Geochemistry, Geophysics, Geosystems

Frische, Matthias; Garofalo, Kristin; Hansteen, Thor H.; Borchers, Reinhard

doi: 10.1029/2005GC001162pmid: N/A

In order to assess the contribution of quiescent degassing volcanoes to the global halo(hydro)carbon inventory, we have quantified volcanic fluxes of methyl halides (CH3Cl, CH3Br, and CH3I), ethyl halides (C2H5Cl, C2H5Br, and C2H5I), and higher chlorinated methanes (CH2Cl2, CHCl3, and CCl4). About every eight months over a 2‐year period (July 2001 to July 2003), gas samples were collected and analyzed from high‐temperature fumaroles (472°C–776°C) at the Nicaraguan subduction zone volcano Momotombo. Using a simultaneous record of trace and main compounds in fumarolic gases as well as SO2 fluxes of the plume, we were able to calculate halo(hydro)carbon fluxes for Momotombo and extrapolate our results to estimate halo(hydro)carbon fluxes for the whole Quaternary Nicaraguan volcanic arc and, in addition, for all volcanoes globally. The most abundant halohydrocarbon was CH3Cl with concentrations up to 19 ppmv. Further major halo(hydro)carbons were CH3Br, CH3I, CH2Cl2, CHCl3, CCl4, C2H5Cl, C2H5Br, C2H5I, and C2H3Cl with an average concentration of 0.20 to 720 ppbv. Estimated mean halo(hydro)carbon fluxes from Momotombo were in the range of 630–5000 g/yr for methyl halides, 49–260 g/yr for ethyl halides, and 2.4–24 g/yr for higher chlorinated methanes. When the results for Momotombo are scaled up to SO2 fluxes of the Nicaraguan volcanic transect, fluxes of 1.7 × 105 g/yr CH3Cl and 82 g/yr CCl4 are attained for Nicaragua. Scaled up to the estimated global SO2 flux, this translates to hypothetical global fluxes of 5.6 × 106 g/yr CH3Cl and 2.7 × 103 g/yr CCl4. These volcanic fluxes are negligible compared to global anthropogenic and natural emissions of about 3 × 1012 g/yr CH3Cl and 2 × 1010 g/yr CCl4.

Wanless, V. Dorsey; Garcia, M. O.; Trusdell, F. A.; Rhodes, J. M.; Norman, M. D.; Weis, Dominique; Fornari, D. J.; Kurz, M. D.; Guillou, Hervé

doi: 10.1029/2005GC001086pmid: N/A

A 2002 multibeam sonar survey of Mauna Loa's western flank revealed ten submarine radial vents and three submarine lava flows. Only one submarine radial vent was known previously. The ages of these vents are constrained by eyewitness accounts, geologic relationships, Mn‐Fe coatings, and geochemical stratigraphy; they range from 128 years B.P. to possibly 47 ka. Eight of the radial vents produced degassed lavas despite eruption in water depths sufficient to inhibit sulfur degassing. These vents formed truncated cones and short lava flows. Two vents produced undegassed lavas that created “irregular” cones and longer lava flows. Compositionally and isotopically, the submarine radial vent lavas are typical of Mauna Loa lavas, except two cones that erupted alkalic lavas. He‐Sr isotopes for the radial vent lavas follow Mauna Loa's evolutionary trend. The compositional and isotopic heterogeneity of these lavas indicates most had distinct parental magmas. Bathymetry and acoustic backscatter results, along with photography and sampling during four JASON2 dives, are used to produce a detailed geologic map to evaluate Mauna Loa's submarine geologic history. The new map shows that the 1877 submarine eruption was much larger than previously thought, resulting in a 10% increase for recent volcanism. Furthermore, although alkalic lavas were found at two radial vents, there is no systematic increase in alkalinity among these or other Mauna Loa lavas as expected for a dying volcano. These results refute an interpretation that Mauna Loa's volcanism is waning. The submarine radial vents and flows cover 29 km2 of seafloor and comprise a total volume of ∼2 × 109 m3 of lava, reinforcing the idea that submarine lava eruptions are important in the growth of oceanic island volcanoes even after they emerged above sea level.

