Geochemistry, Geophysics, Geosystems

Suhr, Günter; Hellebrand, Eric; Snow, Jonathan E.; Seck, Hans A.; Hofmann, Albrecht W.

doi: 10.1029/2001GC000277pmid: N/A

The origin of large mantle dunites is considered critical for models of melt migration in the mantle. Their presence is not compatible with formation synchronous to a fracture‐related melt transport event. In models of porous channel systems for melt transport, they represent a strongly coalesced, high‐flux conduit. Dunites from the lower parts of the mantle sections in the Bay of Islands Ophiolite are investigated by detailed geochemical traverses and with single samples. Dunites tend to cluster in the sense that several smaller dunites are associated with larger dunites or several dunites occur together. The chemistry of the large bodies is very depleted (Mg# in olivine 92–94, CaO in olivine 0.05–0.08%, Cr# (100 Cr/(Cr + Al)) in spinel 65–85, TiO2 in clinopyroxene 0.01–0.04%, Sm/Yb 0.2 to 0.7 relative to N‐MORB). Detailed traverses across the dunites commonly show a decrease of NiO in olivine associated with an increase in the Mg# along the harzburgite‐dunite boundary. Internally, dunite bodies are nearly homogeneous. Thickness of dunite bodies correlates with chemistry, in particular Mg# in olivine and probably Cr# and ferric iron in spinel, but not NiO in olivine. Incompatible element data for the largest dunites argue for their formation by an extremely depleted, high Mg# (boninitic?) melt. We suggest that integrated refractory melt: rock ratios in the largest dunites (up to 40 m) were below 8, because of a low abundance of refractory melts in the crust, and a lack of a systematic change of NiO in olivine with dunite width or across single dunites in detailed chemical traverses. Tectonically, the formation of depleted melts in a late stage of the spreading center is indicated. Their melt feeders failed when approaching the base of the mantle lithosphere and generated large dunites as replacive bodies. The latest expression of this magmatism are orthopyroxenite dykes, in part draining the large dunites. Since the large majority of all deeper mantle dunites are of refractory chemical nature and not akin to MORB, we caution as universally taking large dunite bodies to represent deep‐reaching channels with high melt flux and to take the abundance and size distribution of all dunites in an ophiolitic mantle section to infer melt migration mechanisms. In the Bay of Islands Ophiolite, the largest dunites in the mantle section appear to have little to do with the main constructional stage of the spreading center.

Fitton, J. Godfrey; Saunders, Andrew D.; Kempton, Pamela D.; Hardarson, Björn S.

doi: 10.1029/2002GC000424pmid: N/A

Icelandic basalt ranges in composition from voluminous tholeiite, erupted in the rift zones, to small‐volume, mildly alkaline basalt erupted off‐axis. In addition, small‐volume flows of primitive basalt, highly depleted in incompatible elements, are sometimes found in the actively spreading rift axes. Relative incompatible‐element depletion or enrichment in Icelandic basalt is correlated with variation in radiogenic isotope ratios, implying that the mantle beneath Iceland is heterogeneous and that the relative contribution of the various mantle components relates to eruption environment (on‐ or off‐axis) and hence to degree of melting. Thus small‐degree off‐axis melting preferentially samples an enriched and more fusible mantle component, whereas more extensive melting beneath the rift axes produces magma that more closely represents the bulk Iceland plume mantle composition. The small‐volume flows of depleted basalt represent melts that have preferentially sampled a depleted and more refractory mantle component. A debate has arisen over the nature of the depleted component in the Iceland plume. Some authors (e.g., , 1997) argue that the depleted component is ambient upper mantle, the source of normal mid‐ocean ridge basalt (NMORB) in this region. Others (e.g., , 1995; , 1995; , 1997), however, have used various lines of evidence to suggest that the plume contains an intrinsic depleted component that is distinct from the NMORB source. (2000) attempt to refute the existence of a depleted Iceland plume (DIP) component through a critical evaluation of the Nb‐Zr‐Y arguments advanced by (1997) and the Hf‐Nd‐isotopic evidence presented by (1998). In this paper we examine the case presented by (2000) and conclude that their arguments are flawed. Firstly, their trace‐element data set excludes data from depleted Icelandic basalt samples and so it is not surprising that they find no evidence for a DIP component. Secondly, they present two new Hf‐isotope analyses of a single depleted Icelandic basalt sample and show that the data plot in their NMORB field on an εHf versus εNd diagram. However, new data allow the resolution of distinct NMORB and depleted Icelandic basalt fields on this diagram. We conclude that trace‐element and radiogenic isotope data from Iceland require the existence of a DIP component.

