American Geophysical Union (AGU)

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.

Estimates of the average composition of various Precambrian shields and a variety of estimates of the average composition of upper continental crust show considerable disagreement for a number of trace elements, including Ti, Nb, Ta, Cs, Cr, Ni, V, and Co. For these elements and others that are carried predominantly in terrigenous sediment, rather than in solution (and ultimately into chemical sediment), during the erosion of continents the La/element ratio is relatively uniform in clastic sediments. Since the average rare earth element (REE) pattern of terrigenous sediment is widely accepted to reflect the upper continental crust, such correlations provide robust estimates of upper crustal abundances for these trace elements directly from the sedimentary data. Suggested revisions to the upper crustal abundances of Taylor and McLennan [1985] are as follows (all in parts per million): Sc = 13.6, Ti = 4100, V = 107, Cr = 83, Co = 17, Ni = 44, Nb = 12, Cs = 4.6, Ta = 1.0, and Pb = 17. The upper crustal abundances of Rb, Zr, Ba, Hf, and Th were also directly reevaluated and K, U, and Rb indirectly evaluated (by assuming Th/U, K/U, and K/Rb ratios), and no revisions are warranted for these elements. In the models of crustal composition proposed by Taylor and McLennan [1985] the lower continental crust (75% of the entire crust) is determined by subtraction of the upper crust (25%) from a model composition for the bulk crust, and accordingly, these changes also necessitate revisions to lower crustal abundances for these elements.

We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world's ocean basins as geographic and Mercator grids with 2 arc min resolution. The grids include data from all the major ocean basins as well as detailed reconstructions of back‐arc basins. The age, spreading rate, and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments. The age uncertainties for grid cells coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge flanks, are based on the difference between gridded age and observed age. The uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward hot spots. We also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age‐depth relationships. The new set of grids helps to investigate prominent negative depth anomalies, which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow. A combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems.

Seafloor bathymetric data acquired with modern swath echo sounders provide coverage for only a small fraction of the global seabed yet are of high value for studies of the dynamic processes of seafloor volcanism, tectonics, mass wasting, and sediment transport that create and shape the undersea landscape. A new method for compilation of global seafloor bathymetry that preserves the native resolution of swath sonars is presented. The Global Multi‐Resolution Topography synthesis consists of a hierarchy of tiles with digital elevations and shaded relief imagery spanning nine magnification doublings from pole to pole (http://www.marine‐geo.org/portals/gmrt). The compilation is updated and accessible as surveys are contributed, edited, and added to the tiles. Access to the bathymetry tiles is via Web services and with WMS‐enabled client applications such as GeoMapApp®, Virtual Ocean, NASA World Wind®, and Google Earth®.

We present an estimate for the composition of the depleted mantle (DM), the source for mid‐ocean ridge basalts (MORBs). A combination of approaches is required to estimate the major and trace element abundances in DM. Absolute concentrations of few elements can be estimated directly, and the bulk of the estimates is derived using elemental ratios. The isotopic composition of MORB allows calculation of parent‐daughter ratios. These estimates form the “backbone” of the abundances of the trace elements that make up the Coryell‐Masuda diagram (spider diagram). The remaining elements of the Coryell‐Masuda diagram are estimated through the composition of MORB. A third group of estimates is derived from the elemental and isotopic composition of peridotites. The major element composition is obtained by subtraction of a low‐degree melt from a bulk silicate Earth (BSE) composition. The continental crust (CC) is thought to be complementary to the DM, and ratios that are chondritic in the CC are expected to also be chondritic in the DM. Thus some of the remaining elements are estimated using the composition of CC and chondrites. Volatile element and noble gas concentrations are estimated using constraints from the composition of MORBs and ocean island basalts (OIBs). Mass balance with BSE, CC, and DM indicates that CC and this estimate of the DM are not complementary reservoirs.

