Showing papers in "Geochemistry Geophysics Geosystems in 2003"

An updated digital model of plate boundaries

Abstract: [1] A global set of present plate boundaries on the Earth is presented in digital form. Most come from sources in the literature. A few boundaries are newly interpreted from topography, volcanism, and/or seismicity, taking into account relative plate velocities from magnetic anomalies, moment tensor solutions, and/or geodesy. In addition to the 14 large plates whose motion was described by the NUVEL-1A poles (Africa, Antarctica, Arabia, Australia, Caribbean, Cocos, Eurasia, India, Juan de Fuca, Nazca, North America, Pacific, Philippine Sea, South America), model PB2002 includes 38 small plates (Okhotsk, Amur, Yangtze, Okinawa, Sunda, Burma, Molucca Sea, Banda Sea, Timor, Birds Head, Maoke, Caroline, Mariana, North Bismarck, Manus, South Bismarck, Solomon Sea, Woodlark, New Hebrides, Conway Reef, Balmoral Reef, Futuna, Niuafo'ou, Tonga, Kermadec, Rivera, Galapagos, Easter, Juan Fernandez, Panama, North Andes, Altiplano, Shetland, Scotia, Sandwich, Aegean Sea, Anatolia, Somalia), for a total of 52 plates. No attempt is made to divide the Alps-Persia-Tibet mountain belt, the Philippine Islands, the Peruvian Andes, the Sierras Pampeanas, or the California-Nevada zone of dextral transtension into plates; instead, they are designated as “orogens” in which this plate model is not expected to be accurate. The cumulative-number/area distribution for this model follows a power law for plates with areas between 0.002 and 1 steradian. Departure from this scaling at the small-plate end suggests that future work is very likely to define more very small plates within the orogens. The model is presented in four digital files: a set of plate boundary segments; a set of plate outlines; a set of outlines of the orogens; and a table of characteristics of each digitization step along plate boundaries, including estimated relative velocity vector and classification into one of 7 types (continental convergence zone, continental transform fault, continental rift, oceanic spreading ridge, oceanic transform fault, oceanic convergent boundary, subduction zone). Total length, mean velocity, and total rate of area production/destruction are computed for each class; the global rate of area production and destruction is 0.108 m2/s, which is higher than in previous models because of the incorporation of back-arc spreading.

A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry

Abstract: The various cleaning steps required for preparation of foraminiferal samples for Mg/Ca (and Sr/Ca) analysis are evaluated for their relative importance and effects on measured elemental ratios. It is shown that the removal of silicate contamination is the most important step for the measurement of Mg/Ca ratios. In an example, bulk sample Mg/Ca decreases from 10.5 to 2.5 mmol mol?1 during clay removal. Oxidation of organic material causes a lowering of sample Mg/Ca in the order of 10% or approximately 1°C when converted to temperature. Use of dilute acid leaching to remove adsorbed contaminants causes partial dissolution of the sample carbonate and a corresponding decrease in Mg/Ca. Reductive treatment also causes dissolution of the sample and a decrease in the Mg/Ca ratio of up to 10–15%. Sample preparation for Sr/Ca analysis does not require the same degree of rigor as is necessary for Mg/Ca work. The “within-run” reproducibility of the method described here for G. ruber in a core-top sample from the Arabian Sea was ±1.8% (mean sample ratio was 4.72 mmol mol?1). When converted to temperature, this becomes 28 ± 0.2°C. The equivalent result for Sr/Ca was ±0.5% (mean ratio = 1.44 mmol mol?1).

A new parameterization of hydrous mantle melting

Abstract: [1] Modeling of melt formation and transport in all tectonic settings requires the inclusion of water, since water has large effects on mantle solidi as well as physical properties of liquids. To facilitate the inclusion of water in melting models this paper presents a new parameterization for melt fraction as a function of pressure, temperature, water content and modal cpx, based on knowledge gained from recent advances in the fields of thermodynamic modeling as well as experimental investigations of peridotite melting and hydrous equilibria. The parameterization is computationally efficient and can be modified easily as better experimental data become available. We compare it to other published parameterizations and test it in simple calculations of adiabatic decompression melting (mid-ocean ridge) and hydrous melting (subduction zone).

PaleoMac: A Macintosh™ application for treating paleomagnetic data and making plate reconstructions

Abstract: [1] This brief note provides an overview of a new Macintosh™ application, PaleoMac, (MacOS 8.0 or later, 15Mb RAM required) which permits rapid processing of paleomagnetic data, from the demagnetization data acquired in the laboratory, to the treatment of paleomagnetic poles, plate reconstructions, finite rotation computations on a sphere, and characterization of relative plate motions. Capabilities of PaleoMac include (1) high interactivity between the user and data displayed on screen which provides a fast and easy way to handle, add and remove data or contours, perform computations on subsets of points, change projections, sizes, etc.; (2) performance of all standard principal component analysis and statistical processing on a sphere [Fisher, 1953] etc.); (3) output of high quality plots, compatible with graphic programs such as Adobe Illustrator, and output of numerical results as ASCII files. Beyond its usefulness in treating paleomagnetic data, its ability to handle plate motion computations should be of large interest to the Earth science community.