Johnson, H. P.; Hautala, S. L.; Bjorklund, T. A.; Zarnetske, M. R.

doi: 10.1029/2005GC001065pmid: N/A

New hydrostations plus a comprehensive compilation of existing data have allowed us to characterize the dissolved silica plume located at midwater depths in the North Pacific. The North Pacific silica plume is a global‐scale anomaly, extending from the North American continental margin in the east to beyond the Hawaii‐Emperor seamount chain in the west. Inventory of the plume between 2000 and 3000 m depth indicates that it contains 164 Tmols (164 × 1012 mols) of anomalous dissolved silica and is maintained by a horizontal flux of approximately 1.5 Tmols/yr from the east. The source region of this plume has been previously suggested to be Cascadia Basin in the NE Pacific. Biochemical and geothermal processes within this small region can produce approximately one third of the required flux, but the majority of silica contained within the North Pacific plume may originate in crustal fluid venting from the warm upper basement aquifer that underlies the easternmost Pacific plate.

Ito, Garrett; Mahoney, John J.

doi: 10.1029/2005GC001158pmid: N/A

Parameterized models of mantle flow and melting are used to examine two problems in ocean‐island and mid‐ocean ridge basalt (OIB, MORB) geochemistry: (1) the causes of variations in He, Sr, Nd, and Pb isotopes with one another and with age of the surrounding seafloor and (2) the origin of geochemical distinctions between OIB and MORB. We assume the mantle is a mixture of different isotopic components that have different solidus temperatures and are expressed in magmas to varying degrees depending on mantle temperature, lithospheric thickness, and shallow mantle flow. Isotopic variations along the Hawaiian hot spot chain support the possibility that lithospheric thickness controls magma composition primarily by limiting the melting of the most refractory, depleted mantle (DM) component. For Hawaii and a collection of other hot spots, calculations of melt extraction trajectories reproduce the strong correlations often seen among Sr‐Nd‐Pb isotopes together with the more variable and weaker correlations involving 3He/4He. The latter are predicted to be a direct consequence of only one mantle component with high 3He/4He. We show that both OIB and MORB can arise out of the same heterogeneous mantle layer if three conditions are met: the high‐3He/4He source begins melting deeper than DM, DM is ≥85% of the mantle, and the concentration of He in the high‐3He/4He source is comparable to that in DM.

Cochran, James R.; Edwards, Margo H.; Coakley, Bernard J.

doi: 10.1029/2005GC001114pmid: N/A

The Lomonosov Ridge is a band of continental crust that stretches across the Arctic Ocean and separates the Mesozoic Amerasian Basin from the Cenozoic Eurasian Basin. From about 87°N north of Greenland across the Pole to about 86°N, the Lomonosov Ridge is a single highstanding blocky ridge with minimum depths of ∼950–1400 m. South of 86°N on the Siberian side, the ridge breaks up into a series of ridges spread over a width of about 200 km. In this region a highstanding blocky ridge with minimum depths of ∼650–1400 m bounds the Eurasian Basin and continues to the Siberian continental margin. This ridge is continuous with the single ridge making up the Lomonosov Ridge toward North America and is the former outermost continental shelf of Eurasia bounding the Amerasian Basin. The Eurasian Basin margin of the Lomonosov Ridge consists of a series of rotated fault blocks stepping down to the basin that result from nearly orthogonal rifting to form the Eurasian Basin. No rotated fault blocks are observed on the Amerasian Basin margin of the Lomonosov Ridge. On the Amerasian Basin side, Marvin Spur, a linear ridge separated from Lomonosov Ridge by a deep basin, parallels Lomonosov Ridge on the North American side of the pole. At the bend in the Lomonosov Ridge near the North Pole, Marvin Spur continues along strike across the Makarov Basin. South of 86°N toward Siberia, a continuous outer ridge makes up the Amerasian Basin edge of the Lomonosov complex with a series of basins and ridges between it and the former Eurasian shelf. The outer ridge marks an abrupt boundary between the Lomonosov Ridge complex and the apparently oceanic crust of the Makarov Basin. The outer ridge and Marvin Spur very closely follow small circles about a pole located on the Mackenzie delta. The observed structure on the Amerasian Basin side of the Lomonosov Ridge is analogous to that observed at well‐studied shear margins and supports rotational models for the development of the Amerasian Basin.