Bird, Peter

doi: 10.1029/2001GC000252pmid: N/A

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.

Hall, Chad E.; Parmentier, E. M.

doi: 10.1029/2002GC000308pmid: N/A

Grain size, one of the most important microstructural properties of materials, evolves during creep deformation to minimize the free energy of polycrystalline aggregates. We apply a model of grain size evolution to the study of convective instability of cooling boundary layers. The grain size evolution model is coupled to a composite rheology where the deformation rate is the sum of that due to dislocation creep and grain size sensitive diffusion creep. The onset of convection is sensitive to grain growth rates and the initial grain size. The formation of convective instabilities is enhanced by stresses induced by plate motions; therefore small‐scale convection is more likely to occur beneath fast‐moving plates. In finite amplitude convection, grain size evolution leads to high viscosity in regions where convective stresses are low and can induce viscosity contrasts exceeding one order of magnitude. Such viscosity contrasts are sufficient to influence the dynamics of convection, often leading to domains which remain isolated from the well‐mixed convecting fluid. A composite viscosity including diffusion creep, which has a lower activation energy than dislocation creep, reduces the effective temperature dependence of viscosity.

Stracke, Andreas; Bizimis, Michael; Salters, Vincent J. M.

doi: 10.1029/2001GC000223pmid: N/A

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.

Higgins, John A.; Schrag, Daniel P.

doi: 10.1029/2002GC000403pmid: N/A

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 (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 (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 (1999) and (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.

Su, Yongjun; Langmuir, Charles H.; Asimow, Paul D.

doi: 10.1029/2002GC000323pmid: N/A

PetroPlot is a 4000‐line software code written in Visual Basic for the spreadsheet program Excel that automates plotting and data management tasks for large amount of data. The major plotting functions include: automation of large numbers of multiseries XY plots; normalized diagrams (e.g., spider diagrams); replotting of any complex formatted diagram with multiple series for any other axis parameters; addition of customized labels for individual data points; and labeling flexible log scale axes. Other functions include: assignment of groups for samples based on multiple customized criteria; removal of nonnumeric values; calculation of averages/standard deviations; calculation of correlation matrices; deletion of nonconsecutive rows; and compilation of multiple rows of data for a single sample to single rows appropriate for plotting. A cubic spline function permits curve fitting to complex time series, and comparison of data to the fits. For users of Excel, PetroPlot increases efficiency of data manipulation and visualization by orders of magnitude and allows exploration of large data sets that would not be possible making plots individually. The source codes are open to all users.

Bach, Wolfgang; Peucker‐Ehrenbrink, Bernhard; Hart, Stanley R.; Blusztajn, Jerzy S.

doi: 10.1029/2002GC000419pmid: N/A

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.

Conrad, Clinton P.; Gurnis, Michael

doi: 10.1029/2001GC000299pmid: N/A

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.

Staudigel, Hubert; Helly, John; Koppers, Anthony A. P.; Shaw, Henry F.; McDonough, William F.; Hofmann, Albrecht W.; Langmuir, Charles H.; Lehnert, Kerstin; Sarbas, Baerbel; Derry, Louis A.; Zindler, Alan

doi: 10.1029/2002GC000314pmid: