Geochemistry, Geophysics, Geosystems

Kirby, J. F.; Swain, C. J.

doi: 10.1029/2007GC001773pmid: N/A

A variety of methods exist to estimate the elastic thickness (Te) of the lithosphere. In this contribution, we attempt to provide an indication of how well the fan wavelet coherence method recovers Te, through synthetic modeling. The procedure involves simulating initial topographic and subsurface loads and emplacing them on a thin elastic plate of known Te, generating the postloading topography and gravity. We then attempt to recover that Te distribution from the gravity and topography through the wavelet method, hence discovering where its strengths and weaknesses lie. The Te distributions we use here have elliptical and fractal geometries, while the initial loads are fractal. Importantly, we have found that this widely used synthetic loading calibration method will tend to result in underestimates of Te no matter which recovery method is used. This is due to random correlations between the initial loads which, on average, serve to increase their coherence at all wavelengths and spatial locations. For the fan wavelet method, the degree of underestimation from this “background” source is approximately 10% of the true Te. In addition, the fan wavelet coherence method will provide underestimates of (1) the true Te when the study area size is of the order of the highest flexural wavelength or less, (2) relative Te differences when the Te anomaly is narrow compared to its flexural wavelength, and (3) steep Te gradients. Significantly, we find that the recovery is not greatly affected by the assumption of uniform Te in the inversion of the coherence. We also find that Te recovery from the coherence is only weakly dependent upon the initial subsurface‐to‐surface loading ratio (f). In contrast to the coherence, Te recovery from the admittance is highly “noisy,” with discontinuities and overestimates of Te frequently arising. This is most likely due to the high sensitivity of the admittance to f and is likely to apply to real data as well.

Mamani, Mirian; Tassara, Andrés; Wörner, Gerhard

doi: 10.1029/2007GC001925pmid: N/A

Present‐day ratios of Pb isotopes (324 published samples, 435 new) and Nd‐Sr isotopes (150 published, 180 new) on Proterozoic to Holocene igneous, metamorphic, and sedimentary rocks define (at high spatial resolution) distinct isotopic domains of the crust in the central Andes. These domains correlate with the internal compositional structure of the crust as revealed by a three‐dimensional density model. Pb‐Nd isotopic boundaries thus correspond to variations in crustal compositional structure and reflect Proterozoic mafic‐dominated and Paleozoic felsic‐dominated crustal lithologies. Age and composition (mafic versus felsic) of these domains have controlled the rheology of the Andean crust, have influenced crustal deformation patterns, and correlate with the central Andean plateau segmentation.

Ghosh, Sanghamitra; Xu, Yingfeng; Humayun, Munir; Odom, Leroy

doi: 10.1029/2007GC001827pmid: N/A

The toxicity of mercury's methylated species and its biomagnification in aquatic food chains and global dispersion by the atmosphere are the cause of worldwide health problems. Recent reports have observed natural mass‐dependent fractionation in mercury isotopes, and recent theoretical work has demonstrated that isotopic separation in mercury is due primarily to nuclear field shifts (nuclear volume effect), and the magnetic spin effect gives rise to mass‐independent fractionation (MIF) of odd neutron number isotopes. Now we present analytical evidence of mass‐independent isotopic variations in mercury produced by both nuclear volume and magnetic isotope processes. Even mass number isotopes exhibit a pattern indistinguishable from that produced by mass‐dependent fractionation, with both positive and negative 199Hg and 201Hg anomalies. MIF is easier to reliably determine in Hg isotopes than mass‐dependent fractionation alone, and thus it provides a potentially important key in constraining models of mercury sources and pathways in the environment.

Gehrels, George E.; Valencia, Victor A.; Ruiz, Joaquin

doi: 10.1029/2007GC001805pmid: N/A

U‐Th‐Pb geochronology by laser ablation–multicollector–inductively coupled plasma–mass spectrometry initiated during the mid to late 1990s as a reconnaissance tool, capable of generating ages of only moderate precision from relatively large volumes of zircon. New developments in instrumentation and experimental methodology, as described herein and by other researchers, now make it possible it to correct for common Pb accurately (using measured 204Pb), to acquire geochronologic information rapidly (30–40 unknowns/h), to generate U‐Pb ages with an accuracy of better than 1% for most zircon standards, and to conduct analyses on much smaller (e.g., 10 μm by 6 μm) volumes of material. These capabilities are driving important advances in many aspects of Earth science research.

