Showing papers in "Geochemistry Geophysics Geosystems in 2005"

Geochemical mapping of the Mariana arc‐basin system: Implications for the nature and distribution of subduction components

Abstract: A new ICP-MS database for glasses from the Mariana Trough, together with published and new ICP-MS data from the Mariana arc, provides the basis for geochemical mapping of the Mariana arc-basin system. The geochemical maps presented here are based on the graphic representation of spatial variations in geochemical proxies for the principal mantle and subduction components. The focus is on three elements with high and similar partition coefficients but different behavior in subduction systems, namely, Ba, Th, and Nb. Two elements with different partition coefficients, Ta and Yb, are used as normalizing factors. Ratio maps (Ta/Yb, Nb/Ta, Th/Ta, Ba/Ta, Ba/Th) provide the simplest petrogenetic insights, subduction zone addition maps based on deviations from a MORB array provide more quantitative insights, and component maps represent an attempt to isolate the different subduction components. The maps shown here indicate the presence of a variably depleted asthenosphere and three added components: a Nb-Th-Ba component, a Th-Ba deep-subduction component, and a Ba-only shallow-subduction component. The asthenosphere entering the system is enriched relative to N-MORB and appears to be focused at three sites within the Mariana Trough. The Nb-Th-Ba component is present mainly in the north of the arc (the Northern Seamount province and northern Central Island Province), the northern edge of the Mariana Trough, and two locations within the Southern Seamount Province. It has a distinctively high Nb/Ta ratio and a moderate enrichment in Th and Ba relative to Nb. Its composition and distribution indicate that it may not be part of the present subduction system but instead originates in mantle lithosphere previously enriched above the subduction zone by addition of small-degree, subduction-modified mantle melts. The Th-Ba component is present throughout the arc and, in minor amounts, in parts of the back-arc basin. The Ba-only component is mainly present in the central part of the arc and at the edges of the back-arc basin. Overall, the geochemical maps provide a new perspective on the geochemical processes that accompany the evolution of an arc basin system from prerifting lithospheric enrichment, through arc-rifting to arc volcanism and back-arc spreading

FOZO, HIMU, and the rest of the mantle zoo

Abstract: Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 6 (2005): Q05007, doi:10.1029/2004GC000824.

On the relationships between slab dip, back‐arc stress, upper plate absolute motion, and crustal nature in subduction zones

Abstract: [1] Statistical analysis of modern oceanic subduction zone parameters, such as the age of a downgoing plate or the absolute plate motions, is performed in order to investigate which parameter controls the dip of a slab and, conversely, what the influence of slab geometry is on upper plate behavior. For that purpose, parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge/plateau/seamount subduction. On the basis of tomographic images, slabs that penetrate through, or lie on, the 670 km discontinuity are also identified. The results of the statistical analysis are as follows: (1) Back-arc stress correlates with slab dip, i.e., back-arc spreading is observed for deep dips (deeper than 125 km) larger than 50°, whereas back-arc shortening occurs only for deep dips less than 30°. (2) Slab dip correlates with absolute motion of the overriding plate. The correlation is even better when the slab lies on, or even more penetrates through, the 670 km discontinuity. (3) Slabs dip more steeply, by about 20° on average, beneath oceanic overriding plates than beneath continental ones. (4) Slabs dip more steeply on average by about 10° near edges. (5) Slab dip does not correlate with the magnitude of slab pull, the age of subducting lithosphere at the trench, the thermal regime of the subducting lithosphere, the convergence rate, or the subduction polarity (east versus west). The present study provides evidence that the upper plate absolute motion plays an important role on slab dip, as well as on upper plate strain. Retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates. One can infer that low slab dips correlate well with compression in continental advancing upper plates, whereas steep dips are often associated with extension in oceanic retreating upper plates. Excess weight of old slabs is often counterbalanced by other forces, probably asthenospheric in origin, such as lateral mantle flow near slab edges or anchor forces, to determine slab dip.

Adiabat_1ph: A new public front‐end to the MELTS, pMELTS, and pHMELTS models

Abstract: The program adiabat_1ph is a simple text-menu driver for subroutine versions of the algorithms MELTS, pMELTS, and pHMELTS [Asimow et al., 2004; Ghiorso et al., 2002; Ghiorso and Sack, 1995]. It may be used to calculate equilibrium assemblages along a thermodynamic path set by the user and can simultaneously calculate trace element distributions. The MELTS family of algorithms is suitable for multicomponent systems, which may be anhydrous, water-undersaturated, or water-saturated, with the options of buffering oxygen fugacity and/or water activity. A wide variety of calculations can be performed either subsolidus or with liquid(s) present; melting and crystallization may be batch, fractional, or continuous. The software is suitable for Linux, MacOS X, and Windows, and many aspects of program execution are controlled by environment variables. Perl scripts are also provided that may be used to invoke adiabat_1ph with some command line options and to produce output that may be easily imported into spreadsheet programs, such as Microsoft Excel. Benefits include a batch mode, which allows almost complete automation of the calculation process when suitable input files are written. This technical brief describes version 1.04, which is provided as ancillary material. Binaries, scripts, documentation, and example files for this and future releases may be downloaded at http://www.gps.caltech.edu/~asimow/adiabat. On a networked computer, adiabat_1ph automatically checks whether a newer version is available.

