannel seismic reflection (MCS) and wideangle onshore/offshore seismic experiment conducted in 1996 across the southeast Greenland continental margin. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a two-dimensional velocity structure. We employ a hybrid raytracing scheme based on the graph method and the local ray-bending refinement to efficiently obtain an accurate forward solution, and we employ smoothing and optional damping constraints to regularize an iterative inversion. We invert 2318 Pg and 2078 PmP travel times to construct a compressional velocity model for the 350-km-long transect, and a long-wavelength structure with strong lateral heterogeneity is recovered, including (1) -30-km-thick, undeformed continental crust with a velocity of 6.0 to 7.0 km/s near the landward end, (2) 30-to 15-km-thick igneous crust within a 150-km-wide continent-ocean transition zone, and (3) 15-to 9-km-thick oceanic crust toward the seaward end. The thickness of the igneous upper crust characterized by a highvelocity gradient also varies from 6 km within the transition zone to -3 km seaward. The bottom half of the lower crust generally has a velocity higher than 7.0 km/s, reaching a maximum of 7.2 to 7.5 km/s at the Moho. A nonlinear Monte Carlo uncertainty analysis is performed to estimate the a posteriori model variance, showing that most velocity and depth nodes are well determined with one standard deviation of 0.05-0.10 km/s and 0.25-1.5 km, respectively. Despite significant variation in crustal thickness, the mean velocity of the igneous crust, which serves as a proxy for the bulk crustal composition, is surprisingly constant (-7.0 km/s) along the transect. On the basis of a mantle melting model incorporating the effect of active mantle upwelling, this velocitythickness relationship is used to constrain the mantle melting process during the breakup of Greenland and Europe. Our result is consistent with a nearly constant mantle potential temperature of 1270-1340øC throughout the rifting but with a rapid transition in the style of mantle upwelling, from vigorous active upwelling during the initial rifting phase to passive upwelling in the later phase.

Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbonbearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5-10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory. Key carbon reservoirs and transport mechanisms can now be better quantified. These include carbon concentration ([C]) in altered mantle lithologies (this paper plus refs. 9 and 10), carbon solubility in aqueous fluids at subduction zone conditions (this paper plus refs. 11 and 12), the volume of altered peridotites in subducting oceanic plates (especially ref. 13), the volume of altered peridotite in the mantle wedge, and the nature of metasedimentary diapirs rising from subducting crust (14).In this paper, we reevaluate carbon fluxes in several tectonic settings. We start with carbon uptake during hydrothermal alteration near midocean ridges, followed by an estimate of carbon addition during alteration of shallow mantle peridotite at the "outer rise," where subducting oceanic plates bend before subduction. We then consider carbon transfer in fluids and melts derived from the subducting plate. Finally, we review carbon outputs from arc volcanoes and via diffuse venting.Carbon Uptake During Hydrothermal Alteration of Basaltic Oceanic Crust: 22-29 Mt C/y Following Alt and Teagle (15), we compiled data on [C] in altered oceanic crust (Dataset S1; also see SI Text).[C] is now thought to be higher in volcanic rocks and lower in gabbros. These differences offset each other, so our estimate of 500-600 ppm carbon in oceanic crust agrees with Alt and Teagle. Oceanic plates are consumed at an average of ∼0.05 m/y along the ∼44,500-km length of global subduction zones (4). These values and [C] in altered oceanic crust yield a carbon flux of 22-29 Mt C/y (Dataset S2).Seismic data and seafloor outcrops imply that 5-15% of oceanic crust that formed at slow-and ultraslow-spreading ridges is composed of altered mantle peridotite, with the extent of serpentinization varying from ∼100% at the seafloor to ∼0% at ∼7 km depth (16,17). Because slow-and ultraslow-spreading crust is formed at ∼30% of midocean ridges (18,19), crust composed of altered peridotite is 1-4% of the total, and not a significant part of the global ca...

