Showing papers in "Geochemistry Geophysics Geosystems in 2004"

Composition of the depleted mantle

Abstract: [1] 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.

The isotopic composition of Yb and the precise and accurate determination of Lu concentrations and Lu/Hf ratios by isotope dilution using MC-ICPMS

Abstract: [1] We report Yb isotopic data for the purpose of accurately correcting measurements of Lu and Lu/Hf ratios during Multicollector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) analysis. The Yb isotopic ratios agree with recent results of Chu et al. [2002] and Amelin and Davis [2004], when corrected for mass fractionation to the same reference values. Protocols for Lu measurement involving simultaneous subtraction of Yb interference and application of a fractionation correction based on Yb are described. It is shown that Lu concentrations and Lu/Hf ratios determined by MC-ICPMS using this methodology are more precise and potentially more accurate than results based on analysis of Lu by Thermal Ionization Mass Spectrometry (TIMS). A number of MC-ICPMS Lu-Hf isotopic analyses of standard rocks and other geological samples, previously analyzed by TIMS, are presented. These document reproducibility of Lu/Hf within 0.2%, where most discrepancies can be assigned to the unknown Lu mass fractionation during TIMS analysis.

Evolving force balance during incipient subduction

Abstract: Nearly half of all active subduction zones initiated during the Cenozoic. All subduction zones associated with active back arc extension have initiated since the Eocene, hinting that back arc extension may be intimately associated with an interval (several tens of Myr) following subduction initiation. That such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates. Subduction initiation is known to have occurred in a variety of tectonic settings: old fracture zones, transform faults, and extinct spreading centers and through polarity reversal behind active subduction zones. Although occurring within different tectonic settings, four known subduction initiation events (Izu-Bonin-Mariana (IBM) along a fracture zone, Tonga-Kermadec along an extinct subduction boundary, New Hebrides within a back arc, and Puysegur-Fiordland along a spreading center) were typified by rapid uplift within the forearc followed by sudden subsidence. Other constraints corroborate the compressive nature of IBM and Tonga-Kermadec during initiation. Using an explicit finite element method within a two-dimensional domain, we explore the evolving force balance during initiation in which elastic flexure, viscous flow, plastic failure, and heat transport are all considered. In order to tie theory with observation, known tectonic settings of subduction initiation are used as initial and boundary conditions. We systematically explore incipient compression of a homogeneous plate, a former spreading center, and a fracture zone. The force balance is typified by a rapid growth in resisting force as the plate begins bending, reaching a maximum value dependent on plate thickness, but typically ranging from 2 to 3 × 1012 N/m for cases that become self-sustaining. This is followed by a drop in stress once a shear zone extends through the plate. The formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall. Cumulative convergence, not the rate of convergence, is the dominant control on the force balance. Viscous tractions influence the force balance only if the viscosity of the asthenosphere is >1020 Pa s, and then only after plate failure. Following plate failure, buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction. The total work done is also influenced by the rate of lithospheric failure. A self-sustaining subduction zone does not form from a homogeneous plate. A ridge placed under compression localizes subduction initiation, but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate. The large initial bending resistance can be entirely eliminated in ridge models, explaining the propensity for new subduction zones to form through polarity reversals. A fracture zone (FZ) placed in compression leads to subduction initiation with rapid extension of the overriding plate. A FZ must be underthrust by the older plate for ~100–150 km before a transition from forced to self-sustaining states is reached. In FZ models the change in force during transition is reflected by a shift from forearc uplift to subsidence. Subduction initiation is followed by trench retreat and back arc extension. Moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of Pacific subduction zones. The models provide an explanation for the compressive state of western Pacific margins before and during subduction initiation, including IBM and Tonga-Kermadec in the Eocene, and the association of active back arcs with young subduction zones. On the basis of our dynamic models and the relative poles of rotation between Pacific and Australia during the Eocene, we predict that the northern segment of the Tonga-Kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self-sustaining states.

Recycled metasomatized lithosphere as the origin of the Enriched Mantle II (EM2) end‐member: Evidence from the Samoan Volcanic Chain

Abstract: An in-depth Sr-Nd-Pb-He-Os isotope and trace element study of the EMII-defining Samoan hot spot lavas leads to a new working hypothesis for the origin of this high 87Sr/86Sr mantle end-member. Systematics of the Samoan fingerprint include (1) increasing 206Pb/204Pb with time - from 18.6 at the older, western volcanoes to 19.4 at the present-day hot spot center, Vailulu'u Seamount, (2) en-echelon arrays in 206Pb/204Pb – 208Pb/204Pb space which correspond to the two topographic lineaments of the 375 km long volcanic chain – this is much like the Kea and Loa Trends in Hawai'i, (3) the highest 87Sr/86Sr (0.7089) of all oceanic basalts, (4) an asymptotic decrease in 3He/4He from 24 RA [Farley et al., 1992] to the MORB value of 8 RA with increasing 87Sr/86Sr, and (5) mixing among four components which are best described as the “enriched mantle”, the depleted FOZO mantle, the (even more depleted) MORB Mantle, and a mild HIMU (high 238U/204Pb) mantle component. A theoretical, “pure” EMII lava composition has been calculated and indicates an extremely smooth trace element pattern of this end-member mantle reservoir. The standard recycling model (of ocean crust/sediment) fails as an explanation for producing Samoan EM2, due to these smooth spidergrams for EM2 lavas, low 187Os/188Os ratios and high 3He/4He (>8 RA). Instead, the origin of EM2 has been modeled with the ancient formation of metasomatised oceanic lithosphere, followed by storage in the deep mantle and return to the surface in the Samoan plume.

``Yin-Yang grid'': An overset grid in spherical geometry

Abstract: [1] A new kind of overset grid, named Yin-Yang grid, for spherical geometry is proposed. The Yin-Yang grid is composed of two identical component grids that are combined in a complemental way to cover a spherical surface with partial overlap on their boundaries. Each component grid is a low-latitude part of the latitude-longitude grid. Therefore the grid spacing is quasi-uniform, and the metric tensors are simple and analytically known. One can directly apply mathematical and numerical resources that have been written in the spherical polar coordinates or latitude-longitude grid. The complemental combination of the two identical component grids enables us to make efficient and concise programs. Simulation codes for geodynamo and mantle convection simulations using finite difference scheme based on the Yin-Yang grid are developed and tested. The Yin-Yang grid is suitable for massively parallel computers.