On the uncertainties in hot spot reconstructions and the significance of moving hot spot reference frames

Abstract: [1] It is widely accepted that substantial relative motion has occurred between the Indo-Atlantic and Pacific hot spots since the Late Cretaceous. At the same time, a fixed Indo-Atlantic hot spot reference frame has been argued for and used since the advent of plate tectonics, implying relatively little motion between the hot spots in this domain since about 130 Ma. Most plumes purported to have caused these hot spots, while being advected in the global-scale mantle flow field, are assumed to move an order of magnitude more slowly than plates. However, the lifetime of a plume may be over ∼100 Myr, and the integrated motion of a plume is expected to be significant over these times. The uncertainties inherent in hot spot reconstructions are of a magnitude similar to the expected plume motion, and so any differences between a fixed and moving frame of reference must be discernible beyond the level of these uncertainties. We present a method for constraining hot spot reconstruction uncertainties, similar to that in use for relative plate motion. We use a modified Hellinger criterion of fit for the hot spot problem, using track geometries and radiometric dating, and derive covariance matrices for our Indo-Atlantic rotations for the last 120 Myr. However, any given mantle convection model introduces additional uncertainties into such models, based on its model parameters and starting conditions (e.g., choice of global tomography model, viscosity profile, nature of mantle phase transitions). We use an interactive evolutionary approach, where we constrain the hot spot motion resulting from convection models to fit paleomagnetic constraints, and converge on an acceptable motion solution by varying unknowns over several generations of simulations. Our hot spot motion model shows large motion (5–10°) of the Indo-Atlantic hot spots for times >80 Ma, consistent with available paleomagnetic constraints. The differences between the fixed and moving hot spot reference frames are not discernible over the level of uncertainty in such rotations for times <80 Ma.

Some thoughts on the use of InSAR data to constrain models of surface deformation : noise structure and data downsampling

Abstract: Repeat-pass Interferometric Synthetic Aperture Radar (InSAR) provides spatially dense maps of surface deformation with potentially tens of millions of data points. Here we estimate the actual covariance structure of noise in InSAR data. We compare the results for several independent interferograms with a large ensemble of GPS observations of tropospheric delay and discuss how the common approaches used during processing of InSAR data affects the inferred covariance structure. Motivated by computational concerns associated with numerical modeling of deformation sources, we then combine the data-covariance information with the inherent resolution of an assumed source model to develop an efficient algorithm for spatially variable data resampling (or averaging). We illustrate these technical developments with two earthquake scenarios at different ends of the earthquake magnitude spectrum. For the larger events, our goal is to invert for the coseismic fault slip distribution. For smaller events, we infer the hypocenter location and moment. We compare the results of inversions using several different resampling algorithms, and we assess the importance of using the full noise covariance matrix.

Continuous geomagnetic field models for the past 7 millennia: 2. CALS7K

Abstract: [1] We present two continuous global geomagnetic field models for recent millennia: CALS3K.2, covering the past 3000 years, and CALS7K.2, covering 7000 years from 5000 BC to 1950 AD. The models were determined by regularized least squares inversion of archeomagnetic and paleomagnetic data using spherical harmonics in space and cubic B splines in time. They are derived from a greatly increased number of paleomagnetic directional data, compared to previous efforts, and for the first time a significant amount of archeointensity data is used in this kind of global model, allowing the determination of evolution of geomagnetic dipole strength. While data accuracy and dating uncertainties remain a limitation, reliable low-resolution global models can be obtained. The results agree well with previous results from virtual axial dipole moment (VADM) studies from archeomagnetic intensity data apart from a systematic offset in strength. A comparison of model predictions with the previous 3000 year model, CALS3K.1, gives general agreement but also some significant differences particularly for the early epochs. The new models suggest that the prominent two northern hemisphere flux lobes are more stationary than CALS3K.1 implied, extending considerably the time span of stationary flux lobes observed in historical models. Between 5000 BC and 2000 BC there are time intervals of weak dipole moment where dipole power is exceeded by low-degree nondipole power at the core-mantle boundary. Model coefficients and evaluation code can be obtained from the EarthRef Digital Archive (ERDA) together with animations and snapshots plots for every 100 years at http://www.earthref.org. Detailed URLs for the different material are listed in Appendix A.

High-precision Pb-Sr-Nd-Hf isotopic characterization of USGS BHVO-1 and BHVO-2 reference materials

Abstract: The recent development of multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and increasing use of the technique have created the need for well-characterized rock standards, especially for isotopic systems where no internal fractionation correction can be applied. This paper presents a careful leaching experiment on the U.S. Geological Survey (USGS) reference materials BHVO-1 and BHVO-2 (Hawaiian basalts) and documents the evidence for contamination of the rock powders during processing. This contamination accounts for the difference in Pb isotopic ratios of BHVO-1 and BHVO-2 as well as for their lack of homogeneity both in Pb isotopic compositions and in some trace element contents. Copyright 2005 by the American Geophysical Union.

Submarine fault scarps in the Sea of Marmara pull-apart (North Anatolian Fault): Implications for seismic hazard in Istanbul