The rate of natural carbonation of tectonically exposed mantle peridotite during weathering and low-temperature alteration can be enhanced to develop a significant sink for atmospheric CO 2. Natural carbonation of peridotite in the Samail ophiolite, an uplifted slice of oceanic crust and upper mantle in the Sultanate of Oman, is surprisingly rapid. Carbonate veins in mantle peridotite in Oman have an average 14 C age of Ϸ26,000 years, and are not 30 -95 million years old as previously believed. These data and reconnaissance mapping show that Ϸ10 4 to 10 5 tons per year of atmospheric CO 2 are converted to solid carbonate minerals via peridotite weathering in Oman. Peridotite carbonation can be accelerated via drilling, hydraulic fracture, input of purified CO 2 at elevated pressure, and, in particular, increased temperature at depth. After an initial heating step, CO 2 pumped at 25 or 30°C can be heated by exothermic carbonation reactions that sustain high temperature and rapid reaction rates at depth with little expenditure of energy. In situ carbonation of peridotite could consume >1 billion tons of CO 2 per year in Oman alone, affording a low-cost, safe, and permanent method to capture and store atmospheric CO 2.alteration and weathering ͉ carbon capture ͉ exothermic ͉ carbon sequestration ͉ mineral R ecognition that anthropogenic CO 2 input to the atmosphere has substantially increased atmospheric CO 2 concentration, and that increased CO 2 may drive rapid global warming, has focused attention on carbon capture and storage (1). One storage option is conversion of CO 2 gas to stable, solid carbonate minerals such as calcite (CaCO 3 ) and magnesite (MgCO 3 ) (2). Natural carbonation of peridotite by weathering and lowtemperature alteration is common. Enhanced natural processes in situ may provide an important, hitherto neglected alternative to ex situ mineral carbonation ''at the smokestack.'' In this article, we evaluate the rate of natural carbonation of mantle peridotite in the Samail ophiolite, Sultanate of Oman, and then show that under certain circumstances exothermic peridotite alteration (serpentinization, carbonation) can sustain high temperature and rapid reaction with carbonation up to 1 million times faster than natural rates, potentially consuming billions of tons of atmospheric CO 2 per year. In situ mineral carbonation for CO 2 storage should be evaluated as an alternative to ex situ methods, because it exploits the chemical potential energy inherent in tectonic exposure of mantle peridotite at the Earth's surface, does not require extensive transport and treatment of solid reactants, and requires less energy for maintaining optimal temperature and pressure.Tectonically exposed peridotite from the Earth's upper mantle, and its hydrous alteration product serpentinite, have been considered promising reactants for conversion of atmospheric CO 2 to solid carbonate (3). However, engineered techniques for ex situ mineral carbonation have many challenges. Kinetics is slow unless olivine and serpentine ...

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Near-surface reaction of CO 2 -bearing fluids with silicate minerals in peridotite and basalt forms solid carbonate minerals. Such processes form abundant veins and travertine deposits, particularly in association with tectonically exposed mantle peridotite. This is important in the global carbon cycle, in weathering, and in understanding physical-chemical interaction during retrograde metamorphism. Enhancing the rate of such reactions is a proposed method for geologic CO 2 storage, and perhaps for direct capture of CO 2 from near-surface fluids. We review, synthesize, and extend inferences from a variety of sources. We include data from studies on natural peridotite carbonation processes, carbonation kinetics, feedback between permeability and volume change via reaction-driven cracking, and proposed methods for enhancing the rate of natural mineral carbonation via in situ processes ("at the outcrop") rather than ex situ processes ("at the smokestack"). 545 Annu. Rev. Earth Planet. Sci. 2011.39:545-576. Downloaded from www.annualreviews.org by University of British Columbia on 10/27/12. For personal use only.

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Exposed, subduction-related magmatic arcs commonly include sections of ultrama®c plutonic rocks that are composed of dunite, wehrlite, and pyroxenite. In this experimental study we examined the eects of variable H 2 O concentration on the phase proportions and compositions of igneous pyroxenites and related ultrama®c plutonic rocks. Igneous crystallization experiments simulated natural, arc magma compositions at 1.2 GPa, corresponding to conditions of the arc lower crust. Increasing H 2 O concentration in the liquid changes the crystallization sequence. Low H 2 O concentration in the liquid stabilizes plagioclase earlier than garnet and amphibole while derivative liquids remain quartz normative. Higher H 2 O contents (>3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing derivative corundum normative andesite liquids. The experiments show that alumina in the liquid correlates positively with Al in pyroxene, as long as no major aluminous phase crystallizes. Extrapolation of this correlation to natural pyroxenites in the Talkeetna and Kohistan arc sections indicates that clinopyroxenes with low Ca-Tschermaks component represent near-liquidus phases of primitive, Si-rich hydrous magmas.Density calculations on the residual solid assemblages indicate that ultrama®c plutonic rocks are always denser than upper mantle rocks in the order of 0.05 to 0.20 g/cm 3 . The combination of high pressure and high H 2 O concentration in the liquid suppresses plagioclase crystallization, so that ultrama®c plutonic rocks form over a signi®cant proportion of the crystallization interval (up to 50% crystallization of ultrama®c rocks from initial, mantle-derived liquids). This suggests that in subductionrelated magmatic arcs the seismic Moho might be shallower than the petrologic crust/mantle transition. It is therefore possible that calculations based on seismic data have overestimated the normative plagioclase content (e.g., SiO 2 , Al 2 O 3 ) of igneous crust in arcs.