Composition of altered oceanic crust at ODP Sites 801 and 1149

Abstract: [1] We present a comprehensive major and trace element dataset establishing ODP Site 801 as a geochemical reference for altered oceanic crust. The composition of old crustal sequences like those at Sites 801 and 1149 are critical to developing models of crustal aging and seawater chemistry evolution and to understanding the fate of crust consumed at subduction zones. Our estimate of the bulk composition of oceanic crust at Site 801 comprises ICP-AES and ICP-MS analyses of 117 discrete samples, 14 mixed composites and 5 glasses from the upper 500 m of Jurassic Pacific crust. Comparing the 801 “super-composite” with glass reveals enrichment of U (5x), Li (2x), K2O (4x), Rb (9x), and Cs (7x), similar to DSDP Sites 417/418, but little to no enrichment in Ba or Pb. The data also demonstrate good (∼10%) agreement between U measured on discrete samples and natural gamma logs, suggesting logging data is a reliable means of establishing bulk geochemical characteristics of oceanic crust. Data reported here serve to link other geochemical and mineralogical measurements on Site 801 and 1149 samples. We also document Boston University sample preparation procedures and instrument parameters for ICP-MS and ICP-AES analyses, and provide comparisons with other laboratories and techniques. We present new techniques for basaltic glass analyses using the Boston University 213 nm Nd-YAG LA-ICP-MS system, and show the data agree well with both solution-ICP-MS (5–10%) and ion probe measurements (∼10%).

Recycling oceanic crust: Quantitative constraints

Abstract: [1] Recycled ancient oceanic crust with variable amounts of aging, or inclusion of sediments of differing types and origins has often been invoked as a source for present-day ocean island basalts (OIB), but the current evidence remains largely qualitative. Previous quantitative modeling has shown that much has to be learned in order to better understand the implications of crustal recycling on mantle heterogeneity. Here, we present new model calculations incorporating recent constraints on subduction-zone processes and the composition of subducted sediments. Modeled compositions of the recycled oceanic crust vary widely as a function of the recycling age and composition of the oceanic crust. HIMU-type sources can only be created by recycling igneous oceanic crust if it has undergone substantial modification during subduction. Although the required modifications are qualitatively consistent with dehydration processes in subduction zones, the many uncertainties prevent a precise estimate of the isotopic composition of ancient recycled igneous crust. Inclusion of sediments increases the isotopic variability and although the resulting Sr and Nd isotopic signatures can be similar to enriched mantle (EM) signatures, the Pb isotopic composition of EM-type OIB is difficult to reconcile with the presence of sediment in their sources. The large variability of modeled compositions of the subducted crust suggests that if mantle heterogeneity is largely formed by crustal recycling, each OIB is likely to have a unique isotopic composition resulting from specific combinations of composition, age and subduction modification of the subducted crust. Given the variability of the recycled components, a small number of relatively well-defined enriched compositions can only be explained if either the subduction processing of oceanic crust is a far better defined process than observation would seem to indicate, or, the intramantle disaggregation and mixing of compositionally diverse recycled materials is surprisingly efficient.

Subduction fluxes of water, carbon dioxide, chlorine, and potassium

Abstract: [1] The alteration of upper oceanic crust entails growth of hydrous minerals and loss of macroporosity, with associated large-scale fluxes of H2O, CO2, Cl−, and K2O between seawater and crust. This age-dependent alteration can be quantified by combining a conceptual alteration model with observed age-dependent changes in crustal geophysical properties at DSDP/ODP sites, permitting estimation of crustal concentrations of H2O, CO2, Cl−, and K2O, given crustal age. Surprisingly, low-temperature alteration causes no net change in total water; pore water loss is nearly identical to bound water gain. Net change in total crustal K2O is also smaller than expected; the obvious low-temperature enrichment is partly offset by earlier high-temperature depletion, and most crustal K2O is primary rather than secondary. I calculate crustal concentrations of H2O, CO2, Cl−, and K2O for 41 modern subduction zones, thereby determining their modern mass fluxes both for individual subduction zones and globally. This data set is complemented by published flux determinations for subducting sediments at 26 of these subduction zones. Global mass fluxes among oceans, oceanic crust, continental crust, and mantle are calculated for H2O, Cl−, and K2O. Except for the present major imbalance between sedimentation and sediment subduction, most fluxes appear to be at or near steady state. I estimate that half to two thirds of subducted crustal water is later refluxed at the prism toe; most of the remaining water escapes at subarc depths, triggering partial melting. The flux of subducted volatiles, however, does not appear to correlate with either rate of arc magma generation or magnitude of interplate earthquakes.

Constraints on deformation conditions and the origin of oceanic detachments: The Mid‐Atlantic Ridge core complex at 15°45′N

Abstract: [1] Deformed rocks sampled from a corrugated detachment fault surface near the Mid-Atlantic Ridge (15°45′N) constrain the conditions of deformation and strain localization Samples recovered in situ record deformation restricted to the cold (shallow) lithosphere (greenschist facies), with no evidence for significant high-temperature deformation either at the fault zone or in the footwall near it High-temperature deformation (∼720–750°C) is observed only at two sites, and cannot be directly linked to the detachment Detachment faulting was coeval with dyke intrusions that cross cut it, as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones, and by the presence of chill margins in diabase against fault rock Basalts are very scarce and restricted to clasts in breccias, with no evidence of pillows or extrusive structures Gabbros crop out along mass-wasted and fault scarps structurally below the detachment Footwall rocks show little or no deformation, due to strain localization along a narrow shear zone (<200 m) with fluid flow, as required to form talc- and amphibole schists after an ultramafic protolith We speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time Our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold (shallow) lithosphere is applicable to other corrugated surfaces along slow- and intermediate-spreading ridges These observations preclude detachment models rooting in melt-rich zones (ie, Atlantis Bank, Southwest Indian Ridge) or recording high-temperature deformation We infer that oceanic detachment faults (1) localize strain at T < 500–300°C, (2) persist during active magmatism, and (3) root at shallow rheological boundaries, such as a melt-rich zone or magma chamber (“hot” detachments) or an alteration front (“cold” detachments)

Reconstructing the total shortening history of the NW Himalaya.