A hydrous melting and fractionation model for mid-ocean ridge basalts: Application to the Mid-Atlantic Ridge near the Azores

Abstract: The major element, trace element, and isotopic composition of mid-ocean ridge basalt glasses affected by the Azores hotspot are strongly correlated with H2O content of the glass. Distinguishing the relative importance of source chemistry and potential temperature in ridge-hotspot interaction therefore requires a comprehensive model that accounts for the effect of H2O in the source on melting behavior and for the effect of H2O in primitive liquids on the fractionation path. We develop such a model by coupling the latest version of the MELTS algorithm to a model for partitioning of water among silicate melts and nominally anhydrous minerals. We find that much of the variation in all major oxides except TiO2 and a significant fraction of the crustal thickness anomaly at the Azores platform are explained by the combined effects on melting and fractionation of up to ~700 ppm H2O in the source with only a small thermal anomaly, particularly if there is a small component of buoyantly driven active flow associated with the more H2O-rich melting regimes. An on-axis thermal anomaly of ~35°C in potential temperature explains the full crustal thickness increase of ~4 km approaching the Azores platform, whereas a ≥75°C thermal anomaly would be required in the absence of water or active flow. The polybaric hydrous melting and fractionation model allows us to solve for the TiO2, trace element and isotopic composition of the H2O-rich component in a way that self-consistently accounts for the changes in the melting and fractionation regimes resulting from enrichment, although the presence and concentration in the enriched component of elements more compatible than Dy cannot be resolved.

Strength of the geomagnetic field in the Cretaceous Normal Superchron: New data from submarine basaltic glass of the Troodos Ophiolite

Abstract: [1] We present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the Troodos Ophiolite ( similar to 92 Ma), formed during the Cretaceous Normal Superchron (CNS), a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all. Monte Carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength. Our data suggest a dipole strength equivalent to the present field or nearly twice the post-CNS average. The mean and standard deviation of the dipole moment (81 +/- 43 ZAm(2); Z = 10(21)) from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from DSDP Site 522. The new data set for the CNS suggests a picture of a strong and stable field during the period of time when it stopped reversing. Moreover, the similarity of the CNS data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability.

Seawater-peridotite interactions: First insights from ODP Leg 209, MAR 15°N

Abstract: [1] We present first results of a petrographic study of hydrothermally altered peridotites drilled during Ocean Drilling Program (ODP) Leg 209 in the 15°20′N fracture Zone area on the Mid-Atlantic Ridge (MAR). We find that serpentinization is extensive at all drill sites. Where serpentinization is incomplete, phase relations indicate two major reaction pathways. One is reaction of pyroxene to talc and tremolite, and the other is reaction of olivine to serpentine, magnetite, and brucite. We interpret these reactions in the light of recent peridotite-seawater reaction experiments and compositions of fluids venting from peridotite massifs at a range of temperatures. We suggest that the replacement of pyroxene by talc and tremolite takes place at temperatures >350°–400°C, where olivine is stable. The breakdown of olivine to serpentine, magnetite, and brucite is favored at temperatures below 250°C, where olivine reacts faster then pyroxene. High-temperature hydrothermal fluids venting at the Logatchev and Rainbow sites are consistent with rapid reaction of pyroxene and little or no reaction of olivine. Moderate-temperature fluids venting at the Lost City site are consistent with ongoing reaction of olivine to serpentine and brucite. Many completely serpentinized peridotites lack brucite and talc because once the more rapidly reacting phase is exhausted, interaction with the residual phase will change fluid pH and silica activity such that brucite or talc react to serpentine. At two sites we see strong evidence for continued fluid flow and fluid-rock interaction after serpentinization was complete. At Site 1268, serpentinites underwent massive replacement by talc under static conditions. This reaction requires either removal of Mg from or addition of Si to the system. We propose that the talc-altered rocks are Si-metasomatized and that the source of Si is likely gabbro-seawater reaction or breakdown of pyroxene deeper in the basement. The basement at Site 1268 is heavily veined, with talc and talc-oxide-sulfide veins being the most common vein types. It appears that the systems evolved from reducing (oxygen fugacity buffered by magnetite-pyrrhotite-pyrite and lower) to oxidizing (dominantly hematite). We propose that this transition is indicative of high fluid flux under retrograde conditions and that the abundance of hematite may relate to the Ca-depleted nature of the basement that prevents near-quantitative removal of seawater sulfate by anhydrite precipitation. At site 1272 we find abundant iowaite partly replacing brucite. While this is the first report of iowaite from a mid-ocean ridge setting, its presence indicates, again, fairly oxidizing conditions. Our preliminary results indicate that peridotite-seawater and serpentinite-seawater interactions can take place under a wider range of temperature and redox conditions than previously appreciated.

Subduction Factory 3: An Excel worksheet and macro for calculating the densities, seismic wave speeds, and H2O contents of minerals and rocks at pressure and temperature

Abstract: [1] An Excel macro to calculate mineral and rock physical properties at elevated pressure and temperature is presented. The workbook includes an expandable database of physical parameters for 52 rock-forming minerals stable at high pressures and temperatures. For these minerals the elastic moduli, densities, seismic velocities, and H2O contents are calculated at any specified P and T conditions, using basic thermodynamic relationships and third-order finite strain theory. The mineral modes of suites of rocks are also specifiable, so that their predicted aggregate properties can be calculated using standard solid mixing theories. A suite of sample rock modes taken from the literature provides a useful starting point. The results of these calculations can be applied to a wide variety of geophysical questions including estimating the alteration of the oceanic crust and mantle; predicting the seismic velocities of lower-crustal xenoliths; estimating the effects of changes in mineralogy, pressure and temperature on buoyancy; and assessing the H2O content and mineralogy of subducted lithosphere from seismic observations.