Geodynamic models commonly assume equations of state as a function of pressure and temperature. This form is legitimate for homogenous materials, but it is impossible to formulate a general equation of state for a polyphase aggregate, e.g., a rock, as a function of pressure and temperature because these variables cannot distinguish all possible states of the aggregate. In consequence, the governing equations of a geodynamic model based on a pressure‐temperature equation of state are singular at the conditions of low‐order phase transformations. An equation of state as a function of specific entropy, specific volume, and chemical composition eliminates this difficulty and, additionally, leads to a robust formulation of the energy and mass conservation equations. In this formulation, energy and mass conservation furnish evolution equations for entropy and volume and the equation of state serves as an update rule for temperature and pressure. Although this formulation is straightforward, the computation of phase equilibria as a function of entropy and volume is challenging because the equations of state for individual phases are usually expressed as a function of temperature and pressure. This challenge can be met by an algorithm in which continuous equations of state are approximated by a series of discrete states: a representation that reduces the phase equilibrium problem to a linear optimization problem that is independent of the functional form used for the equations of state of individual phases. Because the efficiency of the optimization decays as an exponential function of the dimension of the function to be optimized, direct solution of the linearized optimization problem is impractical. Successive linear programming alleviates this difficulty. A pragmatic alternative to optimization as an explicit function of entropy and volume is to calculate phase relations over the range of pressure‐temperature conditions of interest. Numerical interpolation can then be used to generate tables for any thermodynamic property as a function of any choice of independent variables. Regardless of the independent variables of the governing equations, a consistent definition of pressure, and the coupling of equilibrium kinetics to deformation, is only possible if the continuity equation accounts for dilational strain.

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).

The Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia has undertaken a systematic analysis of the isotopic (Sr, Nd, and Pb) compositions and concentrations of a broad compositional range of U.S. Geological Survey (USGS) reference materials, including basalt (BCR‐1, 2; BHVO‐1, 2), andesite (AGV‐1, 2), rhyolite (RGM‐1, 2), syenite (STM‐1, 2), granodiorite (GSP‐2), and granite (G‐2, 3). USGS rock reference materials are geochemically well characterized, but there is neither a systematic methodology nor a database for radiogenic isotopic compositions, even for the widely used BCR‐1. This investigation represents the first comprehensive, systematic analysis of the isotopic composition and concentration of USGS reference materials and provides an important database for the isotopic community. In addition, the range of equipment at the PCIGR, including a Nu Instruments Plasma MC‐ICP‐MS, a Thermo Finnigan Triton TIMS, and a Thermo Finnigan Element2 HR‐ICP‐MS, permits an assessment and comparison of the precision and accuracy of isotopic analyses determined by both the TIMS and MC‐ICP‐MS methods (e.g., Nd isotopic compositions). For each of the reference materials, 5 to 10 complete replicate analyses provide coherent isotopic results, all with external precision below 30 ppm (2 SD) for Sr and Nd isotopic compositions (27 and 24 ppm for TIMS and MC‐ICP‐MS, respectively). Our results also show that the first‐ and second‐generation USGS reference materials have homogeneous Sr and Nd isotopic compositions. Nd isotopic compositions by MC‐ICP‐MS and TIMS agree to within 15 ppm for all reference materials. Interlaboratory MC‐ICP‐MS comparisons show excellent agreement for Pb isotopic compositions; however, the reproducibility is not as good as for Sr and Nd. A careful, sequential leaching experiment of three first‐ and second‐generation reference materials (BCR, BHVO, AGV) indicates that the heterogeneity in Pb isotopic compositions, and concentrations, could be directly related to contamination by the steel (mortar/pestle) used to process the materials. Contamination also accounts for the high concentrations of certain other trace elements (e.g., Li, Mo, Cd, Sn, Sb, W) in various USGS reference materials.

Nhiều phương pháp đã được phát triển để đánh giá trạng thái nhiệt của manti dưới các sống núi đại dương, đảo, và cao nguyên, dựa trên thạch học và hóa địa chất của dung nham phun trào. Một phương pháp dẫn đến kết luận rằng nhiệt độ tiềm năng của manti (gọi là T_P) của manti môi trường dưới các sống núi đại dương là 1430°C, giống như Hawaii. Phương pháp khác cho thấy các sống núi có một phạm vi lớn về nhiệt độ tiềm năng của manti môi trường (gọi là T_P = 1300–1570°C), so sánh trong một số trường hợp với các điểm nóng (Klein và Langmuir, 1987; Langmuir et al., 1992). Phương pháp thứ ba cho thấy nhiệt độ thấp đồng nhất cho manti môi trường dưới các sống núi, khoảng 1300°C, với các dị thường 250°C có tính điểm liên quan đến các cột manti. Tất cả các phương pháp đều có các giả định và không chắc chắn mà chúng tôi đánh giá phê phán. Một đánh giá mới được thực hiện về thành phần dung nham mẹ có thể kết tinh olivin với hàm lượng forsterit tối đa quan sát thấy ở các dòng chảy dung nham. Những điều này nhìn chung phù hợp với các thành phần dung nham chính được tính bằng phương pháp cân bằng khối lượng của Herzberg và O'Hara (2002), và sự khác biệt phản ánh các hiệu ứng nổi tiếng của sự kết tinh phân đoạn. Kết quả về thành phần dung nham chính mà chúng tôi thu được cho bazan sống núi giữa đại dương và các đảo, cao nguyên đại dương khác nhau nói chung ủng hộ loại mô hình thứ ba nhưng với nhiệt độ tiềm năng của manti môi trường trong phạm vi 1280–1400°C và các dị thường nhiệt có thể vượt trên khoảng nền này từ 200–300°C. Kết quả của chúng tôi phù hợp với mô hình cột manti.

We describe a newly calibrated model for the thermodynamic properties of magmatic silicate liquid. The new model, pMELTS, is based on MELTS [Ghiorso and Sack, 1995] but has a number of improvements aimed at increasing the accuracy of calculations of partial melting of spinel peridotite. The pMELTS algorithm uses models of the thermodynamic properties of minerals and the phase equilibrium algorithms of MELTS, but the model for silicate liquid differs from MELTS in the following ways: (1) The new algorithm is calibrated from an expanded set of mineral‐liquid equilibrium constraints from 2439 experiments, 54% more than MELTS. (2) The new calibration includes mineral components not considered during calibration of MELTS and results in 11,394 individual mineral‐liquid calibration constraints (110% more than MELTS). Of these, 4924 statements of equilibrium are from experiments conducted at elevated pressure (200% more than MELTS). (3) The pMELTS model employs an improved liquid equation of state based on a third‐order Birch‐Murnaghan equation, calibrated from high‐pressure sink‐float and shockwave experiments to 10 GPa. (4) The new model employs a revised set of end‐member liquid components. The revised components were chosen to better span liquid composition‐space. Thermodynamic properties of these components are optimized as part of the mineral‐liquid calibration. Comparison of pMELTS to partial melting relations of spinel peridotite from experiments near 1 GPa indicates significant improvements relative to MELTS, but important outstanding problems remain. The pMELTS model accurately predicts oxide concentrations, including SiO₂, for liquids from partial melting of MM3 peridotite at 1 GPa from near the solidus up to ∼25% melting. Compared to experiments, the greatest discrepancy is for MgO, for which the calculations are between 1 and 4% high. Temperatures required to achieve a given melt fraction match those of the experiments near the solidus but are ∼60°C high over much of the spinel lherzolite melting interval at this pressure. Much of this discrepancy can probably be attributed to overstabilization of clinopyroxene in pMELTS under these conditions. Comparison of pMELTS calculations to the crystallization and partial melting experiments of Falloon et al. [1999] shows excellent agreement but also suffers from exaggerated calculated stability of clinopyroxene. Finally, comparison of pMELTS calculations to the garnet peridotite experiments of Walter [1998] at 3–7 GPa reveals disparities between calculations and experiments that increase with pressure. The most prominent of these disparities is manifest as overprediction of the stability of garnet and underprediction of that of olivine. Part of this problem may be attributed to inadequacies in the Birch‐Murnaghan equation of state in reproducing the behavior of highly compressible liquids at high pressures and temperatures.