Robinson, Laura F.; Adkins, Jess F.; Fernandez, Diego P.; Burnett, Donald S.; Wang, S.‐L.; Gagnon, Alexander C.; Krakauer, Nir

doi: 10.1029/2005GC001138pmid: N/A

In this study we use microsampling techniques to explore diagenetic processes in carbonates. These processes are important as they can affect the accuracy of U series chronometry. Fission track maps of deep‐sea scleractinian corals show a threefold difference between the minimum and maximum (U) in modern corals, which is reduced to a factor of 2 in fossil corals. We use micromilling and MC‐ICP‐MS to make detailed analyses of the (U) and δ234Uinitial distributions in corals from 218 ka to modern. Within each fossil coral we observe a large range of δ234Uinitial values, with high δ234Uinitial values typically associated with low (U). A simple model shows that this observation is best explained by preferential movement of alpha‐decay produced 234U atoms (alpha‐recoil diffusion). Open‐system addition of 234U may occur when alpha‐recoil diffusion is coupled with a high (U) surface layer, such as organic material. This process can result in large, whole‐coral δ234Uinitial elevations with little effect on the final age. The diagenetic pathways that we model are relevant to both shallow‐water and deep‐sea scleractinian corals since both exhibit primary (U) heterogeneity and may be subject to U addition.

Syracuse, Ellen M.; Abers, Geoffrey A.

doi: 10.1029/2005GC001045pmid: N/A

The location and motion of subducting plates relative to volcanic arcs provide a first‐order constraint on theories of arc magmagenesis. We compile volcano‐specific subduction parameters for 33,000 km of the global arc system at 839 volcanic centers, measuring the depth to the top of the slab (H) beneath each volcano. The compilation also includes estimates of slab strike and dip, incoming plate velocity, and age, all available in accompanying auxiliary material. The slab geometry is contoured from the top surface of Wadati‐Benioff zones (WBZs) for a variety of teleseismic and local seismicity catalogs, which provides a reference surface for evaluating the distribution of seismicity within subducting plates. The WBZ thickness exceeds that expected from hypocentral errors in a manner correlating with plate age, indicating that old plates have thicker regions in which earthquakes can occur. When averaged over 500‐km‐long arc segments, H ranges from 72 to 173 km with a global average of 105 km, increasing by 20 km when hypocentral error effects are taken into account. These depths correlate poorly with most subduction parameters, but significant correlations exist between H and slab dip (correlation coefficient is 0.54 for 45 arc segments). The dip correlation can be explained if the melting region is displaced from the Wadati‐Benioff zone by a constant‐thickness boundary layer. For the north Pacific, H varies inversely with descent rate; this trend may reflect the manner in which wedge thermal structure affects arc location. Over short distances some arc segments exhibit abrupt variations in arc location but not slab geometry, indicating that upper‐plate tectonic processes also exert control on H. These along‐strike trends in H also correlate with geochemical proxies for the degree of melting, at least in one test case. Thus slab geometry and kinematics provide an important control on the melting that produces arc volcanoes.

Mariga, J.; Ripley, E. M.; Li, C.

doi: 10.1029/2005GC001184pmid: N/A

Nickel‐copper‐cobalt sulfide ores in the Voisey's Bay Intrusion are closely associated with troctolitic to gabbroic rocks that contain abundant country rock xenoliths. Potential sources of the xenoliths include pelitic paragneiss, enderbitic orthogneiss, and mafic to quartzofeldspathic gneisses that form immediate country rocks to different parts of the intrusion. Regardless of location, all xenoliths have a similar refractory mineral assemblage composed of hercynite, magnetite, Ca‐rich plagioclase, and corundum. The refractory mineral assemblage formed via partial melting immediately after xenoliths were engulfed by magma. Rapid thermal equilibration allowed the xenoliths to survive prolonged interaction with magma. Corundum was formed by the incongruent melting of Na‐rich plagioclase in pelitic and quartzofeldspathic gneisses. Corundum and Ca‐rich plagioclase assemblages are aluminous; their origin involved either multistage melting of protoliths where the production of a granitic minimum melt was followed by the liberation of a more silica‐rich and Al‐poor melt or one‐stage disequilibrium melting. Density differences between the xenoliths, restite assemblages, and enclosing mafic magma facilitated the separation of partial melt from restite. No evidence for the melts in the form of channels or interstitial glass is observed in the restite. Flow of mafic magma in the conduit system is thought to have dispersed the Si‐ and alkali‐rich melts derived from the xenoliths. The only record of the xenolith‐derived melts is in the form of concentric rims of plagioclase and biotite which crystallized from a hybrid melt in boundary layers around most xenoliths. Hercynite in the restitic assemblage was produced either by partial melting involving Fe‐ and Mg‐bearing minerals such as garnet and pyroxene or by replacement of corundum. FeO and MgO that were excluded from the boundary layer diffused inward and reacted with corundum to form hercynite. The thickness of the hercynite bands suggests formation times between 3,000 and 23,000 years. Where the insulating rims of plagioclase and biotite were not present, diffusion of FeO and MgO from the surrounding crystal mush continued, resulting in near complete conversion of corundum to hercynite. The highly refractory mineral assemblages which characterize xenoliths present in the Voisey's Bay Intrusion provide evidence for a complex history of magma–country rock interaction. The transfer of xenomelts and sulfur to flowing magma may have been essential for the formation of the magmatic sulfide ores.

Chang, Zhaoshan; Vervoort, Jeffery D.; McClelland, William C.; Knaack, Charles

doi: 10.1029/2005GC001100pmid: N/A

In this study we used LA‐ICP‐MS (laser ablation–inductively coupled plasma–mass spectrometry) to determine U‐Pb ages of 5 zircon samples of known age (∼1800 Ma to ∼50 Ma) in order to determine the reproducibility, precision, and accuracy of this geochronologic technique. This work was performed using a ThermoFinnigan Element2 magnetic sector double‐focusing ICP‐MS coupled with a New Wave Research UP‐213 laser system. The laser ablation pit sizes ranged from 30 to 40 μm in diameter. Laser‐induced time‐dependent fractionation is corrected by normalizing measured ratios in both standards and samples to the beginning of the analysis using the intercept method. Static fractionation, including those caused during laser ablation and due to instrumental discrimination, is corrected using external zircon standards. Total uncertainty for each laser analysis of an unknown is combined quadratically from the uncertainty in the measured isotope ratios of the unknown and the uncertainty in the fractionation factors calculated from the measurement of standards. For individual analyses we estimate that the accuracy and precision are better than 4% at the 2 sigma level, with the largest contribution in uncertainty from the measurement of the standards. Accuracy of age determinations in this study is on the order of 1% on the basis of comparing the weighted average of the LA‐ICP‐MS determinations to the TIMS ages. Due to unresolved contributions to uncertainty from the lack of a common Pb correction and from potential matrix effects between standards and unknowns, however, this estimate cannot be universally applied to all unknowns. Nevertheless, the results of this study provide an example of the type of precision and accuracy that may be possible with this technique under ideal conditions. In summary, the laser ablation technique, using a magnetic sector ICP‐MS, can be used for the U‐Pb dating of zircons with a wide range of ages and is a useful complement to the established TIMS and SHRIMP techniques. This technique is especially well suited to reconnaissance geochronologic and detrital zircon studies.

Wortmann, Ulrich G.

doi: 10.1029/2005GC001143pmid: N/A

Reaction‐transport modeling of dissolved species in interstitial water allows for the inversion of transport processes and thus facilitates the detailed investigation of signals which are usually blurred by diffusion and advection. Here I present a case study from the South Australian continental margin (ODP Leg 182, Site 1130) where I use reaction‐transport modeling to derive a depth transect of volumetric sulfate reduction rates. Site 1130, located on the shelf slope in 500 m deep water, is of special interest as an upwelling sulfate‐rich brine allows for an extended sulfate reduction zone which reaches to a depth of at least 300 mbsf. The obtained reduction rates vary from 600 pmol/cm−3 yr−1 at 30 mbsf to 63 pmol/cm−3 yr−1 at 300 mbsf. The depth‐integrated sulfate consumption equals 65 × 10−6 mol/yr cm−2, which is similar to other shelf slope settings without advecting sulfate. This suggests that the primary control on sulfate reduction rates is organic matter reactivity, rather than sulfate availability. However, similar to other ODP Leg 182 sites, the interstitial water chemistry in the upper 30 mbsf is inconsistent with a diffusive/advective transport system. While the actual process causing this remains elusive, pyrite burial rates from this zone suggest that sulfate reduction rates in this zone are at least 60 times higher than those derived from reaction‐transport modeling assuming diffusion and advection alone.