Abstract: Earthquake scarps associated with recent historical events have been found on the floor of the Sea of Marmara, along the North Anatolian Fault (NAF). The MARMARASCARPS cruise using an unmanned submersible (ROV) provides direct observations to study the fine-scale morphology and geology of those scarps, their distribution, and geometry. The observations are consistent with the diversity of fault mechanisms and the fault segmentation within the north Marmara extensional step-over, between the strike-slip Ganos and Izmit faults. Smaller strike-slip segments and pull-apart basins alternate within the main step-over, commonly combining strike-slip and extension. Rapid sedimentation rates of 1?3 mm/yr appear to compete with normal faulting components of up to 6 mm/yr at the pull-apart margins. In spite of the fast sedimentation rates the submarine scarps are preserved and accumulate relief. Sets of youthful earthquake scarps extend offshore from the Ganos and Izmit faults on land into the Sea of Marmara. Our observations suggest that they correspond to the submarine ruptures of the 1999 Izmit (Mw 7.4) and the 1912 Ganos (Ms 7.4) earthquakes. While the 1999 rupture ends at the immediate eastern entrance of the extensional Cinarcik Basin, the 1912 rupture appears to have crossed the Ganos restraining bend into the Sea of Marmara floor for 60 km with a right-lateral slip of 5 m, ending in the Central Basin step-over. From the Gulf of Saros to Marmara the total 1912 rupture length is probably about 140 km, not 50 km as previously thought. The direct observations of submarine scarps in Marmara are critical to defining barriers that have arrested past earthquakes as well as defining a possible segmentation of the contemporary state of loading. Incorporating the submarine scarp evidence modifies substantially our understanding of the current state of loading along the NAF next to Istanbul. Coulomb stress modeling shows a zone of maximum loading with at least 4?5 m of slip deficit encompassing the strike-slip segment 70 km long between the Cinarcik and Central Basins. That segment alone would be capable of generating a large-magnitude earthquake (Mw 7.2). Other segments in Marmara appear less loaded.

Strong biological controls on Sr/Ca ratios in aragonitic marine bivalve shells

Abstract: [1] It is well known that skeletal remains of carbonate secreting organisms can provide a wealth of information about past environments. Sr/Ca ratios have been successfully used as a temperature proxy in corals and sclerosponges. Previous work on aragonitic bivalve shells has not been conclusive but suggests a major control of growth rate on Sr/Ca ratios. As many studies have used bivalve growth rates to determine temperature, we tested if Sr/Ca ratios could predict temperature through its relationship with growth rate. Shells from the two species of clams from the same family (veneroidea) studied here, Saxidomus giganteus and Mercenaria mercenaria, show vastly different seasonal Sr/Ca profiles. A strong relationship between average annual Sr/Ca ratios and annual growth rate was found in S. giganteus shells from both Washington (R2 = 0.87) and Alaska (R2 = 0.64), USA, but not in M. mercenaria shells from North Carolina, USA. Furthermore, the Sr/Ca–growth rate relationship was also evident upon a more detailed inspection of subannual growth rates in S. giganteus (R2 = 0.73). Although there were significant positive correlations between Sr/Ca ratios and temperature in S. giganteus shells, the correlations were weak (0.09 < R2 < 0.27), and thus Sr/Ca ratios cannot be used as a reliable temperature proxy in these species of aragonitic bivalves. It is clear from this study that Sr/Ca ratios are not under thermodynamic control in either clam species, since thermodynamics predict a negative correlation between Sr/Ca ratios and temperature in aragonite. This points toward dominance of biological processes in the regulation of Sr2+. This is also reflected by the largely differing Sr/Ca partition coefficients (DSr) in these shells (DSr ≈ 0.25), when compared to inorganic, coral, and sclerosponge studies (DSr ≈ 1), all of which show a negative dependence of Sr/Ca on temperature. We suggest that caution be taken when using Sr/Ca in any biogenic aragonite as a temperature proxy when the DSr greatly deviates from one, as this indicates the dominance of biological controls on Sr/Ca ratios.

Relationship between bend-faulting at trenches and intermediate-depth seismicity

Abstract: [1] We have studied faulting associated with bending of the incoming oceanic plate along segments of Middle America and Chile subduction zones and its relationship to intermediate-depth intraslab seismicity and slab geometry. Multibeam bathymetry shows that bending-related faulting forms patterns made of sets of faults with orientations ranging from parallel to almost perpendicular to the trench axis. These fault patterns may change along a single subduction zone within along-strike distances of several hundred kilometers or less. Where available, near-trench intraplate earthquakes show normal-fault focal mechanisms consistent with mapped bending-related normal faults. The strike of bending-related faults in the incoming oceanic plate is remarkably similar to the strike of the nodal planes of intermediate-depth earthquakes for each segment of the study areas. This similarity in strike is observed even for faults oriented very oblique to the trench and slab strikes. Thus, in the studied subduction zones, results strongly support that many intraslab earthquakes do not occur along the planes of maximum shear within the slab and that much intermediate-depth seismicity occurs by reactivation of faults formed by plate bending near the trench. Furthermore, a qualitative relationship between trench faulting and intraslab seismicity is indicated by segments of the incoming plate with pervasive bend-faulting that correspond to segments of the slabs with higher intermediate-depth seismicity.

Formation of the Jinchuan ultramafic intrusion and the world's third largest Ni-Cu sulfide deposit: Associated with the ∼825 Ma south China mantle plume?

Abstract: We report here SHRIMP U-Pb geochronological, geochemical, and Nd isotopic data for the Jinchuan ultramafic intrusion (Gansu Province, China), which hosts the world's third largest magmatic Ni-Cu sulfide deposits U-Pb baddeleyite analyses yield an age of 812 ± 26 Ma for the ultramafic intrusion This age is indistinguishable within analytical uncertainties from the U-Pb zircon ages of 827 ± 8 Ma and 828 ± 3 Ma for the sulfide-bearing ultramafic rocks and the dolerite dykes that cut the ultramafic intrusion, respectively These U-Pb dating results show beyond doubt that the Jinchuan ultramafic intrusion and associated Ni-Cu sulfide deposit were formed at ∼825 Ma, rather than ∼1500 Ma as has been widely believed The ultramafic rocks exhibit large negative ɛNd(T) values (−89 to −120) that decrease with increasing La/Sm, suggesting that their parental magmas were derived from a long-term enriched lithospheric mantle and experienced crustal contamination Mineralogical, petrological, and geochemical data all indicate that the Jinchuan intrusion was generated by melting of the enriched lithospheric mantle heated by an anomalously hot plume The U-Pb ages of ∼825 Ma for igneous baddeleyites and zircons and ∼900–880 Ma for inherited zircons in the Jinchuan mafic-ultramafic rocks are comparable with those in the Qaidam block and Qilian belt, the western extension of the Qinling belt that was likely derived from northern Yangtze craton The Jinchuan Ni-Cu sulfide-bearing intrusion, along with coeval regional plume-related mafic dykes and tholeiites, and mafic-ultramafic complexes with associated V-Ti and Ni-Cu-PGE mineralization, is interpreted to be genetically related to the ∼825 Ma south China mantle plume

Radiolytic H2 in continental crust: Nuclear power for deep subsurface microbial communities

Abstract: [1] H2 is probably the most important substrate for terrestrial subsurface lithoautotrophic microbial communities. Abiotic H2 generation is an essential component of subsurface ecosystems truly independent of surface photosynthesis. Here we report that H2 concentrations in fracture water collected from deep siliclastic and volcanic rock units in the Witwatersrand Basin, South Africa, ranged up to two molar, a value far greater than observed in shallow aquifers or marine sediments. The high H2 concentrations are consistent with that predicted by radiolytic dissociation of H2O during radioactive decay of U, Th, and K in the host rock and the observed He concentrations. None of the other known H2-generating mechanisms can account for such high H2 abundance either because of the positive free energy imposed by the high H2 concentration or pH or because of the absence of required mineral phases. The radiolytic H2 is consumed by methanogens and abiotic hydrocarbon synthesis. Our calculations indicate that radiolytic H2 production is a ubiquitous and virtually limitless source of energy for deep crustal chemolithoautotrophic ecosystems.

Continuous geomagnetic field models for the past 7 millennia: 1. A new global data compilation

Abstract: [1] A global data set of archeomagnetic and paleomagnetic data covering the past 7000 years has been compiled. It consists of 16,085 results of inclination, 13,080 of declination, and 3188 of intensity for the time span 5000 BC to 1950 AD. Declination and inclination data come partly from existing databases and partly from original literature. A new global compilation of intensity data for the millennial scale is included. Data and dating uncertainties are discussed as we attempted to obtain an internally coherent data set. The global distribution of the data is very inhomogeneous in both time and space. All the data are compared to predictions from the previous 3000 year global model, CALS3K.1. This collection of data will be useful for global secular variation studies and geomagnetic field modeling, although southern hemisphere data are still underrepresented. In particular, we will use it in a further study to update and extend the existing global model, CALS3K.1. The huge increase in data compared to the previous compilation will result in significant changes from current models. As we might have missed some suitable data, we encourage the reader to notify us about any data that have not been included yet and might fit in, as improving our global millennial scale models remains our aim for the future. The data files described in this paper are available from the EarthRef Digital Archive (ERDA) at http://earthref.org/cgi-bin/erda.cgi?n=331.

Geochemistry of serpentinized peridotites from the Mariana Forearc Conical Seamount, ODP Leg 125: Implications for the elemental recycling at subduction zones

Abstract: [1] Recent examinations of the chemical fluxes through convergent plate margins suggest the existence of significant mass imbalances for many key species: only 20–30% of the to-the-trench inventory of large-ion lithophile elements (LILE) can be accounted for by the magmatic outputs of volcanic arcs. Active serpentinite mud volcanism in the shallow forearc region of the Mariana convergent margin presents a unique opportunity to study a new outflux: the products of shallow-level exchanges between the upper mantle and slab-derived fluids. ODP Leg 125 recovered serpentinized harzburgites and dunites from three sites on the crests and flanks of the active Conical Seamount. These serpentinites have U-shaped rare earth element (REE) patterns, resembling those of boninites. U, Th, and the high field strength elements (HFSE) are highly depleted and vary in concentration by up to 2 orders of magnitude. The low U contents and positive Eu anomalies indicate that fluids from the subducting Pacific slab were probably reducing in nature. On the basis of substantial enrichments of fluid-mobile elements in serpentinized peridotites, we calculated very large slab inventory depletions of B (79%), Cs (32%), Li (18%), As (17%), and Sb (12%). Such highly enriched serpentinized peridotites dragged down to depths of arc magma generation may represent an unexplored reservoir that could help balance the input-output deficit of these elements as observed by Plank and Langmuir (1993, 1998) and others. Surprisingly, many species thought to be mobile in fluids, such as U, Ba, Rb, and to a lesser extent Sr and Pb, are not enriched in the rocks relative to the depleted mantle peridotites, and we estimate that only 1–2% of these elements leave the subducting slabs at depths of 10 to 40 km. Enrichments of these elements in volcanic front and behind-the-front arc lavas point to changes in slab fluid composition at greater depths.