Abstract: The onset of India-Asia contact can be dated with both biostratigraphic analysis of syn-collisional sedimentary successions deposited on each side of the Indus Suture zone, and by radiometric dating of Indian crustal rocks which have undergone subduction to great depths in the earliest subduction-collision stages. These data, together with paleomagnetic data show that the initial contact of the Indian and Asian continental margins occurred at the Paleocene/Eocene boundary, corresponding to 55 ± 2 Ma. Such dating, which is consistent with all available geological evidence, including the record of magnetic anomalies in the Indian ocean and decrease of magmatic activity related to oceanic subduction can thus be considered as accurate and robust. The sedimentary record of the Tethys Himalaya rules out obduction of oceanic allochtons directly onto the Indian continental margin during the Late Cretaceous. The commonly inferred Late Cretaceous ophiolite obduction events may have thus occurred in intra-oceanic setting close to the Asian margin before its final emplacement onto the India margin during the Eocene. Granitoid and sedimentary rocks of the Indian crust, deformed during Permo-Carboniferous rifting, reached a depth of some 100 km about 1 Myr after the final closure of the Neo-Tethys, and began to be exhumed between 50 and 45 Ma. At this stage, the foreland basin sediments from Pakistan to India show significant supply from volcanic arcs and ophiolites of the Indus Suture Zone, indicating the absence of significant relief along the proto-Himalayan belt. Inversion of motion may have occurred within only 5 to 10 Myr after the collision onset, as soon as thicker and buoyant Indian crust chocked the subduction zone. The arrival of thick Indian crust within the convergent zone 50-45 Myr ago led to progressive stabilization of the India/Asia convergent rate and rapid stabilization of the Himalayan shortening rate of about 2 cm.yr-1. This first period also corresponds to the onset of terrestrial detrital sedimentation within the Indus Suture zone and to the Barrovian metamorphism on the Indian side of the collision zone. Equilibrium of the Himalayan thrust belt in terms of amount of shortening vs amount of erosion and thermal stabilization less than 10 Myr after the initial India/Asia contact is defined as the collisional regime. In contrast, the first 5 to 10 Myr corresponds to the transition from oceanic subduction to continental collision, characterized by a marked decrease of the shortening rate, onset of aerial topography, and progressive heating of the convergent zone. This period is defined as the continental subduction phase, accommodating more than 30% of the total Himalayan shortening.

Stress-driven melt segregation in partially molten rocks

Abstract: [1] We demonstrate that deformation of partially molten ductile rocks can produce melt segregation by two-phase flow. In simple shear experiments on several melt-rock systems at high temperature and pressure, melt segregates into distinct melt-rich layers oriented 20° to the shear plane. Melt segregates in samples in which pressure gradients can develop at length scales less than the sample thickness. A simple scaling argument combined with a comparison of length scale data suggests that such pressure gradients can develop in the samples with compaction lengths less than or on the order of the sample thickness. In nature, stress-driven melt segregation may produce both high-permeability pathways that contribute to rapid extraction of melt and localization of deformation that increases the anisotropy in viscosity of partially molten regions of the upper mantle and lower crust.

Incompatible element ratios in oceanic basalts and komatiites: Tracking deep mantle sources and continental growth rates with time

Abstract: [1] Ratios of elements with similar incompatibilities in the mantle can be used to characterize magma sources through time. Nb/Y and Zr/Y distributions in oceanic basalts support the existence of a long-lived, deep depleted source in mantle. Zr/Y, Nb/Y, Zr/Nb, and Nb/Th ratios in oceanic basalts and komatiites suggest that depleted and recycled components, together probably with an enriched component, were present in the deep mantle by 3.5 Ga. Low Zr/Nb and Hf/Sm ratios and high La/Yb and Nb/Y ratios in some plume basalts and Al-depleted komatiites may reflect majorite fractionation. High Zr/Nb ratios and low Nb/Y ratios in Archean Al-undepleted komatiites may record partial melting of a Mg-perovskite source in deep mantle plumes in which Mg-perovskite crystallizes and accumulates in komatiite melts during ascent. Oceanic greenstone basalts show a gradual increase in the Nb/Th ratio with time with a relatively sudden increase at about 2 Ga. This trend is consistent with gradual continental growth and with a major episode of continental growth at 2.7 Ga. Nb/Th ratios in some Early Archean basalts may record extraction of up to 25% of the present volume of continental crust from the early upper mantle. An alternative explanation for the rapid increase in Nb/Th in oceanic basalts at 2 Ga is that a catastrophic 2.7-Ga event in the mantle changed the composition or/and location of the primary volume of mantle from which continental crust was extracted.

Sr‐Nd‐Pb composition of Mesozoic Pacific oceanic crust (Site 1149 and 801, ODP Leg 185): Implications for alteration of ocean crust and the input into the Izu‐Bonin‐Mariana subduction system

Abstract: We report Sr, Nd and Pb isotopic compositions of sediments and variably altered igneous rocks from ODP Site 801 (Marianas) and ODP Site 1149 (Izu-Bonin). These Sites provide the most complete drilled ocean crust sections located in front of the Mariana and Izu-Bonin trenches and characterize the unmodified isotopic input into these subduction zones. The subducted ocean crust belongs to the oldest (130–167 Ma) in situ Pacific Ocean crust and thus has end-member character with respect to alteration and sediment load. The lithostratigraphic division of sedimentary units at Site 1149 into clays, cherts, lower clays and carbonates with clay is reflected on isotope correlation diagrams. The Pb isotope data of the sediments show much greater variation than previously reported from this region. Particularly noteworthy are zeolite-bearing clays and clay bearing carbonates from the lower Units that have Pb isotopic compositions identical to the Izu Volcanic Front. The basaltic basement samples display variable 87Sr/86Sr ratios at near constant 143Nd/144Nd ratios, indicating mixing with seawater derived Sr. Most basaltic samples from Site 1149 and 801 exhibit highly variable 206Pb/204Pb (17.88–20.00) at near constant 207Pb/204Pb and 208Pb/204Pb ratios. Three samples from Site 801 display the most extreme 206Pb/204Pb (23.70–26.86) and 207Pb/204Pb (15.73–15.83) ratios ever measured in altered MORB reflecting an increase of 238U/204Pb ratios (μ), most likely through addition of seawater derived U. Initial Pb isotopes of most samples overlap with the age corrected field of the Pacific MORB source, thus the increase in μ took place shortly after formation of the crust in most samples. According to our new isotope data the radiogenic end-member of the Izu arc volcanic rocks could either represent Pb from the lower sediment column released from the slab by delayed dewatering or an integrated slab fluid in which 90–95% of the Pb comes from the basaltic crust and 5–10% of the Pb from the sediments. The Pb isotope systematics of the Mariana arc output suggest two component mixing. Both components appear to be input derived with the radiogenic component represented by average Site 801 sediment and the unradiogenic component generated by mixing of ∼80% unaltered crust with ∼20% highly altered crust.

Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean

Abstract: [1] We demonstrate that the carbon isotopic signal of mid-depth waters evolved differently from deep waters in the South Atlantic sector of the Southern Ocean during the Pleistocene. Deep sites (>3700 m) exhibit large glacial-to-interglacial variations in benthic δ13C, whereas the amplitude of the δ13C signal at Site 1088 (∼2100 m water depth) is small. Unlike the deep sites, at no time during the Pleistocene were benthic δ13C values at Site 1088 lower than those of the deep Pacific. Reconstruction of intermediate-to-deep δ13C gradients (Δ13CI-D) supports the existence of a sharp chemocline between 2100 and 2700 m during most glacial stages of the last 1.1 myr. This chemical divide in the glacial Southern Ocean separated well-ventilated water above ∼2500 m from poorly ventilated water below. The Δ13CI-D signal parallels the Vostok atmospheric pCO2 record for the last 400 kyr, lending support to physical models that invoke changes in Southern Ocean deep water ventilation as a mechanism for changing atmospheric pCO2. The emergence of a strong 100-kyr cycle in Δ13CI-D during the mid-Pleistocene supports a change in vertical fractionation and deep-water ventilation rates in the Southern Ocean, and is consistent with possible CO2-forcing of this climate transition.

Hydrogen concentration analyses using SIMS and FTIR: Comparison and calibration for nominally anhydrous minerals

Abstract: We report analyses of hydrogen abundance in experimentally annealed and natural mantle minerals using FTIR and use these data to establish calibration lines for measurement of H2O concentrations in olivine, pyroxenes, garnet, amphibole and mica by secondary ion mass spectrometry (SIMS). We have reduced the detection limit for H2O analysis by SIMS to 2–4 ppm H2O (by weight) through careful attention to sample preparation and vacuum quality. The accuracy of the SIMS calibrations depends on the choice of FTIR extinction coefficients; however, all of the calibrations reported here are shown to be consistent with measurements on standards whose H2O abundance has been determined independently via manometry or nuclear reaction analysis. The resulting calibrations are accurate to 10–30% at the 95% confidence limit, with improvements possible through the use of higher-H2O standards. Using our SIMS calibration, we determined hydrogen concentrations in coexisting olivine, orthopyroxene, and glass from a single melting experiment at 2 GPa and 1380°C. Olivine/melt and orthopyroxene/melt partition coefficients are equal to 0.0020 ± 0.0002 and 0.0245 ± 0.0015, respectively, and the orthopyroxene/olivine coefficient is 12 ± 4 (2σ uncertainties).

Testing the tracer ratio method for modeling active compositional fields in mantle convection simulations

Abstract: [1] Tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion. Composition is typically taken to be proportional to the absolute local concentration of tracers, but an increasingly popular method is to have “dense” and “regular” tracers with composition being equal to the local fraction of “dense” tracers. This paper tests this “ratio” method using established benchmarks and by comparing the performance of the two tracer methods and grid-based methods for simulating the long-term evolution of a convecting mantle with a thick, dense, stable layer. For this scenario the ratio method is found to have several advantages, giving sharp, stable long-term layering with no tracer settling, minimal statistical “noise” and low entrainment, even with only ∼5 tracers per cell. The method is equally applicable to finite volume and finite element treatments of the underlying flow. Entrainment in grid-based advection methods is heavily dependent on resolution and numerical details, and is reduced ∼1 order of magnitude by the filter proposed by A. Lenardic. Numerical determination of physically correct entrainment rates remains a challenging problem. Comparing tracer and grid based methods, the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids; however the estimated entrainment differs significantly.

Extreme chemical variability as a consequence of channelized melt transport

Abstract: [1] The spatial and temporal variability of chemical signals in lavas, residues, and melt inclusions provides important constraints on source heterogeneity, the efficiency of convection, and melt transport processes in the mantle. The past decade has seen an impressive increase in the number, precision, and spatial resolution of chemical analyses. However, as resolution has increased, the picture of variation that emerges has become increasingly difficult to understand. For example, mid-ocean ridge basalts can display large variations in trace element concentration on scales from 1000 km of ridge to melt inclusions in 500 micron crystals. These observations suggest that melt transport processes do not readily homogenize partially molten regions. While some of the observed variability is due to source variations, a large proportion could be the consequence of magma transport in channelized systems. We present results of numerical models that calculate the trace element signatures of high-porosity dissolution channels produced by reactive fluid flow. These models were originally developed to explain the organization of melt transport networks, based on observations of “replacive dunites” found in ophiolites. Channelized flow can produce orders of magnitude variation in the concentrations of highly incompatible elements, even for idealized systems with a homogeneous source, constant bulk partition coefficients, and equilibrium transport. Most importantly, the full range of variability may be found in each channel because channelization can transpose the chemical variability produced by melting throughout the melting column into horizontal variability across the width of the channel. The centers of channels contain trace element–enriched melts from depth, while the edges of the channels transport highly depleted melts extracted from the interchannel regions at shallower levels. As dunite channels may be spaced on scales of 1–100 m in the mantle, this mechanism allows highly variable melt compositions to be delivered to the Moho on small length scales. The chemical variation produced in the models is consistent with that seen in melt inclusion suites, lavas, and residual mantle peridotites dredged from the ridges and sampled in ophiolites.

Short-lived and discontinuous intraplate volcanism in the South Pacific: Hot spots or extensional volcanism?

Abstract: [1] South Pacific intraplate volcanoes have been active since the Early Cretaceous. Their HIMU-EMIEMII mantle sources can be traced back into the West Pacific Seamount Province (WPSP) using plate tectonic reconstructions, implying that these distinctive components are enduring features within the Earth’s mantle for, at least, the last 120 Myr. These correlations are eminent on the scale of the WPSP and the South Pacific Thermal and Isotopic Anomaly (SOPITA), but the evolution of single hot spots emerges notably more complicated. Hot spots in the WPSP and SOPITA mantle regions typically display intermittent volcanic activity, longevities shorter than 40 Myr, superposition of hot spot volcanism, and motion relative to other hot spots. In this review, we use 40 Ar/ 39 Ar seamount ages and Sr-Nd-Pb isotopic signatures to map out Cretaceous volcanism in the WPSP and to characterize its evolution with respect to the currently active hot spots in the SOPITA region. Our plate tectonic reconstructions indicate cessation of volcanism during the Cretaceous for the Typhoon and Japanese hot spots; whereas the currently active Samoan, Society, Pitcairn and Marquesas hot spots lack long-lived counterparts in the WPSP. These hot spots may have become active during the last 20 Myr only. The other WPSP seamount trails can be only ‘‘indirectly’’ reconciled with hot spots in the SOPITA region. Complex age distributions in the Magellan, Anewetak, Ralik and Ratak seamount trails would necessitate the superposition of multiple volcanic trails generated by the Macdonald, Rurutu and Rarotonga hot spots during the Cretaceous; whereas HIMU-type seamounts in the Southern Wake seamount trail would require 350–500 km of hot spot motion over the last 100 Myr following its origination along the Mangaia-Rurutu ‘‘hotline’’ in the Cook-Austral Islands. These observations, however, violate all assumptions of the classical Wilson-Morgan hot spot hypothesis, indicating that long-lived, deep and fixed mantle plumes cannot explain the intraplate volcanism of the South Pacific region. We argue that the observed short-lived and discontinuous intraplate volcanism has been produced by another type of hot spot-related volcanism, as opposed to the strong and continuous Hawaiian-type hot spots. Our results also indicate that other geological processes (plate tension, hotlines, faulting, wetspots, self-propagating volcanoes) may act in conjunction with hot spot volcanism in the South Pacific. In all these scenarios, intraplate volcanism has to be controlled by ‘‘broad-scale’’ events giving rise to multiple closely-spaced mantle plumelets, each with a distinct isotopic signature, but only briefly active and stable over geological time. It seems most likely that these plumelets originate and dissipate at very shallow mantle depths, where they may shoot off as thin plumes from the top of a ‘‘superplume’’ that is present in the South Pacific mantle. The absence of clear age progressions in most

A simple model for the CaCO3 saturation state of the ocean: The “Strangelove,” the “Neritan,” and the “Cretan” Ocean

Abstract: [1] A simple model of the CaCO3 saturation state of the ocean is presented. It can be solved analytically and is intended to identify the fundamental controls on ocean carbonate ion concentration. It should also attract researchers unfamiliar with complex biogeochemical models. Despite its limitations, the model-calculated CaCO3 saturation state of today's ocean agrees well with observations. In general, the model reveals three distinctly different modes of operation: The “Strangelove Ocean” of high supersaturation which is dominated by inorganic CaCO3 precipitation, (2) the “Neritan Ocean” of indefinite saturation dominated by biogenic shallow-water CaCO3 precipitation, and (3) the “Cretan Ocean” of low saturation dominated by biogenic pelagic CaCO3 precipitation. In the latter mode, the deep ocean [CO32−] is remarkably stable, provided that the biogenic production of CaCO3 exceeds the riverine flux of Ca2+ and CO32−. This explains the overall constancy of the saturation state of the ocean documented over the last 100 Ma. The model is then used to address diverse questions. One important result is that the recovery of the oceanic carbonate chemistry from fossil fuel neutralization in the future will be accelerated due to expected reduced biogenic calcification.

Crustal motion in the Southern Andes (26°–36°S): Do the Andes behave like a microplate?

Abstract: [1] A new Global Positioning System (GPS)-derived velocity field for the Andes mountains (26°–36°S) allows analysis of instantaneous partitioning between elastic and anelastic deformation at the orogen's opposing sides. Adding an “Andes” microplate to the traditional description of Nazca-South America plate convergence provides the kinematic framework for nearly complete explanation of the observed velocity field. The results suggest the oceanic Nazca boundary is fully locked while the continental backarc boundary creeps continuously at ∼4.5 mm/yr. The excellent fit of model to data (1.7 mm/yr RMS velocity misfit), and the relative aseismicity of the upper crust in the interior Andean region in comparison with its boundaries, supports the notion that the mountains are not currently accruing significant permanent strains. Additionally, the model implies permanent deformation is not accumulating throughout the backarc contractional wedge, but rather that the deformation is accommodated only within a narrow deformational zone in the backarc.