Experimental determination of trace element partitioning between garnet and silica-rich liquid during anhydrous partial melting of MORB-like eclogite

Abstract: [1] We present new experimentally determined trace element partition coefficients for nine garnet/melt and two clinopyroxene/melt pairs at 2.9–3.1 GPa and 1325°–1390°C, applicable to anhydrous partial melting of MORB-like eclogite in the upper mantle. Phase compositions are similar to those documented in partial melting experiments of eclogite at these conditions: garnets with 16–25% grossular component and 0.4–2.0 wt % TiO2 coexist with siliceous partial melts having 52.4–57.1 wt % SiO2 and 1.1–6.7 wt % TiO2. Observed garnet/melt partitioning depends on TiO2 concentrations in garnet. Among the high-TiO2 garnets (1.4–2.0 wt % TiO2), partition coefficients (Dgt/melt) increase with increasing grossular content of garnet (16–24%), but for the low-TiO2 garnets (0.4–0.6 wt % TiO2) there is no difference in partitioning behavior at 19–24% grossular. In general, partition coefficients for the low-TiO2 garnets tend to be higher than for the high-TiO2 garnets. DZr and DHf increase with higher grossular content, as does DZr/DHf, but DZr and DHf always remain smaller than unity, and TiO2 in garnet appears to have little effect on DZr/DHf. DTh and DU have average values of 0.0012 and 0.0111 for the high-TiO2 garnets and 0.0074 and 0.0376 for the low-TiO2 garnets (DU/DTh of 9.1 and 5.1, respectively), affirming that partial melting of eclogite in the upper mantle can produce liquids with significant (230Th)/(238U) excess. D values of highly charged cations (Nb, Ta, Th, U) are lower in high-TiO2 garnets. This is likely because Ti4+ occupying the garnet Y site requires charge balance by 2+ cations in Y or 3+ cations in Z, thereby limiting the availability of these charge-balancing substitutions to accommodate other highly charged cations. The overall range of our new garnet/melt partition coefficients, regardless of Ti and grossular content, is similar to literature garnet/melt partitioning data applicable to garnet peridotite melting, and therefore bulk partition coefficients for eclogite and peridotite depend largely on mineral modes and choice of partitioning data for clinopyroxene. Using our new data and previously published data from the literature, we calculate bulk partition coefficients appropriate for two MORB-like eclogites (82% clinopyroxene + 18% garnet and 75% clinopyroxene + 25% garnet) and a garnet peridotite (60% olivine + 17% orthopyroxene + 13% clinopyroxene + 10% garnet). Bulk partition coefficients are generally much higher for MORB-like eclogite when compared to garnet peridotite, with the exception of Th and U. However, the ratios of bulk partition coefficients of eclogite and garnet peridotite are similar for pairs such as DSm/DYb, DZr/DHf and DU/DTh, indicating that both lithologies can induce similar trace element fractionations during partial melting in the presence of garnet.

An InSAR‐based survey of volcanic deformation in the central Andes

Abstract: We extend an earlier interferometric synthetic aperture radar (InSAR) survey covering about 900 remote volcanos of the central Andes (14°–27°S) between the years 1992 and 2002. Our survey reveals broad (10s of km), roughly axisymmetric deformation at 4 volcanic centers: two stratovolcanoes are inflating (Uturuncu, Bolivia, and Hualca Hualca, Peru); another source of inflation on the border between Chile and Argentina is not obviously associated with a volcanic edifice (here called Lazufre); and a caldera (Cerro Blanco, also called Robledo) in northwest Argentina is subsiding. We explore the range of source depths and volumes allowed by our observations, using spherical, ellipsoidal and crack-like source geometries. We further examine the effects of local topography upon the deformation field and invert for a spherical point-source in both elastic half-space and layered-space crustal models. We use a global search algorithm, with gradient search methods used to further constrain best-fitting models. Inferred source depths are model-dependent, with differences in the assumed source geometry generating a larger range of accepted depths than variations in elastic structure. Source depths relative to sea level are: 8–18 km at Hualca Hualca; 12–25 km for Uturuncu; 5–13 km for Lazufre, and 5–10 km at Cerro Blanco. Deformation at all four volcanoes seems to be time-dependent, and only Uturuncu and Cerro Blanco were deforming during the entire time period of observation. Inflation at Hualca Hualca stopped in 1997, perhaps related to a large eruption of nearby Sabancaya volcano in May 1997, although there is no obvious relation between the rate of deformation and the eruptions of Sabancaya. We do not observe any deformation associated with eruptions of Lascar, Chile, at 16 other volcanoes that had recent small eruptions or fumarolic activity, or associated with a short-lived thermal anomaly at Chiliques volcano. We posit a hydrothermal system at Cerro Blanco to explain the rate of subsidence there. For the last decade, we calculate the ratio of the volume of magma intruded to extruded is between 1–10, and that the combined rate of intrusion and extrusion is within an order of magnitude of the inferred geologic rate.

Icelandic analogs to Martian flood lavas

Abstract: We report on new field observations from Icelandic lava flows that have the same surface morphology as many Martian flood lava flows. The Martian flood lavas are characterized by a platy-ridged surface morphology whose formation is not well understood. The examples on Mars include some of the most pristine lava on the planet and flows >1500 km long. The surfaces of the flows are characterized by (1) ridges tens of meters tall and wide and hundreds of meters long, (2) plates hundreds of meters to kilometers across that are bounded by ridges, (3) smooth surfaces broken into polygons several meters across and bowed up slightly in the center, (4) parallel grooves 1–10 km long cut into the flow surface by flow past obstacles, and (5) inflated pahoehoe margins. The Icelandic examples we examined (the 1783–1784 Laki Flow Field, the Burfells Lava Flow Field by Lake Myvatn, and a lava flow from Krafla Volcano) have all these surface characteristics. When examined in detail, we find that the surfaces of the Icelandic examples are composed primarily of disrupted pahoehoe. In some cases the breccia consists of simple slabs of pahoehoe lava; in other cases it is a thick layer dominated by contorted fragments of pahoehoe lobes. Our field observations lead us to conclude that these breccias are formed by the disruption of an initial pahoehoe surface by a large flux of liquid lava within the flow. In the case of Laki, the lava flux was provided by surges in the erupted effusion rate. At Burfells it appears that the rapid flow came from the sudden breaching of the margins of a large ponded lava flow. Using the observations from Iceland, we have improved our earlier thermal modeling of the Martian flood lavas. We now conclude that these platy-ridged lava flows may have been quite thermally efficient, allowing the flow to extend for >100 km under a disrupted crust that was carried on top of the flow.

Toward an optimal geomagnetic field intensity determination technique

Abstract: [1] Paleointensity determinations based on double heating techniques (in-field/zero-field cooling, zero-field/in-field cooling, and two in-field steps with opposite laboratory fields) are generally considered to be functionally interchangeable producing equally reliable paleointensity estimates. To investigate this premise, we have developed a simple mathematical model. We find that both the zero-field first and in-field first methods have a strong angular dependence on the laboratory field (parallel, orthogonal, and anti-parallel) while the two in-field steps method is independent of the direction of the laboratory-produced field. Contrary to common practice, each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met, even with marginal (10%) tails of partial thermoremanence. Our calculations suggest that the zero field first method with the laboratory-produced field anti-parallel to the natural remanence (NRM) is the most robust paleointensity determination technique when the intensity of the lab-induced field is smaller than ancient field. However, the zero field first method with the laboratory-field parallel to the NRM is the optimum approach when the intensity of the lab-induced field is larger than the ancient field. By far the best approach, however, is to alternatethe infield-zerofield (IZ) steps with zerofield-infield (ZI) steps.

Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research

Abstract: Thirteen laboratories from the USA and Europe participated in an intercomparison study of Mg/Ca and Sr/Ca measurements in foraminifera. The study included five planktonic species from surface sediments from different geographical regions and water depths. Each of the laboratories followed their own cleaning and analytical procedures and had no specific information about the samples. Analysis of solutions of known Mg/Ca and Sr/Ca ratios showed that the intralaboratory instrumental precision is better than 0.5% for both Mg/Ca and Sr/Ca measurements, regardless whether ICP-OES or ICP-MS is used. The interlaboratory precision on the analysis of standard solutions was about 1.5% and 0.9% for Mg/Ca and Sr/Ca measurements, respectively. These are equivalent to Mg/Ca-based temperature repeatability and reproducibility on the analysis of solutions of ±0.2°C and ±0.5°C, respectively. The analysis of foraminifera suggests an interlaboratory variance of about ±8% (%RSD) for Mg/Ca measurements, which translates to reproducibility of about ±2–3°C. The relatively large range in the reproducibility of foraminiferal analysis is primarily due to relatively poor intralaboratory repeatability (about ±1–2°C) and a bias (about 1°C) due to the application of different cleaning methods by different laboratories. Improving the consistency of cleaning methods among laboratories will, therefore, likely lead to better reproducibility. Even more importantly, the results of this study highlight the need for standards calibration among laboratories as a first step toward improving interlaboratory compatibility.

Ridge segmentation and the magnetic structure of the Southwest Indian Ridge (at 50°30′E, 55°30′E and 66°20′E): Implications for magmatic processes at ultraslow‐spreading centers

Abstract: [1] The aim of this paper is to investigate the relationships between the segmentation and the magnetic structure of the ultraslow-spreading Southwest Indian Ridge Contrary to faster spreading ridges, magnetization usually decreases from high values along the neovolcanic axis to low values in the nontransform discontinuities There is a direct correlation between the deepening of the axial valley and the decrease of the magnetization from the neovolcanic axis toward the deepest parts of the axial discontinuities We suggest that less frequent eruptions as the distance from the segment center and the length of these discontinuities increase, result in thinner extrusive lavas and thus control the along-axis magnetization variations by thinning the magnetic source layer A unique segment centered at 50°28′E shows a marked low magnetization anomaly at its center similarly to the segments of the slow-spreading Mid-Atlantic Ridge We suggest that in this segment both the mantle temperature and the magmatic activity are high enough for the lavas not to be highly fractionated A higher rate of melt production to the west of Gallieni transform fault may have created some form of reservoir where mixing of melts occurs and where crystalline fractionation is low producing low-magnetization lavas To the east, magma chambers may be smaller with cooler mantle temperatures resulting in restricted mixing and significant fractionation which may lead to relatively high intensity magnetization lavas Finally, we propose that serpentinization of peridotites has no significant contribution to the variation of the magnetization along the axial valley Off-axis, in thin crust areas, upper mantle rocks may become progressively more altered, as distance from the axis increases The strong faulting and alteration of a thin basaltic cap and underlying upper mantle rocks can produce the disappearance of the magnetic reversal pattern and the increase of the magnetization which is observed along the traces of the largest amagmatic discontinuities By contrast, in thicker crust areas, the upper mantle rocks are shielded from the alteration and the serpentinization process may be delayed resulting, as on the Mid-Atlantic Ridge, in slightly more positive magnetization values along the traces of axial discontinuities, regardless of polarity

Re‐evaluation of SO2 release of the 15 June 1991 Pinatubo eruption using ultraviolet and infrared satellite sensors

Abstract: [1] In this study, ultraviolet TOMS (Total Ozone Mapping Spectrometer) satellite data for SO2 are re-evaluated for the first 15 days following the 15 June 1991 Pinatubo eruption to reflect new data retrieval and reduction methods. Infrared satellite SO2 data from the TOVS/HIRS/2 (TIROS (Television Infrared Observation Satellite) Optical Vertical Sounder/High Resolution Infrared Radiation Sounder/2) sensor, whose data sets have a higher temporal resolution, are also analyzed for the first time for Pinatubo. Extrapolation of SO2 masses calculated from TOMS and TOVS satellite measurements 19–118 hours after the eruption suggest initial SO2 releases of 15 ± 3 Mt for TOMS and 19 ± 4 Mt for TOVS, including SO2 sequestered by ice in the early Pinatubo cloud. TOVS estimates are higher in part because of the effects of early formed sulfate. The TOMS SO2 method is not sensitive to sulfate, but can be corrected for the existence of this additional emitted sulfur. The mass of early formed sulfate in the Pinatubo cloud can be estimated with infrared remote sensing at about 4 Mt, equivalent to 3 Mt SO2. Thus the total S release by Pinatubo, calculated as SO2, is 18 ± 4 Mt based on TOMS and 19 ± 4 Mt based on TOVS. The SO2 removal from the volcanic cloud during 19–374 hours of atmospheric residence describes overall e-folding times of 25 ± 5 days for TOMS and 23 ± 5 days for TOVS. These removal rates are faster in the first 118 hours after eruption when ice and ash catalyze the reaction, and then slow after heavy ash and ice fallout. SO2 mass increases in the volcanic cloud are observed by both TOMS and TOVS during the first 70 hours after eruption, most probably caused by the gas-phase SO2 release from sublimating stratospheric ice-ash-gas mixtures. This result suggests that ice-sequestered SO2 exists in all tropical volcanic clouds, and at least partially explains SO2 mass increases observed in other volcanic clouds in the first day or two after eruption.

High-resolution animated tectonic reconstruction of the South Pacific and West Antarctic Margin

Abstract: [1] An animated reconstruction shows South Pacific plate kinematics between 90 and 45 Ma, using the satellite-derived gravity anomaly field, interpolated isochrons and plate rotation parameters from both published and new work on marine geophysical data. The Great South Basin and Bounty Trough, New Zealand, are shown as the earliest Pacific–Antarctic plate boundary that opened before 83 Ma. The earliest true Pacific–Antarctic seafloor formed within the eastern parts of this boundary, but later and farther west, seafloor formed within its Antarctic flank. After 80 Ma, the Bellingshausen plate converged with an oceanic part of the Antarctic plate to its east, while its motion simultaneously caused rifting in continental Antarctica to the south. The Pacific–Bellingshausen spreading center developed a set of long offset transform faults that the Pacific–Antarctic plate boundary inherited around chron C27 when the Bellingshausen plate ceased to move independently as part of a Pacific-wide plate tectonic reorganization event. Southwest of these transforms the Pacific–Antarctic Ridge saw an increase in transform-fault segmentation by ∼58 Ma. One of the long offset Pacific–Bellingshausen transforms, referred to as “V,” was modified during the C27 reorganization event when a Pacific–Antarctic–Phoenix triple junction initiated on its southern edge. Eastern parts of “V” started to operate in the Pacific–Phoenix spreading system, lengthening it even more, while its western parts operated in the Pacific–Antarctic system. This complicated feature was by-passed and deactivated by ridge axis propagation to its northwest at ∼47 Ma. We interpret our animation to highlight possible connections between these events.

Composition of basaltic lavas sampled by phase-2 of the Hawaii Scientific Drilling Project: Geochemical stratigraphy and magma types

Abstract: [1] This paper presents major and trace element compositions of lavas from the entire 3098 m stratigraphic section sampled by phase-2 of the Hawaii Scientific Drilling Project. The upper 245 m are lavas from Mauna Loa volcano, and the lower 2853 m are lavas and volcanoclastic rocks from Mauna Kea volcano. These intervals are inferred to represent about 100 ka and 400 ka respectively of the eruptive history of the two volcanoes. The Mauna Loa tholeiites tend to be higher in SiO2 and lower in total iron, TiO2, alkalis, and incompatible elements at a given MgO content than Mauna Kea lavas. The transition from Mauna Loa to Mauna Kea lavas is all the more pronounced because the Mauna Loa tholeiites overlie a thin sequence of postshield Mauna Kea alkalic to transitional tholeiitic lavas. The Mauna Loa tholeiites display welldeveloped coherent trends with MgO that are indistinguishable in most respects from modern lavas. With depth, however, there is a slight decline in incompatible element abundances, and small shifts to depleted isotopic ratios. These characteristics suggest small changes in melt production and source components over time, superimposed on shallow melt segregation. The Mauna Kea section is subdivided into a thin, upper 107 m sequence of postshield tholeiites, transitional tholeiites and alkali basalts of the Hamakua volcanics, overlying four tholeiitic magma types that are intercalated throughout the rest of the core. These four magma types are recognized on the basis of MgO-normalized SiO2 and Zr/Nb values. Type-1 lavas (high SiO2 and Zr/Nb) are ubiquitous below the postshield lavas and are the dominant magma type on Mauna Kea. They are inter-layered with the other three lava types. Type-2 lavas (low SiO2 but high Zr/Nb) are found only in the upper core, and especially above 850 m. Type-3 lavas (low SiO2 and Zr/Nb) are very similar to tholeiites from Loihi volcano and are present only below 1974 m. There are only 3 discrete samples of type-4 lavas (high SiO2 and low Zr/Nb), which are present in the upper and lower core. The differences between these magma types are inferred to reflect changes in melt production, depth of melt segregation, and differences in plume source components over about 400 ka of Mauna Kea’s eruptive history. At the start of this record, eruption rates were high, and two distinct tholeiitic magmas (type-1 and 3) were erupting concurrently. These two magmas require two distinct source components, one similar to that of modern Loihi tholeiites and the other close to that of Kilauea magmas. Subsequently, the Loihilike source of the type-3 magmas was exhausted, and these lavas are absent from the remainder of the core. For the next 200 ka or so, the eruptive sequence consists of inter-layered type-1 and -2 lavas that are derived from a common Mauna Kea source, the major difference between the two being the depth at which the melts segregated from the source. At around 440 ka (corresponding with the transition in the core from submarine to subaerial lavas) eruption rates began to decline and low-MgO lavas are suddenly much more abundant in the record. Continuing gradual decline in melting and eruption rates was accompanied by a decline in normalized SiO2 content of the type-1 magmas, and the eventual onset of postshield magmatism. Components: 30,168 words, 16 figures, 4 tables.

Analyzing absolute paleointensity determinations: Acceptance criteria and the software ThellierTool4.0

Abstract: [1] The ThellierTool4.0 is an intuitive and easy-to-use software which provides the possibility to analyze a wide range of different modifications of the Thellier absolute paleointensity experiment (available at http://earthref.org/tools/). Besides the Arai plot for paleointensity determination, orthogonal projections of the direction, decay of NRM during thermal demagnetization, and additional plots regarding alteration and multidomain checks enable the user to visualize the quality of individual determinations. Experimental checks for magnetomineralogical changes, either in-field or zero-field pTRM* checks, are evaluated regarding their differences to the corresponding pTRM* acquisition in two most commonly used ways. Furthermore, a measure for the cumulative alteration differences beginning at room temperature is calculated, and the possibility to correct for magnetomineralogical changes is provided. Two different experimental methods to check for multidomain bias are supported and analyzed by the software. Intensity differences recorded by pTRM*-tail checks are calculated. Accounting for the directional difference between applied laboratory field and magnetization of the sample, the effective pTRM*-tail is determined, and thus failures of Thellier's law of independence are monitored. Failures of the law of additivity, experimentally observed by additivity checks, are also evaluated by the software. The vectorial character of individual measurements is fully considered for all calculations. Uniform selection criteria for acceptance and rejection of determinations can be applied, and a set of such criteria with emphasis on minimal bias due to alteration, multidomain remanence, and analysis/experimental inaccuracies is suggested.

Strain localization on an oceanic detachment fault system, Atlantis Massif, 30°N, Mid-Atlantic Ridge

Abstract: [1] Microstructural observations, mineral chemistry, and the spatial distribution of deformation fabrics recorded in outcrop samples collected from Atlantis Massif, the active inside corner high at 30°N, Mid-Atlantic Ridge, suggest that strain is localized near the subhorizontal domal surface hypothesized to be an exposed detachment fault Deformation textures in peridotite and gabbro indicate that high-temperature (>500°C) strain occurred via crystal-plastic flow and diffusive mass transfer Low-temperature (<400°C) shear zones containing brittle and semibrittle microboudinage textures in which tremolite, chlorite, and/or talc replace fractured serpentine or hornblende cut earlier formed high-temperature deformation fabrics in peridotite Textures indicate strain was localized by cataclasis and reaction softening into zones of intense greenschist and subgreenschist grade metamorphism Gabbro is only weakly deformed below amphibolite facies (<500°C), indicating that strain was partitioned into altered peridotite at low temperature There is a clear relationship between deformation intensity and structural depth beneath the subhorizontal surface of the Massif Discontinuous high-intensity crystal-plastic deformation fabrics are found at all structural depths (0–520 m) beneath the surface, indicating that high-temperature, granulite- and amphibolite-grade deformation was not localized in a single shear zone In contrast, semibrittle and brittle low-temperature shear zones are concentrated less than 90 m structurally beneath the surface, and the most intensely brittlely deformed samples concentrated in the upper 10 m Localization of brittle deformation fabrics near the upper surface of the massif supports the hypothesis that it is the exposed footwall of a detachment fault The structural evolution of Atlantis Massif is therefore analogous to a continental metamorphic core complex Strain was localized onto the fault by reaction-softening and fluid-assisted fracturing during greenschist- and subgreenschist-grade hydrothermal alteration of olivine, clinopyroxene, serpentine, and hornblende to tremolite, chlorite, and/or talc

Radiometric ages for basement rocks from the Emperor Seamounts, ODP Leg 197

Abstract: [1] The Hawaiian-Emperor seamount chain is the “type” example of an age-progressive, hot spot-generated intraplate volcanic lineament. However, our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago. Improvements in instrumentation, sample preparation methods, and new material obtained by recent drilling warrant a reexamination of the age relations among the older Hawaiian volcanoes. We report new age determinations (40Ar-39Ar incremental heating method) on whole rocks and feldspar separates from Detroit (Sites 1203 and 1204), Nintoku (Site 1205), and Koko (Site 1206) Seamounts (Ocean Drilling Program (ODP) Leg 197) and Meiji Seamount (Deep Sea Drilling Project (DSDP) Leg 19, Site 192). Plateaus in incremental heating age spectra for Site 1203 lava flows give a mean age of 75.8 ± 0.6 (2σ) Ma, which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments. Site 1204 lavas produced discordant spectra, indicating Ar loss by reheating and K mobilization. Six plateau ages from lava flows at Site 1205 give a mean age of 55.6 ± 0.2 Ma, corresponding to Chron 24r. Drilling at Site 1206 intersected a N-R-N magnetic polarity sequence of lava flows, from which six plateau ages give a mean age of 49.1 ± 0.2 Ma, corresponding to the Chron 21n-22r-22n sequence. Plateau ages from two feldspar separates and one lava from DSDP Site 192 range from 34 to 41 Ma, significantly younger than the Cretaceous age of overlying sediments, which we relate to postcrystallization K mobilization. Combined with new dating results from Suiko Seamount (DSDP Site 433) and volcanoes near the prominent bend in the lineament [Sharp and Clague, 2002], the overall trend is increasing volcano age from south to north along the Emperor Seamounts, consistent with the hot spot model. However, there appear to be important departures from the earlier modeled simple linear age progression, which we relate to changes in Pacific plate motion and the rate of southward motion of the Hawaiian hot spot.

Volcanic sources of tropospheric ozone‐depleting trace gases

Abstract: [1] Recent investigations report mixing ratios of BrO gas reaching 1 ppb in plume 4–7 km downwind of the summit of Soufriere Hills volcano, Montserrat. The detection of BrO in volcanic plumes is potentially important evidence of halogen-catalyzed tropospheric ozone destruction and suggests that volcanoes either directly emit BrO or emit bromine species that are rapidly converted to reactive bromine in volcanic plumes. Species distribution models constrained by volcanic gas and condensate analytical data for arc, rift, and hot spot volcanoes demonstrate that shallow magma degassing generates volcanic gases with ppb to ppm levels of reactive radicals Br, Cl, H, and HO but no significant BrO or ClO. The conversion of volcanic Br and Cl by reaction with ozone in gas-phase catalytic reaction cycles during plume transport can lead to secondary BrO and ClO at mixing ratios of a ppt and higher, possibly approaching a ppb for especially halogen-rich volcanic gases, several kilometers downwind of degassing sources. In general, however, ppb BrO and ClO levels in volcanic plumes several kilometers downwind probably require near-vent, high-temperature reaction of magmatic gases with air and/or in-plume heterogeneous chemical processes involving aerosols during plume transport. These processes oxidize bromine and chlorine in HBr and HCl, giving rise to increased levels of reactive Br, Cl, and HO, in addition to NOx. They can also produce significant amounts of the photochemically active precursors Br2, BrCl, and Cl2, which are photolyzable to reactive Br and Cl. The precursors may build up in nighttime volcanic plumes and accelerate ozone destruction and BrO and ClO production after sunrise. Downwind conversion of reactive Br, Cl, H, and HO from volcanic gas sources by catalytic reaction chains to HOBr and HOCl may trigger heterogeneous chemical processes, which are probably essential for sustaining ozone destruction in volcanic plumes. Gas-phase reactions of HBr and HCl with HO in volcanic plumes might also boost BrO and ClO downwind. Thus in-plume processes that generate reactive halogens may contribute significantly to ozone destruction in volcanic plumes. Enhanced levels of HOx and NOx in volcanic emissions may trigger nonhalogen catalytic cycles depleting ozone in tropospheric volcanic plumes.

Xenolith constraints on seismic velocities in the upper mantle beneath southern Africa

Abstract: [1] We impose geologic constraints on seismic three-dimensional (3-D) images of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 120 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based on the elastic and thermal moduli of constituent minerals under equilibrium P-T conditions at the mantle source. The largest sources of error in the velocity estimates derive from inaccurate thermo-barometry and, to a lesser extent, from uncertainties in the elastic constants of the constituent minerals. Results are consistent with tomographic evidence that cratonic mantle is higher in velocity by 0.5–1.5% and lower in density by about 1% relative to off-craton Proterozoic samples at comparable depths. Seismic velocity variations between cratonic and noncratonic xenoliths are controlled dominantly by differences in calculated temperatures, with compositional effects secondary. Different temperature profiles between cratonic and noncratonic regions have a relatively minor influence on density, where composition remains the dominant control. Low-T cratonic xenoliths exhibit a positive velocity-depth curve, rising from about 8.13 km/s at uppermost mantle depths to about 8.25 km/s at 180-km depth. S velocities decrease slightly over the same depth interval, from about 4.7 km/s in the uppermost mantle to 4.65 km/s at 180-km depth. P and S velocities for high-T lherzolites are highly scattered, ranging from highs close to those of the low-T xenoliths to lows of 8.05 km/s and 4.5 km/s at depths in excess of 200 km. These low velocities, while not asthenospheric, are inconsistent with seismic tomographic images that indicate high velocity root material extending to depths of at least 250 km. One plausible explanation is that high temperatures determined for the high-T xenoliths are a nonequilibrium consequence of relatively recent thermal perturbation and compositional modification associated with emplacement of kimberlitic fluids into the deep tectospheric root. Seismic velocities and densities for cratonic xenoliths differ significantly from those predicted for both primitive mantle peridotite and mantle eclogite. A model primitive mantle under cratonic P-T conditions exhibits velocities about 1% lower for P and about 1.5% lower for S, a consequence of a more fertile composition and different modal composition. Primitive mantle is also about 2% more dense at 150-km depth than low-T garnet lherzolite at cratonic P-T conditions. Similar calculations based on an oceanic geotherm are consistent with the isopycnic hypothesis of comparable density columns beneath oceanic and cratonic regions. Calculations for a hypothetical “cratonic” eclogite (50:50 garnet/omphacite) with an assumed cratonic geotherm produce extremely high VP and VS (8.68 km/s and 4.84 km/s, respectively, at 150 km depth) as well as high density (∼3.54 gm/cc). The very high velocity of eclogite should render it seismically conspicuous in the cratonic mantle if present as large volume blocks or slabs. We discuss how the seismic velocity data we have compiled in this paper from both xenoliths and generic petrologic models of the upper mantle differ from commonly used standard earth models IASPEI and PREM.

Switching perspectives: Do mineral fluxes determine particulate organic carbon fluxes or vice versa?

Abstract: [1] It has recently been postulated that mineral material like biogenic silica frustules or carbonate shells as well as lithogenic dust act as ballast material determining particulate organic carbon (POC) fluxes below 2000 m in the ocean. However, correlations do not identify cause and effect, and in this commentary it is proposed that on the contrary, POC fluxes determine fluxes of this mineral material. I suggest that during sedimentation marine snow originating from biological activity in the surface layer collect small, non-sinking mineral particles of biogenic and lithogenic origin until its carrying capacity is reached.

Implications of a nonlinear 40Ar/39Ar age progression along the Louisville seamount trail for models of fixed and moving hot spots

Abstract: [1] The Louisville seamount trail has been recognized as one of the key examples of hot spot volcanism, comparable to the classic volcanic Hawaiian-Emperor lineaments. The published total fusion 40Ar/39Ar data of Watts et al. [1988] showed an astonishing linear age progression, firmly establishing Louisville as a fixed hot spot in the South Pacific mantle. We report new 40Ar/39Ar ages based on high-resolution incremental heating 40Ar/39Ar dating for the same group of samples, showing a marked increase in both precision and accuracy. One of the key findings in our reexamination is that the age progression is not linear after all. The new data show a significantly decreased “apparent” plate velocity for the Louisville seamount trail older than 62 Ma but confirm the linear trend between 47 Ma and the present day (although based on only three samples over 2150 km). The most recent volcanic activity in the Louisville seamount trail has now been dated at 1.11 ± 0.04 Ma for the most southeastern seamount located at 50°26′S and 139°09′W. These results indicate that the Louisville age progression should be interpreted on the basis of both plate and hot spot motion. In this paper we examine our new results in conjunction with the numerical mantle flow models of Steinberger et al. [2004] that also predict marked deviations from simple linear age progressions. With these models we can achieve a good fit to the geometry of both the Hawaiian and Louisville seamount trails and their age progressions as well as the ∼15° paleolatitudinal shift observed by Tarduno et al. [2003] for the Hawaiian hot spot between 80 and 47 Ma. If the model is restricted to Pacific hot spots only, we can improve the fit to the nonlinear age trend for the Louisville seamount trail by allowing an additional rotation change of the Pacific plate around 62 Ma and by decreasing the initiation age of the Louisville plume from 120 to 90 Ma. This improved model features a significant eastward hot spot motion of ∼5° between 80 and 30 Ma for the Louisville hot spot, which is quite dissimilar to the southward motion of the Hawaiian hot spot during the same time interval, followed by a minor ∼2° latitudinal shift over the last 30 Myr. If hot spot tracks are considered globally, the age trend observed for the oldest part of the Louisville seamount trail does not entirely follow the numerical model predictions. This may indicate some remaining inaccuracies in the global plate circuit, but it may also indicate that the Louisville hot spot experienced a motion somewhat different than in the numerical model: faster in the interval between 62 and 47 Ma but slower before that.

Strong directivity of ocean‐generated seismic noise

Abstract: [1] We measure direction and amplitude of ocean-generated continuous seismic noise in the western United States. Slowness direction of the noise is determined using array beamforming, and particle motion direction from individual three-component stations. We find two surprising results. First, the noise is highly monodirectional at all sites, regardless of coastal distance. A single narrow generation area dominates for most of the time period, interrupted by a second well defined direction during ocean swell events. Second, we find that a storm off the Labrador coast with not unusual wave heights generates coherent noise across the entire continent. We show the causal relationship between swells arriving at different North American coastal areas and the triggered microseisms in time-lapse movies (Animations 1 and 2) of ocean swells and concurrent microseisms. Our results have a number of implications for different fields of research. A useful by-product of our finding that microseisms are a strongly directional noise source is the possibility of using automated processing of the continuous noise as a near real-time check on station polarity and calibration problems, which would be a simply implemented indicator for the state of health of a seismic network. Consistent monodirectional noise may have an influence on seismic azimuthal measurements such as shear wave splitting. Most importantly, our findings should be taken into account in proposed studies which will use seismic noise as a proxy for ocean wave height in investigations of interdecadal climate change.

Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project

Abstract: [1] This paper presents new magmatic helium isotopic compositions in a suite of lavas from phase II of the Hawaiian Scientific Drilling Project (HSDP2) core, which sampled Mauna Kea volcano to a maximum depth of 3098 m below sea level. Most of the measurements were performed by in vacuo crushing of olivine phenocrysts, but include submarine pillow glasses from the 2200 to 2500 meter depth interval, and orthopyroxene phenocrysts from an intrusive at 1880 m. The magmatic 3He/4He ratios range from 6 to 24.7 times atmospheric (Ra), which significantly extends the range of values for Mauna Kea volcano. The 3He/4He ratios are lowest (i.e., close to MORB values of ∼8 Ra) near the top of the Mauna Kea section and rise slowly, to 10–12 Ra, at 1000 m below sea level, consistent with results from the HSDP1 core. At depths greater than 1000 m in the core, primarily in the submarine lavas, there are brief periods when the 3He/4He ratios are higher than 14.5 Ra, always returning to a baseline value. Twelve such excursions were identified in the core; all but one are in the submarine section, and most (7) are in the deepest section, at depths of 1950 to 3070 m. The baseline 3He/4He value rises from 10–12 Ra near 1000 m depth to 12–14 Ra at 3000 m. The helium spikes are found only in lavas that are older than 380 Ka in age, based on an age model derived from Ar-Ar data (W. D. Sharp et al., manuscript in preparation, 2003). Excluding the excursions defined by single lava flows (3) and intrusive units (3), the average spike duration is approximately 15 (±9) Ka (n = 6). The high 3He/4He spikes are interpreted as pulses of magma from the center of the actively upwelling Hawaiian hot spot. The short duration of the high 3He/4He excursions suggests that Mauna Kea was never directly over high the 3He/4He component of the plume (during the HSDP2 eruptive period), presumed to be the plume center. Assuming that the Mauna Kea helium spikes result from melting of heterogeneities within the plume, their short duration implies that the length scales of heterogeneities in the solid upwelling mantle are between 60 m and 12 km (for upwelling rates of 2 to 40 cm/yr). The high 3He/4He are associated with high 208Pb/204Pb, and relatively low 143Nd/144Nd, Zr/Nb, and SiO2. The correlations with major elements, trace elements and isotopes demonstrate that helium is coupled to the other geochemical variations, and that the Mauna Kea isotopic variability is caused by heterogeneities within the upwelling plume.

Structure and dynamics of sheared mantle plumes

Abstract: [1] An extensive series of laboratory experiments is used to investigate the behavior of sheared thermal plumes. The plumes are generated by heating a small circular plate on the base of a cylindrical tank filled with viscous fluid and then sheared by rotating a horizontal lid at the fluid surface. The motion of passive tracers in the plumes is visualized by the release of several dye streams on the hot plate. We systematically examine the dependence of the convective flow on four dimensionless numbers: a velocity ratio, a Rayleigh number, the viscosity ratio, and an aspect ratio. We identify and delineate two transitions in the convective behavior: from a regime where the plume can spread upstream against the shear to a regime where the entire plume is advected downstream, and from a regime of negligible cross-stream circulation to a regime with significant cross-stream circulation and thermal entrainment. Our analysis of the steady profiles of the plumes shows that they initially rise with a constant vertical rise velocity. This rise velocity depends on the buoyancy flux and ambient viscosity but is almost independent of the centerline plume viscosity, which suggests that most of the thermal plume has a viscosity that is much closer to the ambient viscosity than the centerline viscosity. As the plumes approach the lid, they decelerate as the viscous drag on them steadily increases. The lateral spreading of the plumes under the lid is found to be well described by similarity solutions derived for the spreading of compositional plumes on a rigid surface, if the effective viscosity of the thermal plumes is taken to be the ambient viscosity rather than the centerline viscosity. A similar theoretical model is found to roughly predict the upstream spreading of thermal plumes at low shear, but it breaks down at moderate to high shear, where the entire plumes are advected downstream. When our results are applied to the Earth, we find that mantle plumes are mostly divided into only two flow regimes in the upper mantle: plumes under slow moving plates experience upstream flow and negligible cross-stream circulation, while plumes under faster moving plates (including all Pacific plumes) experience significant cross-stream circulation and are advected downstream. We also demonstrate that geochemical heterogeneities in a plume's source region will result in an azimuthally zoned plume and in an asymmetric geographical distribution of geochemical heterogeneities in the erupted hot spot basalts, as is seen in the Hawaiian, Galapagos, Marquesas, and Tahiti/Society island chains. For individual mantle plumes, we determine their diameter and vertical rise velocity as well as the extent of upstream spreading and the rate of lateral spreading under the lithosphere.

Hydrothermal venting in magma deserts: The ultraslow-spreading Gakkel and Southwest Indian Ridges

Abstract: [1] Detailed hydrothermal surveys over ridges with spreading rates of 50–150 mm/yr have found a linear relation between spreading rate and the spatial frequency of hydrothermal venting, but the validity of this relation at slow and ultraslow ridges is unproved. Here we compare hydrothermal plume surveys along three sections of the Gakkel Ridge (Arctic Ocean) and the Southwest Indian Ridge (SWIR) to determine if hydrothermal activity is similarly distributed among these ultraslow ridge sections and if these distributions follow the hypothesized linear trend derived from surveys along fast ridges. Along the Gakkel Ridge, most apparent vent sites occur on volcanic highs, and the extraordinarily weak vertical density gradient of the deep Arctic permits plumes to rise above the axial bathymetry. Individual plumes can thus be extensively dispersed along axis, to distances >200 km, and ∼75% of the total axial length surveyed is overlain by plumes. Detailed mapping of these plumes points to only 9–10 active sites in 850 km, however, yielding a site frequency Fs, sites/100 km of ridge length, of 1.1–1.2. Plumes detected along the SWIR are considerably less extensive for two reasons: an apparent paucity of active vent fields on volcanic highs and a normal deep-ocean density gradient that prevents extended plume rise. Along a western SWIR section (10°–23°E) we identify 3–8 sites, so Fs = 0.3–0.8; along a previously surveyed 440 km section of the eastern SWIR (58°–66°E), 6 sites yield Fs = 1.3. Plotting spreading rate (us) versus Fs, the ultraslow ridges and eight other ridge sections, spanning the global range of spreading rate, establish a robust linear trend (Fs = 0.98 + 0.015us), implying that the long-term heat supply is the first-order control on the global distribution of hydrothermal activity. Normalizing Fs to the delivery rate of basaltic magma suggests that ultraslow ridges are several times more efficient than faster-spreading ridges in supporting active vent fields. This increased efficiency could derive from some combination of three-dimensional magma focusing at volcanic centers, deep mining of heat from gabbroic intrusions and direct cooling of the upper mantle, and nonmagmatic heat supplied by exothermic serpentinization.