Mid‐ocean microseisms

Abstract: [1] The Hawaii-2 Observatory (H2O) is an excellent site for studying the source regions and propagation of microseisms since it is located far from shorelines and shallow water. During Leg 200 of the Ocean Drilling Program, the officers of the JOIDES Resolution took wind and wave measurements for comparison with double-frequency (DF) microseism data collected at nearby H2O. The DF microseism band can be divided into short-period and long-period bands, SPDF and LPDF, respectively. Comparison of the ship's weather log with the seismic data in the SPDF band from about 0.20 to 0.45 Hz shows a strong correlation of seismic amplitude with wind speed and direction, implying that the energy reaching the ocean floor is generated locally by ocean gravity waves. Nearshore land seismic stations see similar SPDF spectra, also generated locally by wind seas. At H2O, SPDF microseism amplitudes lag sustained changes in wind speed and direction by several hours, with the lag increasing with wave period. This lag may be associated with the time necessary for the development of opposing seas for DF microseism generation. Correlation of swell height above H2O with the LPDF band from 0.085 to 0.20 Hz is often poor, implying that a significant portion of this energy originates at distant locations. Correlation of the H2O seismic data with NOAA buoy data, with hindcast wave height data from the North Pacific, and with seismic data from mainland and island stations, defines likely source areas of the LPDF signals. Most of the LPDF energy at H2O appears to be generated by high-amplitude storm waves impacting long stretches of coastline nearly simultaneously, and the Hawaiian Islands appear to be a significant source of LPDF energy in the North Pacific when waves arrive from particular directions. The highest levels observed at mid-ocean site H2O occur in the SPDF band when two coincident nearby storm systems develop. Deep water, mid-ocean-generated DF microseisms are not observed at continental sites, indicating high attenuation of these signals. At near-coastal seismic stations, both SPDF and LPDF microseism levels are generally dominated by local generation at nearby shorelines.

Identification and removal of Mn-Mg-rich contaminant phases on foraminiferal tests: Implications for Mg/Ca past temperature reconstructions

Abstract: [1] The geochemical composition of foraminifera shells from an Ocean Drilling Program site in the Panama Basin has been analyzed by several analytical techniques (LA-ICP-MS, ICP-MS, XRD, SEM, EDX) in order to identify and evaluate the occurrence of contaminant phases which may bias paleoenvironmental reconstructions. LA-ICP-MS results on uncleaned tests indicate the presence of MnMg-rich contaminant phases at the inner surfaces of the foraminiferal shells (which have Mn/Ca ratios up to 400 mmol mol 1 and Mg/Ca ratios up to 50 mmol mol 1 ). We have rigorously assessed the ability of different cleaning protocols to remove these contaminant phases and have obtained satisfactory results only when a reductive step is included. The analysis of cleaning residuals collected after each of the different cleaning steps applied reveals that high Mn values are associated with at least two different contaminant phases, of which only one is linked to high Mg values. XRD analysis further reveals that the Mn-Mg-rich phase is the Ca-Mn-Mg carbonate kutnahorite (Ca(Mn, Mg)(CO3)2). Our results demonstrate that the presence of kutnahorite-like minerals can bias Mg/Ca ratios toward higher values (by 7–36%) and lead to significant overestimation of past seawater temperatures (by 0.9 up to 6.2� C, in the case of these Panama Basin samples). Components: 11,867 words, 13 figures, 3 tables.

A Cenozoic diffuse alkaline magmatic province (DAMP) in the Southwest Pacific without rift or plume origin

Abstract: [1] Common geological, geochemical, and geophysical characteristics of continental fragments of East Gondwana and adjacent oceanic lithosphere define a long-lived, low-volume, diffuse alkaline magmatic province (DAMP) encompassing the easternmost part of the Indo-Australian Plate, West Antarctica, and the southwest portion of the Pacific Plate. A key to generating the Cenozoic magmatism is the combination of metasomatized lithosphere underlain by mantle at only slightly elevated temperatures, in contrast to large igneous provinces where mantle temperatures are presumed to be high. The SW Pacific DAMP magmatism has been conjecturally linked to rifting, strike-slip faulting, mantle plumes, or hundreds of hot spots, but all of these associations have flaws. We suggest instead that sudden detachment and sinking of subducted slabs in the late Cretaceous induced Rayleigh-Taylor instabilities along the former Gondwana margin that in turn triggered lateral and vertical flow of warm Pacific mantle. The interaction of the warm mantle with metasomatized subcontinental lithosphere that characterizes much of the SW Pacific DAMP concentrates magmatism along zones of weakness. The model may also provide a mechanism for warming south Pacific mantle and resulting Cenozoic alkaline magmatism, where the oceanic areas are characterized primarily, but not exclusively, by short-lived hot spot tracks not readily explained by conventional mantle plume theory. This proposed south Pacific DAMP is much larger and longer-lived than previously considered.

Erosion and exhumation in accretionary orogens: Experimental and geological approaches

Abstract: [1] This study examines the role of erosion on exhumation and deformation history of accretionary orogens through experimental and geological methods. Fault propagation, kinematics, and exhumation rates in an eroded thrust wedge are analyzed through scaled sandbox simulations by testing different parameters (friction, rheologic layering, and sediment input/output ratio). A model Coulomb wedge is submitted to erosion under flux steady state conditions; that is, the volume of eroded material remains equal to the volume of newly accreted material, maintaining a constant surface slope during shortening. Results show that the way of exhumation depends on the internal dynamics of thrust wedges and, conversely, on how this dynamics is modified by erosion. In the eroded thrust wedges the diversity of exhumation patterns is controlled by the mode of fault propagation, depending on the basal friction (high or low). The vertical component of exhumation is generally higher for the wedges with high basal friction than for low-friction wedges. The uplift of material occurs along a cluster of subvertical thrusts in the middle part of the eroded thrust wedge with low basal friction. The material is exhumed along a series of inclined (20°–50°) thrusts in the rear of the high-friction wedge. The zone of maximum exhumation is generally localized in the middle part of the thrust wedge and migrates toward the backstop with continued shortening. The vertical exhumation rate increases with time, and the material accreted later is rapidly transferred to the main exhumation zone compared to the material accreted during the early stages. The exhumation occurs at rates 2 times faster in the wedges with half the thickness than in full thickness wedges. The total quantity of eroded material at the end of convergence constitutes 36–50% of initial model area for the thrust wedges with low basal friction and 20–40% for the high basal friction wedges. The extent of basal underplating increases with total shortening. The area of basal underplated material constitutes up to 30 and 40% of the eroded thrust wedge area for the models with low and high basal friction, respectively. Presence of decollements in the accreted series allows underplating of thrust units developing an anticlinal stack, whose growth and location is favored by erosion. Our results are compared to different present-day active and ancient convergent orogens (Himalaya, Taiwan, Southern Alps of New Zealand, and Cascadia orogen), allowing the different specific exhumation processes to be characterized.