Jean‐Baptiste, P.; Fourré, E.; Dapoigny, A.; Charlou, J. L.; Donval, J.‐P.

doi: 10.1029/2007GC001765pmid: N/A

As part of a multidisciplinary project aimed at studying mid‐ocean ridge processes near the Azores, fifty water column profiles were analyzed for 3He/4He ratios in dissolved helium (a well‐known hydrothermal tracer) from 36°N to 40°N along the Mid‐Atlantic Ridge (MAR) and over the Azores Plateau. As expected, large δ3He anomalies could be observed over the Rainbow, Lucky Strike, and Menez Gwen hydrothermal sites. The main finding of the present study is the discovery of a large hydrothermal 3He plume north of the Açor Fracture Zone (north AFZ site), with a CH4/3He ratio indicative of a basaltic‐hosted hydrothermal system. Clear 3He and CH4 anomalies, likely corresponding to unknown venting sites too, were also detected in the Amar Minor segment and south of the Kurchatov Fracture Zone. Evidence for substantial mantle helium degassing was also observed in the deep nodal basins along the Terceira Rift. On the basis of 3He plumes over the total length of the surveyed segments, the distribution of hydrothermal sites corresponds to a site frequency of 1.3 ± 0.2 site/100 km, in good agreement with the global vent field statistics of Baker and German (2004). For the Rainbow, Lucky Strike, and Menez Gwen sites, the application of a plume model based on the conservation of mass, heat, and momentum shows that the heat output computed by the model is only an estimation of the heat released by the focused part of the flow imputable to one single vent. Applied to the north AFZ venting site for which the height of the plume is not known precisely, the model does not allow us to discriminate between a Menez Gwen/Rainbow type of venting or a more focused vent complex such as the one observed at the TAG site (26°N).

Evans, Helena K.; Hall, Ian R.

doi: 10.1029/2007GC001771pmid: N/A

Three depth transects containing a total of 33 sediment cores were investigated along the Blake Outer Ridge in the western subtropical North Atlantic. Sortable silt mean () grain size and stable isotope records were used to assess the position and relative intensity of the Western Boundary Undercurrent (WBUC) during the Holocene, the Last Glacial Maximum (LGM), and the Younger Dryas (YD) intervals. The Holocene reconstruction is consistent with modern physical and chemical hydrographic measurements in the area, suggesting a deep position for the fast flowing core of the WBUC (3000–4000 m, deepening to ∼4500 m water depth on the ridge flanks) and a water column dominated by North Atlantic Deep Water (NADW). The LGM and YD reconstructions show that a comparable hydrographic regime was present during both these intervals, suggesting a similar mode of circulation that was appreciably different from the Holocene reconstruction. The WBUC's zone of maximum flow speed during these intervals is suggested to have shifted above 2500 m water depth, consistent with nutrient depleted Glacial North Atlantic Intermediate Water formation with an increasing influence of Southern Source Water (SSW) beneath. Below 4000 m water depth, results hint at increased SSW flow vigor during both the LGM and YD with higher flow speeds than during the Holocene. This study provides a framework for aiding the interpretation of time series records of paleocurrent flow speed changes in the region of the WBUC.

Geist, Dennis; Diefenbach, Bridget A.; Fornari, Daniel J.; Kurz, Mark D.; Harpp, Karen; Blusztajn, Jerzy

doi: 10.1029/2007GC001795pmid: N/A

New multibeam bathymetric and side‐scan sonar data from the southwestern edge of the Galápagos platform reveal the presence of ∼60 large, stepped submarine terraces between depths of 800 m and 3500 m. These terraces are unique features, as none are known from any other archipelago that share this geomorphic form or size. The terraces slope seaward at <2° and are surrounded by escarpments that average ∼300 m in height with average slopes of 24°. The stepped morphology, fine‐scale features, and sinuous planform continuity of terrace edges indicate that each terrace results from a sequence of major submarine volcanic eruptions, similar in extent to young deep‐water (>3000 m) lava flow fields west of Fernandina and Isabela Islands. The terraces are formed of thick sequences of lava flows that coalesce to form the foundation of the Galápagos platform, on which the subaerial central volcanoes are built. The compositions of basalts dredged from the submarine terraces indicate that most lavas are chemically similar to subaerial lavas erupted from Sierra Negra volcano on southern Isabela Island. There are no regular major element, trace element, or isotopic variations in the submarine lavas as a function of depth, relative stratigraphic position, or geographic location along the southwest margin of the platform. We hypothesize that magma supply at the western edge of the Galápagos hot spot, which is influenced by both plume and mid‐ocean ridge magmatic processes, leads to episodic eruption of large lava flows. These large lava flows coalesce to form the archipelagic apron upon which the island volcanoes are built.

Hopp, Jens; Trieloff, Mario

doi: 10.1029/2007GC001833pmid: N/A

We evaluated He‐Ne‐Ar elemental compositions of mid‐ocean ridge basalt (MORB) glasses and basalt glasses from the Hawaiian hot spot (Loihi Seamount, Kilauea) to determine the (3He/22Ne)solar, 4He*/21Ne*, and 4He*/40Ar* MORB‐type and “plume”‐type end‐member compositions within a binary mixing model. Although the 3He/22Ne ratios of “plume” magma and MORB magma marginally agree with current estimates of solar compositions, the calculated 4He*/21Ne* ratios are lower than the theoretical production ratio and indicate that the “plume” magma component experienced a predegassing fractionation event. This fractionation event resulted in a deficit in both radiogenic 4He* and primordial 3He and accounts for the differences in 3He/22Ne, 4He*/21Ne*, and 4He*/40Ar* ratios and 3He concentrations for both mantle domains. This resolves an apparent contradiction previously termed “mantle He paradox,” though the derived 3He concentrations are also at odds with a primitive mantle concept that requires still higher concentrations. We propose that the primary fractionation of He/Ne and He/Ar ratios is related to a more compatible behavior of He relative to Ne, Ar during partial melting. One model is presented: at depths below a shallow MORB source region a rising mantle plume causes formation of low‐degree partial melts, which are deficient in He. During melt ascent, these “plume”‐derived melts mix with melts of the shallow (MORB‐type) mantle source that experienced a higher degree of melting and hence show no He deficit. Depending on the relative proportion of mixing the resulting oceanic basalts will encompass the whole observed range from Loihi‐type and MORB‐type noble gas isotope compositions.

Smith, Deborah K.; Escartín, Javier; Schouten, Hans; Cann, Johnson R.

doi: 10.1029/2007GC001699pmid: N/A

The region of the Mid‐Atlantic Ridge (MAR) between the Fifteen‐Twenty and Marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension. Normal faults show large amounts of rotation, dome‐shaped corrugated detachment surfaces (core complexes) intersect the seafloor at the edge of the inner valley floor, and extinct core complexes cover the seafloor off‐axis. We have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments (13° and 15°N segments). Steep outward‐facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30°. The rotation occurs very close to the ridge axis (as much as 20° within 5 km of the volcanic axis) and is complete by ∼1 My, producing distinctive linear ridges with roughly symmetrical slopes. This morphology is very different from linear abyssal hill faults formed at the 14°N magmatic segment, which display a smaller amount of rotation (typically <15°). We suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults. If faults are long‐lived, a dome‐shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex. A single ridge segment can have several active core complexes, some less than 25 km apart that are separated by swales. We present two models for multiple core complex formation: a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales, and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales. Either model can explain the observed morphology.

Wheat, C. Geoffrey; McManus, James

doi: 10.1029/2007GC001892pmid: N/A

We present concentrations of germanium and silicon in sediment pore waters, basaltic formation fluids, and bulk sediment from three ridge flank hydrothermal systems (RFHS). Basaltic formation fluids from warm (>30°C) RFHS have much higher Ge concentrations and Ge:Si molar ratios than overlying sediment pore waters, requiring seawater‐basalt reactions to dominate Ge concentrations in basaltic formation fluids. In contrast to warm RFHS, cool (∼20°C) RFHS have similar Ge concentrations in basal sediment pore waters and underlying basaltic formation fluids, implying that there is little net exchange between these two fluid reservoirs. Despite this low net exchange, Ge:Si molar ratios in basaltic formation fluids are elevated compared to seawater and overlying sediment pore waters, implying that seawater‐basalt reactions must influence Ge and Si cycling. Such seawater‐basalt reactions are likely associated with secondary clay formation because increases in Ge concentration scale with Mg loss from basaltic formation fluids. Processes that control Ge cycling in cold (3–10°C) RFHS are poorly constrained because our data are restricted to sediment pore waters that have been overprinted by diagenetic reactions and possibly sampling artifacts. Although net Ge fluxes from RFHS prevail over a wide temperature range, a refined estimate for the global RFHS Ge flux is currently not possible without data from cold RFHS springs or basaltic formation fluids because cold RFHS transport most of the convective heat and crustal fluid to the oceans.