Automatic processing of high‐rate, high‐density multibeam echosounder data

Abstract: [1] Multibeam echosounders (MBES) are currently the best way to determine the bathymetry of large regions of the seabed with high accuracy. They are becoming the standard instrument for hydrographic surveying and are also used in geological studies, mineral exploration and scientific investigation of the earth's crustal deformations and life cycle. The significantly increased data density provided by an MBES has significant advantages in accurately delineating the morphology of the seabed, but comes with the attendant disadvantage of having to handle and process a much greater volume of data. Current data processing approaches typically involve (computer aided) human inspection of all data, with time-consuming and subjective assessment of all data points. As data rates increase with each new generation of instrument and required turn-around times decrease, manual approaches become unwieldy and automatic methods of processing essential. We propose a new method for automatically processing MBES data that attempts to address concerns of efficiency, objectivity, robustness and accuracy. The method attributes each sounding with an estimate of vertical and horizontal error, and then uses a model of information propagation to transfer information about the depth from each sounding to its local neighborhood. Embedded in the survey area are estimation nodes that aim to determine the true depth at an absolutely defined location, along with its associated uncertainty. As soon as soundings are made available, the nodes independently assimilate propagated information to form depth hypotheses which are then tracked and updated on-line as more data is gathered. Consequently, we can extract at any time a “current-best” estimate for all nodes, plus co-located uncertainties and other metrics. The method can assimilate data from multiple surveys, multiple instruments or repeated passes of the same instrument in real-time as data is being gathered. The data assimilation scheme is sufficiently robust to deal with typical survey echosounder errors. Robustness is improved by pre-conditioning the data, and allowing the depth model to be incrementally defined. A model monitoring scheme ensures that inconsistent data are maintained as separate but internally consistent depth hypotheses. A disambiguation of these competing hypotheses is only carried out when required by the user. The algorithm has a low memory footprint, runs faster than data can currently be gathered, and is suitable for real-time use. We call this algorithm CUBE (Combined Uncertainty and Bathymetry Estimator). We illustrate CUBE on two data sets gathered in shallow water with different instruments and for different purposes. We show that the algorithm is robust to even gross failure modes, and reliably processes the vast majority of the data. In both cases, we confirm that the estimates made by CUBE are statistically similar to those generated by hand.

Geochemical modeling of dehydration and partial melting of subducting lithosphere: Toward a comprehensive understanding of high-Mg andesite formation in the Setouchi volcanic belt, SW Japan

Abstract: [1] Possible mechanisms for the production of mantle-derived, high-Mg andesite magmas, including (1) partial melting of mantle wedge peridotite by addition of aqueous fluids from the subducting lithosphere and (2) partial melting of the subducting sediments and altered oceanic crust, and subsequent melt-mantle interaction, were examined by geochemical formulation of dehydration, partial melting and melt-solid reactions. The modeling results demonstrate that both mechanisms can reasonably explain the incompatible trace element characteristics of high-Mg andesites in the Setouchi volcanic belt, SW Japan. However, simple hydrous melting of mantle wedge peridotite cannot account for the Sr-Nd-Pb-Hf isotopic compositions of such andesites. By contrast, the latter mechanism, which is consistent with thermal structures beneath the Setouchi volcanic belt, can well reproduce the isotopic signature of those high-Mg andesites.

Geochemistry of hydrothermally altered oceanic crust: DSDP/ODP Hole 504B – Implications for seawater‐crust exchange budgets and Sr‐ and Pb‐isotopic evolution of the mantle

Abstract: [1] This paper presents petrographic, chemical, and isotopic (Sr, S) analyses of whole rock samples from a 1.8 km section of upper ocean crust (DSDP/ODP Hole 504B). The samples were selected to cover all lithologies (pillows, flows, breccias, dikes) and alteration/mineralization styles. The chemical and petrographic data were used to calculate weighted averages for upper crustal composition, based on which seawater-ocean crust exchange fluxes were calculated. These results confirm earlier estimates that identify the upper crust as a significant sink for K and Mg and a source of Ca and Si to the oceans. Changes in trace element geochemistry implies that the upper ocean crust in 504B is a sink for CO2, Rb, Cs, and U, although the flux rates are an order of magnitude smaller than suggests by previous estimates for DSDP Sites 417 and 418 in 118 Ma Atlantic crust. Fluxes of these components are similar, within a factor of four, to flux rates estimated for the Juan de Fuca Ridge flank, which may relate to similarities in the thermal and hydrogeological evolution at both sites that is controlled by rapid termination of fluid circulation and conductive reheating of the upper crust. The contrast between the fluxes of trace elements derived for those settings and the open-ocean sites 417/418 likely reflects prolonged fluid-rock interaction at the latter location. If the Mg uptake and Sr exchange reconstructed from 504B core is representative, ridge flank hydrothermal alteration cannot account for the imbalance in the Mg and Sr budgets of the oceans. Up to 10% of the crustal Pb resides in the mineralized parts of the transition zone between the volcanic section and the sheeted dike complex. Combined, the Pb mobilized in the deepest parts of the hydrothermal systems (probably not penetrated in 504B) and hosted in metalliferous sediments and mineralized stockwork may account for the Pb surplus of the continental crust and the evolution of Ce/Pb of the mantle. Hydrothermal alteration results in net increases of Rb/Sr and U/Pb, in particular in the uppermost 600 m of crust, but the increases are not large enough to make altered upper ocean crust a plausible precursor for the HIMU mantle component. Moreover, the fractionation between Th and Pb, if any, is insufficient to account for the development of highly radiogenic 208Pb/204Pb in a HIMU mantle source. Potential HIMU precursors can be derived from altered ocean crust after 1–2 Ga, if on the order of 80–90% Pb, 40–55% Rb, 40% Sr, and 35–40%U are removed during partial dehydration in subduction zones.

Aftermath of a snowball Earth

Abstract: [1] Using a simple 3-box model of the ocean-atmosphere system, we simulate the cycling of carbon and strontium in the aftermath of a global glaciation. Model simulations include the delivery of alkalinity to seawater from intense carbonate and silicate weathering under high pCO2 conditions as well as ocean mixing, air-sea gas exchange, and biological productivity. The δ13C of the first carbonate precipitated after the glaciation depends on the pCO2, temperature, the saturation state of the surface ocean, and kinetic effects associated with mineral precipitation. With no biological productivity, the model produces δ13C values between +1‰ and −3‰, consistent with observations. This is in direct contradiction with arguments by Kennedy et al. [2001a], who suggest that the δ13C value of dissolved carbon in a snowball ocean (and directly afterward) must be −5‰. Kennedy et al. assume the carbon isotope cycle is in steady state, which does not apply to a global glaciation, and also neglect any effect of high pCO2 on the carbonate chemistry of seawater. A major difference between our findings and the qualitative predictions of Hoffman et al. [1998] is our interpretation of the cap dolostone as representing an interval dominated by carbonate weathering of exposed continental shelves. As a result, the ∼2‰ drop in the δ13C observed in the cap dolostone is unlikely to be the product of Rayleigh distillation of atmospheric CO2 via silicate weathering. Instead, we interpret the ∼2‰ drop in the δ13C values as indicative of an increase in sea surface temperature which lowers the fractionation between CO2 and carbonate. Kinetic isotope effects associated with rapid precipitation from a highly supersaturated surface ocean may also be important. Rayleigh distillation of atmospheric CO2 via silicate weathering is a viable explanation for the continued drop in the δ13C values in the limestone sequence above the cap dolostone, with biological productivity and carbonate weathering driving a slow increase in δ13C values once pCO2 levels decline. Our study also simulates the cycling of strontium in seawater. In contrast to the finding of Jacobsen and Kaufman [1999] and Kennedy et al. [2001a], model simulations show a drop in 87Sr/86Sr of less than 0.001 during 5 million years of global glaciation and an increase of less than 0.001 over the entire episode of silicate weathering. Our calculations emphasize the importance of considering the changes in seawater chemistry due to high pCO2 in evaluating the Snowball Earth hypothesis.

Distinguishing between water column and sedimentary denitrification in the Santa Barbara Basin using the stable isotopes of nitrate

Abstract: [1] Below its sill depth, the Santa Barbara Basin (SBB) is commonly suboxic ([O2] ∼ 3 μM), with only brief periods of ventilation. Associated with development of suboxia, the concentration of nitrate decreases with depth into the basin without an associated decrease in phosphate, indicating that a substantial fraction of the nitrate supplied to the basin is removed by denitrification. Coincident with the decrease in nitrate concentration across the “redoxcline” (the interface between oxic and suboxic waters) within the SBB, there is an increase in the 15N/14N of that nitrate, as would be anticipated from the isotopic fractionation associated with denitrification. However, the increase in 15N/14N of nitrate is much smaller than occurs in the open eastern tropical North Pacific (ETNP) for a comparable amount of nitrate loss. Both the concentrations of N species within the basin and measurements of nitrate 18O/16O suggest that the lower-than-expected 15N enrichment in the suboxic SBB involves denitrification, rather than being due to some unknown source of low-15N/14N N to the deep SBB. Calculations with a range of models of nitrate supply and consumption indicate that the degree of nitrate consumption in the basin is too small for differences in water circulation to explain the isotopic differences between the Santa Barbara Basin and the open ETNP. Previous studies indicate that the isotope effect of sedimentary denitrification is negligible due to nitrate diffusion in sediment pore waters. Thus we infer that the small magnitude of the isotopic enrichment of SBB water column nitrate is due to the importance of sedimentary denitrification within the basin. Assuming that water column and sedimentary denitrification have isotope effects of 25 and 1.5 per mil, respectively, our results suggest that sedimentary denitrification accounts for more than 75% of the nitrate loss within the suboxic SBB.

CO2 emissions from the Yellowstone volcanic system

Abstract: [1] Two methods are used to estimate CO2 degassing from the Yellowstone magmatic-hydrothermal system. The amount of magmatic CO2 released as basaltic magma emplaces from the mantle into the crust beneath the Yellowstone caldera is calculated and compared to CO2 fluxes measured in three different types of hydrothermal regions within Yellowstone. Comparison of modeled estimates with surface measurements suggests that 3.7 ± 1.3 × 1011 mol y−1 (45 ± 16 kt d−1) of CO2 are released from Yellowstone due to diffuse degassing. Flux measurements suggest that the diffuse flux in acid-sulfate regions is significant in total calculations (>96% of the total), whereas the diffuse flux in neutral-chloride and travertine-precipitating areas is not significant. Analyses of carbon and helium isotopes suggest that ∼50% of the CO2 emitted is derived from sedimentary sources at locations outside the caldera, whereas locations inside the caldera likely have sedimentary contributions <30%. In addition to release of CO2 with emplacement, magma crystallization in the subsurface is thought to contribute significantly to the CO2 emissions at the surface. The contribution of CO2 from Yellowstone to global volcanic CO2 emissions (∼6–7 × 1012 mol y−1) is comparable to the CO2 contribution from other large volcanic systems like Popocatepetl, Mexico and the combined contribution from the Hawaii hot spot. Likewise, the amount of CO2 emitted per land area from Yellowstone (on average 108 mol CO2 km−2 y−1) is comparable to other large volcanic and hydrothermal systems worldwide.

Seismic tomography, surface uplift, and the breakup of Gondwanaland: Integrating mantle convection backwards in time

Abstract: Mantle density heterogeneities, imaged using seismic tomography, contain information about time-dependent mantle flow and mantle structures that existed in the past. We model the history of mantle flow using a tomographic image of the mantle beneath southern Africa as an initial condition while reversing the direction of flow and analytically incorporating cooling plates as a boundary condition. If the resulting (backwards integrated) model for structures is used as a starting point for a forwards convection model, today's mantle can be adequately reconstructed if we do not integrate backwards more than than about 50–75 Ma. Flow can also be reliably reversed through the Mesozoic, but only if instability of the lower boundary layer can be suppressed. Our model predicts that the large seismically-slow and presumably hot structure beneath southern Africa produced 500–700 m of dynamic topography throughout the Cenozoic. Since ~30 Ma, uplift has moved from eastern to southern Africa, where uplift rates are ~10 m/Myr, consistent with observations. During the Mesozoic, the modeled topographic high is situated near Gondwanaland rifting, raising the possibility that this buoyant structure may have been involved with this breakup.

The 320 kyr Pb isotope evolution of Mauna Kea lavas recorded in the HSDP-2 drill core

Abstract: [1] We analyzed Pb isotopic compositions of 50 samples from the HSDP-2 drill hole, covering the time interval 180 to 550 kyr B.P. in the stratigraphic record of Mauna Kea. All analyses were corrected for instrumental bias using a triple-spike technique. The aims of this study are to document temporal changes in sources contributing to Mauna Kea and to investigate how these may relate to the chemical structure of the Hawaiian plume. Lead isotopic compositions of the lavas have 206Pb/204Pb ratios ranging from 18.41 to 18.63, 207Pb/204Pb from 15.47 to 15.49, and 208Pb/204Pb from 37.97 to 38.22. In 207Pb/204Pb-206Pb/204Pb space, the samples display a broad linear array, while three distinct arrays are found in 208Pb/204Pb-206Pb/204Pb space. These arrays can clearly be distinguished by their 208Pb/204Pb ratios and are referred to as “Kea-lo8,” “Kea-mid8,” and “Kea-hi8.” The 206Pb/204Pb isotope ratios exhibit rapid shifts by ∼0.2 over 100 m depth intervals, and jumps from one Pb isotope array to another and back in less than ∼100 m depth. Despite these rapid Pb isotope fluctuations, a particular Pb isotope array dominates over periods of several tens to hundreds of kiloyears. We interpret the Pb isotope arrays found in HSDP-2 in terms of mixing of end-members lying along the radiogenic and unradiogenic extensions of the arrays. At the radiogenic extension the three HSDP-2 arrays converge to a common end-member. The lower extensions of the arrays diverge in three directions, each with different 208Pb/204Pb ratios. This topology suggests that the HSDP-2 arrays were produced by mixing of at least four end-members. The origin of these end-members was investigated using Monte Carlo simulations of a Pb isotope evolution model. The simulations suggest that the common radiogenic end-member of the three Pb isotope arrays contains material with elevated μ values and has a relatively young age (<1.5 Ga). Such a signature can be plausibly interpreted in terms of the presence of recycled oceanic crust in the source. The HSDP-2 Kea-lo8, Kea-mid8, and Kea-hi8 Pb isotope arrays dominate over different time periods and can be related to the displacement of Mauna Kea relative to the plume center over time. The Kea-lo8 array is present between ∼180 and 370 ka and samples more peripheral parts of the plume, while the Kea-mid8 and Kea-hi8 arrays occur in the deeper parts of the core (∼370 to 550 kyr ago), when Mauna Kea was closer to the plume center. Over the time intervals when each array dominates, we derive corresponding “lengths” of materials in the source by integrating the estimated upwelling velocity across the plume. These calculations suggest Pb isotope heterogeneities of at least several tens of kilometers in vertical length within the Hawaiian plume. The Pb isotope arrays may correspond to relatively small-scale heterogeneities derived from the D″ layer in the lower mantle.

Interaction between the Mid-Atlantic Ridge and the Azores hot spot during the last 85 Myr: Emplacement and rifting of the hot spot-derived plateaus

Abstract: [1] Multiple- and single-beam bathymetric data are compiled over the Azores plateau to produce a 1 km × 1 km grid between latitudes 32°N and 49°N and longitudes 22°W and 43°W. Mantle Bouguer anomalies are then calculated from this grid and the satellite-derived gravity. These grids provide new insights on the temporal and spatial variations of melt supply to the ridge axis. The elevated seafloor of the Azores plateau is interpreted as resulting from the interaction of a mantle plume with the Mid-Atlantic Ridge (MAR). The presence of a large region of elevated seafloor associated with a thick crust between the Great Meteor Seamounts and the Azores platform on the Africa plate, and less developed conjugate structures on the North America plate, favors genetic relations between these hot spot-derived structures. This suggests that a ridge-hot spot interaction has occurred in this region since 85 Ma. This interaction migrated northward along the ridge axis as a result of the SSE absolute motion of the Africa plate, following a direction grossly parallel to the orientation of the MAR. Kinematic reconstructions from chron 13 (∼35 Ma) to the present allow a proposal that the formation of the Azores plateau began around 20 Ma and ended around 7 Ma. A sharp bathymetric step is associated with the beginning of important melt supply around 20 Ma. The excess of melt production is controlled by the interaction of the ridge and hot spot melting zones. The geometry and distribution of the smaller-scale features on the plateau record episodic variations of the hot spot melt production. The periodicity of these variations is about 3–5 Myr. Following the rapid decrease of widespread volcanism, the plateau was subsequently rifted from north to south by the Mid-Atlantic Ridge since 7 Ma. This rifting begins when the MAR melting zone is progressively shifted away from the 200-km plume thermal anomaly. These results bear important consequences on the motion of the Africa plate relative to the Azores hot spot. They also provide an explanation to the asymmetric geochemical signature of the Azores hot spot along the Mid-Atlantic Ridge.