MPI-DING glasses: New geological reference materials for in situ Pb isotope analysis

Abstract: [1] Advances in microanalytical techniques continue to revolutionize trace element and isotope geochemistry. Yet the lack of widely available geological reference materials has made interlaboratory comparisons and assessment of analytical accuracy difficult or impossible in many cases. The preparation and extensive analysis of the MPI-DING glasses has been designed to fill this gap. Here we report high-precision Pb isotope analyses by TIMS and MC-ICPMS techniques on seven geological MPI-DING glasses. The Pb concentrations in the reference glasses vary between 0.4 and 20 μg/g, allowing use in a wide range of analytical situations. In addition, solution ICPMS and LA-ICPMS analyses were used to investigate the analytical performance of a rapidly scanning single-collector sector-field mass spectrometer for the measurement of Pb isotope ratios. Our TIMS and MC-ICPMS measurements demonstrate that variations of 208Pb/206Pb and 207Pb/206Pb in most MPI-DING glasses are less than 0.03%. Preliminary reference values for 208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb, 208Pb/206Pb, and 207Pb/206Pb were obtained by averaging all high-precision data and normalizing them to the mean triple spike data for NIST SRM 981, obtained in the Mainz laboratory. 208Pb/206Pb and 207Pb/206Pb ratios, which are commonly utilized for in situ Pb isotope studies, vary from 2.0110 (GOR132-G) to 2.0810 (T1-G) and 0.81656 (GOR132-G) to 0.84389 (GOR128-G), respectively. Any heterogeneity present in the glasses is generally lower than the reproducibility obtained from in situ microanalytical techniques, such as LA-ICPMS and SIMS (about 0.1–0.3% for 208Pb/206Pb and 207Pb/206Pb measurements). This is demonstrated by the results for our LA-ICPMS Pb isotope analyses using a single-collector sector-field ICP mass spectrometer, which agree to within 0.2% of the reference values. A similar accuracy was obtained from solution ICPMS using the same single-collector mass spectrometer.

Postglacial eruptive history of the Western Volcanic Zone, Iceland

Abstract: New field observations, age constraints, and extensive chemical analyses define the complete postglacial eruptive history of the 170-km-long Western Volcanic Zone (WVZ) of Iceland, the ultraslow-spreading western boundary of the south Iceland microplate. We have identified 44 separate eruptive units, 10 of which are small-volume eruptions associated with the flanking Grimsnes system. Overall chemical variations are consistent with very simplified models of melting of a source approximating primitive mantle composition. The 17 eruptions in the first 3000 years of postglacial time account for about 64% of the total postglacial production and are incompatible-element depleted compared to younger units, consistent with enhanced melting as a consequence of rebound immediately following deglaciation. Steadily declining eruption rates for the last 9000 years also correlate with changes in average incompatible element ratios that appear to reflect continued decline in melting extents to the present day. This result is not restricted to the WVZ, however, and may herald a decline in melting throughout all of western Iceland during later postglacial time. Lavas from the northern part of the WVZ are depleted in incompatible elements relative to those farther south at all times, indicating either a long-wavelength gradient in mantle source composition or variations in the melting process along axis. We find no evidence in the postglacial volcanic record for current failure of the WVZ, despite evidence for continued propagation of the eastern margin of the microplate. The dominance of lava shields in the eruptive history of the WVZ contrasts with the higher number of fissure eruptions in other Icelandic volcanic zones. WVZ shields represent long-duration, low-effusion rate eruptions fed by recharge magma arising out of the mantle. Average effusion rate is the key variable distinguishing shield and fissure eruptions, both within the WVZ and between different volcanic zones. High effusion rate, large-volume eruptions require the presence of large crustal magma reservoirs, which have been rare or absent in the WVZ throughout postglacial time.

Characterization of Mg/Ca distributions in planktonic foraminifera species by electron microprobe mapping

Abstract: [1] The distribution of Mg/Ca within the tests of eight modern planktonic foraminifer species has been characterized using electron microprobe mapping. Species include several that are commonly used for estimation of past seawater temperatures (Globigerinoides ruber, G. sacculifer, Neogloboquadrina incompta (synonym of N. pachyderma dex.), N. dutertrei, and G. truncatulinoides). Each of the investigated species displays large variations in Mg/Ca composition within individual tests. However, the pattern of Mg/Ca variation is notably different between symbiont-bearing and symbiont-free species. In symbiotic species, cyclic Mg/Ca compositional banding occurs that is characterized by narrow (<1–3 μm), high-Mg/Ca (typically 8–11 mmol/mol) bands, intercalated between broader low Mg/Ca (typically 1–5 mmol/mol) bands. This factor of 2–3 difference equates to an apparent calcification temperature change of 10°C or more. Such temperature changes are considered highly improbable and suggest vital effects significantly modify the incorporation of Mg/Ca into the tests of symbiont-bearing species. These vital effects remain poorly understood and demand further careful evaluation as they may need to be accounted for when making reliable reconstructions of past oceanic temperatures. Symbiont-free species typically have fewer and broader compositional bands that may reflect more closely changes in calcification temperature as these species migrate within a water column.

