Juan de Fuca洋脊Endeavour段热液硫化物稀土元素地球化学特征

用 ICP-MS对取自 Juan de Fuca洋脊 Endeavour段 5块热液硫化物样品的 13个分析样进行了稀土元素(REE)测试.结果显示该区硫化物样品的 REE含量较低(0.35～ 14.8 μ g/g),所有样品的 REE球粒陨石标准化分布模式均表现出 Eu正异常和 LREE富集的特征,表明硫化物中的 REE来自热液.不同喷口硫化物的 REE含量变化较大,同一块状硫化物不同部位的含量也有较大差异,主要是由于硫化物形成过程中,热液和海水的混合不均一性以及不同矿物沉淀和 (或 )溶解的结果.硫化物 REE的分布特征主要受热液的影响,烟囱内外层 Eu正异常的变化主要受矿物组成和物理化学条件的控制.     

Geochemistry and morphology of metalliferous sediments and oxyhydroxides from the Endeavour segment, Juan de Fuca Ridge

We present first data on the geochemistry, mineralogy and morphology of near-vent sediments (35 and 200 m from active vent) and ridge flank sediments (approximately 3 km from the vent field) as well as oxyhydroxide deposits from the Endeavour segment, Juan de Fuca Ridge. The purpose of the study was to understand better the origin and characteristic features of metalliferous sediments associated with base and precious metal massive sulfides in volcanic terrains. Hydrothermal components in sediments are Fe-Si ± S-rich and Mn-Fe-Si-rich phases, sulfides and barite, which were exclusively derived from plume fallout. Sulfides are only a minor constituent of near-vent sediments (2-4 wt%) and were not detected in ridge flank sediments. The study suggests that the distribution of hydrothermal phases and associated elements in near-vent and ridge flank sediments is affected mainly by processes of agglomeration, dissolution, absorption and settling that take place within a plume and to a lesser extent post-depositional processes. Rapid deposition of sulfides in the vicinity of the vents is reflected in a sharp drop of the Cu concentrations in sediments with increasing distance from the vents. Besides sulfides, important carriers of Pb, Cu, Zn and Co in near-vent sediments are Fe-Mn oxyhydroxides that occur together with silica as aggregates of gel-like material and flaky particles and as coatings on filaments. Away from the vents, trace metals are mostly in Fe-Mn oxyhydroxides and authigenic Fe-rich montmorillonite. Oxyhydroxides at the Main Endeavour field are interpreted to have originated from oxidation of mound sulfides accompanied by precipitation of primary Fe-oxyhydroxide + silica from low-temperature fluids. At the Mothra field, seafloor deposits and chimney crusts composed of Fe-oxyhydroxide ± Mn + silica are considered to be direct precipitates from hydrothermal fluids that have been less diluted with seawater. Oxyhydroxide deposits exhibit unique microtextures that resemble mineralized microorganisms and may indicate existence of diverse microbial communities.     

Geochemistry of low-molecular weight hydrocarbons in hydrothermal fluids from Middle Valley, northern Juan de Fuca Ridge

Hydrothermal vent fluids from Middle Valley, a sediment-covered vent field located on the northern Juan de Fuca Ridge, were sampled in July, 2000. Eight different vents with exit temperatures of 186-281 °C were sampled from two areas of venting: the Dead Dog and ODP Mound fields. Fluids from the Dead Dog field are characterized by higher concentrations of ΣNH₃ and organic compounds (C₁-C₄ alkanes, ethene, propene, benzene and toluene) compared with fluids from the ODP Mound field. The ODP Mound fluids, however, are characterized by higher C₁/(C₂ + C₃) and benzene:toluene ratios than those from the Dead Dog field. The aqueous organic compounds in these fluids have been derived from both bacterial processes (methanogenesis in low temperature regions during recharge) as well as from thermogenic processes in higher temperature portions of the subsurface reaction zone. As the sediments undergo hydrothermal alteration, carbon dioxide and hydrocarbons are released to solution as organic matter degrades via a stepwise oxidation process. Compositional and isotopic differences in the aqueous hydrocarbons indicate that maximum subsurface temperatures at the ODP Mound are greater than those at the Dead Dog field. Maximum subsurface temperatures were calculated assuming that thermodynamic equilibrium is attained between alkenes and alkanes, benzene and toluene, and carbon dioxide and methane. The calculated temperatures for alkene-alkane equilibrium are consistent with differences in the dissolved Cl concentrations in fluids from the two fields, and confirm that subsurface temperatures at the ODP Mound are hotter than those at the Dead Dog field. Temperatures calculated assuming benzene-toluene equilibrium and carbon dioxide-methane equilibrium are similar to observed exit temperatures, and do not record the hottest subsurface conditions. The difference in subsurface temperatures estimated using organic geochemical thermometers reflects subsurface cooling processes via mixing of a hot, low salinity vapor with a cooler, seawater salinity fluid. Because of the disparate temperature dependence of alkene-alkane and benzene-toluene equilibria, the mixed fluid records both the high and low temperature equilibrium conditions. These calculations indicate that vapor-rich fluids are presently being formed in the crust beneath the ODP Mound, yet do not reach the surface due to mixing with the lower temperature fluids.     

The concentration and isotopic composition of carbon in basaltic glasses from the Juan de Fuca Ridge, Pacific Ocean

The abundance and 13C/12C ratios of carbon were analyzed in basaltic glass from twenty locations along the Juan de Fuca Ridge using a 3-step combustion/extraction technique. Carbon released during the first two combustion steps at 400-500 degrees C and 600-650 degrees C is interpreted to be secondary, and only the carbon recovered during a final combustion step at approximately 1200 degrees C is thought to be indigenous to the samples. For carbon released at approximately 1200 degrees C, glasses analyzed as 1-2 mm chips contained 23-146 ppm C with delta 13C values of -4.8 to -9.3%, whereas samples crushed to 38-63 microns or 63-90 microns yielded 56-103 ppm C with delta 13C values of -6.1 to -9.2%. The concentrations and isotopic compositions of the primary carbon dissolved in the glasses and present in the vesicles are similar to those previously reported for other ocean-ridge basalts. The Juan de Fuca basaltic magmas were not in equilibrium with respect to carbon when they erupted and quenched on the sea floor. Evidence of disequilibrium includes (1) a large range of carbon contents among glasses collected at similar depths, (2) a highly variable calculated carbon isotopic fractionation between melt and vapor determined by comparing crushed and uncrushed splits of the same sample, and (3) a lack of correlation between vesicle abundance, carbon concentration, and depth of eruption. Variations in carbon concentration and delta 13C ratios along the ridge do not correlate with major element chemistry. The observed relationship between carbon concentrations and delta 13C values may be explained by late-stage, variable degrees of open-system (Rayleigh-like) degassing.     

Local and regional variation of MORB parent magmas: evidence from melt inclusions from the Endeavour Segment of the Juan de Fuca Ridge

The development of petrogenetic models of igneous processes in the mantle is dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixing and fractionation during transport to the surface, destroying much of the evidence of their primary diversity. To circumvent this problem and to determine the diversity of melts produced in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de Fuca Ridge. This area was selected for study because of the demonstrated close association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas have been analyzed by electron and ion microprobe for major and trace elements. The depleted and enriched lavas, as well as their melt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than, yet collinear with, the host lava suites. In contrast, the minor and trace element characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contain both depleted and enriched melt inclusions, and therefore exhibit the largest compositional range (K₂O: 0.01 to 0.4 oxide wt%, P₂O₅: <0.01 to 0.2 oxide wt%, La_N: 7 to 35, Yb_N: 1 to 13, and Ti/Zr: <100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore relatively homogeneous (K₂O: 0.32 to 0.9 oxide wt %, P₂O₅: 0.02 to 0.35 oxide wt%, La_N: 11 to 60, Yb_N: 4 to 21, and Ti/Zr: ∼100). In addition, the most primitive E-32 inclusions are similar in composition to the most enriched inclusions from the depleted hosts. Major element data for melt inclusions from both N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative history. When fractionation-corrected, inclusion compositions correlate with their host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in the upper mantle or lower crust. Based on this fact, the lava suites are not produced from a single parent magma, but from a suite of primary magmas. The chemistry of the melt inclusions from the enriched lavas is consistent with a derivation from variable percentages of partial melting within the spinel stability field by a process of open system (continuous or critical) melting assuming a depleted lherzolite source veined with clinopyroxenite. The low percentage melts are dominantly enriched melts of the clinopyroxenite. In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clinopyroxene was consumed. This set of characteristics is consistent with a scenario by which diverse melts produced at different depths travel through the melting regime to the base of the crust without homogenizing en route. The homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate of the melts with the materials in the conduit walls, as well as the residence times and flux rate, in the upper mantle and lower crust. Received: 2 December 1997 / Accepted: 27 August 1998     

Amino acid abundances and stereochemistry in hydrothermally altered sediments from the Juan de Fuca Ridge, northeastern Pacific Ocean.

The Juan de Fuca Ridge is a hydrothermally active, sediment covered, spreading ridge situated a few hundred kilometres off the west coast of North America in the northeastern Pacific Ocean. Sediments from seven sites drilled during the Ocean Drilling Program (ODP) Legs 139 and 168 were analyzed for total hydrolyzable amino acids (THAA), individual amino acid distributions, total organic C (TOC) and total N (TN) contents. The aim was to evaluate the effects of hydrothermal stress on the decomposition and transformation of sedimentary amino acids. Hydrolyzable amino acids account for up to 3.3% of the total organic C content and up to 12% of the total N content of the upper sediments. The total amounts of amino acids decrease significantly with depth in all drilled holes. This trend is particularly pronounced in holes with a thermal gradient of around 0.6 degrees C/m or higher. The most abundant amino acids in shallow sediments are glycine, alanine, lysine, glutamic acid, valine and histidine. The changes in amino acid distributions in low temperature holes are characterized by increased relative abundances of non-protein beta-alanine and gamma-aminobutyric acid. In high temperature holes the amino acid compositions are characterized by high abundances of glycine, alanine, serine, ornithine and histidine at depth. D/L ratios of samples with amino acid distributions similar to those found in acid hydrolysates of kerogen, indicate that racemization rates of amino acids bound by condensation reactions may be diminished.     

东太平洋北部胡安·德富卡洋中脊Mothra热液场硫化物烟囱体成矿物质来源:铅和硫同位素组成

对采自Mothra热液场Faulty Towers硫化物烟囱体群(47°57.447,N,129°06.568W)的一个硫化物烟囱体进行了铅和硫同位素组成的研究工作.分析结果表明,铅同位素组成的分布范围为:206Pb/204Pb=18.665～18.828; 207Pb/204Pb=15.460～15.607; 208...     

Rhyolite xenolith from the neovolcanic basalts of the rift valley of the Juan de Fuca Ridge, northeastern pacific: Reconstructsen MOR silicic rocks and basic magmas

In this paper, we discuss the formation conditions of rhyolites and results of their interaction with later portions of basic magmas on the basis of the investigation of melt and fluid inclusions in minerals from a rhyolite xenolith and host neovolcanic basalts of the Cleft segment of the Juan de Fuca Ridge. In terms of bulk chemistry and the compositions of melt inclusions in pyroxene and olivine phenocrysts, the basic rocks of the southern part of this segment are typical MOR basalts. Their olivine, clinopyroxene, and plagioclase crystallized at temperatures of 1160–1280°C and a pressure range between 20 and 100 MPa. The xenolith is a leucocratic rock with negligible amounts of mafic minerals, which clearly distinguishes it from the known occurrences of silicic rocks in the rift valleys of MOR. The rhyolite melt crystallized at temperatures of 900–880°C. The final stages of rhyolite melt crystallization at temperatures of 780–800°C were accompanied by the release of a saline aqueous fluid with high chloride contents. Based on the geochemical characteristics of melt inclusions and melting products, it can be suggested that the magmatic melt was produced by melting of metamorphosed oceanic crust within the Cleft segment under the influence sof saline aqueous fluid trapped in the pores and interstices of the rock. The rock represented by the xenolith is a late differentiation product of such melts. The ultimate products of silicic melt fractionation show high volatile contents: H₂O > 3.0 wt %, Cl ～ 2.0 wt %, and F ～ 0.1 wt %. The interaction of the xenolith with the host basaltic melt occurred at temperatures equal or slightly higher than those of ferrobasalt melts (1190–1180°C). During ascent the xenolith occurred for a few tens of hours in high-temperature basic magma, and diffusion exchange between the basaltic and silicic melts was very minor.     

Mercury in the Sedimentary Cover and Basalts of the Basement in the Recent Hydrothermal Activity Area,Middle Valley,Juan de Fuca Ridge

The content and distribution of mercury in Holocene–Upper Pleistocene turbidites, hemipelagic sediments intercalating therein, as well as basement basalts are studied. Samples of sediments were taken from the core of Holes 858A, 858B, 858C, 858D, and 858F. Basalt samples were taken from Holes 858F and 858G drilled during Leg 139 ODP in the Middle Valley (Juan de Fuca Ridge) in the Dead Dog hydrothermal field with a high heat flow (4–20 W/m²) and numerous vents with temperature ranging from 234 to 276°C. Samples of sediments and basalts with the background Hg content were taken from the core of Holes 855A, 855C, and 855D are located beyond the hydrothermal system in the base of the fault scarp on the eastern Middle Valley. In rocks, the content of Hg and its occurrence form were determined by the atomic absorption spectrometry with thermal atomization method; the chemical composition, by the XFA and ICP-MS methods. Sections of the sedimentary cover and basalt basement are marked by an alternation of “layer cake” type units with low and high contents of Hg. Mercury occurs in rocks in the physically adsorbed and mineral forms. The Hg concentration in some parts of the sedimentary section is anomalously high: up to 9696 ppb in Hole 858B and 7260 ppb in Hole 858C. In metalliferous sediments, the Hg content is 3130 ppb. Its maximum content (up to 23200 ppb) is recorded in basalts.     

Low-molecular weight hydrocarbons in vent fluids from the Main Endeavour Field, northern Juan de Fuca Ridge

Despite its location on sediment-free basalt, vent fluids from the Main Endeavour Field (MEF) contain chemical species that indicate fluids have interacted with sediments during circulation. We report on the distribution and isotopic abundances of organic compounds (C₁-C₃ alkanes and alkenes, benzene and toluene) in fluids collected from the Main Endeavour Field (MEF) in July, 2000, to understand the processes that regulate their abundances and characterize fluid sources. Aqueous organic compounds are derived from the thermal alteration of sedimentary organic matter and subsequently undergo further oxidation reactions during fluid flow. Fluid:sediment mass ratios calculated using ΣNH₄ concentrations indicate that the sediments are distal to the MEF, resulting in a common reservoir of fluids for all of the vents. Following the generation from sediment alteration, aqueous organic compounds undergo secondary alteration reactions via a stepwise oxidation reaction mechanism. Alkane distributions and isotopic compositions indicate that organic compounds in MEF fluids have undergone a greater extent of alteration as compared to Middle Valley fluids, either due to differences in subsurface redox conditions or the residence time of fluids at subsurface conditions. The distributions of the aromatic compounds benzene and toluene are qualitatively consistent with the subsurface conditions indicated by equilibration of aqueous alkanes and alkanes. However, benzene and toluene do not achieve chemical equilibrium in the subsurface. Methane and CO₂ also do not equilibrate chemically or isotopically at reaction zone temperatures, a likely result of an insufficient reaction time after addition of CO₂ from magmatic sources during upflow. The organic geochemistry supports the assumption that the sediments with which MEF fluids interact has the same composition as sediments present in Middle Valley itself, and highlight differences in subsurface reaction zone conditions and fluid flow pathways at these two sites.     

Timescale of open-reservoir evolution beneath the south Cleft segment, Juan de Fuca ridge

Lavas erupted at the southern end of the intermediate Juan de Fuca ridge (Cleft segment) are mostly cogenetic and their chemical diversity results from melt evolution in an open magma system. In the present study, we apply a theoretical model allowing the time evolution of this periodically recharged and tapped magma chamber to be estimated. In our mathematical procedure, the melt quantity supplied to the reservoir varies through time following a sinusoidal function. The rare earth element concentrations in the refilling melt were calculated on the basis of the REE distribution in lavas. This theoretical composition is akin to that previously estimated for a Mg#70 MORB from mineralogical and chemical data. Then, we approached the temporal evolution of the reservoir using a set of suitable parameters deduced from the geometry of the crust and magma system beneath the Cleft segment. Particularly, we considered two end-members scenarios for the melt repartition through the magma reservoir beneath the Cleft segment: the “gabbro glacier” model (crystal nucleation and growth occur within one single melt lens and crystals subside vertically and laterally) and the “sheeted sill” model (crystallization takes place within a network of connected sills located at various depths within the crust). We estimated that the magma chamber is refilled every thousand years and that the melt resides approximately one hundred years within the reservoir.     

Basalts of the North Fiji Basin: the generation of back arc basin magmas by mixing of depleted and enriched mantle sources

Active spreading ridges in the North Fiji Basin range from well-developed stable ridges where largescale mantle upwelling is in progress to proto-ridges where spreading is incipient. South of 17°S, where the central ridge of the North Fiji Basin has a bathymetric profile normally expected of a fast-spreading, steadystate mid-ocean ridge, basalts are evolved N-type MORBs. North of 17°S, where the central ridge is propagating northward into old North Fiji Basin crust and spreading is in the initial stages, two types of basalt have been recovered: N-type MORBs from this northern arm of the central ridge are believed to be samples of older North Fiji Basin crust; basalts with transitional alkalic chemistry (up to 0.5% Ne in the Norm) and characterized by strong relative enrichments in Rb, Ba, K, Nb, La, Ce, Sr, P, Zr, and Ti are believed to be associated with incipient rifting. Among the latter group are compositions that are intermediate between transitional alkalic types and MORBs and these are geochemically similar to the back-arc basin (BABB) magma type defined by Sinton and Fryer (1987) from a study of Mariana back arc basin basalts. Dredges along the South Pandora Ridge, a transform zone characterized by short spreading segments, are dominated by basalts that are enriched in large-ion lithophile and high field strength minor and trace elements and compositions range from types resembling ocean island tholeiites to transitional alkalic varieties. Basalts from Rotuma are regarded as alkalic end-members of the South Pandora Ridge magmatic spectrum. In areas of the North Fiji Basin where relatively fast spreading must be accompanied by largescale asthenospheric upwelling, depleted (N-type) MORBs dominate, whereas in areas of slow mantle upwelling, or where some other tectonic effect (e.g. a transform fault) causes a transient thermal disturbance within the lithosphere or upper asthenosphere, enriched (alkalic) magmas either dominate or make a significant and noticeable contribution to the overall chemical characteristics of basalts being erupted. The MORBs have a depleted asthenospheric source, and the alkalic component is believed to derive from an enriched lithospheric or shallow asthenospheric source. The BABB magma type may simply be part of the spectrum of mixed magmas that can occur in the transitional tectonic settings represented by the early development of most back-arc basins.     

First Data about Mercury in Modern Hydrothermal Process in the Ocean,Juan de Fuca Ridge

The results of studying Hg in an underwater hydrothermal system in the ocean using the Middle Valley of the Juan de Fuca ridge as an example are presented. A significant part of Hg is accumulated in the basalt fundament (Holes 858F, G), forming anomalously high concentrations (up to 29.30 ppm) in certain parts. The high Hg contents were established in metalliferous sediments (323 ppm) of the sedimentary cover (Hole 858D) and in sulfide deposits (up to 10.30 ppm). In other parts of the section, Hg content is 0.02–0.76 ppm (Holes 858B, D, F), background Hg contents in sediments—0.08–0.28 ppm and in basalts—0.17–0.31 ppm (Holes 855A, C, D).     

Late Cenozoic rotations of the Juan de Fuca Ridge and the Gorda Rise: A case study

In response to at least one change in the direction of sea-floor spreading, the Juan de Fuca Ridge and Gorda Rise have rotated approximately 20° clockwise with respect to geographic North during the last 10 million years. The rotation histories of these ridge segments have been determined from the ages and azimuths of linear magnetic anomalies within the corresponding “zed” patterns. In each case the rotations were systematic and occurred between about 9 and 3 Ma B.P. Significantly, the rotations occurred in a number of discrete stages during each of which the rates of rotation were approximately constant; rotation rates range from 1.3 to 8.6°/Ma.Though the rotation histories of these spreading centers are generally similar, some changes in the rotation rates are not synchronous, and until 3 Ma B.P., the Juan de Fuca Ridge had a 5–10° more easterly trend than the Gorda Rise. For the last 3 million years both ridge segments have had stable trends near 19°E of North.On a time scale of millions of years, ridge reorientation may be regarded as a continuous process wherein the rotation of the spreading center results from asymmetric spreading. Discontinuous changes in the degree of asymmetric spreading are required to account for observed changes in rotation rate. If the orthogonal arrangement of spreading centers and transform faults represents a least-work condition in which the resistance to plate motions is minimized by minimizing the lengths of ridge segments, as suggested previously, and if the rate at which the system seeks to reduce the total resistance after a change in spreading direction is maximum, it follows that the degree of asymmetric spreading, and hence the rate of rotation, are inversely proportional to the resistance to motion on transform faults. Thus, the various stages of rotation of the Juan de Fuca Ridge and Gorda Rise probably reflect different stress conditions on the Blanco Fracture Zone.It is difficult to account for the different trends of the Juan de Fuca Ridge and Gorda Rise largely because the Gorda Block is not behaving as a rigid plate and because the Mendocino Fracture Zone is not a transform fault. However, the fact that the Gorda Rise has had a stable trend for 3 million years, in spite of the deformation of an adjacent plate, suggests that the motion of the Gorda Block is not controlled by the motions of the vast Pacific and North American Plates, and that the Driving mechanism is “felt” directly at the ridge.     

The strontium isotopic composition of basalts from the Gordo and Juan de Fuca Rises,northeastern Pacific Ocean

The strontium isotopic compositions have been determined for twelve tholeiitic basalts dredged from the Gordo and Juan de Fuca Rises. Sr⁸⁷/Sr⁸⁶ ratios range from 0.7012 to 0.7031 and average 0.7026. These data, combined with other data from the East Pacific Rise indicate that tholeiite basalts being erupted along the active rises, in the Pacific Ocean, contain less radiogenic Sr⁸⁷ than basalts erupted on the islands. These isotopic differences between the ocean-ridge tholeiite and the more alkali island basalts indicate that variations in Rb/Sr have persisted in the mantle for billions of years. The possible origins and distribution of these heterogeneties are discussed.     

Peculiarities of Clay Mineral Formation in Sediments from the Hydrothermal System Center,Hole 858B,Juan de Fuca Ridge

Lithology and Mineral Resources - Clay minerals in Holocene–Pleistocene sediments from Hole 858B DSDP drilled at 20 m from the black smoker in the Dead Dog hydrothermal field, axial valley of...     

Lead isotope relations in oceanic Ridge basalts from the Juan de Fuca-Gorda Ridge area N.E. Pacific Ocean

Lead isotopic analyses of a suite of basaltic rocks from the Juan de Fuca-Gorda Ridge and nearby seamounts confirm an isotopically heterogeneous mantle known since 1966. The process of mixing during partial melting of a heterogeneous mantle necessarily produces linear data arrays that can be interpreted as secondary isochrons. Moreover, the position of the entire lead isotope array, with respect to the geochron, requires that U/Pb and Th/Pb values are progressively increased over the age of the earth. Partial melting theory also dictates analogous behavior for the other incompatible trace elements. This process explains not only the LIL element character of MOR basalts, but also duplicates the spread of radiogenic lead data collected from alkali-rich oceanic basalts. This dynamic, open-system model of lead isotopic and chemical evolution of the mantle is believed to be the direct result of tectonic flow and convective overturn within the mantle and is compatible with geophysical models of a dynamic earth.