Stable organic carbon isotope stratigraphy across Oceanic Anoxic Event 2 of Demerara Rise, western tropical Atlantic

Abstract: Ocean Drilling Program (ODP) Leg 207 recovered expanded sections of organic-carbon-rich laminated shales on Demerara Rise (western tropical Atlantic) High-resolution organic carbon isotope and total organic carbon (TOC) records are presented, which span the Cenomanian-Turonian boundary interval (CTBI), including the Oceanic Anoxic Event (OAE) 2, from four sites oriented along a NW striking depth transect These records represent the first high-resolution carbon isotope records across OAE 2 from the South American margin of the tropical Atlantic Due to the scarcity of age significant fossils, the main purpose of this study was to develop a detailed carbon isotope stratigraphy in order to correlate the CTBI across the depth transect and to tie this to biostratigraphically well-defined sections in the Western Interior Basin (Pueblo, USA), boreal shelf seas (Eastbourne, England), and western Tethys (Oued Mellegue, Tunisia) All four sections studied document a 6‰ increase of ?13Corg values at the base of the CTBI, which is followed by an interval of elevated ?13Corg values and a subsequent decrease Our results supply an important stratigraphic base for subsequent paleoceanographic studies on Late Cenomanian to Early Turonian sediments from Demerara Rise and elsewhere

Weak mixing in Lake Kivu: New insights indicate increasing risk of uncontrolled gas eruption

Abstract: [1] The deep waters of the East African Rift Lake Kivu contain large amounts of dissolved carbon dioxide and methane The release of a fraction of these gases, which could be triggered by a magma eruption within the lake, would have catastrophic consequences for the two million people living on its shore Up to now the safety assessment of the lake was based on the assumption that the gas concentrations in the deep waters are in a steady state with a residence time of 400 years Turbulent transport was regarded as the main pathway of vertical exchange Recent measurements and the analysis of the vertical transport processes in the lake radically change this evaluation The vertical turbulent exchange is negligible, as documented by a spectacular set of several hundred double-diffusive layers Gases are mainly transported out of the deep zones by a slow upwelling with a residence time of 800–1000 years Our results indicate that the methane production within the sediment has recently increased, leading to a gas accumulation in the deep waters and consequently decreasing the heat input needed to trigger a devastating gas release With the estimated current CH4 production, the gas concentrations could approach saturation within this century

Oxygen isotope constraints on the sources of Central American arc lavas

Abstract: Oxygen-isotope ratios of olivine and plagioclase phenocrysts in basalts and basaltic andesites from the Central American arc vary systematically with location, from a minimum δ18Oolivine value of 4.6 (below the range typical of terrestrial basalts) in Nicaragua near the center of the arc to a maximum δ18Oolivine value of 5.7 (above the typical range) in Guatemala near the northwest end of the arc. These oxygen-isotope variations correlate with major and trace element abundances and with Sr and Nd isotope compositions of host lavas, defining trends that suggest variations in δ18O reflect slab contributions to the mantle sources of these lavas. These trends can be explained by a model in which both a low-δ18O, water-rich component and a high-δ18O, water-poor component are extracted from the subducting Cocos slab and flux melting in the overlying mantle wedge. The first of these components dominates slab fluxes beneath the center of the arc and is the principal control on the extent of melting of the mantle wedge (which is highest in the center of the arc); the second component dominates slab fluxes beneath the northwestern margin of the arc. Fluxes of both components are small or negligible beneath the southeastern margin of the arc. We suggest that the low-δ18O component is a solute-rich aqueous fluid produced by dehydration of hydrothermally altered rocks deep within the Cocos slab (perhaps serpentinites produced in deep normal faults offshore of Nicaragua) and that the high-δ18O component is a partial melt of subducted sediment on top of the plate.

Recent distribution and accumulation of organic carbon on the continental margin west off Spitsbergen

Abstract: The study compiles the controlling factors for organic matter sedimentation patterns from a suite of organogeochemical parameters in surface sediments off Spitsbergen and direct seabed observations using a Remotely Operated Vehicle (ROV) In addition we assess its storage rates as well as the potential of carbon sinks on the northwestern margin of the Barents Sea with short sediment cores from a selected fjord environment (Storfjord) While sedimentation in the fjords is mainly controlled by river/meltwater discharge and coastal erosion by sea ice/glaciers resulting in high supply of terrigenous organic matter, Atlantic water inflow, and thus enhanced marine organic matter supply, characterizes the environment on the outer shelf and slope Local deviations from this pattern, particularly on the shelf, are due to erosion and out washing of fine-grained material by bottom currents Spots dominated by marine productivity close to the island have been found at the outer Isfjord and west off Prins Karls Forland as well as off the Kongsfjord/Krossfjord area and probably reflect local upwelling of nutrient-rich Atlantic water–derived water masses Accumulation rates of marine organic carbon as well as reconstructed primary productivities decreased since the middle of the last century Negative correlation of the Isfjord temperature record with reconstructed productivities in the Storfjord could be explained by a reduced annual duration of the marginal ice zone in the area due to global warming Extremely high accumulation rates of marine organic carbon between 54 and 172 g m−2 yr−1 mark the Storfjord area, and probably high-latitude fjord environments in general, as a sink for carbon dioxide

Shell of the Great Scallop Pecten maximus as a high‐frequency archive of paleoenvironmental changes

Abstract: [1] We investigate the environmental and biological controls on oxygen isotope ratios in the shells of modern Pecten maximus bivalves collected alive in the Bay of Brest (France). Seasonal δ18O profiles, obtained by drilling the daily calcite ridges deposited at the surface of the left valve, were compared with in situ measurements of temperature and salinity. We show that P. maximus δ18O values accurately track seasonal variations in bottom water temperature. Shell growth rate has no significant effect on shell δ18O values. Our study demonstrates that daily variations in water temperature can be reconstructed within about 0.5°C. Temperatures estimated with the paleotemperature equation established in this study were compared with temperatures derived from previously published equations. The comparison indicates that the most commonly used paleotemperature equation for biogenic calcite (Epstein et al., 1953) provides inaccurate temperature estimates, but the Kim and O'Neil (1997) equation, established from abiogenic calcite precipitation, provides results very similar to ours and should therefore be used for scallop individuals coming from populations where proper empirical calibration cannot be done. Pecten maximus bivalves precipitate calcite in isotopic equilibrium with seawater, produce large daily growth striae, are stenohaline, and are well preserved in archeological and geological deposits, making them an excellent high-frequency archive of paleoenvironmental change.

Acquisition of the neodymium isotopic composition of the North Atlantic Deep Water

Abstract: The North Atlantic Deep Water (NADW) neodymium isotopic composition (Nd IC) is increasingly used in oceanography and paleoceanography to trace large-scale circulation and weathering processes, notably to investigate past variations of the global thermohaline circulation. Although the present-day NADW Nd IC is well characterized at e Nd = A13.5, the acquisition of this isotopic signature (in other words, the causes of this value) has so far been very sparsely documented. Such an understanding is, however, fundamental to the interpretation of paleo records. Nd IC and rare earth element concentrations were measured at 9 stations within the North Atlantic Subpolar Gyre (SIGNATURE cruise, summer 1999). The comparison of this data set with our understanding of water mass circulation provides a description of how the three layers constituting the NADW, the Labrador Sea Water (LSW, e Nd = A13.9 ± 0.4), North East Atlantic Deep Water (NEADW, e Nd A13.2 ± 0.4), and North West Atlantic Bottom Water (NWABW, e Nd A14.5 ± 0.4), acquire their Nd IC through distinct water mass mixings and lithogenic inputs. These different mechanisms, acting upon water masses from very diverse sources, seem to bring the Nd IC of the three NADW layers to values close together and similar to that of the NADW. It is suggested that sediment/seawater interactions significantly lower the NEADW and NWABW Nd IC along the South East Greenland margin. Since these interactions do not significantly modify the Nd content of these water masses, sediment remobilizations leading to the Nd IC variations are probably associated with Nd removal fluxes from the water mass toward the sediment, a process called boundary exchange. On the other hand, LSW seems to acquire its Nd IC from the Subpolar Mode Waters from which it is formed by deep convection, and no other mechanism needs to be invoked. Its unradiogenic signature could ultimately be linked to fresh water runoff from the Canadian Shield. These conclusions should allow more precise interpretations of paleoceanographic Nd IC records, taking into account the distinct histories of the three NADW layers, including distinct water mass mixings and distinct lithogenic inputs. Components: 12,506 words, 6 figures, 4 tables.

New insights into the origin and distribution of the DUPAL isotope anomaly in the Indian Ocean mantle from MORB of the Southwest Indian Ridge

Abstract: [1] We report new Sr, Nd, and Pb isotope data for Mid-Ocean Ridge Basalts (MORB) dredged between 35 and 69°E along a ∼4100 km section of the Southwest Indian Ridge (SWIR), which is one of the slowest spreading ridges of the global mid-ocean ridge system (full rate of 16 mm yr−1). The SWIR appears isotopically more heterogeneous than other mid-ocean ridges over similar length scales and comparable sampling density. Isotopic variations are generally independent of ridge segmentation, and the degree of heterogeneity decreases from west to east. This decrease in heterogeneity correlates with the observed increase in axial depth and decrease in crustal thickness, commonly attributed to decreasing mantle potential temperature. Data for the easternmost SWIR in the vicinity of the Indian Triple Junction confirm that these lavas are isotopically distinct from those of the Triple Junction, the Southeast Indian Ridge, and the Central Indian Ridge, reflecting the presence of an isotopic boundary over an along-axis distance of less than 78 km. Results for lavas from the 39–41°E section extend the isotopic range of MORB to the lowest 206Pb/204Pb values (to 16.58) yet found among oceanic islands and spreading centers worldwide and confirm their extremely anomalously high 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb and low 143Nd/144Nd. This extreme Indian signature does not bear any isotopic affinities with recent products of the nearby Marion hot spot. Furthermore, the presence of old subducted marine sediment as a source for the low 206Pb/204Pb component is excluded, as no sediment group has the appropriate parent-daughter Th/Pb and U/Pb characteristics to yield such compositions, not even when dewatering subduction processes are considered. Incorporation of old subduction-modified mantle into the MORB source does not yield high enough 207Pb/204Pb and 208Pb/204Pb to explain the features of lavas from the 39–41°E section. The unusual isotopic attributes of this section (low 206Pb/204Pb and 143Nd/144Nd and high 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb) are best explained by the presence of stranded lower continental crust embedded in the upper mantle. This component is also inferred to be present in MORB from other spreading centers in the Indian Ocean.